 

 

    

”5mm“ , p , , . .. 5.3.3.13

  

o

 

 

. u
. 3.
.3;

Lu 1
.

 

3-.
rap.

pow—morn.

u: t‘vlw1bou0'oﬂmgn‘hn

 

 

 

Date

MICHIGAN STATEU

I II IIII II IIII II IIIIIIIIII II

59 2789

 

II

 

 

 

 

 

 

 

 

 

 

 

 

This is to certify that the

thesis entitled

The Effects of Fhotoperiod and Cold Treatment
on Fiowering of Twenty-five Species of
Herbaceous Perennials

' presented by
Erik Sanford Runkle

has been accepted towards fulﬁllment
of the requirements for .

Master degree in Science

 

 

ﬂ a/aa Af/Ml

/Major professor

10/29/96

 

0-7 639

MS U is an Afﬁrmative Action/Equal Opportunity Institution

 

 

LIBRARY

Michigan State
University

 

 

 

PLACE N RETURN BOXto move this chockout from your record.
TO AVOID FINES Mum on or baton duo duo.

DATE-DUE DATE DUE DATE DUE

      
 

 

 

 

A??? 0 S 19171
.9" 1:9

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

MSU loAn Namath. Mum/E Opportunity Intuition
M mm:

 

 

 

THE EFFECTS OF PHOTOPERIOD AND COLD TREATMENT ON
FLOWERING OF TWENTY-FIVE SPECIES OF HERBACEOUS PERENNIALS

By

Erik Sanford Runkle

A THESIS
Submitted to
Michigan State University

in partial fulﬁllment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Horticulture

1 996

ABSTRACT

THE EFFECTS OF PHOTOPERIOD AND COLD TREATMENT ON
FLOWERING OF TWENTY-FIVE SPECIES OF HERBACEOUS PERENNIALS

BY

Erik Sanford Runkle

Twenty-ﬁve herbaceous perennial species were treated at 5 °C for 0 or 15
weeks and placed under photoperiods of 10, 12, 13, 14, 16, or 24 hours of
continuous light or 9 hours plus a four-hour night interruption. Species were
categorized into several response groups based on the effects of cold and
photoperiod on ﬂowering. The cold treatment was required for ﬂowering of
seven species and improved ﬂowering of 16 species. The perennials were
obligate long-day, facultative long—day, or day-neutral plants. The effects of cold
and photoperiod on the percentage of ﬂowering, time to ﬂower, node
development, ﬂower number, and plant height are presented.

A separate study was conducted to determine the effect of night-
interruption duration and cyclic lighting on ﬂowering of six long-day herbaceous
perennial plants. Photoperiods were nine-hour natural days with night
interruptions for the following durations: 0.5, 1, 2, or 4 hours; 6 min on, 54 min off
for 4 hours (10% cyclic lighting); or 6 min on, 24 min off for 4 hours (20% cyclic
lighting). Response to night interruptions varied by species, but ﬁve of the six

species ﬂowered most rapidly and uniformly under four-hour night interruption.

ACKNOWLEDGMENTS

Many thanks to my major professor, Dr. Royal Heins, for his support,
encouragement, advice, and friendship throughout my program.

I would also like to express gratitude to Dr. Art Cameron and Dr. William
Carlson, whose guidance and camaraderie are greatly appreciated, and to Dr.
Jan Zeevaart for his valuable input and analysis. I would like to recognize the
many greenhouse growers who supported this work, for their generosity and
vision have made this research possible.

Thanks to all of the horticulture graduate students, past and present, who
have provided me with valuable advice, support, and especially friendship. A
special thanks to Bill Argo, Cheryl Hamaker, Paul Koreman, Bin Liu, Katie Nott,
Shi-Ying Wang, Cathy Whitman, and Mark Yelanich. I also owe many thanks to
Tom Wallace and his crew of students who provided the daily nurturing of the
plants in these experiments, and to Cara Wallace for her masterful editing.

Last but not least, I thank my family for their constant love and blessing.

TABLE OF CONTENTS

LIST OF TABLES ............................................... vii

LIST OF FIGURES ............................................... x

SECTION I

LITERATURE REVIEW ........................................... 1
Introduction ............................................... 2
Vemalization .............................................. 2
Photoperiodism ............................................ 7
Day-neutral Plants .......................................... 9
Vegetative Growth ......................................... 10
Reproductive Growth ....................................... 12
Daylength ................................................ 14
Manipulation of Daylength ................................... 16
Critical Photoperiod ........................................ 19
Phytochrome ............................................. 21
Types of Phytochromes ..................................... 25
Short—day Plants .......................................... 28
Hourglass Theory .......................................... 29
Endogenous Oscillator Theory ................................ 31
Long-day Plants ........................................... 34
The Role of Phytochrome in LDP ............................. 37
Flowering Stimulus Theory .................................. 41
Inhibitory Process Theory ................................... 45
Chemical Induction of Flowering .............................. 46
The Role of Temperature in Flowering ......................... 47
Summary ................................................ 56
References .............................................. 57

SECTION II

THE EFFECTS OF PHOTOPERIOD AND COLD TREATMENT ON
FLOWERING OF TWENTY-FIVE SPECIES OF HERBACEOUS

PERENNIALS .................................................. 65
Introduction .............................................. 66
Materials and Methods ...................................... 68

iv

Results and Discussion ..................................... 76

Day-Neutral Species That Beneﬁt from a Cold Treatment ..... 76
Armen’a xhybn'da ‘Dwarf Ornament Mix’ .............. 76
Annen'a pseudarmen’a ........................... 81
Scabiosa columbaria ‘Butterﬂy Blue’ ................ 81
Veronica spicata ‘Blue’ ........................... 87

Day-Neutral Species That Require a Cold Treatment ......... 90
La vandula angustifolia ‘Munstead Dwarf’ ............. 90
Phlox subulata ‘Emerald Blue’ ..................... 93
Veronica Iongifolia ‘Sunny Border Blue’ .............. 93

Facultative Long-day Species That Beneﬁt from a Cold

Treatment .......................................... 98
Leucanthemum xsuperbum ‘Snow Cap’ .............. 98
Leucanthemum xsuperbum ‘White Knight’ ........... 102
Lobelia xspeciosa ‘Compliment Scarlet’ ............. 102

Facultative Long-day Species That Require a Cold Treatment . 106
Coreopsis grandiﬂora ‘Sunray’ .................... 106
Gail/ardia xgrandiﬂora ‘Goblin’ .................... 1 11
Physostegia virginiana ‘Alba’ ..................... 114
Salvia x superba ‘Blue Queen’ .................... 118

Obligate Long—day Species That Beneﬁt from a Cold

Treatment ......................................... 1 18
Campanula carpatica ‘Blue Clips’ .................. 118
Coreopsis verticillata ‘Moonbeam’ ................. 125
Echinacea pumurea ‘Bravado’ .................... 128
Gypsophila paniculata ‘Double Snowﬂake’ ........... 129
Helenium autumnale ............................ 136
Oenothera missouriensis ........................ 139
Phlox paniculata ‘Eva Cullum’ .................... 142
Phlox paniculata ‘Tenor’ ......................... 142
Rudbeckia fulgida ‘Goldsturrn’ .................... 149

Other Responses ................................... 152
Asclepias tubemsa ............................. 152
Hibiscus xhybn'da ‘Disco Belle Mixed’ ............... 153

Conclusions ............................................. 158
References ............................................. 160

SECTION III
EFFECT OF NIGHT INTERRUPTION DURATION AND CYCLIC LIGHTING
ON FLOWERING OF LONG-DAY HERBACEOUS PERENNIAL PLANTS . . 163

Introduction ............................................. 164
Materials and Methods ..................................... 167
Results and Discussion .................................... 170

Campanula carpatica ‘Blue Clips’ ....................... 170

V

Coreopsis grandiﬂora ‘Early Sunrise’ .................... 171

Coreopsis verticillata ‘Moon beam’ ....................... 176
Echinacea purpurea ‘Bravado’ ......................... 176
Hibiscus xhybn'da ‘Disco Belle Mixed’ .................... 181
Rudbeckia fulgida ‘Goldsturrn’ .......................... 181
Conclusions ............................................. 186
References ............................................. 189

vi

Table

10.

11.

12.

13.

14.

15.

16.

LIST OF TABLES

Page
SECTION II

Species studied and characteristics of starting material ............ 69
Plug size, propagation date, dates of forcing, and average air
temperatures from date of forcing to average date of ﬂowering for
each species under each photoperiod .......................... 73
The effects of photoperiod and cold treatment on ﬂowering of
Annen’a xhybrida ‘Dwarf Ornament Mix’ ......................... 80
The effects of photoperiod and cold treatment on ﬂowering of
Annen'a pseudarmen'a ...................................... 83
The effects of photoperiod and cold treatment on ﬂowering of
Scabiosa columban'a ‘Butterﬂy Blue’ ........................... 86
The effects of photoperiod and cold treatment on ﬂowering of
Veronica spicata ‘Blue’ ...................................... 89
The effects of photoperiod and cold treatment on ﬂowering of
Lavandula angustifolia ‘Munstead Dwarf ....................... 92
The effects of photoperiod and cold treatment on ﬂowering of
Phlox subulata ‘Emerald Blue’. ............................... 95
The effects of photoperiod and cold treatment on ﬂowering of
Veronica Iongifolia ‘Sunny Border Blue’ ......................... 97
The effects of photoperiod and cold treatment on ﬂowering of
Leucanthemum xsuperbum ‘Snow Cap’ ........................ 100
The effects of photoperiod and cold treatment on ﬂowering of
Leucanthemum xsuperbum ‘White Knight’ ...................... 104
The effects of photoperiod and cold treatment on ﬂowering of
Lobe/fa xspeciosa ‘Compliment Scarlet’ ........................ 108
The effects of photoperiod and cold treatment on ﬂowering of
Coreopsis grandiﬂora ‘Sunray’ ............................... 1 10
The effects of photoperiod and cold treatment on ﬂowering of
Gaillardia xgrandiﬂora ‘Goblin’ ............................... 1 13
The effects of photoperiod and cold treatment on ﬂowering of
Physostegia virginiana ‘Alba’ ................................ 1 17
The effects of photoperiod and cold treatment on ﬂowering of
Salvia x superba ‘Blue Queen’ ............................... 120

vii

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

30.

31.

32.

33.

34.

35.

The effects of photoperiod and cold treatment on ﬂowering of

Campanula carpatica ‘Blue Clips’: 1994-95 ..................... 122
The effects of photoperiod and cold treatment on ﬂowering of

Campanula carpatica ‘Blue Clips‘: 1995-96 ..................... 124
The effects of photoperiod and cold treatment on ﬂowering of

Coreopsis verticillata ‘Moonbeam’ ............................ 127
The effects of photoperiod and cold treatment on ﬂowering of

Echinacea purpurea ‘Bravado’: 1994-95 ....................... 131
The effects of photoperiod and cold treatment on ﬂowering of

Echinacea purpurea ‘Bravado’: 1995-96 ....................... 133
The effects of photoperiod and cold treatment on ﬂowering of

Gypsophila paniculata ‘Double Snowﬂake’ ..................... 135
The effects of photoperiod and cold treatment on ﬂowering of

Helenium autumnale ...................................... 138
The effects of photoperiod and cold treatment on ﬂowering of

Oenothera missoun'ensis ................................... 141
The effects of photoperiod and cold treatment on ﬂowering of

Phlox paniculata ‘Eva Cullum’ ............................... 144
The effects of photoperiod and cold treatment on ﬂowering of

Phlox paniculata ‘Tenor’ .................................... 147
The effects of photoperiod and cold treatment on ﬂowering of

Rudbeckia fulgida ‘Goldsturrn’ ............................... 151
The effects of photoperiod on ﬂowering of cold-treated Asclepias

tuberosa ................................................ 1 55
The effects of photoperiod on ﬂowering of Hibiscus xhybrida

‘Disco Belle Mixed’ ........................................ 157

The recommended photoperiods for the most rapid, complete, and
uniform ﬂowering of cold-treated long-day herbaceous perennial
plants ................................................. 1 59

SECTION III
Species studied, characteristics of starting material, and average air
temperatures from date of forcing to average date of ﬂowering for

each species under each photoperiod ......................... 168
The effects of night-interruption duration and cyclic lighting on
ﬂowering of Campanula carpatica ‘Blue Clips' ................... 173
The effects of night-interruption duration and cyclic lighting on
ﬂowering of Coreopsis grandiﬂora ‘Early Sunrise’ ................ 175
The effects of night-interruption duration and cyclic lighting on
ﬂowering of Coreopsis verticillata ‘Moonbeam’ .................. 178
The effects of night-interruption duration and cyclic lighting on
ﬂowering of Echinacea purpurea ‘Bravado’ ..................... 180

viii

36.

37.

The effects of night-interruption duration and cyclic lighting on

ﬂowering of Hibiscus xhybrida ‘Disco Belle Mixed’ ................ 183
The effects of night-interruption duration and cyclic lighting on
ﬂowering of Rudbeckia fulgida ‘Goldsturm’ ..................... 185

ix

LIST OF FIGURES

Figure Page
SECTION I
1. Flower induction index of Lilium longiﬂorum ‘Nellie White’ as a
function of cold temperature and duration (Lange, 1993) ............ 4

2. Graphical illustrations of long-day plants (LDP), short-day plants
(SDP), and day-neutral plants. CDL = critical daylength. From

Vince-Prue, 1975 ........................................... 8
3. Biological daylength on clear days at 43 °N latitude ............... 15
4. The spectral distribution of four lamp types (Whitman, 1995) ........ 17
5. Phytochrome absorption spectrum (Vierstra and Quail, 1983) ....... 22
6. The differing rhythmic sensitivities to ﬂower induction in the LDP

white mustard and the SDP pigweed (Sweeney, 1987) ............. 36
7. The photothermal effects on ﬂowering of the SDP soyabean (top)

and the LDP lentil (bottom) (Roberts and Summerﬂeld, 1987) ....... 53

SECTION II

8. Flowering response categories of herbaceous perennial plants ...... 77
9. The effects of photoperiod and cold treatment on ﬂowering of

Annen'a xhybrida ‘Dwarf Ornament Mix’ ......................... 79
10. The effects of photoperiod and cold treatment on ﬂowering of

Anneria pseudanneria ...................................... 82

11. Percentage ﬂowering, time to ﬂower, and ﬂowering uniformity of
Annen'a pseudanneria under different photoperiods with or without
cold treatment. Numbers next to symbols represent photoperiods
consisting of nine-hour natural days that were extended with
incandescent lamps. NI = nine-hour natural days with four hours of

night interruption. Error bars are 95% conﬁdence intervals. ......... 84
12. The effects of photoperiod and cold treatment on ﬂowering of

Scabiosa columban‘a ‘Butterﬂy Blue’ ........................... 85
13. The effects of photoperiod and cold treatment on ﬂowering of

Veronica spicata ‘Blue’ ...................................... 88
14. The effects of photoperiod and cold treatment on ﬂowering of

Lavandula angustifolia ‘Munstead Dwarf’ ....................... 91

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

The effects of photoperiod and cold treatment on ﬂowering of

Phlox subulata “Emerald Blue‘ ................................ 94
The effects of photoperiod and cold treatment on ﬂowering of
Veronica Iongifolia ‘Sunny Border Blue’ ......................... 96
The effects of photoperiod and cold treatment on ﬂowering of
Leucanthemum xsuperbum ‘Snow Cap’ ......................... 99

Percentage ﬂowering, time to ﬂower, and ﬂowering uniformity of
Leucanthemum xsuperbum ‘Snow Cap’ under different photoperiods
with or without cold treatment. Numbers next to symbols represent
photoperiods consisting of nine-hour natural days that were extended
with incandescent lamps. NI = nine-hour natural days with four hours

of night interruption. Error bars are 95% conﬁdence intervals ...... 101
The effects of photoperiod and cold treatment on ﬂowering of
Leucanthemum xsuperbum ‘White Knight’ ...................... 103

Percentage ﬂowering, time to ﬂower, and ﬂowering uniformity of
Leucanthemum xsuperbum ‘White Knight’ under different
photoperiods with or without cold treatment. Numbers next to
symbols represent photoperiods consisting of nine-hour natural days
that were extended with incandescent lamps. NI = nine-hour natural
days with four hours of night interruption. Error bars are 95%
conﬁdence intervals. Symbols without error bars indicate that the

conﬁdence intervals were too large for the graph ................ 105
The effects of photoperiod and cold treatment on ﬂowering of
Lobelia xspeciosa ‘Compliment Scarlet’ ........................ 107
The effects of photoperiod and cold treatment on ﬂowering of
Coreopsis grandiﬂora ‘Sunray’ ............................... 109
The effects of photoperiod and cold treatment on ﬂowering of
Gail/ardia xgrandiﬂora ‘Goblin’ ............................... 112

Percentage ﬂowering, time to ﬂower, and ﬂowering uniformity of
Gail/ardia xgrandiﬂora ‘Goblin’ under different photoperiods with or
without cold treatment. Numbers next to symbols represent
photoperiods consisting of nine-hour natural days that were
extended with incandescent lamps. NI = nine-hour natural days
with four hours of night interruption. Error bars are 95% conﬁdence
intervals. Symbols without error bars indicate that the conﬁdence

intervals were too large for the graph ......................... 115
The effects of photoperiod and cold treatment on ﬂowering of
Physostegia virginiana ‘Alba’ ................................ 116
The effects of photoperiod and cold treatment on ﬂowering of
Salvia x superba ‘Blue Queen’ ............................... 119
The effects of photoperiod and cold treatment on ﬂowering of
Campanula carpatica ‘Blue Clips’. 1994-95 ..................... 121
The effects of photoperiod and cold treatment on ﬂowering of
Campanula carpatica ‘Blue Clips’. 1995-96 ..................... 123

xi

29.

30.

31.

32.

33.

34.

35.

36.

37.

38.

39.

40.

41.

The effects of photoperiod and cold treatment on ﬂowering of

Coreopsis verticillata ‘Moonbeam’ ............................ 126
The effects of photoperiod and cold treatment on ﬂowering of
Echinacea purpurea ‘Bravado’. 1994-95 ....................... 130
The effects of photoperiod and cold treatment on ﬂowering of
Echinacea purpurea ‘Bravado’. 1995-96 ....................... 132
The effects of photoperiod and cold treatment on ﬂowering of
Gypsophila paniculata ’Double Snowﬂake” ..................... 134
The effects of photoperiod and cold treatment on ﬂowering of
Helenium autumnale ...................................... 137
The effects of photoperiod and cold treatment on ﬂowering of
Oenothera missoun‘ensis ................................... 140
The effects of photoperiod and cold treatment on ﬂowering of
Phlox paniculata ‘Eva Cullum' ............................... 143

Percentage ﬂowering, time to ﬂower, and ﬂowering uniformity of
Phlox paniculata ‘Eva Cullum’ under different photoperiods with or
without cold treatment. Numbers next to symbols represent
photoperiods consisting of nine-hour natural days that were
extended with incandescent lamps. Nl = nine-hour natural days
with four hours of night interruption. Error bars are 95% conﬁdence
intervals. Symbols without error bars indicate that the conﬁdence

intervals were too large for the graph ......................... 145
The effects of photoperiod and cold treatment on ﬂowering of
Phlox paniculata ‘Tenor' .................................... 146

Percentage ﬂowering, time to ﬂower, and ﬂowering uniformity of
Phlox paniculata ‘Tenor‘ under different photoperiods with or
without cold treatment. Numbers next to symbols represent
photoperiods consisting of nine-hour natural days that were
extended with incandescent lamps. NI = nine-hour natural days

' with four hours of night interruption. Error bars are 95% conﬁdence

intervals. Symbols without error bars indicate that the conﬁdence

intervals were too large for the graph ......................... 148
The effects of photoperiod and cold treatment on ﬂowering of

Rudbeckia fulgida ‘Goldsturrn’ ............................... 150
The effects of photoperiod on ﬂowering of cold-treated

Asclepias tuberosa ........................................ 154
The effects of photoperiod on ﬂowering of non-cold treated

Hibiscus xhybrida ‘Disco Belle Mixed’ ......................... 156

xii

42.

43.

44.

45.

46.

47.

SECTION III
Flowering of Campanula carpatica ‘Blue Clips’ under various
durations of night interruption or cyclic lighting. Night interruption
was provided by incandescent lamps that were turned on during
the middle of 15-hour dark periods. For the cyclic lighting
treatments, lights were on for six minutes every 30 or 60 minutes
for a four hour period during the middle of the night. Error bars

are 95% conﬁdence intervals ................................ 172

Flowering of Coreopsis grandiﬂora ‘Early Sunrise’ under various
durations of night interruption or cyclic lighting. Night interruption
was provided by incandescent lamps that were turned on during
the middle of 15-hour dark periods. For the cyclic lighting
treatments, lights were on for six minutes every 30 or 60 minutes
for a four hour period during the middle of the night. Error bars

are 95% conﬁdence intervals ................................ 174

Flowering of Coreopsis verticillata ‘Moonbeam’ under various
durations of night interruption or cyclic lighting. Night interruption
was provided by incandescent lamps that were turned on during
the middle of 15-hour dark periods. For the cyclic lighting
treatments, lights were on for six minutes every 30 or 60 minutes
for a four hour period during the middle of the night. Error bars

are 95% conﬁdence intervals ................................ 177

Flowering of Echinacea purpurea ‘Bravado’ under various
durations of night interruption or cyclic lighting. Night interruption
was provided by incandescent lamps that were turned on during
the middle of 15-hour dark periods. For the cyclic lighting
treatments, lights were on for six minutes every 30 or 60 minutes
for a four hour period during the middle of the night. Error bars

are 95% conﬁdence intervals ................................ 179

Flowering of Hibiscus xhybrida ‘Disco Belle Mixed’ under various
durations of night interruption or cyclic lighting. Night interruption
was provided by incandescent lamps that were turned on during
the middle of 15-hour dark periods. For the cyclic lighting
treatments, lights were on for six minutes every 30 or 60 minutes
for a four hour period during the middle of the night. Error bars

are 95% conﬁdence intervals ................................ 182

Flowering of Rudbeckia fulgida ‘Goldsturm’ under various
durations of night interruption or cyclic lighting. Night interruption
was provided by incandescent lamps that were turned on during
the middle of 15-hour dark periods. For the cyclic lighting
treatments, lights were on for six minutes every 30 or 60 minutes
for a four hour period during the middle of the night. Error bars

are 95% conﬁdence intervals ................................ 184

xiii

48.

Time to ﬂower and uniformity of ﬂowering under the six night

interruption treatments for six species of herbaceous perennials.

Night interruption was provided by incandescent lamps that were

turned of during the middle of 15-hour dark periods. Cyclic lighting

was for six minutes every 30 or 60 minutes for a four hour period

during the middle of the night . Data are results from Experiment

1. The absence of a point indicates that no plants ﬂowered under

that treatment ............................................ 187

xiv

SECTION I

LITERATURE REVIEW

Introduction

Some plants ﬂower independent of the surrounding environmental
conditions, which is known as autonomous ﬂowering. Others ﬂower in response
to one or more environmental stimuli. The environmental conditions that induce
ﬂowering are species-speciﬁc. Plants of the same species but with different
genotypes (varieties, subspecies, and cultivars) may have different ﬂowering
requirements (Vince-Prue, 1975). Some plants such as African violets, roses,
and cyclamen ﬂower only in response to temperature in the presence of
adequate radiant energy. Other plants, including many herbaceous perennials,
ﬂower only after exposure to temperatures less than 7 °C for a certain period of
time. This is known as vemalization. Many plants, including poinsettias and
Chrysanthemums, ﬂower in response to the duration and timing of light and dark
periods in a day or series of days, which is known as photoperiodism (Vince-
Prue, 1984). Temperature and photoperiod interact to play signiﬁcant roles in

the ﬂowering process of many plants.

Vemalization

Vemalization is a cold treatment given to an imbibed seed, bulb, or plant
that promotes ﬂowering at subsequent higher temperatures (Vince-Prue, 1975).
Vemalization leads to ﬂower induction sometime after the cold temperature
treatment. In many plants, ﬂoral initials are not present immediately after a plant

is vemalized; they differentiate only when the plant is exposed to higher

3

temperatures (Zeevaart, 1978). Other plants form ﬂoral initials during the cold
temperature treatment. Vemalization does not affect the ﬂowering process of all
plants. Some plants ﬂower only when vernalized; others ﬂower faster if
vernalized. For example, there are three types of vemalization responses in
winter wheat (Triticum aestivum L.) cultivars: 1) cold-obligate, or qualitative, 2)
cold-stimulated, or quantitative, and 3) cold-neutral, or unresponsive (Gardner
and Barnett, 1990).

Vemalization occurs in apices of shoots. A cold treatment is perceived by
shoot tips; the leaves of a plant do not affect vemalization. Chilling the roots of
penny cress (Thlaspi arvense L.) was ineffective, whereas chilling only the shoot
tips initiated reproductive development (Metzger, 1988). However, leaf cuttings
taken from vernalized penny cress plants exhibited signs of ﬂower development,
while cuttings taken from unvemalized plants developed into vegetative rosettes
(Metzger, 1988), which suggests the shoot apex is not the only tissue capable of
being vernalized, and some new meristems are potential sites for vemalization.

The length and effective temperature range for vemalization varies by
species. In general, plants require several weeks of cold to saturate the
vemalization response. Forty-six percent of the ‘Gloriosa’ blazing-star (Liatn’s
spicata erld.) herbaceous perennials that received six weeks of 3 to 5 °C
ﬂowered, whereas 90% that received eight weeks of 3 to 5 °C ﬂowered
(Waithaka and Wanjao, 1982). This suggests that ‘Gloriosa' blazing-star

requires at least eight weeks of cold for most plants to become vernalized. The

4

most effective temperature range for vemalization of most plants is 1 to 7 °C

(Lang, 1965), although higher and lower temperatures are effective for some

plants. ‘Nellie White’

 

 

Easter lily bulbs (Lilium

 

Iongiﬂorum Thunb.)
vernalized for eight
weeks at 5 °C had the

highest ﬂower induction

 

 

Flower Induction Index

 

 

index, a relationship 0.0 ' . - .
0 5 1 O l 5 20 25

that represents the Temperalure (C)

 

 

 

relative ﬂower-induction Figure 1. Flower induction index of Lilium Iongiﬂorum
‘Nellie White’ as a function of cold temperature and
effectiveness of a cold duration (Lange, 1993).

treatment (Figure 1) (Lange, 1993). As the temperature varied from 5 °C and the
length of vemalization decreased, the ﬂower induction index rapidly decreased.
Vemalization is not effective for juvenile plants, and time to maturity is

species dependent. Some seeds can be vernalized, but the juvenile phase may
last weeks or months. In winter wheat cultivars, seeds became saturated with
cold treatment (most were vernalized) after 49 days, while seedlings with two or
seven leaves were saturated after 42 or 35 days of vemalization, respectively
(Wang et al., 1995). The minimum vemalization duration required for saturation
in winter wheat cultivars decreased linearly as plant age increased (Wang et al.,

1995). In silver-dollar plant (Lunan’a annua L.), a biennial, three- and ﬁve-week-

5

old plants did not ﬂower after being vernalized, and were apparently juvenile.
Plants nine weeks old and older were completely mature, and all ﬂowered after
being vernalized; those seven weeks old were judged intermediate, and half of
them ﬂowered (Wellensiek, 1958).

A plant may become devemalized, which means loss of the vernalized
condition. The most common way is from a few days’ exposure to high
temperatures (30 to 35 °C) immediately following a vemalization treatment
(Thomas and Vince-Prue, 1984). However, once the cold requirement becomes
saturated, the vernalized condition is extremely stable, and devemalization is
nearly impossible (Thomas and Vince-Prue, 1984).

Vemalization also can be deﬁned as the biochemical processes that occur
during a cold treatment (Napp-Zinn, 1987). The internal processes are
controlled by the genetics of a plant and signaled by environmental conditions.
Vemalization is triggered by either dominant or recessive alleles (Napp-Zinn,
1987). Garden peas’ (Pisum sativum L.) dominant alleles cause synthesis of a
ﬂower inhibitor, which is not produced when only recessive alleles are present.
Vemalization reduces ﬂower inhibitor synthesis by the dominant gene that
causes ﬂowering (Napp-Zinn, 1987). Vemalization requirements may be caused
by recessive alleles, as are those of winter wheat and mouse-ear cress
(Arabidopsis thaliana Heynh.) (Napp-Zinn, 1987). In this case, a ﬂower-
promoting substance formed in the presence of the dominant allele is absent.

Several genes are involved in vemalization, and the identity and function of each

6

is not well understood. Nearly genetically identical lines of a species may react
differently to a cold treatment. After three near-isogenic lines of winter wheat
were vernalized for zero to 11 weeks at 4 °C, one line showed a quantitative
vemalization response, and two showed an all-or-nothing ﬂowering response
(Flood and Halloran, 1984).

Applying gibberellic acid (GA3) may substitute for either vemalization or
inductive photoperiods for some species (see summary by Lang, 1965).
Vemalization of penny cress alters GA metabolism in the shoot tip, which may be
the mechanism that induces ﬂowering (Hazebroek and Metzger, 1990). The
endogenous levels of kaurenoic acid, a GA precursor, in penny cress shoot tips
decreased 10-fold and 50-fold two and 10 days, respectively, after plants were
returned to 21 °C following four weeks of vemalization at 6 °C (Hazebroek et al.,
1993). There was no change in the endogenous levels of kaurenoic acid in the
leaves. The activity of an enzyme that dictates changes in the conversion of
kaurenoic acid to 7-OH kaurenoic acid rapidly increased in shoots tips following
the vemalization treatment; there was no increase in activity in the leaves.
Hazebroek et al. (1993) have proposed that the conversion of kaurenoic acid to
7-OH kaurenoic acid is the primary step in GA metabolism regulated by
vemalization in penny cress shoot tips.

Recently, it has been postulated that DNA methylation provides a
developmental control that prevents ﬂower initiation, and vemalization releases

the block to ﬂower initiation by demethylation. The demethylation of a promoter

7

of a gene responsible for ﬂowering allows its transcription, and the plant is
induced to ﬂower. Mouse-ear cress and penny cress plants treated with 5-
azacytidine, a DNA demethylating agent, induced unvemalized plants to ﬂower
signiﬁcantly faster than untreated control plants (Burn et al., 1993). Plants
insensitive to vemalization did not respond to the demethylating agent. In a
separate experiment, 5-azacytidine induced ﬂowering of penny cress cultivars
signiﬁcantly earlier than that of unvemalized plants, although not nearly as
rapidly as that of plants vernalized for six weeks at 2 °C (Brock and Davidson,
1994). Gamma ray treatments also induced ﬂowering rapidly compared with that
of unvemalized ‘Wlnco’ penny cress plants, which suggests gamma rays may
act as a demethylating agent similar to 5-Azacytidine (Brock and Davidson,
1994). Under certain conditions, both treatments partially substituted for cold

treatment in promoting winter wheat ﬂowering.

Photoperiodism

Plants sexually and asexually reproduce when environmental conditions
are favorable for production and distribution of seeds and formation of bulbs,
tubers, runners, etc. Many plants therefore have mechanisms that interpret
seasonal changes by measurement of photoperiod. Photoperiod is the only
completely reliable environmental signal with respect to calendar date at a given
latitude. Plants that originate above or below around 30° north and south

latitude, respectively, are exposed to pronounced changes in daylength as the

8

seasons change. Plants that originate closer to the equator are exposed to

small changes in daylength as the seasons change, but there are examples of

photoperiodic control of ﬂowering even close to the equator. One often can

predict accurately if and how photoperiod affects ﬂowering by knowing from

where a plant evolved.
Plants have been
divided into three main
categories on the basis of
ﬂowering in response to
photoperiod (Figure 2).
Day-neutral plants (DNP)
ﬂower regardless of the
photoperiod to which they
I are exposed. Short—day
plants (SDP) only ﬂower, or
ﬂower most rapidly, when
exposed to fewer than a
certain number of hours of
light in a 24-hour cycle. In
contrast, long-day plants
(LDP) only ﬂower, or ﬂower

quicker, when exposed to

 

 

 

LDP Hyoscyamus niger

 

 

Relative flowering response

 

 

 

 

 

 

CDL
I I l r
SDP Xanthium strumarium
CDL
l l l l
T _ _ ‘7
/
/
l 7
/

Day-neutral plant
1 L l u
6 12 18 24

Duration of daily photoperiod/h

 

Figure 2. Graphical illustrations of long-day plants
(LDP), short-day plants (SDP), and day-neutral
plants. CDL = critical daylength. From Vince-

Prue, 1975.

 

9

more than a certain number of hours of light in each 24-hour cycle. However, it
has been shown that the length of the dark period is the critical factor for ﬂower
induction: SDP require uninterrupted nights longer than a certain duration, and
LDP require a limited darkness duration. The number of photoperiod cycles
required for ﬂowering varies tremendously by species, from as little as one to
more than 70. SDP and LDP can be subdivided further: plants may have either
a qualitative or a quantitative response to photoperiod. A qualitative response,
also known as an absolute or obligate response, means the plant requires
daylengths that are either shorter or longer than a certain duration to ﬂower. For
example, a qualitative LDP must have photoperiods that meet or exceed a
particular duration to ﬂower. A quantitative photoperiodic response describes a
particular daylength that hastens, but is not essential for, ﬂowering. A
quantitative LDP will ﬂower under short days, but will ﬂower quicker under long

days.

Day-neutral Plants

Some plants exhibit little or no ﬂowering response to daylength. DNP may
ﬂower any time of the year under any daylength. Virtually all DNP, including
African violets, cyclamen, and roses have no speciﬁc environmental
requirements for ﬂower induction, other than adequate light levels and
temperatures. “Sentimental Blue’ balloon ﬂower (Platycodon grandiﬂorus A. DC.

‘Sentimental Blue’) plants grown under 10-hour (SD) or 16-hour (LD)

10

photoperiods ﬂowered roughly simultaneously (Song et al, 1993). Because
ﬂowering was not affected by photoperiod, ’Sentimental Blue’ is a DNP. Other
DNP are cucumber, annual bluegrass, rice, tomato, and some varieties of corn
and tobacco (Salisbury, 1981; Vince-Prue, 1975).

Some DNP are induced to ﬂower by high or low temperatures or by
temperature ﬂuctuations. Bulbous plants have mechanisms to survive low and
high temperatures, drought, or both. Shoots of bulbs that actively grow above
the soil in the spring generally rest in the summer, when temperatures are high.
Growth resumes in the fall but often is underground. Many bulbs, including
Tulipa, Freesia, Narcissus, and Hyacinthus, require a warrn-cold-warm sequence
to complete their life cycle (LeNard and De Hertogh, 1993). Other bulbs,
including Allium, Gladiolus, and Lilium, need a cold-warm-cold temperature
sequence to ﬂower and complete their life cycle (LeNard and De Hertogh, 1993).
The three general stages of the life cycle of bulbous plants are the initiation of
leaves in the bulb, ﬂower formation in the bulb, and elongation, growth, and
above ground development. Temperature is generally the most important
environmental factor for bulb growth, development, and ﬂowering (LeNard and

De Hertogh, 1993).

Vegetative Growth
In most plants, some vegetative growth is required before ﬂowering can

take place (Vince-Prue, 1975). Photoperiod affects vegetative growth as well as

11

reproductive growth. Vertical growth of LDP generally is restricted by short days
(SD) and is promoted by long days (LD). Many LDP develop only as leaf
rosettes during short photoperiods. LD favor stem growth of gymnosperrns and
runner development of strawberries, and induce bulb formation in onions.
Photoperiod also can inﬂuence the number of vegetative and reproductive
stems. Dense—ﬂowered loosestrife (Lysimachia congestiﬂora Hemsl.) averaged
27 vegetative stems and one ﬂowering stem under 8-hour photoperiods, 22
vegetative and ﬁve ﬂowering stems under 12-hour photoperiods, and one
vegetative stem and 21 ﬂowering stems under 16-hour photoperiods (Zhang et
al., 1995). Leaf initiation and expansion also were affected by photoperiod. The
minimum leaf number required before ﬂowering can occur has been determined
for some species. This vegetative phase is deﬁned as “juvenile” or “basic” as
discussed in the vemalization section, and is assumed insensitive to photoperiod
for ﬂowering. We are most concerned with the phases beyond the juvenile
phase.

E.H. Roberts and R.J. Summerﬁeld have distinguished four phases
through which all seed plants that ﬂower in response to photoperiod proceed
(Roberts et al., 1986). First is a preemergence phase that lasts from germination
to emergence through the soil surface. The seedlings are in darkness and
therefore are presumably insensitive to photoperiodic stimulation. The second
phase is the preinductive phase, also known as the juvenile or basic vegetative

phase, that begins at shoot emergence and exposure to light. This phase is a

12

period of relative, if not complete, insensitivity to photoperiod. The length of the
preinductive phase is species-speciﬁc and may either not exist if the ﬁrst leaves
are photoperiodically sensitive or last for several years, as is the case with many
woody plants. In the quantitative SDP soybean (Glycine max Merrill) the
duration of the photoperiod-insensitive preinductive phase lasted approximately
18 days (Ellis et al., 1992). Following the preinductive phase is an inductive
phase in which the plant is very sensitive to photoperiod, and its duration varies
too. The inductive phase persists in less-inductive regimes; for a quantitative
SDP, LD increase the length of the inductive phase, and in quantitative LDP, SD
increase the length of the inductive phase. Many, but not all, plants proceed
through the fourth phase, called the postinductive phase. This phase extends
through the ﬂowering process and is insensitive to photoperiod. In studies with
rice (Oryza sativa L.) the duration of the two photoperiod-insensitive phases
decreased as temperature increased. No consistent effects of temperature were
apparent for the duration of the photoperiod-sensitive inductive phase (Collinson

etaL,1992)

Reproductive Growth

There are also several phases during the ﬂowering process. Any of these
stages may be affected by photoperiod, depending on the species. The ﬁrst
stage is ﬂower induction, which is the biochemical change in a plant. The

second stage is ﬂower initiation, the ﬁrst physical evidence of the morphological

13

change, in which one can discern the ﬂoral inﬂorescence, the buds, or both. A
microscope allows this stage to be divided into many substages primarily by
redifferentiation of the reproductive meristem and ﬂower bud size (Salisbury and
Ross, 1978). The next stage is ﬂower development, in which the inﬂorescence
and ﬂowers develop and expand. The ﬂowers then open, which is the fourth
stage. The ﬁnal stage is anthesis, when pollen is shed by the ﬂower.

In studying the LDP spinach (Spinacia oleracea L.), Knott (1934)
discovered that photoperiodism is perceived by the leaves of a plant. When a
spinach plant had its leaves removed, exposure to LD photoinduction cycles did
not cause ﬂoral initiation. If the plant was defoliated except for one leaf, the
photoinduction cycle caused ﬂoral initiation. A leafs sensitivity to daylength
often varies with age. In general, plants become more sensitive to daylength as
they grow older, perhaps because young leaves do not export carbohydrates
(Vince-Prue, 1975). Other studies show that peak photoperiod sensitivity occurs
in the newest leaves and those half-expanded. Photoperiodic sensitivity of the
leaves of several cultivars of Chrysanthemum gradually decreases with
increasing age, until there is no sensitivity (Ochesanu and Barbat, 1965).
According to Lang (1965), peak sensitivity in most plants is reached when a leaf

has just attained full size.

14
Daylength

In photoperiodism, a plant perceives day and night duration and, in
response to one or both, initiates ﬂowering (Salisbury, 1981). There are
substantial changes in the spectral composition of natural light as the day begins
and ends, particularly in the red to far-red ratio (RzFR) (Hughes et al., 1984).
However, it is most likely the. change from night to day and vice verse is signaled
by exceeding or falling below a particular value of irradiance or photon ﬂuence
rate, not by a change in the spectral quality (Hughes et al., 1984). Plants’
sensitivity to light varies tremendously by species, but in general, they perceive a
very low illuminance. The threshold light value may be deﬁned as the lowest
intensity at which a plant still perceives the light. The SDP Mexican bush sage
did not ﬂower when exposed to four-hour night breaks with an intensity of 2.3
umol-m'Z-S'1 or higher in the 400-700 nm wave band, but did ﬂower under a night
break intensity of 1.3 umol-m"’-s‘1 or lower; the results suggest the threshold
light level for Mexican bush sage is somewhere between 1.3 and 2.3 ,umol-m‘z’s‘1
(Armitage and Laushman, 1989).

The "natural daylength“ commonly has been deﬁned as the length of the
day between civil twilights. Civil twilight begins in the morning when the center of
the sun is 6° below the horizon and lasts until sunrise, and begins in the evening
at sunset and lasts until the center of the sun is 6° below the horizon (Grifﬁths,
1976). The illuminance at the beginning of civil twilight in the morning and at the

end of civil twilight in the evening is around 0.06 umol-m"‘-S'1 (Grifﬁths, 1976).

15
The higher the latitude, the longer the civil twilight. Civil twilight lasts 21 to 23

minutes at the equator, depending on the time of year (Grifﬁths, 1976); 27 to 33
minutes at 40° latitude; and 41 to 108 minutes at 60° latitude (Grifﬁths, 1976).

