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thesis entitled

Developing Production Systems for Tabletop Christmas

Trees

presented by

Marcus wayne Duck

has been accepted towards fulfillment
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MS degree in Horticulture

 

 

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6/01 cJCIRC/DateDuopss-sz

DEVELOPING PRODUCTION SYSTEMS FOR TABLETOP CHRISTMAS
TREES

BY

Marcus Wayne Duck

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE
Department of Horticulture

2002

ABSTRACT

DEVELOPING PRODUCTION SYSTEMS FOR TABLETOP
CHRISTMAS TREES

By

Marcus Wayne Duck

Live tabletop Christmas tree production could be lucrative for conifer and
greenhouse growers in the upper Midwestern United States. Our goal was to
determine various production systems for tabletop Christmas trees by using
species hardy to the upper Midwest. We evaluated the efficacy of seven plant
growth retardants (PGRs) in controlling seedling growth. Several PGRs
effectively controlled height; however significant species x PGR interactions
make it difficult to generalize recommendations. Chilling and short-day (SD)
requirements were determined for producing a marketable product in the
shortest time possible. All conifer seedlings we tested must receive a duration of
cold treatment for timely budbreak, but only a few seedlings require SD
treatments before chilling. By using two methods of measuring seedling
transpiration, treatments of selected antitranspirants were evaluated for efficacy
in extending seedling shelf life: Moisturin, NuArbor, and Wilt-Pruf. Treatments
were effective on one of three species tested. Effects of postholiday storage
conditions and seedling response to planting outdoors after storage were
determined. Seedlings responded most favorably to cool, well-lit storage

environments.

ACKNOWLEDGMENTS

I am extremely grateful for the guidance, support, and friendship of my
major professor, Dr. Bert Cregg. Working with Bert over the past two and a half
years has been a wonderful learning experience. I couldn’t have asked for a
better mentor. I would also like to thank my committee members, Dr. Bridget
Behe, Dr. Tom Fernandez, and Dr. Mel Koelling for their guidance, input, and the
many hours reviewing my thesis. Bridget — thank you for your friendship and for
getting my butt back to college. Torn — thank you for always being there when I
needed your help and friendship.-

I would like to thank the many others who helped me make it through
graduate school: My mom, dad, Dennis, Mary, and Star for their love, support,
and understanding with my decision to go back to school and move so far away
from home; Andy and Jason, my two closest friends, for their love and friendship;
Arm for her big push in getting me back to school; my fellow graduate students
and office mates: Randall Vos, Charlie Rohwer, Carmela Rios, Beth Fausey,

' Janelle Glady, Lee Ann Pramuk, Kathy Kelley, Andrea Beckwith, Roberto Lopez,
and Genhua Niu; David Joeright and Mike Olrich for their hard work caring for all
those prickly conifers, but most importantly for their friendship; and Fernando
Cardoso for the many, many hours spent helping me analyze and make sense of
all my data.

Finally, and most importantly, I would like to thank my loving wife, Donna.
Her love, support, and understanding throughout my master’s program helped

get me through this difficult and challenging step in our lives.

TABLE OF CONTENTS

LIST OF TABLES
......................................................... vi
LIST OF FIGURES .............................................. viii
LIST OF ABBREVIATIONS ........................................ xi
INTRODUCTION ................................................ 1
LITERATURE REVIEW ........................................... 6
PLANT GROWTH RETARDANTS ............................. 7
Introduction .......................................... 7
Ethylene Releasing Compounds .......................... 8
Inhibitors of GA Translocation ............................ 9
Inhibitors of GA Biosynthesis ........................... 11
Synthetic Cytokinins .................................. 22
PHOTOPERIOD AND CHILLING ............................. 27
Definitions .......................................... 27
Role in Growth and Dormancy .......................... 29
Seed Source Effects on Photoperiod and Temperature ....... 38
Abscisic Acid, Gibberellins, and Cytokinins ................. 40
ANTITRANSPIRANTS ...................................... 42
Definitions .......................................... 42
Metabolic Antitranspirants .............................. 43
Film-Forming Compounds .............................. 45
Reflecting Materials ................................... 48
LITERATURE CITED ....................................... 49
CHAPTER ONE
CONTROLLING GROWTH OF CONIFERS WITH SIX COMMON PLANT
GROWTH RETARDANTS ................................... 57
Abstract ................................................. 58
introduction .............................................. 59
Materials and Methods ...................................... 61
Results and Discussion ..................................... 63
Literature Cited ........................................... 70
List of Figures ............................................ 75

CHAPTER TWO
APPLICATIONS OF UNICONAZOLE AND 6-BA TO CONTROL HEIGHT

OF PICEA SPECIES AND CHAMAECYPARIS LAWSON/ANA ...... 79
Abstract ................................................. 80
Introduction .............................................. 81
Materials and Methods ...................................... 83
Results .................................................. 85
Discussion ............................................... 87
Literature Cited ........................................... 92
List of Figures ............................................ 96
CHAPTER THREE
FORCING CONIFERS FOR TABLETOP CHRISTMAS TREES ..... 100
Abstract ................................................ 101
Introduction ............................................. 102
Materials and Methods ..................................... 103
Results ................................................. 107
Discussion .............................................. 1 09
Literature Cited .......................................... 1 15
List of Figures ........................................... 119
CHAPTER FOUR
USE OF ANTITRANSPIRANTS TO EXTEND THE SHELF LIFE OF
TABLETOP CHRISTMAS TREES ............................ 127
Abstract ................................................ 128
Introduction ............................................. 1 29
Materials and Methods ..................................... 130
Results ................................................. 134
Discussion .............................................. 135
Literature Cited .......................................... 141
List of Figures ........................................... 146
CHAPTER FIVE
POSTHOLIDAY CARE OF LIVE TABLETOP CHRISTMAS TREES . . 149
Abstract ................................................ 150
Introduction ............................................. 1 51
Materials and Methods ..................................... 152
Results and Discussion .................................... 155
Literature Cited .......................................... 159
List of Figures ........................................... 163

LIST OF TABLES

TAB LE PAGE

CHAPTER ONE

Analysis of variance for mean relative growth rate.

73
Mean initial and final heights, differences between control and most
effective PGR treatments, and PGRs that caused phytotoxicity.
74
CHAPTER TWO
Analysis of variance for mean relative growth rate (RGR) (cm d") and
bud density (buds cm").
94
Mean RGR, height increase, number of buds, and bud densities for all
species and treatments.
95
W
Analysis of variance for days to budbreak data.
117
Recommended tabletop Christmas tree production schedules for all
species tested.
118
CHAPTER FOUR
Analysis of variance for whole-plant water loss and gas exchange data.
144

Mean daily whole-plant water loss and shoot transpiration rates of three
conifers under two vapor pressure deficits.
145

vi

TABLE PAGE

CHAPTER FIVE

1 Analysis of variance for the rating system used to evaluate the storage

and planting trials, which were designed to determine postholiday care for
tabletop Christmas trees.

160
2 Condition of seedlings at the end of storage trials as indicated by a visual
rating scale (0 = dead to 5 = 100% alive).
161
3 Condition of seedlings at the end of planting trials as indicated by a visual

rating scale (0 = dead to 5 = 100% alive), and percentage of seedlings
achieving budbreak by the end of the planting trial.

162

vii

LIST OF FIGURES

FIGURE PAGE

CHAPTER ONE

Effects of plant growth regulator applications on mean relative growth
rates of concolor fir, Fraser fir, grand fir, noble fir, and Douglas-fir
seedlings. Error bars represent standard error of the mean.

76
Effects of plant growth regulator applications on mean relative growth
rates of Black Hills spruce, Serbian spruce, and Colorado blue spruce
seedlings. Error bars represent standard error of the mean.

77

Effects of Plant growth regulator applications on mean relative growth
rates of Port Orford cedar and arborvitae seedlings. Error bars represent
standard error of the mean.

78

CHAPTER TWO

Effects of Plant growth regulator applications on height growth over time
for Black Hills spruce, Serbian spruce, Colorado blue spruce, and Port
Orford cedar seedlings.

97

Effects of Plant growth regulator applications on mean relative growth
rates of Black Hills spruce, Serbian spruce, Colorado blue spruce, and
Port Orford cedar seedlings. Error bars represent standard error of the
mean.

98

Effects of Plant growth regulator applications on mean bud densities of
Black Hills spruce, Serbian spruce, and Colorado blue spruce seedlings.
Error bars represent standard error of the mean.

99

CHAPTER THREE

Median days to budbreak for Black Hills spruce seedlings exposed to
chilling and short-day treatments. Error bars on the scatter plot represent
standard error of the median as calculated by using SAS’s LIFEREG
procedure.

120

viii

FIGURE PAGE

2

Median days to budbreak for Serbian spruce seedlings exposed to chilling
and short-day treatments. Error bars on the scatter plot represent
standard error of the median as calculated by using SAS’s LIFEREG
procedure.

121

Median days to budbreak for Meyer spruce seedlings exposed to chilling

and short-day treatments. Error bars on the bar graph represent standard

error of the median as calculated by using SAS’s LIFEREG procedure.
122

Median days to budbreak for Colorado blue spruce seedlings exposed to
chilling and short-day treatments. Error bars on the bar graph represent
standard error of the median as calculated by using SAS’s LIFEREG
procedure.

123

Median days to budbreak for Wilson spruce seedlings exposed to chilling

and short-day treatments. Error bars on the bar graph represent standard

error of the median as calculated by using SAS’s LIFEREG procedure.
124

Median days to budbreak for noble fir seedlings exposed to chilling and

short-day treatments. Error bars on the bar graph represent standard

error of the median as calculated by using SAS’s LIFEREG procedure.
125

Median days to budbreak for Nordmann fir seedlings exposed to chilling

and short-day treatments. Error bars on the bar graph represent standard

error of the median as calculated by using SAS’s LIFEREG procedure.
126

CHAPTER FOUR

Effects of antitranspirant treatments on mean daily water loss of Black
Hills spruce, Serbian spruce, and noble fir seedlings as determined by
whole-plant water loss measurements. Error bars represent standard
error of the mean.

147

Effects of antitranspirant treatments on mean transpiration rates of Black
Hills spruce, Serbian spruce, and noble fir seedlings as determined by gas
exchange measurements. Error bars represent standard error of the
mean.

148

FIGURE

PAGE
CHAPTER FIVE
1 Maximum and minimum daily temperatures recorded during a storage trial
for seedlings in a garage environment without light.
164
2.

Maximum and minimum daily temperatures recorded during a storage trial
for seedlings in a garage environment with moderate light.

165

LIST OF ABBREVIATIONS

ABA ................................................ Abscisic acid
a.i ................................................. Active ingredient
BA ................................................. Benzyladenine
BAP ............................................ Benzylaminopurine
CCC ......................................... Chlorochoiine chloride
CD .............................................. Critical daylength
CK ..................................................... Cytokinin
CN ............................................. Critical night length
FR ................................................... Far-red light
GA ................................................... Gibberellin
kPa .................................................... Kilopascal
LD ..................................................... Long day
MPa ................................................. Megapascal
ND .............................................. Natural daylength
NI ............................................... Night interruption
nm .................................................... Nanometer
PGR ......................................... Plant growth retardant
PMA ........................................ Phenylmercuric acetate
R ....................................................... Red light
RGR ........................................... Relative growth rate
RH .............................................. Relative humidity
SD ..................................................... Short-day
USDA .......................... United States Department of Agriculture
VPD ......................................... Vapor pressure deficit
WUE ........................................... Water use efficiency

xi

INTRODUCTION
Michigan Growers

In 1999, nursery operators were producing 16,750 acres of woody plant
material in Michigan (Kleweno and Matthews, 2000). About 11,000 of those
acres were planted with narrow-leaved evergreens. Furthermore, Christmas tree
producers alone had $41 million in sales in 19\99,/additional evidence that conifer
production has a strong presence in the state. Michigan is the fifth largest
producer of poinsettias in the United States [United States Department of
Agriculture (USDA), 2002]. However, they have not experienced the profits that
other floriculture markets have in recent years. In the past nine years, poinsettia
growers in Michigan have seen a decrease in the wholesale price of their crop
(USDA, 2002). Inflation-adjusted wholesale prices (United States Department of
Labor, 2002) for poinsettias in pots five inches or smaller, decreased from $1.93
a pot in 1993 to $1.19 a pot in 2001. Prices of poinsettias larger than five inches
decreased from $4.60 to $4.04 during the same time, which has caused some
poinsettia growers to consider producing alternative greenhouse crops for the

holidays or close their doors for several winter months.

Consumer Preferences

According to demographic evidence, Americans are aging, which can
affect the products people need and buy, even at Christmastime. An aging
American population should cause some traditional Christmas tree growers, and
perhaps some poinsettia growers, to begin thinking of new ways to meet the

changing needs of their customers during the Christmas season. In 2001, 58%

1

of Americans were older than 36 years, and 28% of Americans were older than
55 years (Mitchell, 1998). Older customers may require more assistance in
handling larger cut trees and may actually prefer to have smaller, easier-to-
handle tabletop trees (Crothers, 1990; Florkowski and Lindstrom, 1995).
Because small trees are less likely to have the problems associated with needle
drop, Florkowski and Lindstrom also suggested that live tabletop trees are
preferable to fresh-cut trees.

Another consideration is where live tabletop Christmas trees can be used.
Many of the Baby Boomers (people born between 1946 and 1964) are upgrading
from starter homes to larger ones (Mitchell, 1998). For consumers who live in
large homes with many rooms to decorate during the holidays, tabletop trees
could provide easier, less expensive decorations that can be used in addition to
a fresh-cut tree (Crothers, 1990). Apartments and retirement homes are perfect
venues for tabletop trees because of their limited space where large, fresh-cut
trees are often impractical (Crothers, 1990). Finally, businesses and office areas
could also benefit from tabletop trees because of space restrictions and ease of

handfing.

Current Market

There are few companies marketing live tabletop Christmas trees, most of
which are sold through mail-order catalogs and some mass merchandisers. The
majority of those trees are not cold hardy in the upper Midwest (zone 5), where
the average annual minimum temperature ranges from -29 °C to -23 °C (Bailey

and Bailey, 1976; Crothers, 1990; USDA, 2002). The current mail-order

2

selections are also somewhat expensive for the average homeowner. Some
mailorder catalogs, such as Jackson & Perkins (2000), Harry and David (2000),
and Calyx and Corolla (2000), sell decorated and undecorated tabletop tree
species that retail for $50 to $85, including delivery.

Mass merchandiser products are relatively inexpensive compared with
that of mailorder catalogs (Crothers, 1990). Also, the tabletop Christmas tree
mass merchandiser market has seen exceptional growth in recent years (15% in
1990) (Crothers, 1990). Some growers have experienced even greater
increases, such as Pinery Tree Farms in Escondido, Calif., which saw a 35%
increase in crop production in 1990. Consumers in Europe purchase tabletop
trees by the millions, with sales generally lasting from November to June
(Hamrick, 2002). Port Orford cedar [Chamaecyparis Iawsoniana (Murray) Parl.
‘Ellwoodii’] is the primary species for this market and can be purchased
undecorated, flocked, or covered in glitter.

Because of the increase in consumer interest in tabletop Christmas trees
and nonhardy species available on the market, this research project was
designed to determine methods for adapting more traditional Christmas tree
species for the tabletop tree market. The conifer species we selected for the
study were chosen for their natural growth habit, popularity as cut Christmas
trees, and cold hardiness in Midwestern winters. A brief description of each area

of the production schedule we investigated is provided below.

Production Schedules

The first goal of our project was to develop a production system schedule

3

to produce live tabletop Christmas trees in the shortest time possible. Most of
the traditional tree species require a cold treatment to break bud dormancy for
the promotion of new growth (Hanover, 1980; Hart and Hanover, 1979; Kramer
and Kozlowski, 1979; Nienstaedt, 1967; Wareing, 1956). In the field, it would
take five years (not including the time it would take to produce the planting stock)
to produce a mature, 4- to 5-foot fir tree (M. R. Koelling, personal

W).— Most growers could not devote this much greenhouse time and
space to one crop. Thus, one goal is to produce a full tree in approximately 18
months.

Lowering production costs by minimizing manual labor inputs, such as
shearing, is also a goal of this project. Sundback (2002) found that many
consumers actually prefer the natural branching habit of conifers as opposed to
dense, heavily sheared live-cut Christmas trees sold by large-scale tree farms
and massmerchandisers. This preference is a result of two factors: naturally
branched trees have more spaces for ornaments to hang straight and organized,
and many sheared trees have partially cut and bruised, unattractive needles.
The natural growth habit of spruce seedlings makes them prime candidates for

this project.

Plant Growth Retardants

Plant growth retardants could be a major benefit for use on conifers grown
as tabletop Christmas trees by reducing the vigorous central leader growth that
usually occurs when conifers are grown in extremely optimum greenhouse

environments (Landis et al., 1992; Landis et al., 1999). These compounds could

4

also prove beneficial by increasing lateral branching for a fuller yet naturally

shaped specimen (Hinesley, 1998), unlike trees that are heavily sheared.

Antitranspirants and Postholiday Care

Once the tabletop trees are ready for distribution to retail markets, efforts
must be made to ensure that quality specimens remain high quality for
consumers. Antitranspirants are one possibility for increasing plant storability,
survival, and quality (Gale and Hagan, 1966; Simpson, 1984). Applying these
compounds could prevent water stress that may result from shipping or plant
neglect in retail stores.

Another issue that can be addressed is a growing concern for the
environment. By offering live, attractively potted tabletop trees that are hardy in
the upper Midwest, customers have the option of planting the tree in the spring,
reducing the amount of debris contributing to the diminishing landfill space
(Florkowski and Lindstrom, 1995). Therefore, consideration should be given to
how tabletop trees will be cared for by the consumer after the holidays. By
addressing the concerns and suggestions given above, Michigan growers can

produce a very marketable and profitable tabletop Christmas tree.

LITERATURE REVIEW

PLANT GROWTH RETARDANTS
Introduction

Plant growth retardants (PGRs) are used on many horticultural crops as
an aid in altering normal growth behavior (Schott and Walter, 1991 ). They are
often used as a production tool by creating a more uniform crop and as a
handling and shipping aid by producing smaller, easier-to-handle crops (Blake
and South, 1991; Nickell, 1983). The effects of the PGRs are usually temporary,
which is why the chemicals are referred to as retardants (Cathey, 1964; Hield et
al., 1977). Generally, PGRs work by reducing the shoot length of plants without
causing any detrimental adverse effects such as phytotoxicity. or without altering
normal patterns of development (Grossman, 1990; Rademacher, 1991).
However, if the chemicals are improperly applied, stunting can occur, causing
adverse effects such as leaf distortion or discoloration (Cathey, 1964). When
PGRs are applied to the selected crop, the plant responds by a decrease in the
rate of cell division along with a reduction in cell elongation (Rademacher, 1991).
This alteration in plant processes can be accomplished by interfering with the
normal functions of gibberellins (GAS), auxins, or both. These are the two most
important plant hormones involved in shoot elongation. Gibberellins promote a
wide array of plant growth and development processes, including apical
dominance (Pharis et al., 1964; Ruddat and Pharis, 1966), cell division, and cell
enlargement, and auxins promote growth primarily from cell enlargement
(Grossman, 1990; Kramer and Kozlowski, 1979; Kozlowski and Pallardy, 1997;
Nickell, 1983; Pharis and Kuo, 1977). Auxins also prevent lateral branching by

controlling axillary bud growth (Barrett, 1992). When auxin levels are reduced by

7

either pruning the stem tip (the site of auxin production in plants) or applying
chemicals, apical dominance is removed and lateral branching can occur to
produce a fuller, shorter plant.

The most common PGRs on the market today can be classified into three
major categories in relation to their effects on natural hormones in the plant:
ethylene-releasing compounds, inhibitors of GA translocation, and inhibitors of

GA biosynthesis (Rademacher, 1991).

Ethylene Releasing Compounds

Ethephon [trade name: Florel (Monterey Lawn and Garden Products, Inc.,
Fresno, Calif.) or Ethrel (Aventis CropScience Pty Ltd. A.C.N., Victoria, Canada)]
is the most important ethylene-releasing PGR. This chemical is effective by
inhibiting cell elongation processes (Rademacher, 1991). Ethephon reduces cell
and stem elongation by preventing auxins from reaching growth sites and by
blocking biosynthesis of the auxins (Burg and Burg, 1967; Morgan and
Gausman, 1966).

Ethephon effectively controls growth and other plant functions in various
floriculture and ornamental crops. Azaleas (Rhododendron spp. L.) and
geraniums (Pelargonium spp. L’Herit.) treated with ethephon responded with an
increase in lateral branching, resulting in shorter, fuller specimens (Neumann,
1988). Foliar application of ethephon increased height and lateral branching in a
wide variety of herbaceous perennials (Hayashi et al., 2001 ). Chlorophyll loss in
certain crops was retarded when they were treated with ethephon, causing an

enhancement in the greenness of the foliage (Neumann, 1988). Plants that are

8

 

under stress at chemical application may have undesirable secondary treatment

effects such as leaf abscission (Neumann, 1988).

Inhibitors of GA Translocation

Daminozide (B-Nine, Uniroyal Chemical, Middlebury, Conn.) is the most
important chemical in this category of PGRs. This product has been used
successfully on many horticultural crops, including omamentals (Cathey, 1964;
Rademacher, 1991). Daminozide works by reducing the translocation of GAs or
GA precursors to actively growing tissues and may also promote breakdown and
conjugation of GA (Takeno et al., 1981 ). However, some research has
suggested that daminozide works through the interconversion of GAs (Kuo and
Pharis, 1975). Regardless of the specific mode of action, the movement of GA
within the plant is affected by the presence of daminozide, thereby controlling the
growth of the plant. Daminozide became an important product for floriculture
crops as a foliar spray to help control height for hydrangeas and bedding plants
(Larson, 1992). Foliar applications are recommended because of toxicity effects
that appeared after soil drenching (Cathey, 1964). Daminozide also causes
darker green foliage on treated plants. It is used only as a foliar-applied
chemical because it breaks down readily in soil. Also, results from applications
depend on plant age and temperature. Applications made to herbaceous
floriculture crops are more effective under lower temperatures (Barrett, 1992).

Hare (1982) determined the effects of foliar treatments of daminozide and
ethephon, along with seven other PGRs, on slash pine (Pinus elliotti Engelm.

var. elliotti!) and loblolly pine (P. taeda L.). Daminozide reduced growth slightly

9

compared with the control; however, the effect was not statistically significant
(ethephon actually caused an increase in shoot growth). The other seven PGRs
that were used all greatly damaged both species. Plants commonly died. The
lack of phytotoxic effects and the marginal growth reduction show that
daminozide should be considered for further testing for growth control of
conifers.

Ruddat and Pharis (1966) examined the effects of PGRs and 6A8 on
apical dominance. Redwood [Sequoia sempervirens (D. Don) Endl.] seedlings
were treated with various concentrations of daminozide and GA3 for three
months. During this time, lateral buds escaped apical dominance in the
presence of daminozide. After three months, treatments ceased and the plants
were decapitated to remove the terminal bud, thus removing apical dominance.
A new terminal developed sooner on plants that had received GA3 and the PGR
treatments than those subjected only to daminozide, which shows that PGRs
have an effect on overriding apical dominance and GA plays an important role in
restoring it.

The effects of root-applied daminozide and chlormequat (chemical
background and behavior discussed in more detail below) on Norway spruce
(Picea abies Karst.) were determined (Dunberg and Eliasson, 1972).
Daminozide effectively reduced shoot and root growth at all application rates, but
300 mg L‘1 was extremely toxic and killed roots and then shoots. The fresh and
dry weights were reduced at 10 mg L“, but an increase in fresh and dry weights
was observed at 1 mg L". Chlormequat had similar effects on shoot growth and

fresh and dry weight, but it was not as toxic at 300 mg L". Dunberg and Eliasson

1O

(1972) propose two explanations: 1) specific retardant effects of blocking GA
synthesis and 2) an indirect effect caused by toxic effects to the root system.
These results further emphasize the consensus that daminozide works by
interfering with GA translocation and chlormequat interferes with GA
biosynthesis. However, the extreme toxic effects that occurred from the high
rate of daminozide cannot be explained by the blockage of GA, which shows that
further research should be performed to determine the PGR’s exact mode of

action (Kuo and Pharis, 1975).

Inhibitors of GA Biosynthesis

This category of inhibitors can be broken down into two groups according
to the location of inhibition in GA biosynthesis: 1) onium compounds, such as
chlormequat, and 2) compounds with an N—containing heterocycle, such as
ancymidol, paclobutrazol, and uniconazole, which lower the content of
biologically active GAs (Rademacher, 1991). These types of retardants attack
the metabolism of terpenoids, which is the pathway by which the phytohormone
groups of GA, cytokinin, and possibly abscisic acid (ABA) and the sterols are
derived (Grossman, 1990). Therefore, the availability of GAs can be affected by
low concentrations of these retardants, which has a subsequent effect on cell
elongation. Relatively high rates of GA inhibitors can also reduce cell division,
but the rates of application are somewhat higher (>105 M as opposed to <10”5 M,
the effective rate for reducing cell elongation). These retardation effects are
believed to be caused by an alteration in sterol synthesis, causing a change in

the membrane properties of the cells (Grossman, 1990; Nitsche et al., 1985).

11

 

Endogenous GAs in conifers have an important role in promoting vegetative
shoot growth (Pharis and Kuo, 1977), so the application of PGRs that inhibit GA
synthesis could prove useful in controlling height in tabletop Christmas tree

specimens.

Chlormequat

Chlormequat [CCC or Cycocel (Olympic Horticultural Products Company,
Mainland, Pa.)] is one of the most important chemicals in this category for the
floriculture and agriculture industries. It was first introduced into floriculture crop
production in the early 19603 (Larson, 1992). Before to the introduction of
chlormequat, growers had to regulate plant growth by limiting water and nutrients
supplied to the plants, reducing temperature, and folding over plants for
shipping. The PGR applications allowed the plants to be grown in optimal
conditions, but at the same time regulated plant height (Barrett, 1992).
Chlormequat not only controls plant height but also enhances the green color of
the foliage (Cathey, 1964).

Chlormequat is effective on many plants as a soil drench and a foliar
application. When applied to the soil, chlormequat generally is leached from the
medium within four weeks (Cathey, 1964). Soil drench is the preferred method
of application because there is little or no foliar phytotoxicity, whereas foliar
applications are highly phytotoxic (Asher, 1963). Phytotoxicity generally appears
as chlorotic spotting within three to five days of the foliar application, mainly in
the foliage that was expanding at application. Some crops have shown even

greater height control and less phytotoxicity when chlormequat is used with

12

daminozide because the chemicals affect different points in the pathway of GA
biosynthesis. Also, applicators should be aware of the behavior of these two
chemicals when they are applied to the foliage. Both chemicals are highly water-
soluble and move slowly into the wax layer of the leaf, causing them to be more
effective the longer the foliage remains wet, which also allows the chemicals to
be washed from the foliage if the plants are exposed to rain or watered soon
after application (Barrett, 1992).

The effect of chlormequat varies among species. Initial studies that were
performed when chlormequat was released onto the market have shown minimal
efficacy in height control with plants that have flushes of growth, such as oak.
Height control in the more vigorous plants was obtained only after high rates
were applied. Plants that have slow, continuous growth, however, were more
responsive to low PGR rates (Cathey, 1964). Plants that were grown under long
photoperiods and treated with chlormequat were also unaffected by the chemical
treatment (Cathey, 1964).

