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MICHIGAN IIIIIIIIIIIIIIIIIIIIIII

01591 4066

L I B R A R Y
Michigan State
University

FLORAL INITIATION AND EXPRESSION IN WEIGELA AS AFFECTED
BY PHOTOPERIOD AND EXOGENOUS APPLICATIONS OF
GIBBERELLII AND MALEIC HYDRAZIDE

By

DAVID CAMERON MACLEAN

A THESIS

Submitted to the School for Advanced Graduate Studies of
Michigan State University of Agriculture and
Applied Science in partial fulfillment of

the requirements for the degree 0!

MASTER OF SCIENCE

Department of Herticulture

1952 ' {Hf f” :

 

ACKNOWLEDGEMENTS

The author wishes to express his sincere appreciation
to Dr. Harold Davidson for his accurate guidance, invaluable
suggestions and aid throughout this investigation.
'The writer is also indebted to Dr. Donald P. Watson
for his helpful suggestions concerning the anatomical studies,
to Dr. M. John.Bukovac for his constructive criticisms, and
to Dr. Richard R. Dedolph for assistance with statistical

methods.

11

TABLE

INTRODUCTION . . . . . .
REVIEW OF LITERATURE . .
PhotoPeriodism . .

General . . .

OF CONTENTS

0

The Flowering Response .

PhotOperiodism in Woody Plants
Effects of Gibberellin on.Woody

Maleic Hydrazide .
MATERIALS AND METHODS .

Plant Material and Culture

Photoperiods

Bud Analysis

Photoperiod-Gibberellin,Experiment

'0‘. O O

O O O O O O O

O O O O O O 0' O

O

O

Photoperiod-Maleic Hydrezide Experiment

RESULTS

Photoperiod-Gibberellin Experiment

0 O O O O O O O O O O O O O O

Vegetative RespOnses e e e e 0

Floral Responses . . . .

O

PhotOperiod-Maleic Hydrazide Experiment

Vegetative Responses . . . . . . .

Floral Responses .

DISCUSSION AND CONCLUSIONS .

SUMMARY

LITERATURE CITED . . . .

O

Page

(DUNNN

12
13
17
17
17
18
19
21
22
22
22
27
3O
30
34
58
‘2
45

1.

2.

3.

4.

5.

6.

LIST OF FIGURES

Growth rates of Weigela grown under long-
and short-day exposure... . . . . . . . .

Effect of long— and short-days on separate
stems of the same plant . . . . . . . . .

Three stem plant which received photoper-
iodic treatments. . . . . . . . . . . . .

Effect of pinching, gibberellin, and
photoperiod on the growth of Weigela. . .

Responses of Wei ela after three weeks
exposure to long-days, short-days and
long-days plus 3000 p.p.m. maleic
hydratido o o o o e e o o o e o o o o e o

Photomicrographs of longitudinal sections
or buds of "613613. e o o o o o e e e e 0

Growth rates of Weigela subjected to
long-days, short-days, and long-days
plus 3131610 hYdI‘fiZide . o o o o o o e o 0

Percent incidence of flower expression

by nodes of Ieigela exposed to long-days,
and long-days plus maleic hydrazide . . .

iv

Page

24

26

26

26

26

29

32

36

INTRODUCTION

The growth and development of plants can be modified
in various ways. Two such methods are: by controlling the
relative duration of darkness and illumination.(photoperiod);
and by the application of chemical substances with growth
regulatory properties such as gibberellin and maleicrhydr-
aside.

The numerous literature references germain to photo-
periodism and to the responses of plants to applications
of gibberellin and maleic hydrazide have, for the most part,
been concerned with herbaceous species, and relatively
little has been published of their effects on woody plants.
Undoubtedly, the limited amount of research on woody plants
is due to the time required to obtain valid results and the
relatively small number of researchers concerned with woody
plants. In the past decade there has been an increasing
interest in.the responses of woody plants to photoperiod
and to plant growth substances, but there is much yet to be
learned of these responses on both the theoretical and
practical levels.

This investigation concerns the Joint effects of photo-
period and maleic hydrazide or gibberellin on the initiation

and expression of flowering inflleigel .

1.

(.4

d'

74

REVIEW OF LITERATURE

Photoperiodism

General

The use of the electric light to lengflhen the daily
duration of illumination in order to promote plant growth
was reported as early as 1880 by Siemens (1880). Shortly
thereafter, Bailey (1891, 1892, 1893) conducted an extensive
series of experiments using the electric arc light on
greenhouse plants. He noted that growth and maturity were
hastened and that flower color was often intensified by
these treatments.

Later Bonnier (1895) published a series of papers on
the modifications of plant structure when grown under
electric lights. He reported on the responses to different
intensities, but not to different durations. Klebs (1914),
in 1914 reported that European beech, oak, ash, and horny
beam grew all winter under continuous illumination.