In East Lansing, Michigan, which is N 43° latitude, heavily overcast skies
signiﬁcantly reduced the length of daylight above 0.25 umol-m'zs1 (Faust and
Heins, 1994). Under clear skies in a glass greenhouse in September, light levels
exceeded 0.25 umol-m‘z-s" for nearly 20 minutes before sunrise through 20
minutes after sunset; under heavily overcast skies, the duration was only 5
minutes before sunrise through 5 minutes after sunset. Outside the greenhouse,

light levels exceeded 0.25 umol-m""-s‘1 approximately 12 minutes longer before

 

"Biological" Daylength on Clear Days
at 43 degrees North Latitude

17
16

 

Jan Mar May Jul Sep Nov
Feb Apr Jun Aug Oct Dec

 

 

 

Figure 3. Biological daylength on clear days at 43 °N latitude.

16

and after sunrise and sunset than inside the greenhouse. Therefore, the natural
daylength under clear skies for many plants at 40° latitude is roughly 40 minutes
longer than the daylength duration from sunrise to sunset, and cloudy

weathercan reduce the duration of the natural daylength. Figure 3 illustrates the

biological daylength, the approximate duration that plants perceive light.

Manipulation of Daylength

In the greenhouse industry, the photoperiod often is shortened or
lengthened artiﬁcially to keep plants vegetative or induce ﬂowering. Under
natural LD, SD are created by blocking out light; i.e., by covering the plants with
blackcloth. Under natural SD, LD are created by adding light beyond the
daylength. There are four ways to extend natural SD into LD: lighting before
dusk and into the night (day extension), interrupting the night with a period of
light (night interruption or night break), lighting before the end of the night until
after dawn (predawn lighting), and lighting continuously (24 hours a day).
Traditionally, night interruption has been the method of choice for delivering LD,
and many of the studies of LDP, including that of the role of phytochrome, have
been with night interruption. Plants respond differently to the timing of the light
period at night; some methods of creating LD more effectively induce ﬂowering of
some LDP species than others. Continuously lighting baby’s-breath, an LDP,

caused plants to ﬂower in 91 days; 4-hour predawn and 4-hour night-interruption

17

lighting caused plants to ﬂower in 125 days; 4-hour day-extension lighting

caused plants to ﬂower in 148 days (Shillo and Halevy, 1982).

For most plants, yellow, and especially the red, regions of the spectrum

most effectively promote ﬂowering in LDP and prevent ﬂowering in SDP when

used to extend natural SD (Vince-Prue, 1975). When plants are irradiated with

similar red-light intensities of blue, green, or violet, many hardly perceive the

light, and in many instances, the light is equivalent to darkness (Vince-Prue,

1975). For some species, blue light must be 20 to 250 times more intense than

red light to be equally effective for promoting or preventing ﬂowering (Vince-

Prue, 1975).

There are
several types of
electrical lamps used
to provide
supplemental
greenhouse light to
plants. The four
most common lamps
are ﬂuorescent,
metal halide, high-
pressure sodium,

and incandescent.

 

 

1 m-Z s-1

pmol nm'

-1

pmol nm'1 m‘zs

 

.0 .o
—I A
O U!
I I

.°
c:
on
I

 

(A) Cool white ﬂuorescent I

 

 

(B) High pressure sodium

 

AA.

 

 

 

9
.5
01
I

.o .o
o .5
01 O
I I

 

0.00 -

(C) Incandescent

 

__/—I

r 1 | a
400 500 600 700 800
n rn

 

(D) Metal halide

 

 

—I I I . '—"
400 500 600 700 800

nm

 

Figure 4. The spectral distribution of four lamp types
(Whitman, 1995).

 

18

There are many differences among lamp types, including spectral distributions
(Figure 4). Cool-white ﬂuorescent lamps emit primarily blue, green, and yellow
light. Metal halide lamps emit mostly blue and violet light and some green and
yellow light. High-pressure sodium lamps, the most common for photosynthetic
lighting in ﬂoriculture, emit yellow and orange light. Incandescent lamps emit
relatively high amounts of red and far-red light. Because red light most
effectively promotes ﬂowering in LDP, the most effective artiﬁcial light source for
extending the number of hours of natural light should be incandescent lamps. A
blend of red and far-red light is desired for decreasing the length of the dark
period, so incandescent lamps most effectively promote ﬂower induction
(Deitzer, 1984). However, the value of lighting with incandescent lamps must be
weighed in relation to their effect on stem elongation (Vince-Prue, 1975).

It may be beneﬁcial to use ﬂuorescent lamps if the plants adequately
perceive the light; ﬂuorescent lamps emit very low amounts of far-red light, which
may limit overall plant height. However, some plants do not respond to light from
various sources. In a glass greenhouse, a 4-hour lighting treatment with cool-
white ﬂorescent lamps to create LD caused all baby’s-breath plants to remain
vegetative, regardless of when the treatment was delivered (Shillo and Halevy,
1982). In contrast, LD delivered with incandescent lamps induced ﬂowering. For
inducing ﬂowering, a combination of one 40-W cool-white ﬂuorescent lamp and
two 60-W incandescent lamps was equal to or better than only incandescent

lamps of the same intensity (Shillo and Halevy, 1982). The LDP black-eyed

19

Susan (Rudbeckia hirta var. pulchern'ma L.) perceived LD when grown under
ﬂuorescent illumination at 161 umol-m'z-S'1 (Podol’nyi and Chetverikov, 1986).
However, this high an intensity of ﬂuorescent light easily would contain enough
red light to elicit the ﬂowering response. Whitman (1995) found that, even at low
intensities (<1.0 umOI'lTl'z'S'", incandescent, cool white ﬂuorescent, metal halide,
and high pressure sodium lamps were effective for ﬂower induction of four
species of long-day herbaceous perennials: Campanula carpatica ‘Blue Clips’,
Coreopsis grandiﬂora ‘Early Sunrise’, Coreopsis verticillata ‘Moonbeam’, and

Rudbeckia fulgida ‘Goldsturm’.

Critical Photoperiod

The critical photoperiod is deﬁned by Vince-Prue as the daylength at
which 50% of the same species ﬂowers (Vince-Prue, 1975). The critical
photoperiod marks the transition from vegetative growth to reproductive growth
in a population of one genotype. This deﬁnition applies to SDP and LDP and
does not consider time as a factor. Roberts and Summerﬁeld (1987) deﬁne
critical photoperiod in SDP as “that photoperiod at or below which the time to
ﬂower is minimal and is not affected by variations in daylength; photoperiods
longer than [the critical photoperiod] delay ﬂowering.” Roberts and Summerﬁeld
(1987) propose several deﬁnitions for the critical photoperiod of LDP; of those,
the following deﬁnition is useful: “that photoperiod above which time to ﬂowering

is minimal and not affected by further increases in photoperiod, and below which

20

ﬂowering is delayed.” From this deﬁnition, an LDP that ﬂowers most rapidly
under 24-hour continuous light would have a critical photoperiod of 24 hours.
Horticulturally, percent ﬂowering, time to ﬂower, and uniformity are all required
components of a deﬁnition. Thus, critical photoperiod will be referred to as that
photoperiod that elicits a population of the same genotype to ﬂower completely,
rapidly, and uniformly. Thus, the critical photoperiod of LDP is that photoperiod
which, if met or exceeded, elicits an identical population of plants to ﬂower
completely, rapidly, and uniformly. Plants provided daylengths shorter than the
critical daylength may still ﬂower, but more slowly, less uniformly, or only
partially. The critical photoperiod can differ with different species, or even
different cultivars within the same species.

Roberts and Summerﬁeld (1987) also propose two additional ﬂowering
photoperiod concepts for SDP and LDP: the base photoperiod and the ceiling
photoperiod. The base photoperiod for SDP is that photoperiod at which, if
lengthened, plants remain permanently vegetative; for LDP, that photoperiod at
which, if shortened, plants remain permanently vegetative. The base
photoperiod concept can apply only to qualitative LDP or SDP, since quantitative
SDP or LDP eventually ﬂower under SD and LD. The ceiling photoperiod for
SDP is that photoperiod below which ﬂowering is hastened; for LDP, above
which ﬂowering is hastened. Again, the ceiling photoperiod can apply only to

qualitative SDP or LDP.

21

The critical photoperiod of a species may change to some degree with
changes in environmental conditions or plant age. As ﬂoriculturists, we are more
concerned with what daylength keeps a species vegetative when one desires
vegetative growth, and what daylength is required for ﬂowering when one wants
the plant to ﬂower, perhaps most rapidly. Therefore, knowing the critical
photoperiod of a plant is useful so that one can either prevent or initiate
ﬂowering, whichever is desired. Cuttings propagated under photoinductive
cycles favor reproductive growth, not desired vegetative growth. Cuttings of two
cultivars of obedience plant (Physostegia virginiana L. ‘Summer Snow’ and
‘Vrvid’), both LDP, rooted well under SD but rooted poorly under LD (Beattie et
al., 1989). Production time decreased and plant quality increased when stock
plants from which the cuttings were taken were grown under SD, and rooted
cuttings then were forced to ﬂower under LD (Beattie et al., 1989). For most
LDP, exceeding the critical daylength induces a higher percentage of the same
species to ﬂower, and faster. For most SDP, reducing the critical daylength
below the base photoperiod, yet still long enough for active photosynthesis,

increases the percentage of plants that ﬂower and hastens ﬂowering.

Phytochrome
The quality of light describes the spectral energy distribution curve. The
wavelengths of electromagnetic radiation (light) that humans can detect is similar

to the photosynthetically active radiation wave band in plants: 400-700 nm. Light

22

quality has a profound inﬂuence on plant morphology and, thus, on the ﬂowering
process. Plants detect light quality through photoreceptors, particularly the major
photoreceptors found in nearly all plants, phytochromes. The amounts of
phytochromes in plants vary by species. Phytochrome is involved in many
physiological responses, including seed germination, photomorphogenesis, bud
dormancy, many enzyme activities, and ﬂowering.

In unirradiated plants, phytochrome is present in a red light-absorbing
form, PR. This form is converted by red light to a far-red light-absorbing form,
Pm- The PFR form can be converted to PR by far-red light, so phytochrome is
, somewhat photoreversible, but most PFR is metabolized. Phytochrome
establishes a photoequilibrium based on the RzFR. Red light typically is deﬁned
as photon irradiance between 655 and 665 nm; far-red light, 725 and 735 nm
(Smith, 1994). Interestingly, leaves absorb hardly any radiation between 700

and 800 nm; virtually all the

 

incoming far-red radiation is either

 

transmitted through or reﬂected
from the leaf (Smith, 1994). The

PR form of phytochrome absorbs

Absorbance

very little in the far-red region of

the light spectrum, but the spectra

 

 

 

 

. . 300 400 500 600 700 800
of PR and PFR overlap srgnrﬁcantly Wavelength (nm)

 

 

 

I" the red region (Figure 5)' The Figure 5. Phytochrome absorption

spectrum (Vierstra and Quail, 1983).

23

proportion of the PFR form, after saturating red light illumination, is only 85% (T aiz
and Zeiger, 1991). Therefore, phytochrome never can be 100% in one form or
another once a plant has been exposed to light.

Many problems arise when R:F R and estimates of phytochrome
photoequilibrium are used to compare plant responses (Rajapakse and Kelly,
1994). First, the range of wavelengths chosen for peak absorbances of PR and
PFR varies from a 5-nm wave band to over a 100-nm wave band. Smith (1982)
used a 10-nm width centered around the peak absorbencies of red and far-red of
660 and 730 nm, respectively, while Mortensen and Stromme (1987) used broad
widths of 100 nm, in which red was deﬁned as 600-700 nm and far-red as 700-
800 nm. Therefore, there is no consistency among researchers when relating
R:FR to phytochrome-mediated responses. Second, the R:FR of a light source
can vary considerably. For example, etiolated corn coleoptile tips exposed to
cool-white ﬂuorescent light, sunlight, and high-pressure sodium light contained
76%, 57%, and 74% PFR at photochemical photoequilibrium, respectively,
assuming that red light produces 80% Pm at photoequilibrium (Gardner and
Graceffo, 1982). This assumption can lead to erroneous conclusions when
responses of plants grown under light sources with little red or far-red light are
explained. Third, estimation methods for determining phytochrome equilibrium
(szPmm, where Pmm= PR + Pm) vary among researchers (Gardner and
Graceffo, 1982; Mortensen and Stromme, 1987; Smith, 1982). Finally, poor

understanding of the physiological roles and photochemical properties of

24

phytochromes may result in further erroneous phytochrome equilibrium
estimates.

In all known cases, PFR is the physiologically active form of phytochrome,
but it is very unstable; most is destroyed when the plant is irradiated with red
light. The amount of PR and PFR can be regulated by synthesis, breakdown, and
dark reversion (T aiz and Zeiger, 1991). PR is synthesized in darkness, and there
may be some slow dark reversion from PFR to PR over a period of several hours.
In many SDP, a ﬂash of red light during a long night prevents ﬂowering, which
can be restored by a ﬂash of far-red light. In a few LDP, (i.e. Fuchsia hybn'da
‘Lord Byron’), a ﬂash of red light during a long dark period induces ﬂowering, and
the effect is reversed by a ﬂash of far-red light (Vince-Prue, 1994). In general,
however, most LDP require much longer periods (half an hour to several hours)
of light to break up the long night and, thus, induce ﬂowering.

Circumstantial evidence implied that phytochrome existed in more than
one form. In garden peas (Pisum sativum L.) an initial level of phytochrome was
detected in dark-grown seedlings, but once the plants were exposed to light, the
phytochrome levels were no longer measurable, even though those plants still
had phytochrome responses. The amount of phytochrome in plants varies by
species, and those deﬁcient in phytochrome, such as ﬂorist’s Chrysanthemum
(Dendranthema x mon'folium Ramat.), still may contain the pigment, but in

undetectable levels or different forms (Lane et al., 1963). The physiological

25

functions of the different phytochromes are being elucidated slowly through the

use of mutant plants with reduced phytochrome levels and transgenic plants.

Types of Phytochromes

There are two known groups or types of phytochrome found in plants.
Type I phytochrome, also called light-labile phytochrome, is abundant in dark-
grown tissue and is present at low levels in light-grown tissue (Parks and Quail,
1993; Smith, 1995). The PFR form of phytochrome l is unstable compared to the
PR form (Smith, 1995). Type II phytochromes, also called light-stable
phytochromes, are present at relatively equal levels in dark- and light-grown
tissue (Parks and Quail, 1993; Smith, 1995). Type II phytochromes are stable in
the PFR form (O’Neill, 1992). The genes that encode these phytochromes have
been at least partially identiﬁed in several plants, including tomato, oat,
cucumber, ﬁeld mustard, and sorghum, while the most intensive study has been
with Arabidopsis (Smith, 1995). To date, there have been ﬁve different
phytochrome genes identiﬁed in this quantitative LDP, and, thus, ﬁve different
phytochromes (Reed et al., 1994). Phytochromes A-E, which are encoded by
genes PHY A-E, respectively, have very similar structures (Clack et al., 1994).
The amino acid sequences of these ﬁve Arabidopsis phytochromes have been
determined to be from 46 to 80% identical (Clack et al., 1994).

Phytochrome A is a type I phytochrome believed to play an important role

in seed germination and early seedling establishment (Smith, 1994) and may be

26

the primary, if not exclusive, far-red photoreceptor (Parks and Quail, 1993). In
addition, this phytochrome may regulate a component of photoperiodic
perception in LDP (Smith, 1995). Phy A Arabidopsis mutants (plants that
contained no or very low levels of phytochrome A) were signiﬁcantly less
responsive to night interruption than were wild-type plants (Reed et al., 1994).
Under SD, wild-type and phy A plants ﬂowered at the same time after producing
the same number of vegetative leaves. When grown under night interruption to
provide artiﬁcial long days, wild-type plants ﬂowered six days earlier and grew
eight fewer leaves than those grown without night interruption. Phy A mutants
ﬂowered only two days earlier and grew four fewer leaves under long days than
those grown under short days. Because phy A mutants were less sensitive to
daylengths than wild-type plants, PHY A may interact with the circadian rhythm
involved in sensing daylengths (Reed et al., 1994). Phytochrome A also appears
to play an important role in the ﬂowering of winter wheat (Carr-Smith et al.,
1994).

Phytochromes B, C, D, and E are considered type II phytochromes
because they are all light-stable (Clack et al., 1994). Of these four
phytochromes, most is known about phytochrome B, the most abundant form in
green plants. Phytochrome B is believed to be at least partially responsible for
detection of R:FR and the R/FR reversible responses (Smith, 1995).
Phytochrome B has been implicated in ﬂowering in two separate studies with

mutants of two different plant species. However, in the ﬁrst case (garden pea),

27

the light-stable phytochrome mutants, known as lv mutants, behaved similarly to
phy A mutants of Arabidopsis which lack the Iig ht-labile phytochrome A.

Garden pea, a quantitative LDP, showed a substantial reduction in ﬂowering
response to photoperiod in lv mutants compared to wild-type plants (Weller and
Reid, 1993). The hastening of ﬂowering under LD compared to SD was not as
pronounced with mutant plants as it was with wild-type plants. VWd-type plants
ﬂowered six nodes earlier under 24-hour photoperiods than under 8-hour
photoperiods; mutants ﬂowered only 1.5 nodes earlier. Perhaps these mutants
really lacked the light-labile phytochrome A, not the light-stable B. Alternatively,
the phytochrome forms may have different functions in separate species.

In Arabidopsis, phy B mutants ﬂowered earlier and with fewer rosette
leaves than wild-type plants, regardless of photoperiod (Reed et al., 1993). The
apical meristematic cells of mutants undenrvent vegetative to reproductive
differentiation prematurely compared to wild-type plants. The experiment was
repeated later and yielded similar results (Reed et al., 1994). This suggests that
phytochrome B plays an inhibitory role in ﬂowering, since plants that contained
this phytochrome ﬂowered signiﬁcantly later than mutants.

Phytochromes A and B may interact to control ﬂowering. Reed et al.
(1994) believe phytochromes A and B act synergistically or antagonistically to
affect ﬂowering. Johnson et al. (1994) suggest phytochrome A action is

antagonistic to the action of phytochrome B. However, Parks and Quail (1993)

28

postulate that phytochromes A and B have reciprocal and independent roles in
mediating ﬂowering.

Little is known about phytochromes C, D, or E, and only recently have the
PHY D and PHY E sequences been elucidated in Arabidopsis (Clack et al.,
1994). The physiological roles for genes PHY C, D, and E are not yet known,
and mutants deﬁcient in these phytochromes have not yet been identiﬁed (Smith,
1994). The proteins encoded by PHY D and E are more similar to phytochrome
B than A or C (Clack et al., 1994). Phytochromes D and E are the least
abundant forms of phytochrome in Arabidopsis (Clack et al., 1994).

The roles of the various phytochromes will be better understood as more
phytochrome mutants are discovered and studied. Transgenic plants may be
engineered that “turn on or off” certain phytochromes, and their subsequent
responses could be monitored. However, other photoreceptors, such as blue-

light and UV photoreceptors, may also be involved in the ﬂowering process.

Short-day Plants

Short—day plants ﬂower only, or ﬂower more rapidly, under fewer than a
certain number of hours of light in each 24-hour period. However, the length of
the darkness is the critical factor for ﬂower induction, not the length of the light
period. Thus, these plants more accurately could be labeled long-night plants.
Although the duration of night or darkness promotes or inhibits ﬂowering, light

must precede the dark period (Vince-Prue, 1975). The intensity and duration of

29

light required varies by species. In general, the amount of light required for
inhibition of ﬂower induction is much less than that needed for promotion of it
(Cockshull, 1984). For some plants, including those in the genus
Chrysanthemum, ﬂowering may be delayed considerably if the light intensity is
low. The length of illuminance required to initiate ﬂowering, given the critical
night length, varies tremendously by species, from one second to 8 to 12 hours
(Vince-Prue, 1975). Thus, photoperiodism in SDP must be analyzed in terms of
dark reactions counteracted by light and a light requirement, primarily for

photosynthesis.

Hourglass Theory

There are two photoperiodism theories that attempt to explain how short-
day plants perceive durations of light and darkness. The ﬁrst, known as the
"hourglass theory," holds that time is measured by a series of curves, which must
be completed in sequence in order to measure the durations of light and
darkness. The transfer to darkness initiates a noncyclic process or series of
processes that function as an hourglass. The effective element in ﬂower
induction is the duration of darkness. When darkness begins, the hourglass is
tipped upside down, and it continues to empty to the bottom half as long as there
is darkness. If the darkness extends long enough for the hourglass to empty to
the bottom half, the critical duration of darkness is completed and ﬂower-

induction processes are initiated. lf light is perceived by the plant before the

30

hour-glass has emptied, the critical duration of darkness is not reached, and the
plant is not induced to ﬂower. The hourglass may represent the time taken for
PFR to fall below a critical threshold that no longer inhibits ﬂowering in SDP
(Vince-Prue, 1994).

The critical dark period may begin a few hours after the onset of darkness
(Vince-Prue, 1975). As discussed previously, PR is synthesized in darkness.
When a low critical threshold of PFR is reached, the hourglass then may be
tipped, and the timing process may begin. If the plant is exposed to red light, PFR
is destroyed; this destruction may cease or reverse the ﬂower-induction process.
However, because phytochrome reversion does not always begin at the onset of
darkness and may be delayed for several hours, it is difﬁcult to associate it with
the critical dark period (Vince-Prue, 1975).

If SDP receive red light several hours into the dark period, ﬂowering is
inhibited. This inhibition may be nulliﬁed by a subsequent exposure to far-red
light. Reversibility is possible for several cycles; a plant repeatedly exposed to a
red/far-red sequence will not ﬂower. This reversibility is most effective when the
far-red light is given soon after red light. The response becomes irreversible if
time between the red/far-red sequence exceeds a critical duration, known as the
escape time. Far-red light given at the end of the photoperiod or early in the
dark period may inhibit ﬂowering in some SDP species (Vince-Prue, 1975).
Phytochrome may have a dual action on ﬂowering in SDP. Within the ﬁrst

several hours of the dark period, a reaction that depends on the presence of PFR

31

is required for ﬂoral induction. After this reaction is completed, further reactions
leading to induction require reduction of PFR below a certain threshold. When red
light is given, these later reactions are interrupted or stopped, and ﬂoral induction
fails. It is still unclear why far-red and red light are inhibitory at times, and some
authors believe a second pigment may be involved (Thomas, 1993; Vince-Prue
and Takimoto, 1987).

The hourglass theory is not considered correct, because plants’ time-
keeping mechanisms are not affected signiﬁcantly by changes in temperature.
All biological reactions are hastened with an increase in temperature to a certain
point. If the theory were correct, raising the temperature should shorten the
critical dark period required for ﬂower induction; in other words, the hourglass
would empty faster with an increase in temperature. In cocklebur and morning
glory, changes in temperature only marginally affect the length of the critical
photoperiod (Salisbury and Ross, 1969). Thus, evidence leads to dismissal of

the theory.

Endogenous Oscillator Theory

The second theory that attempts to explain how SDP measure time is the
"clock" or "endogenous oscillator" theory, in which an internal oscillator computes
the daily durations of light and darkness. Time is measured on a circadian (24-
hour) clock, and there is an oscillation between phases of inhibition and

promotion of ﬂowering by light. If ﬂowering is to occur, the light and dark pattern

32

must be synchronized in some way with the internal oscillator. There may be a
light-sensitive phase, known as external coincidence, in the photoperiodic rhythm
Mnce-Prue, 1994). This proposition holds that there is a single photoperiodic
rhythm, and light directly prevents ﬂower induction in SDP when it coincides with
a particular light-sensitive phase of the rhythm. Another proposition, for which
there is more evidential support, is known as internal coincidence. This theory
maintains that there is an interaction of two rhythms, and ﬂower induction occurs
only when critical phase points coincide (Vince-Prue, 1994).

Many organisms are subjected to daily alterations of light and darkness
that often cause rhythmic behavior. Under long periods of darkness, the internal
rhythm continues and is said to be free-running. Thus, the rhythms are innate
but may need an initiation signal, such as a light-to-dark or dark-to-light transfer.
The circadian rhythm is started by the ﬁrst dark period, which will act as a long
night for ﬂower induction only if it coincides with the night phase of the circadian
rhythm. Duckweed (Lemna perpusilla Torr.) ﬂowers only when the dark period
longer than the critical night length coincides with the circadian clock’s night
phase; darkness during the day ineffectively initiates ﬂowering (Sweeney, 1987).
The period of circadian rhythms is insensitive to temperature, strengthening the
theory that the circadian clock is responsible for measuring the night length
(Sweeney, 1987).

There are two essential components of the photoperiodic process in SDP

(Vince-Prue, 1994). First, time is measured in darkness, and when SDP are

33

exposed to a sufﬁciently long dark period or succession of dark periods, ﬂower
induction occurs. Second, the night length must be preceded by a minimal
photoperiod. Many rhythms respond identically to skeleton photoperiods, or
recurrent pulses of light, and entire photoperiods (Vince-Prue, 1975). In the SDP
pigweed (Chenopodium rubrum L.), the light-to-dark signal sets the phase, and
the timing of the dark-to-light signal determines if ﬂowering occurs (Cumming et
al., 1965). Thus, it is the timing of "dawn" and “dusk" signals that is important.
There are many other examples of similar rhythmic ﬂowering responses (King,
1984; Vince-Prue, 1975). However, not all plants are dominated by light-onllight-
off signals (King, 1984).

Phytochrome may be involved in light detection and, to some degree,
inhibits or promotes ﬂowering, depending on the circadian time. Phytochrome’s
link to the ﬂowering clock is unknown, although night-break inhibition of ﬂowering
in SDP depends on PR. However, phytochrome apparently is not involved in
photocontrol of the circadian rhythm in some species that respond identically to
blue and red light (Vince-Prue, 1994). Plants that respond identically to blue and
red light are all members of the Brassicaceae family (Thomas, 1993), one of
many aspects of the "clock" theory that requires further study and explanation.
Nevertheless, ﬂowering in SDP appears to be connected to circadian rhythms,
and the “clock" theory has received support from numerous experiments and is

currently the accepted theory.

34

Long-day Plants

Long—day plants ﬂower, or ﬂower more rapidly, only when the length of
irradiance exceeds a critical number of hours. Qualitative LDP remain vegetative
when the duration of darkness exceeds a particular value and ﬂower when it is
less than a critical value. Again, the critical photoperiod varies among species
and genotypes. ‘Esther Read’ daisy Chrysanthemum (Chrysanthemum
maximum Ramond ‘Esther Read’) remained vegetative under 12-hour
photoperiods and ﬂowered under photoperiods of 13 hours or longer; ‘T.E.
Killian’ daisy Chrysanthemum plants ﬂowered only under 15-hour photoperiods
and remained vegetative under 14-hour or shorter photoperiods (Grifﬁn and
Carpenter, 1964). Many LDP ﬂower under continuous 24-hour light, which
suggests there is not an absolute dark-period requirement for ﬂowering in many
LDP. Therefore, some people term LDP, perhaps more accurately, light-
dominant plants.

‘Moonbeam’ tickseed (Coreopsis verticillata L. ‘Moonbeam’) is an example
of a qualitative LDP; no plants grown with 8-hour photoperiods after receiving 0,
6, or 12 weeks of 4.5 °C cold treatment ﬂowered, whereas all those grown under
16- or 24-hour photoperiods ﬂowered, regardless of cold treatment (lversen and
Weller, 1994). Many LDP show a quantitative response to light after the critical
photoperiod until a maximum has been reached. Forty percent of a clone of
shasta daisy (Chrysanthemum x superoum Bergmans) plants grown under 12-

hour photoperiods ﬂowered, and 80% of the plants under 14-hour photoperiods

35
ﬂowered (Shedron and Weiler, 1982). Thus, the critical photoperiod as deﬁned

by Vince-Prue (1975) is between 12 and 14 hours. Plants were grown from seed
for 80 days under 10-hour photoperiods, then transferred to 12-, 14-, 16-, or 18-
hour photoperiods. As the photoperiod duration increased, the number of days
to reach visible bud decreased: 100 at 12 hours, 92 at 14 hours, 49 at 16 hours,
and 28 at 18 hours (Shedron and Weiler, 1982). Flowering was most rapid
under 18-hour photoperiods, so horticulturally, the critical photoperiod is 218
hours.

Some LDP may be induced to ﬂower by vemalization, exposure to cold
temperatures, or LD. ‘Bristol Fairy’ baby’s-breath (Gypsophila paniculata L.
‘Bristol Fairy’) can be induced to ﬂower by LD or cool night temperatures (120).
Plants grown at 18 °C or above did not ﬂower under 11-hour photoperiods (SD),
whereas all plants ﬂowered when grown under 24-hour continuous light (LD)
(Moe, 1988). All plants grown under SD with cool night temperatures (12/18 °C
night/day) ﬂowered, but took 38 days longer than those grown under LD at the
same temperature regime (Moe, 1988). The photoperiodic induction of ﬂowering
in LDP is much less well understood than that in SDP. The mechanism for the
time-measuring process in LDP appears similar to that in SDP. It is theorized
LDP perceive a critical nightlength that, if exceeded, prevents ﬂowering, whereas
in SDP it promotes ﬂowering. Flowering may depend on whether light is given
during a ﬂowering-promotion phase of a circadian rhythm, although fewer

species have been examined to test this theory (Vince-Prue, 1994). If there is a

circadian clock
involved in ﬂowering
in LDP, the rhythm
appears to be out of
phase with that
found in SDP (Vince-
Prue, 1975). The
differing rhythmic
sensitivities to ﬂower
induction can be
illustrated in the

graph to the right in

36

 

 

X-I

’

T l I I
x CHENOPODIUM RUBIN/M (SDP) 9’
o SWAPS/5 ALBA (LDP) ,
I
x ‘ I

O)
O
T

M
n

I
l
I
I
l
I
l

7. FLOWERING RESPONSE
8
I

 

 

 

 

 

20 \
\ o
X \‘
\ r
O 2. II 3.;

IO 20 4o 5&0 so
TIME OF LIGHT INTERRUPTION (h)

 

 

 

 

 

Figure 6. The differing rhythmic sensitivities to ﬂower
induction in the LDP white mustard and the SDP
pigweed (Sweeney, 1987)

the LDP white mustard and the SDP pigweed (Sweeney, 1987).

LDP can be divided into two ﬂowering response types on the basis of the

role of light and darkness in ﬂowering (Vince-Prue, 1994). Flowering of some

LDP is controlled primarily by dark processes, and a long night can be prevented

by a short night break at an appropriate time. These plants are referred to as

dark-dominant response types. For other LDP, a long light period to initiate

ﬂowering is very important. These plants can be labeled light-dominant LDP.

LDP are usually less sensitive to night interruptions than SDP. Only a small

number of LDP species is capable of ﬂower induction with a single night break of

fewer than 30 minutes, and then only under speciﬁc conditions (Deitzer, 1984).

 

37

LDP usually require longer light exposures, higher light intensities, or both to
promote ﬂowering than are required by SDP to inhibit ﬂowering (Kasperbauer et
al., 1963; VInce-Prue, 1975). For most species, the number of ﬂowers increases
as the amount of irradiance striking a plant increases. For many light-dominant
LDP, earliness of ﬂowering increases as the amount of irradiance striking the
plant increases. The ﬂowering process was accelerated in ‘Bridal Veil' and
‘Bristol Fairy’ baby’s-breath when the photosynthetic photon ﬂux increased from

210 to 710 umol-m'Z-S'1 at 12, 20, or 28 °C (Hicklenton et al., 1993).

The Role of Phytochrome in LDP

Similar to that in SDP, phytochrome conversion and reversion has been
demonstrated in ﬂowering of LDP. For some LDP, a brief exposure of far-red
light immediately following a brief period of red light can reverse the promoting
effect of red light on ﬂower induction. However, brief night-breaks are often
ineffective at promoting ﬂowering in LDP. Most LDP require longer durations,
higher intensities, or both, of light to interrupt the night and promote ﬂowering
than SDP require to interrupt the night and inhibit ﬂowering. mm long night
breaks, the action spectrum for a maximal night-break effect to promote
ﬂowering in LDP is near 720 nm (\fInce-Prue, 1994). If long photoperiods do not
include far-red light, LDP either do not ﬂower or ﬂower more slowly (Vince-Prue,
1975). The addition of far-red light not only directly promotes ﬂowering, but also

affects the phase of the time-keeping mechanism that controls the sensitivity of

38

the plant for ﬂower promotion (Deitzer, 1984). Flowering is frequently most rapid
under continuous 24-hour light, as long as both red and far-red light are
delivered.

The optimum R:F R for earliest ﬂowering changes dramatically during the
course of the daily cycle (Vince, 1969). Light—dominant LDP have a distinctive
pattern of sensitivity to light quality (Thomas, 1993). Long periods of light given
as a day extension with a blend of red and far-red light generally induce
ﬂowering in most LDP, including lettuce and carnation, far better than red light
alone (Thomas, 1993). The addition of far-red light has a promoting effect on
ﬂowering when delivered from about the eight hour of the daily photoperiod
through about the sixteenth hour (Vince-Prue, 1994). However, the addition of
far-red light to the ﬁrst eight hours of a 16-hour period of red light often had little
or sometimes no effect on promotion of ﬂowering in LDP (Vince-Prue, 1994).
Far-red light’s ﬂowering promotion or lack thereof may be interpreted as a form
of high-irradiance response, presumed to act through Pm. and the far-red action
spectrum for promotion of ﬂowering in LDP by long light exposures may not
apply solely in terms of PFR (Weller and Reid, 1993). However, why far-red is
required during photoperiods for optimal ﬂowering in LDP is still unknown (Vrnce-
Prue, 1994).

VInce-Prue suggests that, at the end of a short day of sunlight, a high
concentration of PFR in leaves inhibits ﬂowering of LDP (Vince-Prue, 1975).

Later in the night, PFR is necessary for ﬂower induction, and at this point the

39

addition of far-red light often has little or no effect on ﬂowering (Vince-Prue,
1975). The results suggest a dual response to Pm. as in SDP, except the
sequence of promotion and inhibition by PFR is reversed in LDP (Vince-Prue,
1975). Deitzer (1984) believes there is a low-PFR- and a high-PFR-requiring
period involved in LDP ﬂower induction. There may be two sequential
phytochrome-mediated events necessary for ﬂowering in LDP: one toward the
middle of the dark period, requiring comparatively higher levels of PFR to initiate
ﬂowering, and a relatively lower-PFR-requiring period that occurs at the end of the
day and promotes ﬂoral development (Deitzer, 1984). Pm inhibits ﬂowering of
the LDP ryegrass (Lolium temulentum L.) at some phases of a circadian rhythm
and promotes it in others (VInce-Prue, 1994). Photoperiodic sensing in LDP may
be the result of two circadian rhythms (Vince-Prue and Takimoto, 1987). It is
proposed that the ﬁrst rhythm runs in the light, is responsive to far-red light, and
may be related to the LD requirement. The second rhythm runs in darkness, is
responsive to red light, is suspended in continuous light, and relates to the
measurement of the critical night length. The role of phytochrome is not clearly
understood in LDP; we know only that it plays some role in ﬂowering or the lack
thereof.

Recent evidence suggests that gene expression shows a rhythmic
response that may be involved in ﬂowering in LDP. The expression levels of
distinct leaf mRNAs oscillated in a circadian rhythm with respect to photoperiod

in mouse-ear cress (Lechner and Rau, 1993). In the LDP white mustard, levels

40

of an mRNA undergo circadian oscillations in light/dark cycles with maxima
between 2000 HR and 2400 HR and minima around 0800 HR (Heintzen et al.,
1994). The underlying oscillatory mechanism(s) operate(s) synchronously in
different plant organs, including the epidermis and spongy parenchyma cells in
the leaves and regions of the cortex in stems and petioles (Heintzen et al.,
1994). No novel mRNA appeared and mRNA did not decrease to undetectable
levels during changes from SD to inductive LD. After the onset of LD, there were
alterations in the phase and amplitude of circadian oscillations of mRNA
expression levels either within hours after the beginning of the extended light
period or after the ﬁrst LD was complete (Lechner and Rau, 1993). These
ﬁndings indicate that a distinct time-measuring mechanism at least partially
regulates levels of mRNA, which may participate in temporary processes in the
leaves and thereby transform a photoperiodic perception into a ﬂowering
stimulus (Lechner and Rau, 1993).

Although there is strong evidence for the involvement of a circadian
rhythm in ﬂower induction, there is also strong evidence for the involvement of a
semidian rhythm that cycles twice each day. The semidian rhythmic process
persists in prolonged light with a period of about 12 hours and has a pronounced
effect on ﬂowering, at least in LDP (Heide et al., 1986). At various times before
the beginning of the dark periods, mouse-ear cress plants exposed to 90
minutes of far-red light during continuous white light deﬁcient of far-red displayed

signs of distinct inhibitory and promotive effects on ﬂowering (Heide et al., 1986).

41

Far-red light given for 90 minutes 4, 16, and 28 hours before the dark period
promoted ﬂowering, and when given 8, 22, and 34 hours before the dark period,
it inhibited ﬂowering. The semidian rhythm is set by a light-on signal, in contrast
to the phasing of the circadian rhythm, which is set by a light-off signal. Far-red
interruptions’ effect on ﬂower promotion increases with duration, and
temperature may inﬂuence the period length of the semidian rhythm (Heide et

al., 1986).

Flowering Stimulus Theory

There is evidence from many physiological experiments that leaves
produce a ﬂower-inducing hormone, or a ﬂoral stimulus, under photoinduced
cycles. This proposed hormone was termed "ﬂorigen" by Chailakhyan around
1937 (see Lang, 1965). Despite decades of research, the ﬂoral stimulus has not
yet been identiﬁed. Numerous grafting experiments demonstrate that the ﬂoral
stimulus can be transmitted through a graft union. A plant kept under
noninductive conditions could be induced to ﬂower by a graft union with an
induced leaf. Examples exist in SDP, LDP, and plants that require long then
short days to ﬂower (LSDP), within species, and between species of different
families (Lang, 1965; Zeevaart, 1976). In some cases, a leaf that was taken
from the graft-induced plant and was never under inductive conditions still could
induce ﬂowering indirectly when grafted onto another uninduced plant. Such

grafts have been successful in the SDP cocklebur (Xanthium struman'um L.), the

42

LDP garden catchﬂy (Silene annen'a L.), and the LSDP Devil’s-backbone
(Kalanchoe daigremontiana Hamet & Perr.) (Zeevaart, 1976). The ﬂoral stimulus
may be the same or very similar in LDP, SDP, and DNP, since it can be
transmitted from SDP to LDP, SDP to DNP, LDP to DNP, and vice versa (Lang,
1965). Transmission of the ﬂower-promotiing stimulus has also been
demonstrated between DNP (Lang, 1965). Additional evidence to support the
existence of a ﬂoral stimulus comes from plants that initiate ﬂowers after one
inductive cycle. Immediate removal of the induced leaves after the end of the
cycle can prevent a ﬂowering response, but if the leaves are removed a certain
number of hours after the end of the cycle, the plants ﬂower as if their leaves still
are intact (Lang, 1965).

The ﬂowering stimulus appears to be translocated with the ﬂow of
carbohydrates, through the phloem, to the bud meristem (Vince-Prue, 1975).
There was a rapid, dramatic increase in apical sap transmitted from the phloem
during ﬂoral induction in the LDP white mustard (Brassica hirta Moench.,
formerly Sinapis alba) (Lejeune et al., 1993). These results suggest sucrose
plays a messenger-type role in transmitting the ﬂoral stimulus from the leaves to
the apex, since there is an accumulation of sucrose in the meristem early in the
vegetative to reproductive process (Lejeune et al., 1993).

Once the ﬂoral stimulus arrives at the apex, cell activity increases; nucleic
acid, RNA, and protein synthesis increase; and soon there is an increase in cell

size (Vrnce-Prue, 1975). The increase in RNA synthesis in the LDP black-eyed

43

Susan (Rudbeckia hirta L.) is apparent after eight LD (Harkess and Lyons,
1993). The increase in RNA in other species occurs just before or on the arrival
of the ﬂoral stimulus and is necessary for ﬂowering (Harkess and Lyons, 1993).
Genes that are inactive when the plant is vegetative may become activated once
the ﬂoral stimulus arrives at the apex. Two major groups of white mustard genes
whose expression was affected during ﬂower formation were identiﬁed (Melzer et
al., 1990). The ﬁrst group of genes, present at low concentrations in the apex in
uninduced plants, quickly accumulated after the end of the inductive
photoperiod. The second group of genes was not detected in uninduced plants
but was detected ﬁrst 10 days after the onset of inductive photoperiods. The
group rapidly accumulated, then dropped to undetectable levels before the
ﬂower reached maturity. Alterations in gene expression during photoperiodic
induction appear to be temporary (Lechner and Rau, 1993). Following the ﬂoral
stimulus, the apex reorganizes and differentiates ﬂoral organs. Once cells begin
their increased activity, ﬂowering moves into the initiation stage, and the distinct
anatomical zonation in the meristem is lost (Harkess and Lyons, 1993).