Asher (1963) determined the effects of chlormequat foliar and soil drench
applications on the growth and development of slash pine. Foliar applications,
with or without a surfactant, reduced shoot growth, but there was substantial
foliar damage. Soil drench applications, however, were not phytotoxic and
reduced shoot growth and needle length. Another benefit observed from the
PGR applications was a reduction in seedling transpiration, which could be due
to several factors: 1) reduction in stomatal opening either directly or because of
reduced photosynthetic rate, 2) reduced water use, or 3) retarded root

development.

13

Ancymidol

The second PGR in this category is ancymidol (A-Rest; SePRO
Corporation, Carmel, Ind.), which is commercially important in ornamental
horticulture, floriculture, and turf grass production (Rademacher, 1991).
Ancymidol became available to growers in the 1970s as another effective tool in
controlling height as either a soil drench or foliar application. Some early
research with floriculture crops has shown the chemical to be less effective when
used as a soil drench on plants potted in media with high percentages of pine
bark humus (Larson, 1992), which is important considering the types of media
that are often used when conifer crops are containerized.

Some precautions need to be considered when this PGR is used.
Floriculture crops have exhibited necrotic spotting when ancymidol is applied to
the foliage, especially when temperatures exceed 21 °C. Also, cost is a great
deal higher for ancymidol compared with other PGRs discussed here (Barrett,
1992)

The effects of ancymidol, Ethrel (an ethylene-releasing compound), CCC
(chlormequat), and daminozide were determined on the growth of lodgepole pine
[Pinus contorta var. Iatifolia (Engelm.) Critchf.] and white spruce [Picea glauca
(Moench) Voss] (Weston et al., 1980). The purpose of this trial was to reduce
root binding and increase transplanting success of containerized pine and spruce
trees. Ancymidol was applied as a soil drench, whereas the other chemicals
were applied as a foliar spray. For lodgepole pine, all four PGRs reduced shoot
height and caused little or no leaf chlorosis. Ancymidol and Ethrel reduced the

shoot to root ratio more than the other treatments did. The PGR applications

14

reduced shoot height and decreased the number of lateral branches produced
by white spruce, which incurred little or no foliar damage.

Ancymidol reduced shoot growth in Monterey pine (Pinus radiata D. Don)
when used as a foliar spray (Hield et al., 1977). Four- or five-year—old Monterey
pines were treated with ancymidol. Shoot growth was reduced (by 69%) for up

to six months, and there was no injury, except for a possible minor leaf curl.

Uniconazole and Paclobutrazol

Uniconazole and paclobutrazol [Sumagic (Valent USA Corporation,
Walnut Creek, Calif.) and Bonzi (Uniroyal Chemical, Middlebury, Conn.)] are
classified as triazoles and are extremely important in growth regulation of
ornamental crops (Rademacher, 1991). Paclobutrazol was introduced into the
industry in the early 19803 as a foliar spray or soil drench, and can be used on
an extensive number of plants, including ornamental trees and shrubs (Keever
and West, 1992; Kimball, 1990; Larson, 1992). Uniconazole was released a few
years later and effectively controlled height at lower concentrations than
paclobutrazol and many other PGRs. Foliar treatments of uniconazole at 5 or 10
ppm were as effective as paclobutrazol at 100 ppm, ancymidol at 125 ppm,
chlormequat at 2000 ppm, or daminozide at 5000 ppm (Larson, 1992). The main
benefits from uniconazole and paclobutrazol are shortened internodes, darker
green foliage, an increase in epicormic bud development, or all three (Hickman,
1986; Kimball, 1990; Ruter, 1994). These treatment effects diminish after a
couple of years, when normal growth resumes.

Applicators should be cautious when using the triazoles. Mixing and

15

application of both chemicals must be accurate because of potential severe
damage (Hickman, 1986; Larson, 1992). The method of chemical transportation
within plant tissues should also be considered when paclobutrazol and
uniconazole are used. These chemicals do not move in the phloem, limiting the
effects to the foliage that the chemical contacts. However, the chemicals readily
move through the xylem, which transports material from the roots upward into
the growing tips of the plant, making soil drenches more effective in distributing
the PGRs throughout the plant (Barrett, 1992; Hickman, 1986; Sterrett, 1985).
Drought stress and high fertilizer levels can also hinder the effects of
paclobutrazol. In research performed on floriculture crops, healthier, more
vigorous plants showed a greater reduction in height than stressed plants
(Barrett, 1992).

Four concentrations of uniconazole (0, 2, 4, or 16 mg L") reduced
terminal shoot length of Fraser fir [Abies frasen' (Pursh) Poir] by 22% to 45% and
increased the number of lateral buds by 20% to 24%. However, this effect on
lateral buds varied by date of application. The earlier application in May
increased lateral bud formation, whereas the June and September application
caused a 45% and 34% reduction in lateral bud formation, respectively (Hinesley
et al., 1998). Application of the chemical early in the growing season hindered
apical dominance, which resulted in increased lateral bud formation, produced a
shorter, more dense tree than the control, and created the desired look for
tabletop Christmas tree specimens. At higher concentrations (greater than or
equal to 8 mg L"), foliar problems such as chlorosis. necrosis, or abscission

occurred.

16

Uniconazole treatments applied as a soil drench to 3-year-old loblolly pine
had decreased height growth by 54% to 55% by the three months following
treatment (Barnes and Kelley, 1992). Diameter growth was also reduced,
although not as much as height. Needle density (needles/cm of shoot)
increased by 24% to 28%. This reduction in height was accompanied by a
denser arrangement of needles, resulting in a smaller, fuller specimen.

Keever and West (1992) treated Leyland cypress [X Cupressocypan's
leylandii (A.B. Jacks. & Dallim.) Dallim. & A.B. Jacks] and thorny elaeagnus
(Elaeagnus pungens Thunb. ‘Fruitlandii’) with uniconazole as a soil drench and
foliar application to determine the effects on established landscape plants. The
higher-rate (45 mg a.i. per plant) soil drench treatment reduced shoot dry
weights of elaeagnus more than the other treatments, and the growth
suppression effects lasted up to two years after treatment. However, the shoot
dry weight of the foliar-treated plants was unaffected. For the leyland cypress,
neither the application rate nor the application method affected the shoot dry
weights. In a subsequent short-term experiment, a group of containerized
leyland cypress was treated in the same manner as the field-grown specimens.
Growth was reduced by 37% to 48%. Keever and West (1992) suggest that this
may be due to the age of the plants and the soil environment. In Wheeler’s
(1987) study with paclobutrazol on loblolly pine, the plants treated in the
containers may have shown a better response because the chemical was able to
reach a larger percentage of the root system, which allowed the plant to take up
more of the PGR to transport to the growing tips of the shoots. As with many

other studies of uniconazole effects (Warren, 1990; Warren et al., 1991), there

17

were no phytotoxic or abnormal foliar effects on either species in any of the
treatments.

In an effort to reduce maintenance pruning costs in container-grown
ornamental nurseries, Warren (1990) studied uniconazole as a soil drench and
foliar application on 13 species common in the industry. Ten of the species
tested showed a reduction in shoot dry weight: glossy abelia (Abe/fa x grandiflora
R. Br.), Japanese barberry (Berben‘s thunbergii DC. ‘Atropurpurea’), forsythia
(Forsythia x intermedia Zab. ‘Spectabilis’), Carolina yellow jessamine
[Gelsemium sempervirens (L.) St.-Hil.], winter jasmine (Jasminum nudiflorum
Lindl.), crapemyrtle (Lagerstroemia indica L. Natchez”), pyracantha (Pyracantha
coccinea Roem. ‘Lalandei’), and azalea (Rhododendron mucronatum G. Don.
“Delaware Valley White’, R. X ‘Formosa’, and R. X ‘Gilbraltar’). After the plants
were treated, the growth-controlling effects were observed for up to 120 days for
most species. The soil drench method was generally more effective than the
foliar applications for most species (Warren, 1990; Warren et al.,1991).

Paclobutrazol as a soil drench treatment effectively controls the height of
Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] and loblolly pine seedlings,
but it is not effective on older trees (Wheeler, 1987). This tree age x treatment
interaction is believed to be related to the activity of the chemical once it enters
the soil and to the root growth habit of the trees studied. Paclobutrazol as a
wettable powder binds tightly to colloidal particles, causing it to move slowly
through the soil profile, especially in highly organic soils. Douglas-fir and loblolly
pine roots can extend several feet into the soil; therefore, the chemical may not

have come into contact with enough of the root system to have affected growth.

18

Wheeler also suggested that the lack of response in conifer species treated with
PGRs may be due to differences in gymnosperm and angiosperm vascular
systems. Because of the lower hydraulic conductivity of tracheids compared with
vessels, slower movement of chemicals into the canopy system of gymnosperms
may be reduced; therefore paclobutrazol should be used in containers in which
the media can be modified for greater chemical mobility and better contact
between the chemical and roots.

Pyracantha and Juniperus L. have shown responses to treatment
applications of paclobutrazol (Ruter, 1994). Paclobutrazol was applied as a
liquid soil drench to containerized Pyracantha and resulted in a 36% height
reduction and a total biomass reduction of about 30%. Pyracantha treated with
PGRs were evaluated 17 months after treatment and showed a continued
reduction in shoot dry weight. Treatments did not reduce the height of
Juniperus, but there was a decrease in horizontal shoot growth and an increase
in the amount of white roots covering the root ball of the treated plants. This
increase in root quality could prove beneficial in helping to reduce plant stress
that is generally associated with container-grown crops. The Juniperus were
also evaluated 17 months after treatment, but there was no effect on final shoot
dry weight.

Groninger and Seller (1997) studied the effects of soil drench applications
of paclobutrazol to sweetgum (Liquidambar styraciflua L.) and white pine (Pinus
strobus L.). The plants were potted in two soil types (clay loam or sandy clay
loam) and received adequate amounts of water or were drought stressed. For

the well-watered trees, paclobutrazol reduced shoot growth by 53% for

19

sweetgum and 23% for white pine. The drench applications were more effective
for sweetgum in the clay loam than the sandy clay loam, and soil type did not
influence the effectiveness on shoot growth for white pine. Regardless of soil
type and water availability, paclobutrazol reduced the total leaf area and specific
leaf area of sweetgum and reduced the needle length of white pine. The main
conclusions from this trial are that paclobutrazol effectively reduces shoot growth
for the species tested, even if drought occurs before and after the PGR
application.

As mentioned earlier, the mechanisms of action for paclobutrazol cause
the chemical to be more effective as a soil drench (Barrett, 1992; Hickman,
1986) than a foliar application. Sterrett (1985) determined the effectiveness of
stem injection treatments of paclobutrazol to several woody plant species.
California privet (Ligustrum ovalifolium Hassk.), red maple (Acer rubrum L.),
white ash (Fraxinus americana L.), yellow poplar (Liriodendron tulipifera L.), and
American sycamore (Platanus occidentalis L.) showed height reductions with
increasing concentrations of paclobutrazol. This information could prove useful
in the attempt to use PGRs without causing adverse effects to target and
nontarget species. The foliar applications can damage target species by causing
phytotoxicity in leaves, and damage can occur to nearby species because of
chemical drift.

Paclobutrazol has also been shown to aid in other plant functions.
Douglas-fir and white spruce grown in sand were treated with paclobutrazol as a
shoot dip or a soil drench (Van den Driessche, 1989). The most productive a.i.

rate was 5 mg L", which not only caused a reduction in shoot and root dry weight

20

but also reduced stomatal conductance (thought to be due to an increase in
abscisic acid), transpiration, and needle areazweight ratio. Once again, the
drench treatment showed better results than the shoot dip treatment; however,
stomatal conductance of the drenched white spruce increased with increasing
paclobutrazol concentration. The drench treatments had an additional benefit:
they increased the xylem water potential by 0.7 MPa above that of the controls (-
1.8 MPa).

In a more recent study, Van den Driessche (1996) determined the effect
of paclobutrazol on survival of conifers under different levels of moisture stress.
Paclobutrazol did not increase seedling survival under drought, probably
because the seedlings were planted in large sand beds, which gave the roots an
unlimited area for growth and allowed the plant to exploit a larger area of soil for
water. In a more limited environment, such as a one-gallon container, there
would be greater control of PGR uptake. Transpiration rates would be reduced
more effectively, causing the plant to use the available water in the container
more slowly. There was an increase in water use efficiency (WUE) because of
paclobutrazol applications. Whole-plant water use was determined by
periodically weighing the plants, and WUE was calculated as the increase in dry
weight divided by water use between start of the experiment and the harvest.
During the first 31 days of treatment, WUE was increased by 240%. However,
this effect decreased over time.

The induction of drought resistance in jack pine (Pinus banksiana Lamb.)
from paclobutrazol applications was determined by Marshall et al. (1991).

Paclobutrazol was applied to the seedlings as a soil drench, and then drought

21

conditions were initiated two weeks later. After five days of drought, transpiration
rates of the paclobutrazol-treated seedlings fell below that of the control plants,
and the treated seedlings had higher water potentials than the controls did. After
two weeks of drought, 25% of the control seedlings had reached permanent
wilting point, whereas none of the paclobutrazol-treated plants had wilted or died.
During a second study, jack pine seedlings treated with paclobutrazol were
subjected to extremely high temperatures and drought stress in a greenhouse.
After 10 days of drought and high temperatures, all of the control plants had
died, but 89% of the PGR-treated trees had survived. These results may have
been due to the ability of paclobutrazol to influence stomatal closure, which is a

physiological mechanism normally regulated by ABA.

Synthetic Cytokinins

Benzyladenine (BA) and benzylaminopurine (BAP) are synthetic
cytokinins used to manipulate growth in ornamental and floriculture crops.
Unlike the PGRs mentioned above, BA and BAP act like natural plant hormones
to stimulate plant growth instead of inhibiting other natural hormones. Cytokinins
have been shown to release bud dormancy in woody plants (Kramer and
Kozlowski, 1979) and are responsible for promoting lateral branching (Barrett,
1992), which makes BA and BAP possible treatments for promoting more dense,
less leggy conifers.

In a study performed on loblolly pine, BA effectively controlled height
growth when the trees were treated at the high rate (1000 ppm), not the low rate

(500 ppm), in August (Blake and South, 1991). The treatment in August at the

22

lower rate and both treatments in September did not effectively in control height.
However, data from this same study showed that at much higher rates, BA
caused abnormal bud development, inhibition of secondary needle extension,
delayed budbreak, and reduced root development.

Lateral shoot production of balsam fir can be increased by dipping shoots
in a liquid solution of BA (Little, 1985). When the BA was applied to trees, an
increase in lateral bud number, branch diameter, and lateral shoot production
was observed. However, the lateral and whorl shoot elongation decreased
during the growth cycle of the following season, which could have been due to
more competition for nutrients because there were more lateral shoots. The
most responsive treatments were those applied after the untreated branches had
reached 70% of their final length. Phytotoxicity was observed after multiple
applications of BA, whereas the single treatments caused minimal or no
phytotoxicity. Some phytotoxicity was also caused when BA was applied too
soon to the new growth; the shoots temporarily changed from green to purple.

Mulgrew and Williams (1984,1985) determined the effects of BA on bud
(development and branching of Scotch pine (Pinus sylvestn‘s L.) and Colorado
spruce (Picea pungens Engelm.). Different concentrations of 6-BA, 6-BA plus
various surfactants, and 6-BA plus Wilt-Pruf were applied to Scotch pine.
According to the product’s material safety data sheet, Wilt-Pruf is typically used
as an antitranspirant, but it was used in this trial as a spreader-sticker. The
surfactants and Wilt-Pruf were tested to increase the effectiveness of BA
applications in promoting bud development. All of the treatments resulted in an

increase of fascicular buds, the addition of Wilt-Pruf did not improve the

23

effectiveness of the applications, and only a few of the surfactants improved the
effectiveness of the BA treatments. However, not all of the buds formed lateral
branches the following growing season, and the branches that did form were
short and reduced the aesthetic appearance of the trees. The foliage of treated
plants was also unattractive because of shorter, chlorotic needles. The
applications of 6-BA to Colorado spruce had similar results. In this trial, 6-BA
was applied to the foliage of the trees at various developmental stages. The
1000-mM application at budbreak and to the pruned summer hardwood trees
resulted in the highest increase in bud number. After application of the chemical,
the area of increased axillary bud development turned bright red. As with Scotch
pine, the buds did not form lateral branches the following growing season.
Another possible use for BA is to help stimulate growth in fir seedlings
(Bryan and Seller, 1991). During the first few growth cycles of Fraser fir
seedlings, growth is slow. Also, once dormancy is established, it can be broken
only by chilling treatments (Bryan and Seller, 1991; Cazell and Seller, 1992).
Additional cultural treatments such as heat, supply of nutrients, or extended
photoperiods cause stunting, loss of apical dominance, and lack of symmetry
(Hinesley, 1982), all undesirable traits for tabletop Christmas tree production. To
promote faster and more uniform growth, BA at 50 ppm and 100 ppm was
applied to Fraser fir seedlings’ foliage. After 20 weeks of applications, 86% of
the seedlings sprayed with BA at 100 ppm had actively growing shoots and 58%
of the seedlings sprayed with 50 ppm had actively growing shoots. The control
treatment had only 31% active shoot growth. The stem diameter of the new

growth was substantially increased, whereas the diameter of the older stem

24

tissue did not increase as much. One detrimental effect of the higher application
rate was a decreased root-weight to shoot-weight ratio, which could cause
survival complications once the seedlings are planted outdoors.

Benzylaminopurine has been found to increase lateral bud densities and
inhibit central leader elongation in five- or six-year-old balsam fir, resulting in
more densely branched trees (Little, 1984). However, when very young Fraser
fir seedlings were treated with BAP, there was a decrease in the formation of
lateral branches (Cazell and Seller, 1992), suggesting that the age of the tree
could prove to be an important factor in the results obtained. The well-fertilized
trees, treated at the beginning of the shoot elongation period with a large dose of
BAP (600 mg BAP L"), produced the greatest number of lateral buds. The
foliage of the treated trees was shorter and also bluer compared with the
needles produced from overwintering buds. There is a limit to the increase in
Christmas tree density that these treatments can elicit. Not all of the BAP-
produced buds developed into shoots after the required chilling period. Little
(1984) suggests that maybe localized BAP treatments, to specific areas that
need more branches be used.

In relation to tabletop Christmas tree production, the control of apical
growth and bud density could prove beneficial in producing a small, densely
branched specimen. Also, the control of water relations with applications of
paclobutrazol, and possibly other PGRs, could provide higher success rates for
survival during stressful shipping conditions, extended periods in retail stores,
and growth following planting. Unfortunately, it is hard to make general PGR

application recommendations for an entire species because of the highly specific

25

nature of the retardant compounds, meaning there is rarely an obvious
correlation between the plant response and taxonomic classification (Cathey,
1964). For better recommendations to be made for the horticulture industry,
more research needs to be performed for a better understanding of how the

various PGRs work on different species.

26

PHOTOPERIOD AND CHILLING
Definitions

Plant response to the relative length of day and night is called
photoperiodism (Landis et al., 1992; 1999). Along day (LD) photoperiod causes
specific plant functions to occur as a result of more than a certain number of
hours of light in each 24-h period (Odlum, 1991; Thomas and Vince-Prue, 1997;
Wareing, 1956). A short-day (SD) photoperiod treatment also causes specific
functions, but as a result of less than a certain number of hours of light in each
24-h period (Thomas and Vince-Prue, 1997). Photoperiod regulation can be the
single most important environmental factor when plants are grown indoors (Hart
and Hanover, 1979). The manipulation of photoperiod can result in three to four
years of growth obtained in a continuous production period of four to nine
months (Hart and Hanover, 1979).

The photoreceptor responsible for daylength perception and the control of
dormancy is the phytochrome system (Landis et al., 1992; Thomas and Vince-
Prue, 1997; Young and Hanover, 1977b). The regulation of plant growth caused
by the response of phytochrome to photoperiod length occurs only in conifer
seedlings less than three years of age (Young and Hanover, 1976, 1977a,
1977b). Most determinate, single-flush tree species develop an endogenous
system for growth regulation at about three years of age (Hanover, 1980).
Phytochrome can be found throughout the plant, but the largest amounts are
located in the meristematic tissues (Landis et al., 1992), and for most species,
the specific location for perception is thought to be the youngest fully expanded

or partially expanded leaves (Kramer and Kozlowski, 1979; Thomas and Vince-

27

Prue, 1997). The blue-green pigmented phytochrome forms a system that
responds to light two ways: 1) red light (R; 660 nm) converts phytochrome from
the inactive to the active form, causing budbreak, shoot growth, prevention of
cold hardiness, and seed germination, and 2) far-red light (FR; 735 nm) converts
phytochrome from the active to the inactive form, causing bud set, cessation of
shoot growth, induction of cold hardiness, and seed dormancy. Photoperiodic
lighting, as well as natural daylight, contains a high R:FR ratio, which effectively
shortcuts the natural system by keeping the phytochrome in the active form
longer (Landis et al., 1992).

Low temperatures can cause different responses in plants, depending on
their state of growth. The chilling requirement for a plant is the amount and
duration of cold that is needed to break dormancy (Landis et al., 1999; Thomas
and Vince-Prue, 1997). Most plants cannot resume growth once dormancy has
been established until their chilling requirement has been met (Hanover, 1980;
Hart and Hanover, 1979; Kramer and Kozlowski, 1979; Nienstaedt, 1967;
Wareing, 1956). Cold temperatures, alone or combined with 80s, are also
involved in the induction of dormancy (Kramer and Kozlowski, 1979; Landis et
al., 1992). Once dormancy has been broken by sufficient cold treatment, a
critical period of warm temperatures must be experienced for normal growth to
occur (Kramer and Kozlowski, 1979).

Dormancy is defined as an adaptation the plant makes to detrimental
environmental conditions causing the dormant organ to be more resistant than
the nondormant organ (Thomas and Vince-Prue, 1997). The dormancy period of

a plant can be divided into three types of growth inactivity: 1) ecodormancy, the

28

inactive growth condition of a plant as a direct consequence of the surrounding
environmental conditions; 2) ectodorrnancy, which causes cessation of growth as
a result of factors external to the dormant tissue but within the plant; and 3)
endodormancy, which causes growth to cease because of factors within the
dormant tissue (Thomas and Vince-Prue, 1997). Even though buds do not
elongate once they have entered dormancy, plants are still somewhat active
because of appreciable meristematic and metabolic activity (Kramer and
Kozlowski, 1979). The complex process of inducing bud dormancy differs,
depending on species, and can include cessation of shoot growth, morphological
modifications of the resting buds, an increase in the degree of frost hardiness,
increased drought resistance, leaf abscission, and the breaking of
endodorrnancy for growth to resume when the environment is suitable. All of the
processes involved in dormancy have been shown to be influenced by
photoperiod and temperature in some manner (Landis et al., 1992; Thomas and

Vince-Prue, 1997).

Role in Growth and Dormancy
Photoperiod

Photoperiod control is the main trigger for inducing terminal bud dormancy
and determines the duration of extension growth (Landis et al., 1992; Thomas
and Vince-Prue, 1997). Typically, LDs cause shoot growth and prolong
dormancy, whereas SDs cause the cessation of shoot growth and speed up the
dormancy process (Bongarten and Hanover, 1985; Hanover, 1980; Kramer and

Kozlowski, 1979; Landis et al., 1992; Tinus, 1995; Wareing, 1956; Young and

29

Hanover, 1977a). Many growers mimic 808 by shading plants or using growth
chambers to experimentally induce ectodormancy (Kramer and Kozlowski,
1979). Short-day conditions are important in determining the formation of winter
resting buds and the development of resistance to winter cold damage.
However, artificial SD treatments encourage early budbreak in the spring,
allowing possible damage from spring frost (Odlum, 1991). The assumption has
been made that factors responsible for controlling dormancy are stimulated and
exported from leaves and that these stimuli are different between LDs and 803
(Thomas and Vince-Prue, 1997).

Species such as Nonlvay spruce and Scotch pine can achieve cold
hardiness by subjecting them to SDs or low-temperature (about 5 °C) treatments
(Christersson, 1978; Kramer and Kozlowski, 1979). When Norway spruce
seedlings were placed under LDs and low temperature, hardiness was induced
but without causing the trees to become dormant. The photoperiod and
temperature effects seemed to be additive and also independent of one another
for achieving cold hardiness. The maximum amount of cold hardiness with many
species has been observed to occur with cold treatments following SD
treatments, just as in nature (Hart and Hanover, 1979; Kramer and Kozlowski,
1979; Thomas and Vince-Prue, 1997).

Many species of conifers experience a rapid growth phase after the
seedlings become established (Landis et al., 1992; Young and Hanover, 1976).
It is important to increase the photoperiod treatment to the saturation point to
maximize photosynthesis and to increase the photoperiod gradually to prevent

damage to sensitive, new growth. It is also crucial to avoid moisture stress when

30

possible because this can cause free growth to cease and induce dormancy
(Young and Hanover, 1978). The LD photoperiod treatments can shorten the
dormant periods between the growth flushes of the rapid growth phase to such a
short interval that growth appears to be continuous (Thomas and Vince-Prue,
1997). The LD treatments should be applied for the entire duration of the rapid
growth phase (Landis et al., 1992). When Norway spruce was exposed to only a
brief SD, its extension growth decreased or stopped. This response is
comparable to the natural summer dormancy that, unlike winter dormancy, is
more easily induced to break bud when exposed to an L0 treatment (Wareing,
1956). Growth resumed when the plants were placed back under LDs; however,
the duration of growth cessation was positively correlated with the amount of
$03 that were experienced by the plant (Thomas and Vince-Prue, 1997).

In one- to two-year-old blue spruce seedlings, the free growth
characteristic described above is influenced only by environmental factors, not
internal plant functions (Young and Hanover, 1976). However, once the
seedlings reach three years of age or older, they have a brief period of growth in
the spring and no free growth the rest of the season. The induction of true
dormancy in the one— to two-year-old seedlings is not as deep as that observed
in older seedlings (Young and Hanover, 1977a). Also, a minimum period of 20
SDs must be given for bud scales to form. The transition from needle initiation to
bud scale formation seems to be independent of environmental factors in mature
blue spruce but under rigid environmental control in younger seedlings.

Bongarten and Hanover (1985) determined the effects of an accelerated

growth regimen on blue spruce seedlings. This program consisted of extended

31

photoperiod, frequent watering, and fertilization to encourage the most growth
possible. In a greenhouse environment, this treatment resulted in seedlings
reaching field planting size within six months as opposed to three years
outdoors. The trees with accelerated growth continued to have increased growth
compared with the naturally grown conifers for up to seven years after initial
treatment. The greenhouse-grown trees also had increased cold hardiness and
later spring budbreak, which, following the treatments, made them less prone
than seedlings from outdoors to frost damage in the spring.

Often, ectodormancy is not achieved until a minimum exposure to 803
has occurred. When the plant is not in ectodormancy, it can resume growth after
simply being subjected to LD treatments (Kramer and Kozlowski, 1979; Landis et
al., 1992; Odlum, 1991), which is why continuing SD treatments even after
visible shoot extension has ceased is important. However, once endodormancy
has been achieved, only the required cold period will break dormancy (Kramer
and Kozlowski, 1979; Thomas and Vince-Prue, 1997).