The effect of the relative duration of day and night
on plants was not conclusively demonstrated until Garner
and Allard (1920) published their classical report. For
this effect they suggested the term photoperiod, and to
designate "the response of organisms to the relative length
of day and night", the term photoperiodism was preposed.

on the basis of their flowering responses, plants were

2.

3.
classified as: (a) long-day plants, those which flowered
when the photoperiod was longer than the critical period
(eg. that light period which is the dividing line between
photoperiods favorable for flowering and photoperiods favor—
able for vegetative activity); (b) short-day plants, those
which flowered only when the photoperiod was shorter than
the critical period; and (c) indeterminate plants, those
which flowered over a wide range of photOperiods (Garner and
Allard, 1920). Later (Allard, 1935), a fourth group
designated intermediate was added. Intermediate plants
were those which flowered when the photoperiod was inter-
mediate (12 to 14 hours), but remained vegetative at photo-
periods longer than 12 to 14 hours. More recently, new
terms have been introduced such as long-short-day plants,
non-obligate short-day plants, and many others which more

precisely describe their photoperiodic responses.

5313 g: the 2ar___k Period

The first mention of the role of the dark period was
made by Blackman (1936) who postulated that "in the case
of short-day plants there is some reason to believe that
it is...the period of darkness to which attention should be
directed". Subsequently, it was realized that the length of
the dark period was the primary determining factor for photo-

periodic flower induction for both long- and short-day
plants, and they might be more properly termed short- or

long-night plants reapectively (Hamner and Bonner, 1938).

Short-night (long-day) plants have been defined as those

4.

plants which require a dark period shorter than the critical
period (Bonner, 1959a), and long—night (short-day) plants
have been defined as those which require an uninterrupted
dark period longer than the critical period. For convenience,
the original terms long-day and short-day are still accepted
and commonly used.

For photoperiodic responses it is the duration of light
(or more accurately the duration of continuous darkness),
and not light intensity, which is effective. Supplimentary
artifical light of an intensity as low as 0.1 foot candles,
which is only twice the intensity of bright moonlight, has
been effective in hastening the flowering of Callistephus
chinensis (Withrow and Benedict, 1936). Using bright moon,

 

light itself for supplimentary illumination, Gaertner (1935)
found that Hordeum.distichum, Triticum vulgare, and Iberis
ggggg, all long-day species, flowered earlier; he also
noted that two short-day species, S213_hispida and Pharbites
hispida, exhibited a delay in flowering when exposed to
bright moonlight.

Working initially with tobacco and soybean, and later
with a wide variety of plants, Garner and Allard (1920, 1923)
observed various plant responses to photoperiods of differ-
ent durations. For the most part, emphasis was placed on
the influence of photOperiod on: increase in stature;
tuberization and formation of bulbs and thickened roots;
character and extent of branching; root growth; pubescence;

pigment synthesis; senescence and rejuvinescence; abscission

5.
and leaf fall; dormancy and death; and geographical dis-
tribution. Since then the influence of photoperiod on
sex expression, root development, rhizome formation, seed
germdnation, and seedling elongation have been reported
(Hendricks and Borthwick, 1954). More recently, some
teratological effects have been attributed to photoperiod
by Youmis (1955), Davidson and Watson (1959), and Piringer
and Borthwick (1961).

The Flowering Response

The most basic stage of flowering is induction, or the
transition of a meristem from vegetative to reproductive
development. If Xanthium, a short-day plant, which has
been growing under long-days is exposed to one short-day
followed by a long-night and then returned to long-day
exposure, flowering will be initiated and expressed even if
long-days are continued (Hamner, 1940). This phenomenon is
photoperiodic induction, and the number of photoinductive
cycles required for photoinduction varies from species to
species.

The leaves are the loci of photOperiodic induction
(Bonner, 1959b; Hamner and Bonner, 1938; Borthwick and
Parker, 1938a; Hamner and Naylor, 1939; Naylor, 1941b) and
the stimulus, which is interchangeable for both long- and
short-day types, is of a hormonal nature (Lang, 1952; Bonner

and Liverman, 1953).

§gg£tfidgy Plants

In.short-day plants it is the long night that controls
photoperiodic induction (Hamner and Bonner, 1938). Hamner
(1942) postulated a hormonal theory in which substance "A"
increases in amount when.p1ants are exposed to light, and
another substance, "B”, that increases during dark periods
and is destroyed by brief periods of light. When "B” reaches
a threshold value, it interacts with "A" resulting in.the
substance "0", and floral initiation occurs.

Later, Liverman and Bonner (1953), suggested that the
presence of an auxin-receptor complex controls the flowering
response. When exposed to light, an auxin nonpreceptive
precursor becomes receptive and combines with auxin resulting
in.the auxin-receptor complex. In who dark, this complex
begins to decompose below the threshold level required for
short-day plant induction. However, a flash of red light
during the dark period interrupts this decomposition and the
auxinpreceptor complex is formed again.