After a sufﬁcient number of favorable cycles, photoperiodically sensitive
plants may continue to ﬂower, even if returned to noninductive cycles (Vince-
Prue, 1975). Nearly all seed plants transition from the vegetative to reproductive
state is almost completely irreversible (Krishnamoorthy and Nanda, 1968).
Dense-ﬂowered loosestrife, a quantitative LDP, given one week of LD followed

by SD ﬂowered at the same time as those given two, three, or four weeks of LD

44

followed by SD, or continuous LD (Zhang et al., 1995). Therefore, this species
requires seven or fewer LD to initiate 100% ﬂowering. However, ﬂower number
decreased as the duration of LD decreased.

Some species’ inﬂorescence requires continued favorable cycles through
the late stages of ﬂower development. The qualitative SDP garden balsam
(Impatiens balsamina L.), must be exposed to an appropriate photoperiod until
anthesis; if not, the plant will revert to vegetative growth, even after anthers and
ovules have formed (Krishnamoorthy and Nanda, 1968). Mexican bush sage
(Salvia leucantha Cav.) SDP exposed to ﬁve weeks of SD following ﬂower
initiation then were followed by LD and did not reach anthesis; 57% of plants
exposed to six weeks of SD followed by LD reached anthesis; and all plants
exposed to nine weeks of SD when the calyx became visible reached anthesis
(Armitage and Laushman, 1989). Roberts and Summerﬁeld (1987) proposed the
existence of a postinductive phase, which is insensitive to photoperiod.
However, initiation of the phase varies by species, from immediately after ﬂoral
induction to the beginning of anthesis. Therefore, induction is not an "all or
none" process; there are degrees. A plant exposed to inductive cycles less than
the number that elicits a full ﬂowering response may still ﬂower, but in a different
manner. For example, kalanchoe (Kalanchoe blossfeldiana Poelln.), when
exposed to one or two fewer cycles than the number that would provide full
ﬂowering, ﬂowers sparsely and from axillary shoots; terminal inﬂoresences are

absent (Carlson et al., 1979).

45

Inhibitory Process Theory

Another theory, for which there is less evidence, is that an inhibitory
process occurs in plants under noninductive daylengths, which implies that a
plant ﬂowers when the inhibitor is absent. There are some examples of LDP and
SDP that ﬂower in noninductive cycles when their leaves are removed,
suggesting an inhibitory substance originates in the leaves and acts at the apex.
In a grafting experiment with the SDP morning glory (Pharbitis nil Choisy.),
different strains produced different intensities of ﬂowering stimuli or amounts of
ﬂowering hormone. In many cases, the productivity of the ﬂoral stimuli by the
leaves was more important than the reactability of the bud. The experimenters
concluded inhibitory factors, when transmitted through the graft, played some
signiﬁcant role in ﬂowering (lmamura et al., 1966). As ‘Marrnalade’ black-eyed
Susan plants, an LDP, experienced longer periods of uninterrupted LD, the effect
of photoperiodic inhibition diminished (Orvos and Lyons, 1989). The longer
plants perceived the inductive photoperiod, the faster they came into ﬂower, and
the effects of photoperiodic inhibition on ﬂowering were strongest for plants that
received the fewest inductive days (Orvos and Lyons, 1989).

WIth many of the grafting experiments, non-induced, particularly mature,
leaves were usually removed since their presence had an unfavorable effect on
the ﬂowering response, whereas removal of young leaves often had an adverse
effect on ﬂower initiation (Lang, 1965). The inhibitory action of non-induced

leaves was reduced when they were provided low light intensity, complete

46
darkness, or extreme SD in the case of LDP (Lang, 1965). Thus, the inhibitory

effect appears to be translocated and interferes with ﬂorigen transport from
induced leaves to the buds. However, no recent evidence suggests that the

removal of an inhibitor induces ﬂowering.

Chemical Induction of Flowering

Application of a variety of substances can induce ﬂowering in some
plants, including the plant hormones gibberellin, cytokinin, auxin, abscisic acid,
and ethylene, as well as sugars, growth retardants, and some mineral elements
(\frnce-Prue, 1975). However, most substances are effective at inducing
ﬂowering in only a small number of often related species. Numerous attempts
have been made to extract from ﬂowering plants various chemicals that would
induce ﬂowering in plants under noninductive conditions. To date, there has
been very limited success, and no hormone that has an inductive effect over a
broad range of plants has been discovered.

In the 1950s, gibberellic acid was discovered and was believed by some
to be the ﬂowering hormone. In some cases, GA can substitute for a cold
requirement; in others, for LD to induce ﬂowering. For example, application of
GA to two cultivars of blanket ﬂower (Gaillardia x grandiﬂora Van Houtte
‘Dazzler' and ‘Goblin’) substituted for LD and promoted ﬂowering under SD in the
same amount of time untreated, photoperiodically induced plants required

(Evans and Lyons, 1988). In some LDP, GA applications have little effect on

47

ﬂowering. Flowering and stem elongation are induced by photoperiod in garden
catchﬂy, but the ﬂowering response mainly is LD-qualitative and is not induced
by applied GA, and stem elongation is related to the duration of the LD treatment
(Talon and Zeevaart, 1990). However, GA can replace either cold or LD, not
both, and does not cause SDP to ﬂower. Levels of GA increase in many LDP
exposed to LD. The rate of accumulation of ent-Kaurene, a point of regulation in
the GA pathway, was three times higher in the LDP spinach and two and one-
half times higher in the LDP corn cockle (Agrostemma githago L.) when plants
were grown under LD compared to SD (Zeevaart and Gage, 1993). Most of the
plants that respond to GA are rosettes. The primary effect of GA is intemode
elongation; secondary, ﬂowering. lf GA biosynthesis inhibitors (growth
retardants) are applied to LDP under LD, the plants do not bolt, but they ﬂower.
The growth retardant tetcyclacis, a GA biosynthesis inhibitor, inhibited stem
elongation induced by LD in Silene, but had no effect on ﬂowering (Talon and
Zeevaart, 1990). Therefore, GA directly affected stern growth, and indirectly

inﬂuenced ﬂowering.

The Role of Temperature in Flowering

Many plants ﬂower in response to photoperiod, and in a vast majority of
those, temperature plays a signiﬁcant role in the rate of ﬂower induction,
initiation, development, and maturation. The duration of the ﬂowering process

can be measured by either the number of days to ﬂowering (F) or its inverse, the

48

rate of progress toward ﬂowering (1/F). The rate of progress toward ﬂowering is
a positive linear function, extending from the base to optimum temperature of a
species (Roberts and Summerﬁeld, 1987). The base temperature (Tm) is
species-speciﬁc and describes the temperature at which growth begins; below
that base temperature there is no growth. The optimum temperature (T on.) also
varies by species and describes the point at which growth and the ﬂowering
process are most rapid; beyond Tom, both are delayed and eventually aborted.
The ﬂowering process is accelerated as the average daily temperature increases
from To... to a maximal rate, Tom. Herbaceous perennial ‘Bristol Fairy’ baby’s-
breath plants grown at 12 °C under 450 or 710 ,umol'm'z-S'1 took 81 or 70 days to
reach visible bud, respectively; at 20 °C, 63 or 43 days, respectively; and at 28
°C, 24 or 25 days, respectively (Hicklenton et al., 1993). There is a possibility
that the increased light levels increased plant temperature and confounded the
results. As the temperature increased, the average number of ﬂorets per plant
decreased from 3,022 and 8,977 at 450 or 710 umol-m'Z-S'1 at 12 °C to 720 and
1,874 at 280, respectively (Hicklenton et al., 1993). ‘Sentimental Blue’ balloon
ﬂower ﬂowered earlier when plants were grown at 23/25 °C night/day (137 days)
than at 15/17 °C (159 days) (Song et al., 1993).

High temperatures (25-35C) generally are inhibitory to SDP toward the
end of the inductive night (\fInce-Prue, 1975). In contrast, several LDP, including
calamint (Calamintha nepeta glandulosa P.W. Ball), underwater rose (Samolus

parviﬂorus Raf), and garden catchﬂy ﬂowered under SD with night temperatures

49

above 30 °C (Zeevaart, 1976). Over a wide range of temperatures, the rate of
progress toward ﬂowering increases usually in a linear manner with an increase
in temperature until an optimum temperature is reached (Roberts and
Summerﬁeld, 1987). Beyond the optimum temperature, ﬂowering is delayed as
temperatures get warmer (Roberts and Summerﬁeld, 1987). The optimum
temperature varies by species.

Roberts and Summerﬁeld (1987) have proposed mathematical equations
that attempt to predict the time it takes a plant to ﬂower based on temperature
and photoperiod. Three factors that modulate the rate of progress toward
ﬂowering in the quantitative LDP lentil (Lens culinan’s Medic.) were found:
vemalization, postvemalization mean temperature, and photoperiod (Roberts et
al., 1986). The photoperiodic sensitivity of lentil, deﬁned in terms of the
difference in days to ﬂower between two different photoperiods, was affected
markedly by temperature (Roberts et al., 1986). Roberts and Summerﬁeld
(1987) believe that the critical photoperiod of SDP decreases with an increase in
temperature. Their results contradict those of Vince-Prue, who believes the
critical photoperiod remains relatively resistant to changes in temperature (Vince-
Prue, 1975). The majority of evidence suggest that temperature may shorten or
lengthen the critical photoperiod of some species to at least a small extent.
Photoperiodic responses in general often are modiﬁed by changes in

temperature.

50

The interaction of daylength and temperature was investigated in three
cultivars of the SDP poinsettia (Euphorbia pulchem'ma erld.). Langhans and
Miller (1963) deﬁned the critical daylength for SDP as that daylength above
which the plant remains vegetative and below which the plant ﬂowers. For all
three cultivars studied, the critical daylength for ﬂower initiation and development
decreased as the temperature increased from 16 to 27 °C (Langhans and Miller,
1963). For example, the critical photoperiod of ‘Barbara Ecke Supreme’ shifted
from above 12 hours at 16 °C to 11.5 hours at 21 °C to between 10 and 12 hours
at 27 °C (Langhans and Miller, 1963).

A similar experiment was conducted on three cultivars of the SDP
Chrysanthemum (Dendranthema grandiﬂora Tzvelev): ‘White Wonder', a 6-week
variety; ‘Encore’, a 10-week variety; and ‘Snow’, a 15-week variety.

Temperature altered the critical photoperiods required for ﬂower initiation and
ﬂower development in all three cultivars (Cathey, 1957). Cathey (1957) deﬁned
the critical photoperiod of SDP as the minimum light length necessary for
ﬂowering. In ‘Encore’, as the temperature increased from 10 to 27 °C, the critical
photoperiod for ﬂower initiation increased from 13.75 to 15.25 hours and the
critical photoperiod for ﬂower development decreased from 13.75 to 12 hours
(Cathey, 1957). In contrast, the critical photoperiods for ﬂower initiation of
‘Snow’ decreased from 12 to 10 hours as temperatures increased from 10 to 27
°C and the critical photoperiod for ﬂower initiation decreased from 12 to 9 hours

(Cathey, 1957). The poinsettia and Chrysanthemum examples provide evidence

51

that, at least in SDP, temperature modiﬁes the critical photoperiods for ﬂower
initiation and development.

Describing the rate of progress toward ﬂowering (the inverse of days to
ﬂower, or 1IF) is perhaps more useful than describing ﬂowering as days to
ﬂower. These ﬂowering rates vary by species and are affected by temperature
and possibly photoperiod. In experiments with chickpeas (Cicer arietinum L.)
and soybeans, there was no apparent correlation between relative sensitivity of
temperature and photoperiod for ﬂowering (Roberts et al., 1985; Upadhyay et al.,
1994). These studies suggest that, although both factors affect time to ﬂowering,
they are under separate genetic control (Roberts et al., 1985; Upadhyay et al.,
1994)

The rate of progress toward ﬂowering can be related linearly to mean
temperature, t, in °C by the equation

1IF=a + bt
where a and b are constants, a is the slope coefﬁcient, and b is the intercept
coefﬁcient. The constants a and b vary by species. The base temperature, The“,
as described previously, can be determined by the equation

Tbm=—alb

At suboptimal temperatures, the ﬂowering response rate decreases
linearly until Tm, is reached, at or below which the rate is zero (Upadhyay et al.,

1 994).

I I
II.__E

52

The rate of progress toward ﬂowering is clearly a linear function of mean
temperature for photoperiod-insensitive genotypes (DNP); daylength has no
effect on the rate of ﬂower development. SDP exhibit a basic temperature
response similar to that of DNP and a photoperiodic response in which the rate
of progress toward ﬂowering is a negative linear function of photoperiod (Roberts
and Summerﬁeld, 1987). In soybean, increases in daylength beyond the optimal
daylength in which ﬂowering was most rapid progressively delayed ﬂowering until
the ﬂowering response rate reached a minimum (Upadhyay et al., 1994).
Temperature also may have some effect on the rate of progress toward ﬂowering
when the photoperiod exceeds the critical photoperiod for that genotype. The
following equation describes the rate of progress toward ﬂowering in SDP:

1/F=a' + b't + c'p
where t is the mean temperature in °C, p is photoperiod in hours, and a‘, b', and
c‘ are species-speciﬁc constants that apply when photoperiods are shorter than
the critical photoperiods (Roberts and Summerﬁeld, 1987). For SDP, the
temperature constant, b‘, always will be positive between Thm and T , and the
photoperiodic constant, 0', always will be negative. In a photoperiod-sensitive
genotype of the SDP soybean (T Gx 46-3C), data from plants grown under
various temperature and photoperiodic regimes yielded the top graph shown on
the next page (Figure 7), which illustrates photothermal effects on ﬂowering

(Roberts and Summerﬁeld, 1987).

53

The photothermal responses of
LDP are essentially mirror images to
those of the SDP soybean. However,
the value of the photoperiodic constant,
c', is positive; the longer the
photoperiod in many LDP, the faster the
rate of ﬂowering. The lower graph in
Figure 7 illustrates the effects of
photoperiod and temperature on
ﬂowering in a photoperiod-sensitive
genotype of the LDP lentil (ILL 4605)
(Roberts and Summerﬂeld, 1987). The
response shown for lentil is similar to
that of other LDP, including chickpeas,
barley (Hordeum vulgare L.), and faba
bean (Vicia faba L.) (Roberts and
Summerﬁeld, 1987). No critical

photoperiod is apparent, and as the

 

Soyobeon TGx 1.6 - 3C

 

 

 

 

 

 

 

 

Figure 7. The photothermal effects on
ﬂowering of the SDP soyabean (top)
and the LDP lentil (bottom) (Roberts
and Summerﬁeld, 1987).

length of the photoperiod increases, ﬂowering rates increase, so this genotype of

lentil is likely a quantitative LDP.

Recent experiments have focused on what effect, if any, carbon dioxide

(002) levels have on annual plants’ development toward ﬂowering. Reekie et al.

54
(1994) suggested that the effect of 002 on ﬂowering is a function of the

photoperiodic response of a species. In four SDP, increasing levels of CO2
delayed ﬂowering somewhat, whereas in four LDP, increasing levels of CO2
hastened ﬂowering (Reekie et al., 1994). Flowering was delayed by one, two,
four, and ﬁve days in Chrysanthemum, cocklebur (Xanthium pensylvanicum
Gandoger), kalanchoe, and morning glory, respectively, when plants were grown
at 350 umol COzlmol of air compared to those grown at 1000 umol COzlmol of
air (Reekie et al., 1994). Flowering was hastened by 6, 8, 10, and 14 days in the
LDP common yarrow (Achillea millefolium L.), China aster (Callistephus
chinensis Nees), throatwort (Trachelium caeruleum L.), and Italian bellﬂower
(Campanula isophylla Moretti), respectively, when plants were grown at 350
umol COJmol of air compared to those grown at 1000 umol COzlmol of air
(Reekie et al., 1994). In another study, as CO2 levels increased from 210 to 720
umol COzlmol of air, ﬂowering was delayed by 17 or 19 days in two cultivars of
the SDP sorghum (Sorghum bicolor Moench.) and by three days in soybean
(Ellis et al., 1995). Flowering was hastened by two days in the SDP cowpea
(Vigna unguiculata Walp.) (Ellis et al., 1995). For the two genotypes of sorghum
studied, as CO2 concentrations increased, panicle initiation occurred 17 to 22
days earlier at 210 than at 720 ,umol COZImol of air (Ellis et al., 1995). The
effects of CO2 concentrations on rates of development clearly vary by species,

and no signiﬁcant generalizations can be made (Ellis et al., 1995).

55
The preceding equations that attempt to quantify and predict the rate of

progress toward ﬂowering are perhaps the best (and only) models developed to
date. Upon analysis, the models have several faults. First, each assumes that
plants are sensitive to photoperiod throughout the four phases of plant growth
and development. As described earlier, most SDP and LDP go through phases
in which they are relatively insensitive to photoperiod. Second, the three
constants, a', b', and c', vary by genotype, and these constants must be derived
for application. Third, little research delineates the effect of vemalization on the
models. Roberts and Summerﬁeld (1987) predict modeling of crop phenology
will become more simpliﬁed and reliable when thermal and photoperiodic time
are integrated into models.

Quantiﬁcation of the effects of photoperiod on rates of ﬂowering is not as
well understood, but some conclusions have been reached. Several LDP ﬂower
faster as the length of the photoperiod increases beyond the critical photoperiod
(Roberts and Summerﬁeld, 1987). However, in the LDP garden pea,
photoperiods longer than the critical photoperiod have no effect on ﬂowering,
and the time to ﬂowering is solely a function of mean temperature (Roberts and
Summerﬁeld, 1987). These LDP contradictions may be explained if species in
which ﬂowering is hastened as the photoperiod increases are light-dominant
plants, and garden pea plants are an example of a dark-dominant LDP. In 10
genotypes of soybean, a SDP, the rate of progress toward ﬂowering increased

as the photoperiod decreased below the critical photoperiod (Roberts and

56

Summerﬁeld, 1987). In cowpea and soybean, both SDP, there is a temperature-
dependent critical photoperiod until there is no longer a photoperiodic-hastening
response, when time to ﬂower is solely a function of mean temperature (Roberts
and Summerﬁeld, 1987). The rate of progress toward ﬂowering is not affected
by photoperiod in DNP, as expected. Thus, rates of progress toward ﬂowering
tend to be nearly linear functions of temperature, photoperiod, or both (Roberts

and Summerﬁeld, 1987).

Summary

There is not yet a clear understanding of how plants ﬂower in response to
photoperiod; we are only beginning to explain this very complex issue. To date,
we know that leaves respond to light and dark and transmit the signals to the
apex. Depending on the plant and its internal oscillator, the signal either
promotes or inhibits ﬂowering. Phytochrome is involved in the ﬂowering process,
but exactly how is unknown. If a universal plant hormone that induces ﬂowering
exists and can be synthetically replicated, then it may be applied to plants, which
would make them ﬂower. Conversely, a hormone that inhibits ﬂowering and thus
promotes vegetative growth may be identiﬁed. The idea of bringing a crop to
ﬂower with a chemical is fascinating and would change the plant world as we

know it today drastically.

57

References

Armitage, AM. and J.M. Laushman. 1989. Photoperiodic control of ﬂowering of
Salvia leucantha. J. Amer. Soc. Hort. Sci. 114(5):?55—758.

Beattie, D.J., C.F. Deneke, E.J. Holcomb, and J.W. White. 1989. The effects of
photoperiod and temperature on ﬂowering of Physostegia virginiana
‘Summer Snow’ and ‘ind’ as potted plants. Acta Hort. 252:227—233.

Brock, RD. and J.L. Davidson. 1994. 5-Azacytidine and gamma rays partially
substitute for cold treatment in vemalizing winter wheat. Environ. Expt.
Bot. 342195-199.

Burn, J.E., D.J. Bagnall, J.D. Metzger, E.S. Dennis, and W.J. Peacock. 1993.
DNA methylation, vemalization, and the initiation of ﬂowering. Proc. Natl.
Acad. Sci. 90:287-291.

Carlson, W.H., C.A. Turner, and J. Schnabel. 1979. Effect of inductive
photoperiodic cycles on ﬂowering of kalanchoe cultivars. HortScience
14(1):62-63.

Carr-Smith, H.D., C.B Johnson, C. Plumpton, G.W. Butcher, and B. Thomas.
1994. The kinetics of type I phytochrome in green, light—grown wheat
(Triticum aestivum L.). Planta 194:136-142.

Cathey, HM. 1957. Chrysanthemum temperature study. F. The effect of
temperature upon the critical photoperiod necessary for the initiation and
development of ﬂowers of Chrysanthemum mon'folium. Proc. Amer. Soc.
Hort. Sci. 69:485-491.

Clack, T., S. Mathews, and RA. Sharrock. 1994. The phytochrome apoprotein
family Arabidopsis is encoded by ﬁve genes: the sequences and
expression of PHYD and PHYE. Plant Mol. Biol. 25:413-427.

Cockshull, K.E. 1984. The photoperiodic induction of ﬂowering in short-day
plants, p. 33-49. In: D. Vrnce-Prue, B. Thomas, and K.E. Cockshull (eds).
Light and the ﬂowering process. Academic Press, San Diego.

Collinson, S.T., R.H. Ellis, R.J. Summerﬁeld, and EH. Roberts. 1992. Durations
of the photoperiod-sensitive and photoperiod-insensitive phases of
development to ﬂowering in four cultivars of rice (Oryza sativa L.). Ann.
Bot. 70:339-346.

58

Cumming, B.G., S.B. Hendricks, and HA. Borthwick. 1965. Rhythmic ﬂowering
responses and phytochrome changes in a selection of Chenopodium
rubrum. Can. J. Bot. 43:825-853.

Deitzer, GP. 1984. Photoperiodic induction in long-day plants, p. 51-63. In: D.
Vince-Prue, B. Thomas, and K.E. Cockshull (eds.). Light and the ﬂowering
process. Academic Press, San Diego.

Ellis, R.H., P.Q. Craufurd, R.J. Summerﬂeld, and EH. Roberts. 1995. Linear
relations between carbon dioxide and rate of development towards
ﬂowering in sorghum, cowpea, and soyabean. Ann. Bot. 75:193-198.

Ellis, R.H., S.T. Collinson, D. Hudson, and WM. Pateﬂeld. 1992. The analysis of
reciprocal transfer experiments to estimate the durations of the
photoperiod-sensitive and photoperiod-insensitive phases of plant
development: An example in soya bean. Ann. Bot. 70:87-92.

Evans, MR. and RE. Lyons. 1988. Photoperiodic and gibberellin-induced
growth and ﬂowering responses of Gail/ardia x grandiﬂora. HortScience
23(3):584-586.

Faust, J.E. and RD. Heins. 1994. Determining the effect of cloud conditions on
the variation in daylength perceived by plants. HortScience 29(5):504.

Flood, R.G. and GM. Halloran. 1984. The nature and duration of gene action for
vemalization response of wheat. Ann. Bot. 53:363-368.

Gardner, PP. and RD. Barnett. 1990. Vemalization of wheat cultivars and a
triticale. Crop Sci. 30:166-169.

Gardner, G. and MA. Graceffo. 1982. The use of a computerized
spectoradiometer to predict phytochrome photoequilibria under
polychromatic irradiation. Photochem. Photobiol. 36:349-354.

Grifﬁn, CW. and W.J. Carpenter. 1964. Photoperiodic response of shasta daisy
clones Esther Read and T. E. Killian. Proc. Amer. Soc. Hort. Sci. 85:591-
593.

Grifﬁths, J.F. 1976. Sunrise, sunset. Weather 31:427-429.

Harkess, R.L. and RE. Lyons. 1993. Anatomical changes in Rudbeckia hirta L.
during transition to ﬂowering. J. Amer. Soc. Hort. Sci. 228(6):835-839.

Hazebroek, JP. and JD. Metzger. 1990. Therrnoinductive regulation of
gibberellin metabolism in Thlaspi arvense. Plant Physiol. 94:154-165.

59

Hazebroek, J.P., J.D. Metzger, and ER. Mansager. 1993. Therrnoinductive
regulation of gibberellin metabolism in Thlaspi arvense. Plant Physiol.
102:547-552.

Heide, O.M., R.W. King, and LT. Evans. 1986. A semidian rhythm in the
ﬂowering response of Pharbitis nil to far-red light. Plant Physiol. 80:1020-
1024.

Heintzen, C., R. Fischer, S. Melzer, S. Kappeler, K. Apel, and D. Staiger. 1994.
Circadian oscillations of a transcript encoding a germin-Iike protein that is

associated with cell walls in young leaves of the long—day plant Sinapis
alba L. Plant Physiol. 106:905-915.

Hicklenton, P.R., S.M. Newman, and L.J. Davies. 1993. Growth and ﬂowering of
Gypsophila paniculata L. “Bristol Fairy’ and ‘Bridal Veil’ in relation to
temperature and photosynthetic photon ﬂux. Scientia Hort. 53:319-331.

Hughes, J.E., D.C. Morgan, P.A. Lambton, C.R. Black, and H. Smith. 1984.
Photoperiodic time signals during twilight. Plant Cell Environ. 7:269-277.

Imamura, S., M. Muramatsu, SJ. Kitajo, and A. Takimoto. 1966. Varietal
difference in photoperiodic behavior of Pharbitus nil. Bot. Magazine,
Tokyo 79:714-721.

lversen, RR. and TC. Weiler. 1994. Strategies to force ﬂowering of six
herbaceous garden perennials. HortTechnology 4(1):61-65.

Johnson, E., M. Bradley, N.P. Harberd, and 6.0. Whitelam. 1994.
Photoresponses of light-grown phyA mutants of Arabidopsis. Plant
Physiol. 105:141-149.

Kasperbauer, M.J., H.A. Borthwick, and HM. Cathey. 1963. Cyclic lighting for
promotion of sweetclover, MeIiIotus abla Desr. Crop Sci. 32230-232.

King, R.W. 1984. Light and photoperiodic timing, p.91-105. In: D. Vince-Prue, B.
Thomas, and K.E. Cockshull (eds.). Light and the ﬂowering process.
Academic Press, San Diego.

Knott, J.E. 1934. Effect of localized photoperiod on spinach. Proc. Amer. Soc.
Hort. Sci. 31 :152-154.

Krishnamoorthy, HM and K.K. Nanda. 1968. Floral bud reversion in Impatiens
balsamina under non-inductive photoperiods. Planta (Berlin) 80:43—51.

Lane, H.C., H.W. Siegelman, W.L. Butler, and EM. Firer. 1963. Detection of
phytochrome in green plants. Plant Physiol. 38:414-416.

60

Lang, A. 1965. Physiology of ﬂower initiation, p.1380-1535. In: W. Ruhland (ed.).
Encyclopedia of plant physiology. Springer-Verlag, Berlin.

Lange, NE. 1993. Modeling ﬂower induction in Lilium Iongiﬂorum. MS Thesis,
Michigan State University, East Lansing.

Langhans, R.W. and R.G. Miller. 1963. Inﬂuence of daylength, temperature, and
number of short days on the ﬂowering of poinsettia (Euphorbia
pulchen‘mma). Proc. Amer. Soc. Hort. Sci. 75:753-760.

Lechner, F .J., and W. Rau. 1993. A complex pattern of changes in polysomal
mRNA populations is evident in the leaves of Arabidopsis thaliana (L.)
Heynh. during photoperiodic induction of ﬂowering. Planta 189:522-532.

Lejeune, P., G. Bernier, M. Requier, and J. Kinet. 1993. Sucrose increase during
ﬂoral induction in the phloem sap collected at the apical part of the shoot
of the long-day plant Sinapis alba L. Planta 190:71-74.

Le Nard, M. and AA. De Hertogh. 1993. Bulb growth and development and
ﬂowering, p. 29-43. In: A. De Hertogh and M. Le Nard (eds). The
physiology of ﬂower bulbs. Elsevier Science Publishers, Amsterdam,
Netherlands.

Melzer, 8., 0M. Majewski, and K. Apel. 1990. Early changes in gene expression
during the transition from vegetative to generative growth in the long-day
plant Sinapis alba. Plant Cell 22953-961.

Metzger, JD. 1988. Localization of the site of perception of therrnoinductive
temperatures in Thlaspi arvense. Plant Physiol. 88:424-428.

Moe, R. 1988. Flowering physiology of Gypsophila. Acta Hort. 218:153-158.

Mortensen, L.M. and E. Stromme. 1987. Effects of light quality on some
greenhouse crops. Scientia Hort. 33:27-36.

Napp-Zinn, K. 1987. Vemalization—environmental and genetic regulation, p. 123-
132. In: J.G. Atherton (ed.). Manipulation of ﬂowering. Robert Hartnoll,
Bodmin, Cornwall.

Ochesanu, C. and I. Barbat. 1965. The photoperiodical sensitivities of leaves
depending on their age. Revue Roumaine de Biologie, serie de Botanique
10:403-409.

O’Neill, SD. 1992. The photoperiodic control of ﬂowering: progress toward
understanding the mechanism of induction. Photochem. Photobiol.
56(5):789-801 .

61

Orvos, AR. and RE. Lyons. 1989. Photoperiodic inhibition of stem elongation
and ﬂowering in Rudbeckia hirta ‘Marmalade’. J. Amer. Soc. Hort. Sci.
1 14(2):219-222.

Parks, 3M. and PH. Quail. 1993. hy8, a new class of Arabidopsis long
hypocotyl mutants deﬁcient in functional phytochrome A. Plant Cell 5:39-
48.

Podol’nyi, V2. and AG. Chetverikov. 1986. Inﬂuence of photoperiodic induction
and gibberellin on growth of young coneﬂower leaves and formation of
their photosynthetic apparatus. Soviet Plant Physiol. 33:686—692.

Rajapakse, NC. and J.W. Kelly. 1994. Problems of reporting spectral quality and
interpreting phytochrome-mediated responses. HortScience 29(12):1404-
1407.

Reed, J.W., A. Nagatani, T.D. Elich, M. Fagan, and J. Chory. 1994.
Phytochrome A and phytochrome B have overlapping but distinct
functions in Arabidopsis development. Plant Physiol. 104:1139-1149.

Reed, J.W., P. Nagpal, D.S. Poole, M. Furuya, and J. Chory.1993. Mutations in
the gene for the red/far-red light receptor phytochrome B alter cell
elongation and physiological responses throughout Arabidopsis
development. Plant Cell 51147-157.

Reekie, J.Y.C., P.R. Hicklenton, and E.G. Reekie. 1994. Effects of elevated CO2
on time to ﬂowering in four short-day and four long-day species. Can. J.
Bot.72:533-538.

Roberts, E.H. and R.J. Summerﬂeld. 1987. Measurement and prediction of
ﬂowering in annual crops, p.17-50. In: J.G. Atherton (ed.). Manipulation of
ﬂowering. Robert Hartnoll, Bodmin, Cornwall.

Roberts, E.H., P. Hadley, and R.J. Summerﬂeld. 1985. Effects of temperature
and photoperiod on ﬂowering in chickpeas (Cicer arietinum L.). Ann. Bot.
55:881-892.

Roberts, E.H., R.J. Summerﬂeld, F.J. Muehlbauer, and R.W. Short. 1986.
Flowering in lentil (Lens culinan's Medic): The duration of the
photoperiodic inductive phase as a function of accumulated daylength
above the critical photoperiod. Ann. Bot. 58:235-248.

Salisbury, F.B. 1981. Responses to photoperiod, p. 135-167. In: O.L. Lange,
P.S. Nobel, C.B. Osmund, and H. Ziegler (eds.). Physiological plant
ecology, vol. I: Responses to the physical environment. Springer-Verlag,
New York.

62

Salisbury, F .B. and CW. Ross. 1969. Plant physiology. Wadsworth Publishing,
Belmont, California.

Salisbury, F.B., and CW. Ross. 1978. Plant physiology. 2nd ed. Wadsworth
Publishing, Belmont, California.

Shedron, K.G. and TC. Weiler. 1982. Regulation of growth and ﬂowering in
Chrysanthemum x superbum Bergmans. J. Amer. Soc. Hort. Sci.
1 07(5):874-877.

Shillo, R. and AH. Halevy. 1982. Interaction of photoperiod and temperature in
ﬂowering-control of Gypsophila paniculata L. Scientia Hort. 16:385-393.

Smith, H. 1982. Light quality, photoperception, and plant strategy. Ann. Rev.
Plant Physiol. 33:481-518.

Smith, H. 1994. Sensing the light environment: the functions of the phytochrome
family, p. 377-416. In: R.E. Kendrick and G.H.M. Kronenberg (eds.).
Photomorphogenesis in plants. 2nd ed. Kluwer Academic Publishers,
Netherlands.

Smith, H. 1995. Physiological and ecological function within the phytochrome
family. Annu. Rev. Plant Physiol. Plant Mol. Biol. 46:289-315.

Song, C.Y., M.S. Roh, S.K. Chung, and RH. Lawson. 1993. Effect of
temperature and light on growth and ﬂowering of potted plant production
of Platycodon. J. Korean Soc. Hort. Sci. 34(6):446-453.

Sweeney, BM. 1987. Rhythmic phenomena in plants. 2nd ed. Academic Press,
San Diego.

Taiz, L. and E. Zeiger. 1991. Plant physiology. Benjamin/Cummings Publishing,
Redwood City, California.

Talon, M. and J.A.D. Zeevaart. 1990. Gibberellins and stem growth as related to
photoperiod in Silene armen'a L. Plant Physiol. 92:1094-1100.

Thomas, B. 1993. The role of phytochrome and other photoreceptors in the
control of ﬂowering in long-day plants. Flowering Newsletter 1626-10.

Thomas, B, and D. Vince-Prue. 1984. Juvenility, photoperiodism and
vemalization, p. 408-439. In: M.B. Wilkins (ed.). Advanced plant
physiology. Pitman Publishing, London.

63

Upadhyay, A.P., R.H. Ellis, R.J. Summerﬁeld, E.H. Roberts, and A. Qi. 1994.
Characterization of photothermal ﬂowering responses in maturity isolines
of soyabean [Glycine max (L.) Merrill] cv. Clark. Ann. Bot. 74:87-96.

\flerstra, RD. and PH. Quail. 1983. Puriﬁcation and structural characterization of
124-kDa phytochrome from Avena. Biochemistry 22:2498-2505.

Vince, D. 1969. The regulation of ﬂowering in long—day plants. Acta Hort. 14:91-
95.

Vlnce-Prue, D. 1975. Photoperiodism in plants. McGraw, London.

Vince-Prue, D. 1984. Light and the ﬂowering process--setting the scene, p. 3-15.
In: D. Vince-Prue, B. Thomas, and K.E. Cockshull (eds.). Light and the
ﬂowering process. Academic Press, San Diego.

Vince-Prue, D. 1994. The duration of light and photoperiodic responses, p. 447-
490. In: R.E Kendrick and G.H.M. Kronenberg (eds.).
Photomorphogenesis in plants. 2nd ed. Kluwer Academic Publishers,
Netherlands.

Vince-Prue, D. and A. Takimoto. 1987. Roles of phytochrome in photoperiodic
ﬂoral induction, p.259-273. In: M. Furuya (ed.). Phytochrome and
photoregulation in plants. Academic Press, London.

Waithaka, K. and L.W. Wanjao. 1982. The effect of duration of cold treatment on
growth and ﬂowering of liatris. Scientia Hort. 18:153-158.

Wang, S., R.W. Ward, J.T. Ritchie, R.A. Fischer, and U. Schulthess. 1995.
Vemalization in wheat: l. A model based on the interchangeability of plant
age and vemalization duration. Field Crops Res. 41 :91-100.

Wellensiek, SJ. 1958. Vemalization and age in Lunan'a biennis, p. 561-571. In:
Proc. koninklijke nederlandse akademie van wetenschappen, vol. 61.

Weller, J.L. and J.B. Reid. 1993. Photoperiodism and photocontrol of stem
elongation in two photomorphogenic mutants of Pisum sativum L. Planta
1 89:1 5-23.

Whitman, CM. 1995. Inﬂuence of photoperiod and temperature on ﬂowering of
Campanula carpatica ‘Blue Clips; Coreopsis grandiﬂora ‘Early Sunrise’,
Coreopsis verticllata ‘Moonbeam’, Rudbeckia fulgida ‘Goldsturm’ and
Lavandula angustifolia ‘Munstead’. MS Thesis, Michigan State University,
East Lansing.

64

Zeevaart, J.A.D. 1976. Physiology of ﬂower formation. Annu. Rev. Plant Physiol.
272321 -348.

Zeevaart, J.A.D. 1978. Phytohorrnones and ﬂower formation. P. 291-327. In:
Letham, Goodwin, and Higgins (eds.). Phytohonnones and related
compounds—a comprehensive treatise, vol. II. Elsevier/North-Holland
Biomedical Press.

Zeevaart, J.A.D. and DA. Gage. 1993. ent-Kaurene biosynthesis is enhanced by
long photoperiods in the long-day plants Spinacia oleracea L. and
Agmstemma githago L. Plant Physiol. 101225-29.

Zhang, 0., AM. Armitage, J.M. Affolter, and MA. Dirr. 1995. Environmental
control of ﬂowering and growth of Lysimachia congestiﬂora Hemsl.
HortScience 30(1):62-64.

SECTION II

THE EFFECTS OF PHOTOPERIOD AND COLD TREATMENT
ON FLOWERING OF TWENTY-FIVE SPECIES
OF HERBACEOUS PERENNIALS

66

Introduction

Herbaceous perennials continue to increase in popularity. Between 1993
and 1994, 86% of ﬁrms surveyed saw an average increase of 33% in their sales
of perennials (Rhodus and Hoskins, 1995). In the northern states most
herbaceous perennials are sold in the spring, when a majority are not in ﬂower.
Herbaceous perennials in ﬂower have much more appeal and marketing
potential than those sold green, but the ﬂowering requirements of most garden
herbaceous perennials are unknown. The ﬂowering requirements for the
herbaceous perennials Dendranthema spp. and Easter lily (Lilium Iongiﬂorum
Thunb.) have been intensely studied. This knowledge has enabled greenhouse
growers to schedule crops to ﬂower on a certain date with desired ﬂowering
characteristics. By knowing the ﬂower induction requirements of other species of
perennials, a greenhouse grower could force a variety of perennials into ﬂower
on a predetermined date.

Some plants ﬂower only after exposure to temperatures less than 7 °C for
a certain period of time (Lang, 1965). This is known as vemalization. Other
plants ﬂower faster following a cold temperature treatment (e.g., Easter lily),
while for others, a cold temperature treatment does not affect ﬂowering. The
length and effective temperature range for vemalization varies by species. In
general, plants require several weeks of cold to saturate the vemalization
response. For example, forty-six percent of the ‘Gloriosa’ blazing-star (Liatn's
spicata WIlId.) herbaceous perennials that received six weeks of 3-5 °C ﬂowered,

whereas 90% that received eight weeks of 3-5 °C ﬂowered (Waithaka and

67

Wanjao, 1982). This suggests that ‘Gloriosa’ blazing-star requires at least eight
weeks of cold for most plants to become vernalized. The most effective
temperature range for vemalization of most plants is 1 to 7 °C (Lang, 1965).

Many herbaceous plants ﬂower in response to the duration and timing of
light and dark periods in a day or series of days, which is known as
photoperiodism (Vince-Prue, 1984). Plants have been divided into three main
categories on the basis of ﬂowering in response to photoperiod. Day-neutral
plants ﬂower regardless of the photoperiod to which they are exposed. For
example, ‘Sentimental Blue’ balloon ﬂower (Platycodon grandiﬂorus A. DC.
‘Sentimental Blue’) plants grown under 10-hour (short day) or 16-hour (long day)
photoperiods ﬂowered roughly simultaneously; thus, the plant is considered day-
neutral (Song et al, 1993). Short-day plants (e.g. Chrysanthemums) only ﬂower,
or ﬂower most rapidly, when exposed to fewer than a certain number of hours of
light in a 24-hour cycle. In contrast, long-day plants only ﬂower, or ﬂower
quicker, when exposed to more than a certain number of hours of light in each
24-hour cycle. It has been shown that the length of the dark period is the critical
factor for ﬂower induction: short-day plants require uninterrupted nights longer
than a certain duration, and long-day plants require a limited darkness duration.
The number of photoperiod cycles required for ﬂowering varies tremendously by
species, from as little as one to more than 70 (Vince-Prue, 1975).

Short- and long-day plants can be subdivided further: plants may have
either a qualitative or a quantitative response to photoperiod. A qualitative

response, also known as an absolute or obligate response, means the plant

iov.
met
ver
pla'
coll
phi
A c

has

VGI

an:
in

QIE

Ma

are

‘Su

68

requires daylengths that are either shorter or longer than a certain duration to
ﬂower. For example, a qualitative long-day plant must have photoperiods that
meet or exceed a particular duration to ﬂower. ‘Moonbeam’ tickseed (Coreopsis
verticr'llata L. ‘Moonbeam’) is an example of a qualitative long-day plant; no
plants grown with 8-hour photoperiods after receiving 0, 6, or 12 weeks of 4.5 °C
cold treatment ﬂowered, whereas all those grown under 16- or 24-hour
photoperiods ﬂowered, regardless of cold treatment (lversen and Weiler, 1994).
A quantitative photoperiodic response describes a particular daylength that
hastens, but is not essential for, ﬂowering. Dense-ﬂowered loosestrife
(Lysimachia congestiﬂora Hemsl.) is an example of a quantitative long-day plant;
days to visible bud decreased from 61 to 27 and ﬂower number increased from
21 to 416 as the photoperiod increased from 8 to 16 hours (Zhang et al., 1995).
The objectives of these experiments were to determine 1) the effects of a
vemalizing cold-treatment on ﬂowering, 2) the photoperiodic response category
for ﬂowering, 3) the inﬂuence of photoperiod on ﬂower number and plant height,
and 4) the photoperiod(s) that induced the most complete, rapid, and uniform
ﬂowering. The herbaceous perennial species were chosen based on popularity,

greenhouse grower interest, and suitability as a potted plant.