Odlum and Colombo (1988) determined the effects of SD treatments on
black spruce [Picea man'ana (Mlll.) B.S.P.] before overwintering. Compared with
the trees that received natural daylength (ND), SD trees broke bud sooner (by 14
d) in the spring and continued growth longer into the fall. Although the promotion
of increased growth the following year would allow larger trees to be produced in
a shorter time, there are some drawbacks. Frost damage can occur because of
buds breaking too early in the spring and also to trees still growing in the fall,
when early frosts can occur. Also, SD-treated trees may have broken bud

sooner in the spring, but they did not reach 100% bud initiation until three to six

32

weeks later than the ND trees, resulting in a less uniform crop.

Some plants can experience the same effects caused by LD conditions
from treatments of SD with a brief period of light during the dark period, revealing
that the dark period is a major factor in the photoperiodic regulation of dormancy
(Thomas and Vince-Prue, 1997; Young and Hanover, 1977b). Night-interruption
(NI) treatments are effective because the brief light stimulation causes the
phytochrome to become active, disrupting the normal inactive state that occurs
at night when the R:FR ratio is low (Landis et al., 1992).

Tlnus (1995) studied the effect of periodic lighting on blue spruce,
developing a light fixture with a 400—W high-pressure sodium arc lamp and an
oscillating parabolic mirror as the single light source for a 6-by—15-m greenhouse.
The light was placed at the far end of the greenhouse and oscillated a beam of
light from one end to the other repeatedly throughout the dark period. Each
cycle took a total of 1 min. The single light source successfully prevented
dormancy in the apical bud from as far as 13 m away, and at light intensities as
low as 0.5 [E m'2 sec". Seedlings that were shielded from light treatments
during the dark period stopped growing and began setting buds within about 4
weeks. The number of seedlings setting bud increased with increasing distance
from the light source. However, trees that were exposed to light treatments
during the dark period continued to grow in height until January (28 to 32 weeks
after the treatments began).

One study has shown that simply extending the photoperiod may not be
enough of a stimulus to prevent dormancy in Picea spp. (Thomas and Vince-

Prue, 1997). An LD photoperiod of 17h was more effective when FR light was

33

present in the latter part of the day than earlier in the day, which suggests that
the timing of FR light exposure, rather than simply extending the photoperiod,

may be an important factor in dormancy control.

Temperature

Temperature can have a profound effect on the processes that occur in
plants. Not only is the temperature during bud expansion important in
determining the amount of shoot growth but also the temperature experienced
during bud formation is crucial. Species that have fully preformed shoots in their
winter resting buds experience better shoot growth when optimum temperatures
are given during bud formation rather than shoot expansion (Kramer and
Kozlowski, 1979; Landis et al., 1999). Bud development, dormancy, and cold
hardiness are affected by the environmental temperatures maintained during the
hardening phase (Dormling, 1993; Landis et al., 1992). The majority of conifers
experience little seedling growth when the temperature drops below 10 °C, but
basic processes like photosynthesis and respiration continue at much slower
rates at lower temperatures. For conifers, controlling the temperature of the root
zone is also crucial, since the slightest exposure to warm temperatures can
change seedling dormancy (Landis et al., 1992; 1999).

The ambient temperature is an important factor to consider when plants
are placed under SDs (Thomas and Vince-Prue, 1997). Plants that are exposed
to higher temperatures while under SD treatments grow more the following year,
which is due to more prlmordia being able to differentiate under the warmer

conditions, resulting in an increase in the next year’s shoot growth. If dormancy

34

induction is desired, it is important to have cool temperatures for true dormancy
to occur (Hart and Hanover, 1979; Thomas and Vince-Prue, 1997). High
temperatures combined with SDs could cause only cessation of growth rather
than induction of endodormancy.

As mentioned previously, many conifers experience continuous growth or
a series of sequential flushes, which makes Optimizing the temperature of the
growth environment important (Landis et al., 1992). If the temperature drops too
low, shoot growth could cease and cause the terminal bud to set, making it
difficult to restart growth because of possible endodormancy induction, which
would require that the plant have sufficient chilling before growth could resume
(Kramer and Kozlowski, 1979; Landis et al., 1992). Shoot growth is directly
proportional to temperature, so warm conditions can cause these fast-growing
species to rapidly grow beyond target points (Landis et al., 1999). The
temperature required to obtain the best growth in conifers and many other
species depends on what type of growth is desired (Landis et al., 1992).

Young and Hanover (1978) found that extremes in temperature can have
a significant effect on dormancy induction and bud formation of blue spruce.
Seedlings that were grown under a 24-h photoperiod and constant temperatures
of 12, 18, 25, and 31 °C did not set bud, and less growth occurred at 12 and 18
°C than at 25 °C. Most of the seedlings from the 31 °C treatment experienced
leaf browning and death. The seedlings were also subjected to nutrient stress by
a lowering of the amount of nitrogen supplied to the plants. This stress caused
growth to cease, chlorosis to develop within two weeks, and 100% of the buds to

set within six weeks. However, when the stress was relieved, all of the seedlings

35

became green and broke bud within two weeks.

Fraser fit is a difficult species to grow in greenhouses because of the lack
of uniformity in crop growth if specific growing conditions are not met. Fraser fir
seedlings that did not receive chilling lost apical dominance, had deformed or
aborted terminal buds, had stunted growth, and lacked the characteristic
symmetry found in naturally occurring Fraser fir trees (Cazell and Seller, 1992;
Hinesley, 1982; Seller and Kreh, 1987). However, seedlings that received
chilling treatments had the greatest height growth compared with unchilled and
benzylaminopurine-treated trees (Cazell and Seller, 1992). Also, the time to
reach budbreak decreased with increased chilling (Hinesley, 1982). Hinesley
(1982) states that the cold requirement for Fraser fir is approximately two to four
weeks for containerized seedlings and four to six weeks for nursery-grown 1-0
seedfings.

Photoperiod also affected Fraser fir. Under low durations of cold
treatment, Fraser fir seedlings are quite sensitive to photoperiod changes
(Hinesley, 1982). Trees that received SDs to induce bud formation before
chilling experienced budbreak and improved growth (Cazell and Seller, 1992;
Hinesley, 1982). Seedlings placed under LD treatments after receiving little or
no chilling experienced 100% budbreak; however, photoperiod did not have an
effect on trees with more than six weeks of cold treatment. Artificial chilling of
Fraser fir has been shown to accelerate normal growth processes, producing
specimens in a greenhouse within 15 months, similar to four-year-old plants.
However, trees exposed to natural chilling conditions generally have more

uniform growth and better symmetry than greenhouse-grown trees (Seller and

36

Kreh, 1987).

Noble fir (Abies procera Rehd.) also exhibits specific environmental
requirements to induce ectodormancy when grown out of its native high-elevation
environment (Tung and Deyoe, 1991). Various combinations of temperature,
photoperiod, and moisture stress were applied to trees. Ectodormancy was
induced in noble fir under 1) warm/dry (25/20 °C and —1.2 MPa) and cool/wet
(18/12 °C and —0.6 MPa) in the first two weeks of treatment, 2) SDs/warm (11 h)
and LDs/cool (15 h) in the third and fourth weeks of treatment, and 3) SDs and
cool thermoperiods that independently accelerated dormancy induction in weeks
five through 12. Tung and Deyoe were able to explain between 10% and 71% of
the response noble fir had to photoperiod, temperature, and moisture stress
treatment combinations, which illustrates the complex environmental and
physiological interactions involved in inducing terminal bud set of fir species.

A day/night temperature differential could prove useful in maximizing
desired growth response in conifers (Hellmers and Rook, 1973; Mellerowicz et
al., 1992). The scheduling of bud initiation was shown to be unaffected by day
temperature but was influenced by night temperatures (Odlum, 1991). When
day temperature was below 10 °C or above 22 °C, then bud initiation was
delayed by 18 to 46 d. The most rapid bud initiation occurred when night
temperatures were between 12 and 14 °C. Hellmers and Rook (1973)
determined that radiata pine had increased growth rates when exposed to cool
day (17 to 23 °C) and cool night (5 to 11 °C) temperatures, with night
temperature being the most important daily temperature characteristic. The

cooler night temperatures also resulted in higher root to shoot ratios. A possible

37

mechanism is that carbohydrates are conserved during the cool nights, which
becomes more important as the seedlings grow larger.

For greenhouse-grown conifers, Landis et al. (1992) have defined a
protocol for inducing dormancy and the hardening-off process. First, the
temperature should be lowered to a level just below optimum for four to six
weeks to reduce shoot growth but remain warm enough to allow caliper and root
growth. Second, temperatures should be lowered to just above freezing, also for
four to six weeks, whereupon bud-chilling requirements are met, the
development of cold hardiness is achieved, root growth potential is increased,

and a higher resistance to mechanical damage is acquired.

Seed Source Effects on Photoperiod and Temperature

Within species with a wide physiographic range, critical daylength (CD)
can vary widely among ecotypes (Nienstaedt, 1967; Thomas and Vince-Prue,
1997). This characteristic makes the use of photoperiodic lighting crucial in
maximizing growth and uniformity when a variety of species and ecotypes is
being grown together (Landis et al., 1992, 1999). The nonresponsive species
are not harmed, the responsive species benefit, and the operating costs to the
grower are minimal. For example, blue spruce from southern latitudes grew
taller than northem-latitude species in an outdoor nursery in Michigan
(Bongarten and Hanover, 1985). Races of a species from lower latitudes can
become subject to autumn frost damage when moved to higher latitudes
because they tend to grow longer into the season, with terminal meristem tissue

generally incurring in the most damage because it is the last to quit growing and

38

become hardy (Kramer and Kozlowski, 1979; Landis et al., 1992, 1999). Light
quality may also be altered, depending on the latitudinal source of a species.
Picea seedlings from a northern ecotype (lat. 66°N) had 100% bud set when
placed under a 16-h day by using white fluorescent lamps lacking FR light.
Plants from lower latitudes (47 to 59°N) had 0% bud set under the same
conditions.

Dormling (1979) found that Norway spruce populations of southern origin
are more sensitive to light, temperature, and photoperiod than populations of
northern origin. The critical night length (CNL, the night length that results in
50% bud set in a plant population) for Scandinavian populations can vary from
1.7 to 5.8 h (69°N lat. to 56°N lat.). Lower light intensity treatments give shorter
CNLs and higher light intensities give longer ones. Spruces of southern origin
that are moved north will cease growth later in the season because of a longer
CNL requirement and their ability to register more of the twilight as day. Also,
when southern species are moved north, the cooler night temperatures (10 °C
rather than the normal 20 °C) cause growth to continue longer than it would in
the plants’ native range, which causes problems by increasing the probability of
early frost injury.

Elevation of the seed source of ponderosa pine (Pinus ponderosa var.
scopulorum Engelm.), Engelmann spruce [Picea engelmannii (Parry) Engelm.],
and Douglas-fir can have an effect on acclimatization and deacclimatization of a
species (Burr et al., 1989). Species from higher elevations take longer to
transition from full dormancy to quiescence than those from lower elevations.

However, transition time from quiescence to 50% budbreak under

39

deacclimatizing conditions was quicker for species from lower elevations than

those from higher elevations.

Abscisic Acid, Gibberellins, and Cytokinins

Because leaves are the site of daylength perception, it is believed that
dormancy-controlling stimuli are produced in the leaves and then translocated to
the buds and shoot apices because of LDs, $03, or both (Thomas and Vince-
Prue, 1997). The three main hormones involved in dormancy induction are ABA,
GA, and cytokinins (CK) (Hanover, 1980; Thomas and Vince-Prue, 1997). Some
species have shown the existence of a dormancy-inducing stimulus produced by
the leaves under appropriate photoperiods. Exogenous applications of ABA
have been shown to influence some of the changes involved with dormancy,
mainly a reduction or cessation of shoot extension, but dormancy components
are unaffected. Observations in ecotypes of Picea have indicated a correlation
between photoperiodic behavior and transient increases in ABA following SD
treatments (Thomas and Vince-Prue, 1997). The exact role of ABA in dormancy
regulation is, however, still unclear. Work performed with some woody species
has shown an SD-induced block of the biosynthesis of GAZO. Since it is the
precursor of GA,, the blocking of this GA could be an early step in stopping shoot
growth and inducing dormancy. Dormancy does not appear to be controlled by
one factor alone but by a dormancy inhibitor (possibly GA) and a dormancy
promoter. As is the case with LD treatments, GA applications could not
substitute for the low temperature requirement needed by the plant once it has

entered deep dormancy. Finally, CK appears to be involved in the breaking of

40

bud dormancy in woody plants rather than in dormancy induction. Synthetic
applications of CKs have been found to stimulate dormant bud growth in some

woody species but not others (Thomas and Vince-Prue, 1997).

41

ANTITRANSPIRANTS
Definitions

Transplration is the loss of water from a plant in the form of vapor and
primarily depends on the difference between the humidity levels inside and
outside the leaf (Barrett, 1990). Transplration is important as a cooling
mechanism for plants; however, too much water loss creates a water deficit in
plants, which inhibits growth and can cause injury or death from severe
dehydration (Barrett, 1990; Das and Raghavendra, 1979; Gale and Hagan, 1966;
Kozlowski and Pallardy, 1997).

Stomata are microscopic openings surrounded by two guard cells in the
epidermis of plant leaves (Kozlowski and Pallardy, 1997). As guard cells take in
water and become turgid, they separate to cause the stomata to open. These
openings allow plants to transpire and CO2 to enter the leaf interior for
photosynthesis to occur (Das and Raghavendra, 1979; Kozlowski and Pallardy,
1997; Zelitch, 1974). Transplration is reduced relatively more than
photosynthesis when an increase in stomatal resistance occurs (Gale and
Hagan, 1966). Since this is the primary path for water to escape from leaves, it
is the primary focal point for managing plant water loss (Das and Raghavendra,
1979)

An antitranspirant is any material applied to plants to retard transpiration
and should operate at the leaf-air interface (Gale and Hagan, 1966). Reducing
transpiration rates could prove beneficial to plant water management and relieve
plant water stress by keeping the plant tissue more turgid (Das and

Raghavendra, 1979). However, normal growth (Gale and Hagan, 1966) and

42

overall photosynthesis rates should not be detrimentally affected when lowering
of transpiration rates is attempted. Applying antitranspirants can reduce
transpiration rates within 24 h, with gradual recovery of normal rates within 10 to
20 d. Most growers use antitranspirants when plant survival, not maximum
growth, is critical. Antitranspirants can be grouped into three categories in
relation to their mode of action: 1) metabolic antitranspirants that inhibit or
restrict the opening of stomata; 2) film-forming compounds that form an external
physical barrier over leaf surfaces, resulting in a decrease in water vapor loss;
and 3) reflecting materials to decrease transpiration rates caused by lower leaf

temperature.

Metabolic Antitranspirants

During transpiration, water vapor moves through a series of resistances
as it diffuses out of a leaf: 1) stomatal aperture resistance, 2) boundary layer
resistance, and 3) possibly a mesophyll resistance (Gale and Hagan, 1966;
Poljakoff-Mayber and Gale, 1967). Carbon dioxide encounters the same
resistances as it diffuses into the leaf, but mesophyll resistance can be as great
as stomatal and boundary layer resistance combined because of the presence of
a physical resistance to CO2 diffusion and to mass transport within mesophyll
cells and a chemical resistance to the acceptance of CO2 by chloroplasts.
Therefore, a decrease in stomatal conductance should reduce transpiration
relatively more than photosynthesis, since stomatal effects on 002 diffusion can
be negligible compared to mesophyll resistance. However, there is a certain

degree of stomatal closure at which CO2 diffusion, and subsequently

43

photosynthesis, will eventually be affected.

Metabolic antitranspirants function by causing stomata to close, thereby
decreasing stomatal conductance to prevent excessive water loss. Compounds
of this nature should remain confined to the leaf epidermis and have low mobility
within the plant to avoid toxic adverse effects to other plant systems (Gale and
Hagan, 1966). Abscisic acid and phenyl mercuric acetate (PMA) are the most
broadly used antitranspirants in this category (Das and Raghavendra, 1979).
Although it effectively decreases transpiration rates, PMA may penetrate into
mesophyll cells of the leaf, disrupting normal metabolism and resulting in
phytotoxicity (Squire and Jones, 1971; Zelitch, 1974). Various other metabolic
antitranspirants have been tested. Inhibitors of cyclic photophosphorylation
reduced transpiration of Peruvian trumpets (Datura arborea L.) seedlings for up
to 12 (I because of their effects on stomatal closure (Raghavendra and Das,
1977). The antitranspirant EMD 7301 (morphactin) reduced transpiration rates
of seven-week-old cotton plants (Gossypium hirsutum L. Var. Lakshmi) for up to
nine days by decreasing stomatal aperture size (Das et al., 1977).

ABA has proven a reliable and nontoxic compound in this category
(Davies and Kozlowski, 1975; Das and Raghavendra, 1979). Photosynthesis is
generally not directly inhibited by the application of ABA as an antitranspirant,
and there has also been no mention of leaf abscission caused by these ABA
applications. Davies and Kozlowski (1975) showed a 60% reduction in
transpiration when ABA was applied to white ash, sugar maple (Acer saccharum
Marsh), and citrus (Citrus mitis Blanco.). Water loss of white ash was reduced

for more than 21 d after ABA treatments without signs of phytotoxicity, which was

44

determined by measuring leaf resistance with a porometer. Before treatment,
leaf conductance was 1.216 mmol m'2 s", but conductance decreased to 0.013

mmol rn’2 s" after the application of ABA.

Film-Forming Compounds

The film-forming antitranspirants are inert compounds that reduce
transpiration by forming a water-impermeable coating over the leaf surface, and
they generally have long-lasting effects (Das and Raghavendra, 1979). The film
formed over the leaf surface should have greater resistance to water vapor than
to CO2 and O2 to allow normal metabolic processes to continue. Ideally, these
compounds should come from nonphytotoxic emulsions, remain elastic, and
resist degradation from solar ultraviolet radiation, oxidation, and microorganisms
(Gale and Hagan, 1966).

Some of the earliest work performed to determine the efficacy of
antitranspirants showed contradicting results. The antitranspirant SN Ceremul
‘C’ was applied to Monterey pine seedlings in two trials to increase nursery stock
survival during shipping (Jack, 1955). There was no difference between
treatments in the first trial; however, SN Ceremul ‘0’ increased the survival of
‘seedlings in the second trial, which was a result of the poorer-quality plant
material that was treated in the second trial. Since the plants were healthy in the
first trial, it was more difficult to see an improvement in seedlings treated with the
antitranspirant. Three antitranspirants, an emulsified wax, a vinyl latex, and
lanolin, were ineffective in increasing plant survival of ponderosa pine, Jeffrey

pine (Pinus jeffreyi Balf.), or sugar pine (P. Iambertiana Douglas.) (Fowells and

45

Schubert, 1955). Lanolin treatments to ponderosa pine resulted in an 80%
survival compared with a 37% survival of the control treatment.

Davies and Kozlowski (1974) found that silicone emulsions and polyvinyl
chloride solutions reduced transpiration of ash (Fraxinus americana L.) seedlings
for up to 32 d and red pine (Pinus resinosa Alt.) seedlings for well beyond 32 (1.
Red pine seedlings had lower transpiration to photosynthesis ratios when treated
with Dow Silicone, Improved Wilt-Pruf, Keykote, or Folicote. Further analysis of
the red pine needles revealed that the silicone emulsion clogged the stomatal
pores, preventing water vapor and CO2 exchange which eventually caused
chlorosis, altered metabolism, browned the needles, and reduced needle growth.
This was not the case with the ash seedlings, since the flexibility of the foliage
allowed cracking and subsequent breakdown of the film layer for transpiration
and gas exchange to resume over time. Unfortunately, the major drawback with
the film-forming compounds is that they allow more water vapor exchange than
CO2 exchange (Kramer and Kozlowski, 1979).

Wilt-Pruf (terpenic polymer), along with many other antitranspirants
[Plantco (acrylic emulsion), Cloud Cover (information not available), Vapor Gard
(di-1-p-menthene), Dow X2-1337 (polydimethyl slloxane emulsion), Clear Spray
(acrylic emulsion), Folicote (wax emulsion), Plantgard (polyethylene emulsion),
and XEF-4-3561-A (silicone emulsion)], has given variable results on its
effectiveness in preventing plant water stress (Colombo and Odlum, 1987;
Englert et al., 1993a, 1993b; Odlum and Colombo, 1987; Simpson, 1984;
Williams et al., 1990). Wilt-Pruf effectively reduced transpiration of black spruce

for up to 14 (I; however, seedling survival was greatly reduced (Colombo and

46

Odlum, 1987; Odlum and Colombo, 1987). Simpson (1984) found Wilt-Pruf to
have inconsistent effects on white spruce seedlings. One experiment showed a
positive effect on white spruce seedlings by reducing moisture stress for up to 5
d. However, other applications decreased white spruce seedling survival and
storability and also reduced growth and storability of western hemlock [Tsuga
heterophylla (Raf) Sarg] and Douglas-fir. Simpson determined Wilt-Pruf
effective in reducing moisture stress development in lodgepole pine seedlings
without reducing root growth capacity or storability. White spruce and white pine
seedlings that were treated with Wilt-Pruf and subjected to greenhouse drought
conditions experienced high survival rates, low phytotoxic effects, and high
biomass growth along with moderate height growth (Williams et al., 1990).
Moisturin is a commonly used antitranspirant for horticulture crops;
however, there is little evidence that it can be used to aid in crop performance
under stressful conditions. Arnold and Culbertson (1994) found that applications
of Moisturin to bare-root northern red oak seedlings (Quercus rubra L.) did not
affect first-year growth during subsequent container or field production. Douglas-
fir seedlings, whose foliage was dipped into 1:3 concentrations of Moisturin, had
higher mean water potential values (—1.43 MPa) compared with that of the
control treatment (—1.95 MPa) (Rose and Haase, 1995). This result shows that
treated plants were under less water stress than the control. Moisturin reduced
water loss from Washington hawthorn (Crataegus phaenopyrum Medic.) and
other deciduous seedlings (Englert et al., 1993b). There was up to an 80%
reduction in water loss for plants treated with Moisturin, compared with control

seedlings. The investigators determined that a thicker layer of film on stems was

47

more effective at preventing stem dieback.

Reflecting Materials

Reflective antitranspirants can be used alone or in combination with other
antitranspirants and are divided into two categories: 1) generally refiectant
pigments, like kaolinite or whitewash; and 2) compounds that selectively reflect
radiation above 700 nm and below 400 nm but allow the transmittance of all
radiation between (Das and Raghavendra, 1979; Gale and Hagan, 1966).

Abou-Khaled et al. (1970) found that kaolinite applied to plant foliage
grown under high light intensities (1660 umol m“2 8") reduced leaf temperature
by 3 to 4 °C and transpiration by 28%. Photosynthesis rates were not affected
except at low light intensities (249 umol tn‘2 8") because of an increase in the
light compensation and light saturation points.

Naturally occurring glaucous surface waxes have been found to regulate
moisture exchange and increase light reflectance (Reicosky and Hanover, 1978).
Compared with semiglaucous and nonglaucous foliage, glaucous foliage of blue
spruce had the greatest reflectance, at 350 nm, but also showed significant
reflectance up to 800 nm. Species that naturally reflect light energy may be
better choices for plant material that must endure intense solar radiation or when
lower transpiration rates are desired: glaucous foliage can better manage

energy that enters the leaf and regulate leaf temperature.

48

LITERATURE CITED

Abou-Khaled, A., R.M. Hagan, and DC. Davenport. 1970. Effects of kaolinite as
a reflective antitranspirant on leaf temperature, transpiration, photosynthesis,
and water-use efficiency. Water Resources Research 6(1):280—289.

Arnold, MA, and R.K. Culbertson. 1994. Effects of antitranspirant sprays and
hydrophilic polymer root dips on the growth of bare-root northern red oak
seedlings transplanted to the field or containers. SNA Research Conference.

Asher, WC. 1963. Effects of 2-Chloroethyltrimethylammonium chloride and 2,4-
Dichlorobenzyltributyl phosphonium chloride on growth and transpiration of slash
pine. Nature 2002912.

Barnes, AD, and W.D. Kelley. 1992. Effects of a triazole, uniconazole, on shoot
elongation and root growth in loblolly pine. Canadian Journal Forest Resources
22:1—4.

Barrett, J. 1990. Plants and water 101: Understanding VPD. Grower Talks.
September248—52.

Barrett, J.E. 1992. Mechanisms of action; Tips on the use of chemical growth
regulators on florlculture crops. Ohio Florists' Association, Colombus, Ohio.
12—18. In: H. K. Tayama, Roy A. Larson, P. Allen Hammer, and Teresa J. Roll.

Bailey, L.H., and E2. Bailey. 1976. Hortus Third. New York, Macmillan.

Blake, J. I., and DB. South. 1991. Effects of plant growth regulators on loblolly
pine seedling development and field performance. Proc., 6th Biennial S. Silv.
Res. Conf.:100—107.

Bongarten, 3.0., and J.W. Hanover. 1985. Accelerating seedling growth through

photoperiod extension for genetic testing: A case study with blue spruce (Picea
pungens). Forest Science 31 (3):631—643.

Bryan, J.A., and J.R. Seller. 1991. Accelerating Fraser fir seedling growth with
benzylaminopurine sprays. HortScience 26(4):389—390.

Burg, SP, and EA. Burg. 1967. Inhibition of polar auxin transport by ethylene.
Plant Physiology 42: 1 224-1 228.

Burr, K.E., R.W. Tinus, S.J. Wallner, and RM. King. 1989. Relationships among
cold hardiness, root growth potential and bud dormancy in three conifers. Tree
Physiology 5(3):291—306.

Calyx and Corolla. 2000. Holiday 2000 catalog. Calyx and Corolla: 29.

49

Cathey, HM. 1964. The physiology of growth retarding chemicals. Annual
Review of Plant Physiology 15:271-302.

Cazell, B.H., and J.R. Seller. 1992. lnterrnittent short-days and chilling, and
benzylaminopurine affect the growth and morphology of Fraser fir seedlings.
Journal of environmental horticulture 10(4):205—207.

Christersson, L. 1978. The influence of photoperiod and temperature on the
development of frost hardiness in seedlings of Pinus sylvestn's and Picea abies.
Physiologia plantarum 44:288-294.

Colombo, SJ, and KD. Odlum. 1987. Efficacy of six antitranspirants on black
spruce container seedlings. New forests 1(3):239—244.

Crothers, D. 1990. Big profits in little packages. Produce Business. 6:50,52,54.

Das, V.S.R., l. Madhusudana Rao, and PM. Swamy. 1977. Antitranspirant
activity of Morphactin on cotton plants. Indian Journal of Experimental Biology
15:642—644.

Das, V.S.R., and AS. Raghavendra. 1979. Antitranspirants for improvement of
water use efficiency. Outlook on Agriculture 10:92—98.

Davies, W.J. and T.T. Kozlowski. 1974. Short— and long-term effects of
antitranspirants on water relations and photosynthesis of woody plants. J. Amer.
Soc. Hort. Sci. 99(4):297—304.

Davies, W.J. and T.T. Kozlowski. 1975. Effects of applied abscisic acid and plant
water stress on transpiration of woody angiosperms. Forest Science
21(2):191-195.