Materials and Methods
Plant material. The species studied, plug size, and age of plant material
are provided in Table 1. To eliminate juvenility problems, Coreopsis grandiﬂora

‘Sunray’, Gaillardia xgrandiﬂora ‘Goblin’, and Rudbeckia fulgida ‘Goldsturm’ were

C P F P F R S S ‘17 Iv~rmau b c d e 3! AM! P J...

I

69

Table 1. Species studied and characteristics of starting material.

 

 

 

Propagation Plug Avg.
Species Date Method Environment‘ size’ nodes
Annen‘a xhybrida ‘Dwarf Ornament Mix’ 10/3/94 seed a 128 12.0
Annen'a pseudanneria Mansf. 6/20/94 seed b 50 35.6
Asclepias tuberosa L. 7/10/94 seed c 50 0
Campanula carpatica Jacq. “Blue Clips’ (94-5) 9/26/94 seed a 128 4.9
Campanula carpatica Jacq. ‘Blue Clips’ (95-6) 8/7l95 seed d 70 13.0
Coreopsis grandiﬂora Hogg ex Sweet ‘Sunray’ 6/25/95 seed b 50 7.9“"
Coreopsis verticillata L. ‘Moonbeam’ (no cold) unknown cutting unknown 128 2.7"
Coreopsis verticillata L. ‘Moonbeam’ (with cold) unknown cutting unknown 70 3.3"
Echinacea purpurea Moench. ‘Bravado’ (94-5) 10/17/94 seed a 128 4.2
Echinacea purpurea Moench. ‘Bravado’ (95-6) 10/9/95 seed a 128 4.1
Gail/ardia xgrandiﬂora Van Houtte ‘Goblin’ 6/25/95 seed b 50 18.8‘"
Gypsophila paniculata L. “Double Snowﬂake' 10/17194 seed a 128 8.2"
Helenium autumnale L. 6/15/95 seed b 50 5.1
Hibiscus xhybn’da ‘Disco Belle Mixed’ 11/7/94 seed a 128 4.5
Lavandula angustifolia Mill. ‘Munstead Dwarf 6/10/94 seed b 50 21 .8"
Leucanthemum xsuperbum ‘Snow Cap’ unknown tissue unknown 8 cm 11.9

culture
Leucanthemum xsuperbum ‘White Knight’ 10/9/95 seed a 128 6.1
Lobelia x speciosa Sweet ‘Compliment Scarlet’ 10/3/94 seed a 128 6.6
Oenothera missouriensis Sims 10/10/94 seed a 128 4.4
Phlox paniculata ‘Eva Cullum’ 6/95 cutting d 50 8.8"
Phlox paniculata ‘Tenor’ unknown cutting d 50 4.7"
Phlox subulata L. ‘Emerald Blue’ unknown cutting d 70 15.5"
Physostegia virginiana Benth ‘Alba’ 10/10/94 seed a 128 4.9"
Rudbeckia fulgida Ait. ‘Goldsturm’ 6/1/95 seed b 50 10.0"
Salvia xsuperba ‘Blue Queen' 10/17/94 seed a 128 4.7"
Scabiosa columban’a L. “Butterﬂy Blue” 8/94 tissue e 8 cm 58"

culture
Veronica Iongifolia L. ‘Sunny Border Blue’ 8/94 cutting b 50 46"
Veronica spicata L. ‘Blue’ 10/24/94 seed a 128 7.0

 

2a = natural photoperiods, temperatures beginning at 24 °C and gradually decreasing to 19 °C.
b = natural photoperiods, minimum temperatures of 19 °C until last two weeks, when minimum

temperatures decreased to 13 °C.

c = same as b, but with 4-hour night interruption lighting from 8/25 to 10/1.

d = natural daylengths, no exposure to temperatures below 12 to 15 °C.

e = natural photoperiods, propagated at 18 °C, held at four weeks with 7 °C night temperatures

and 7 to 21 °C day temperatures, then grown at 18 °C for ﬁnal two weeks.
Nolume of 128-, 70-, and 50-cell trays or 8-cm containers are 10, 50, 85, or 350 ml, respectively.
"Plants have opposite phyllotaxy, so the number of leaves is twice the number of nodes; all others
have alternate phyllotaxy, so the number of nodes equals the number of leaves.
”Plants were grown under photoperiods <11 hours for 6 or 7 weeks to attain indicated node count.

ligl

SO

70

grown under natural short-day photoperiods (approximately 10 to 11 hours of
light) for seven, six, or six weeks, respectively, before cold treatment or forcing
so that they met the recommendations of Yuan (1995).

Plant culture. Plants were grown in a commercial soilless medium
composed of composted pine bark, horticultural vermiculite, Canadian sphagnum
peat moss, processed bark ash, and washed sand (MetroMix 510, Scotts-Sierra
Horticultural Products Company, Marysville, Ohio). Plants were top-watered with
well water acidiﬁed (two parts H3PO4 plus one part H2804, which provided z2.5
mol P-m’a) to a titratable alkalinity of approximately 130 mg calcium bicarbonate
per liter and fertilized with 14N-0P-6K20 (mol-m'3) from potassium nitrate (14N-
0P-55K20) (Vicksburg Chemical Co., Vicksburg, MS) and ammonium nitrate
(34N-0P-0K20) (Cargill, Lexington, KY). Fertilization and acidiﬁcation rates were
adjusted in response to weekly soil test results, so regimes varied during
experiments. High-pressure sodium lamps provided a photosynthetic photon ﬂux
(PPF) of approximately 50 meI'm'Z'S'1 at plant level when the ambient
greenhouse PPF was lower than 400 umol-m‘z-s”.

Cold treatments. Plants received either no cold treatment or were placed
in a controlled-environment chamber for 15 weeks at 5 °C. The chamber was lit
from 0800 to 1700 HR at approximately 10 ,um0|°m'2'S'1 from cool-white
ﬂuorescent lamps (VHOF96T12; Philips, Bloomﬁeld, NJ), as measured by a Ll-
COR quantum sensor (model Ll-189; Ll-COR, lnc., Lincoln, NE). Plants were

cold-treated in the containers in which they were received. While in the cooler,

71

plants were watered with well water acidiﬁed (H2804) to an approximate pH of 6.0.

Photoperiod treatments. In 1994-95, sixty plants of each species and cold
treatment were removed from their containers, thinned to a single plant per cell
(singulated), and transplanted into 10-cm round containers (470 ml). In 1995-96,
seventy plants of each species and cold treatment were removed from their
containers, singulated, and transplanted into 13-cm square containers (1.1
liters). Ten plants were placed under each photoperiod treatment that was
assigned randomly to benches in the greenhouse. In 1994-95, photoperiods
were 10, 12, 14, 16, or 24 hours of continual light or 9 hours with a 4-hour night
interruption (NI) from 2200 to 0200 HR. In 1995-96, photoperiods were 10, 12,
13, 14, 16, or 24 hours of continual light or 9 hours with a 4-hour NI. Black cloth
was pulled at 1700 HR and opened at 0800 HR every day on all benches to
provide similar daily light integrals. Photoperiods were completed with
incandescent lamps at 1 to 3 umol-m'Z-s". For the continual photoperiodic
treatments, lamps provided day-extensions; they were turned on at 1700 HR and
turned off after each photoperiod was completed.

Greenhouse temperature control. All plants were grown in glass
greenhouses set at 20 °C. Air temperatures on each bench were monitored with
36-gauge (0.013-mm-diameter) type E thermocouples connected to a CR10
datalogger (Campbell Scientiﬁc, Logan, UT). To provide uniform temperatures,
the datalogger controlled a 1500-watt electric heater under each bench, which
provided supplemental heat as needed throughout the night. The datalogger

collected temperature data every 10 seconds and recorded the hourly average.

72

Actual average daily air temperatures from the beginning of forcing to the
average date of ﬂowering under every photoperiod were calculated for each
species and are presented in Table 2.

Data collection and analysis. The leaves of each plant were counted at
the onset of forcing. Date of the ﬁrst visible bud or inﬂorescence and date of
opening of the ﬁrst ﬂower were recorded for each plant. At ﬂowering, the number
of visible ﬂower buds or inﬂorescences, the number of leaves on the main stem
below the ﬁrst ﬂower, and total plant height were determined. Plants that did not
have visible buds or inﬂorescences after 15 weeks of forcing were discarded and
considered nonﬂowering, but those with visible buds or inﬂorescences were kept
until ﬂowering. Days to visible bud, days from visible bud to ﬂower, days to
ﬂower, and increase in node count were calculated.

For each species, a randomized complete block design was used in which
blocks were photoperiods with ten observations for each cold treatment. Data
were analyzed using SAS’s (SAS Institute, Cary, NC) analysis of variance and
general linear models procedures.

Presentation of results. For each species, a page with six ﬁgures provides
illustrations of means and trends; the following apply to these ﬁgures. Unless
othenrvise indicated, all data points represent means of the number of plants that
ﬂowered out of ten. (A) and (B) show days to visible bud, days to ﬂower, and
percentage of ﬂowering in non-cold treated and cold-treated plants, respectively.
(C) shows the average number of initial nodes (n=120) and nodes at ﬂower for

non-cold treated plants. (D) shows the number of initial nodes (n=120), nodes

 

{th'ullcc ‘lt ell-I I
l- .11! h\ ‘hnulhr

73

62950: $53 oz.n
.EoEtoaxo c_ 33.05 so: notoqouocd.
.cozaEEE £9: Eocene“:

 

 

 

.8 .. m. 8 o. 8 .. .. .. 2 888
:8 m8 8.8 o. 8 8.8 :8 .. o 8% :8? .0885. 3:93 $83.3
N8 -- 98 s. 8 - o. 8 m. 8 3 85%
m8 .. m8 m8 - .. .. o 35 :8, .2855. «8:83 802.com
38 Q8 o. 8 o. 8 - 8.8 m8 9 85:8
8.8 :8 N8 88 - m8 m8 o 858 .5888? asset? «.5828
«.8 m. 8 m8 m8 8. 8 «.8 8.8 m: 888
m8 m8 .8 :8 - o8 .. o 8?. :8? 395m. 205288 3828
o8 8.8 8.8 v8 .. -- .. 2 888
Q8 v8 28 88 28 .. .. o 888. .36 Ba. 8888 «3:3st
«.8 o. 8 m. 8 8. 8 - .. -- 2 83s
«.8 0.8 8.8 m8 - .. .. o 35:9 8:0 86. 8.888 «3:358
m8 :8 F. 8 m8 - .. .. 9 85:8
.. .. .. .. - .. .I o 3%: 8985 83%?
8.8 :8 8.8 m8 - «.8 :8 2 85:8
«.8 38 o. 8 8. 8 - m8 m8. o 35: 88:8:me 885.
v8 v. 8 0.8 F. 8 - 0.8 :8 2 83v
«.8 0.8 m8 m8 .. v.8 m8 o 33. :8. .52 .85an ES. auras €85-
~_Z .VN @v 3. 0r Nw or On we 0528 m0_owaw
A285 3.8221 9.82, so 93

 

Gov 9:22 9E3 2383an omﬂo><

 

.uotoaoaoca some .625 36QO some .2

9:26: .o Emu omega 2 @528 .0 086 ES.— moLESXEE .._m omega new .0528 Co «San .23 5:88.05 6.5m man. .N can...

74

ace—5:098 c_ 3.0205 8: 358828
89026: 0:8... 02.
.cozanFs 59: 502.508

 

 

 

F8 0.8 v. 8 o. 8 - 0.8 08 0F 0030
0.8 0.8 8.8 0.8 - .. .. o 38 F8F 5088855 50588
0.8 0.8 0. 8 F. 8 - 0.8 o. 8 0F 08%
0.8 08 F8 88 - .. .. 0 38:8 00:80 505858. 858qu 0.583
8. 8 0.8 0.8 F. 8 0.8 0.8 0. 8 0F 0088
0.8 0.8 8.8 o. 8 8.8 0.8 .. 0 0.2058 .595. 253 5:883 8505883
0.8 8. 8 0.8 v. 8 F. 8 0.8 o. 8 0F 00:8
0.8 0.8 0.8 .. .. .. .. o 095: .80 380. 5:883 53505883
8.8 0.8 F. 8 0.8 - 8.8 28 0F 08:8
0.8 8.8 .. .. - -- -- o 883 5030 822:5. 085088 58:83
.. .. .. -- - .. .. 0F 002%
0.8 8.8 8.8 0.8 - 0.8 .. o 35sz .0055 0:00 805. 858$ 0855:
0.8 0. 8 F8 F. 8 .. I .. 0F 00:8
8.8 0.8 0.8 .. .. .. .. o 020: F 58850 5:520:
F. 8 8.8 0.8 F. 8 - .. F. 8 0F 0030
0.8 0.8 0.8 0.8 x- -- .. 0 88:9 20.5650 0.88. 558.58 5.5888
0.8 F. 8 0.8 0.8 0. 8 0.8 0.8 0F 0088
F8 0.8 0.8 o. 8 F8 5. 0.8 o 0005 .538. 85859 55558
u_Z VN or 3. Mr NF 0.. Um no @598 mmﬁwam
3.85 80822“. 9.003 5 0.8

 

 

6.» 959.8 0555 0.300.880. omEo><

 

.5580 8 0.8»

75

.E0E..0ax0 5 0003.0... 8: 08.08888...

80.030: 02.0.0 02.
80.32.02. 29: 59.530"...

 

 

 

F8 0.8 F.. 8 o. 8 - F8 88 8 0888
«.8 0.8 88 0.8 - v8 08 o .88 :8 .020. 58.8 8.8.0:.
0.8 8.8 F. 8 8.8 - 8.8 0.8 8 0888
l l I I - l l o 3:85 .0:_m .088 3:5. 8.83:8 08:88.
8.8 o. 8 8.8 o. 8 - 0.8 0.8 .8 08:8
0.8 0.8 F. 8 F8 - 0.8 0.8 o 88:: .020 .5250. 88558 80.580
F8 08 v. 8 o. 8 - F8 0.8 8 0888
0.8 0.8 .. .. - .. 0.8 o .88 :8 .880 020. 05088 0.3.8
0.8 «.8 F8 8. 8 F. 8 0. 8 -- 0F 08.88
8.8 0.8 8.8 o. 8 .. .. .. 0 0888 5.38.00. 888. 8.888
0.8 0.8 F8 0. 8 - 8. 8 0. 8 8 0888
0.8 0.8 ...8 .. .- v.8 .. o .88 :8 .82. 8555.: 0.8.888
0. 8 0.8 8.8 0.8 0.8 8.8 o. 8 0F 08:8
0.8 o. 8 0.8 8.8 0.8 0.8 0.8 o 088:: .020 0.5050. 08880 8.8.
08 F8 08 8. 8 F. 8 0.8 .. 8 08:8
0.8 0.8 0.8 8.8 0.8 .. .. o 088: F .808 08.858 .680.
0.8 F. 8 0.8 8. 8 F. 8 88 .. 8 08:8
0.8 0.8 8.8 0.8 .. .. .l o 088: F .5280 80. 08.858 x58.
”.2 VN m_. .3. Mr NF or On *0 050.8 aﬂoaw
.585 08822.. 883 .o 0.8

 

6... 9.0.8 mezzo 0.30.0088 0W80><

 

.058. 8 285

 

Res

phc
0H

0f

da

tre

76

after cold treatment (n=60), and nodes at ﬂower for cold-treated plants. (E) and

(F) show the number of inﬂorescences per plant and plant height at ﬂower.

Results and Discussion

Plants were placed into one of six categories based on the effects of
photoperiod and cold treatment on ﬂowering (Figure 8). Most plants ﬁt into one
of two cold treatment response categories: cold treatment was either beneﬁcial
or required for ﬂowering. Plants ﬁt into one of three photoperiodic response
categories for ﬂowering; species were day-neutral, facultative (quantitative) long-
day, or obligate (qualitative) long—day plants. No species required both cold
treatment and long days for ﬂowering. Percent ﬂowering, days to ﬂower, ﬂower
number, and uniformity in time to ﬂower were the four primary ﬂowering
parameters considered when species were placed into categories.

Several species responded to photoperiod differently before or after cold
treatment. For example, Lobelia xspeciosa “Compliment Scarlet’ ﬂowered as an
obligate long—day plant without a cold treatment, and a facultative long-day plant
after cold treatment. For these situations, plants were placed into response

categories based on the photoperiodic responses after cold treatment.

Day-Neutral Species That Beneﬁt from a Cold Treatment
Armeria xhybrida ‘Dwarf Ornament Mix’. Time to ﬂower in ‘Dwarf

Ornament Mix’ was highly variable, regardless of photoperiod or cold treatment

77

0.20.0 3.5.0.00 030000.00 .0 000000.00 0020000. 020026.“. .0 0.09....

 

 

.5000
02m. 00.0000 x 0.3.00
.00.<. .005 .028
0203.9... 0.00.0808. .6200. 0.3.320. 003000..
.5300. .00.@
0.00.0203... 0.0.0.000 0.0025. 0.03000 .63....
00.200. 0.026 0020205.. 02.000".
0.02.0203. 0.0000200 0.3300020 0302020.. 0.o0
.00x_5. 0..0m
000.0. 00.2030A 0000.0...
.E.0.00_00. 00.035020 0.0000.
00.0.0. 00.000000. 0.030.200 03.000000
.o00>0.m.
..oc0.r 0.030.200 .808. 0030.00 00002.2.0m. .005. 0.00.2.0 00390..
.8230 30002005.. 002000 3050.02.00. .005 200.39
0>m.. 0.030.200 x008. 0.0....0.:0> 0.0000200 000.0009. 0.300.. 020023.00 000.3000.
09.0 020. .295. 223 250.0009.
0.0200000002 0.02.0200 00000.00 032002.00 2350230000.. 00022000000 0.2022<
.000 2620. E08000? .00... 30502.0 3.0.3200
0.052030 $030.0... 000.000. 00.00300. 232.02.20000.. 0025. 00.2030. 0.20220. 0.o0
«20.0

.5... 800.05.. 80930

 

80.93.. 0.55.000“.

 

«20.0 88002-80

 

78
(Figure 9, Table 3). For example, the 95% conﬁdence intervals (CI) of days to

ﬂower for plants under the NI treatment were :I: 17 or :I: 18 days, without or with
cold treatment, respectively.

Percentage of ﬂowering increased from 70 to 100 as photoperiod
increased from 10 to 24 hours for plants that did not receive 15 weeks of cold
treatment. However, the percentage decreased from 100 to 40 as photoperiod
increased for plants that did receive the cold treatment. Cold treatment
signiﬁcantly reduced (by approximately two weeks) days to visible bud and
ﬂower. It also reduced the number of new nodes formed before ﬂowering from
27 to 21 but did not affect days from visible bud to ﬂower, ﬁnal plant height, or
number of inﬂorescences.

There were no photoperiodic trends in days to visible bud or ﬂower for
unchilled plants, but for cold-treated plants time to ﬂower increased linearly as
photoperiod increased. This trend suggests that ‘Dwarf Ornament Mix’ is day-
neutral before cold treatment and is a quantitative short-day plant thereafter.
However, the latter conclusion is not supported by a reduction in nodes formed
under shorter daylengths. There was a linear increase in ﬁnal plant height as
photoperiod increased in unchilled plants. Photoperiod had no effect on ﬂower
number.

In Anneria maritime Willd. ‘Diisseldorfer Stolz’, 34% of plants ﬂowered (in
approximately 25 weeks) when forced under natural photoperiods in a 14 to 16

°C greenhouse beginning in November (Christensen et al., 1989). In January,

 

 

 

A 0 Weeks of 5C

100 - 100%
C UNI g

75; — — ——————— 0M 75% -
. S
a~ so ___________________ .5091, u.
D o-
25 l— —————————————————— 25% g
o.

 

 

 

10 12 14 16 18 20 22 24
Photoperiod

 

ﬂi— Day-inmate“ a DlbeFloer uo-PuumFlmll-ig

 

 

 

c Plant Node Development
_ 0 Weeksiof 5C --

888

Number of Nodes

   

1o ’ 12 ‘ 14 16
Photoperiod

:Elnﬂlﬂmﬂu .Nodeulllemrl

 

 

 

 

E Number of lnﬂoresences

§ 2.5

 

N

.0
0|
-

 

£2

 

Number of lnﬂoresen

 

1012141618 20 22 24
Photoperiod

 

 

F30 weeks cold -15 weeks cola]

 

 

 

 

79

 

 

 

'3 15 Weeks of 5c
100 100%
755 - ____-_ {(7993‘1 75%?
0 , 0N1 g
0 so —————————————— w 50% u.
0 E
25 ———————————————————— 25% g
n.
0%

 

 

 

0 v A A, A A A A
10 12 14 16 18 20 22 24
Photoperiod

 

HfDlyItoVuihleBud a DlyeloFIower .0.me

 

 

 

 

D Plant Node Development

, 15 Weeks of 5c -

1
:4-- .7.

Number of Nodes

    

2"1'4’15'24 NI,
Photoperiod

iIIMUnodu

 

Gunman-mum]

 

 

F Plant Height at Flower

 

 

 

 

30
€25 ——————————————————————
f° ““““““““ gar
I15 L- ____ ______________
10':

1012141618 20 22 24
Photoperiod

 

Ede-Weeks cold .15 weeks cold“

 

 

 

 

 

 

Figure 9. The effects of photoperiod and cold treatment on ﬂowering of Armen'a

xhybrida ‘Dwarf Ornament Mix’.

80

Table 3. The effects of photoperiod and cold treatment on ﬂowering of Armen'a xhybrida ‘Dwarf Ornament
Mix'.

 

Days to Days from Days Increase Final plant

 

 

 

Weeks Percentage visible visible bud to in node height Flower
Q! 59 Phgtoggrig ﬂgwering bud to Mr (cm) mm;
0 - 81 78 12 91 27 17 1.6
15 - 68 65 12 77 21 18 1.6
- 10 85 71 12 83 25 15 1.4
12 75 74 12 86 23 15 1.9
14 75 65 12 77 24 18 1.5
16 65 73 12 85 24 17 1.3
24 70 83 14 97 22 24 2.2
Nl‘ 79 64 12 76 25 18 1.5
0 10 70 80 10 90 29 12 1.3
12 80 81 11 92 26 14 2.0
14 80 74 12 86 25 18 1.4
16 80 75 12 87 28 15 1.4
24 100 84 16 100 24 28 2.3
NI 78 76 13 89 29 17 1.3
15 10 100 62 14 76 21 17 1.6
12 70 67 12 79 19 16 1.8
14 70 56 13 68 23 18 1.6
16 SO 70 12 83 20 19 1.2
24 40 82 12 93 20 20 2.0
N_! 80 51 12 63 21 19 1.6
Signiﬁcance
Weeks cold (WC) ”" NS m “ NS NS
Photoperiod (P) " NS " NS * NS
WC x P Ns * NS NS NS NS
95% Conﬁdence interval for NI
Zero weeks SC 14 3.8 17 6.8 3.7 0.5
15 weeks SC 18 1.9 18 4.5 2.8 0.6
Contrasts
Zero weeks SC
NI vs. 16 NS NS NS NS Ns NS
NI vs. 24 NS * NS NS *“ NS
Pu... (10 to 24 h) NS “1' NS NS *** NS
Pan-drafts (10 to 24 h) NS NS NS NS NS NS
15 weeks SC
NI vs. 16 * NS * NS NS NS
NI vs. 24 *" NS ** NS NS NS
P0... (10 to 24 h) " NS * NS NS NS
Pam-0 (10 to 24 h) NS NS NS NS NS NS
0 and 15 weeks SC
Pu... (10 to 24 h) " us * NS '” NS
P0000110 to 24 h) N§ N_§ N_s_ N_§ N_ N§

 

‘Nl = 4-h night interruption.
"5' " ”- '°' Nonsigniﬁcant or signiﬁcant at Ps0.05, 0.01, or 0.001 . respectively.

 

81

89% of plants ﬂowered in an average of 14 weeks, and 75% ﬂowered (in about
seven weeks) when forced at similar temperatures in March.

Armeria pseudanneria. Flowering characteristics of A. pseudanneria
were highly variable, regardless of photoperiod or cold treatment (Figure 10 and
Table 4). The 95% CI of days to ﬂower for plants under NI was reduced after
cold treatment, but was still :t 21 days. The relatively large error bars in time to
ﬂower in Figure 11 illustrate the nonuniforrnity of ﬂowering under all photoperiods
with both cold treatments.

The 15 weeks of cold treatment increased the percentage of ﬂowering by
about one-half. The cold treatment signiﬁcantly reduced (by approximately 20
days) days to visible bud and ﬂower, increased ﬁnal plant height by 30%, and
increased the average number of inﬂorescences by 0.5. Cold treatment also
reduced the number of new nodes formed before ﬂowering, particularly under the
longer photoperiods. After cold treatment, days from visible bud to ﬂower
increased an average of two days.

Photoperiod did not affect time to visible bud or ﬂower without or with cold
treatment; thus, A. pseudanneria is a day-neutral species with a quantitative
response to cold treatment. There was a linear increase in total plant height at
ﬂower as photoperiod increased.

Scabiosa columbaria ‘Butterﬂy Blue’. Scabiosa ﬂowered uniformly
under all photoperiods, especially after cold treatment (Figure 12, Table 5). For
example, the 95% CI of days to ﬂower for cold-treated plants under NI was :t: 1

day.

 

 

 

 

  

A 0 Weeks of 5c
100%
75%
"50%
25%
,._,_,. . . . .091.

 

 

t—v—tvP—k

0 -,.,
10 12 14 16 18 20 22 24
Photoperiod

“9

Percent F

 

[* Days I) Vinnie Bud :3 Days to Flower .0- Percent Flowering

 

 

 

 

c Plant Node Development
0 Weeks of 5C

 

 

 

 

E Number of lnﬂorescences

 

 

Number of Inﬂoreeencee
u

 

o A l ._._

 

 

14 16 18 20 22 24
Photoperiod

 

Ina-0 weeks cold -15 weeks coldj

 

 

 

82

 

 

'3 15 Weeks of 5c

- 100%

100 -—‘
V 0 N1
75 ‘ ______________________ 75%

- 50%

 

Pemt FIowen‘ng

 

::
2F

0 1 . . Y , t
10 12 14 16 18 20 22 24
Photoperiod

 

 

 

”it Days loWIible Bud ﬁ- DeystoVFIowoF *Poroent Flowering E

 

 

 

 

D Plant Node Development
15 Weeks of SC

O
O

 

Number of Nodes
N # O)
O O O

O

 

{31:10.le- :] Nodes moronic I Nodes 01102071

 

 

 

F Plant Height at Flower

 

 

5 ¢ I Y e
10 12 14 18 18 2O 22 24
Photoperiod

F6Weeks cold -0- 15 Wéks cold]

 

 

Figure 10. The effects of photoperiod and cold treatment on ﬂowering of Annen'a

pseudanneria.

 

 

 

83

Table 4. The effects of photoperiod and cold treatment on ﬂowering of Annen‘a pseudanneria.

 

Days to Days from Days Increase Final plant

 

 

 

Weeks Percentage visible visible bud to in node height Flower
9f 5;; Phgjgpgn'gg ﬂmring bud to ﬂmer ﬂgwgr number (cm) number_
0 - 60 51 11 62 21 14 1.3
15 - 92 3O 13 43 13 20 1.8
- 1O 74 37 12 49 17 16 1.6
12 70 42 12 S4 17 15 1.5
14 75 36 11 47 13 15 1.8
16 75 32 13 45 13 17 1.4
24 90 38 14 52 17 26 1.7
Ni1 70 S7 11 67 24 15 1.3
0 10 67 35 11 46 15 10 1.3
12 40 58 10 68 23 14 1.3
14 50 42 9 S1 12 11 1.6
16 70 48 12 60 17 15 1.3
24 80 43 14 S7 20 24 1.3
NI 50 80 1O 90 37 13 1.0
15 10 80 38 13 S1 19 22 2.0
12 100 26 14 40 11 16 1.7
14 100 30 13 43 13 18 1.9
16 8O 17 13 31 9 20 1.5
24 100 33 15 48 15 28 2.1
N_I 90 33 11 44 12 18 1.7
Significance
Weeks cold (WC) *“ *” m “ “* "
Photoperiod (P) NS “ NS NS **" NS
WC x P NS NS NS " NS NS
95% Conﬁdence interval for NI
Zero weeks 5C 27 1.9 27 24 13 O
15 weeks SC 20 2.1 21 S 3 0 8
Contrasts
Zero weeks SC
Nl vs. 16 " NS NS “ NS NS
NI vs. 24 * “ ' “ *" NS
Pu... (10 to 24 h) NS *‘ NS NS *“ NS
Pan-erase (10 to 24 h) NS NS NS NS NS NS
15 weeks SC
NI vs. 16 NS NS NS NS NS NS
NI vs. 24 NS " NS NS *“ NS
Pun... (10 to 24 h) NS NS NS NS m NS
Pom (10 to 24 h) NS NS NS NS " NS
0 and 15 weeks SC
Pu». (10 to 24 h) NS ** us NS “’ NS
P m (10 to 24 h) N_S N_§ N_§ N_S * NS

 

zNI = 4-h night interruption.
"3- '° ”' '" Nonsigniﬁcent or signiﬁcant at Ps0.05, 0.01, or 0.001. respectively.

 

84

 

 

 

 

 

 

 

 

 

 

 

 

120 _
7 O No cold treatment
0 1Sweeks of5°C
100 - ................................................................................... ,
NI.
5 so - ..................................................................................
a)
E 126 l
2 60 -6 ...................... 24 ...............................
0
> 14 10
8 1° Nlo $2
40 _ .......... :: ......................................................... 12 ..........
16
20 - ..................... _ ._ .............................................................
0 l T I f l I T
30 40 50 60 70 80 90 100 1 10

Percentage Flowering

Figure 11. Percentage ﬂowering, time to ﬂower, and ﬂowering uniformity of
Annen'a pseudanneria under different photoperiods with or without cold treatment.
Numbers next to symbols represent photoperiods consisting of nine-hour natural
days that were extended with incandescent lamps. Nl = nine-hour natural days
with four hours of night interruption. Error bars are 95% conﬁdence intervals.

 

 

 

 

 

A 0 Weeks of 5c

 

 

Percent Flowering

 

 

 

o A . .A . . 1 A
1012141618 20 22 24 "
Photoperiod

 

li—DeyIIoWblswanmek-mor *PeroontFlow-ring

 

 

 

 

c Plant Node Development
0 Weeks of SC

N
O

.0
0|

Number of Nodes
a. 3

  

O

 
  

14 7e
Photoperiod

_ InltlaI—_ W. M.

   

 

 

 

E

Number of lnﬂorescences

 

 

 

 

0 . , ‘— 1‘ 5 ~45— ﬁ
10 12 14 16 1 B 20 22 24
Photoperiod

 

Number of Inﬂoresences

 

IEOWeekscold -o-15weekscold

 

 

 

 

85

 

 

 

 

 

 

'3 15 Weeks of 5c
100 - : H! 100%

75 ____________________ 75% 2’
.. E
e 50 ———————————————————— 50% 17.
D on
25 [WEN 25% g
0
AL—'—H~—k—-———A A Nl 0-

101214161820222 “
Photoperiod

 

I
“~A-MstM-BDq-DFM *PMFW |
l

 

 

 

 

D Plant Node Development

1 5 Weeks of SC

N
O

 

.0
01

Number of Nodes
01 8

 

 

O

 

 

 

[Elniielnedu Dunc-media! Neda-111mm i

 

 

 

 

Plant Height at Flower

 

Height (cm)

 

 

 

 

o A g

10 12 14 16 18 20 22 24
Photoperiod

 

Lug-0 weeks cold -15 weeks coldg

 

 

Figure 12. The effects of photoperiod and cold treatment on ﬂowering of

Scabiosa columban'a ‘Butterﬂy Blue’.

 

 

 

86
Table 5. The effects of photoperiod and cold treatment on ﬂowering of Scabiosa columban’a ‘Butterfly Blue’.

 

Days to Days from Days Increase Final plant

 

 

 

Weeks Percentage visible visible bud to in node height Flower
m & Phgjgmrigg ﬂmring bud to ﬂower flower ngmmr (cm) numb_Qr_
O - 100 39 21 60 10 32 8
15 - 100 12 15 27 3 28 15
- 10 100 26 19 45 7 18 15
12 100 28 18 46 6 19 11
14 100 27 18 44 6 27 12
16 100 22 18 40 6 35 12
24 100 26 18 44 7 53 12
NIz 100 26 17 43 6 30 10
0 10 100 40 22 63 11 15 1O
12 100 44 21 65 10 17 7
14 100 41 21 62 10 26 8
16 100 32 21 53 9 39 9
24 100 39 20 60 10 62 9
NI 100 40 20 60 1O 32 7
15 10 100 12 16 27 3 21 20
12 100 12 16 28 3 20 14
14 100 12 15 27 3 27 15
16 100 12 15 26 3 31 14
24 100 13 16 28 3 45 15
N1 109 12 15 26 3 28 13
Signiﬁcance
Weeks cold (WC) '“ *” *“' ”* “" "*
Photoperiod (P) NS * NS NS '“ ***
WC x P NS NS NS NS **" NS
95% Confidence interval for NI
Zero weeks 50 6.8 1.8 7.6 0.6 3.4 2.6
15 weeks 50 1.0 0.9 1.0 0.5 3.1 3.8
Contrasts
Zero weeks 50
Nl vs. 16 NS " NS NS “* NS
NI vs. 24 NS NS NS NS *” NS
PM (10 to 24 h) NS NS NS NS m NS
Pom-6c (10 to 24 h) NS NS NS * NS NS
15 weeks 50
Nl vs. 16 us NS us NS * NS
NI vs. 24 NS NS NS NS **' NS
Pu». (10 to 24 h) NS NS NS NS ““ "
PM (10 to 24 h) NS NS NS NS NS **
0 and 15 weeks 5C
PLhet (10 to 24 h) NS " NS NS *** NS
P tic (10 to 24 h) N_§ N_§ N_§ * N_§ "

 

‘NI = 4-h night interruption.
"s- " ”' '” Nonsigniﬁcant or signiﬁcant at Ps0.05. 0.01. or 0.001, respectively.

87

All plants ﬂowered, irrespective of cold treatment or photoperiod. Cold
treatment out time to ﬂower in half, reduced the number of new nodes formed
from ten to three, and increased ﬂower number nearly two-fold. Plants
developed an average of ﬁve nodes (ten leaves) during cold treatment, which
partially explains the reduction in nodes formed after cold.

Photoperiod did not affect days to visible bud or ﬂower, which suggests
that ‘Butterﬂy Blue’ is day-neutral. Final plant height increased over four-fold as
photoperiod increased from 10 to 24 hours without a cold treatment and two-fold
with the cold treatment.

Veronica spicata ‘Blue’. Cold treatment dramatically increased the
percentage of ﬂowering and improved uniformity of all ﬂowering characteristics
measured (Figure 13, Table 6). Only half of the plants ﬂowered without cold
treatment, but all plants ﬂowered after cold treatment. Under NI, cold treatment
reduced the 95% (CI) for days to ﬂower from about 16 to 2 days. Cold treatment
also reduced plant height by an average of 13 cm. Engle (1994) observed a
similar effect of cold treatment on ﬂowering; 43% or 98% of plants ﬂowered
without or with 15 weeks of 5 °C cold treatment, respectively.

Final plant height increased linearly as photoperiod increased for plants
that did not receive cold. Cold treatment and photoperiod interacted with each of
the following: days to visible bud, days to ﬂower, and increase in node number.
Without cold treatment, days to visible bud and ﬂower increased linearly as
photoperiod increased. However, the percentage of ﬂowering increased as

photoperiod increased. If plants had remained on the benches longer, more

 

Il‘rh

 

A 0 Weeks of 5c
100 100%
75 _____ , — — —.m 75% E
3 50 ___________ 50% IT.
a ..
25 __________________ 25% g
n.

  

0 I r r |
10 12 14 16 18 20 22 24
Photoperiod

 

FWDWMBMDFM *PercentFlaw-r‘ng

 

 

 

 

 

c Plant Node Development
0 Weeks of SC

{A}
0

Number of Nodes

 

 

 

 

 

E Number of lnﬂorescences

_n
U!

 

.n
0

0|

    

Number of Inﬂoresences

 

   

0

 

14 16 18 2O 22 24
Photoperiod

@wsekseord '15weekscoldl

10 12

 

 

L

 

88

 

 

'3 15 Weeks of 5c

 

    

Percent Flowering

 

O r l . . r
10 12 14 16 18 20 22 24
Photoperiod

 

[k Day-toVIdbleBud B DeyItnFlower .0.ij

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

   

 

 

 

 

 

 

D Plant Node Development
15 Weeks of 5C
30
8
§ 20
g 10
2 M 7 4 3
o 14 1E: “27;
Photoperiod
"Ennis-Imus: Bum-mm-Nmumj
F Plant Height at Flower
100
’8‘ so ————————————————
3’ so ———————————

      

 

14 16 18 20 22 24
Photoperiod

l-E-Oweekscolcl uo-15weekscoldl

 

 

Figure 13. The effects of photoperiod and cold treatment on ﬂowering of

Veronica spicata ‘Blue’.

 

 

 

89
Table 6. The effects of photoperiod and cold treatment on ﬂowering of Veronica spicata 'Blue’.

 

Days to Days from Days Increase Final plant

 

 

Weeks Percentage visible visible bud to in node height Flower
9f 5Q Phgtgperig ﬂgwgring bud to ﬂower ﬂgwer number (cm) number
0 - 57 61 15 76 36 77 8
15 - 100 34 16 50 23 64 10
- 10 65 39 16 55 24 59 8
12 70 48 16 64 31 70 10
14 85 53 16 69 33 71 9
16 80 39 15 54 23 60 5
24 85 54 16 70 32 86 9
NIz 85 53 15 68 34 79 12
0 10 30 43 16 59 23 51 7
12 40 59 15 74 39 79 8
14 70 74 17 90 43 78 7
16 60 48 15 64 26 66 5
24 70 69 15 84 40 97 8
NI 70 73 14 87 45 91 14
15 10 100 35 17 51 25 66 10
12 100 38 16 54 24 ' 62 11
14 100 32 16 49 23 63 1 1
16 100 30 14 44 20 54 6
24 100 39 17 56 23 74 10
ﬂ EX) 3 16 49 23 66 1O
Signiﬁcance
Weeks cold (WC) “* NS ”* “* *" NS
Photoperiod (P) ** NS *“ **" "' *“
WC x P “ NS *‘ “* NS NS
95% Conﬁdence interval for NI
Zero weeks 5C 16.5 1.6 16.4 10.8 25.1 7.5
15 weeks SC 2.7 1.5 2.4 2.8 5.0 1.3
Contrasts
Zero weeks 5C
NI vs. 16 *” NS *“ *** ** “‘
NI vs. 24 NS NS NS NS NS *
Pu... (10 to 24 h) * NS " NS *“ NS
PM (10 to 24 h) NS NS NS NS NS NS
15 weeks 5C
NI vs. 16 NS * NS NS NS N8
N1 vs. 24 NS NS NS us NS NS
Pulsar (10 to 24 h) NS NS NS NS NS NS
PM (10 to 24 h) NS * NS NS " NS
0 and 15 weeks 50
Pu... (10 to 24 h) * NS " NS *“ NS
' (10 to 24 h) N_§ N_§ N_§ N_S N_§ NS

 

zNl = 4-h night interruption.
"8' '- "- ”' Nonsigniﬁcant or signiﬁcant at Ps0.05, 0.01, or 0.001, respectively.

 

woulc

parl‘r

day-

wit

D:

90

would have ﬂowered, which would have increased the average time to ﬂower,
particularly under the shorter photoperiods. After cold treatment, plants were
day-neutral because their ﬂowering was unaffected by daylength. The number
of new nodes formed decreased after cold treatment, but the reduction varied

with photoperiod.

Day-Neutral Species That Require a Cold Treatment

Lavandula angustifolia ‘Munstead Dwarf'. Few plants ﬂowered without
cold treatment (Figure 14, Table 7), and those that did required more than 80
days and were rangy. After cold treatment, all plants ﬂowered uniformly, in about
50 days. Cold treatment reduced the number of new nodes formed before ﬂower
by 14 (28 leaves). ‘Munstead’ plants grown under NI from 50-cell plugs that
received 15 weeks of 5 °C ﬂowered over 40 days earlier than non-cold treated
plants (Whitman, 1995). Engle (1994) found that 15%, 15%, 17%, 35%, 56%, or
84% of ‘Munstead’ plants ﬂowered after receiving 0, 2, 4, 6, 8, or 10 weeks of 5
°C cold treatment, respectively.

After cold treatment, photoperiod did not signiﬁcantly affect days to
ﬂower; thus, ‘Munstead Dwarf is day-neutral. Photoperiod also did not inﬂuence
the number of new nodes formed or the number of inﬂorescences. Plant height
increased linearly from 32 to 41 cm as the photoperiod increased from 10 to 24
hours. Whitman (1995) found that ‘Munstead’ plants grown under 4-hour NI had
a greater percentage of ﬂowering and ﬂowered three to seven days earlier than

plants grown without NI.

 

125
10

Oxala

aegz .c LEE. .2

\E

‘l‘f‘c¢"hcc:‘ \C

.hlI‘ cube. -2

Fig
La

 

 

A 0 Weeks of so

125 D", 100%

 

 

 

 

 

o t .
1012141618 20 22 24
Photoperiod

 

[A Day- te Vlelble Bud a Days In Flower ..— Penxnr Flmdngj

 

 

 

 

 

 

 

 

 

 

 

 

c Plant Node Development
0 Weeks of SC
50
1£40 Jr — — — ,. , ,
g so I 777777777777777777
3 2° 1%: *5”: , ’f
s I r- r s
= ‘° I ii” i S
0 10 E124 {16'
Photoperiod
(E1 mm: nodel .NoEEsi-rml
Number of lnﬂorescences
g 12
5
g 8 M _ - -
:2 ' INI
E 4 _____________________ qt“.
3 Cl
E
g #

 

 

 

0 v—* r r ' r r
10 12 14 16 18 20 22 24
Photoperiod

 

[Fl 0 weeks cold .- 15 weeks cold}

 

 

 

 

 

91

 

 

 

 

 

B
15Weeks ofSC

125 :95! 100%
100 ____________________ sass 2’
g 75 ——————————————————— 60%;
LL
8 50r=r=—E~B. E ........ ”753m 40% E
251——;.;,--——-—————,—AN, mg

9%

 

0 4' t 4?- , ,
10 12 14 16 18 20 22 24
Photoperiod

 

limbmuu ommm -Pmntmlf
, .

 

 

 

 

D Plant Node Development

15 Weeks of 5C

 

 

 

 

 

 

  

 
 
    

:1. 7“th ,17'
16 24 Nl
Photoperiod

 

 

 

 

   

LE; , "
14

 

Eglniﬁalnodes [j Nodeelfleroold- Nodes-mower]

 

 

 

F Plant Height at Flower

a
O

 

Height (cm)
A
O

 

l

 

 

zorl—e—l—e—q—q—i-«b—q—‘b—l—e—a—é—tt
1012141618 20 22 24
Photoperiod

f: 0 weeks cold -15 weeks cold]

 

 

 

 

 

Figure 14. The effects of photoperiod and cold treatment on ﬂowering of

Lavandula angustifolia ‘Munstead Dwarf’.

92

Table 7. The effects of photoperiod and cold treatment on ﬂowering of Lavandula angustifolia ‘Munstead

 

 

 

 

Dwarf.
Days to Days from Days Increase Final plant
Weeks Percentage visible visible bud to in node height Flower
of 59 Photgpgrig ﬂowering bud to ﬂmr flower number (cm) numb;
0 - 15 65 26 92 23 47 2.9
15 - 100 19 30 49 9 34 8.0
- 10 50 21 31 52 9 32 7.2
12 50 20 33 53 9 32 8.2
14 SO 19 30 49 9 33 9.5
16 50 19 28 47 10 35 8.0
24 90 37 28 65 15 43 5.8
NIz 55 26 30 55 10 35 6.4
0 10 0 «V - - - -- —
12 0 - - - - -- -
14 0 - — - - -- —
16 0 - - - -- -- -
24 80 61 27 88 23 46 2.6
NI 10 99 20 119 23 56 5.0
15 10 100 21 31 52 9 32 7.2
12 100 20 33 53 9 32 8.2
14 100 19 30 49 9 33 9.5
16 100 19 28 47 10 35 8.0
24 100 18 29 47 8 41 8.3
N_! 100 18 31 49 8 33 6.5
Signiﬁcance
Weeks cold (WC) *“ m *” “* *** NS
Photoperiod (P) ' “* Ns NS Ns Ns
WC x P ** ** * NS ** Ns
95% Conﬁdence interval for NI
15 weeks SC 3.3 2.7 5.0 1.1 3.0 3.6
Contrasts
15 weeks SC
Nl vs. 16 Ns NS NS Ns Ns Ns
NI vs. 24 Ns Ns NS NS *“ NS
PM (10 to 24 h) Ns *" NS Ns *“ Ns
PM (10 to 24 h) N§ N_§ us N_S I1§ N§

 

I‘Nl = 4-h night interruption.

’- = No plants showed visible bud after 105 days of forcing.
"3- °' ”- "' Nonsigniﬁcant or signiﬁcant at Ps0.05, 0.01, or 0.001, respectively.

 

 

spa
ne;

95

93

Phlox subulata ‘Emerald Blue’. Cold treatment increased percentage of
ﬂowering, ﬂower number, and ﬂowering uniformity (Figure 15, Table 8). Two-
thirds of plants ﬂowered without cold treatment, but ﬂowering was sporadic and
sparse; plants averaged less than 10 ﬂowers per plant. After the cold treatment
nearly all plants ﬂowered and ﬂower number increased by over four-fold. The
95% CI of days to ﬂower for plants under NI was reduced by over 8-fold.

There was an interaction with cold treatment and photoperiod for days to
visible bud, days to ﬂower, the number of new nodes formed, and ﬁnal plant
height. Without cold treatment, ﬂowering was progressively hastened as the
photoperiod increased, which suggests that ‘Emerald Blue’ is a quantitative long-
day plant. Flower buds were immediately visible on plants in all photoperiods
after cold treatment, so photoperiod did not inﬂuence time to ﬂower. Thus,
‘Emerald Blue' is day-neutral after cold. Flower number showed a quadratic
response to photoperiod, reaching a maximum under the 14-hr photoperiod.

Veronica Iongifolia ‘Sunny Border Blue’. No plants ﬂowered without
cold treatment and all plants ﬂowered uniformly after cold treatment (Figure 16,
Table 9). Plants developed approximately two nodes (four leaves) during the
cold treatment. Photoperiod did not inﬂuence days to visible bud, days to ﬂower,
the number of new nodes formed, or ﬂower number. Plant height at ﬂower
increased from 33 to 38 cm as the photoperiod increased from 10 to 24 hours.
Engle (1994) found that a cold treatment of ﬁve weeks at 5 °C increased the

percentage of ﬂowering from 3 to 100.

 

A 0 Weeks of 5c

100%

 

 

s,
U!
39

Percent Flowering

 

 

 

0 r r 1 r 1
10 12 14 18 18 20 22 24
Photoperiod

 

Ii— DaystoneibloBud (J Daysto Flower -PercentFvaenng ‘E

 

 

 

 

 

C Plant Node Development
0 Weeks of SC

A
O

(A
0

Number of Nodes
8 B

 

 

 

 

 

 

 

Number or Flower Buds "1

 

 

 

1012141513 20 22 24
Photoperiod

 

[-E-o weeks cold -I-15 weeks colﬂ

 

 

 

94

 

 

 

B 15 Weeks of 5C
so Me". 100%
60 ————————————————————— .7591. 3
E40 ————————————————————— 50% g

 

 

 

 

o 't'ﬂ—r'ﬂ—i .AMI 9%
1012141618 20 22 24
Photoperiod

 

[-kDeyebVleﬂeBudBDey-loFlow uo-Percentmnng ‘

 

 

 

 

D Plant Node Development
15 Weeks of 5C

§
0

 

(A
0

Number of Nodes
3 3

  

 

[Ernie-r node: T’ Noclel Iﬂercold I Noun allow

 

 

 

F Plant Height at Flower

 

Height (cm)

    

 

1012141618 20 22 24
Photoperiod

[:36 weeks cold -0- 15 weeks cold I

 

Figure 15. The effects of photoperiod and cold treatment on ﬂowering of Phlox

subulata ‘Emerald Blue’.

 

 

 

95
Table 8. The effects of photoperiod and cold treatment on ﬂowering of Phlox subulata ‘Emerald Blue’.

 

Days to Days from Days Increase Final plant

 

 

 

Weeks Percentage visible visible bud to in node height Flower
ef e9 Photeeen'm ﬂowering bed 19 ﬂower ﬂower number (cm) number
0 - 64 38 12 S1 1 1 14 5
15 - 93 3 14 16 5 13 22
- 10 63 29 13 42 8 12 6
12 82 34 12 46 10 14 13
13 88 26 14 41 6 14 13
14 93 22 12 34 7 12 16
16 78 11 13 25 6 12 15
24 75 8 13 21 6 15 15
Nl1 94 13 13 26 6 14 14
0 10 33 55 1 1 66 14 13 2
12 71 65 10 74 1 7 14 8
13 71 50 12 66 7 17 6
14 75 40 12 S2 12 12 2
16 75 20 13 34 8 12 4
24 50 12 13 24 8 1S 7
N1 83 24 12 38 10 17 3
15 10 90 3 14 18 S 1 1 10
12 90 4 14 18 6 13 19
13 100 2 15 17 S 12 21
14 100 3 13 16 5 13 30
16 80 3 14 16 S 14 25
24 9O 4 13 17 6 15 23
_ N_I 100 1 13 14 5 12 26
Signiﬁcance
Weeks cold (WC) *“ “'" *"* m “* *“
Photoperiod (P) m Ns *‘* *“ * Ns
WC x P *” Ns **" ** *“ NS
95% Conﬁdence interval for NI
Zero weeks SC 11.8 5.0 17.7 5.7 O 1.5
15 weeks SC 1.0 1.5 2.1 0.7 1.2 9.0
Contrasts
Zero weeks 5C
NI vs. 16 Ns Ns Ns NS “* Ns
NI vs. 24 * NS " Ns NS NS
PM (10 to 24 h) *“ Ns *** ** Ns Ns
PM (10 to 24 h) * Ns Ns NS Ns Ns
15 weeks SC
Nl vs. 16 NS Ns Ns NS * N5
N1 vs. 24 NS NS Ns Ns m Ns
Pu... (10 to 24 h) Ns NS NS NS “* Ns
Poem (10 to 24 h) Ns Ns Ns NS Ns **
0 and 15 weeks SC
Pu... (10 to 24 h) “* Ns **" “ ‘ NS
- BM (10 to 24 h) M§ N§ N§ N_§ J§ N§

 

:Nl = tn night interruption.
8- " ”' Nonsigniﬁcant or signiﬁcant at Ps0.05, 0.01 , or 0.001. respectively.

 

 

A 0 Weeks of 5c
100 100%
75 —————————————————— _ 75% .§’
§ so —————————————————— e 50% g
25 —————————————————— s 25% g
D.

 

 

 

0
10 12 14 16 18 20 22 24
Photoperiod

 

+DeythnlbIeBud-30eyetoFlovwr *PercentFlowerlngﬁ

 

 

 

c Plant Node Development
0 Weeks of 5C

 

16

.a
N

Number of Nodes
on

 

 

 

 

 

 

 

 

E

 

 

.a

 

Number of lnﬁoresences

 

0

 

1012141618 20 22 24
Photoperiod

 

PaOmkscold -15weekseord]

 

 

 

 

96

 

B 15 Weeks of 5c

100 :“' 100%

 

Days
0'!
O

I
I
l
l
l
l
I
l
I
l
I
l
I
I
0|
0
33
Percent Flowering

 

 

ulv

0 , f . . . r .
10 12 14 16 18 20 22 24
Photoperiod

 

 

5* Daylbwubleaud a DeysteFlow-r *PereentFlewsrlng!

 

 

 

 

D Plant Node Development

15 Weeks of 5C

‘° ‘D f “.7.

Number of Nodes

   

 

Photoperiod

 

DNed-seﬂerenld-Nodesltlower]

 

 

 

 

Plant Height at Flower

42

 

Height (cm)
Y: 8

 

 

 

 

30 . I , 1 1 4 s—r
1O 12 14 16 18 20 22 24
Photoperiod

 

[EiOweekscold -o-15weekscold"

 

 

Figure 16. The effects of photoperiod and cold treatment on ﬂowering of
Veronica Iongifolia ‘Sunny Border Blue’.

 

 

97

Table 9. The effects of photoperiod and cold treatment on ﬂowering of Veronica Iongifolia “Sunny Border
Blue'.

 

Days to Days from Days Increase Final plant

 

 

 

 

Weeks Percentage visible visible bud to in node height Flower
of 5C PhojegerMinc bug to flower ﬂower number (cm) numgr
0 - 0 -‘ - - - - -
1S - 100 38 26 63 7 36 2 8
15 10 100 38 27 64 8 33 2 9
12 100 38 26 64 8 35 3 1
14 100 37 25 62 8 35 3 1
16 100 37 25 62 7 39 2 2
24 100 39 25 64 7 38 2 9
N_l’ 100 38 27 6S 7 34 2 7
Signiﬁcance
Photoperiod (P) Ns *" Ns Ns *‘* NS
95% Conﬁdence interval for NI
15 weeks SC 3.8 1.2 3.7 0.6 2.8 0.6
Contrasts
15 weeks SC
NI vs. 16 Ns * Ns Ns ** Ns
NI vs. 24 Ns *" NS Ns ** Ns
Pu... (10 to 24 h) Ns " Ns Ns “‘ NS
Fem (10 to 24 h) Ns ﬁ Ns N§ * N§

 

 

f—No plants showed visible bud after 105 days-of forcing.
’NI = 4-h night interruption.
"s- " ”' "' Nonsigniﬁcant or signiﬁcant at Ps0.05, 0.01, or 0.001, respectively.

98
Facultative Long-day Species That Beneﬁt from a Cold Treatment

Leucanthemum xsuperbum ‘Snow Cap’. Without a cold treatment, no
plants ﬂowered under photoperiods 514 hours, but at least 60% of plants
ﬂowered under photoperiods 216 hours or NI (Figure 17, Table 10). All plants
ﬂowered after cold treatment. Cold accelerated (by approximately ten days) time
to visible bud and ﬂower, reduced ﬁnal plant height by four to six cm, and
improved ﬂowering uniformity (Figure 18). Under photoperiods 216 hours or NI,
cold treatment more than doubled the number of inﬂorescences.

Days to visible bud and ﬂower, days from visible bud to ﬂower, and the
number of new nodes formed decreased linearly as photoperiod increased.

Plant height increased linearly from 10 to 17 cm as the photoperiod increased
from 10 to 24 hours. Flower number was greatest under photoperiods 216 hours
or NI.

The effects of cold treatment and photoperiod on ﬂowering of L.
xsuperbum (formerly Chrysanthemum x superbum or C. maximum) varies
considerably by cultivar or clone. Non-cold treated ‘Esther Read’ daisy
Chrysanthemum (C. maximum Ramond, ‘Esther Read’) remained vegetative
under 12-hour photoperiods and ﬂowered under photoperiods of 13 hours or
longer (Grifﬁn and Carpenter, 1964). Non-cold treated ‘T.E. Killian’ daisy
Chrysanthemum plants ﬂowered only under 15-hour photoperiods and remained
vegetative under 14-hour or shorter photoperiods (Grifﬁn and Carpenter, 1964).

Shedron and Weiler (1982) propagated ﬁve clones of ‘G. Marconi’ shasta daisy

 

A
75
so

$45

030

0 Weeks of SC

 

15

 

o 2.2..
1012141618 20 22 24
Photoperiod

 

I" DthlelbI-BudE-Deysbm *wml

  

 

 

C Plant Node Development
0 Weeks of SC

 

Number of Nodes

 

lglnmelneeu I Nodualllowerl

 

 

 

Number of Flower Buds

—a
N

 

 

&
A

NumberofFlower Buds "1
o1

 

 

0

 

6

101214118202224
Photoperiod

 

 

[4+0 weeks cold .15 weeks cold]

 

 

 

99

 

 

 

 

'3 1SWeeksofSC
75 :H‘ 100%
so _______________ —— 80% 2’
Essaaa -
9.45""""\§:" —,—,ﬁDN1 50%;
a 7 LL
““hR'N‘ME
A
15 ,- —————— _,______,20 m
n

 

O I I 1 r 1 '
10 12 14 16 18 20 22 24
Photoperiod

 

bwuwmmawbm .Q-P-rcemFlmring]

 

 

Plant Node Development
15 Weeks of SC

‘
O

 

U
0

Number of Nodes
8 B

 

lj Nor; altercold I NodeistKiE

 

 

 

  

  

F Plant Height at Flower

25 .

20]: _______ WU”,
§10 ————————————————————————
I

5

 

+, . 2, . , .
1o 12 14 16 18 20 22 24
Photoperiod

{4.1. o’weeks cold V-o- 15 weeks cold I

 

 

 

Figure 17. The effects of photoperiod and cold treatment on ﬂowering of

Leucanthemum xsuperbum ‘Snow Cap’.

100

Table 10. The effects of photoperiod and cold treatment on ﬂowering of Leucanthemum xsuperbum ‘Snow
Cap’.

 

Days to Days from Days Increase Final plant

 

 

 

Weeks Percentage visible visible bud to in node height Flower
Wt 5 Ph 0 ri ﬂ rin bud Mr (cm) rum
0 - 31 35 28 62 18 20 5
15 - 100 26 25 51 20 13 7
- 10 50 30 26 56 20 10 4
12 50 29 25 54 20 1 1 4
13 50 29 26 SS 21 12 4
14 50 28 26 SS 20 12 4
16 80 30 28 58 18 18 9
24 95 26 24 51 18 19 7
N1‘ 85 29 26 55 18 16 8
0 10 0 --’ - - - - -
12 0 -- - - - — --
13 0 - - - - - -
14 0 - - - - - —
16 60 36 31 67 18 21 6
24 90 33 26 59 18 21 4
NI 70 35 28 63 18 19 4
15 10 100 30 26 56 20 10 4
12 100 29 25 54 20 11 4
13 100 29 26 55 21 12 4
14 100 28 26 55 20 12 4
16 100 24 25 49 19 15 11
24 100 19 23 42 19 17 10
N1 100 23 24 47 19 13 11
Signiﬁcance
Weeks cold (WC) “* “* 1... * *“ “*
Photoperiod (P) *‘* m *” Ns ... “‘
WC x P Ns * Ns Ns Ns NS
95% Conﬁdence interval for NI
Zero weeks SC 3.3 1.4 3.2 1.3 2.3 1.5
15 weeks SC 1.1 0.8 1.0 0.9 0.8 1.4
Contrasts
Zero weeks SC
NI vs. 16 Ns ** NS Ns Ns N3
N1 vs. 24 Ns * " NS Ns Ns
15 weeks 5C
NI vs. 16 NS Ns Ns Ns * Ns
NI vs. 24 * Ns ** Ns *” NS
PLheer (10 to 24 h) m m *“ " *“ **"
PQggrltlc (10 19 24 h) N§ N_s N§ _s " “*

 

zNI = 4-h night interruption.
’- .= No plants showed visible bud after 105 days of forcing.
”s- 3" ”' Nonsigniﬁcant or signiﬁcant at Ps0.05, 0.01, or 0.001, respectively.

101

 

 

 

 

 

 

 

 

 

 

75
e No cold treatment
0 1 °
70 .............................................................. .5.Yt%?!‘?.?f§-9---
«all
65 -. ...................................................... .‘ ...........................
. NI
‘2’
60 .4 .............. - :- ....................................................................
u—‘f 24s
.3 10
5‘ 55 - ------------------------------------------------------- I ------ 131g -------------
O
50 -, ..................................................................................
16
Nl
45 _, ...................................................................................
24D
40 I I l l I
50 60 70 80 90 100 110

Percentage Flowering

Figure 18. Percentage ﬂowering, time to ﬂower, and ﬂowering uniformity of
Leucanthemum xsuperbum 'Snow Cap' under different photoperiods with or
without cold treatment. Numbers next to symbols represent photoperiods
consisting of nine-hour natural days that were extended with incandescent lamps.
NI = nine-hour natural days with four hours of night interruption. Error bars are
95% conﬁdence intervals.

102

(Chrysanthemum x superbum Bergmans) and placed them under photoperiods
of 10, 12, 14, 16, or 18 hours. Two clones were photoperiodic, one responded to
cold temperatures, and two responded to cold treatment and photoperiod.
Damann and Lyons (1995) found that C. xsuperbum ‘Snow Lady’ ﬂowered as a
facultative long—day plant .

Leucanthemum xsuperbum ‘White Knight’. Flowering of ‘White Knight’,
propagated from seed, was not as rapid, complete, or uniform as ‘Snow Cap’,
which was a tissue-cultured clone (Figures 18 and 20). One-half of the plants
ﬂowered without cold treatment, and three-fourths ﬂowered after cold treatment
(Figure 19, Table 11). The uniformity of time to ﬂower increased nearly two-fold
after cold treatment. Cold treatment hastened ﬂowering by about nine days but
had no effect on the number of new nodes formed, ﬁnal plant height, or ﬂower
number.

Under photoperiods of 514 hours, many plants did not ﬂower without cold
treatment; after cold treatment, while more plants ﬂowered, ﬂowering was not
uniform. The greatest percentage of ﬂowering was achieved under photoperiods
216 hours or NI, regardless of cold treatment. Days to visible bUd and ﬂower
and the number of new nodes formed decreased linearly as the photoperiod
lengthened. Therefore, ‘White Knight’ responded as a quantitative long-day
plant. Photoperiod did not inﬂuence plant height or ﬂower number.

Lobelia xspeciosa ‘Compliment Scarlet’. Cold treatment did not

accelerate ﬂowering or improve ﬂowering uniformity, but it did increase ﬂower

 

 

 

 

 

01 ~ e—e—e—Fs -. 4 r e 4
10 12 14 16 1B 20 22 2
Photoperiod

 

 

[’7‘ DethanbleBud {g- DeystoFIower uo-Percentml

 

 

 

 

C Plant Node Development
0 Weeks of SC

 

 

 

 

 

 

 

1012141818 20 22 24
Photoperiod

L {25.3515 "care .0; GWeeks cold]

 

 

 

103

 

  

 

B 15 Weeks of 5c

 

50%,?
60%
4011.?

20%“.

 

 

0 ~4——~—+—o—+—~—l—o—+—+
1O 12 14 16 18 2o 22 24
Photoperiod

 

 

 

’A DIthVleﬂeBudBDeystoFlawer *PmmFlonmgl

 

 

 

D Plant Node Development

 

        

15Weeklof5c

4o ,--_-2, ‘_ﬁ_.
geolﬂ ”j
z . l
32017 , , - J
g .
510‘» - E~ , ,
o' g .

24 Ni

.4 .
O

12 *"13 14 1
Photoperiod

Eulnitlelnodes ;_ iNeesurrereeIaINeaee-rrlemrl

 

 

 

 

Height (cm)

 

 

E0 weeks eeld a} 1amks ﬁn

 

 

Figure 19. The effects of photoperiod and cold treatment on ﬂowering of
Leucanthemum xsuperbum ‘White Knight’.

 

 

104

Table 11. The effects of photoperiod and cold treatment on ﬂowering of Leucanthemum xsuperbum ‘White
Knight’.

 

Days to Days from Days Increase Final plant
Weeks Percentage visible visible bud to in node height Flower

1 Ph riod ﬂow rin bug to ﬂower ﬂmer nummr (cm) nummr
29 82 20 6

 

 

 

0 - 52 57 14
15 - 75 S3 22 73 20 14 6
- 10 38 63 22 85 22 13 7
12 26 61 26 80 26 14 6
13 50 69 23 86 21 14 7
14 63 69 25 86 28 17 7
16 100 S4 28 82 20 13 6
24 82 39 24 63 16 14 5
NIz 85 46 27 72 18 14 6
0 10 0 -’ - - - - -
12 10 74 25 99 27 11 5
13 50 78 28 99 21 1 1 9
14 40 73 29 94 31 13 7
16 100 55 29 84 20 15 5
24 70 41 29 70 17 14 6
NI 90 51 29 80 19 15 6
15 10 86 63 22 85 22 13 7
12 44 58 26 74 25 15 7
13 50 59 21 81 21 15 7
14 89 68 23 83 26 19 6
16 100 54 25 78 21 9 7
24 100 37 19 56 14 13 4
NI 80 4O 24 63 17 12 6
Signiﬁcance
Weeks cold (WC) ‘* *“ ‘“ NS NS Ns
Photoperiod (P) *“ NS *“ “* Ns NS
WC x P Ns Ns Ns Ns *‘ Ns
95% Conﬁdence interval for NI
Zero weeks SC 9.6 3.5 10.2 3.2 2.7 2.0
15 weeks SC 5.9 1.5 5.7 1.9 2.0 2.3
Contrasts
Zero weeks SC
NI vs. 16 Ns NS NS Ns Ns Ns
NI vs. 24 Ns Ns * Ns Ns Ns
PLheu' (12 to 24 h) *” Ns *“' *“ Ns Ns
PM (12 to 24 h) Ns Ns Ns NS Ns NS
15 weeks SC
NI vs. 16 " NS *‘ Ns NS Ns
NI vs. 24 NS ” Ns NS NS NS
PLheer (10 to 24 h) m " *“' **" NS "
BM (10 to 24 h) N_S ‘ LIL lie N§ N§

 

2N1 = 4-h night interruption.
’- = No plants showed visible bud alter 105 days of forcing.
"5' '- ”- '°° Nonsigniﬁcant or signiﬁcant at Ps0.05. 0.01. or 0.001. respectively.

105

 

 

 

 

 

 

 

 

 

 

 

110
e No cold treatment
0 1SweeksofS°C
100 _....12.. ........................ 1 3. .............................................. .
140
go- ..................................................................................
a; 101% 16
o 80.1 ................................ 1 .3.§ ....................... NI ..................
E “r 16
9. 126
g 70. ............................................. 24$ ...............................
m ‘f'
D “ NI
so- ..................................
24C?
50- ------------------------------------------------------------------------------------
40 T l l I I
0 20 40 60 80 100 120

Percentage Flowering

Figure 20. Percentage ﬂowering, time to ﬂower, and ﬂowering uniformity of

Leucanthemum xsuperbum White Knight’ under different photoperiods with or
without cold treatment. Numbers next to symbols represent photoperiods
consisting of nine-hour natural days that were extended with incandescent lamps.
NI = nine-hour natural days with four hours of night interruption. Error bars are
95% conﬁdence intervals. Symbols without error bars indicate that the
conﬁdence intervals were too large for the graph.

106

number and the number of new nodes formed (Figure 21, Table 12). Plants
without or with the cold treatment averaged 21 and 48 ﬂowers, respectively.
Some of this increase is likely due to naturally higher light levels when cold-
treated plants were grown.

‘Compliment Scarlet’ ﬂowered as an obligate long-day plant before cold
treatment and as a facultative long-day plant after cold treatment. VVlthout cold
treatment, ‘Compliment Scarlet’ only ﬂowered under photoperiods 214 hours or
NI. In contrast, all cold-treated plants ﬂowered and as the photoperiod duration
increased, days to visible bud and ﬂower and the number of new nodes formed
decreased linearly. For example, as the photoperiod increased from 10 to 24
hours, days to ﬂower decreased from 83 to 64. The number of ﬂower buds and
plant height were greatest under 14-hour photoperiods and both decreased
under shorter or longer photoperiods.

Engle (1994) found that no ‘Compliment Scarlet’ plants ﬂowered without a
cold treatment. After 15 weeks of 5 °C, 80 or 100% of plants ﬂowered without or
with a 4-hour NI, respectively. However, ‘Queen \flctoria’ ﬂowered completely

under Nl without a cold treatment (Engle, 1994).

Facultative Long-day Species That Require a Cold Treatment

Coreopsis grandiﬂora ‘Sunray’. ‘Sunray’ requires a cold treatment for
ﬂowering (Yuan, 1995). However, plants that were not cold-treated but exposed
to seven weeks of short days prior to transfer to the experimental photoperiods

ﬂowered under all but 10-hour photoperiods (Figure 22, Table 13). Exposure to

 

 

 

 

A 0 Weeks of 5c

100 :99 100%

75 - - — — —————————————— 75%
UNI g
§50~-— rig—“Lua—-— 50%|;

A
25 _ — — ——————————————— 25% E
D.
O 0%

 

 

 

10 12 14 16 18 20 22 24
Photoperiod

 

Himbmwamum *PMM

 

 

 

'3 15 Weeks of so

100 :N! 100%

 

 

Percent Flowering

 

 

 

O I . r 4 A
1 0 1 2 14 1 6 1 8 20 22 24
Photoperiod

 

».— DthieibIeBudE} DIyImFIow-r *Pmml

 

 

 

 

 

 

 

 

             

 

 

 

 

 

c Plant Node Development
50 0 Weeks of 50

g 40

z 30

a

g 20

E

z 10

0
Elmm .Nodultﬂm

E

Number of Flower Buds

 

 

LL
15 ______ a _______________ El ML

0 . . - .
1O 12 14 16 18 20 22 24
Photoperiod

 

 

 

 

[-g-Oweekscold -15weekscold

 

 

 

 

 

 

D
Plant Node Development
15 Weeks of 5C
sore ,,, .77 a. , 71
€40 E ....... 7 4
g 30 I , 7 ,
I— L 7
£20 I
7? ‘°i *
o l :_ , iii. ,
10 14
Photoperiod
F Plant Height at Flower
so
E
g

 

 

10 12 14 16 18 20 22 24
Photoperiod

 

ﬁ-S-Oweekscold -15weekeeold]

 

 

 

Figure 21. The effects of photoperiod and cold treatment on ﬂowering of Lobelia

xspeciosa ‘Compliment Scarlet'.

 

 

108

Table 12. The effects of photoperiod and cold treatment on ﬂowering of Lobelia xspeciosa ‘Compliment

 

 

 

 

:Nl=

NS,“

Scanet'.
Days to Days from Days Increase Final plant
Weeks Percentage visible visible bud to in node height Flower
9f 59 Phgtgmrlod ﬂmn‘nq bud to ﬂower ﬂower ngmber (cm) numbe_r_
O - 76 47 21 6816 49 21
15 - 100 51 22 73 24 59 48
- 1O 67 62 21 83 32 47 30
12 67 61 20 81 34 57 46
14 90 47 23 70 21 62 44
16 100 48 22 70 18 56 40
24 100 46 21 67 16 58 31
Nl1 100 44 21 64 17 48 35
O 10 0 -’ —- - - - -
12 0 - - - - - -
14 80 48 21 69 18 48 19
16 100 51 20 71 16 44 24
24 100 49 22 71 15 56 24
Nl 100 41 20 61 16 47 17
15 10 100 62 21 83 32 47 3O
12 100 61 20 81 34 57 46
14 100 47 24 71 23 71 61
16 100 46 23 69 21 69 57
24 100 43 21 64 18 6O 38
N_I 100 46 21 67 19 49 52
Signiﬁcance
Weeks cold (WC) NS *” NS " “* *"
Photoperiod (P) ”" * *" *** “ "
WC x P NS * NS NS ** NS
95% Conﬁdence interval for NI
Zero weeks 50 7.1 1.3 6.4 3.0 9.7 5.9
15 weeks 5C 5.8 2.0 6.3 3.2 10.9 23.3
Contrasts
Zero weeks 5C
Nl vs. 16 NS NS NS NS NS NS
NI vs. 24 NS NS NS NS NS NS
Pm. (14 to 24 h) NS NS NS NS NS NS
Pm (14 to 24 h) NS NS NS NS NS NS
15 weeks 5C
NI vs. 16 NS NS NS NS *“ NS
NI vs. 24 NS NS us NS NS NS
Pu». (10 to 24 h) *‘* NS *“ “" us NS
PM (10 to 24 h) ” *"* N_§ " m *”

 

4-h night interruption.

TﬂNo plants showed visible bud after 105 days of forcing.

”Nonsigniﬁcant or signiﬁcant at Ps0. 05, 0. 01. or 0. 001, respectively.

 

 

A 0 Weeks of SC

 

 

 

 

1* Day-6mm .5. ombrlow *Pmrmll

 

 

 

 

C Plant Node Development
0 Weeks of 5C

 

Number of Nodes

 

 

 

"I

L
O

 

8

—e
O
-

 

Number of Flower Buds
N
O

 

 

0

 

1012141618202224
Phat riod

 

kid-0 weeks cold .15 weeks cold]

 

 

 

 

109

 

B 15 Weeks of 5C

100 :95! 100%

 

Percent Flowering

 

 

 

0 I I I I I . I
10 12 14 16 18 20 22 24
Photoperiod

 

*mbmch-ywnm *PeruntFlcmering I

 

 

 

D Plant Node Development
15 Weeks of 5c

 
   

Number of Nodes

Illllllllllllllllll '

   

 

 

 

 

 

 

    

13 16
Photoperiod
“531mm: [I Nodesdbrmld- Nodes-Ilium]
F Plant Height at Flower
60
CINI
A 50 ‘ ____________________ .. N_I _
5,
E 40 ————————————————————
i
so ——————————————————————————
2O 7

 

4—o—4a—1—+~l—v—+——e——+—e——+—~.+
1o 12 14 16 18 20 22 24
Photoperiod

l-I:-I-o weeks cold .75 week; cold

 

Figure 22. The effects of photoperiod and cold treatment on ﬂowering of

Coreopsis grandiﬂora ‘Sunray’.

 

1 10
Table 13. The effects of photoperiod and cold treatment on ﬂowering of Coreopsis grandiﬂora ‘Sunray’.

 

Days to Days from Days Increase Final plant

 

 

 

 

Weeks Percentage visible visible bud to in node height Flower
of ‘ Ph t ri ﬂowerin bud to ﬂower ﬂower number (cm) number
0 - 68 33 23 56 4 50 13
15 - 100 22 20 42 6 48 26
- 1O 50 35 24 60 7 28 1 1
12 85 39 25 64 6 38 17
14 100 24 21 46 5 51 26
16 95 23 21 43 5 54 20
24 90 22 20 42 6 56 14
Nl’ ~ 85 23 21 44 5 54 21
0 1O 0 -" - - -- - -
12 70 54 27 81 5 34 8
14 100 31 22 53 4 49 19
16 9O 27 22 49 4 54 13
24 80 27 22 49 4 55 6
NI 70 28 22 5O 4 56 17
15 10 100 35 24 6O 7 28 1 1
12 100 25 22 47 6 43 25
13 100 20 20 4O 6 46 36
14 100 18 21 39 6 53 33
16 100 19 19 38 6 54 27
24 100 18 18 35 7 58 23
NJ 100 18 19 37 6 53 26
Signiﬁcance
Weeks cold (WC) "’” ""’ *” *‘* NS **”
Photoperiod (P) *“ “" **" NS *” “*
WC x P *” Ns '** NS Ns Ns
95% Conﬁdence interval for NI
Zero weeks 50 3.2 2.9 3.1 3.5 4.8 6.7
15 weeks 5C 0.9 1.0 1.2 1.5 2.8 5.4
Contrasts
Zero weeks SC
Nl vs. 16 Ns NS Ns Ns Ns NS
NI vs. 24 NS Ns NS NS NS “
PUneer (12 to 24 h) *“ **" *“ Ns ”* *
Pom (12 to 24 h) *“ **" m " m *"
15 weeks 50
Nl vs. 16 NS NS Ns Ns Ns Ns
NI vs. 24 Ns NS NS " Ns NS
PLIneer (10 to 24 h) *" m “" " “* NS
PM (10 to 24 h) “” *" *"* " **” “"

zAll plants were grown under natural short-day photoperiods for 51 days prior to forcing or cold treatment.
”M = 4-h night interruption.

‘- = No plants showed visible bud after 105 days of forcing.

"3- " ”' '" Nonsigniﬁcant or signiﬁcant at Ps0.05, 0.01, or 0.001, respectively.

sho

anc

lei}

pe

111

short days can be substituted by a cold requirement, or vice-versa (Ketellapper
and Barbaro, 1966). However, in another experiment, ‘Sunray’ plants that
received 10 weeks of nine-hour short days followed by long days did not ﬂower,
perhaps because of heat stress from warm day temperatures (26 to 30 °C)
(Damann and Lyons, 1993).

Cold treatment increased the percentage of ﬂowering from 68 to 100,
improved ﬂowering uniformity, doubled the ﬂower number, but did not inﬂuence
plant height. Cold treatment reduced the time to visible bud and ﬂower by
approximately 30 days under 12-hour photoperiods and by 10 to 15 days under
photoperiods 213 hours or NI.

Days to visible bud and ﬂower decreased at a decreasing rate as the
photoperiod increased from 12 to 24 hours in non-coId-treated plants and from
10 to 24 hours in cold-treated plants. For example, as the photoperiod
increased, time to ﬂower decreased from 81 to 49 days or 60 to 35 days for non-
coId-treated or cold-treated plants, respectively. Plant height increased at a
decreasing rate as photoperiods increased, but photoperiod did not signiﬁcantly
affect the number of nodes formed. Plants had the most ﬂowers under 13- and
14—hour photoperiods.

Gaillardia xgrandiﬂora ‘Goblin’. Only 43% of non-cold-treated plants
ﬂowered, and those that did ﬂowered sporadically (Figure 23, Table 14). For
example, without cold treatment, the 95% CI of days to ﬂower for plants under NI

was :1: 14 days. After cold treatment, 91% of the plants ﬂowered and much more

 

A 0 Weeks of 5c

 

Percent Flowering

  

 

 

 

 

0 . I . . I
10 12 14 16 18 20 22 24
Photoperiod

 

[*Whmwamhm *PMFW

 

 

 

c Plant Node Development
0 Weeks of SC
75 2 Vi

   

  

, "
if ,7

3‘1 ' 16
Photoperiod

 

 

 

 

 

 

E Number of Flower Buds
15
3 am
g 10 _________________
i E """""""""

 

 

 

1012141618 20 22 24
Photoperiod

 

[-90 weeks oold -.-15weekscold]

 

 

 

112

 

 

  

 

 

B 15 Weeks of so
125 #295! 100%
100. ———————————————————— 50% g
,3, 75 ————————————————————— 60% E
0 sons. , . - ___________ 40% a
25 “ESE 20% g
. 0%

 

0 + . . v
10 12 14 16 18 20 22 24
Photoperiod

 

itDthIﬂbleBudBDlylebw-r uonPeroemFIMIng—l

 

 

 

D Plant Node Development

 

 

 

 

15WeeksofSC
75
.360 ————————————————————————
Z4 _________________________
‘6
iso_ _ _ _ — _ _ .
E
=15~-’=“A77E»—— ,,__1
2, gig: i j
10 12 13 14'16 24 NI
Photoperiod

 

l' . ‘
H- InIInInedu [j Nodes-Mould ! Nodesmﬂmrj

 

 

 

F Plant Height at Flower

 

Height (cm)

 

 

 

 

20 22 24

10 12 14 16 18
Photoperiod

 

I49- 0 weeks cold -0- 15 weeks cold]

 

 

 

 

Figure 23. The effects of photoperiod and cold treatment on ﬂowering of

Gail/ardia xgrandiﬂora ‘Goblin’.

113

Table 14. The effects of photoperiod and cold treatment on ﬂowering of Gaillardia xgrandiﬂora ‘Goblin’.

 

Days to Days from Days Increase Final plant

 

 

 

Weeks Percentage visible visible bud to in node height Flower
gf 5Q Phgtggn'g ﬂggering bud to ﬂower ﬂower ngmber (cm) numbi
O - 43 79 24 100 43 27 7
1S - 91 24 22 47 23 25 1O
- 10 50 42 21 63 23 1 1 5
12 50 34 24 51 23 20 8
13 65 44 21 55 27 25 9
14 55 35 24 59 28 29 1 1
16 65 42 25 59 29 29 10
24 95 43 22 64 3O 30 10
Nl‘ 9O 48 23 68 31 26 10
0 10 20 79 23 102 34 15 4
12 1O 92 -' - - - -
13 4O 89 19 99 38 22 6
14 20 83 26 109 49 35 11
16 40 84 24 108 44 25 4
24 90 71 23 94 43 31 9
NI 80 79 25 100 45 27 6
15 1O 80 33 20 53 20 1 1 5
12 9O 27 24 51 23 20 8
13 90 24 21 45 24 26 10
14 90 24 24 48 23 27 1 1
16 9O 23 25 49 24 31 13
24 100 17 21 38 21 30 1 1
100 24 21 44 22 25 13
Signiﬁcance
Weeks cold (WC) *" NS “" “* Ns ”
Photoperiod (P) Ns ” NS Ns “1 NS
WC x P Ns Ns NS NS Ns Ns
95% Conﬁdence interval for NI
Zero weeks SC 16.7 3.5 13.9 14.2 5.5 4.4
15 weeks SC 5.0 2.1 4.1 2.7 3.7 4.2
Contrasts
Zero weeks SC
NI vs. 16 Ns Ns NS NS NS Ns
Nl vs. 24 Ns NS Ns Ns NS Ns
Pulsar (10 to 24 h) NS Ns Ns Ns * Ns
PM (10 to 24 h) NS Ns NS NS Ns Ns
15 weeks SC
NI vs. 16 NS " NS Ns Ns Ns
NI vs. 24 Ns Ns Ns Ns Ns NS
Pu... (10 to 24 h) m NS " Ns m *

PM! (10 to 24 h)
‘NI = 4.h night interruption.
”The only plant with visible bud died before ﬂowering.
"3' '- "- "' Nonsigniﬁcant or signiﬁcant at Ps0.05, 0.01. or 0.001. respectively.

m N§ N§

114

uniformly (the 95% CI for plants under NI was :I: 4 days)(Figure 24). Cold
treatment reduced days to ﬂower from 100 to 47 days, reduced the number of
new nodes formed from 43 to 23, and increased ﬂower number from seven to
ten. Cold treatment did not affect ﬁnal plant height. Evans and Lyons (1988)
found that multiple gibberellin applications (100 ppm GAM) could replace the
cold-treatment requirement for ‘Goblin’.