Dormling, l. 1979. Influence of light intensity and temperature on photoperiodic
response of Nowvay spruce provenances. lUFRO Joint Meeting on Norway
Spruce Provenances and Norway Spruce Breeding, Bucharest.

Dormling, l. 1993. Bud dormancy, frost hardiness, and frost drought in seedlings
of Pinus sylvestn's and Picea abies. Advances in giant cold hardiness. P.H. Li,
and Christersson, L. Boca Raton, CRC Pressz285—298.

Dunberg, A., and L. Eliasson. 1972. Effects of growth retardants on Norway
spruce (Picea abies). Physiologia Plantarum 26:302-305.

Englert, J. M., L.H. Fuchigami, and T.H.H. Chen. 1993a. Bare-root basics.
American Nurseryman. February256—59.

Englert, J.M., K. Warren, L.H. Fuchigami, and T.H.H. Chen. 1993b.
Antidesiccant compounds improve the survival of bare-root deciduous nursery

50

trees. J. Amer. Soc. Hort. Sci. 118(2):228—235.

Florkowski, W.J. and OM. Lindstrom. 1995. Opinions and perceptions of having
a live—cut Leyland cypress as a Christmas tree. HortTechnology 5(2):161—164.

Fowells, HA. and G.H. Schubert. 1955. Planting trials with transpiration
retardants in California. Tree Planters' Notes 20:19—22.

Gale, J. and RM. Hagan. 1966. Plant Antitranspirants. Annual Review Plant
Physiology 17:269—282.

Groninger, J.W. and J.R. Seller. 1997. Soil texture and moisture availability
impacts on the efficacy of soil-applied paclobutrazol. Journal of arboriculture
23(3):89—92.

Grossman, K. 1990. Plant growth retardants as tools in physiological research.
Physiologia Plantarum 78:640—648.

Hamrick, D. 2002. Conifers are hot as a pot plant at Sluitter. FloraCulture
International. Mayz22.

Hanover, J. W. 1980. Control of tree growth. BioScience 30(11):756—762.

Hare, R. C. 1982. Effect of nine growth retardants applied to loblolly and slash
pine. Canadian Journal Forest Resources 12:112—114.

Harry and David. 2000. Christmas 2000 Catalog. Harry and David: 45.

Hart, J. W. and J. Hanover. 1979. Practical requirements for controlled
environment nursery stock production. International Plant Propagators' Society.

Hayashi, T., R.D. Heins, A.C. Cameron and W.H. Carlson. 2001. Ethephon
influences flowering, height, and branching of several herbaceous perennials.
Scientia Horticulturae 91:305—323.

Hellmers, H. and DA. Rook. 1973. Air temperature and growth of Radiata pine
seedlings. New Zealand Journal of Forestry Science 3(3):271—285.

Hickman, G.W. 1986. A new growth regulator for greenhouse plants. California
Agriculture (Nov—Dec):16-17.

Hield, H., R.M, Sachs and S. Hemstreet. 1977. Growth reduction comparisons of
9 plant regulators applied to Pinus radiata. Proceedings of the Plant Growth
Regulators Working Group, Hot Spring, Arkansas.

Hinesley, LE. 1982. Dormancy in Abies frasen' seedlings at the end of the first
growth cycle. Canadian Journal Forest Resources 12:374-383.

51

Hinesley, LE. 1998. Effect of uniconazole on shoot growth and budset of
containerized Fraser fir. HortScience 33(1):82—84.

Jack, JB 1955. Tests of a transpiration inhibitor. Tree Planters' Notes 20:23—25.

Jackson & Perkins. 2000. Holiday Gifts 2000 Catalog. Jackson and Perkins:
3,9,16,27,29,33,34,36,39,47.

Keever, G.J. and MS. West 1992. Response of established landscape plants to
uniconazole. HortTechnology (Oct/Dec):465-468.

Kimball, S.L. 1990. The physiology of tree growth regulators. Journal of
arboriculture 16(2):39—41 .

Kleweno, DD. and V. Matthews. 2000. Michigan Rotational Survey: Nursery and
Christmas Tree Inventory 1999-2000. Lansing, Michigan Department of
Agriculture: 1.

Kozlowski, T.T. and 8G. Pallardy 1997. Physiology of Woody Plants. San Diego,
Academic Press: 411.

Kramer, P.J. and T.T. Kozlowski 1979. Physiology of Woody Plants. Orlando,
Academic Press, Inc: 811

Kuo, C. . and RP. Pharis 1975. Effects of AMO-1618 and B-995 on growth and
endogenous gibberellin content of Cupressus anizonica seedlings. Physiologia
plantarum 34:288—292.

Landis, T.D., R.W. Tinus, S.E. McDonald and JP. Barnett. 1992. The Container
Tree Nursery Manual: Atmospheric Environment. Washington, USDA Forest
Service.

Landis, T.D., R.W. Tinus and JP. Barnett. 1999. The Container Tree Nursery
Manual Volume Six: Seedling Propagation. Hamden, USDA Forest Service.

Larson, RA 1992. History. Tips on the use of chemical growth regulators on
floriculture craps. H.K. Tayama, R.A. Larson, P. A. Hammer and T.J. Roll.
Columbus, Ohio Florists' Association: 8—1 1.

Little, C.H.A. 1984. Promoting bud development in balsam fir Christmas trees
with 6-benzylaminopurine. Canadian Journal Forest Resources 14:447-451.

Little, C.H.A. 1985. Increasing lateral shoot production in Balsam fir Christmas
trees with cytokinin application. HortScience 20(4):713—714.

Marshall, J.G., J.B. Scarratt and EB. Dumbroff. 1991. Induction of drought
resistance by abscisic acid and paclobutrazol in jack pine. Tree Physiology

52

82415—421.

Mellerowicz, E.J., W.K., Coleman, R.T., Riding and C.H.A. Little 1992.
Periodocity of cambial activity in Abies balsamea I. Effects of temperature and
photoperiod on cambial dormancy and frost hardiness. Physiologia plantarum
85:515—525.

Mitchell, S. 1998. American Generations Who thev are. How they live. What
they think. Ithaca, New Strategist Publications, Inc.

Morgan, P.W. and H.W. Gausman. 1966. Effects of ethylene on auxin transport.
Plant Physiology 41:45—52.

Mulgrew, SM. and DJ. Williams. 1984. Promoting bud development and
branching of scotch pine with 6-BA. Journal of arboriculture 10(11):294-297.

Mulgrew, SM. and DJ Williams. 1985. Effect of Benzyladenine on the
promotion of bud development and branching of Picea pungens. HortScience
20(3):380-381 .

Neumann, P.M., Ed. 1988. Plant Growth and Leaf-Applied Chemicals. Boca
Raton, CRC Press, Inc.

Nickell, LG. 1983. Plant Growth Regulators Agricultural Uses. New York,
Springer-Verlagz32—38, 47—48, 55—58, 1 12-125.

Nienstaedt, H. 1967. Chilling Requirements in Seven Picea Species. Silvae
Genet 16:65—68.

Nitsche, K., K. Grossmann, E. Sauerbrey and J. Jung. 1985. Influence of the
growth retardant tetcyclacis on cell division and cell elongation in plants and cell
cultures of sunflower, soybean, and maize. Journal of Plant Physiology
118:209—218.

Odlum, K. D. 1991. Hardening and overwintering container stock in Ontario:
Practices and research. 11th Annual Conference of the Forest Nursery
Association of British Colombia, Prince George, BC.

Odlum, KD. and SJ. Colombo 1987. The effect of three film-forming
antitranspirants on moisture stress of outplanted black spruce seedlings. Tree
Planters' Notes 38(4):23—26.

Odlum, KD. and SJ. Colombo 1988. Short-day exposure to induce budset
prolongs shoot growth in the following year. General Technical Report RM:
Rocky Mountain Forest and Range Experiment Station, Forest Service, US.
Dept. of Agriculture 167(Dec):57—59.

53

Pharis, RP. and CG. Kuo. 1977. Physiology of gibberellins in conifers.
Canadian Journal Forest Resources 7:299—325.

Pharis, R.P., M. Ruddat and C. Phillips. 1964. Gibberellin, growth retardants, and
apical dominance in Arizona cypress. Die Natuwvlssenschaften 52:88—89.

Poljakoff-Mayber, A. and J. Gale. 1967. Final technical report on effect of plant
antitranspirants on certain physiological processes of forest seedlings and other
plant material. Jerusalem, The Hebrew University of Jerusalemz107.

Rademacher, W. 1991. Biochemical Effects of Plant Growth Retardants. Plant
Biochemical Regulators. H.W. Gausman. New York, Marcel Dekker,
lnc:169—200.

Raghavendra, A. S. and V.S.R. Das. 1977. Antitranspirant activity of inhibitors of
cyclic photophosphorylation. Journal of Experimental Botany 28(103):480—483.

Reicosky, DA and J.W. Hanover. 1978. Physiological effects of surface waxes.
Plant Physiology 62:101—104.

Rose, R. and UL. Haase. 1995. Effect of the antidesiccant Moisturin on conifer
seedling field performance. Tree Planters' Notes 46(2):97—101.

Ruddat, M. and RP. Pharis 1966. Participation of gibberellin in the control of
apical dominance in soybean and redwood. Planta 71:222—228.

Ruter, J.M. 1994. Growth and landscape establishment of Pyracantha and
Juniperus after application of paclobutrazol. HortScience 29(11):1318—1320.

Schott, P.E., and H. Walter. 1991. Bioregulators: Present and Future Fields of
Application. Plant Biochemical Regulators. H.W. Gausman. New York, Marcel
Dekker, lnc:247—321.

Seller, J.R. and RE. Kreh. 1987. The effect of chilling and seed source on the
growth of containerized fraser fir (Abies fraseri (Pursh) Poir.) seedlings. Tree
Planters' Notes(Spring 1987): 1 9—21 .

Simpson, D.G. 1984. Filmforming antitranspirants: Their effects on root growth
capacity, storability, moisture stress avoidance, and field performance of
[containerized conifer seedlings. The Forestry Chronicle 60:335—339.

Squire, GR. and MB. Jones. 1971. Studies on the mechanisms of action of the
antitranspirant phenylmercuric acetate, and its penetration into the mesophyll.
Journal of Experimental Botany 22(73):980—991.

Sterrett, JP. 1985. Paclobutrazol: A promising growth inhibitor for injection into
woody plants. J. Amer. Soc. Hort. Sci. 110(1):4-8.

54

Sundback, E. 2002. The future is open. Michigan Christmas Tree Journal.
49:5—7.

Takeno, K., R.L. Legge and RP. Pharis. 1981. Effect of the growth retardant B-9
(SADH) on endogenous GA level, and transport and conversion of exogenously
applied {3H} GA20 in Alaska pea. Plant Physiology 67(suppl.):103.

Thomas, B. and D. Vince-Prue. 1997. Chapter 11: Bud Dormancy.
Photoperiodism in Plants. San Diego, Academic Press:279—316.

Tinus, R.W. 1995. A new greenhouse photoperiod lighting system for prevention
of seedling dormancy. Tree Planters' Notes 46(1):11—14.

Tung, OH. and DR. Deyoe. 1991. Dormancy induction in container-grown Abies
seedlings: Effects of environmental cues and seedling age. New forests
5(1):13—-22.

United States Department of Agriculture. 2002. Floriculture Crops, National
Agriculture Statistics Service. httpzllusda.mannlib.cornell.edu.

USDA Plant Hardiness Zone Map. 2002.
http://www.usna.usda.gov/Hardzone/ushzmap.html?

United States Department of Labor. 2002. Inflation Calculator, Bureau of Labor
Statistics. httpzl/www.bls.gov/cpi/home.htm#data.

Van den Driessche, R. 1989. Paclobutrazol and triadimefon effects on conifer
seedling growth and water relations. Canadian Journal Forest Resources
20:722—729.

Van den Driessche, R. 1996. Drought resistance and water use efficiency of
conifer seedlings treated with paclobutrazol. New forests 11:65—83.

Wareing, PF. 1956. Photoperiodism in woody plants. Annual Review of Plant
Physiology 7:191—214.

Warren, S.L. 1990. Growth response of 13 container=grown landscape plants to
uniconazole. Journal of environmental horticulture 8(3):151—153.

Warren, S.L., F .A. Blazich and M. Thetford. 1991. Whole-plant response of
selected woody landscape species to uniconazole. Journal of environmental
horticulture 9(3):163—167.

Weston, G.D., L.W. Carlson and EC. Wambold. 1980. The effect of growth

retardants and inhibitors on container-grown Pinus contorta and Picea glauca.
Canadian Journal Forest Resources 10:510—516.

55

Wheeler, NC. 1987. Effect of paclobutrazol on Douglas-fir and loblolly pine.
Journal of Horticultural Science 62(1):101—106.

Williams, P.A., A.M. Gordon and AW. Moeller. 1990. Effects of five
antitranspirants on white spruce and white pine seedlings subjected to
greenhouse drought. Tree Planters' Notes 41 (1 ):34—38.

Young, E. and J.W. Hanover. 1976. Accelerating maturity in Picea seedlings.
Acta Horticulturae 56:105—114.

Young, E. and J.W. Hanover. 1977a. Development of the shoot apex of blue
spruce (Picea pungens). Canadian Journal Forest Resources 72614—620.

Young, E. and J.W. Hanover. 1977b. Effects of quality, intensity, and duration of
light breaks during a long night on dormancy in blue spruce (Picea pungens
Engelm.) seedlings. Plant Physiology 60:271—273.

Young, E. and J.W. Hanover. 1978. Effects of temperature, nutrient, and
moisture stresses on dormancy of blue spruce seedlings under continuous light.
Forest Science 24(4):458—467.

Zelitch, l. 1974. Effect of biochemical inhibitors on stomatal opening.
Experimental plant physiology. A. S. Pietro. Saint Louis, Mosbyz161—163.

56

CHAPTER ONE
CONTROLLING GROWTH OF CONIFERS WITH SIX COMMON PLANT

GROWTH RETARDANTS

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Controlling Growth of Conifers with Six Common Plant Growth Retardants
Additional index words: Picea, Abies, paclobutrazol, daminozide, uniconazole,

ancymidol, chlormequat, ethrel

Abstract

Tabletop Christmas tree growers whose greenhouse-grown conifers have
undesirable shoot growth may alleviate this problem by applying plant growth
retardants (PGRs). Some of the most common PGRs in the horticulture industry
were evaluated to determine their effectiveness in controlling height growth:
ancymidol at 100 ppm, daminozide at 5000 ppm, paclobutrazol at 60 ppm,
chlormequat at 1500 ppm, uniconazole at 15 ppm, ethephon at 500 ppm, and a
control. The following conifer species were tested: Colorado blue spruce [Picea
pungens Engelm.], Black Hills spruce [P. glauca (Moench) Voss var. densata],
Serbian spruce [P. omorika (Pancic) Purkyne], noble fir (Abies procera Rehd.),
grand fir [A. grandis (Dougl. Ex D. Don) Lindl.], Fraser fir [A. fraseri (Pursh) Poir.],
concolor fir [A. concolor (Gord. & Glend.) Lindl. Ex Hildebr.], arborvitae (Thuja
occidentalis L.), Port Orford cedar [Chamaecyparis Iawsoniana (Murray) Parl.],
and Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco]. Chlormequat was the
only PGR that caused phytotoxicity and damage to the foliage was minimal. It is
difficult to make generalized recommendations for using these PGRs because of
the interaction between treatment and species. Noble fir, Douglas-fir, Colorado
blue spruce, and arborvitae were unaffected by any PGR treatment. Daminozide
reduced growth of Port Orford cedar and concolor fir, uniconazole reduced

growth of Black Hills and Serbian spruce, paclobutrazol reduced growth of Fraser

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fir, and ethephon reduced growth of grand fir.

Introduction

The use of plant growth retardants (PGRs) is a common practice in the
horticulture industry as an aid in altering normal growth behavior of selected
crops (Schott and Walter, 1991). They are used for promoting more uniform
plants, controlling fruit and flower production, and most important, reducing
height growth (Larson, 1992). Applying PGRs aids in marketing a species by
creating a more uniform crop and also helps in handling and shipping by
producing smaller, easier-to-handle crops (Blake and South, 1991; Nickell,
1983). Generally, PGRs work by reducing the shoot length of plants without
causing any detrimental adverse effects such as phytotoxicity or altering normal
patterns of development (Grossman, 1990; Rademacher, 1991). Plant growth
retardants cause a decrease in the rate of cell division along with a reduction in
cell elongation (Rademacher, 1991). This alteration in plant processes can be
accomplished by interfering with the normal functions of gibberellins (GAs),
auxins, or both. These are the most important plant hormones involved in shoot
elongation.

The most common PGRs on the market today can be classified into three
major categories in relation to their effects on natural hormones in plants: 1)
ethylene-releasing compounds, 2) inhibitors of GA translocation, and 3) inhibitors
of GA biosynthesis (Rademacher, 1991). Ethephon (trade name: Florel;
Monterey Lawn and Garden Products, Inc, Fresno, Calif.) is an ethylene-

releasing compound that inhibits the cell elongation process by preventing

59

auxins from reaching growth sites and by blocking biosynthesis of auxins (Burg
and Burg, 1967; Morgan and Gausman, 1966; Rademacher, 1991). Daminozide
(B-nine; Uniroyal Chemical, Middlebury, Conn.) reduces translocation of GAs or
GA precursors to actively growing tissues and may also promote breakdown and
conjugation of GA (Takeno et al., 1981). Chlormequat (Cycocel; Olympic
Horticultural Products Company, Mainland, Pa.), ancymidol (A-rest; SePRO
Corporation, Carmel, Ind.), paclobutrazol (Bonzi; Uniroyal Chemical, Middlebury,
Conn), and uniconazole (Sumagic; Valent U.S.A. Corporation, Walnut Creek,
Calif.) are inhibitors of GA biosynthesis (Rademacher, 1991). Such inhibitors
have been shown to attack the metabolism of terpenoids, which is the pathway
by which the phytohormone groups of GA, cytokinin, and possibly abscisic acid
and the sterols are derived (Grossman, 1990).

Considerable research has been performed on PGRs to determine their
effectiveness in controlling growth of conifer species (Asher, 1963; Dunberg and
Eliasson, 1972; Groninger and Seller, 1997; Hare, 1982; Keever and West,
‘1992; Ruddat and Pharis, 1966; Ruter, 1994; Van den Driessche, 1989, 1996;
Weston et al., 1980). Growers who produce coniferous tabletop Christmas tree
species in a greenhouse environment could benefit financially by using PGRs.
Most conifer species naturally experience a rapid growth phase after seedlings
become established (Landis et al., 1992; Young and Hanover, 1976). When
seedlings are placed in greenhouse environments under optimum growing
conditions, growth can be stimulated even more than is normal. Applying PGRs
allows plants to be grown in optimal conditions but at the same time regulates

plant height (Barrett, 1992). Therefore, the use of PGRs could help produce a

60

uniform, aesthetically pleasing, tabletop Christmas tree crop.

The goal of our project is to develop a production system for conifer and
greenhouse growers to produce a marketable and affordable live tabletop
Christmas tree hardy in the upper Midwest. In this trial, our objectives were to
determine effectiveness of PGR applications in controlling seedling height and to
determine any phytotoxic effects that may occur from these applications. Ten
conifer species that show promise as tabletop Christmas trees and are common
in the landscape industry were subjected to treatments of the six PGRs
discussed above. The experiment was conducted to determine the most

effective PGRs for each species.

Materials and Methods

Plant material and culture. The following plants were received from Vans
Pines Nursery (West Olive, Mich.) in Nov. 1999 and given 10 weeks of chilling:
Colorado blue spruce (Picea pungens), Black Hills spruce (P. glauca var.
densata), Serbian spruce (P. omorika), noble fir (Abies procera), grand fir (A.
grandis), Fraser fir (A. frasen), concolor fir (A. concolor), arborvitae (Thuja
occidentalis), Port Orford cedar (Chamaecypan’s Iawsoniana), and Douglas-fir
(Pseudotsuga menziesii). Conifers at the beginning of the trial ranged in size
from 11.5 to 61.0 cm. After chilling, the seedlings were planted on Jan. 27,
2000, in 13-by—13-cm square pots (1100 cm3) with a commercial medium
(Strong-Lite High Porosity Mix, Pine Bluff, Ark) composed of pine bark, fibrous
Canadian Sphagnum peat, horticultural vermiculite, and screened course perlite,

along with a wetting agent and a starter fertilizer charge. The greenhouse

61

environment was set at 20 °C with a long-day photoperiod treatment (16-h day-
extension photoperiod using high-pressure sodium lamps). The seedlings were
irrigated as needed with a nutrient solution of well water (electrical conductivity of
0.65 mS cm" and 105, 35, 23 [mg L"] Ca, Mg, and S, respectively) acidified with
H2804 to a titratable alkalinity of 03802 at 130 mg L" and water-soluble fertilizer
providing 125-12-125-13 (mg L") N-P-K-Ca plus 1.0-0.5-0.5-0.1-0.1 Fe-Mn-Zn-
Cu-B-Mo mg L" (MSU Special, Greencare Fertilizers, Chicago, Ill).

Treatments. Seedlings were arranged in a completely randomized design
using five plants per species in each of the following treatments: 1) ancymidol at
100 ppm, 2) daminozide at 5000 ppm, 3) paclobutrazol at 60 ppm, 4)
chlormequat at 1500 ppm, 5) uniconazole at 15 ppm, 6) ethephon at 500 ppm,
and 7) a control. These rates were determined according to protocols of prior
PGR work performed by the Michigan State University floriculture grdup. Plant
growth retardants were applied to the seedling foliage beginning May 15, 2000.
Treatments were applied at two-week intervals throughout a 10-week period for
a total of five applications.

Data collection and analysis. Initial plant heights were recorded at the
beginning of the experiment, and heights were also recorded the day before
each PGR application. Final heights were determined two weeks after the last
chemical application. From the height data, relative growth rate (RGR) was
determined by an exponential height growth model:

RGR = [In (final height) — In (initial height)] (1'1
Relative growth rate data were transformed by using the square root

(SORT) function (SAS/PC software, SAS Institute, Inc., Cary, NC.) to achieve

62

normal distribution on the residual terms. Every species was then allowed to
have its own residual variance, which is referred to as a heteroscedasticity
model. Once normality was determined, RGR data were analyzed by using
SAS’s mixed procedure (PROC MIXED) to calculate significance of main effects

and interactions.

Results and Discussion

Species, PGR, and interactions between the two affected mean relative
growth rate of seedlings (Table 1). Height growth was decreased by at least one
PGR treatment for six of the 10 species we tested (Figs. 1, 2, and 3). However,
the RGR of Douglas-fir, noble fir, Colorado blue spruce, and arborvitae was
unaffected by PGR treatments (Table 2). Chlormequat, daminozide, and
ethephon caused minimal phytotoxicity, which occurred only with true fir species
(Abies). Because of interactions between species and PGR applications, it is
difficult to make generalized recommendations for the overall experiment;
therefore, results are presented below by PGR.

Ethephon. The effects of an ethylene-releasing compound were
determined by applying ethephon to seedling foliage, with effects differing across
all species. Concolor fir expressed minor phytotoxic symptoms on newly
expanded foliage. Foliar tissue developed light-pink to purple discolorations after
the third application. Similar discoloration has been observed in balsam fir
[Abies balsamea (L.) Mill.] after applications of benzyladenine (BA) (Little, 1985).
Temporary symptoms were observed when BA was applied too soon to new

growth.

63

Ethephon reduced growth of Fraser, concolor, and grand fir (Fig. 1) and
Black Hills and Serbian spruce (Fig. 2) species (Pinaceae); however, neither Port
Orford cedar nor arborvitae (Cupressaceae) species were affected (Fig. 3).
Research performed to determine effects of ethephon applications on plant
growth has mainly been directed toward floriculture and ornamental crops, giving
little insight into responses observed with coniferous species. Azaleas
(Rhododendron spp. G. Don.) and geraniums (Pelargonium spp. L’Herit.) treated
with ethephon responded with an increase in lateral branching, resulting in
shorter, fuller specimens (Nuemann, 1988). Hayashi and others (2001)
observed increased branching in herbaceous perennials they treated with
ethephon, although height growth also increased. However, lodgepole pine
[Pinus contorta var. Iatifolia (Engelm.) Critchf.] and white spruce [Picea glauca
(Moench) Voss] had reduced shoot growth when treated with ethephon (Weston
et al., 1980). White spruce seedlings also experienced a decrease in the
number of lateral branches produced. Unlike Pinaceae species we tested, slash
pine (P. elliotti Engelm. var. elliottii) and loblolly pine (P. taeda L.) treated with
ethephon experienced an increase in growth (Hare, 1982).

Daminozide. As an inhibitor of GA translocation, daminozide was
effective in controlling height growth of Fraser and concolor fir (Fig. 1), Serbian
spruce (Fig. 2), and Port Orford cedar (Fig. 3). All other species tested were
unaffected by these treatments. As we found with ethephon, little research has
been performed on conifer species to determine efficacy of daminozide
applications. Hare (1982) observed that growth of slash and loblolly pine was

reduced compared with that of the control. Increased branching of redwood

64

[Sequoia sempervirens (D. Don) Endl.] seedlings, caused by lateral buds
escaping apical dominance, occurred after daminozide was applied to seedling
foliage (Ruddat and Pharis, 1966). Norway spruce (Picea abies Karst.) treated
with soil drench applications of daminozide experienced reduced shoot and root
growth (Dunberg and Eliasson, 1972). Phytotoxicity appeared in roots and
subsequently in the shoots at high concentrations (300 mg L"), indicating
daminozide is more beneficial when applied to seedling foliage. Although similar
species have experienced the same responses we observed, Kuo and Pharis
(1975) recommended that further research be conducted to more accurately
determine the PGR’s exact mode of action, which would allow treatment effects
to be understood more completely.

Chlormequat. Although chlormequat effectively controls height of many
plant species (Barrett, 1992; Larson, 1992), concolor fir (Fig. 1) and Serbian
spruce (Fig. 2) were the only species we tested that experienced height
reduction, which could be explained by the growth habit of these seedlings.
Most seedlings of conifer species, including those we tested, experience a rapid
growth phase (Landis et al., 1992; Young and Hanover, 1976). Cathey (1964)
found that height control in more vigorous plants was obtained only after high
"rates of PGRs were applied. Also, plants that were grown under long
photoperiods and treated with chlormequat were unaffected by chemical
treatments. Asher (1963) observed reductions in growth of slash pine given
foliar treatments of chlormequat at 2000 to 16,000 ppm. In our trial, chlormequat
was applied at 1500 ppm and seedlings were grown under a 16-h photoperiod.

Long photoperiod, along with such a low application rate, could explain why we

65

detected reduced growth with so few species.

Phytotoxic symptoms developed in some of the species we treated.
Grand, Fraser, and noble fir experienced yellowing of foliage tips, mainly on the
most recent flushes of growth. These symptoms were not observed on any other
species we tested, nor were they observed in the control treatment. Previous
researchers have determined that soil drench applications are more beneficial
because of the high incidence of phytotoxicity after chlormequat is applied to
foliage. Slash pine seedlings treated with high rates of chlormequat (16,000
ppm) expressed foliar damage and even death (Asher, 1963). Phytotoxic
symptoms on most plant species generally appear as chlorotic spotting within
three to five days of foliar application and appear mainly in foliage that was
expanding at application (Barrett, 1992). For chlormequat to be considered as a
means for controlling height growth of conifer species, higher, more effective
rates must be determined, and treatments should be soil-applied to avoid foliar
damage.