As the photoperiod increased from 10 to 24 hours, cold-treated plants
ﬂowered faster, had more ﬂowers, and were taller. Days to ﬂower decreased
from 53 to 38, ﬂower number increased from 5 to 13, and plant height increased
from 11 to 31 cm. Photoperiod did not signiﬁcantly affect the number of new
nodes formed. Thus, ‘Goblin’ requires cold treatment and photoperiods 213
hours or NI for complete, rapid, and uniform ﬂowering.

Physostegia virginiana ‘Alba’. Cold treatment increased the percentage
of ﬂowering from 47% to 90%, increased ﬂower number from an average of 2.9
to 7.0, and improved ﬂowering uniformity (Figure 25, Table 15). VWthout a cold
treatment, all plants ﬂowered only when under continual light; 3 50% of the
plants ﬂowered under other photoperiods.

Sixty, 80, or 100 percent of cold-treated plants ﬂowered under 10-, 12-, or
214-hour photoperiods or NI, respectively. Days to visible bud and ﬂower
decreased linearly as the photoperiod increased from 10 to 24 hours. The
number of new nodes formed decreased from 23 nodes (46 leaves) under 12
hours to 12 nodes (26 leaves) under continual light. Flower number increased

linearly from 3.0 to 9.5 as the photoperiod increased from 10 to 24 hours. An

115

 

 

 

 

 

 

 

 

 

 

120
110- ---------- T4."'°”"”1'6 --------------------------------------------------------
100- ---------- 1 Q’- --------- 13 ----------------------- NI? ---------------------------
244
b 90.. ....................................................................................
d3) _.
2 30- ....................................................................................
u.
2 --
a, 70. .................................................................................. I
>
8
604 ..................................................................................
1O
50— ...................................................... he. ................... .
1%
40“ O Nocoldtreatrnent mi .............
O 15weeksof5°C 24
30 l l I l I
O 20 40 60 80 100 120

Percentage Flowering

Figure 24. Percentage ﬂowering, time to ﬂower, and ﬂowering uniformity of
Gail/ardia xgrandiﬂora ’Goblin’ under different photoperiods with or without cold
treatment. Numbers next to symbols represent photoperiods consisting of nine-
hour natural days that were extended with incandescent lamps. NI = nIne-hour
natural days with four hours of night interruption. Error bars are 95% conﬁdence
intervals. Symbols without error ars indicate that the conﬁdence intervals were

too large for the graph.

 

 

A
0 Weeks of 5C

125 100%

100 ———————————————— C' VNSI 30% g
ANI

..75_.’g _____ ,,__ ——60%_§
5‘ A .NI re
a so _____________ «A 17 _ _ 40% a
E

25 —————

 

 

nQL
v :-

 

o . _ .
1O 12 14 16 18 20 22 24
Photoperiod

 

Pomemeuda DIyIIoFlower IO-PMFIowemg

 

 

 

C Plant Node Development
0 Weeks of SC

30

Number of Nodes

 

 

 

 

E Number of lnﬂorescences

_e
N

 

5

 

Number of Inﬂoresenoes
O

 

 

0

 

1012141618 20 22 24
Photoperiod

 

FEI-Oweeks cold -15weekscold]

 

 

 

116

 

15 Weeks of 5C

=.”' 100%

 

 

m

D

E
Percent Flowering

 

 

 

o . e . . I
10 12 14 16 18 20 22 24
Photoperiod

 

>‘-DlythmBudHDmem -PMWI

 

 

 

D Plant Node Development

        

 

 

 

 

 

 

15 Weeks of SC
30. “747—. ,2,7, 7:
£20 , l
‘6 l
E 10 O 7 r l 1
g , ,, _ _
ol : - :5 ,El
14 15 24 Nl
Photoperiod
F Plant Height at Flower
so
’g‘eoq _ - — - __________________
E. gill
g 40 _________________
20 I: A A

 

 

 

10 12 14 16 1B 20 22 24
Photoperiod

 

H-EI-Oweekscold .15 weeks cold!

 

 

Figure 25. The effects of photoperiod and cold treatment on ﬂowering of

Physostegia virginiana ‘Alba'.

 

117

Table 15. The effects of photoperiod and cold treatment on ﬂowering of Physostegia virginiana ‘Alba’.

 

Days to DaysTom Days Increase Final plant

 

 

 

Weeks Percentage visible visible bud to in node height Flower
9f 5C Phgtogn'g ﬂowering bug to ﬂower ﬂgwer number (cm) number
0 - 47 63 22 85 13 42 2.9
15 - 9O 59 22 80 14 53 7.0
- 10 4O 95 16 1 1 1 22 60 3.0
12 6O 84 19 102 16 56 4.0
14 67 63 19 82 16 58 4.5
16 75 54 24 78 14 48 5.5
24 100 43 25 68 12 43 6.7
NI‘ 75 57 22 79 14 50 5.7
0 10 0 —’ - — - - -
12 20 S7 25 82 3 24 1.0
14 O - - - -- - -
16 50 65 20 85 14 39 1 8
24 100 50 23 73 12 43 3 8
NI 50 89 22 1 10 18 47 2 4
15 1O 6O 95 16 111 22 60 3 0
12 8O 87 18 105 23 72 5.5
14 100 63 19 82 16 58 4.5
16 100 49 26 75 15 53 7 3
24 100 36 27 63 12 43 9 5
NI 1m 41 22 64 13 51 7 4
Signiﬁcance
Weeks cold (WC) *"” NS *** *“ “" *“
Photoperiod (P) "" *** *"* *"* NS ***
WC X P m H m m m NS
95% Conﬁdence interval for NI
Zero weeks SC 6.6 2.1 6.8 5.7 6.5 2.6
15 weeks SC 2.4 1.3 3.2 2.4 4.4 1.4
Contrasts
Zero weeks SC
Nl vs. 16 Ns Ns NS *"* " NS
NI vs. 24 *" Ns ' * Ns Ns
15 weeks SC
NI vs. 16 **" “" *** “* m '“
NI vs. 24 *“ NS *" *" ** *
Pu... (10 to 24 h) *‘* *“ *‘* *“ NS ”
N_§ m N_§ e on Q

P9u_s_dnuc(10 to 24 h)

 

‘Nl = 4-h night interruption.
’— = No plants showed visible bud after 105 days of forcing.
"3' '- ~. °°' Nonsigniﬁcant or signiﬁcant at Ps0.05, 0.01, or 0.001, respectively.

118

experiment with P. virginiana ‘Summer Snow’ and ‘Vivid’ also concluded that this
species is a quantitative long-day plant: percentage of ﬂowering decreased and
days to ﬂower increased as exposure to long—days increased (Beattie et al.,
1989)

Salvia x superba ‘Blue Queen’. Only 22% of the plants that were not
treated with cold ﬂowered, and those that did took an average of 101 days to
ﬂower (Figure 26, Table 16). In contrast, all cold-treated plants ﬂowered. In a
separate experiment, all ‘Blue Queen’ ﬂowered regardless of cold treatment or
photoperiod (Engle, 1994).

Plants under photoperiods 214 hours or NI ﬂowered uniformly (the 95% CI
of days to ﬂower for cold-treated plants under MI was :1: 2 days). As the
photoperiod increased from 10 to 24 hours, days to ﬂower decreased from 58 to
29, ﬁnal plant height increased linearly from 29 to 47 cm, and the number of new
nodes formed decreased from 13 (26 leaves) to seven (14 leaves). Cold and

photoperiod had no effect on ﬂower number.

Obligate Long-day Species That Beneﬁt from a Cold Treatment
Campanula carpatica ’Blue Clips’. Plants grown from 50-cell plug trays

in 1995-96 ﬂowered 12 to 15 days faster than plants grown from 128-cell plug

trays in 1994-95 (Figures 27 and 28, Tables 17 and 18). The following results

and discussion apply to both years in which Campanula was studied.

 

 

0 Weeks of SC

 

 

 

 

 

 

 

l+ombmeudaomlenm *mml

 

 

 

 

 

C Plant Node Development
0 Weeks of SC

0
O

 

3

.3
0

Number of Nodes

 

 

O

 

 

[Ira-Incas .Nodesotﬁmrﬂ

 

 

 

 

 

E Number of Inﬂorescences

 

 

 

 

 

lEIOweekscold -I-15weeksooldl

 

 

 

 

119

 

 

B 15 Weeks of 5C

125 - - . :=NI100%

 

Percent Flowering

 

 

 

Oﬁse 0%

 

1012141618202224
Photoperiod

 

NbeWﬂiBud-B-Dmbf-‘M *mmﬂ

‘L

 

 

 

D Plant Node Development

15 Weeks of SC

 

3

________________________

Number of Nodes

    

‘1 14 16 24 N
Photoperiod

 

 

O

 

n-lnﬂslnodos [:INodeuIIerceIdINoduI-Im]

 

 

 

 

Plant Height at Flower

 

Height (cm)

 

 

20 ,4:. -...

1012141618202224
Photoperlod

 

lngeeksooId o-ISweekscoldj

 

 

Figure 26. The effects of photoperiod and cold treatment on ﬂowering of Salvia x

superba ‘Blue Queen'.

 

 

 

120

Table 16. The effects of photoperiod and cold treatment on ﬂowering of Salvia x superba ‘Blue Queen'.

 

Days to Days from Days Increase Final plant

 

 

 

Weeks Percentage visible visible bud to in node height Flower
of Ph riod ﬂowerin bud to ﬂower ﬂower numﬁr (cm) number
0 - 22 88 13 101 19 56 3.2
15 - 100 29 12 4O 9 40 4.2
- 1O 60 54 14 67 14 29 3.3
12 67 41 13 55 12 40 4.1
14 67 26 1 1 36 9 40 4.8
16 67 20 10 30 8 37 4.4
24 80 44 1 1 55 12 56 3.6
NIz 60 31 13 44 9 47 4.7
0 10 20 100 14 1 14 20 32 3 0
12 0 --’ - - - - --
14 0 - -- -- - - —
16 O - - - - -- -
24 6O 87 12 99 20 71 3 3
NI 20 80 16 96 15 33 3 O
15 10 100 45 13 S8 13 29 3.4
12 100 41 13 55 12 40 4.1
14 100 26 1 1 36 9 40 4.8
16 100 20 10 30 8 37 4.4
24 100 19 11 29 7 47 3.8
N_I 100 22 12 33 7 50 5.0
Signiﬁcance
Weeks cold (WC) *“ " ""” *“ NS NS
Photoperiod (P) **" *"" *** *”* “" Ns
WC x P * Ns Ns *“ *“ NS
95% Conﬁdence interval for NI
15 weeks SC 2.0 0.7 1.9 1.1 4.6 0.8
Contrasts
Zero weeks 5C
NI vs. 24 Ns " NS *** *** NS
15 weeks SC
NI vs. 16 Ns * NS NS *“ Ns
NI vs. 24 NS Ns NS NS Ns NS
PLhoer (10 to 24 h) *"" ** *“ **' “" Ns

PM (10 19 24 h) N§ Ns
zNI = 4-h night interruption. '
’- = No plants showed visible bud after 105 days of forcing.

”s- '- "- °" Nonsigniﬁcant or signiﬁcant at Ps0.05, 0.01 , or 0.001, respectively.

 

 

A 0 Weeks of so
125 -
100 .

75<
is.
25<
O

 

 

 

 

 

 

l‘qunthbbBuLE}anbeuu .o.nm~mnmunm]

 

 

 

 

 

 

 

  

 

 

 

 

 

 

 

 

 

 

0 Plant Node Development
,0 0 Weeks of 50
igao. _____________________
2520. ______________
E
g 10 I ---------
Photoperiod
Eigumunaus I'INahsdﬁum]
E Number of Flower Buds
g 80
2 60
u. 40 «
s
3 2° *
E
3 0 - 4

 

 

 

1012141618202224
Photoperiod

 

[-69-0wukscold -1Sweekscold]

 

 

 

121

 

 

B 15 Weeks of SC

125 33914—40036

 

 

 

 

 

 

 

r;roqmehbbBuLE}anthwr uquahmunnanum ﬂ

 

 

 

 

Plant Node Development
15 Weeks of 5c

 

 

 

 

 

[Iluﬂlnahe llamasdunudllahmeuewnrl

 

 

 

 

F Plant Height at Flower

 

 

 

 

10+.......-.ﬁ..-.
1012141618202224
Phouxxxhu

 

 

“sivauukscoklucptsvmnkscokq

 

 

Figure 27. The effects of photoperiod and cold treatment on ﬂowering of

Campanula carpatica ‘Blue Clips’. 1994-95.

 

 

 

122

Table 17. The effects of photoperiod and cold treatment on ﬂowering of Campanula carpatica ’Blue Clips’:
1994-95.

 

Days to Days from Days Increase Final plant

 

 

 

Weeks Percentage visible visible bud to in node height Flower
of Ph n’ ﬂ rin bud to ﬂower ﬂower number (cm) number
0 - 64 56 20 76 16 15 28
1S - 67 48 17 65 17 17 41
- 1O 0 -‘ - - - - -
12 0 - — - - - -
14 35 78 21 100 24 14 17
16 100 43 18 61 13 14 24
24 95 62 18 80 21 20 56
Nl’ 100 43 19 62 14 15 32
0 1O 0 - - - - - -
12 0 - - - - -« —
14 3O 97 27 124 25 16 23
16 100 48 19 67 14 13 18
24 90 63 19 82 18 18 38
Nl 100 45 20 64 14 14 30
15 10 0 - - - - - -
12 0 - - - - - --
14 40 S9 16 75 22 13 1 1
16 100 38 18 55 13 15 3O
24 100 62 16 78 23 22 74
N1 109 42 19 69 13 15 33
Signiﬁcance
Weeks cold (WC) ”* “* * NS Ns *
Photoperiod (P) "* *‘" “* *** m m
WC x P N m m m t “It
95% Conﬁdence interval for NI
Zero weeks SC 5.0 1.1 5.7 1 3 2.3 5.9
15 weeks SC 5.6 1.3 5.0 1 8 1.5 10.2
Contrasts
Zero weeks 5C
NI vs. 16 Ns Ns NS Ns NS '
NI vs. 24 “" NS “* *“ *‘ Ns
PM (14 to 24 h) “ *“ “* * ‘ *
PM (14 to 24 h) m m m m * Ns
15 weeks 5C
NI vs. 16 NS NS Ns NS Ns NS
NI vs. 24 m *" .1... “‘ “‘ “*
PLheer (14 to 24 h) " Ns NS ** *** 1...
Poe-ewe (14 to 24 h) m " *** “* NS NS
0 and 15 weeks SC
Pm (14 to 24 h) NS **" Ns Ns “" “‘
PM”: (1449 24 h) *"* ** “* **" N§ Ns

 

 

‘- = No plants showed visible bud after 105 days of forcing.
’Nl = 4-h night interruption.
"5' '- "- '" Nonsigniﬁcant or Signiﬁcant at Ps0.05, 0.01, or 0.001, respectively.

 

 

A OWeeksofSC
125 :N'10096
100 ——————————————————— 80%.?
75———— —————————————— 60%;
a [,8 L
350____ - ........ Mum‘-
25__— --_-__________A_Nl 20%;
nee

 

 

 

0 I I I .
10 12 14 16 18 20 22 24
Photoperiod

 

IIEWhMMBMy-bm sow-1mm

 

 

 

C Plant Node Development
0 Weeks of SC
50 .

II?" 1

      

‘ '
Photoperiod

 

 

 

Number of Flower Buds

&
O

 

W
O

N
O

.s
O

 

Number or Flower Buds m

 

 

 

14 16 18 20 22 24
Photoperiod

 

ea-Oweekscold -o-15weekscold

 

 

 

 

123

 

 

'3 15Weeks ofSC
125 :95! 100%
1ooI ——————————————————— 50% '8'
2. 75 _____________________ 6091.2
u.
3 so —————— gswamm 4096‘:
25H" ----,,_-______A_N120%l
0%.

 

 

 

0 I . I .
10 12 14 16 18 20 22 24
Photoperiod

 

k DoystoVislbIeMBDmtoFba-r so-Percentml

 

 

 

D Plant Node Development
15 Weeks of SC

50
340
E 30
0

g 20
5 1°

0

 

”1010901106

 

 

 

 

F Plant Height at Flower

 

Height (cm)

 

 

 

 

 

|-I-:I-0weekscold -15weeksooldl

 

 

 

 

Figure 28. The effects of photoperiod and cold treatment on ﬂowering of

Campanula carpatica ‘Blue Clips’. 1995-96.

124

Table 18. The effects of photoperiod and cold treatment on ﬂowering of Campanula carpatica ‘Blue Clips’:
1995-96.

 

Days to Days from Days Increase Final plant

 

 

Weeks Percentage visible visible bud to in node height Flower
o! 59 Phgjggrig ﬂowering bgd to ﬂower ﬂower number (cm) numge_r_
O - 77 43 20 63 21 20 29
15 - 72 38 17 50 15 16 25
- 1O 0 -‘ - - — - -
12 O - - - - -- -
13 13 64 20 84 33 14 7
14 55 69 20 85 29 16 14
16 100 32 19 51 16 18 32
24 100 41 17 58 18 20 28
Nl’ 100 32 18 50 15 17 28
0 1O 0 — - - - - -
12 0 - - — - - -
13 25 64 20 84 33 14 7
14 60 69 21 90 30 17 16
16 100 34 20 54 18 20 33
24 100 47 18 66 21 23 34
Nl 100 32 20 52 17 20 29
15 1O 0 - - -- - - -
12 O - - - - - -
13 0 - - - - -- -
14 50 70 17 72 26 13 1O
16 100 30 18 49 14 16 31
24 100 35 16 51 14 18 22
N_I 100 32 16 47 14 14 27
Signiﬁcance
Weeks cold (WC) NS *" *"” “'* *“ Ns
Photoperiod (P) *“ NS *“ "" ** *"
WC x P Ns Ns Ns Ns NS NS
95% Conﬁdence interval for NI
Zero weeks SC 4.4 1.1 3.9 0.9 2.4 6.5
15 weeks 5C 4.8 1.1 4.6 2.0 1.0 4.7
Contrasts
Zero weeks 5C
NI vs. 16 NS NS NS Ns Ns Ns
NI vs. 24 “” Ns **‘ *" " Ns
PUnosr (13 to 24 h) ** NS *“ “* ** *‘
Pod-dude (13 to 24 h) m NS ”* *“ NS *
15 weeks 5C
NI vs. 16 Ns " NS Ns Ns
NI vs. 24 NS Ns Ns NS " NS
PLInesr (14 to 24 h) *” NS *" *” NS NS
Pguedmlc (14 t_O 24 h) m g m m N_S "

 

‘— = No plants showed visible bud after 105 days of forcing.
'Nl = 4-h night interruption.
"s- '- "' ”° Nonsigniﬁcant or signiﬁcant at P5065. 0.01 , or 0.001, respectively.

125

Cold treatment did not affect the percentage of ﬂowering but reduced
days to ﬂower by ﬁve to ﬁfteen days under photoperiods 216 hours or NI.
However, slightly warmer bench temperatures (0.5 to 1 °C) likely contributed to
this hastening of ﬂowering. Based on experiments by Whitman (1995), a
temperature increase of 1 °C would have accelerated days to ﬂower by 1.5 days.
Cold treatment did not consistently affect any other ﬂowering characteristic
measured. Whitman (1995) found that plants grown from 128-cell plugs that
received 14 weeks of 5 °C ﬂowered approximately 10 days faster than plants that
did not receive a cold treatment, but cold did not hasten ﬂowering of plants
grown from SO-cell plugs.

Campanula is an obligate long-day plant; no plants ﬂowered under
photoperiods s12 hours and essentially all plants ﬂowered under photoperiods
216 hours or NI. Under continual light, ﬂowering was delayed and nonuniform.
Thirty to 60% of plants ﬂowered under 14-hour photoperiods and ﬂowering was
delayed by at least 30 days compared to plants under 16-hour photoperiods.
Thus, photoperiods 216 hours, but not continual light, or NI are recommended
for rapid, uniform ﬂowering. Plant height increased linearly from 14 to 20 cm as
photoperiod increased from 14 to 24 hours. There were no consistent
photoperiodic trends for ﬂower number.

Coreopsis verticillata ‘Moonbeam’. Horticulturally, ‘Moonbeam’ is an
obligate long-day plant. Flowering was complete, rapid, and uniform under
photoperiods 214 hours or NI, regardless of cold treatment (Figure 29, Table 19).

Plants ﬂowered in 45 to 50 days, developed an average of ﬁve to six nodes (10

 

0 Weeks of 5C

 

--- 100%

 

 

 

 

O I I . I
10 12 14 16 18 20 22 24
Photoperiod

 

ﬂi— DeyslonsbloBuda DmtoFlow-r uo-PsroentFlowsring]

 

 

 

 

 

c Plant Node Development
16 ., __O,Weeksof5Cir

 

Number of Nodes

 

 

 

 

15 Weeks of SC

 

 

 

 

 

0 I I I I I .
10 12 14 16 18 20 22 24
Photoperiod

 

ﬁs—o-nmmwao-y-Ionm sow-1mm!

 

 

 

 

D Plant Node Development

15 Weeks of 5C

   
   

 

2:1. 7. 77., , .7:

2. .._.--~

.—,‘.__

 

 

 

Number of Nodes

 

 

Photoperiod

 

ialnhialnodu DNod-e-Irereeld-Nsdemml

 

 

 

 

 

 

E Number of Flower Buds

g 125

“3 1oo .....................
3.. -- ......... ell
IL

~0- 5o _ _ - ___________________
E 25 . ____________________
E

g 2

 

 

 

0 . . . . I .
10 12 14 16 18 20 22 24
Photoperiod

 

[kg-Oweeks cold -15 weeks cold

 

 

 

 

 

 

F Plant Height at Flower

 

Height (cm)
«5 01
O O

8 A851

 

 

 

 

10 12 14 1 18 20 22 24
Photoperiod

 

[nil-Oweeksoold -15weeksooldﬂ

 

 

 

Figure 29. The effects of photoperiod and cold treatment on ﬂowering of

Coreopsis verticillata ‘Moonbeam’.

 

127

Table 19. The effects of photoperiod and cold treatment on ﬂowering of Coreopsis verticillata ’Moonbeam’.

 

 

 

 

Days to Days from Days Increase Final plant
Weeks Percentage visible visible bud to in node height Flower
9f SC Phgjgmriod ﬂowering bud to ﬂower ﬂower nummr (cm) numbe;
0 - 77 38 25 64 7.6 39 57
15 - 9O 28 27 55 7.0 43 67
- 10 25 58 25 83 12.3 32 9
12 75 60 25 86 8.1 29 19
14 100 22 26 48 5.9 49 101
16 100 20 24 44 5.8 45 85
24 100 19 27 47 5.6 47 81
Nl1 100 22 28 50 6.2 47 80
0 1O 10 S1 26 77 13.0 35 5
12 50 9O 22 1 12 9.0 22 8
14 100 23 25 49 5.9 49 98
16 100 21 22 43 5.5 41 8O
24 100 20 29 49 5.8 45 79
Nl 100 25 28 53 6.1 46 75
15 10 40 64 24 88 1 1.5 29 13
12 100 30 29 59 7.2 35 3O
14 100 21 27 48 5.9 49 104
16 100 20 26 45 6.1 49 90
24 100 18 26 44 5.3 48 83
N_I 190 18 28 46 6.2 48 85
Signiﬁcance
Weeks cold (WC) ”* NS *“ * "’ "
Photoperiod (P) *"* ’*" "* *"" m m
WC x P m H. m i M NS
95% Conﬁdence interval for NI
Zero weeks SC 1.8 1.8 2.7 0.2 4.1 8.0
15 weeks 5C 2.2 1.5 2.1 0.6 3.1 13.5
Contrasts
Zero weeks SC
NI vs. 16 Ns "* *** NS * NS
NI vs. 24 NS NS Ns NS NS Ns
PLheer (10 to 24 h) m * m m m *“
Pom (10 to 24 h) m " **" *“ Ns ””
15 weeks 5C
NI vs. 16 Ns NS Ns NS Ns NS
NI vs. 24 NS Ns Ns * NS NS
PM (10 to 24 h) *“ NS *“ *“ m “"
Pol-ass: (10 to 24 h) “" NS “* m “* *“
O and 15 weeks SC
PLInesr (10 to 24 h) “" Ns **" m ”" ”"
PM (10 to 24 h) *‘* N_§ "'* *” m *"

 

‘Nl = 4-h night interruption.

"SI -. ~. ”' Nonsigniﬁcant or signiﬁcant at Ps0.05, 0.01, or 0.001, respectively.

 

128

to 12 leaves), grew 40 to 50 cm tall, and averaged 75 to 100 ﬂowers. Iversen
and Weiler (1994) found that ‘Moonbeam’ plants did not ﬂower under 8-hour
photoperiods after receiving 0, 6, or 12 weeks of 4.5 °C cold treatment, whereas
all those grown under 16- or 24-hour photoperiods ﬂowered, regardless of cold
treatment.

The cold treatment shifted the minimum photoperiod under which all
plants ﬂowered from 14 hours to 12 hours. Under photoperiods 214 hours, cold
treatment did not dramatically affect time to ﬂower. The cold treatment increased
the average ﬂower number from 57 to 67, increased plant height by an average
of four cm, and slightly reduced the number of new nodes formed. However, the
effects of cold treatment on plant height and the number of new nodes formed
varied by photoperiod, with differences primarily under 10- or 12-hour
photoperiods.

Photoperiod inﬂuenced all ﬂowering characteristics measured but, except
for ﬂower number, the effects varied with cold treatment. As the photoperiod
increased, days to visible bud and ﬂower and the number of new nodes formed
decreased at a decreasing rate. For example, for cold-treated plants, time to
ﬂower decreased from 88 to 44 days as the photoperiod increased from 10 to 24
hours. Plant height increased linearly as photoperiod increased from 10 to 24
hours. Plants had few ﬂowers under 10- or 12-hour photoperiods and had the
greatest number of ﬂowers under 14-hour photoperiods.

Echinacea purpurea ‘Bravado’. The following results and discussion

apply to both years in which Echinacea was studied. The cold treatment

129
reduced days to visible bud and ﬂower by 15 to 25 days and decreased the

number of new nodes formed by one or two (Figures 30 and 31, Tables 20 and
21). Cold treatment did not consistently affect any other ﬂowering characteristic
measured.

Echinacea has an optimum photoperiod at or near 14 hours for complete,
rapid, and uniform ﬂowering. Regardless of cold treatment, all plants ﬂowered
under 14-hour photoperiods, and the percentage of ﬂowering decreased as
photoperiods decreased or increased. The percentage of ﬂowering plants under
NI never reached 100%, and only one plant in forty ﬂowered under continual
light. Photoperiodic trends are difﬁcult to establish because of the variable
percentage of ﬂowering plants under all but the 14-hour photoperiods.

Plants grown under 14-hour photoperiods ﬂowered more uniformly than
plants grown under NI. For example, in 1994-95, the 95% CI of days to ﬂower
for plants under 14-hour photoperiods were :I: 8 or :t 3 days, without or with the
cold treatment, respectively. The same intervals for plants under N l were :t 15 or
5:7 days, respectively. In another experiment, 2% of ‘Bravado’ plants ﬂowered
under 9-hour short days and 98% ﬂowered under 4-hour NI (Engle, 1994).

Gypsophila paniculata ‘Double Snowﬂake’. Flowering of Gypsophila
was highly variable, regardless of cold treatment or photoperiod (Figure 32,
Table 22), and those that did ﬂower were rangy and unattractive. Cold treatment
doubled the percentage of ﬂowering and reduced the time to ﬂower by an
average of 25 days. Days from visible bud to ﬂower decreased from 22 to 16 for

cold-treated plants, but part of this acceleration may have been due to higher

 

0 Weeks of 5C

 

  

 

 

 

 

10 12 14 16 18 20 22 24
Photoperiod

 

Liqunovuueun43.uwsthwr eoennumnannu I

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

C Plant Node Development
30 0 Weeks of 5c
3..
3
§§10«
2
Phouxnnod
“Egmuunabe lllNahsuﬁum1
E Number of Flower Buds
§§4
33 .
:2 .

 

 

 

10 12 14 16 18 20 22 24
Phokxnnod

 

|4§h0vwnkscow dlhtsvnnkscobl

 

 

 

 

130

 

 

15 Weeks of SC

 

  

 

 

 

 

10 12 14 16 18 20 22 24
Phouxnnod

 

[[15 DsyislbIeMBDsystlower sonPercsntFlawertng]

 

 

 

 

Plant Node Development
15 Weeks of 5C

 

 

 

 

 

“:1uwumnu [:JNausdunudllammsumuul

 

 

 

 

Plant Height at Flower

23

 

tkﬂﬂﬂlan)
SI 2:

3

 

 

 

1O - I : I I I I I I I 2 I . I s .g
10 12 14 18 18 20 22 24
Phokxnmmd

 

 

[e-Oweekscold 015mkscold!

 

 

Figure 30. The effects of photoperiod and cold treatment on ﬂowering of
Echinacea pumurea ‘Bravado’. 1994-95.

 

 

 

131

Table 20. The effects of photoperiod and cold treatment on ﬂowering of Echinacea purpurea ‘Bravado’:
1994—95.

 

Days to Days from Days Increase Final plant

 

 

 

Weeks Percentage visible visible bud to in node height Flower
of Ph ri ﬂ ering bud to ﬂower ﬂower number (cm) numbe_r_
0 - 38 77 28 105 14 76 3.5
15 - 55 55 26 79 13 74 3.2
- 10 17 90 --‘ ~- -- -- --
12 22 80 22 101 21 17 1.5
14 100 57 28 85 13 78 3.6
16 55 67 26 93 13 68 3.2
24 0 -’ — - - -- -
NI" 80 63 29 92 13 83 3.4
0 10 0 - - - - — -
12 0 - - - - - -
14 100 71 28 99 14 79 3.6
16 60 84 27 1 1 1 15 67 3.3
24 0 — — -- — — -
Nl 70 80 30 110 14 78 3.6
15 10 38 90 - -- -- -- -—
12 50 80 22 101 21 17 1.5
14 100 43 27 71 13 77 3.5
16 50 46 26 72 12 70 3.0
24 0 - — - - - -
JJ 40 49 28 78 12 86 3.2
Signiﬁcance
Weeks cold (WC) “" NS *“ ** NS Ns
Photoperiod (P) *‘* *** m **“ *"" '*
WC x P Ns Ns Ns Ns Ns NS
95% Conﬁdence interval for NI
Zero weeks SC 12.8 3.1 14.7 1.5 6.9 1.3
15 weeks SC 6.4 0.9 6.5 1.5 5.3 0.3
95% Conﬁdence interval for 14 h
Zero weeks SC 8.5 1.4 8.4 1.6 7.3 0.5
15 weeks SC 3.1 1.9 3.0 0.9 5.7 0.4
Contrasts
Zero weeks SC
14 vs. 16 * Ns " NS " Ns
14 vs. Nl NS NS NS Ns NS Ns
15 weeks SC
14 vs. 16 NS *** "* Ns NS Ns
14 vs. Nl Ns “ *‘* NS " NS
PLhesr (12 to 16 h) **” NS *** “* NS NS
Pgbgnec (12 to 16 h) NS *‘* *‘ *“ “” ”

 

‘-—- = Experiment was terminated before plants ﬂowered.

’-- = No plants showed visible bud after 105 days of forcing.

"Nl = 4-h night interruption.

"s- '- ”° "' Nonsigniﬁcant or signiﬁcant at Ps0.05, 0.01, or 0.001, respectively.

 

 

A 0Weeks ofSC
125 - AD", 100%
100 -__ —_ ________ 50% g
AANI
:75--- —— __ sssssssss soseé
LL
3 soI___ _______________ 40% *-
25 »— ———————————— --—— 2mg
9%

 

 

 

0 ﬁg I—I I . I I
10 12 14 16 18 20 22 24
Photoperiod

 

—‘_DsysmVleibleBudQDlystoFlower .o-PercentFlmlerlng

 

 

 

 

 

C Plant Node Development

 

 

   

 

 

 

 

 

 

0Weeksof5C

30F “_-27,22, " I
E

4310 -

3

z 2 E

0 Li S» :7

10 12 13 14 1e 24 NI
Photoperiod
.Inltislnodes .Nodustﬂtmer

E Number of Flower Buds

s
13
E5
124
E
33
E

3
z2

 

 

 

1012141618 20 22 24
Photoperiod

 

[-E-Oweeks cold -15 weeks cold]

 

 

 

 

132

 

 

'3 15 Weeks of so
125 - 100%
100———— ———————————— 13m aossg
g 75 -- — —————————— m 60% g
IL
3 so e“ _ _____________ 40% ﬁg
25 - - — — — — ————————————— 20% g
0 0%

 

 

 

10 12 14 16 18 20 22 24
Photopenod'

 

I‘.DnystoVisfbleBudE}Dsystlow .oePercentFlowerlng

 

 

 

 

D Plant Node Development

 

 

 

 

 

 

 

 

 

 

 

15WeeksofSC
so
E20
”6
E10
2
o
Photoperiod
[Elm-Incas DNodusﬂercoldiNodustﬂowern
F Plant Height at Flower
70
so.
550
'5
.540
I
so
I
20 - I I I I I
101214161820 22 24

Photopenod

 

i490 weeks cold -o-15 weeks com

 

 

 

Figure 31. The effects of photoperiod and cold treatment on ﬂowering of
Echinacea purpurea ‘Bravado'. 1995—96.

 

 

 

133

Table 21. The effects of photoperiod and cold treatment on ﬂowering of Echinacea purpurea ‘Bravado’:
1 995-96.

 

Days to Days from Days Increase Final plant

 

Weeks Percentage visible visible bud to in node height Flower
1 Ph riod flow n'n bud 0 flow r ﬂower num r cm number
0 - 53 78 29 106 15 53 4.6
15 - 32 69 31 90 13 45 4.0
- 10 0 J -— — - - -
12 1 1 83 28 1 15 21 30 4.0
13 60 75 29 104 15 46 5.1
14 100 69 30 97 14 56 4.4
16 5O 75 28 103 13 52 4.1
24 5 91 32 123 16 39 4.0
NI’ 70 8O 29 1 15 14 50 4.2
O 10 0 - - - -- - -
12 10 87 28 115 21 30 4.0
13 80 76 29 104 16 46 5 3
14 100 70 29 99 14 63 4 6
16 9O 78 28 106 14 53 4 2
24 1O 91 32 123 16 39 4.0
NI 80 86 30 119 15 51 4.3
15 1O 0 - - —- - -- -
12 11 78 --" -— -- -- --
13 40 73 -- -- -- -- --
14 100 68 33 92 14 45 4.1
16 1O 47 30 77 9 44 3 0
24 O - - - - - —-
N1 60 72 24 ﬁ97 13 46 4 0
Signiﬁcance
Weeks cold (WC) ' NS *" " NS *
Photoperiod (P) * NS NS * NS *"
WC x P NS NS NS NS NS NS
95% Conﬁdence interval for NI
Zero weeks 50 12.2 6.1 17.5 2.7 9.2 1.1
15 weeks 50 18.8 2.2 w 1.8 w 0.9
95% Conﬁdence interval for 14 h
Zero weeks 5C 5.3 1.9 6.3 1.0 5.1 1.1
15 weeks 50 6.8 7.7 5.2 1.7 6.4 0.5
Contrasts
Zero weeks 50
14 vs. 16 NS NS NS NS NS **
14 vs. NI ** NS “ NS NS “
Pm (12 to 24 h) NS NS NS NS NS NS
P (12 to 24 h) N§ NS N§ m N_S ”*

 

'- = No plants showed visible bud after 105 days of forcing.

”M = 4-h night interruption.

"-- = Experiment was terminated before plants ﬂowered.

w = Insufﬁcient data.

"3' '- "- "' Nonsigniﬁcant or signiﬁcant at Ps0.05. 0.01. or 0.001. respectively.

 

 

 

 

A 0 Weeks of 5C

150 100%

120 _____B\__S\___\:__—DNI 80% .9
ANI

g 90 ____________ — — _ _ 60% g
u.
3 so . ................. 40% 5
so _ _ _ ___________ w 20% a

 

 

 

o . + I I 01¢
10 12 14 16 18 20 22 24
Photoperiod

 

irowathmbBM-E}chbﬂomr -.-Pmuthmﬂw l

 

 

 

 

C Plant Node Development
0 Weeks of SC

 

  

o 12 14 24 Nl

 

.lnttlalnodu Imus-rm

 

 

 

E Number of Inﬂorescences

 

 

.n
-

 

 

 

Number of lnﬁonesences
N U

0 1 i 1 r , .
10 12 14 16 18 20 22 24
Photoperiod

 

[-B-Oweekscold -15weekscold

 

 

 

 

134

 

 

B 15 Weeks of 5c

 

Percent Flowering

 

 

 

 

10 12 14 16 18 20 22 24
Photoperiod

 

W‘sbwlbkuemdfslhanbwr .onPuumﬁmnﬂu]

 

 

 

 

Plant Node Development
15 Weeks of SC

#0!
OO

 

88

.A

Number of Nodes
O O
+—' l

 

 

 

 

 

 

 

Height (cm)

 

 

4_24

 

 

40 ##49 . 4 I
10 12 14 16 18 20 22 24
Photoperiod

 

Leeweekscold -15weeksoord

 

 

Figure 32. The effects of photoperiod and cold treatment on ﬂowering of

Gypsophila paniculata ‘Double Snowﬂake’.

 

 

 

135

Table 22. The effects of photoperiod and cold treatment on ﬂowering of Gypsophila paniculata ‘Double
Snowﬂake”.

 

Days to Days from Days Increase FinanIant

 

 

 

Weeks Percentage visible visible bud to in node height Flower
of QC Phgtogrig flowering bud to ﬂower ﬂower number (cm) number
0 - 25 96 22 1 18 23 66 2.3
15 - 54 80 16 93 3O 71 1 .6
- 1O 5 71 14 85 27 96 1 .0
12 15 102 z z z z z
14 45 91 18 109 33 78 1 .8
16 60 91 19 109 27 69 1.5
24 79 73 18 91 23 59 2.3
NI’ 35 87 16 103 29 75 1 .7
0 10 0 -" - - - - -
12 0 - - - - - -
14 20 1 1 1 27 138 27 78 1.5
16 40 103 26 130 24 65 2.0
24 70 86 17 104 20 65 2.8
NI 20 101 20 121 22 59 2.0
15 1O 10 71 14 85 27 96 1.0
12 30 102 z z z z z
14 70 85 16 101 36 78 2.0
16 80 84 15 99 3O 73 1 .0
24 89 62 19 81 25 53 1.9
N! 50 81 14 95 32 83 1.5
Signiﬁcance
Weeks cold (WC) “‘ m “" * NS NS
Photoperiod (P) “* NS ** NS W NS
WC x P NS " NS NS "* NS
95% Conﬁdence interval for NI
15 weeks 50 12.3 3.1 13.7 11.1 8.2 0.9
Contrasts
Zero weeks 5C
NI vs. 16 NS NS NS NS NS NS
NI vs. 24 NS NS NS NS NS NS
PLineu (14 to 24 h) * ” ** NS NS "
Pom-tic (14 to 24 h) NS NS NS NS NS NS
15 weeks 50
Nl vs. 16 NS NS " " *“ NS
NI vs. 24 NS NS * NS “* NS
PLlneer (1410 24 h) NS NS NS NS ' NS
Pm (14 to 24 h) * NS NS NS NS NS
0 and 15 weeks 5C
Pu... (14 to 24 h) NS NS NS NS “ NS
PM (14 to 24 h) N_s ﬂ N_s N§ * NS

 

 

‘Experiment terminated before plants reached anthesis

’Nl = 4-h night interruption.

"- = No plants showed visible bud after 105 days of forcing.

“a '- "' '” Nonsigniﬁcant or signiﬁcant at Ps0.05. 0.01. or 0.001, respectively.

136

plant temperatures. Cold-treated plants were more vigorous and developed
approximately ﬁve to seven more nodes before visible bud than non-cold-treated
plants.

For plants that did not receive cold treatment, the percentage of ﬂowering
increased from zero to 70 as the photoperiod increased from 12 to 24 hours, and
days to ﬂower decreased linearly from 138 to 104 as photoperiods increased
from 14 to 24 hours. For cold-treated plants, the percentage of ﬂowering
increased from 10 to 90 as the photoperiod increased from 10 to 24 hours, with
fastest ﬂowering occurring under continual light. Plant height decreased linearly
as photoperiods increased, regardless of cold treatment.

Several experiments have been conducted on the effects of photoperiod
and cold treatment on ﬂowering of Gypsophila. Moe (1988) found that long-days
were required for ﬂower initiation and development of ‘Bristol Fairy’, except for
plants grown at 12 °C, where ﬂowering was complete but delayed relative to
plants grown under long days. For seven selections of ‘Bristol Fairy’, few or no
plants ﬂowered under photoperiods of eight or ten hours, and as photoperiods
increased from 12 to 24 hours, days to visible bud decreased (Kusey et al.,
1981). Cold treatment for 2 to 8 weeks at 5 °C hastened days to visible bud but
did not affect percentage of ﬂowering.

Helenium autumnale. Cold treatment shifted the minimum daylength
required for ﬂowering from 16 to 14 hours, hastened days to visible bud and
ﬂower by approximately 20 days, and reduced the number of new nodes formed

(Figure 33, Table 23). On average, cold-treated plants were 15 cm shorter and

 

 

 

 

A 0 Weeks of so
125 ~ 100%
100 ______ __— ______5_N|_-so% g
% 75 —————— N;;-‘-‘-Nl 60% g
a so ——————————————————— 40% a
25 ___________________ 20% g
nee

 

 

 

0 r ' , l
1 O 1 2 1 4 1 6 1 8 20 22 24
Photoperiod

 

*Daylbmaudablysbm -PerumFlou-rlng

 

 

 

 

C Plant Node Development
0 Weeks of 50

N
0|

 

83.8

Number of Nodes
a

 

O

 

 

 

 

 

 

 

 

 

 

[-e-o weeks cold .15 weeks cold I

 

 

 

137

 

 

B 15WeeksofSC
125 =..l100%
1oo ____________________ 430%?
:75"-— ______ le.eo%§
IT.
850*-” _- -2- u wwxg
25---- ———————————————— Izossg

9%

 

 

 

O . I . 1 . I
1 O 1 2 1 4 1 6 1 8 20 22 24
Photoperiod

 

[fDIyIDViﬂNeBudBDlystlmr .o-PMFWI

 

 

 

 

D Plant Node Development
15 Weeks of 50

    

o . -
Photoperiod

 

“E’Irmm .m-mwiummmﬂ

 

 

 

F Plant Height at Flower

 

an

o
[I
E

Herght (cm)
\I
O
l
l
|
l
l
l
l
l
g
l
l
l
I
l
l
I
o :
E 1
I

O)
O

 

 

50 ,g_, . I 1 1 1
1 0 1 2 14 16 1 8 20 22 24
Photoperiod

 

 

I-B-Oweekscold uo-15weekscﬂ

 

 

 

 

Figure 33. The effects of photoperiod and cold treatment on ﬂowering of

Helenium autumnale.

138

Table 23. The effects of photoperiod and cold treatment on ﬂowering of Helenium autumnale.

 

Days to Days from Days Increase Final plant

 

 

 

 

Weeks Percentage visible visible bud to in node height Flower
of 5C Photgpgrigd ﬂgwgring bud to ﬂower ﬂower number (cm) numbe_r_
0 - 40 74 26 101 52 83 16
15 - 55 54 25 82 42 67 27
- 10 0 -‘ - -- - — —
12 0 - - — - - —
13 0 — - - — - -
14 40 68 24 95 59 69 23
16 100 67 27 96 51 75 19
24 95 54 27 85 35 79 14
NI’ 95 66 25 94 52 76 27
0 10 0 -- - -— - - --
12 0 - - - - - -
13 0 -- — — - - --
14 0 - - — - - -
16 100 79 26 106 58 82 16
24 90 62 28 90 36 83 9
NI 90 81 25 105 60 83 21
15 10 0 - — - - - -
12 0 - - - - - -
13 0 - - - - -- -
14 80 68 24 95 59 69 23
16 100 54 27 81 38 64 24
24 100 46 21 70 31 70 24
N_I 100 5_2 26 78 40 66 36
Signiﬁcance
Weeks cold (WC) m Ns *"" “" m .1...
Photoperiod (P) ““ NS **“ *“ NS *
WC x P ** * NS NS Ns NS
95% Conﬁdence interval for NI
Zero weeks 5C 5.2 1.7 6.0 8.4 7.5 6.7
15 weeks 5C 3.4 3.4 6.3 9.6 8.9 13.3
Contrasts
Zero weeks 50
Nl vs. 16 NS Ns Ns NS Ns NS
NI vs. 24 *“ * *“ *“ NS “
15 weeks 5C
NI vs. 16 Ns NS Ns NS Ns *
NI vs. 24 “ Ns Ns Ns Ns Ns
PLlnear (14 to 24 h) “* Ns *“ *** Ns NS
P - (14 to 24 h) *** N_§ * ** N§ N§

 

‘- = No plants showed visible bud after 105 days of forcing.
’NI = 4—h night interruption.
"S- '1 "- '” Nonsigniﬁcant or signiﬁcant at Ps0.05, 0.01 , or 0.001, respectively.

139

had nine more ﬂowers than plants that did not receive the cold treatment. Cold
treatment did not improve ﬂowering uniformity.

Without cold, no plants ﬂowered under photoperiods s 14 hours; after cold,
80% of plants ﬂowered under 14-hour photoperiods but none ﬂowered under
shorter photoperiods. Nearly all plants ﬂowered under photoperiods 216 hours
or Nl. For cold-treated plants, days to ﬂower decreased linearly from 95 to 70
and the number of new nodes formed decreased linearly from 59 to 31 as the
photoperiod increased from 14 to 24 hours. There were no photoperiodic trends
for plant height or ﬂower number.

Oenothera missouriensis. The cold treatment increased the percentage
of ﬂowering from 62 to 72, hastened ﬂowering by 25 days, reduced the number
of new nodes formed by two or three, and improved ﬂowering uniformity (Figure
34, Table 24). For example, the 95% CI of days to ﬂower for plants under Nl
decreased from :t 10 to :r: 4 after cold treatment.

Fewer than 15% of the plants ﬂowered under photoperiods of 10 or 12
hours, and except for non-cold-treated plants under continual light, 290% of
plants ﬂowered under photoperiods 214 hours or NI. Only 30% of non-cold-
treated plants under continual light ﬂowered.

For cold-treated plants, days to visible bud and ﬂower, the number of new
nodes formed, plant height, and ﬂower number were similar under photoperiods
214 hours or NI. On average, plants that ﬂowered under 10- or 12-hour

photoperiods ﬂowered 30 days later, developed six more nodes, were six cm

 

 

 

A 0Weeksof5C
1oo - 100%
oNl
75____ _ 2 ————— {Ml 75962
§-soI_ __ _, ——- ~—-A—Nl~50%I.L
o
25-" _______________ 2511.5
0.
9%

 

 

 

O I I I I I
10 12 14 16 1B 20 22 24
Photoperiod

 

iqusthbhdeEIDqlhﬁmnr nthuuthnﬂwJ

 

 

 

c Plant Node Development
30 .gr 0 Wgeiksﬁoj§L ,

III

   
   

’ _’ ' 16
Photoperiod

g] 1mm. .Nedunrrowu

 

 

 

 

E Number of Flower Buds

 

&

 

M
-

 

Number of Flower Buds
a

 

 

0 I I I .
10 12 14 16 18 20 22 24
Photoperiod

 

eoweekaooro -15weekecoldl

 

 

 

 

14o

 

15 Weeks of SC

 

 

Percent Flowering

 

 

 

0 I I . I I I I
10 12 14 16 18 2O 22 24
Photoperiod

 

“tomelebleBudaDmelow-r -PemFWl

 

 

 

Plant Node Development
15 Weeks of SC

    

Number of Nodes

 

Photoperiod

 

Inltidnodes Nodes-Mould mum
II: E] g!

 

 

 

 

 

F Plant Height at Flower

0
O

 

N
U!

N
O
I
I
1

Height (cm)
I]
E

1

 

 

 

15 I I I I . I
10 12 14 16 18 20 22 24
Photoperiod

 

[Ga-Oweekscold -15weekscold]

 

 

Figure 34. The effects of photoperiod and cold treatment on ﬂowering of

Oenothera missoun'ensis.

 

 

141

Table 24. The effects of photoperiod and cold treatment on ﬂowering of Oenothera missoun'ensis.

 

Days to Days from Days Increase Final plant

 

 

 

Weeks Percentage visible visible bud to in node height Flower
gf 5Q Pmariod ﬂowering bud to ﬂower ﬁmr ngmber (cm) numb9_r_
0 - 62 52 25 77 18 20 4.3
15 - 72 27 25 52 17 23 5.0
- 10 13 50 22 72 22 18 2.0
12 14 46 30 76 23 18 2.0
14 90 42 24 65 18 22 4.5
16 100 34 24 58 17 22 5.5
24 65 33 24 57 16 23 4.9
NF 95 36 25 61 17 22 4.6
0 10 0 -’ - - - - -
12 0 — - - - - -
14 90 60 24 84 19 20 4 5
16 100 44 25 69 17 20 4 9
24 30 65 26 91 19 20 3 0
NI 90 50 24 75 19 21 3 9
15 10 20 50 22 72 22 18 2 0
12 22 46 30 76 23 18 2 0
14 90 23 24 47 17 25 4 5
16 100 23 23 45 16 24 6 2
24 100 23 23 46 16 24 5.3
NL M 24 25 49 16 22 56
Signiﬁcance
Weeks cold (WC) “" Ns “* "’* *“
Photoperiod (P) *‘* .1... ”‘ m ***
WC x P * NS NS Ns " NS
95% Conﬁdence interval for NI
Zero weeks SC 9.8 2.3 10.3 2.2 1.5 0.8
15 weeks 50 2.6 2.6 3.6 2.2 1.5 2.2
Contrasts
Zero weeks 5C
NI vs. 16 NS NS NS NS NS NS
NI vs. 24 " NS * NS NS Ns
Pu... (14 to 24 h) NS NS Ns Ns Ns Ns
Poe-em: (14 to 24 h) '** Ns “ NS NS NS
15 weeks SC
NI vs. 16 Ns Ns NS NS NS Ns
NI vs. 24 NS NS NS Ns Ns Ns
PLheer (10 to 24 h) “" Ns *“ “" ““I “
EMHO to 24 h) *“ Ni “ " "* *

 

zNI = 4-h night interruption.
’- = No plants showed visible bud after 105 days of forcing.
"51"” "° Nonsigniﬁcant or signiﬁcant at Ps0.05, 0.01. or 0.001, respectively.

 

142

shorter, and had less than half as many ﬂowers as plants under other
photoperiods.

Phlox paniculata ‘Eva Cullum’. Cold treatment increased percentage of
ﬂowering and improved the uniformity of time to ﬂower (Figures 35 and 36, Table
25). For example, without cold treatment, 70% of plants under NI ﬂowered in an
average of 69 days with a 95% CI of :t 9 days. After cold treatment, all plants
ﬂowered under NI in an average of 73 days with a 95% CI of d: 3 days. Plants
treated with cold had many more ﬂowers and were more vigorous than non-cold-
treated plants, which may be partly due to higher light levels. Cold treatment
also increased plant height by approximately one-half and increased the number
of new nodes formed by four to six (eight to twelve leaves).

For plants that did not receive cold treatment, no plants ﬂowered under
photoperiods 513 hours and the percentage of ﬂowering increased from zero to
78 as the photoperiod increased from 14 to 24 hours. For cold-treated plants,
0%, 50%, or 100% of plants ﬂowered under 10-, 12-, or 213-hour photoperiods
or NI. For plants that received the cold treatment, days to ﬂower decreased
linearly from 88 to 61 and the number of new nodes formed decreased from 21
to 14 as the photoperiod increased from 12 to 24 hours. Plants under continual
light ﬂowered nonunifon'nly. Photoperiod did not affect days from visible bud to
ﬂower, ﬁnal plant height, or ﬂower number.

Phlox paniculata ‘Tenor’. Flowering of ‘Tenor’ was similar to ‘Eva
Cullum’. Cold treatment doubled the percentage of ﬂowering and improved time-

to-ﬂower uniformity (Figures 37 and 38, Table 26). For example, the 95% Cls of

 

 

 

 

A 0WeeksofSC

100 100%
so. ........... ——-80%E
eNI ‘
a so ———————————— "11.16016;
E40 __________________ 4oss~
zoL—n— —————————————— 20%;

0 0%

 

 

 

1012141618202224
Photoperiod

 

[imbmme-bm *Pmmrml

 

 

 

 

15 Weeks of SC

9!! 100%

 

Percent Flowering

 

[t Deysthcbl-Budt} Deystlewer -PercsrlFlow-ring

 

 

 

 

C Plant Node Development

   

OWeeks of5C
so , ﬁr "- 44‘
8 I
E 20 r 7777777
‘5
.310 E g; E g
z o g.‘ ::7 p—uakj
10 12 6 24 Ni

13 14 ’1’
Photoperiod

 

 

 

 

 

 

 

 

D Plant Node Development

15 Weeks ofSC

r W I

I .

ézoI. _ . - . - I

s I J

3 _ = I

z oI- m,,, EEEI
13 14 16 24 Nl

10 ' 12
Photoperiod

 

Earmarked-u ﬂNodIedbrcsld-Nodeeltlml

 

 

 

 

 

 

 

 

 

 

 

t (ii-Oweekscold +15weekseorJ]

 

 

F

Height (cm)
(a) A
O O

M
0

Plant Height at Flower

 

 

 

iNl‘

 

 

1O

12 14 16 18 20 22 24
Photoperiod

 

[:90weekscold 9.151119“:me

 

 

 

Figure 35. The effects of photoperiod and cold treatment on ﬂowering of Phlox

paniculata ‘Eva Cullum’.

 

 

 

144

Table 25. The effects of photoperiod and cold treatment on ﬂowering of Phlox paniculata ‘Eva Cullum’.

 

 

 

 

Days to Days from Days Increase Final plant
Weeks Percentage visible visible bud to in node height Flower
of SC Phgtgpgmﬁ' ﬂowgring bud to ﬂower ﬂgwgr number (cm) nummr
0 - 29 65 13 77 10 25 23
15 - 80 67 9 76 17 38 73
- 10 0 -‘ - - - - -
12 14 76 12 88 21 30 48
13 40 84 8 91 21 38 66
14 54 68 10 78 14 32 44
16 60 70 1 1 81 14 36 65
24 87 58 11 69 12 33 41
Nl’ 82 61 10 71 12 31 63
0 10 0 - - - - - -
12 0 — - - - - -
13 0 - - - - - -
14 14 76 12 88 9 23 18
16 33 82 14 96 9 22 17
24 78 64 13 77 10 27 22
Nl 70 56 12 69 10 23 26
15 10 0 - - — - - -
12 50 76 12 88 21 30 48
13 100 83 8 91 21 38 66
14 100 67 9 76 15 34 49
16 100 64 9 73 16 42 89
24 100 52 9 61 14 41 63
A 100 65 8 73 16 39 99
Signiﬁcance
Weeks cold (WC) NS “* "' *“ *“ *""
Photoperiod (P) *“ NS "“ ** NS NS
WC x P " NS * NS Ns NS
95% Conﬁdence interval for NI
Zero weeks 5C 8.6 2.4 8.8 0.7 4.0 11.0
15 weeks 50 2.2 0.9 3.0 1.5 3.7 29.8
Contrasts
Zero weeks 5C
NI vs. 16 ”* Ns *“ NS Ns NS
NI vs. 24 NS Ns NS NS NS NS
PUnec (14 to 24 h) Ns Ns Ns NS Ns NS
PM (14 to 24 h) NS NS NS NS NS NS
15 weeks SC
Nl vs. 16 NS NS NS Ns NS Ns
NI vs. 24 * Ns " Ns NS *
PLineer (12 to 24 h) “" Ns “* *" Ns Ns
Pgumuc (12 to 24 h) N N§ N§ N_S N§ Ns

 

A:
‘- = No plants showed visible bud after 105 days of forcing.

'NI = 4-h night interruption.

"s- '- ~. '“ Nonsigniﬁcant or signiﬁcant at Ps0.05, 0.01, or 0.001 , respectively.

 

 

 

145

 

 

 

 

 

 

 

 

 

 

 

120
110 _ ..................................................... O No cold treatment
O 15 weeks of 5 °C
100 _ ..................................................................... :1 .............
16 C
a 2 .................................................................. 1315 .............
3 90 14. 120 2"
.0
IL --
2 80 _ ..................................................................... =-. .............
9. 24. 16 111
re
0 7o 2 ............................................. NI ...., ...........................
60 — ................................................................ 24 ............ I
50 _ ...................................................................................
40 T I l I l
0 20 40 60 80 100 120

Percentage Flowering

Figure 36. Percentage ﬂowerin , time to ﬂower, and ﬂowering uniformity of Phlox
paniculata 'Eva Cullum’ under ifferent photoperiods with or without cold
treatment. Numbers next to symbols represent photoperiods consisting of nine-
hour natural days that were extended with incandescent lamps. NI = nine-hour
natural days with four hours of night interruption. Error bars are 95% conﬁdence
intervals. Symbols without error bars indicate that the conﬁdence intervals were
too large for the graph.

 

 

 

A 0 Weeks of 5c

 

  

Percent Flowering

 

 

 

L I 2

1O 12 14 16 18 20 22 24
Photoperiod

 

 

[*DqlbwuueBmB-Dqshm *PucentFlowsring]

 

 

 

c Plant Node Development

 
    

 

      

 

 

 

 

OWeeks ofSC
so 7, en‘s. ,- , e.
s I ‘
22o ,, IIIIIIIII
”6 I
3
z E . E 7, I
o "10' 1 14 ""6 I24
Photoperiod
[Elmiﬂnodu iNodu-tml
E Number of Flower Buds
120
§
E100
IL 80
15
E so
§ 1:
z 40

 

 

 

1O 12 14 16 18 20 22 24
Photoperiod

 

[ii-Omskscold +15weekseord]

 

 

 

 

146

 

B 15 Weeks of 5C

 

125 A3!!! 100%
100 L _ , ———————————————— 80% g
I:INI
g 75 >** - —————————— 1M 60% g
If IL
0 so I" __________________ >40°/0 ﬁg
25 _ _ —————————————————— 20% g
0 . 0%

 

 

 

1012141618 20 22 24
Photoperiod

 

*DeythlstbleaudQDaysleFlmr ﬁwml

 

 

 

 

D Plant Node Development
15 Weeks of SC

(41
O
' “I

  

Number of Nodes
3 8
I

     

° 10'

 

A I

14 16 ‘
Photoperiod

 

[Elniﬁalnodu Dmmw-Nodultllmji

 

 

 

 

F Plant Height at Flower
60

Height (cm)

 

 

 

 

Photoperiod

 

l-e-Oweeksoord -o-15 weeks coldJ

 

 

 

 

Figure 37. The effects of photoperiod and cold treatment on ﬂowering of Phlox

paniculata ‘Tenor’.

147

Table 26. The effects of photoperiod and cold treatment on ﬂowering of Phlox paniculata ‘Tenor’.

 

Days to Days from Days Increase Final plant

 

 

 

 

Weeks Percentage visible visible bud to in node height Flower
of 5C Phgjgmrig ﬂowgring big to ﬂmer ﬂgﬂr nqur (cm) ngmmr
0 - 38 65 15 80 12 34 56
15 - 75 74 12 86 16 46 96
- 10 5 91 16 107 20 32 47

12 6 106 11 117 21 48 43
13 70 87 12 99 18 43 99
14 74 70 14 84 15 44 96
16 75 66 15 80 14 40 72
24 85 64 14 77 14 41 64
Nl1 70 65 12 77 14 43 90
0 10 10 91 16 107 20 32 47
12 0 -’ - - — - --
13 40 83 14 97 17 37 79
14 44 65 14 78 12 30 58
16 60 58 16 74 11 33 60
24 70 65 15 80 12 36 43
Nl 40 52 14 66 10 33 51
15 10 0 - - - - - -
12 13 106 11 117 21 48 43
13 100 88 11 99 19 46 107
14 100 73 14 86 17 50 111
16 90 71 14 84 15 44 79
24 100 63 12 75 15 45 80
Nl 100 70 11 81 16 47 106
Signiﬁcance
Weeks cold (WC) *" “* *" *"* m ”"
Photoperiod (P) m ' *“ “" Ns "
WC x P ** Ns " NS NS NS

95% Conﬁdence interval for NI

Zero weeks 5C 8.7 0.8 8.7 1.6 10.5 30.2
15 weeks SC 3.1 1.0 2.8 1.0 3.1 23.3
Contrasts
Zero weeks 5C
NI vs. 16 NS ‘ NS NS NS Ns
NI vs. 24 Ns Ns NS NS NS NS
Pu... (10 to 24 h) **“ NS *** ”* Ns Ns
PM (10 to 24 h) *“' NS m ‘* Ns NS
15 weeks 5C
NI vs. 16 Ns ** NS Ns NS
NI vs. 24 " Ns Ns Ns NS "
Pu... (12 to 24 h) *“ NS “‘ *" Ns NS
Emma: (12 to 24 h) *"" & **" " NL N§

 

 

zNI = 4-h night interruption.
’— = No plants showed visible bud after 105 days of forcing.
”51“” Nonsigniﬁcant or signiﬁcant at Ps0.05, 0.01, or 0.001, respectively.

148

 

 

 

 

 

 

 

 

 

 

130
120 o No cold treatment
' """ 1'56 """"""""""""""""""""""""""""" O 15 weeks of5°C "I
110 - ....................................................................................
100 -_
100 _ ..............................................................................
'0') 13 13%
3
(I) 90 _ ...................................................................................
g " 16§ 1“
m 80 - ----------------------------------------- ~24 ---------------- Nl -------------
>. 140
(U 16. 24
D
70 - ...................................................................................
NI
so _ ........................................ T .........................................
5o _ .............................. .. ....................................................
40 l l l l l
O 20 4O 60 80 100 120

Percentage Flowering

Figure 38. Percentage ﬂowering, time to ﬂower, and ﬂowering uniformity of Phlox
paniculata 'Tenor’ under different photoperiods with or without cold treatment.
Numbers next to symbols represent photoperiods consisting of nine-hour natural
days that were extended with incandescent lamps. NI = nine-hour natural days
with four hours of night interruption. Error bars are 95% conﬁdence intervals.
Symbols without error bars indicate that the conﬁdence intervals were too large
for the graph.

 

 

149
days to visible bud and ﬂower for plants under NI were :I: 9 and :l: 3 without or

with cold treatment, respectively. Plants that received the cold treatment were
more vigorous, grew approximately four more nodes, averaged 12 cm taller, and
were more ﬂoriferous than plants that did not receive the cold treatment. Cold
treatment delayed ﬂowering by an average of six days, but the delay varied by
photoperiod.

Few plants ﬂowered under photoperiods 312 hours, so ‘Tenor' requires
short nights to ﬂower. For non-cold-treated plants, the percentage of ﬂowering
increased from zero to 70 as the photoperiod increased from 12 to 24 hours.
Nearly all cold-treated plants ﬂowered under photoperiods 213 hours. Days to
visible bud and ﬂower and the number of new nodes formed decreased at a
decreasing rate as the photoperiod increased. For example, for cold-treated
plants, time to ﬂower decreased from 117 to 75 days as the photoperiod
increased from 12 to 24 hours. There were no photoperiodic trends for plant
height or ﬂower number.

Rudbeckia fulgida ‘Goldsturm’. Rudbeckia ﬂowered very uniformly,
regardless of cold treatment (Figure 39, Table 27). However, cold treatment
shifted the minimum photoperiod required for 100% ﬂowering from 14 to 13
hours. Under photoperiods 214 hours or NI, cold treatment hastened time to
ﬂower by three to four weeks and plant height was reduced by an average of ﬁve
cm. Cold-treated plants developed fewer nodes, but the reduction varied by

photoperiod.

 

 

A OWeeksofSC
125 39910098
100—___ _ _ _-;é—D.N!ao%_e"
., 75———— ————————————— m5”;
550---- __._T,j_‘.s4o%u'
25 ———————————————————— 20%;
0 MIL

 

 

 

'1'

1O 12 14 16 18 20 22 24
Photoperiod

 

[* DIbeVhbleBudBDeysnFlew *Percemm

 

 

C Plant Node Development
0 Weeks of SC

A
O

 

8

Number of Nodes
8 B

O

 

 

 

Number of Flower Buds

0
O

 

.s
O

Number of Flower Buds ITI
8

 

 

 

0

1012141618 20 22 24
Photoperiod

 

 

 

l-S-Oweekscold o15wsekseord]

 

 

150

 

 

 

B 15 Weeks of 5c

125 Lk :95! 100%
100 _ _ , ————————————————— 80% 2’
£75-- WKHBDN' 60%;
u.
3 so — — ————————————————— N i 40% E
,_ I A §
25 ———————————————————— 20% g

 

 

 

1012141618 20 22 24
Photoperiod

 

IfDam-HIIIIII-BmuaIII-vulnFlmv-r opus-111M I

 

 

D Plant Node Development
15 Weeks of 5C

Number of Nodes

      

I , I E , VT;
13 4 16
Photoperiod

 

ifjrwm DNmmm-mum

 

 

F Plant Height at Flower

 

u
0

N
O

 

Height (crrr)

 

10

 

 

1012141618 20 22 24
Photoperiod

 

l-E-i-Oweekscold -15weekscﬂ

 

 

 

Figure 39. The effects of photoperiod and cold treatment on ﬂowering of

Rudbeckia fulgida ‘Goldsturm'.

 

 

1 51
Table 27. The effects of photoperiod and cold treatment on ﬂowering of Rudbeckia fulgida ‘Goldsturm’.

 

Days to Days from Days Increase Final plant

 

 

 

Weeks Percentage visible visible bud to in node height Flower
of 5C Phgtopgrig ﬂowering Lug to ﬂower ﬂower number (cm) numgr
0 - 57 64 36 100 18 35 17
15 - 81 45 34 79 15 30 17
- 10 0 -‘ - - - - -
12 35 84 27 1 1 1 28 20 2
13 50 44 31 75 19 32 17
14 100 53 34 87 16 35 19
16 100 51 38 89 15 31 19
24 100 46 36 82 13 35 16
Nl’ 100 56 36 92 15 30 20
0 10 0 - -- - - - -
12 0 - — - — - -
13 0 - - - - - -
14 100 67 34 101 21 38 18
16 100 64 39 103 18 32 18
24 100 55 38 93 17 38 14
Nl 100 70 35 105 18 33 19
15 10 0 — — — - - -
12 70 84 27 111 28 20 2
13 100 44 31 75 19 32 17
14 100 39 34 73 12 32 19
16 100 38 37 76 13 30 21
24 100 37 34 71 1 1 32 18
Nl 100 LZ 37 80 1 1 28 21
Signiﬁcance
Weeks cold (WC) *“ “ m *” *"* *“
Photoperiod (P) *“ *‘* “" *“ *** *"*
WC x P “" *“' * “ NS NS
95% Conﬁdence interval for NI
Zero weeks 50 2 0 1.2 1.5 0.7 1.8 2.1
15 weeks 50 2 4 1.5 3.0 2.0 1.5 3.1
Contrasts
Zero weeks 50
Nl vs. 16 **" **“ Ns Ns NS Ns
NI vs. 24 m " “* NS *“ **
Pu... (14 to 24 h) *" ** m ** NS *"
PM (14 to 24 h) NS *“ *" ** m NS
15 weeks 5C
NI vs. 16 ” Ns " * NS NS
NI vs. 24 **" m *“ NS **" NS
PM (12 to 24 h) '*"' m *" **" *"" “*

ngsdnucl12 39 24 h)

z-- = No plants showed visible bud after 105 days of forcing.
’Nl = 4~h night interruption.
"s- '- ~. °°' Nonsigniﬁcant or signiﬁcant at Ps0.05, 0.01, or 0.001, respectively.

 

 

152

Without the cold treatment, no plants ﬂowered under s13-hour
photoperiods and all plants ﬂowered under 214-hour photoperiods or NI. No
cold-treated plants ﬂowered under 10-hour photoperiods, 70% ﬂowered under
12-hour photoperiods, and all ﬂowered under 213-hour photoperiods or NI.
Plants that ﬂowered under 12 hours were delayed, had few ﬂowers, and were
shon.

Days to visible bud and ﬂower and the number of new nodes formed
decreased as the photoperiod increased from 14 to 24 hours in non-cold-treated
plants and from 12 to 24 hours in cold-treated plants. For example, the number
of new nodes formed decreased at a decreasing rate from 28 to 11 as the
photoperiod increased from 12 to 24 hours. With the exception of plants that
ﬂowered under 12-hour photoperiods, photoperiod did not dramatically affect
plant height or ﬂower number.

The critical daylengths for ﬂowering of several other Rudbeckia spp. have
been investigated (Kockankov and Chailakhyan, 1986). Without exception, all

are obligate long-day plants with minimal critical photoperiods ranging from 10 to

14.5 hours.

Other Responses

Asclepias tuberosa. When plugs were received, plants had already been
exposed to short days and induced into dormancy. Cold treatment was required

to overcome dormancy. However, plants ﬂower without a cold treatment if they

are never exposed to short days (Whitman, unpublished data).

 

 

153

No cold-treated plants ﬂowered under photoperiods 312 hours, and only
20% of the plants ﬂowered under 14 hours (Figure 40, Table 28). Several plants
under 14-hour photoperiods initiated ﬂower buds that later aborted, and these
were considered as nonﬂowering. Photoperiods 216 hours or NI induced
complete ﬂowering. Asclepias was only moderately uniform in time to ﬂower: the
95% CI of days to ﬂower for plants under NI was 17.5 days. Flower number
varied tremendously within each photoperiod.

Vemalized plants grown at 17l25 °C day/night under 4- or 8-hour NI
during the middle of 15-hour dark periods ﬂowered in 71 or 61 days, respectively
(Albrecht and Lehmann, 1991). No plants ﬂowered under 9-hour photoperiods.
In contrast to the above ﬁndings, Lyons (1986) labeled A. tuberosa as a day-
neutral plant with respect to ﬂowering, but noted that photoperiod inﬂuenced
vegetative and tuberous root development.

Hibiscus xhybrida 'Disco Belle Mixed’. Cold-treated plants died from
chilling injury. No plants ﬂowered under 10-hour photoperiods and all plants
ﬂowered under photoperiods 214 hours or NI (Figure 41, Table 29). As
photoperiod increased from 12 to 24 hours, days to ﬂower decreased from 127
to 85, the number of new nodes formed decreased from 18 to 11, and the
average ﬂower number increased from 5.6 to 12.6. Photoperiod had no effect on

ﬁnal plant height.

 

 

 

 

A
0 Weeks of 5C
100 100%
8’
75 — - - Plugswereexposedt — - 7593'
g short days prior to 50
3 5° __2 forcingandwerethus ' ' % “L
25 .2 _ _ induced into dormancy. ‘ _ 25%
III 0%

 

 

 

0
10 12 14 16 18 20 22 24
Photoperiod

 

I‘D-yobwueM-B-Deyebl’bw *Pmmm

 

 

 

 

 

 

C
Plant Node Development
0 Weeks of SC
80

£60 I' ~-— - Plugswereexposedto - — — »
g short days prior to
.6 4° ””” forcing and were thus ’ ‘ ‘ ‘
§ 20 _____ induced into dormancy. _ _ A
z

0

 

 

 

 

 

 

F Number of Flower Buds

 

 

 

 

 

 

 

 

§so
0‘)

Eco

u.40

‘5

320

E

3 0 I . I . I 47

1012141618 20 22 24
otoperiod

L I-E-Oweekscold -15weekscoldl

 

 

154

 

 

 

 

 

0 I I I
10 12 14 16 18 20 22 24
Photoperiod

B
15 Weeks of 5C
100 4:!!! 100%
75 _ _ — — ——————— mm 75% g
7% 50 ——————————— — — — 414175011. u.
0 a
25 ——————————————————— 25% g
D.
0%

 

ﬂ. DeythisﬂeBud announcer-r uo-Peruntnruerlng

=

 

 

 

Plant Node Development
15 Weeks of SC
so . 7 7

Number of Nodes
N h a:
O O o O
‘ I ;
I I I i
—I
- , I
I ‘ ‘
I I I
_I
I I
Z
I I #2-” 2

Photoperiod

 

[ENE-ind“ leoas-wwd-Noauuml

 

 

 

 

F Plant Height at Flower

A
U'l

 

b
O

 

Height (cm)
U
U!

 

 

A A. 4_2

N
O

 

10 12 14 16 18 20 22 24
Photoperiod

 

Isa-o weeks cold -15weeksﬂ

 

 

Figure 40. The effects of photoperiod on ﬂowering of cold-treated Asclepias

tuberosa.

 

 

 

1 55
Table 28. The effects of photoperiod on ﬂowering of cold-treated Asclepias tuberosa.

 

Days to Days from Days Increase Finai Writ

 

 

Weeks Percentage visible visible bud to in node height Flower
f 5 Ph n'od ﬂ en'n b d to ﬂower ﬂower num r (cm) nummr
O - 0 z z z z z z
15 - 52 49 24 73 68 4O 49
15 10 0 -’ - - - -- -
12 0 - — -- - - -
14 20 45 30 76 62 43 4
16 90 43 20 64 64 38 53
24 100 55 22 77 74 44 62
N_l" 100 5_2 22 74 72 35 76
Signiﬁcance
Photoperiod (P) NS ** NS NS * NS
95% Conﬁdence interval for NI
15 weeks 5C 7.5 1.1 7.5 13 4.3 29
Contrasts
15 weeks SC
NI vs. 16 NS NS NS NS NS NS
NI vs. 24 NS NS NS NS ‘* NS
Pm (14 to 24 h) NS *" NS NS NS NS
PM (1410 24 h) N§ *“ N_L Ls N_§ N§

 

zPlugs were exposed to short days prior to forcing and were thus induced into dormancy.
’- = No plants showed visible bud after 105 days of forcing.

‘Nl = 4-h night interruption.

”s- '- ”- '°' Nonsigniﬁcant or signiﬁcant at Ps0.05. 0.01, or 0.001, respectively.

 

 

A 0 Weeks of 5C
150 :95: 100%
120 _ _ _ ——————————————— 80% g
a so H — ——————— 13M 60% g
3 so I - — ———————————— A _N_l 40% E
30 ——————————————————— 20% g

 

 

4 L mu.
v m

0 r . V .
1O 12 14 16 18 20 22 24
Photoperiod

 

 

li—Deysbwwbaude- DeyetoFlom *Pemntka-mg

 

 

 

C Plant Node Development

25 i ﬂ 0 Weeks of 5C

  

Number of Nodes

 

 

E

.5
0|

 

.5
O

 

OI

 

Number of Flower Buds

 

0 . I I I I I
10 12 14 16 18 20 22 24
Photoperiod

 

 

.90 weeks cold I 15 weeks cold

 

 

 

 

156

 

 

'3 15 Weeks of 5c

150 100%
120 ———————————————————— 80% E
m 90 _ _ _ _All plugs died during_ , _ 30% E
3‘ cold treatment from "-
oso~___ .. .. ———40%°"
chrllrng injury. E
gar--- ~>~20°k8

0%

 

 

 

0 I I I I I I—I
10 12 14 16 1 8 20 22 24
Photoperiod

 

IfDIyobVleibleaudBDeystlow *Pereentm

 

 

 

 

 

 

 

 

 

 

 

 

 

D Plant Node Development
15 Weeks of 56
25
g 20 ————————————————————————
z 15 _____ All plugs died during ______
'3' 10 cold treatment from
g ————— chilling injury. _____
:2 5 ----------
Z
°1o‘12’1416‘24'NI'
Photoperiod
”Em-1mm Bum-wwlnmnm]
F Plant Height at Flower
so
E55
7%.
g so
45 . CJNI

 

 

10 12 14 16 18 20 22 24
Photoperiod

 

Hie-o weeks cold . 15 weeks coldJ

 

 

Figure 41. The effects of photoperiod on ﬂowering of non-cold treated Hibiscus

xhybrida ‘Disco Belle Mixed’.

 

1 57
Table 29. The effects of photoperiod on ﬂowering of Hibiscus xhybrida ‘Disco Belle Mixed’.

 

Days to Days from Days Increase Final plant

 

 

 

Weeks Percentage visible visible bud to in node height Flower
9159 Photomriﬂ ﬂgwgring bud to tiger ﬂower ngmber (cm) numbe_r_
0 - 77 53 43 96 13 50 9.3
15 - O z z z z z z
0 10 0 J -- - - -- -
12 60 83 45 127 18 52 5.6
14 100 53 42 95 13 51 9.4
16 100 44 41 85 1 1 48 8.5
24 100 40 45 85 1 1 55 12.6
N_I" 100 47 442 89 13 46 1L1
Signiﬁcance
Photoperiod (P) m * m “* NS *“
95% Conﬁdence interval for NI
0 weeks 5C 9.6 2.4 10.4 2.3 3.7 1.1
Contrasts
Zero weeks 50
Nl vs. 16 NS NS NS NS NS NS
NI vs. 24 NS * NS * ** "
Pu... (12 to 24 h) *“ NS “' m NS *“
P nucl12to 24 h) m ** m ‘” N_§ NS

 

___9&_

‘All plugs died during cold treatment from chilling injury.

’- = No plants showed visible bud after 105 days of forcing.

"Nl = 4-h night interruption.

"s- '- ”' '°' Nonsigniﬁcant or signiﬁcant at Ps0.05. 0.01, or 0.001 . respectively.

 

 

158

Conclusions
The cold treatment was required for or improved ﬂowering of all

herbaceous perennial species studied. Horticulturally, seven of the 25 plants
required a cold treatment for ﬂowering; no plants ﬂowered or ﬂowering was
erratic and sparse without a cold treatment. The cold treatment improved, but
was not required for, ﬂowering of sixteen plants by increasing the percentage of
ﬂowering, hastening ﬂowering, improving uniformity, and/or increasing ﬂower
number.

Photoperiod did not affect the percentage of ﬂowering, time to ﬂower, or
ﬂower number of seven species studied, which were thus deﬁned as day-neutral.
The remaining eighteen plants were long-day plants. Seven species ﬂowered as
facultative long-day plants and eleven species required long days for ﬂowering.

Table 30 provides the photoperiods that induced the most complete, rapid, and

uniform ﬂowering of the long-day herbaceous perennials studied.

159

Table 30. The recommended photoperiods for the most complete, rapid, and
uniform ﬂowering of cold-treated long-day herbaceous perennial plants.

 

 

Species Photoperiod
Asclepias tuberosa 216 or NIz
Campanula carpatica ‘Blue Clips’ 16 or NI
Coreopsis grandiﬂora ‘Sunray’ 214 or NI
Coreopsis verticillata ‘Moonbeam’ 216 or NI
Echinacea purpurea ‘Bravado’ 14
Gail/ardia xgrandiﬂora ‘Goblin’ 24
Gypsophila paniculata ‘Double Snowﬂake’ 24
Helenium autumnale 24
Hibiscus xhybrida ‘Disco Belle Mixed’ 216
Leucanthemum xsuperbum ‘Snow Cap’ 24
Leucanthemum xsuperbum ‘White Knight’ 24
Lobelia xspeciosa “Compliment Scarlet’ 214 or NI
Oenothera missoun'ensis 214 or NI
Phlox paniculata ‘Eva Cullum’ 16 or NI
Phlox paniculata ‘Tenor’ 24
Physostegia virginiana ‘Alba’ 24 or NI
Rudbeckia fulgida ‘Goldsturm’ 213 or NI
Salvia x superba ‘Blue Queen’ 216 or NI

 

zNI = four-hour night interruption.

160

References

Albrecht, ML. and J.T. Lehmann. 1991. Daylength, cold storage, and plant-
production method inﬂuence growth and ﬂowering of Asclepias tuberosa.

HortScience 26(2): 1 20-1 21 .

Beattie, D.J., C.F. Deneke, E.J. Holcomb, and J.W. White. 1989. The effects of
photoperiod and temperature on ﬂowering of Physostegia virginiana
‘Summer Snow' and ‘Vivid’ as potted plants. Acta Hort. 252:227-233.

Christensen, O.V., L.N. Kristensen, K. Friis, and O. Bovre. 1989. Propagation
and forcing of Armen'a man'tima as a pot plant. Danish J. Plant Soil Sci.

93:239-242.

Damann, MP. and RE. Lyons. 1995. Juvenility and photoperiodic ﬂowering
requirements of Chrysanthemum xsuperbum ‘G. Marconi’ and ‘Snow
Lady’ grown under short— and long-day conditions. J. Amer. Soc. Hort. Sci.

120(2):241-245.

 

Damann, MP. and RE. Lyons. 1993. Juvenility, ﬂowering, and the effects of a
limited inductive photoperiod in Coreopsis grandiﬂora and C. Ianceolata.

J. Amer. Soc. Hort. Sci. 118(4):513-518.

Engle, BE. 1994. Use of light and temperature for hardening of herbaceous
perennial plugs prior to storage at -2.SC. MS Thesis, Michigan State

University, East Lansing

Evans, MR. and RE. Lyons. 1988. Photoperiodic and gibberellin-induced
growth and ﬂowering responses of Gaillardia xgrandiﬂora. HortScience

23(3):584-586.

Faust, J.E. and RD. Heins. 1994. Determining the effect of cloud conditions on
the variation in daylength perceived by plants. HortScience 29(5):504.

Gardner, F.P. and RD. Barnett. 1990. Vemalization of wheat cultivars and a
triticale. Crop Sci. 30:166-169.

Grifﬁn, G.W. and W.J. Carpenter. 1964. Photoperiodic response of shasta daisy
clones Esther Read and T. E. Killian. Proc. Amer. Soc. Hort. Sci. 85:591-

593.

lversen, RR. and TC. Weiler. 1994. Strategies to force ﬂowering of six
herbaceous garden perennials. HortTechnology 4(1):61-65.