Ancymidol. Like chlormequat, ancymidol is a compound that inhibits GA
biosynthesis, which subsequently reduces cell elongation. Concolor and Fraser
fir (Fig. 1) and Black Hills and Serbian spruce (Fig. 2) experienced decreased
shoot growth, but Cupressaceae species were unaffected (Fig. 3). Although we
applied ancymidol to seedling foliage, this chemical has been less effective when
used as a soil drench on plants potted in media with high percentages of pine
bark humus, which is oftentimes the type of media used when conifer crops are
containerized (Larson, 1992). Similar work has been performed on Pinaceae

species, successfully reducing shoot growth. Both lodgepole pine and white

66

spruce seedlings had reduced shoot growth after soil drench applications of
ancymidol (Weston et al., 1980). Four- or five-year-old Monterey pines (P.
radiata D. Don) treated with foliar applications of ancymidol experienced reduced
shoot growth (Hield et al., 1977). Compared with that of the control, shoot
growth was reduced (by 69%) for up to six months.

Uniconazole. Uniconazole is another inhibitor of GA biosynthesis and is
an extremely important PGR for ornamental crops (Rademacher, 1991).
Applications of uniconazole effectively controlled height of Pinaceae and
Cupressaceae species. Shoot growth of Fraser fir (Fig. 1), Black Hills and
Serbian spruce (Fig. 2), and Port Orford cedar (Fig. 3) was reduced. Unlike the
PGRs discussed earlier, a great deal of work has been performed to determine
uniconazole’s effects on conifers. Fraser fir terminal shoot length was reduced
by 22% to 45% after uniconazole was applied to seedling foliage (Hinesley,
1998). Uniconazole applied as a soil drench to three-year-old loblolly pine
decreased height growth by 55%, as determined three months after treatments
ended (Barnes and Kelley, 1992). Keever and West (1992) applied uniconazole
as a soil drench to containerized Leyland cypress [X Cupressocypan's leylandii
(A.B. Jacks. & Dallim.) Dallim. & A.B. Jacks]. Growth was reduced as much as
48%.

Many researchers have also determined that uniconazole is more
effective as a soil drench (Barnes and Kelley, 1992; Hinesley, 1998; Keever and
West, 1992; Warren, 1990; Warren et al., 1991). Unlike the PGRs discussed
above, uniconazole readily moves through xylem tissue rather than phloem

tissue (Barrett, 1992). Xylem tissue is responsible for transporting material from

67

roots upward into the growing tips of the plant. Therefore, soil drenches are
more effective in distributing the PGR throughout the plant. Considering the
success other researchers have experienced from uniconazole applications on
many plant species, applying uniconazole to plant foliage instead of soil may
help explain why we did not observe more pronounced effects on many of the
species that we tested.

Paclobutrazol. Paclobutrazol is similar to uniconazole in that it inhibits GA
biosynthesis and readily moves through xylem tissue rather than phloem tissue.
Concolor and Fraser fir (Fig. 1) and Black Hills and Serbian spruce (Fig. 2) had
reduced shoot growth after receiving treatments of paclobutrazol; however,
neither of the Cupressaceae species responded to these applications (Fig. 3).
Previous research projects have shown similar responses in species of Pinaceae
(Groninger and Seller, 1997; Wheeler, 1987), but there has also been some
success in reducing shoot growth of Cupressaceae species and Douglas-fir.
Ruter (1994) observed decreased horizontal shoot elongation in Juniperus (L.)
that received soil-applied paclobutrazol. Paclobutrazol applied as a soil drench
to Douglas-fir seedlings resulted in reductions of shoot dry weight and shoot
growth (Van den Driessche, 1989; Wheeler, 1987). Paclobutrazol as a soil
drench is more effective than as a foliar spray in controlling shoot growth.
Although we observed reductions in shoot growth with some of the species,
effects could probably be detected with more of the species we tested if
paclobutrazol were applied as a soil drench.

Conclusion. The use of PGRs as a means for controlling growth of

conifers could prove beneficial for tabletop Christmas tree producers. As we

68

observed in this trial and also from work performed by other researchers, PGRs
can be effectively used on conifer species for reducing height growth and
increasing lateral branching. Of the six PGRs we tested, daminozide and
uniconazole were most beneficial because of their effectiveness in controlling
growth with the widest variety of species (Pinaceae and Cupressaceae species).
Chlormequat, uniconazole, and paclobutrazol were also effective PGRs but need
further testing to determine more effective rates as soil-applied treatments and to
determine whether soil drenching can decrease phytotoxic effects observed after
chlormequat treatments. Treatments of ethephon and ancymidol also effectively
controlled seedling height, but only for Pinaceae species. All PGRs that we
tested and have recommended, as with any new chemical chosen for use on a
crop, should be tested on a small group of plants before a large scale chemical

application program is implemented.

69

Literature Cited

Asher, WC. 1963. Effects of 2-chloroethyltrimethylammonium chloride and 2,4-
dichlorobenzyltributyl phosphonium chloride on growth and transpiration of slash
pine. Nature 200:912.

Barnes, AD. and W.D. Kelley. 1992. Effects of a triazole, uniconazole, on shoot
elongation and root growth in loblolly pine. Can. J. For. Res. 22:1—4.

Barrett, J.E. 1992. Mechanisms of action, p. 12—18. In: H.K. Tayama, R.A.
Larson, PA. Hammer and Teresa J. Roll. Tips on the use of chemical growth
regulators on floriculture crops. Ohio Florists' Association, Columbus, Ohio.

Blake, J.l. and 0.8. South 1991. Effects of plant growth regulators on loblolly
pine seedling development and field performance. Proc. 6th Biennial S. Silv.
Res. Conf. 100—107.

Burg, SP. and EA. Burg. 1967. Inhibition of polar auxin transport by ethylene.
Plant Physiol. 42:1224-1228.

Cathey, HM. 1964. The physiology of growth retarding chemicals. Annu. Rev.
Plant Physiol. 15:271—302.

Dunberg, A. and L. Eliasson. 1972. Effects of growth retardants on Norway
spruce (Picea abies). Physiol. Plant. 26:302—305.

Groninger, J.W. and J.R. Seller. 1997. Soil texture and moisture availability
impacts on the efficacy of soil-applied paclobutrazol. J. Arboriculture
23(3):89—92.

Grossman, K. 1990. Plant growth retardants as tools in physiological research.
Physiol. Plant. 78:640—648.

Hare, RC. 1982. Effect of nine growth retardants applied to loblolly and slash
pine. Can. J. For. Res. 12:112—114.

Hayashi, T., R.D. Heins, A.C. Cameron and W.H. Carlson. 2001. Ethephon
influences flowering, height, and branching of several herbaceous perennials.
Scientia Hort. 91 :305-323.

Hield, H., R.M, Sachs and S. Hemstreet. 1977. Growth reduction comparisons
of 9 plant regulators applied to Pinus radiata. Proc. Plant Growth Regulators
Working Group, Hot Spring, Arkansas.

Hinesley, LE. 1998. Effect of uniconazole on shoot growth and budset of
containerized Fraser fir. HortScience 33(1):82—84.

7O

Keever, G.J. and MS. West. 1992. Response of established landscape plants to
uniconazole. HortTechnology (Oct/Dec): 465—468.

Kuo, CG. and RP. Pharis 1975. Effects of AMO-1618 and B-995 on growth and
endogenous gibberellin content of Cupressus an‘zonica seedlings. Physiol. Plant.
34:288—292.

Landis, T.D., R.W. Tinus, S.E. McDonald and JP Barnett. 1992. The Container
Tree Nursery Manual: Atmospheric Environment. USDA Forest Service,
Washington.

Larson, RA. 1992. History, p. 8—11. In: H.K. Tayama, R.A. Larson, PA. Hammer
and T.J. Roll. Tips on the use of chemical growth regulators on floriculture
crops. Ohio Florists' Association, Columbus Ohio.

Little, C.H.A. 1985. Increasing lateral shoot production in balsam fir Christmas
trees with cytokinin application. HortScience 20(4):713—714.

Morgan, P.W. and H.W. Gausman. 1966. Effects of ethylene on auxin transport.
Plant Physiol. 41:45—52.

Neumann, P.M., (ed.). 1988. Plant growth and leaf-applied chemicals. CRC
Press, Inc., Boca Raton, Fla.

Nickell, LG. 1983. Plant growth regrlators: agricultural uses. Springer-Venag,
New York, NY.

Rademacher, W. 1991. Biochemical effects of plant growth retardants, p.
169—200. In: H.W. Gausman. Plant biochemical regulators. Marcel Dekker, Inc,
New York, NY.

Ruddat, M. and RP. Pharis. 1966. Participation of gibberellin in the control of
apical dominance in soybean and redwood. Planta 71 :222—228.

Ruter, J.M. 1994. Growth and landscape establishment of Pyracantha and
Juniperus after application of paclobutrazol. HortScience 29(11):1318—1320.

Schott, PE. and H. Walter. 1991. Bioregulators: Present and future fields of
application. p. 247—321 In: H.W. Gausman Plant biochemical regulators. Marcel
Dekker, Inc., New York, NY.

Takeno, K., R.L. Legge and RP. Pharis 1981. Effect of the growth retardant B-9
(SADH) on endogenous GA level, and transport and conversion of exogenously
applied {3H} GA20 in Alaska pea. Plant Physiol. 67(suppl.):103.

Van den Driessche, R. 1989. Paclobutrazol and triadimefon effects on conifer
seedling growth and water relations. Can. J. For. Res. 20:722—729.

71

Van den Driessche, R. 1989. Paclobutrazol and triadimefon effects on conifer
seedling growth and water relations. Can. J. For. Res. 20:722—729.

Van den Driessche, R. 1996. Drought resistance and water use efficiency of
conifer seedlings treated with paclobutrazol. New For. 11:65—83.

Warren, S.L. 1990. Growth response of 13 container=grown landscape plants to
uniconazole. J. Env. Hort. 8(3):151—153.

Warren, S.L., F.A. Blazich and M. Thetford. 1991. Whole-plant response of
selected woody landscape species to uniconazole. J. Env. Hort. 9(3):163—167.

Weston, G.D., L.W. Carlson and EC. Wambold. 1980. The effect of growth
retardants and inhibitors on container-grown Pinus contorta and Picea glauca.
Can. J. For. Res. 10:510—516.

Wheeler, NC. 1987. Effect of paclobutrazol on Douglas fir and loblolly pine. J.
Hort. Sci. 62(1):101-106.

Young, E. and J.W. Hanover 1976. Accelerating maturity in Picea seedlings.
Acta Hort. 56:105—114.

72

Table 1. Analysis of variance for mean relative growth rate.

 

Effect Degrees of Freedom F Value
PGRz 6 "3.27
Species 9 ***177.84
GR x i 4 **2.

**, ***Signiflcant at P < 0.01 or 0.0001 respectively.

ZPGR indicates plant growth retardant.

73

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74

List of Figures

FIGURE PAGE

1

Effects of plant growth retardant applications on mean relative growth
rates of concolor fir, Fraser fir, grand fir, noble fir, and Douglas-fir
seedlings. Error bars represent SE of the mean.

76
Effects of plant growth retardant applications on mean relative growth
rates of Black Hills spruce, Serbian spruce, and Colorado blue spruce
seedlings. Error bars represent SE of the mean.
77

Effects of plant growth retardant applications on mean relative growth
rates of Port Orford cedar and arborvitae seedlings. Error bars represent
SE of the mean.

78

75

Relative Growth Rate (cm cm'1 d")

 

0.006

 

- Control
[2:] Ancymidol
0'005 ‘ _ Daminozide
[:1 Paclobutrazol 5 I
- Chlormequat
0-004 ‘ — Ethephon
- Uniconazole

 

 

 

0.003 a

0.002 -

Elli . H llll" lllllnl .1

Concolor fir Fraser fir Grand fir Noble fir Douglas-fir

‘ .
>1

 

 

 

 

 

 

 

 

 

r

Species

Figure 1
76

 

Relative Growth Rate (cm cm'1 d")

 

0.010

0.008 -

0.006 -

0.004 ~

0.002 -

 

0.000 -

 

 

 

 

 

 

 

 

- Control

CZ] Ancymidol
- Daminozide
[III Paclobutrazol
- Chlormequat
- Ethephon
- Uniconazole

 

 

 

 

 

 

 

Black Hills spruce Serbian spruce Colorado blue spruce
Species
Figure 2

77

Relative Growth Rate (cm cm'1 d")

 

0.010

0.008 a

0.006 -

0.004 a

0.002 -

 

 

 

 

 

 

 

- Control
:3 Ancymidol
- Daminozide
[:1 Paclobutrazol
- Chlormequat
- Ethephon
- Uniconazole

 

 

 

 

 

 

0.000

 

Port Orford cedar
Species

Figure 3
78

Arborvitae

CHAPTER TWO
APPLICATIONS OF UNICONAZOLE AND 6-BA TO CONTROL HEIGHT OF

PICEA SPECIES AND CHAMAECYPARIS LAWSON/ANA

79

Applications of Uniconazole and 6-BA to Control Height of Picea spp. and
Chamaecypan's Iawsoniana

Additional index words. Sumagic, spruce, plant growth regulator, benzyladenine

Abstract

The use of plant growth retardants (PGRs) could benefit conifer growers
by enabling them to produce more uniform, live tabletop Christmas tree
specimens. We applied two PGRs to determine effectiveness on height control
and terminal bud density for the following species of conifer seedlings: Black
Hills spruce [Picea glauca (Moench) Voss var. densata], Serbian spruce [P.
omorika (Pancic) Purkyne], Colorado blue spruce (P. pungens Engelm.), and
Port Orford cedar [Chamaecyparis Iawsoniana (Murray) Parl.]. The chemicals
were applied at two-week intervals throughout an eight-week period for a total of
four applications. Uniconazole was applied as a foliar spray and soil drench (5
and 1 ppm, respectively), 6-benzyladenine (BA) was applied only as a foliar
spray (500 and 1000 ppm), and the control did not receive a PGR application.
Uniconazole was generally effective on the Spruces but not on Port Orford cedar,
whereas BA effectively controlled height on all species and increased spruce bud
densities. Benzyladenine treatments caused phytotoxic effects in all species
tested. Uniconazole caused minor phytotoxicity in Black Hills spruce from the
drench treatment, and Port Orford cedar experienced minor phytotoxic effects

because of foliar treatments.

80

Introduction

The production of live tabletop Christmas trees in the greenhouse
environment can cause undesirable terminal shoot growth, resulting in an
asymmetrical, unattractive plant. Many conifer species experience a rapid
growth phase after seedlings become established (Landis et al., 1992; Thomas
and Vince-Prue, 1997) When growers produce conifers in a greenhouse
environment, the optimum growing conditions can further increase seedling
growth, which can result in an undesirable growth habit for tabletop Christmas
tree production. Sundback (2002) found that consumers prefer a natural,
symmetrical specimen with full yet open branches. This growth habit allows
ornaments to hang straight and shoot tips to look natural instead of having
sheared or damaged tips. Plant growth retardant applications could eliminate
shearing of seedlings, which would reduce labor costs and still produce a
product appealing to consumers.

Although plant growth retardants (PGRs) are an important tool in growth
regulation of ornamental craps (Rademacher, 1991), little work has been done to
determine the efficacy of PGRs applied to conifer species. Uniconazole is a
common and effective PGR that could prove beneficial to conifer growers
(Barnes and Kelley, 1992; Duck, 2002; Hinesley, 1998; Keever and West, 1992)
in the production of tabletop Christmas trees. It was introduced into the industry
in the early 19803 as a foliar- or soil-drench-applied chemical that inhibits
gibberellin (GA) biosynthesis (Larson, 1992; Rademacher, 1991). The main
benefits from the use of uniconazole are shortened internodes, darker green

foliage, and an increase in epicormic bud development (Kimball, 1990; Ruter,

81

1994). Compared with other PGRs, uniconazole can be applied at much lower
rates to achieve comparable height control. Because of the nature of chemical
transport within plant tissues, uniconazole is more effective when applied as a
soil drench (Barrett, 1992) because the chemical is transported throughout the
plant via xylem tissue rather than phloem tissue. Therefore, when uniconazole is
applied directly to the foliage, it cannot move throughout the plant, resulting in
growth control only in tissue that comes into direct contact with the chemical.
Another PGR that has proven useful in the omamentals industry is
benzyladenine (BA). Unlike uniconazole, which inhibits natural hormones, BA is
a synthetic cytokinin that can mimic natural plant hormones to stimulate plant
growth. Cytokinins are the hormones responsible for promoting lateral branching
(Barrett, 1992), which makes BA a possible treatment for promoting greater bud
densities. Researchers have proven BA effective in controlling growth and
increasing bud densities of various conifers but have also observed undesirable
adverse effects such as deformed bud development, decreased root growth,
delayed budbreak, inhibition of secondary needle extension (Blake and South,
1991), and shortened, chlorotic needles (Mulgrew and Williams, 1984, 1985).
Our objectives for this project were to determine the effectiveness of
uniconazole and BA applications in controlling height and increasing bud
densities and to determine whether these PGR applications cause phytotoxicity
on some common landscape conifers used in the production of tabletop
Christmas trees. We performed the experiment to determine optimum rates and

application methods for each PGR.

82

Materials and Methods

Plant material and culture. The plant species we evaluated were Serbian
spruce (Picea omon‘ka), Black Hills spruce (P. glauca var. densata), Colorado
blue spruce (P. pungens), and Port Orford cedar (Chamaecyparis Iawsoniana).
Conifers at the beginning of the trial ranged in size from 15.0 to 74.0 cm.
Seedlings were received from Vans Pines Nursery (West Olive, Mich.) and
planted in Apr. 2000 in one-gallon containers (2830 cm3) with a 70:30 (volzvol)
peat/perlite substrate (Peat-Perlite Mix, Michigan Grower Products, Galesburg,
Mich.)

After planting, all seedlings were grown outdoors under natural conditions
at the Michigan State University Horticulture Teaching and Research Center
(HTRC) (East Lansing, Mich.) until the experiment began in Feb. 2001.
Seedlings were treated in June 2000 with Ronstar preemergent herbicide at 2 lb
a.i. acre ". Also in June 2000, seedlings were given 17-7-8 slow-release (3- to 7-
month) granular fertilizer (Wilbro Horticultural Products, NonNay, S.C.) at 10
g/tree. Seedlings were irrigated as needed with water obtained from wells at the
HTRC. Once the seedlings entered the greenhouse environment in Feb. 2001,
they were irrigated as needed with a nutrient solution of well water (electrical
conductivity of 0.65 ms cm" and 105, 35, 23 [mg L"] Ca, Mg, and S,
respectively) acidified with H2804 to a titratable alkalinity of CaSO2 at 130 mg L"
and water-soluble fertilizer providing 125-12-125-13 (mg L") N-P-K-Ca plus 1.0-
0.5-0.5-0.1-0.1 (mg L") Fe-Mn-Zn-Cu-B-Mo (MSU Special, Greencare Fertilizers,
Chicago, Ill.)

Treatments. The experiment was set up as a completely randomized

83

design with 10 plants per species and treatment. Once sufficient natural chilling
had occurred, plants were placed into a 16-h (extended photoperiod)/20 °C
greenhouse on Jan. 30, 2001. The chemicals were applied at two-week intervals
throughout an eight-week period for a total of four applications. Uniconazole
(Sumagic, Valent USA Corporation, Walnut Creek, Calif.) was applied as a foliar
spray and soil drench (5 and 1 ppm, respectively), 6-BA (Valent USA
Corporation) was applied only as a foliar spray (500 and 1000 ppm), and the
control did not receive a PGR application. Each treatment was separated with a
barrier during application of the PGRs to avoid chemical drift.

Data collection and analysis. At the beginning of the trial, initial heights
were measured on all seedlings, and terminal bud counts were taken on the
spruce seedlings. Height measurements were taken repeatedly throughout the
trial at two-week intervals. Seedlings were visually rated during the trial for
detection of phytotoxic effects from the PGR applications. Because phytotoxic
effects occurred, seedlings were monitored for 10 weeks after the final
treatments were applied. Final heights and terminal bud counts were recorded
at the completion of the experiment on May 2, 2001. From the height data,
relative growth rate (RGR) was determined by an exponential height growth
model:

RGR = [In (final height) — ln (initial height)] d"
This method calculates growth more accurately because it takes into account
that all seedlings were not the same size. Bud density for each section of new
growth was calculated from bud count data by dividing the number of buds by

shoot length.

84

Relative growth rate and bud density data were analyzed by using the
mixed procedure (PROC MIXED) (SAS/PC software, SAS Institute Inc., Cary,
NC). Normal distribution on the residual terms for RGR data was achieved only
after three extreme outliers were removed. However, significance levels did not
subsequently change; therefore, data analysis was performed on the complete
data set. Normal distribution for bud density data was achieved after a log
transformation was performed and one extreme outlier was removed. As was
the case with RGR data, removal of the outlier did not change significance

levels, so only a log transformation was performed.

Resuns

Each of the main effects examined (species and PGR) affected (P <
0.0001) mean RGR and mean bud density (Table 1). Uniconazole effectively
reduced height growth for Black Hills and Serbian spruce but was ineffective for
Colorado blue spruce and Port Orford cedar (Table 2). However, uniconazole
did not affect bud density for any species tested. Applications of BA increased
bud density, reduced height growth (Fig. 1), and caused phytotoxic effects for
every species we tested. Because of these interactions between main effects, it
is difficult to make generalized recommendations for the overall experiment;
therefore, results are presented below by species.

Black Hills spruce. Both BA treatments caused severe phytotoxic effects
with Black Hills spruce seedlings. The most severely damaged foliage turned
brown and then died. Phytotoxic symptoms appeared after the final 500 ppm

application, with 80% of those seedlings dying within the 10-week monitoring

85

period after final measurements were taken. Seedlings treated with BA at 1000
ppm developed phytotoxic symptoms after the third application, with 100% of
those seedlings dying within the 10-week monitoring period. Needles that
formed after BA treatments began were somewhat stunted compared with those
of control seedlings, and needles grew parallel rather than perpendicular with the
stems. Only one seedling developed minor phytotoxic symptoms in the
uniconazole drench treatment. All PGR treatments reduced height growth (Fig.
2). Bud density was increased by both BA treatments, but uniconazole
treatments had no effect (Fig. 3). New buds occurred in clusters on shoot growth
that formed after BA treatments began. Areas of increased bud development
were deformed and unattractive.

Serbian spruce. Phytotoxic effects were also detected in Serbian spruce
seedlings due to BA applications, although symptoms differed from those
observed with Black Hills spruce seedlings. New growth of affected seedlings
was light-pink to purple. Discoloration in seedlings treated at 500 ppm was
minor and began to disappear after final measurements were taken in May.
However, all seedlings treated with BA at 1000 ppm developed severe phytotoxic
symptoms, resulting in a large amount of dead foliage. Damage initially occurred
on branch tips, but all of these seedlings eventually died within the 10-week
monitoring period. There were no phytotoxic effects as a result of either
uniconazole treatment. All PGR treatments reduced height growth (Fig. 2). Bud
density was increased by both BA treatments, but uniconazole treatments had
no effect (Fig. 3). New buds occurred in clusters on new growth that formed

after BA treatments were applied. These areas were light-pink to purple, and

86

bud clusters were somewhat deformed.

Colorado blue spruce. Seedlings developed phytotoxic symptoms from
both BA treatments. Symptoms were similar to those observed with Serbian
spruce seedlings; however, tissue discoloration was light to dark red. Foliar
damage was severe in both BA treatments, with all of the seedlings dying within
the 10-week monitoring period after final measurements were recorded. Height
was reduced by both BA treatments, but uniconazole did not affect RGR (Fig. 2).
Benzyladenine application increased bud density at 500 ppm (Fig. 3). Increased
bud formation occurred in clusters on growth that formed after BA treatments.
Bud density was not affected by any other PGR treatment.

Port Orford cedar. Phytotoxic symptoms developed after the third
application on seedlings treated with both rates of BA. All seedlings tested in
both BA treatments died within the 10-week monitoring period. New growth that
formed during the trial was extremely stunted and was in clusterlike formations
on the outermost branch tips. There were no phytotoxic effects from
uniconazole drench treatments; however, foliar applications of uniconazole
caused minor phytotoxic symptoms on three of the 10 seedlings tested.
Uniconazole treatments did not affect seedling growth, but both BA treatments

reduced shoot growth (Fig. 2).

Discussion
Benzyladenine. This synthetic cytokinin can mimic natural plant
hormones to stimulate plant growth by either increasing bud density or reducing

height growth (Barrett, 1992; Kramer and Kozlowski, 1979). Height growth was

87

effectively controlled with all species we tested (Fig. 1), and bud density was also
increased for each of the spruce species (Fig. 3). However, BA treatments
caused severe phytotoxic effects on all species. Work performed on loblolly pine
(Pinus taeda L.) resulted in similar findings (Blake and South, 1991).
Benzyladenine was applied to seedlings at 500 and 1000 ppm, with height
control occurring only in the 1000 ppm treatment. Shoots of balsam fir [Abies
balsamea (L.) Mill.] seedlings were dipped in a liquid solution of BA, which
caused an increase in lateral bud number, branch diameter, and lateral shoot
production (Little, 1985). The most responsive treatments were those applied
after the untreated branches had reached 70% of their final length, and
phytotoxic symptoms were observed after multiple applications of BA. Single
treatments caused minimal or no phytotoxicity. We applied a total of four
applications in our trial, with phytotoxic symptoms occurring after the third
treatment with most species. Too many chemical applications is more than likely
the main reason why we observed such severe phytotoxic effects from BA
treatments. Also, since seedlings were actively growing when treatments were
applied, this new growth may have been too young and sensitive, resulting in
damaged tissue similar to that observed with Fraser fir seedlings (Little, 1985).
As we observed in Serbian spruce seedlings, balsam fir (Little, 1985),
concolor fir (Duck, 2002), and Colorado blue spruce (Mulgrew and Williams,
1985) seedlings in related experiments also experienced red to purple
discoloration on new growth. Discoloration of plant tissue occurred around the
areas of increased bud development with Colorado blue spruce seedlings and

occurred on new growth of concolor fir seedlings. For fir seedlings, phytotoxic

88

symptoms were thought to be a result of applying BA too soon to new shoot
growth (Little, 1985).

Buds from areas of increased bud formation on spruce seedlings we
tested were somewhat deformed and occurred in clusters. Loblolly pine
seedlings treated with BA at 1000 ppm experienced abnormal bud development,
inhibition of secondary needle extension, and delayed budbreak (Blake and
South, 1991). Shortened, unnatural needle growth, as observed with Black Hills
spruce seedlings we tested, was also detected in Scotch pine (Pinus sylvestris
L.) seedlings treated with BA (Mulgrew and Williams, 1984).

Uniconazole. Treatments of uniconazole reduced shoot growth of Black
Hills and Serbian spruce. Height growth of Fraser fir [Abies fraseri (Pursh) Poir]
seedlings was reduced (by 22% to 45%) after the plants received uniconazole
treatments at four concentrations (0, 2, 4, or 16 ppm) (Hinesley, 1998).
Uniconazole reduced apical dominance, which resulted in decreased terminal
shoot growth compared with lateral shoot growth. Uniconazole treatments
applied as a soil drench to three-year-old loblolly pine seedlings decreased
height growth by 55% after the three months following treatment (Barnes and
Kelley, 1992).