 

161

Ketellapper, H.J. and A. Barbaro. 1966. The role of photoperiod, vemalization
and gibberellic acid in ﬂoral induction in Coreopsis grandiﬂora Nutt.

Phyton 23(1):33-41.

Kochankov, V.G. and MK. Chailakhyan. 1986. Rudbeckia, p. 295-320. In: A.H.
Halevy (ed.). Handbook of ﬂowering, vol. V. CRC Press, Boca Raton,

Florida.

Kusey, W.E., T.C. Weiler, and PA. Hammer. 1981. Seasonal and chemical
inﬂuences on the ﬂowering of Gypsophila paniculata ‘Bristol Fairy’
selections. J. Amer. Soc. Hort. Sci. 106(1):84-88.

Lang, A. 1965. Physiology of ﬂower initiation, p.1380-1535. In: W. Ruhland (ed.).
Encyclopedia of plant physiology. Springer-Verlag, Berlin.

Lyons, RE. 1986. Asclepias tuberosa, p. 22-28. In: A.H. Halevy (ed.). Handbook
of ﬂowering, vol. V. CRC Press, Boca Raton, Florida.

Moe, R. 1988. Flowering physiology of Gypsophila. Acta Hort. 218:153—158.

Rhodus, T. And J. Hoskins. 1995. Views on management. Perennial Plants:
Quarterly Journal of the Perennial Plant Association 3(4):28-38.

Shedron, K.G. and TC. Weiler. 1982. Regulation of growth and ﬂowering in
Chrysanthemum x superbum Bergmans. J. Amer. Soc. Hort. Sci.

1 07(5):874-877.

Song, C.Y., M.S. Roh, S.K. Chung, and RH. Lawson. 1993. Effect of
temperature and light on growth and ﬂowering of potted plant production

of Platycodon. J. Korean Soc. Hort. Sci. 34(6):446-453.

Vince-Prue, D. 1975. Photoperiodism in plants. McGraw, London.

Vince-Prue, D. 1984. Light and the ﬂowering process—setting the scene, p. 3-15.
In: D. \ﬁnce-Prue, B. Thomas, and K.E. Cockshull (eds.). Light and the

ﬂowering process. Academic Press, San Diego.

Waithaka, K. and L.W. Wanjao. 1982. The effect of duration of cold treatment on
growth and ﬂowering of Liatn's. Scanty Hort. 18:153-158.

Whitman, CM. 1995. Inﬂuence of photoperiod and temperature on ﬂowering of
Campanula carpatica ‘Blue Clips; Coreopsis grandiﬂora ‘Early Sunrise’,
Coreopsis verticillata ‘Moonbeam’, Rudbeckia fulgida ‘Goldsturm’ and
Lavandula angustifolia ‘Munstead’. MS Thesis, Michigan State University,

East Lansing.

 

 

162

Yuan, M. 1995. Effect of juvenility, temperature and cultural practices on
ﬂowering of Coreopsis, Gail/ardia, Heuchera, Leucanthemum, and
Rudbeckia. MS Thesis, Michigan State University, East Lansing.

Zeevaart, J.A.D. 1978. Phytohorrnones and ﬂower formation. P. 291-327. In:
Letham, Goodwin, and Higgins (eds.). Phytohormones and related
compounds—a comprehensive treatise, vol. II. Elsevier/North-Holland
Biomedical Press.

Zhang, 0., AM. Armitage, J.M. Affolter, and MA. Dirr. 1995. Environmental
control of ﬂowering and growth of Lysimachia congestiﬂora Hemsl.
HortScience 30(1):62-64.

 

 

SECTION III

EFFECT OF NIGHT INTERRUPTION DURATION
AND CYCLIC LIGHTING ON FLOWERING
OF LONG-DAY HERBACEOUS PERENNIAL PLANTS

1 64
Introduction

In the greenhouse industry, the photoperiod often is lengthened artiﬁcially
to keep plants vegetative or to induce ﬂowering. For most photoperiodic plants,
the duration of the perceived uninterrupted dark period determines whether plant
growth is vegetative or reproductive. Under natural short days (SD), long days
(LD) are created by lighting during natural dark periods. Traditionally, a four-
hour night interruption (NI) (e.g., from 2200 to 0200 HR) has been the most
popular method of delivering LD.

The effectiveness of NI lighting primarily depends on timing, duration and
intensity. For most plants, lighting during the middle of the dark period most
effectively breaks up the long, dark period (Vince-Prue and Canham, 1983). To
interrupt the dark period satisfactorily, long-day plants (LDP) often require longer
durations and/or higher intensities of light for promotion of ﬂowering than short-
day plants (SDP) require for inhibition of ﬂowering (Vince-Prue, 1975).

Many LDP show a quantitative response to the duration and intensity of
the night-break exposure (Vince-Prue and Canham, 1983). For example,
Trachelium caeruleum L. showed a quantitative relationship between duration
and intensity of NI and the magnitude of the ﬂowering response (Shillo, in press).
However, Kadman-Zahavi (in press) found that for most SDP and LDP studied,
NI of 15 min, 2, 4, or 10 hours were equally effective when provided at the same
total light ﬂuence. The light intensity required for effective NI may change during

the year. To keep the SDP Chrysanthemum xmon'folium vegetative, Sachs et al.

 

165
(1980) found that plants grown during a period of high daytime irradiance (e.g.,

in July) required a greater intensity of NI lighting than plants grown during
periods with a lower daytime irradiance (e.g., in January).

In contrast to continual NI lighting, cyclic, or intermittent, lighting is a
strategy in which lamps are cycled, or ﬂashed on, so that light and dark cycles
are provided throughout the usual lighting period. The primary advantage of
cyclic lighting is a savings of 60 to 80% in energy consumption compared to
continual NI lighting (Bickford and Dunn, 1972; Canham, 1966). Cyclic lighting
regimes have varied; lights may be on for 2 to 50% of the time for part or all of
the dark period (Bickford and Dunn, 1972; Vince-Prue and Canham, 1983).

The efﬁcacy of cyclic lighting at promoting or inhibiting ﬂowering depends
on the plant and the duration, frequency, and intensity of light. Cyclic lighting is
frequently used to maintain vegetative growth in some SDP, such as
Chrysanthemum spp. However, the effectiveness of cyclic lighting at initiating
ﬂowering in LDP has been investigated in only a few species and has been
found to vary considerably.

In the LDP baby’s breath (Gypsophila paniculata L. ‘Bristol Fairy’), cyclic
lighting of 5 min light and 10 min dark (33% cyclic) for four hours with
incandescent lamps, which provided 2 umol-m'z-s", induced ﬂowering similarly to
continual four-hour NI (Shillo and Halevy, 1982). In another study, the LDP
sweet clover (Melilotus alba Desr.) was provided with ﬁve different 10% cyclic
lighting treatments during 16-hour dark periods with incandescent lamps, which

provided 8.6 umol-m'Z-s”, (Kasperbauer et al., 1963). Plants provided with

 

 

 

, in ‘

ET”—

166

shorter (1.5 min) but more frequent (every 15 min) cyclic lighting cycles ﬂowered
similarly to plants under continual light. The ﬂowering response decreased as
both the duration of light during each cycle increased and frequency decreased.
Kasperbauer et al. (1963) found that days to ﬂower decreased and ﬂower
number increased as the cyclic lighting intensity increased from 1 to 17

umol-m'Z-s“.

In two cultivars of the obligate LDP China aster (Callistephus chinensis
Nees), very brief and infrequent cyclic lighting cycles (one minute every hour
during 16-hour dark periods) induced ﬂowering faster than plants under one hour
of continual NI, but not as rapidly as plants under continual light (Cockshull and
Hughes, 1969). Cyclic lighting also hastened ﬂowering in the facultative LDP
snapdragon (Antinhinum majus L.); plants provided with at least 10 seconds of
light per minute for four hours ﬂowered simultaneously to those provided with a
continual four-hour Nl (Maginnes and Langhans, 1967).

The effectiveness of short durations (<4 hours) of NI lighting has been
studied in a few LDP. In sweetclover, days to ﬁrst ﬂower decreased at a
decreasing rate from >60 days to 32 days and average ﬂower number increased
as the NI duration increased from 2 to 16 hours (Kasperbauer et al., 1963). In
the LDP carnation (Dianthus caryophyllus L.), plants were provided with 0.5-hour
or two-hour NI treatments during the middle of 16-hour dark periods with
incandescent lamps which emitted 7.6 umOI'I'I'I'z'S'1 (Harris, 1969). The 0.5-hour

NI did not promote ﬂowering but the two-hour NI was sufﬁcient to produce a

 

167

long-day ﬂowering response. Shillo (in press) reported that short durations (not
speciﬁed) of NI induced ﬂowering in butterﬂy weed (Asclepias tuberosa L.).

The objective of this experiment was to determine the effectiveness of
various durations of NI or cyclic lighting at initiating ﬂowering in six species of

long-day herbaceous perennial species.

Materials and Methods

Plant material. The species studied, plug size, and age of plant material
are provided in Table 31. The experiment was replicated in time with
Experiments I and II beginning on 20 December, 1995 and 16 February, 1996,
respectively. Plants were grown under natural short-day photoperiods ($11.5
hours of light) until the beginning of each experiment.

Plant culture. Plants were grown in a commercial soilless medium
composed of composted pine bark, horticultural vermiculite, Canadian sphagnum
peat moss, processed bark ash, and washed sand (MetroMix 510, Scotts-Sierra
Horticultural Products Company, Marysville, Ohio). Plants were top-watered with
well water acidiﬁed (two parts H3P04 plus one part H2804, which provided 22.5
mol P-m'a) to a titratable alkalinity of approximately 130 mg calcium bicarbonate
per liter and fertilized with 14N-0P-6K20 (mol-m'3) from potassium nitrate (14N-
0P-55K20) (Vicksburg Chemical Co., Vicksburg, MS) and ammonium nitrate
(34N-0P-0K20) (Cargill, Lexington, KY). Fertilization and acidiﬁcation rates were
adjusted in response to weekly soil test results, so regimes varied during

experiments. High-pressure sodium lamps provided a PPF of approximately 50

 

168

 

 

688.6: 888 oz.

.8 v .8 £82688. .88 N new O .8 to use :0 8:94..
.2 v .8 532888. .88 em new. m .8 to new no 85m:>
802.8898 502.0823

532888. ._8 mm .o .8 .9 8a 888 __8-Om .0 .AK £8 .6 0820?
.93 8 058808 28886 one Go em 8 9.858 8.88888 6.08.8828 .888 u o
.00 9 8 38808. 8.88888 8:888 :83 .383 92 8mg :8: 0.. mp 80 8.88888 8:888 8.2.8808 888. u a
do 2. 8 NP 32.8 8.88888 8 83898 9. .mzﬁcoSmU 888. u 8
89.8.8 88» .5 68880.8 83 £285 ..8m8coo_2. .8 88.8 .38 .3 68880... 88> 8:98 =84.N

 

Odw
QON
NOV
N.ON
N.ON
0.0_.
MON
0.0N
VON
0.0N
m.ON

VON
NON
NON
.mON
MON
VON
m.ON
VON
MON
VON
m.ON

adv
VON
Odw
N.ON
m.ON
0.0_.
VON
VON
VON
VON
YON

MON
VON
VON
VON
mON
NON
m.ON
VON
OON
VON
OON

0.0N
«ON
_..ON
NON
«ON
_..ON
MON
N.ON
VON

VON

NON
NON
OON
EON
OON
EON
OON
NON

_..ON
VON

a

N
P
N
r
_.
N
_.
N
_.
N

_.

O. 3
O._._.
OO
Of
3

v.0
NO
WON
0.0_.

on

ONP
me

ON

on

ON

a

O

mQEO .EBmEOO. .g< 8898 88068881

OQOQOr .0822 8.6m coma. mural—«x @3088:
35.3 .8828? 5:082 8.:qu emceeEom

mm? Em .Emoncooz. 3. 8889...? 88809.00

.885 2.8..
8.86 $on .6 moo: 88.888. 888.00

35m .85 cam. .38. 8.888 88.qu8

 

.88 82 c v e N f 5 no 8m .58. 88: 8% 2655.38 28

 

2.96

 

82.82.25 292

 

Gov 9.88 988. 888888 owm.m><

.88. 3..

888m

 

ecozmmmaoi

 

62.8888 some .88: 868» some .8 988.6:

.0 88. 888m 8 9.88 .o 883 88. 8.88888 .8 888m ocm ._m_.8m8 9.88m 80 8880882.... .3628 «808m .3 038.

169

Mmol-m'z-s‘1 at plant level when the ambient greenhouse PPF was lower than
400 umol-m'z-s".

Cold treatments. R. fulgida ‘Goldsturm’ that averaged z11 nodes (leaves)
received either no cold treatment (Experiment I) or were placed in a controlled-
environment chamber for 8 weeks at 5 °C (Experiment II). The chamber was lit
from 0800 to 1700 HR at approximately 10 umol-m"’-s‘1 from cool-white
ﬂuorescent lamps (VHOF96T12; Philips, Bloomﬁeld, N.J.), as measured by a Ll-
COR quantum sensor (model Ll-189; LI-COR, lnc., Lincoln, NE). No other
species received a cold treatment.

Light treatments. Seventy plants of each species were removed from their
containers, singulated, and transplanted into 13-cm square containers (1.1
liters). Ten plants were placed under each treatment that was assigned
randomly to benches in the greenhouse. Black cloth was pulled at 1700 HR and
opened at 0800 HR every day on all benches to provide similar daily light
integrals. During the middle of the dark period, benches were lighted with
incandescent lamps at 1 to 3 pmol-m'z-s‘1 for the following durations: O, 0.5, 1, 2,
or 4 hours, 6 min on, 54 min off for four hours (10% cyclic lighting), or 6 min on,
24 min off for four hours (20% cyclic lighting).

Greenhouse temperature control. All plants were grown in a glass
greenhouse set at 20 °C. Air temperatures on each bench were monitored with
36-gauge (0.013—mm—diameter) type 8 thermocouples connected to a CR10
datalogger (Campbell Scientiﬁc, Logan, UT). To provide uniform temperatures,

the datalogger controlled a 1500-watt electric heater under each bench, which

 

170
provided supplemental heat as needed throughout the night. The datalogger

collected temperature data every 10 seconds and recorded the hourly average.
Actual average daily air temperatures from the beginning of forcing to the
average date of ﬂowering under every photoperiod were calculated for each
species and are presented in Table 31.

Data collection and analysis. The leaves of each plant were counted at
the onset of forcing. Date of the ﬁrst visible bud or inﬂorescence and date of
opening of the ﬁrst ﬂower were recorded for each plant. At ﬂowering, the number
of visible ﬂower buds or inﬂorescences, the number of leaves on the main stem
below the ﬁrst ﬂower, and total plant height were determined. Plants that did not
have visible buds or inﬂorescences after 15 weeks of forcing were discarded and
considered nonﬂowering. Days to visible bud, days from visible bud to ﬂower,
days to ﬂower, and increase in node count were calculated.

For each species, I used a randomized complete block design in which
blocks were light treatments with ten observations for each treatment and
experiment. Data were analyzed using SAS’s (SAS Institute, Cary, NC) analysis

of variance and general linear models procedures.

Results and Discussion
Campanula carpetice ‘Blue Clips’

Experiment 1. No plants ﬂowered with $0.5 hours of NI. Flowering was
similar with 2 or 4 hours of NI or the 20% cyclic lighting treatment: plants

ﬂowered in 49 to 59 days, developed 17 to 20 nodes, averaged 17 or 18 cm tall,

171

and had an average of 38 or 39 ﬂowers (Figure 42, Table 32). Flowering was
most uniform for plants under four hours of NI. For plants under one hour of NI
or 10% cyclic lighting, ﬂowering was incomplete, non-uniform, and delayed by 20
to 50 days. Nl treatment did not affect days from visible bud to ﬂower

Experiment 2. For plants provided with 2 or 4 hours of NI or 20% cyclic
lighting, time to ﬂower, ﬂower number, and the number of new nodes formed
were similar to those in Experiment 1. Although plants were more mature, they
had approximately half the number of ﬂowers as plants in Experiment 1. Few or
no plants ﬂowered with one hour of NI or 10% cyclic lighting.
Coreopsis grandiﬂora ‘Early Sunrise’

Experiment 1. No plants ﬂowered without Nl and all plants ﬂowered with
20.5 hours of NI or 10 or 20% cyclic lighting (Figure 43, Table 33). Flowering
was delayed under 0.5 hours of NI or 10 or 20% cyclic lighting compared to
plants under four-hour NI. Plant height increased from 21 to 31 cm as the
duration of NI increased. Plants under 0.5 hours of NI had the fewest ﬂowers.

Experiment 2. All plants that received NI ﬂowered. Plants ﬂowered more
uniformly and about ten days faster than plants in Experiment 1, which may be at
least partially explained by the use of more developed (by z two nodes, or four
leaves) plants. Under 0.5 hours of NI or 10 or 20% cyclic lighting, days to visible
bud and ﬂower were delayed compared to those under four-hour Nl. Flower
number was reduced (by ﬁve to seven) and days from visible bud to ﬂower was

greatest for plants under 0.5 hours or 10% cyclic lighting.

 

172

 

 

 

 

 

 

 

 

 

 

 

A B
+ Experiment1 “r
120 - ............... . + Experiment2 .( ......................................... _ 120
'c 1'
3 5
(mac - ---------------------------------------- 4 ----------------------------------------- . 1000)
o 3
3 2
.a u-
5 80 - ~ 80 g
o m
‘5; 8‘
(>360 - _ 60 D
D
40 - - 40
4o - - 20
8
“g’ 8
o O
E 30 ~ - 15::
.c
"5 .9:
I... a)
a)
‘13 20 _ ~10;
: 5!
Z n.
10 - _ 5
l I A l l T

 

 

 

 

O 1 2 3 4 6/60 6/30 0 1 2 3 4 6l60 6/30

 

 

 

Hours of Cyclic Hours of Cyclic
Night Interruption Night Interruption

Figure 42. Flowering of Campanula carpatica 'Blue Clips' under various durations
of night interruption or cyclic lighting. Night interruption was rovided by
incandescent lamps that were turned on during the middle 0 15-hour dark
periods. For the cyclic lighting treatments, lights were on for six minutes every 30
or 60 minutes for a four hour period during the middle of the night. Error bars are
95% conﬁdence intervals.

 

 

173

Table 32. The effects of night-interruption duration and cyclic lighting on ﬂowering of Campanula
carpatica ‘Blue Clips'.

 

 

 

 

ans to Days from Increase Final plant
Night Flowering visible visible bud Days to in node height Flower
interruption‘ (%) bud t_o flower_ flower number (cm) number
Experiment 1
0 h 0 -’ - - - - -
0.5 h 0 - - - - - -
1h 70 61bx 18a 79b 27a 14a 14a
2h 100 39c 20a 59c 20b 17b 38b
4h 100 30c 183 49c 17b 18b 39b
10% cyclic" 40 76 a 17 a 93 a 29 a 13 a 9 a
20% cyclic” 100 40 c 20 a 59 c 20 b 18 b 39 b
Experiment 2
0 h 0 — - - — - -
0.5 h 0 - - — - - -
1 h 0 - — - - - -
2h 83 38a 19a 57a 20a 16a 20a
4h 100 34a 19a 52a 16a 14a 18 ab
10% cyclic 17 44a 19a 63a 17a 6b 1b
20% gclic 89 39 a 20 a 59 a 19 a 14 ar 15 L

 

 

z9-h natural days with night-interruption lighting during the middle of the dark period.
’— = no plants showed visible bud after 105 days of forcing.
"Mean separation within each photoperiod by Duncan’s multiple range test (P = 0.05).

wLights on and off for 6 and 54 min, respectively, for 4 h.
"Lights on and off for 6 and 24 min, respectively, for 4 h.

 

 

174

 

 

 

 

 

 

    

 

A B
+ Experiment1
90 ., ................ . _

—A— Experiment2 90

g 80 ------------------------------------------ i - 80
m 'a‘:
1%} ~ 70 E)
.7, u.
5 ~ 60 2
9 a
m — 50 to
5‘ D

D — 40

- 30

- 35
g A
5 £3.
E - 30 ._,
.c
“6 .9
as :“c’
.0 — 25 id
§ .5.
2 CL

l- 20

 

 

 

 

O 1 2 3 4 6/60 6/30 0 1 2 3 4 6/60 6/30

 

 

 

Hours of Cyclic Hours of Cyclic
Night Interruption Night Interruption

Figure 43. Flowering of Coreopsis grandiﬂora 'Early Sunrise' under various
durations of night interruption or cyclic lighting. Night interruption was provided
by incandescent lamps that were turned on during the middle of 15-hour dark
periods. For the cyclic lighting treatments, lights were on for six minutes every 30
or 60 minutes for a four hour period during the middle of the night. Error bars are

95% conﬁdence intervals.

 

175

Table 33. The effects of night-interruption duration and cyclic lighting on ﬂowering of Coreopsis
grandiﬂom “Early Sunrise'.

 

 

 

 

Days to Days from Increase Final plant
Night Flowering visible visible bud Days to in node height Flower
interruption‘ (%) bud to ﬂow ﬂower number (cm) number
Experiment 1
0 h 0 —’ - - - — -
0.5 h 100 53 a" 31 a 83 a 8 b 21 c 6 b
1h 100 40cd 25b 65cd 9a 26b 9a
2h 100 39cd 25b 640d 9ab 29ab 9a
4h 100 36d 25b 61d 8ab 31a 11a
10% cyclicw 100 46 b 28 b 74 b 9 ab 27 b 10 a
20% cyclic" 100 43 bc 26 b 69 bc 8 ab 30 a 10 a
Experiment 2
0 h 0 - - - - - -
0.5h 100 40a 29a 70a 93 23 cb 12b
1h 100 32c 26b 58c 8b 24 ab 17a
2h 100 30cd 25b 55cd 8b 22bc 17a
4h 100 29d 24b 53d 8b 25a 18a
10% cyclic 100 35 b 28 a 63 b 8 ab 21 c 11 b
20% cyclic 100 31 cd 26 b 56 c 8 Q 23 bc 17 a

 

 

z9-h natural days with night-interruption lighting during the middle of the dark period.
’- = no plants showed visible bud after 105 days of forcing.

"Mean separation within each photoperiod by Duncan’s multiple range test (P = 0.05).
"Lights on and off for 6 and 54 min, respectively, for 4 h.
"Lights on and off for 6 and 24 min, respectively, for 4 h.

176

Coreopsis verticillata “Moon beam’

Experiment 1. No plants ﬂowered without Nl. Percentage of ﬂowering
increased from 40 to 100%, days to ﬂower decreased from 114 to 68, and plant
height increased from 31 to 77 cm as the duration of NI increased from 0.5 to 4
hours (Figure 44, Table 34). Furthermore, the 95% CI of time to ﬂower
decreased dramatically as the NI duration increased. All ﬂowering
characteristics measured under 20% cyclic lighting were similar to those under
four-hour NI. Flowering under 10% cyclic lighting was incomplete and delayed
by approximately four weeks compared to plants under four-hour NI. Plants
under two hours of NI were delayed by approximately 17 days compared to

plants under four hours of NI.

Experiment 2. No plants ﬂowered without Nl. All plants ﬂowered under
21 hour of NI or either cyclic lighting treatment. Nl duration did not inﬂuence
days to ﬂower as it did in Experiment 1. Flower number increased from 27 to 63
as the NI duration increased from 0.5 to 4 hours. Plants were shortest under 0.5
hours of NI or 10% cyclic lighting. Flowering under 20% cyclic lighting was
similar to plants under four hours of NI.

Echinacea purpurea ‘Bravado’

Plant mortality was excessively high in Experiment 2, so only results of
Experiment 1 are presented. No plants ﬂowered without and essentially all
ﬂowered with NI (Figure 45, Table 35). All NI durations and cyclic lighting
regimes induced plants to ﬂower at approximately the same time. Plant height

increased from 41 to 61 cm as the NI duration increased from 0.5 to 4 hours.

 

177

 

 

+ Experiment1
120 - .................. —A— Experiment2 .. .. ....................................... _ 120

 

 

 

   

L-

b 100

Visible Bud
8
O
we

r
on
O

on
0
Days to Flo

 

Days to
I»
O

 

 

 

I
A
O

 

+45

L40

00
UI
Plant Height (cm)

Number of Flowers

 

 

r
0.)
O

 

 

 

 

 

l l l I I I

0 1 2 3 4 6l60 6/30 0 1 2 3 4 6/60 6/30

 

 

Hours of Cyclic Hours of Cyclic
Night lntenuption Night Interruption

Figure 44. Flowering of Coreopsis verticillata 'Moonbeam' under various durations
of night interruption or cyclic lighting. Night interruption was rovided by
incandescent lamps that were turned on during the middle 0 15-hour dark
periods. For the cyclic lighting treatments, lights were on for six minutes every 30
or 60 minutes for a four hour period during the middle of the night. Error bars are
95% conﬁdence intervals.

178

Table 34. The effects of night-interruption duration and cyclic lighting on ﬂowering of Coreopsis
verticillata ‘Moonbeam’.

 

 

 

 

Days to Days from Increase Final plant
Night Flowering visible visible bud Days to in node height Flower
interruption‘ l%) M tp ﬂower ﬂower number (cm) number
Experiment 1
0 h 0 -’ — - - - -
0.5h 40 84a" 30a 114a 6b 33c 31d
1h 90 68b 293 97b 6a 40ab 58bc
2h 100 56bc 29a 85bc 6a 45a 6Sabc
4h 100 40d 29a 68d 7a 45a 77a
10% cyclic" 80 68 b 28 a 96 b 6 a 36 bc 48 c
20% cyclic" 100 46 ed 30 a 76 ed 6 a 43 a 68 ab
Experiment 2
0 h 0 - - - - - -—
0.5 h 80 39 bc 29 c 68 b 5 a 30 b 27 c
1h 100 50a 30bc 80a 6a 40a 48 ab
2h 100 33c 30bc 63b 6a 42a 56a
4h 100 33c 32a 66b 6a 42a 63a
10% cyclic 100 42 b 30 bc 72 b 6 a 40 a 39 bc
20% cyclic 100 36 be 32 ab 66 p 6 a 42 a 56 a

 

 

§-h natural days with night-interruption lighting during the middle of the dark period.
’— = no plants showed visible bud after 105 days of forcing.

‘Mean separation within each photoperiod by Duncan’s multiple range test (P = 0.05).

"Lights on and off for 6 and 54 min, respectively, for 4 h.
“Lights on and off for 6 and 24 min, respectively, for 4 h.

 

 

179

 

   

 

 

 

 

 

A B

110- - 110
3100 100
m b if:
Q 3
a so- .905
S .9
o _
.5; 80 -80 g
> 8
(U
Q 70 -. ~70

60- —60

P60
§ A
.3 5
LI... ”504-
“5 a
.. '5
o
D ~40£
E c
3 E
z 0.
~30

 

 

 

0 1 2 3 4 6/60 6/30 0 1 2 3 4 6/60 6/30

 

 

Hours of Cyclic Hours of Cyclic
Night Interruption Night Interruption

Figure 45. Flowering of Echinacea purpurea 'Bravado' under various durations of
night interruption or cyclic lighting. Night interruption was provided by
incandescent lamps that were turned on during the middle of 15-hour dark
periods. For the cyclic lighting treatments, lights were on for six minutes every 30
or 60 minutes for a four hour period during the middle of the night. Error bars are
95% conﬁdence intervals.

180

Table 35. The effects of night-interruption duration and cyclic lighting on ﬂowering of Echinacea
purpurea ‘Bravado’.

 

 

Days to Days from Increase Final plant
Night Flowering visible visible bud Days to in node height Flower
interruption‘ Clo) bud to ﬂower ﬂower number (cm) number
0 h 0 J - - - — -
0.5h 100 76 a‘ 26b 101 ab 17a 41c 6a
1h 100 66b 30 ab 97 ab 14b 53b 6ab
2 h 90 65 b 29 ab 94 b 14 b 59 ab 5 ab
4h 100 71 ab 32a 103 ab 14b 61 3 4b
10% cyclic" 100 70 ab 30 ab 100 ab 14 b 41 c 5 ab
30% cvcﬂp" 100 73 ab 32a 104a 13b 54ab 4p

 

 

 

‘9-h natural days with night-interruption lighting during the middle of the dark period.
’— = no plants showed visible bud after 105 days of forcing.

‘Mean separation within each photoperiod by Duncan’s multiple range test (P = 0.05).
wLights on and off for 6 and 54 min, respectively, for 4 h.

”Lights on and off for 6 and 24 min, respectively, for 4 h.

 

 

181

Plants under 0.5 hours of NI had more ﬂowers than plants under four hours of NI
or 20% cyclic lighting.
Hibiscus xhybrida ‘Disco Belle Mixed’

Experiment 1. Ten percent of plants ﬂowered without NI, 50% ﬂowered
with 0.5 hours of NI, and 280% ﬂowered with 21 hour of NI or 10 or 20% cyclic
lighting for four hours (Figure 46, Table 36). Days to ﬂower decreased from 154
to 114 and ﬂower number increased from 5 to 17 as the NI duration increased
from 0 to 4 hours. Cyclic lighting induced ﬂowering at approximately the same

time as plants under the four hours of NI.

 

Experiment 2. Plants ﬂowered more uniformly and 25 to 40 days earlier
and than plants in Experiment 1, which may be at least partially due to starting
with more mature plants. Seventy percent of plants ﬂowered without and all
plants ﬂowered with a Nl. All plants ﬂowered at approximately the same time
and developed approximately the same number of nodes.

Rudbeckia fulgida ‘Goldsturm‘

Experiment 1. No plants ﬂowered without NI or with 0.5 hours of NI
(Figure 47, Table 37). All plants that received 21 hour of NI ﬂowered, and 60 or
90% of plants ﬂowered under 10 or 20% cyclic lighting, respectively. Plants
under four hours of NI ﬂowered earlier (219 days) and developed at least four
fewer nodes than plants under other lighting treatments. Plants under one or

two hours of NI or 20% cyclic lighting ﬂowered simultaneously. For plants that

 

182

 

  
 
  

 

 

 

 

_..A .............. _
16° + Experiment1 160
—A— Experiment2

.3140 _ ............... - 14o
“3 5
2120 - ------------------------------------------ l --------- — 120;
,9 .0
g LL
>10 » 1009
9 a
m to
g. 8 - 80 D

60 1 - 60

 

.5
O
l
l
b
O

 

r r
-h 01
01 O

A
0
Plant Height (cm)

 

 

r
0)
0|

 

Number of Flowers

 

 

r
(a)
O

 

 

 

I T j l I I j T I

0 1 2 3 4 6/60 6/30 0 1 2 3 4 6/60 6/30

 

 

Hours of Cyclic Hours of Cyclic
Night Interruption Night Interruption

Figure 46. Flowering of Hibiscus xhybrida ‘Disco Belle Mixed' under various
durations of night interruption or cyclic lighting. Night interruption was provided
by incandescent lamps that were turned on during the middle of 15-hour dark
periods. For the cyclic lighting treatments, lights were on for six minutes every 30
or 60 minutes for a four hour period during the middle of the night. Error bars are
95% conﬁdence intervals.

183

Table 36. The effects of night-interruption duration and cyclic lighting on ﬂowering of Hibiscus
xhybrida ‘Disco Belle Mixed’.

 

 

Days to Days from Increase Final plant
Night Flowering visible visible bud Days to in node height Flower
interruption‘ (%) M to ﬂower ﬂower number (cm) number
Experiment 1
0h 10 103 ay 51 a 154a 21 a 33b 5b
0.5h 50 91 ab 44a 135ab 19a 46a 10 ab
1h 90 78bc 46a 126b 15a 41 ab 11 ab
2h 90 76bc 44a 122b 17a 37 ab 14 ab
4h 90 62c 52a 114b 15a 38ab 17a
10% cyclic" 80 90 ab 40 a 131 ab 17 a 46 a 12 ab
20% cyclicw 90 71 bc 47 a 118 b 15 a 44 a 12 ab
Experiment 2
0h 70 60a 52a 101a 14a 34b 7b
0.5h 100 46bc 50a 94a 14a 38 ab 8b
1h 100 53 ab 56a 100a 14a 35 ab 12 ab
2h 100 39c 56a 94a 14a 36ab 10ab
4h 100 38c 56a 92a 12a 37 ab 10 ab
10% cyclic 100 42 bc 48a 90a 13a 42a 14a
20% gclic 100 38 c 51 a 89 a 12 a 42 a 11 ab

 

 

‘9-h natural days with night-interruption lighting during the middle of the dark period.

’Mean separation within each photoperiod by Duncan’s multiple range test (P = 0.05).

"Lights on and off for 6 and 54 min, respectively, for 4 h.

“Lights on and off for 6 and 24 min, respectively, for 4 h.

184

 

 

  
  

 

 

 

A
+ Experiment1
120 _ ................ , “A” Experiment2 .. _120

3

CD 'a':

2100 ~ ~ 100;

.o 2

E u.
.9

.9. ~ 80 g

rn m

3 D

D

r
O)
O

 

 

 

~40
c D

~35
$2
a) A
5 E
E ~30;
“6 .9
h a)
03 :I:
E *252
3 E
Z 0.

~20

 

 

 

 

 

I l I I I T

0 1 2 3 4 6/60 6/30 0 1 2 3 4 6/60 6I30

 

 

 

Hours of Cyclic Hours of Cyclic
Night Interruption Night Interruption

Figure 47. Flowering of Rudbeckia ful ida 'Goldsturm' under various durations

of night interruption or cyclic lighting. ight interruption was provided by
incandescent lamps that were turned on during the middle of 15-hour dark
periods. For the cyclic lighting treatments, lights were on for six minutes every 30
or 60 minutes for a four hour period during the middle of the night. Error bars are
95% conﬁdence intervals.

185

Table 37. The effects of night-interruption duration and cyclic lighting on ﬂowering of Rudbeckia
fulgida ‘Goldsturm'.

 

 

 

Days to Days from Increase Final plant
Night Flowering visible visible bud Days to in node height Flower
interruption‘ (%) bud to ﬂower ﬂower number (cm) number
Experiment 1
0 h 0 —’ - - - - -
0.5 h 0 — - - - — -
1 h 100 91 b" 33 b 124 b 23 a 24 b 23 ab
2h 100 86b 38a 124b 23a 24b 24a
4h 100 65c 37a 102c 18b 24b 18 ab
10% cyclic" 60 100 a 32 b 132 a 24 a 27 a 22 ab
20% cyclic” 90 89 b 32 b 121 b 22 a 24 b 17 b
Experiment 2
0 h 0 - — - - - -
0.5h 100 72a 32c 104a 22a 29a 24a
1h 100 48c 36b 84c 18c 28a 25a
2h 100 42d 37b 79d 15de 28a 18b
4h 100 44d 40a 85c 13e 24b 17b
10% cyclic 100 60 b 37 b 96 b 20 b 27 ab 27 a
20% pyclic 100 42 g 42 af 84 c 16 d 26 a_b 19 b_

 

 

‘9—h natural days with night-interruption lighting during the middle of the dark period.
V— = no plants showed visible bud after 105 days of forcing.

"Mean separation within each photoperiod by Duncan's multiple range test (P = 0.05).
"Lights on and off for 6 and 54 min, respectively, for 4 h.

"Lights on and off for 6 and 24 min, respectively, for 4 h.

186

ﬂowered under 10% cyclic lighting, ﬂowering was delayed and plants were
tallest.

Experiment 2. No plants ﬂowered without and all plants ﬂowered with NI.
The cold treatment shifted the minimum duration of NI for ﬂowering from 1 to 0.5
hours and hastened ﬂowering by more than two weeks. Plants ﬂowered earliest
under two hours of NI, approximately ﬁve to six days earlier than plants under
one or four hours of NI or 20% cyclic lighting. For most plants under four hours
of NI and some plants under 20% cyclic lighting, the ﬂowering phenotype was
atypical: the inﬂorescence was branched at the base. This may explain why
days from visible bud to ﬂower was delayed under these two lighting treatments.
Flowering under 10% cyclic lighting or 0.5 hours of NI was delayed by
approximately 12 or 20 days, respectively. Plants under 0.5 or 1 hour of NI or

10% cyclic lighting had the most ﬂowers.

Conclusions

The response of six species of long-day perennials to the six NI lighting
treatments in Experiment 1 is illustrated in Figure 48. The continual four-hour NI
induced complete, rapid, and uniform ﬂowering of all species. For Coreopsis
verticillata ‘Moonbeam' and Hibiscus xhybrida ‘Disco Belle Mixed” the 20% cyclic
lighting treatment was nearly as effective as the continual four-hour NI. Except

for Echinacea, which ﬂowered at approximately the same time under all NI

187

..coEﬁob $5 .22.: 60.030: 853 o: 35 8.865
.58 m .o accomnm 2F . .coEtoaxm Ea... £53. 9... 2m Soc .29.. 65 .o 262... mg. macaw moron Soc .38 m .2 3.35:. oo .o o ago
855:. x_m .2 mm; 95cm: 26 o .muotoa Emu 50..-? .o 0.62:. 65 9.5.. :0 35:. 2m; 55 3:6. E33385 .3 0330... mm; :2. E95
292 83.5.6.3 3389.0: .6 86on can .2 3:95.83 533:25 29.. xv. oz. .09.: 9526: .o Echo—E: new .626: 0. 6E... .3 2:9“.
was. to. 2.03 on... o _z .0 e32. N 4
26.. v .o. 2.96 8.8 a _z .o 59. . a

_z .o 2:9. v o _z .o 2:2. ad 0
.oBoE o. 960

 

 

 

 

 

 

 

 

 

09 our 0: cm? our m2. 8.. mm mop om mu on on on no om om on on on on .
o — p p L- . _ p _ — p _ _ b p _ L _ p . b r _ o
O o I no 0
ml .50.. ...................................... .qu.....r ......... .....Z. ............. . ..... 10
~ n. o 4

L. ........................................................... r ................. 1 ................... r ................... l 6
or O O D or Cu
O I D G M/J

m F ......................................................................................................................... I n F
4 I m m
S u.
1. D.

......................................................................................................................... r 0
cm om .... m
m. a
ON ......................................................................................................................... I MN m M
6
M
on ......................................................................................................................... l on DIV
I
mm ............................................................. r ........................................................... 1 mm
0
av ov
..Eamsoo. .352 2.8 coma. .3965. .Emoncooz. .omtcam 2.3. .35 0:5.
«has. «Recount mutaxﬁ «3835 358:6 accustom 6.2.8.5., «.3360 88.6286 6.32:8 acumen... 35:6658

188

treatments, NI durations of one hour or less or 10% cyclic lighting substantially
delayed ﬂowering and decreased uniformity.

The cold treatment increased the responsiveness of Rudbeckia to shorter
durations of NI and to 10% cyclic lighting for four hours. This suggests that cold-
treated herbaceous perennials may require shorter durations of NI for complete,
rapid, and uniform ﬂowering than plants not provided with a cold treatment.

Further studies are needed to test this theory.

189

References

Bickford, ED. and S. Dunn. 1972. Lighting for plant growth. Kent State Univ.
Press.

Canham, A.E. 1966. The ﬂuorescent tube as a source of night-break light. Exp.
Hortic. 16:53-68.

Cockshull, K.E. and AP. Hughes. 1969. Growth and dry-weight distribution in
Callistephus chinensis as inﬂuenced by lighting treatment. Ann. Bot.
33:367-379.

Harris, GP. 1968. Photoperiodism in the glasshouse carnation: the effectiveness
of different light sources in promoting ﬂower initiation. Ann. Bot. 32:187-
197.

Kadman-Zahavi, A. 1994. Comparing lamps and timing of illumination to elicit a
long-day response in long-day and short-day plants. Acta Hort. “(In
press)”

Kasperbauer, M.J, H.A. Borthwick, and HM. Cathey. 1963. Cyclic lighting for
promotion of ﬂowering of Sweetclover, Melilotus alba Desr. Crop Sci.
32230-232.

Maginnes, EA. and R.W. Langhans. 1967. Flashing light affects the ﬂowering of
snapdragons. NY State Flower Grow. Bull.:261z1-3

Sachs, R.M., A.M. Kofranek, and J. Kubota. 1980. Radiant energy required for
the night-break inhibition of ﬂoral initiation is a function of daytime light
input in Chrysanthemum. HortScience 15(5):609-610.

Shillo, R. 1994. Flowering of some LDPs in response to photoperiodic lighting
regimes. Acta Hort. “(In press)”

Shillo, R. And A.H. Halevy. 1982. Interaction of photoperiod and temperature in
ﬂowering-control of Gypsophila paniculata L. Scientia Hort. 16:385-393.

Vince-Prue, D. 1975. Photoperiodism in plants. McGraw, London.

Vince-Prue, D. And A.E. Canham. 1983. Horticultural signiﬁcance of
photomorphogenesis, p. 518-544. In: Shropshire, W. and H. Mohr (eds.).
Encyclopedia of plant physiology; new series, vol. 16. Springer-Venag,
Berlin.

 

 

 

MICHIGAN S

TATE UNIV. LIBRRRIES
I Ill"IWI"IIIHI”NIHIIIIIIWW
9301 5592789

312