We also observed minor phytotoxic symptoms in Black Hills spruce and
Port Orford cedar seedlings treated with uniconazole. Concentrations of
uniconazole greater than or equal to 8 ppm caused foliar problems, such as
chlorosis, necrosis, or abscission, in Fraser fir (Hinesley, 1998).

Unlike Black Hills and Serbian spruce, Colorado blue spruce or Port

Orford cedar seedling growth was unaffected by uniconazole. Shoot growth was

89

not affected in Colorado blue spruce seedlings treated with 15 ppm foliar
applications of uniconazole, (Duck, 2002). Shoot dry weight of leyland cypress
[X Cupressocypan's leylandii (A.B. Jacks. & Dallim.) Dallim. & A.B. Jacks] plants
already established in the landscape was unaffected by foliar or soil drench
treatments of uniconazole (Keever and West, 1992). However, growth of
containerized Leyland cypress treated in the same manner as the field-grown
specimens was reduced by 37% to 48%. Effectiveness of uniconazole
applications on Colorado blue spruce and Port Orford cedar seedlings could
depend on age, type of application method, and rate of application.

Bud density of all species tested was unaffected by either uniconazole
treatment. The main benefits generally observed from the use of uniconazole
have typically been shortened internodes, darker green foliage, and an increase
in epicormic bud development. One reason we did not observe increased bud
densities may be because of the time of year treatments were applied. Fraser fir
seedlings treated with uniconazole in May had an increase in lateral bud number
(by 20% to 24%), whereas seedlings treated in June and September had a 45%
to 34% reduction in lateral bud formation (Hinesley, 1998).

Conclusion. Plant growth retardants could benefit tabletop Christmas tree
producers by reducing pruning requirements of excessively long terminal shoots
and increasing lateral branch formation for fuller, more attractive specimens.
Uniconazole treatments decreased shoot growth of Black Hills and Serbian
spruce, had no effect on growth of Colorado blue spruce and Port Orford cedar,
and did not increase bud density of any species tested. We observed significant

decreases in shoot growth of all species treated with BA and also increases in

90

bud density of spruce seedlings treated with BA. However, BA treatments at
both rates caused extreme phytotoxic effects in all species. Because all species
we tested were very responsive to BA treatments, it is important to consider the
potential this PGR can have for controlling height growth. Studies should be
performed with BA and uniconazole to determine more effective rates and the
most effective timing of applications for BA in regard to age of new growth. Plant
growth retardant protocols can then be more confidently recommended for
tabletop Christmas tree producers to generate an aesthetically pleasing and

uniform crop.

91

Literature Cited

Barnes, A. D. and W.D. Kelley. 1992. Effects of a triazole, uniconazole, on shoot
elongation and root growth in loblolly pine. Can. J. For. Res. 2221—4.

Barrett, J.E. 1992. Mechanisms of action, p. 12—18. In: H.K. Tayama, R.A.
Larson, PA. Hammer and Teresa J. Roll. Tips on the use of chemical growth
regulators on floriculture crops. Ohio Florists' Association, Columbus, Ohio.

Blake, J.l. and DB. South 1991. Effects of plant growth regulators on loblolly
pine seedling development and field performance. Proc. 6th Biennial S. Silv.
Res. Conf. 100-107.

Duck, M. W. 2002. Developing production systems for tabletop Christmas trees.
Thes. Michigan State University, East Lansing.

Hinesley, L. E. 1998. Effect of uniconazole on shoot growth and budset of
containerized Fraser fir. HortScience 33(1):82—84.

Keever, G. J. and MS. West. 1992. Response of established landscape plants
to uniconazole. HortTechnology(Oct/Dec):465—468.

Kimball, S. L. 1990. The physiology of tree growth regulators. J. Arboriculture
16(2):39—41.

Kramer, P. J. and T.T. Kozlowski. 1979. Physiology of Woody Plants. Orlando,
Academic Press, Inc.

Landis, T.D., R.W. Tinus, S.E. McDonald and JP. Barnett. 1992. The Container
Tree Nursery Manual: Atmospheric Environment. USDA Forest Service,
Washington.

Larson, RA 1992. History, p. 8-11. In: H.K. Tayama, R.A. Larson, PA. Hammer
and T.J. Roll. Tips on the use of chemical growth regulators on floriculture
crops. Ohio Florists' Association, Columbus Ohio.

Little, C. H. A. 1985. Increasing lateral shoot production in Balsam fir Christmas
trees with cytokinin application. HortSc 20(4):713—714.

Mulgrew, S. M. and DJ Williams. 1984. Promoting bud development and
branching of scotch pine with 6-BA. J. Arboriculture 10(11):294—297.

Mulgrew, S. M. and DJ Williams. 1985. Effect of Benzyladenine on the
promotion of bud development and branching of Picea pungens. HortSci
20(3):380—381.

Rademacher, W. 1991. Biochemical Effects of Plant Growth Retardants. Plant

92

Biochemical Regulators. H. W. Gausman. New York, Marcel Dekker, Inc.
169—200.

Ruter, J. M. 1994. Growth and landscape establishment of Pyracantha and
Juniperus after application of paclobutrazol. HortSci 29(11):1318—1320.

Sundback, E. 2002. The future is open. Michigan Christmas Tree Journal.
49:5—7.

Thomas, B. and D. Vince-Prue. 1997. Bud dormancy. Photoperiodism in Plants.
San Diego, Calif. Academic Press, 279—316.

93

Table 1. Analysis of variance for mean relative growth rate (RGR) (cm d")

and bud density (buds cm").

 

 

 

RGR Bud density
Effect df F df F
PGR 4 ***107.51 4 ***27.13
Species 3 ***82.27 2 ***28.05
P R x ie *** . 2 ***12.4
***Significant at P < 0.0001.

94

Table 2. Mean height growth, number of buds, and bud densities for all species and treatments.

 

 

 

 

 

 

Species Treatment Mean Height Mean Number Mean Bud
(ppm) Growth (cm) of Buds (buds Density (buds
shoot") cm")
Black hills spruce Control 12.75 30.90a 2.43b
BA 500 2.30 14.33c 5.67a
BA 1000 1 .90 16.00bc 3.38b
Uniconazole 1 10.15 28.33ab 2.53b
Uniconazole 5 9.60 26.40abc 2.84b
Serbian spruce Control 10.95 20.20b 1.87b
BA 500 2.25 26.90a 15.30a
BA 1000 0.85 10.000 13.95a
Uniconazole 1 8.00 16.44b 2.23b
Uniconazole 5 8.11 19.75b 2.34b
Colorado blue spruce Control 10.06 19.20a 2.09bc
BA 500 5.55 16.88a 3.04a
BA 1000 2.05 4.71b 1.72c
Uniconazole 1 11.15 23.308 2.09bc
Uniconazole 5 8.75 21 .80a 2.51ab
Port Orford cedar Control 5.60
BA 500 2.40
BA 1000 1.15
Uniconazole 1 5.70

____I.Lnlmnazelfl__.§AQ.—____

 

95

List of Figures

FIGURE PAGE

1

Effects of plant growth retardant applications on height growth over time
for Black Hills spruce, Serbian spruce, Colorado blue spruce, and Port
Orford cedar seedlings.

97

Effects of plant growth retardant applications on mean relative growth
rates of Black Hills spruce, Serbian spruce, Colorado blue spruce, and
Port Orford cedar seedlings. Error bars represent SE of the mean.

98

Effects of plant growth retardant applications on mean bud densities of
Black Hills, Serbian, and Colorado blue spruce seedlings. Error bars
represent SE of the mean.

99

96

Shoot Growth (cm)

 

55

 

Black Hills spruce

 

 

50 - //’2_2—,"6—=——:_:6
45 4
40 -
35 -

—0— IControl ' I Serbian spruce
40 _ ........ v ........ BA 500 ppm

—--I-—- BA 1000 ppm
Uniconazole 1 ppm
Uniconazole 5 ppm

   

 

 

 

 

 

 

 

 

 

02/01/01 02/19/01 03/09/01 Date 03/27/01
Figure 1
97

04/14/01 05/02/01

 

Relative Growth Rate (cm cm'1 d")

 

0.005

0.004 -

0.003 -

0.002 -

0.001 -

 

0.000 -

l

 

 

 

. L ‘
Black Hills
spruce

 

 

- Control

I [:3 BA 500 ppm

- BA 1000 ppm
:3 Uniconazole 1 ppm
— Uniconazole 5 ppm

 

 

   

 

 

 

 

 

 

 

 

.1 w L, .
Serbian spruce Colorado blue Port Orford
spruce cedar
Species
Figure 2

98

Bud Density (buds cm")

20

 

_L
01
l

 

.3
o
J

OI
1

 

 

 

 

ill

 

 

- Control

BA 500 ppm

- BA 1000 ppm
[:1 Uniconazole 1 ppm
- Uniconazole 5 ppm

 

    

 

Black Hills spruce Serbian spruce
Species

Figure 3
99

 

Colorado blue spruce

 

CHAPTER THREE

FORCING CONIFERS FOR TABLETOP CHRISTMAS TREES

100

Forcing Conifers for Tabletop Christmas Trees

Additional index words. Picea, Abies, photoperiod, chilling

Abstract

Several interacting environmental factors, including chilling, photoperiod,
and temperature, control budbreak in conifers. We compared the bud flush
response of several conifers: Black Hills spruce [Picea glauca (Moench) Voss
var. densata], Serbian spruce [P. omorika (Pancic) Purkyne], Wilson spruce (P.
wilsonii Mast), Meyer spruce (P. meyeri Rehd. and Wils.), Colorado blue spruce
(P. pungens Engelm.), noble fir (Abies procera Rehd. ‘Frijsenborg Blue'), and
Nordmann fir [A. nordmanniana-ambrolauria (Steven) Spach.]. The trees were
subjected to various lengths of chilling in the middle of their normal growing
cycle. The seedlings were chilled for 0, 4, 6, or 8 weeks at 3 °C. Before chilling,
seedlings received a short-day (SD) (9-hour photoperiod) treatment for 0, 2, 4, or
6 weeks at 20 °C. After the combination of short-days and chilling, the seedlings
were placed under a long-day (16-hour photoperiod) treatment at 20 °C.
Species varied in response to the treatments. Black Hills spruce required chilling
for uniform and timely budbreak, and SD treatments influenced budbreak when
combined with 4 or 6 weeks of chilling. Serbian spruce required chilling and SD
treatments, except when subjected to 6 weeks of chilling when SD treatments
were not effective. Meyer spruce, Wilson spruce, and noble fir require chilling
but did not need SD treatments for budbreak to occur. Both Colorado blue
spruce and Nordmann fir required chilling and SD treatments for uniform
budbreak to occur. Results of the study suggest that certain conifers can be

101

programmed to respond or flush on a target date according to combinations of

SDs and chilling treatments.

Introduction

Live tabletop Christmas tree production could be a lucrative market for
conifer and greenhouse growers in Michigan. Consumers in Europe purchase
tabletop trees by the millions, Port Orford cedar [Chamaecyparis Iawsoniana
(Murray) Parl. ‘Ellwoodii’] being the primary species (Hamrick, 2002). There are
few choices of tabletop Christmas trees on the market, and most are marketed
through mail-order catalogs and some mass merchandisers. In addition to Port
Orford cedar, Italian stone pine (Pinus pinea L.) is sold as a tabletop tree.
Neither of these trees are cold-hardy in the upper Midwest (zone 5) where the
average annual minimum temperature ranges from -29 to -23 °C (Bailey and
Bailey, 1976; Crothers, 1990; USDA, 2002). The current mail-order selections
are also somewhat expensive for the average homeowner, ranging from $55 to
$85 for a 61-cm-tall specimen (Calyxand Corolla, 2000; Harry and David, 2000;
Jackson 8 Perkins, 2000). However, the mass merchandiser market is
comparatively inexpensive and has seen exceptional growth in recent years
(15% in 1990) (Crothers, 1990). Some individual growers have experienced
even greater increases in growth (35% in 1990).

For a grower to successfully produce a marketable tabletop specimen, a
production schedule must be determined for each species to obtain a finished
product in the shortest time possible. Observations from our initial studies

indicate that consumers prefer trees that have recently flushed. Producing a tree

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with this type of growth can be accomplished by determining how conifer
seedlings respond to photoperiod and chilling treatments. To do this, growers
must force the trees to break bud out of their normal growth phase. A great deal
of work has been performed to determine how conifers respond to photoperiod
(Bongarton and Hanover, 1985; Christersson, 1978; Kramer and Kozlowski,
1979; Landis et al., 1992; Young and Hanover, 1976) and chilling (Cazell and
Seller, 1992; Dormling, 1993; Hart and Hanover, 1979; Young and Hanover,
1978) treatments. Photoperiod duration is a major influence in growth and
dormancy processes of conifer species. A long-day (LD) photoperiod causes
continued shoot growth, whereas a short—day (SD) photoperiod initiates a state
of dormancy. Once dormancy has been reached, seedlings must be subjected
to some period of chilling treatment to break dormancy and continue growth in
the spring. By manipulating these processes, a grower could possibly obtain
multiple flushes of growth in one season.

The objective for this experiment was to determine the optimum
combination of dormancy induction and chilling needed to force conifers to break
bud and flush on a target date and characterize species variation in response to
forcing. In 2001, we compared bud flush response of several conifers
throughout two 20-week production schedules with two start dates (weeks 27
and 29), which were used to determine whether the additional two weeks of

natural growing conditions would affect growth response of the seedlings.

Materials and Methods

Plant material and culture. Seedlings of Wilson spruce (P. wilsonii) were

103

received from Vans Pines Nursery, Inc. (West Olive, Mich.) and planted Aug.
2000 in one-gallon containers (2830 cm”) with a 70:30 (volzvol) peat/perlite
substrate (Peat-Perlite Mix, Michigan Grower Products, Galesburg, Mich.)
Meyer spruce (P. meyen) seedlings were received from Treehaven Evergreen
Nursery (Elma, NY.) and planted May 2001 in one-gallon containers with a
15:85 (volzvol) peat/pine bark substrate (Renewed Earth, lnc., Kalamazoo,
Mich.) Colorado blue spruce (P. pungens) seedlings were received from
Wahmhoff Farms Nursery (Gobles, Mich.) and planted May 2001 in one-gallon
containers with the peat/pine bark substrate. Noble fir (Abies procera
‘Frijsenborg Blue’) and Nordmann fit (A. nordmanniana-ambrolaun’a) were
received from Lawyer Nursery, Inc. (West Plains, Mont.) and planted Mar. 2001
in one-gallon containers with the peat/pine bark substrate. The Serbian spruce
(P. omorika) used in this trial came from two shipments. Eighty percent of the
seedlings were dug from a field production area at Vans Pines Nursery in June
2001 and planted on the same day in one—gallon containers with the peat/pine
bark substrate. The remaining Serbian spruce seedlings were received from
Vans Pines Nursery and planted Apr. 2000 in one-gallon containers with a
peat/perlite substrate. The Black Hills spruce came from two sources. Half of
the seedlings were received from Vans Pines Nursery and planted in Apr. 2000
in one-gallon containers with the peat/perlite substrate. The other half of the
seedlings were received from Northland Evergreens, Inc. (West Olive, Mich.) in
June 2001. The seedlings were planted at their nursery in the fall of 2000 in
one-gallon containers with a 40% rice hull, 50% pine bark, and 10% sand

substrate.

104

After planting, all seedlings were grown outdoors under natural conditions
at the Michigan State University Horticulture Teaching and Research Center
(HTRC) (East Lansing, Mich.) until the experiment began in July 2001.
Seedlings were treated in June 2000 and 2001 with Ronstar preemergent
herbicide at 2 lb a.i. acre ". Also in June 2000 and 2001, seedlings were given
17-7-8 slow-release (3- to 7-month) granular fertilizer (Wilbro Horticultural
Products, Norway, 8.0.) at 10 g/tree. Seedlings were irrigated as needed with
water obtained from wells at the HTRC. Once the seedlings were transferred to
the greenhouse, they were irrigated as needed with a nutrient solution of well
water (electrical conductivity of 0.65 mS cm" and 105, 35, 23 [mg L"] Ca, Mg,
and 8, respectively) acidified with H280, to a titratable alkalinity of CaSO2 at 130
mg L" and water-soluble fertilizer providing 125-12-125-13 (mg L") N-P-K-Ca
plus 1.0-0.5-0.5-0.1-0.1 (mg L") Fe-Mn-Zn-Cu-B—Mo (MSU Special, Greencare
Fertilizers, Chicago, Ill.).

Treatments. Seedlings were exposed to various combinations of chilling
and photoperiod to determine the optimum production schedule to obtain a
finished product in time for Christmas. Conifers at the beginning of the trial
ranged in size from 9.0 to 64.0 cm. Seedlings received an SD (9-h photoperiod)
treatment for 0, 2, 4, or 6 weeks at 20 °C followed by a cold treatment for 0, 4, 6,
or 8 weeks at 2 °C. After the combination of SD and chilling, the seedlings were
placed under an LD treatment (16-h day-extension photoperiod using high-
pressure sodium lamps) at 20 °C. The experiment was set up as a completely
randomized design with the following distribution of plant material: 1) five Black

Hills spruce per treatment from Vans Pines Nursery and five Black Hills spruce

105

per treatment from Northland Evergreens, Inc; 2) eight Serbian spruce per
treatment from Vans Pines Nursery 2001 stock and two Serbian spruce per
treatment from Vans Pines Nursery 2000 stock; and 3) 10 Colorado blue spruce,
Meyer spruce, Wilson spruce, noble fir, and Nordmann fir per treatment (only the
0 and 8 weeks of chilling and 2, 4, 6, or 8 weeks of SD combinations).

Data collection and analysis. At the beginning of the trial, initial heights
(measured from the substrate surface) and terminal bud counts were taken.
Height measurements were taken repeatedly throughout the trial at two-week
intervals. Seedlings were monitored for days to budbreak once they entered the
16-h photoperiod treatment. At the completion of the experiment in November
and December of 2001 (weeks 47 and 49), final heights and terminal bud counts
were recorded.

The budbreak data were analyzed with the LIFEREG procedure (SAS/PC
software, SAS Institute, Inc., Cary, NC). This procedure is typically used on
survival analysis and was selected for our analysis because of the design of this
experiment. Since there was a specific termination date for the trial and some
seedlings never broke bud, we encountered right-censored failure time data.
The LIFEREG procedure fits parametric models to this type of data and
estimates the parameters by maximum likelihood by using a Newton-Raphson
algorithm. The MODEL statement syntax in this trial indicates that the response
variable days is censored when the variable yn (yes/no) takes the value of zero.
Main effects and interaction terms can be specified in the MODEL statement,
similar to the general linear model (GLM) procedure. The default distribution fits

a type 1 extreme value to the log of the response, which is equivalent to fitting

106

the Weibull distribution.

Results

Except for start date, all of the main effects examined (SD exposure,
chilling, and species) significantly (P < 0.0001) affected median days to
budbreak (Table 1). Each species tested requires chilling for uniform and timely
budbreak. However, Colorado blue spruce and Nordmann fir require an SD
treatment in combination with chilling. Because of these interactions between
main effects, it is difficult to make generalized recommendations for the overall
experiment; therefore, results are presented below by species.

Black Hills spruce. When seedlings were subjected to 0 or 8 weeks of
cold, SD treatments did not affect days to budbreak (Fig. 1). Budbreak was
sporadic and extremely slow under the 0-week chilling treatment. Short-day
treatment reduced days to budbreak when combined with 4 or 6 weeks of
chilling. Seedlings that received 4, 6, or 8 weeks of chilling treatments broke bud
much more quickly and uniformly than those not subjected to chilling.

Serbian spruce. Both SD and chilling treatments affected days to
budbreak (Fig. 2). Seedlings took longer to break bud when they received 0
weeks of SD and 0 weeks of chilling than seedlings subjected to 2, 4, or 6 weeks
of SD and 0 weeks of chilling. Seedlings that did not receive chilling were slower
to break bud than those subjected to chilling treatments. However, seedlings
subjected to 0 weeks of SD and 4 weeks of chilling took longer to break bud than
all other seedlings subjected to cold treatment. Short-day treatments had an

effect on days to budbreak except when combined with 6-week chilling

107

treatments, where seedlings in each 6-week chilling treatment had similar
median days to budbreak.

Meyer spruce. Short-day treatments did not influence median days to
budbreak when seedlings were not subjected to chilling (Fig. 3). However, 2
weeks of 803 reduced median days to budbreak when combined with 8 weeks
of chilling. Seedlings broke bud much more quickly when subjected to 8 weeks
of chilling, unlike those that did not receive a cold treatment.

Colorado blue spruce. Short-day treatments reduced number of days to
budbreak (Fig. 4). Seedlings that did not receive SD treatments took three times
as long to break bud as seedlings subjected to 2, 4, or 6 weeks of SDs with or
without chilling. Chilling treatments also reduced days to budbreak. Seedlings
subjected to 8 weeks of chilling broke bud two to three times sooner than those
not subjected to chilling.

Wilson spruce. Short-day treatments did not significantly affect days to
budbreak; however, chilling treatments did affect days to budbreak (Fig. 5).
Seedlings that received 8-week chilling treatments broke bud more than 10 times
as quickly as those under 0 weeks of chilling. Also, chilled seedlings had more
uniform budbreak over the whole tree, and more uniform budbreak occurred
among seedlings within the same treatment.

Noble fir. Chilling had an effect on days to budbreak, regardless of SD
treatments (Fig. 6). Days to budbreak for seedlings subjected to 0 weeks of
chilling took twice as long as those given 8 weeks of chilling. Days to budbreak
were similar regardless of SD treatment.

Nordmann fir. Both SD and chilling treatments affected number of days to

108

budbreak (Fig. 7). Seedlings that did not receive SD treatments took more than
twice as long to break bud as those subjected to SD treatments. Without
chilling, only three of 40 seedlings broke bud, which resulted in insufficient data
for the model to accurately predict days to budbreak. Seedlings subjected to 8
weeks of chilling and 0 weeks of SD were extremely slow to break bud, whereas
seedlings given 8 weeks of chilling and 2, 4, or 6 weeks of SD broke bud quickly

and uniformly.

Discussion

Start date. The production schedule for this trial was determined for two
reasons. First, we chose weeks 27 and 29 as start dates so seedlings would
have achieved a sufficient amount of natural growth before being placed into the
greenhouse environment. Because start date did not affect days to budbreak,
either week could be used. However, growers must use some caution when
determining start time for production because the natural growing conditions
seedlings experience in the spring can differ from year to year. It is important to
make sure seedlings have grown as much as possible before entering the
greenhouse to bulk starting material. Second, production duration was set for 20
weeks from start date to have a marketable product ready for shipment about the
end of November for holiday sales. Suppliers of tabletop trees generally begin
shipping their trees during November (Crothers, 1990).

Short-day response. Short-day treatments can be given in combination
with chilling to induce dormancy in some species. From a practical standpoint,

only two species we evaluated, Colorado blue spruce and Nordmann fir, require

109

an SD period before chilling for uniform budbreak. Research performed on
Colorado blue spruce seedlings has also indicated SDs as an important factor
for inducing dormancy (Bongarten and Hanover, 1985; Young and Hanover,
1977). A minimum period of 20 SDs must be given to Colorado blue spruce
seedlings of any age for bud scales to form (Young and Hanover, 1977). Conifer
seedlings that require SD treatments before chilling must receive their complete
duration to achieve full dormancy (Landis et al., 1992; Odlum, 1991). If SD
treatments are terminated too quickly, brief exposure to LD conditions could
cause seedlings to resume growth, which makes it important to continue SD
treatments even after visible shoot extension has ceased. Once deep winter
dormancy has been reached, only the required cold period will break dormancy
for growth to begin again (Kramer and Kozlowski, 1979; Thomas and Vince-
Prue, 1997). The response of Nordmann fir to SD in this trial is similar to that of
Fraser fir. Under short durations of cold treatment, Fraser fir seedlings are quite
sensitive to photoperiod changes (Hinesley, 1982). Trees that received 803 to
induce bud formation before chilling experienced budbreak and improved growth
(Cazell and Seller, 1992; Hinesley, 1982).

Black Hills spruce, Serbian spruce, and Meyer spruce also broke bud
sooner when given an SD treatment before chilling. However, these seedlings
can achieve budbreak as soon or sooner if simply given two additional weeks of
chilling without an SD treatment. This procedure is more practical from a
grower’s standpoint in that it will reduce the number of times seedlings must be
transferred from one treatment to another. Wilson spruce and noble fir were not

affected by SD treatments. The responses we observed with noble fir are

110

contradictory to those seen by Tung and Deyoe (1991). They found that 803
were necessary in accelerating dormancy induction, which could be due to age
differences between seedlings they tested (12 weeks old) and seedlings we
tested (two years old). Young and Hanover (1976) found that young Colorado
blue spruce seedlings (one to two years old) are more responsive to
environmental treatments than older trees.

Chilling response. Chilling is crucial for proper growth and development of
most conifer species. The responses we observed in this trial correlate with
results obtained in prior work performed on the same or similar species of
conifers. All spruce species we tested require some duration of chilling for timely
and uniform budbreak. A cold treatment is not only responsible for inducing
dormancy (Kramer and Kozlowski, 1979) but also an important process to break
dormancy of many species. Hanover (1980), along with many other researchers
(Hart and Hanover, 1979; Kramer and Kozlowski, 1979; Landis et al., 1992;
Nienstaedt, 1967; Wareing, 1956), has determined that most plants cannot
resume growth once dormancy has been established until their chilling
requirement has been met. Shorter periods of cold may be adequate for species
that received only 0- and 8-week treatments, considering the growth responses
we observed with Black Hills and Serbian spruce seedlings.

Nordmann and noble fir seedlings require chilling treatments to achieve
uniform budbreak. Research performed on other fir species has shown similar
responses (Cazell and Seller, 1992; Hinesley, 1982; Seller and Kreh, 1987).
Fraser fir is a difficult species to grow in greenhouses because of the lack of

uniformity in crop growth if specific growing conditions are not met. Seedlings

111

that did not receive chilling lost apical dominance, had deformed or aborted
terminal buds, had stunted growth, and lacked the characteristic symmetry found
in naturally occurring trees (Cazell and Seller, 1992; Hinesley, 1982; Seller and
Kreh, 1987). Seedlings that received chilling treatments had the greatest height
growth (Cazell and Seller, 1992), and time to reach budbreak decreased with
increased chilling (Hinesley, 1982). However, Fraser fir seedlings exposed to
natural chilling conditions generally have more uniform growth and better
symmetry than greenhouse-grown trees (Seller and Kreh, 1987).

Ecotype. The SD response we observed with Colorado blue spruce and
Nordmann fir could be explained by their native ecotype. Colorado blue spruce
are originally from high elevations (1800-3000 m) in the Rocky Mountains in
Idaho, Colorado, Utah, Wyoming, Arizona, and New Mexico. Nordmann fir are
native to high elevations (900-2100 m) in Turkey. Species from higher
elevations, such as ponderosa pine (Pinus ponderosa var. scopulorum Engelm.),
Engelmann spruce [Picea engelmannii (Parry) Engelm.], and Douglas-fir
[Pseudotsuga menziesii (Mirb.) Franco], take longer to complete the transition
from full dormancy to quiescence than those from lower elevations (Burr et al.,
1989). However, transition time from quiescence to 50% budbreak under
deacclimating conditions was quicker for species from lower elevations.

Forcing. Many consumers prefer the appearance of seedlings that have
soft new growth. Photoperiod extension is important when more growth than
normal is desired in one growing cycle (Landis et al., 1992). After seedlings
have received chilling and SD treatments, optimum growing conditions should be

provided to force this second growth flush, which can be achieved by subjecting

112

seedlings to an LD treatment (16-h photoperiod). Considerable research that
has been performed on conifers shows that LD treatments promote shoot growth
and delay dormancy induction (Bongarten and Hanover, 1985; Hanover, 1980;
Tinus, 1995; Wareing, 1956; Young and Hanover, 1977). The forcing treatment
should be long enough for sufficient new growth to occur and become strong
enough to withstand rigorous shipping and storage conditions.

Data analysis. Overall, the LIFEREG procedure we used provided a
useful means for analyzing budbreak data for this experiment. However,
because the nature of the model, in that it is typically used to model survival
analysis rather than days to budbreak, we encountered some inaccurate
predictions for days to budbreak of Nordmann fir. Of the 40 seedlings that
received 0 weeks of chilling and 0, 2, 4, or 6 weeks of SD, only three broke bud
before the experiment terminated. Seedlings that did achieve budbreak were
those given 6 weeks of SD. Since the LIFEREG procedure considered seedlings
that never broke bud as right-censored data, only those that broke bud were
recognized as observable variables, which resulted in insufficient data for the
model to accurately predict days to budbreak. Although trend predictions
generated from the model show that SDs are needed for budbreak to occur
without chilling, predicted median days to budbreak are extremely large and
unreasonable. Nordmann fir seedlings subjected to 0 weeks of SDs and 0
weeks of chilling have a predicted value of 2932 median days to budbreak
(including SE of the median as 1006 days).

Conclusion. The overall objective for this trial was to determine

production schedules for producing tabletop Christmas trees in a greenhouse

113

environment. We have listed our prescribed production schedules for all seven
species as they would be most practical from a grower’s standpoint (Table 2).
As mentioned previously, even though Black Hills, Serbian, and Meyer spruce
broke bud more quickly when given SD treatments combined with 4 weeks of
chilling, it would be more practical for a grower to avoid SD treatments and
simply extend chilling to 6 weeks. Colorado blue spruce and Nordmann fir must
receive at least 2 weeks of SDs before chilling for timely and uniform budbreak.
Wilson spruce and noble fir were the only species evaluated that were
unaffected by SD treatments, so growers can force these seedlings to break bud
by chilling alone. Seedlings that were subjected only to 0 or 8 weeks of chilling
could respond similarly if given a shorter cold period, but further research should

be performed to determine those exact requirements.

114

 

Literature Cited

Bailey, L.H. and E2. Bailey. 1976. Hortus Third. New York, Macmillan.
Bongarten, BC. and J.W. Hanover 1985. Accelerating seedling growth through
photoperiod extension for genetic testing: A case study with blue spruce (Picea
pungens). For. Sci. 31(3):631-643.

Burr, K.E., R.W. Tinus, S.J. Wallner and RM. King. 1989. Relationships among
cold hardiness, root growth potential and bud dormancy in three conifers. Tree
Physiol. 5(3):291—306.

Calyx & Corolla 2000. Holiday 2000 catalog. Calyx & Corolla: 29.

Cazell, B.H. and J.R. Seller. 1992. Intermittent short-days and chilling, and
benzylaminopurine affect the growth and morphology of Fraser fir seedlings. J.
Environ. Hort. 10(4):205—207.

Christersson, L. 1978. The influence of photoperiod and temperature on the
development of frost hardiness in seedlings of Pinus sylvestris and Picea abies.
Physiol. Plant. 44:288—294.

Crothers, D. 1990. Big profits in little packages. Produce Business. 6:50,52,54.
Dormling, I. 1993. Bud dormancy, frost hardiness, and frost drought in seedlings
of Pinus sylvestris and Picea abies. Advances in Plant Cold Hardiness. P. H. Li,
and Christersson, L. Boca Raton, CRC Press, 285—298.

Hamrick, D. 2002. Conifers are hot as a pot plant at Sluitter. FloraCulture lntl.
Mayz22.

Hanover, J.W. 1980. Control of tree growth. BioScience 30(11):756—762.

Hart, J.W. and J. Hanover 1979. Practical requirements for controlled
environment nursery stock production. lntl. Plant Propagators' Society.

Harry and David 2000. Christmas 2000 catalog. Harry and David: 45.

Hinesley, LE. 1982. Dormancy in Abies fraseri seedlings at the end of the first
growth cycle. Can. J. For. Res. 12:374-383.

Jackson & Perkins 2000. Holiday Gifts 2000 catalog. Jackson & Perkins:
3,9,16,27,29,33,34,36,39,47.

Kramer, P.J. and T.T. Kozlowski. 1979. Physiology of Woody Plants. Academic
Press, Inc., Orlando, Fla.

115

Landis, T.D., R.W. Tinus, S.E. McDonald and JP. Barnett. 1992. The Container
Tree Nursery Manual: Atmospheric Environment. USDA Forest Service,
Washington.

Nienstaedt, H. 1967. Chilling Requirements in Seven Picea Species. Silvae
Genet 16:65-68.

Odlum, KB. 1991. Hardening and overwintering container stock in Ontario:
Practices and research. 11th Annual Conference of the Forest Nursery
Association of British Colombia, Prince George, 30:11 29—35.

Seller, J.R. and RE. Kreh 1987. The effect of chilling and seed source on the
growth of containerized Fraser fir (Abies fraseri (Pursh) Poir.) seedlings. Tree
Planters' Notes (Spring 1987):19—21.

Thomas, B. and D. Vince-Prue. 1997. Bud dormancy. Photoperiodism in plants.
San Diego, Academic Press, 279—316.

Tinus, R.W. 1995. A new greenhouse photoperiod lighting system for prevention
of seedling dormancy. Tree Planters' Notes 46(1):11—14.

Tung, CH. and DR. Deyoe. 1991. Dormancy induction in container-grown Abies
seedlings: Effects of environmental cues and seedling age. New For.
5(1):13—22.

United States Department of Agriculture. Plant hardiness zone map 2002.
httpzllwww.usna.usda.gov/Hardzone/ushzmap.html?

Wareing, PF. 1956. Photoperiodism in woody plants. Annu. Rev. Plant Physiol.
7:191—214.

Young, E. and J.W. Hanover. 1976. Accelerating maturity in Picea seedlings.
Acta Horticulturae. 562105—114.

Young, E. and J.W. Hanover. 1977. Development of the shoot apex of blue
spruce (Picea pungens). Can. J. For. Res. 7:614—620.

Young, E. and J.W. Hanover. 1978. Effects of temperature, nutrient, and

moisture stresses on dormancy of blue spruce seedlings under continuous light.
For. Sci. 24(4):458—467.

116

Table 1. Analysis of variance for days to budbreak data.

 

Effect DF Wald X2 Pr > X2

Start date 1 0.0087 0.9258
Species 6 102.6109 <.0001
Short-day (SD) 3 68.3036 <.0001
Species x SD 18 108.3937 <.0001
Chill 3 975.2937 <.0001
Species x chill 8 239.7857 <.0001

D x chill 7 .4 <. 1

117

Table 2. Recommended tabletop Christmas tree production schedules for all

species tested.

 

Species Weeks of short Weeks of chilling Weeks of

days (9-h at 2 °C forcing (16-h

photoperiod) photoperiod)
Black Hills spruce 0 6 14
Serbian spruce 0 6 14
Meyer spruce 0 8 12
Colorado blue spruce 2 8 10
Wilson spruce 0 8 12
Noble fir 0 8 12

Norgmann fit 2 8 19

 

118

List of Figures

FIGURE PAGE

1

Median days to budbreak for Black Hills spruce seedlings exposed to
chilling and short-day treatments. Error bars on the scatter plot represent
SE of the median as calculated with SAS’s LIFEREG procedure.

120

Median days to budbreak for Serbian spruce seedlings exposed to chilling
and short-day treatments. Error bars on the scatter plot represent SE of
the median as calculated with SAS’s LIFEREG procedure.

121

Median days to budbreak for Meyer spruce seedlings exposed to chilling
and short-day treatments. Error bars on the bar graph represent SE of
the median as calculated with SAS’s LIFEREG procedure.

122

Median days to budbreak for Colorado blue spruce seedlings exposed to
chilling and short—day treatments. Error bars on the bar graph represent
SE of the median as calculated with SAS’s LIFEREG procedure.

123

Median days to budbreak for Wilson spruce seedlings exposed to chilling
and short-day treatments. Error bars on the bar graph represent SE of
the median as calculated with SAS’s LIFEREG procedure.

124

Median days to budbreak for noble fir seedlings exposed to chilling and
short-day treatments. Error bars on the bar graph represent SE of the
median as calculated with SAS’s LIFEREG procedure.

125

Median days to budbreak for Nordmann fir seedlings exposed to chilling
and short-day treatments. Error bars on the bar graph represent SE of
the median as calculated with SAS’s LIFEREG procedure.

126

119

Median Days to Budbreak

 

200

 

 

 

 

 

 

Black Hills Spruce __._ awkso

. -O— 2wk SD

+ 4wkSD

150 _ ‘ —v— 6wkSD
100 .
50 .

V ‘V=\ V

O l I I T I

 

 

 

O
N
b
0)
CD

Weeks of Chilling

Figure 1
120

Median Days to Budbreak

 

140

 

 

 

 

 

 

 

 

 

 

Serbian Spruce + Owk so
120 4 —o— 2 wk so
+ 4 wk so
—v— 6 wk so
100 ~
80 -
0\
so -
4o - .
§_
20 -
0 I I I l l
o 2 4 6 8

Weeks of Chilling

Figure 2
121

Median Days to Budbreak

300

 

250 -

200 a

150 a

100 -

50*

 

 

 

 

 

Meyer Spruce

 

- OwkSD
2wkSD
- 4wkSD
C: 6wkSD

 

 

 

 

8
Weeks of Chilling

Figure 3

122

film

 

 

 

350

 

300 ~

250 ~

200 -

150 a

100 -

Median Days to Budbreak

50-

 

 

 

 

Colorado Blue Spruce

 

Weeks of Chilling

Figure 4

123

 

 

- OwkSD
CZ] 2wkSD
- 4wkSD
[:1 6wkSD

 

 

 

 

 

Median Days to Budbreak

400

 

300 -

200 4

100 *

 

 

 

 

Wilson Spruce

 

 

 

Weeks of Chilling

Figure 5

124

 

 

- OwkSD
CZ} 2wkSD
- 4wkSD
[:1 6wkSD

 

 

 

 

mm

140

 

120 a

100 -

80~

60-

40-

Median Days to Budbreak

20~

 

 

 

 

Noble Fir

 

 

 

Weeks of Chilling

Figure 6
125

 

 

- OwkSD
[:3 2wkSD
- 4wkSD
[:1 6wkSD

 

 

 

 

 

 

 

 

 

 

 

Median Days to Budbreak

300

 

250 a

200 a

150 -

100 .

501

 

Nordmann Fir

 

 

 

 

 

 

 

 

 

- 0wkSD
C3 2wkSD
- 4wkSD
[:1 6wkSD

 

 

 

 

 

 

Weeks of Chilling

Figure 7
126

 

 

CHAPTER FOUR
USE OF ANTITRANSPIRANTS TO EXTEND THE SHELF LIFE OF TABLETOP

CHRISTMAS TREES

127

The Use of Antitranspirants to Extend the Shelf Life of Tabletop Christmas Trees

Additional index words. Moisturin, Wilt-Pruf, NuArbor, Picea, Abies, VPD

Abstract

In this trial, we compared the effect of antitranspirants on extending the
shelf life of live tabletop Christmas trees. We selected Black Hills spruce [Picea
glauca (Moench) Voss var. densata], Serbian spruce [P. omorika (Pancic)
Purkyne], and noble fir (Abies procera Rehd. ‘Frijsenborg Blue’) as the plant
species to be studied because of their performance as tabletop Christmas trees
in previous trials. Two growth chambers were set up for the experiment to obtain
a high and a low vapor pressure deficit (VPD) environment. The antitranspirants
were applied before the plants entered the growth chambers. Whole-plant water
use was determined by periodic weighing. Per-unit leaf area transpiration rates
were determined by gas exchange measurements with a Ll-COR LI-6400
Portable Photosynthesis System. Significance of main effects differed,
depending on measurement method. Analysis of whole-plant water loss data
indicated that Wilt-Pruf effectively reduced mean water loss of Black Hills spruce
under both VPD conditions, whereas Serbian spruce and noble fir were
unaffected by antitranspirant treatments. Gas exchange measurements
indicated that in high VPD conditions, NuArbor reduced transpiration rates of
Serbian spruce. Antitranspirant treatments were not effective on any other

species, regardless of VPD environment.

128

Introduction

Live conifer seedlings are growing in popularity as tabletop Christmas
trees (Crothers, 1990; Hamrick, 2002). Many of these trees are being sold by
mail-order catalogs (Calyx & Corolla, 2000; Harry and David, 2000; Jackson 8
Perkins, 2000) and mass merchandisers. These trees must endure stressful
environmental conditions during shipping and once they reach the customer or
retail outlet. Seedlings inside consumers’ homes during the holidays can
experience vapor pressure deficits (VPD) between 2.4 and 3.6 kPa because of
differences between cold outdoor and warm indoor conditions. The retail store
environment is also stressful to seedlings because of very low light levels from
fluorescent lighting (20 pmol m'2 3"), high VPDs similar to those of home
environments, and water stress as a result of negligence while seedlings are
displayed inside the store. Chastagner (1985) found that fresh-cut grand fir
[Abies grandis (Dougl. Ex D.Don) Lindl.] and Douglas-fir [Pseudotsuga menziesii
(Mirb.) Franco] Christmas trees under humid conditions can lose up to three
quarts of water in a 24-h period, suggesting that even greater water loss may
occur under high VPD conditions, which are typical of a retail store or home
environment. These types of environmental stresses can be detrimental to
seedling appearance and may reduce survival if consumers plant the seedlings
after the holidays. Therefore, it is important for growers to determine a means of
reducing water stress to extend seedling shelf life.

Applying antitranspirants to seedlings is one tool growers may use to
reduce plant water stress. Antitranspirants are materials applied to plants to

retard transpiration and should operate at the leaf-air interface (Gale and Hagan.

129

1966). Different forms of antitranspirants are available (Gale and Hagan, 1966),
but the most commonly used antitranspirants are film-forming compounds.
These inert compounds reduce transpiration by forming a water-impermeable
coating over the leaf surface and generally have long lasting-effects (Das and
Raghavendra, 1979). However, some compounds have been found to not only
reduce water loss but also interfere with CO2 assimilation, subsequently
decreasing photosynthesis rates (Davies and Kozlowski, 1974; Kramer and
Kozlowski, 1979).

Numerous studies have been performed throughout the past 50 years to
determine the effectiveness of film-forming antitranspirants on increasing survival
of various conifer species, but results have been inconsistent (Colombo and
Odlum, 1987; Englert et al., 1993a, 1993b; Fowells and Schubert, 1955; Jack,
1955; Simpson, 1984; Williams et al., 1990). Therefore, our goal for this project
was to determine the effectiveness of various film-forming antitranspirants
applied to three common conifer species, which was achieved by subjecting
treated seedlings to high and low VPDs and measuring their transpiration rates

throughout a seven-week period.

Materials and Methods

Plant material and culture. Black Hills spruce (Picea glauca var. densata)
seedlings were received from Vans Pines Nursery (West Olive, Mich.) and
planted in Apr. 2000 in one-gallon containers (2830 cm") with a 70:30 (volzvol)
peat/perlite substrate (Peat-Perlite Mix, Michigan Grower Products, Galesburg,

Mich.) Serbian spruce (P. omorika) seedlings were dug from a field production

130

area at Vans Pines Nursery in June 2001 and planted on the same day in one-
gallon containers with a 15:85 (volzvol) peat/pine bark substrate (Renewed Earth,
lnc., Kalamazoo, Mich.) Noble fir (Abies procera ‘Frijsenborg Blue’) were
received from Lawyer Nursery, Inc. (West Plains, Mont.) and planted in Mar.
2001 in one-gallon containers with the peat/pine bark substrate. Conifers at the
beginning of the trial ranged in size from 16.5 to 62.0 cm. After planting in one-
gallon containers, all seedlings were grown outdoors under natural conditions at
the Michigan State University Horticulture Teaching and Research Center
(HTRC) (East Lansing, Mich.) Black Hills spruce seedlings were treated in June
2000 and 2001 and Serbian spruce and noble fir seedlings were treated in June
2001 with Ronstar preemergent herbicide at 2.24 kg a.i. hectare ". Also in June
2000 and 2001, seedlings were given 17-7-8 slow-release granular fertilizer (3-
to 7-month) (Wilbro Horticultural Products, Norway, S.C.) at 10 g/tree. Seedlings
were irrigated as needed with water obtained from wells at the HTRC.
Beginning in July 2001, seedlings were used to determine the optimum
production system for generating a marketable crop of tabletop trees for sale
during the Christmas season (Duck, 2002). This trial consisted of various
combinations of chilling and photoperiod treatments throughout a 20-week
period that ended in Nov. and Dec. 2001. At the end of this experiment, the
seedlings that had 100% budbreak and a new flush of growth were selected for
the antitranspirant experiment because they represented the criteria for trees
that would be selected for shipment and sale during Christmas. While the
seedlings were in the greenhouse environment, they were irrigated as needed

with a nutrient solution of well water (electrical conductivity of 0.65 mS cm" and

131

105, 35, 23 [mg L"] Ca, Mg, and S, respectively) acidified with H2804 to a
titratable alkalinity of CaSO2 at 130 mg L" and water-soluble fertilizer providing
125-12-125-13 (mg L") N-P—K-Ca plus 1.0-0.5-0.5-0.1-0.1 (mg L") Fe-Mn-Zn-Cu-
B-Mo (MSU Special, Greencare Fertilizers, Chicago, Ill.)

Treatments. The seedlings were placed in the growth chambers on Jan.
22, 2002, and the experiment was arranged by using five seedlings per
treatment in each of the two growth chambers. Seedlings were treated with
antitranspirants in the greenhouse before being placed into the growth
chambers. All compounds were applied as a foliar spray until the point of drip-
off. Treatment rates were based on label directions. A total of four treatments
were applied to the conifers: 1) one part Moisturin (Burke’s Protective Coatings,
Washougal, Wash.) (vinyl acrylic vinyledene chloride resin) to 10 parts water, 2)
one part Wilt-Pruf (Wilt-Pruf Products, Inc, Essex, Conn.) (terpenic polymer) to
10 parts water, 3) one part Nu-Arbor (Nu-Arbor Tree 8 Shrub Care Products,
Grand Rapids, Mich.) (acrylic copolymer) to 20 parts water, and 4) a control.
Two growth chambers were set to achieve a high (1.90 kPa) and low (0.95 kPa)
VPD: 1) 25 °C with a relative humidity (RH) of 40% for the high VPD, and 2) 25
°C with an RH of 70% for the low VPD. Growth chamber photoperiod was 16h
’with cool-white fluorescent lights (approximately 175 to 195 pmol m'2 3").

Data collection and analysis. Transplration rates were determined by two
methods. The first method figured whole-plant water loss (Mortensen, 1994).
Seedlings were irrigated with the same greenhouse water as described above
until drainage occurred. After drainage from the soil ceased, the containers were

sealed in plastic bags by tying the bag at the base of the seedling stem to

132

prevent water from escaping by any means other than through the plant itself.
Each plant (including the plastic bag) was then weighed to determine the initial
weight. After seven days, the plants were reweighed. The amount of water lost
through transpiration was calculated as the difference between the initial and
final weights on a gram-per-day basis. This process was repeated throughout a
seven-week period for a total of seven moisture-loss calculations.

Transplration was also determined by measuring gas exchange by using
an Ll-6400 Portable Photosynthesis System equipped with an Ll-6400-05 conifer
chamber (LI-COR, Inc., Lincoln, Neb.) Gas exchange measurements have been
determined by other researchers using similar Ll-COR systems (Samuelson and
Seiler,1994; Sulzer et al., 1993; Tan et al., 1992). One shoot segment
approximately 5 cm long was marked on each seedling to make repeated
measures. Once the shoot was inserted into the conifer chamber, the strip chart
option on the Ll-6400 was used to accurately determine a stabilization point for
recording transpiration readings. These measurements were taken on each
seedling each day preceding the whole-plant weight measurements for a total of
seven measurements.

At the end of the trial, the marked shoot segments used to measure gas
exchange rates with the Ll-6400 were harvested. Needle area was determined
from total projected needle area (Guehl et al., 1991; Samuelson and Seller,
1992). Needles were plucked from the shoot segments and placed on a white
sheet of paper so that no needles overlapped each other, and a digital image
was taken. The camera was mounted on a photocopy stand so that the image of

each needle sample was taken from the same position, which was done for each

133

of the shoot segments. Each image was analyzed with SigmaScan Pro 5.0
(SPSS Inc., Chicago, III.) by using a macro developed by Douglas Karcher
(personal communication). A reference object of known area was included in
each image for converting number of pixels to needle area. Once total projected
needle area was measured, the needles were dried to obtain needle dry weight
of the shoot segments. At the end of the experiment, all trees were harvested
and dried to determine seedling needle weight (SNW). To determine accurate
transpiration rates of the selected shoot segments by using the gas exchange
measurements, specific leaf area (SLA) was calculated as total needle surface
area/needle dry weight. Whole-tree needle area was estimated from SNW as
SLA x SNW to calculate whole-plant water loss on a per-unit leaf area basis.
Significance levels of main effects on seedling water loss were
determined by repeated-measures analysis of variance. Differences among
treatments were established at the 5% level of probability. Whole-plant water
loss data and gas exchange data were transformed with the square root function
(SQRT) to achieve normal distribution on the residual terms and then analyzed
with the mixed procedure (PROC MIXED) (SAS/PC software, SAS Institute, Inc.,

Cary, NC.)

Results

Significance levels of main effects we examined (days that measurements
were recorded, species tested, antitranspirant treatment, and VPD) varied
slightly, depending on the method used to determine seedling transpiration

(Table 1). Regardless of antitranspirant treatment, Serbian spruce seedlings had

134

the highest transpiration rates as determined on a per-unit leaf area basis (Table
2). Black Hills spruce seedlings had lower transpiration rates, even though they
had the highest average leaf area. Whole-plant water loss for Black Hills spruce
seedlings was reduced by Wilt-Pruf treatments (Fig. 1), but water loss was not
different according to gas exchange data (Fig. 2). In high VPD conditions,
NuArbor reduced water loss for Serbian spruce seedlings. This effect was not
different from that of the control after day 15. Antitranspirant treatments did not
effectively reduce water loss of Noble fir seedlings. Water loss of all seedlings
we tested decreased over time, regardless of VPD or antitranspirant treatment.
Unlike water loss rates, transpiration rates fluctuated over time; however, the
same trend of decreased transpiration rates over time was detected here, as

described above with whole-plant water loss.

Discussion

Antitranspirant effects. Overall, antitranspirants did not effectively
decrease transpiration of seedlings we tested; however, Wilt-Pruf and NuArbor
may show some promise as effective treatments for certain species (Figs. 1 and
2). The inconsistency in treatment effects indicated by our observations
correlates well with work other researchers have performed (Colombo and
Odlum, 1987; Das and Raghavendra, 1979; Davies and Kozlowski, 1974; Englert
et al., 1993a, 1993b; Fowells and Schubert, 1955; Jack, 1955; Odlum and
Colombo, 1987; Simpson, 1984; Williams et al., 1990).

Applications of Wilt-Pruf have varied in their effectiveness of reducing

transpiration rates of conifer species. Wilt-Pruf effectively reduced transpiration

135

of black spruce [Picea mariana (Mill.) B.S.P.] for up to 14 days; however,
seedling survival was greatly reduced (Colombo and Odlum, 1987; Odlum and
Colombo, 1987). Simpson (1984) found that Wilt-Pruf had inconsistent effects
on white spruce [Picea'glauca (Moench) Voss] seedlings. One experiment
showed a positive effect on white spruce seedlings by reducing moisture stress
for up to five days. Another trial indicated that applications decreased white
spruce seedling survival and storability and also reduced growth and storability of
western hemlock [Tsuga heterophylla (Raf.) Sarg] and Douglas-fir [Pseudotsuga
menziesii (Mirb.) Franco]. Simpson determined Wilt-Pruf effective in reducing
moisture stress development in lodgepole pine (Pinus contorta Doug.) seedlings
without reducing root growth capacity or storability. Wilt-Pruf had positive effects
on white spruce and white pine (Pinus strobus L.) seedlings subjected to
greenhouse drought conditions (Williams et al., 1990). These seedlings
experienced high survival rates, low phytotoxic effects, and high biomass growth
along with moderate height growth.

Transplration rates of species we tested were unaffected by Moisturin
treatments. Prior work performed using Moisturin has given mixed results.
Arnold and Culbertson (1994) determined that applications of Moisturin to bare-
root northern red oak (Quercus rubra L.) seedlings did not improve or decrease
first-year growth during subsequent container or field production. Washington
hawthorn (Crataegus phaenopyrum Medic.) and other deciduous seedlings
experienced reduced water loss when treated with Moisturin (Englert et al.,
1993b). There was up to an 80% reduction in water loss for plants treated with

Moisturin.

136

lneffectiveness of treatments could be due to several factors. Seedling
health may influence effectiveness of antitranspirant treatments. Jack (1955)
found that unhealthy Monterey pine (Pinus radiata D. Don.) seedlings were more
responsive to antitranspirant treatments than healthy seedlings. Because
Monterey pine seedlings were healthy, it was more difficult to see an
improvement in seedlings treated with antitranspirants.

Application method could also influence effectiveness of antitranspirant
treatments. Treatments in our trial were applied as a spray to seedling foliage
until the point of drip-off. Douglas-fir seedlings, whose foliage was dipped into
1:3 concentrations of Moisturin, had higher mean water potential values (—1.43
MPa) than the control treatment did (-1.95 MPa) (Rose and Haase, 1995), which
shows that treated plants were under less water stress than the control
seedlings. Foliar dipping could prove more effective because of better coverage
of needle surfaces. Also, foliar applications result in waste of material, since a
great deal drips off the foliage. Foliar dipping would help ensure that drip-off
material is recaptured as seedlings are removed.

Measurement methods. Although specific results may have differed,
depending on measurement method, the same overall treatment effects and
trends can be detected by both data sets (Figs. 1 and 2). Degree of correlation
between gas exchange and whole-plant water loss measured on the same plant
on the same day was relatively low (R2 = 0.14); however, measurement methods
were correlated with one another (P < 0.0001). Gas exchange measurements
may have differed from whole-plant water loss measurements because of the

following: needle age and position, location of branches where measurements

137

were taken, LI-6400 conifer chamber design, and date in dry-down cycle.

First, branch tips selected for gas exchange measurements may not be
representative of foliage for the entire seedling. Previous research performed on
red spruce (Picea rubens Sarg.) and Douglas-fir has indicated that stomatal
conductance is higher for current-season growth compared with one-year-old
foliage (Day, 2000) and that specific leaf area decreases with increasing
distance from branch base and increasing needle age (Hiroaki et al., 2002). All
of the seedling foliage is represented by whole-plant measurements, so needle
age and position are not an issue.

Second, needles located on branch tips are more photosynthetically
active than needles located in more interior sections of the canopy. This interior
foliage is shaded by other foliage and plant parts, resulting in decreased
photosynthesis rates. When photosynthesis decreases, so do other plant
processes, such as transpiration. Therefore, measuring only the most active
foliage via gas exchange measurements may result in higher values.

Third, transpiration rates may be somewhat higher for needles inside the
Ll-6400 conifer chamber because of increased air circulation. A small fan is
used to circulate air inside the chamber to stabilize air movement 30 gas
exchange measurements can be recorded, which is not a factor in whole-plant
measurements because seedling foliage is influenced only by environmental
conditions of the growth chambers.

Last, transpiration rates may have differed because of the day
measurements were taken during the dry-down cycle. Whole-plant weights were

taken on the last day of dry-down and represent an average water loss

138

throughout the number of days in the cycle. If weights had been recorded each
day, water loss would probably have been highest on the first day and gradually
declined on each subsequent day. However, gas exchange measurements were
taken near the end of each cycle and represent that single point in time.

Change in effects over time. Decreases in transpiration rates throughout
the course of the experiment are probably due to plant acclimatization. Before
entering chamber environments, seedlings were growing in a glass greenhouse
under natural daylight conditions (about 1000 umol rn’2 s") where photosynthesis
and other plant processes were active. However, light levels inside the growth
chambers (175 to 195 pmol m’2 s") were much lower than natural daylight
conditions. Once seedlings were placed in the growth chambers, photosynthesis
and subsequently other plant processes gradually decreased to compensate for
lower light levels.

The overall trend of decreasing transpiration over time differed between
measurement methods because of what each method specifically measures.
Whole-plant water loss determines the average amount of moisture lost
throughout a period, resulting in a gradual decrease of moisture loss over time.
Gas exchange measurements represent transpiration rates for one specific
moment, which is why transpiration values fluctuated more with this method.

Conclusion. It is important to remember that our goal was focused on
antitranspirant effectiveness, and that measurement method, VPD, and species
were used to provide a wide range of test conditions. Multiple measurement
methods were used to provide independent confirmation of treatment effects,

’which is what we observed in regard to the trend of decreasing transpiration

139

rates over time and general lack of antitranspirant effectiveness. Separate VPD
environments were used to determine antitranspirant effectiveness under a
range of environmental conditions. An antitranspirant that successfully reduces
transpiration rates regardless of VPD conditions would be more beneficial for
tabletop Christmas tree producers, considering the range of environmental stress
these conifers may encounter. Although Wilt-Pruf and NuArbor effectively
reduced transpiration rates of Black Hills and Serbian spruce, respectively, these
effects were not consistently observed in both VPD environments, nor were they
long-lasting. Because of variable effectiveness with VPD conditions and the
species tested, it appears that antitranspirant treatments are not a practical tool
to extend shelf life of tabletop trees. Growers should consider other options for
reducing plant water stress during shipping and display during the holidays, such
as improved plant management education for mass merchandisers caring for
seedlings or containers designed with water reservoirs to decrease the need for

frequent watering.

140

Literature Cited

Arnold, MA. and R.K. Culbertson. 1994. Effects of antitranspirant sprays and
hydrophilic polymer root dips on the growth of bare-root northern red oak
seedlings transplanted to the field or containers. SNA Research Conference.
Calyx 8 Corolla. 2000. Holiday 2000 catalog. Calyx 8 Corolla, 29.

Chastagner, G. 1985. Research on Christmas tree keepability. Amer. Christmas
Tree J. 29(4):31—33,35.

Colombo, SJ. and KD. Odlum. 1987. Efficacy of six antitranspirants on black
spruce container seedlings. New For. 1(3):239—244.

Crothers, D. 1990. Big profits in little packages. Produce Business. 6:50,52,54.

 

Das, V.S.R. and AS Raghavendra. 1979. Antitranspirants for improvement of
water use efficiency. Outlook Agr. 10:92—98.

Davies, W.J. and T.T. Kozlowski. 1974. Short- and long-term effects of
antitranspirants on water relations and photosynthesis of woody plants. J. Amer.
Soc. Hort. Sci. 99(4):297—304.

Day, ME. 2000. Influence of temperature and leaf-to-air vapor pressure deficit
on net photosynthesis and stomatal conductance in red spruce (Picea rubens).
Tree Physiol. 20:57—63.

Duck, MW. 2002. Developing production systems for tabletop Christmas trees.
Thes. Michigan State University, East Lansing.

Englert, J.M., L.H. Fuchigami and T.H.H. Chen. 1993a. Bare-root basics. Amer.
Nurseryman. Februaryz56—59.

Englert, J.M., K. Warren, L.H. Fuchigami and T.H.H. Chen. 1993b. Antidesiccant
compounds improve the survival of bare-root deciduous nursery trees. J. Amer.
Soc. Hort. Sci. 118(2):228—235.

Fowells, HA. and G.H. Schubert. 1955. Planting trials with transpiration
retardants in California. Tree Planters' Notes 20:19—22.

Gale, J. and RM. Hagan. 1966. Plant Antitranspirants. Annu. Rev. Plant Physiol.
17:269—282.

Guehl, J.M., G. Aussenac, J. Bouachrine, R. Zimmerman, J.M. Pennes, A. Ferhi
and P. Grieu. 1991. Sensitivity of leaf gas exchange to atmospheric drought, soil
drought, and water-use efficiency in some Mediterranean Abies species. Can. J.
For. Res. 21 :1507-1515.

141

Hamrick, D. 2002. Conifers are hot as a pot plant at Sluitter. FloraCulture lntl.
Mayz22.

Harry and David. 2000. Christmas 2000 catalog. Harry and David, 45.

Hiroaki lshii, E.D.F., M.E. Boscolo, A.C. Manriquez, M.E. Wilson and TM.
Hinckley. 2002. Variation in specific needle area of old—growth Douglas-fir in

relation to needle age, within-crown postition and epicormic shoot production.
Tree Physiol. 22:31—40.

Jack, J.B. 1955. Tests of a transpiration inhibitor. Tree Planters' Notes 20:23—25.

Jackson 8 Perkins. 2000. Holiday Gifts 2000 catalog. Jackson 8 Perkins
3,9,16,27,29,33,34,36,39,47.

Kramer, P.J. and T.T. Kozlowski. 1979. Physiologyof Woody Plants. Academic
Press, Inc., Oriando, Fla.

Mortensen, L.M. 1994. Effects of carbon dioxide concentration on assimilate
partitioning, photosynthesis and transpiration of Betula pendula Roth. and Picea
abies (L.) Karst. seedlings at two temperatures. Acta Agr. Scand. 44:164-169.

Odlum, KB. and SJ. Colombo. 1987. The effect of three film-forming
antitranspirants on moisture stress of outplanted black spruce seedlings. Tree
Planters' Notes 38(4):23—26.

Rose, R. and D.L. Haase. 1995. Effect of the antidesiccant Moisturin on conifer
seedling field performance. Tree Planters' Notes 46(2):97—101.

Samuelson, L.J. and J.R. Seller. 1992. Fraser fir seedling gas exchange and
growth in response to elevated COZ. Environ. Expt. Bot. 32(4):351-356.

Samuelson, L.J. and J.R. Seller 1994. Red spruce seedling gas exchange in
response to elevated COZ, water stress, and soil fertility treatments. Can. J. For.
Res. 24:954—959.

Simpson, D.G. 1984. Film-forming antitranspirants: Their effects on root growth
capacity, storability, moisture stress avoidance, and field performance of
containerized conifer seedlings. For. Chronicle 60:335-339.

Sulzer, A.M., M.S. Greenwood and W.H. Livingston. 1993. Early selection of
black spruce using physiological and morphological criteria. Can. J. For. Res.
23:657—664.

Tan, W., T.J. Blake and T.J.B. Boyle. 1992. Drought tolerance in faster- and

slower-growing black spruce (Picea mariana) progenies: l. Stomatal and gas
exchange responses to osmotic stress. Physiol. Plant. 85:639-644.

142

Williams, P.A., A.M. Gordon and AW. Moeller. 1990. Effects of five
antitranspirants on white spruce and white pine seedlings subjected to
greenhouse drought. Tree Planters' Notes 41 (1 ):34—38.

143

Table 1. Analysis of variance for whole-plant water loss and gas exchange data.

Effect df F value (whole- F value (gas

plant water loss) exchange)

Among subjects

Species 2 “10.21 ***44.45
Treatment 3 0.70 0.70
Vapor pressure deficit (VPD) 1 0.08 “14.37
Species x treatment 6 *2.52 *2.45
Species x VPD 2 0.18 1.63
VPD x treatment 3 0.50 1.41
Species x treatment x VPD 6 0.85 *2.58
Within subjects

Days 6 ***56.75 ***23.44
Days x species 12 ***7.71 "3.51
Days x treatment 18 *2.26 0.71
Days x VPD 6 *3.96 “"781
Days x species x treatment 36 1.03 0.40
D x i 2 * 1.

*'**'***Significant at P < 0.05, 0.01 or 0.0001.

144

 

 

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145

List of Figures

FIGURE PAGE

1

Effects of antitranspirant treatments on mean daily water loss of Black
Hills spruce, Serbian spruce, and noble fir seedlings as determined by
whole-plant water loss measurements. Error bars represent SE of the
mean. VPD indicates vapor pressure deficit.

147

Effects of antitranspirant treatments on mean transpiration rates of Black
Hills spruce, Serbian spruce, and noble fir seedlings as determined by gas
exchange measurements. Error bars represent SE of the mean. VPD
indicates vapor pressure deficit.

148

146

Transplration (g cm'2 d")

0 04 High VPD (1.90 kPa) Low VPD (035 kPa)

 

 

Black Hills Spruce

+ Control
—v— Moisturin

+ NuArbor ‘
—<>- Wilt-Pruf i

l

     

0.03 1-

 

 

0.02 1-

0.01 1*

 

 

0.00

0.04 1-

0.03 11

0.02 1

0.01 1

 

 

0.00

0.03 11

0.02 1-

 

0.01 1

 

 

 

 

0.00 r r r 1 1 1 r r I r

 

Transplration (mmol cm'2 s")

0.16

0.08 1-

0.04 1-

0.00

0.16 1-

0.08 r

0.04 1-

0.00 ‘

0.12 1-

0.08 1-

0.04 1-

0.00

High VPD (1.90 kPa)

Low VPD (0.95 kPa)

 

Black Hi

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—v— Moisturin l
—I— NuArbor ,
—<>— WiIt-Pruf ,

     

 

Serbian Spruce

has

 

 

 

 

Noble Fir
0 1o 20 30 40 o 10 20 30 40
Days
Figure 2

148

 

CHAPTER FIVE

POSTHOLIDAY CARE OF LIVE TABLETOP CHRISTMAS TREES

149

 

Postholiday Care of Live Tabletop Christmas Trees

Additional index words. Picea, postharvest, cold storage

Abstract

One of the factors consumers may consider when purchasing a live
tabletop Christmas tree is whether they can plant the tree in the landscape after
the holiday season has ended. Using two conifer species hardy in the upper
Midwest, Black Hills spruce [Picea glauca (Moench) Voss var. densata] and
Serbian spruce [P. omorika (Pancic) Purkyne], we evaluated the following five
environmental conditions to simulate a range of postholiday storage conditions
until time to plant the seedlings in the spring: 1) storage in a garage with no
windows or other consistent light source and daily temperatures ranging from —-5
to 19 °C; 2) storage in a garage with windows facing south, east, and north and
daily temperatures ranging from —5 to 28 °C; 3) storage in a lab area with low
light levels (approximately 5-10 umol rn'2 s") at 22 °C; 4) storage in a lab area
with high light levels (approximately 25—30 pmol 111'2 s") at 22 °C; and 5) storage
in a controlled-atmosphere hoop house (natural daylength with a minimum
temperature of 13 °C). In May 2002, the seedlings were planted in an outdoor
research plot on the Michigan State University campus. The trees were
evaluated for their growth and overall health. Conditions of the seedlings varied

by treatment and species.

150

Introduction

Consumers are purchasing more and more live tabletop Christmas trees
during the holiday season, whether from mail-order catalogs, mass
merchandisers, or their local garden center (Crothers, 1990; Hamrick 2002).
However, there are few choices on the market, and most are not cold hardy in
the upper Midwest (zone 5), where the average annual minimum temperature
ranges from -29 to -23 °C (Bailey and Bailey, 1976; Crothers, 1990; USDA,
2002). Also, the current nonhardy mail-order selections are somewhat
expensive for the average homeowner, with retail prices ranging from $50 to $85
(Calyx 8 Corolla, 2000; Harry and David, 2000; Jackson 8 Perkins, 2000). For
some consumers, this price is substantial for a product that will be thrown away
after the holidays. Species that are cold hardy would be more economical, since
they could be planted outdoors in the spring.

Many consumers may prefer this option of planting their tabletop tree in
the landscape after Christmas has ended. Some Christmas tree growers offer
potted trees for consumers; however, these trees are typically dug directly from a
field production area just before the holiday season. Therefore, they are already
somewhat acclimated to winter conditions, making them less susceptible to cold
damage when they are placed in the landscape after Christmas. This is not the
case with greenhouse-grown tabletop Christmas trees. Most live tabletop trees
are grown under greenhouse conditions or in warm climates until they are
shipped to consumers during November and December. Nurserymen’s
Exchange typically ships their tabletop trees during November (Crothers, 1990).

These seedlings would require some form of postholiday storage until they could

151

be safely planted outdoors in the spring.

Previous research we performed determined production schedules for
tabletop Christmas trees grown from low-cost starting material hardy to the upper
Midwest (Duck, 2002). We evaluated five species of spruce and two species of
fir, with Black Hills spruce (Picea glauca var. densata), Serbian spruce (P.
omorika) and, Wilson spruce (P. wilsonii Mast.) being the most favorable
species. These species had minimal chilling requirements, an aesthetically
pleasing natural growth habit, and uniformity of new growth flushes. These traits
make species attractive to growers because of the low production inputs needed
for growing a marketable specimen .

Once that trial was completed, we chose Black Hills and Serbian spruce
seedlings that most closely represented what consumers would receive for
display during the holidays. The objectives of our project were to determine how
these seedlings responded to five postholiday storage environments and how

well seedlings performed after being planted outdoors in the spring.

Materials and Methods

Plant material and culture. Black Hills spruce seedlings were received
from Vans Pines Nursery (West Olive, Mich.) and planted in Apr. 2000 in one-
gallon containers (2830 cm3) with a 70:30 (volzvol) peat/perilte substrate (Peat-
Perlite Mix, Michigan Grower Products, Galesburg, Mich.) Serbian spruce
seedlings were dug from a field production area at Vans Pines Nursery in June
2001 and planted on the same day in one-gallon containers with a 15:85 (volzvol)

peat/pine bark substrate (Renewed Earth, lnc., Kalamazoo, Mich.) After

152

planting in one-gallon containers, all seedlings were grown outdoors under
natural conditions at the Michigan State University Horticulture Teaching and
Research Center (HTRC) (East Lansing, Mich.) Black Hills spruce seedlings
were treated in June 2000 and 2001 and Serbian spruce seedlings were treated
in June 2001 with Ronstar preemergent herbicide at 2 lb a.i. acre". Also in June
2000 and 2001, seedlings were given 17-7-8 slow-release granular fertilizer (3-
to 7-month) (Wilbro Horticultural Products, Norway, S.C.) at 10 g/tree. Seedlings
were irrigated as needed with water obtained from wells at the HTRC.

Beginning in July 2001, seedlings were used to determine the optimum
production system for generating a marketable crop of tabletop trees for sale
during the Christmas season. This trial consisted of various combinations of
chilling and photoperiod treatments throughout a 20—week period, which ended in
Nov. and Dec. 2001. While the seedlings were in the greenhouse, they were
irrigated as needed with a nutrient solution of well water (electrical conductivity of
0.65 mS cm" and 105, 35, 23 [mg L"] Ca, Mg, and S, respectively) acidified with
H280, to a titratable alkalinity of Ca802 at 130 mg L" and water-soluble fertilizer
providing 125-12-125-13 (mg L") N-P—K-Ca plus 1.0-0.5-0.5-0.1-0.1 (mg L") Fe-
Mn-Zn-Cu-B-Mo (MSU Special, Greencare Fertilizers, Chicago, Ill.) At the end
of this experiment, the seedlings that had 100% budbreak and a new flush of
growth were selected because they represented the criteria for trees that would
be selected for shipment and sale during Christmas and eventually reach the
consumer.

Treatments. There were nine Black Hills spruce and 10 Serbian spruce

assigned to each of five treatments: 1) storage in a garage below a home with

153

only one wall of the garage exposed aboveground, no windows or other
consistent light source, and daily temperatures ranging from —5 to 19 °C (Fig. 1);
2) storage in a detached garage, completely aboveground with windows facing
south, east, and north and daily temperatures ranging from —5 to 28 °C (Fig. 2);
3) storage indoors under low light levels provided by cool-white fluorescent lights
(approximately 5-10 umol m'2 s") at 22 °C; 4) storage indoors under high light
levels provided by cool-white fluorescent lights (approximately 25-30 umol m'2 s‘
1) at 22 °C; and 5) storage in a controlled-atmosphere hoop house at the HTRC
(natural daylength with a minimum temperature of 13 °C).

Treatments began on Feb. 7, 2002. Conifers at the beginning of the trial
ranged in size from 30.0 to 67.5 cm. Seedlings in the lab area were irrigated as
needed with the same formulation as the greenhouse water described above. In
the hoop house environment, seedlings were irrigated as needed with water
obtained from wells at the HTRC. Tap water was used to irrigate seedlings as
needed in the garage environments. On May 1, 2002, all seedlings were planted
at the HTRC in a completely randomized design. The seedlings were planted
four feet apart from one another in three rows in a freshly tilled area (sandy
loam: 54% sand, 27% silt, and 19% clay). Weed control was limited to the initial
preplant tilling and periodic hand-pulling of broadleaf weeds located in the
immediate vicinity of the seedlings. Seedlings were irrigated as needed with
overhead irrigation by using impact rotors, and no additional fertilizer was given
to the trees after planting.

Data collection and analysis. Seedlings were placed into the five storage

treatments and evaluated throughout the trial on their overall condition by using a

154

six-point rating system: 0 = dead, 1 = 20% live foliage, 2 = 40% live foliage, 3 =
60% live foliage, 4 = 80% live foliage, and 5 = 100% live foliage. Conifer
seedlings were planted at the HTRC in May 2002. The trees were monitored for
budbreak (0 = no, 1 = yes), and their overall condition was again evaluated with
the rating system described above.

At the end of the trial in July 2002, seedling condition data (for storage
and planting treatments) were analyzed with the GENMOD procedure, which is
an ordinal model for multinomial data (SAS/PC software, SAS Institute, Inc.,
Cary, NC.) The SAS program statements fit a cumulative logit model to the
ordinal data, with rating as the response variable and treatment and species as
classification variables.

Results and Discussion

Seedling condition was not affected by the two species we tested, Black
Hills and Serbian spruce (Table 1). However, all treatments affected seedling
performance during storage and once they were planted outdoors (Tables 2 and
3). The overall rating data of seedling condition was divided into two categories:
5 to 4 for acceptable, healthy seedlings and 3 to 0 for unacceptable seedlings.
Because species was not significant, results and discussion for storage and
planting trials are arranged below by treatment.

Garage without light. When seedlings stored in the no-light garage
environment were planted outdoors in May, they appeared to be alive and
healthy, with 71% of them being in an acceptable condition. However, foliage
quickly became discolored and began to drop, with less than 10% having an

acceptable condition when the experiment concluded in July. Just over half of

155

the seedlings broke bud after being planted outdoors (Table 3).

The combination of temperature and very low light levels may have
affected seedling health during storage. Temperatures during the first two
months of storage ranged from just about freezing to almost 10 °C (Fig. 1). Most
conifers experience little seedling growth when temperatures drop below 10 °C,
but basic processes like photosynthesis and respiration remain active at much
slower rates at lower temperatures (Dormling, 1993; Landis et al., 1992). Also,
controlling root zone temperature of conifers is crucial, since the slightest
exposure to warm temperatures can change seedling dormancy (Landis et al.,
1992’ 1999). Temperatures increased substantially during April, without any
change in light availability. Because seedlings did not remain at optimum
dormancy conditions (generally below 5 °C) (Kramer and Kozlowski, 1979), plant
processes may have continued to function even though sufficient light was
insufficient. This increase in plant stress could help explain the high incidence of
death observed once these seedlings were planted outdoors.

Garage with moderate light. Seedlings performed much better during and
after storage in the garage environment with moderate light levels (Tables 2 and
3). A high percentage of these seedlings had acceptable conditions (80%) and
also achieved budbreak (100%) after being planted outdoors. Storage
conditions were more favorable for seedling health because of higher light levels
(three windows, facing north, south, and east) and cooler temperatures (Fig. 2).
Below-freezing temperatures occurred more frequently than in the no-light
garage. As mentioned earlier, seedlings can achieve a moderate state of

dormancy when temperatures are lower than 10 °C (Dormling, 1993; Landis et

156

al., 1992). Cooler temperatures would result in a less stressful environment. As
temperatures increased toward the latter part of storage, daylength also
increased, supplying seedlings with a longer photoperiod. Because seedlings
were exposed to light in the moderately lit garage, normal photosynthetic
processes were able to occur, resulting in healthier seedlings.

Low and high light indoors. The two indoor environments were selected
to simulate seedling storage inside a home. Seedlings in the low light
environment performed worse than those in any other treatment (Tables 2 and
3), with only eight out of 19 seedlings surviving storage treatments. All eight of
these seedlings died during the planting trial. Light levels were'extremely low
(5—10 umol m'2 s"), comparable with most poorly lit indoor situations, and
temperature was set around typical room temperature (20 °C). Seedlings under
high light conditions (25—30 umol m'2 3") performed better than seedlings in low
light, but only one seedling performed well after being planted outdoors. The
majority of seedlings in both light levels experienced severe needle drop during
storage.

Seedlings were actively growing when they entered the indoor light
treatments because dormancy had not been induced. The indoor conditions,
typical of most indoor growing environments, were not favorable for actively
growing conifers. Both light conditions were well below optimum levels. Also,
relatively warm room temperature combined with low relative humidity (around
15% RH) created a high vapor pressure deficit (VPD) (between 2.4 and 3.6 kPa).
Plant transpiration primarily depends on the difference between humidity levels

inside and outside a leaf (Barrett, 1990). Humidity levels inside a leaf are

157

generally around 100%. With ambient VPD being at such high levels,
transpiration would increase. Seedlings may have experienced water stress
because of the high VPD, which would explain why we observed major
desiccation of needles. Chastagner (1985) found that the extent of drying before
rehydration may be one factor resulting in the loss of fresh-looking needles on
conifer species.

Hoop house. For consumers who may have access to higher-quality
storage environments for their tabletop Christmas trees, we evaluated seedlings
stored in a plastic-covered hoop house. This environment was beneficial to
seedling performance (Table 2) because of sufficient light quality and duration
(natural daylight and photoperiod), along with moderate temperatures (13 to 23
°C) throughout the storage period. Seedlings stored in the hoop house
performed better than those in other treatments, with close to 100% of these
seedlings achieving budbreak after being planted outdoors in May (Table 3).

Conclusion. Most tabletop Christmas trees purchased by consumers are
actively growing. Without induction of dormancy, these seedlings require an
environment conducive for plant processes to function adequately for improved
seedling survival. Although our indoor treatments resulted in poor seedling
performance, indoor environments could prove to be successful if light levels
were increased. Placing seedlings in brighter areas of the home or using
supplemental light sources available on the market would make indoor storage
conditions more acceptable for conifer seedlings. By supplying seedlings with
sufficient light levels and moderate temperatures, like conditions given in the

garage environment with moderate light levels and hoop house environment,

158

consumers can successfully keep their tabletop trees for spring planting and
years of enjoyment.

Literature Cited

Bailey, L.H. and E2. Bailey. 1976. Hortus Third. Macmillan, New York, NY

Barrett, J. 1990. Plants and water 101: Understanding VPD. Grower Talks.
Septemberz48—52.

Calyx 8 Corolla. 2000. Holiday 2000 catalog. Calyx 8 Corolla, 29.

Chastagner, G. 1985. Research on Christmas tree keepability. Amer. Christmas
Tree J. 29(4):31—33,35.

Crothers, D. 1990. Big profits in little packages. Produce Business. 6:50,52,54.
Dormling, l. 1993. Bud dormancy, frost hardiness, and frost drought in seedlings
of Pinus sylvestris and Picea abies. Advances in plant cold hardiness. P. H. Li,
and Christersson, L. CRC Press, Boca Raton, L.A., 285—298.

Duck, MW. 2002. Developing production systems for tabletop Christmas trees.
Thes. Michigan State University, East Lansing.

Hamrick, D. 2002. Conifers are hot as a pot plant at Sluitter. FloraCulture lntl.
Mayz22.

Harry and David. 2000. Christmas 2000 catalog. Harry and David, 45.

Jackson 8 Perkins. 2000. Holiday Gifts 2000 catalog. Jackson 8 Perkins
3,9,16,27,29,33,34,36,39,47.

Kramer, P.J. and T.T. Kozlowski. 1979. Physiology of Woody Plants. Academic
Press, Inc., Orland, Fla.

Landis, T.D., R.W. Tinus, S.E. McDonald and JP. Barnett. 1992. The container
tree nursery manual: Atmospheric environment. USDA Forest Service, Wash.

Landis, T.D., R.W. Tinus and JP. Barnett. 1999. The container tree nursery
manual volume six: Seedling propagation. USDA Forest Service, Hamden.

United States Department of Agriculture. Plant hardiness zone map 2002.
,http://www.usna.usda.gov/Hardzone/ushzmap.html?

159

Table 1. Analysis of variance for the rating system used to evaluate the storage
and planting trials, which were designed to determine postholiday care for

tabletop Christmas trees.

 

Effect df Storage trial Planting trial
X2 X2
Species 1 0.04 3.27
Tr m n 4 *** ***
***Significant at P < 0.0001.

160

Table 2. Condition of seedlings at the end of storage trials as indicated by a

visual rating scale (0 = dead to 5 = 100% alive).

Storage trial
treatment
No-light garage
Moderate-light garage
Low light indoors
High light indoors
h

Acceptable
(% rated 5—4)

161

71
96
16
60

7

Not acceptable
(% rated 3—0)
29

4

84

40

3

Table 3. Condition of seedlings at the end of planting trials as indicated by a
visual rating scale (0 = dead to 5 = 100% alive), and percentage of seedlings

achieving budbreak by the end of the planting trial.

 

Planting trial Acceptable Not acceptable Budbreak (%)
treatment (% rated 5—4) (% rated 3-0)
No-light garage 7 93 58
Moderate-light garage 80 20 100
Low light indoors 0 100 0
High light indoors 8 92 43

Hoop hogsg 95 5 ,L

162

 

List of Figures

FIGURE PAGE
1 Maximum and minimum daily temperatures recorded during a storage trial
for seedlings in a garage environment without light.
164
2. Maximum and minimum daily temperatures recorded during a storage trial
for seedlings in a garage environment with moderate light.
165

163

 

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