More recently, Bonner (1959b) summarized the events
which occured during photoperiodic treatments of short-day
plants as: (a) the conversion of a pigment from.a predominat-
ely far-red absorbing form to a red absorbing form.during
the early stages of the dark period; (b) the occurance of a
temperature insensitive timedmeasuring reaction of which
little is known; (c) the formation of the "flowering horme
one" in the leaf in the dark; (d) the stabilisation of the

product of the dark period; and (e) the translocation of

the hormone from.the leaf.

Long-day Plants
‘Unlike short-day plants, the dark period is not necess-

arily a critical factor in the photoinduction of long-day
plants. In fact, flowering of many long-day plants is
Optimal under continuous light (Hamner and Bonner, 1938;
Naylor, 1941a). Experiments with Hyosgymus.niggg,show that
it is the period of illumination and not the short-night
that determines photoperiodic induction, and other evidence
indicates that long dark periods exert an inhibitory effect
on photoperiodic responses (Lang, 1952). Long-day plants
exposed to short-days can be induced to flower by inter-
ruptions of the dark period by low intensity light (Bonner
and Liverman, 1953). Lang (1952) states that the action of
light in controlling floral initiation in long-day plants
is the "counteraction of the inhibitory effect of dark

periods in the formation of the floral stimulus".

The Action Spectra

Measurements of the action spectra of photoperiodic
responses have given mich to the understanding of the
flowering response. Barley, a long-day plant, has an action
spectrum for flowering which is the same as the action
spectrum for flower inhibition of Xanthium, a short-day
plant. The action spectra have two parts; one favors the
photoperiodic response and the other nullifies it, indicating
that the mechanism of flowering of both types is the same

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(Hendricks and Borthwick, 1954). Radiation of 540 to 695 mp
with a maximum at 660 mp (red) favors the flowering of
barley, but inhibits flowering of cocklebur. The induction
of these responses can.be reversed before the actual
response occurs by far-red (695 to 800 mu) radiation.(Borthp
wick and Hendricks, 1960; Butler gg_gl, 1959). The antag-
onism.between red and far-red light is completely reversible.
This photoreversibility is due to phytochrome, a blue or
blue-green pigment which can exist in either red or far-red
absorbing forms. Upon exposure to red light it is converted
to the far-red absorbing form, which if irradiated with far-
red light, changes back to the red absorbing form.(Borthwick
23.51, 1952; Borthwick and Hendricks, 1960).

The reversible photoreaction which applies to most
observed photoperiodic phenomena can be written, with some

uncertainty, as

650-660 mu max

720-740 mu max

3'

Pigment RH2 + A Pigment + FR e AHZ

 

where R refers to the red absorbing form, FR to the far-red
absorbing form, and A to the hydrogen acceptor (Hendricks,
1959).

The above reaction has been shown to control; flowering
of both long- and short-day plants, seed and fern spore
germination, elongation, plumular hook unfolding, and

anthocyanin formation. Evidence indicates that it may also

9.
be involved with epinasty, leaf abscission, bulbing, sex
expression, root development, and plastid generation

(Hendricks, 1959).

Photoperiodism;in'Woody Plants

Vegetative Responses

In 1914, before the discovery of photoperiodisn, Klebs
(1914) noted that European.beech, oak, ash, and hornbeml
grew all winter when given continuous illumination. In
one of their early reports Garner and Allard (1923) reported
that long-days promoted and short-days retarded or caused a
cessation of vegetative growth of Rhgg labra, Aggg'neggndo,
and Liriodendron tulipifera. They also reported that Egggg
sylvestris grew better under short-days than long-days.
However, since these early reports there have been relatively
few publications concerning the effects of photoperiod on
trees and shrubs. Nevertheless, vegetative responses such
as root and shoot extension, the induction and breaking of
dormancy, cambial activity and leaf abscission, all of which
are affected by photoperiod, have been reported. Two
general reviews by Nitsch (1957b) and Wareing (1956) have
summarized these photoperiodic responses of woody plants.

In general, photoperiods longer than the critical
photoperiod favor shoot growth by increases in length and
node number, and short photOperiods usually result in

reduced extension due to earlier cessation of apical growth

10.
and reduced internode extension (Garner and Allard, 1923;
Wareing, 1941; Nitsch and Kitsch, 1959; Waxman, 1958).
The reaponse of roots to photOperiod has received little
attention other than on the rooting of cuttings (see later).

Leaf abscission of a number of deciduous species can
be prolonged by lengthening the photoperiod and hastened by
short photoperiods (Garner and Allard, 1923). Short-days
can also bring about the onset of dormancy (eg. imposed
dormancy) and long-days are capable of breaking dormancy of
many woody plants (Wareing, 1949, 1956). Imposed dormancy
is much more readily broken by long photoperiods than is
winter dormancy (Wareing, 1956).

The natural reduction in the length of day prior to
the onset of freezing temperatures in the Northern.Homi-
sphere retards growth and thereby reduces the probability
of winter injury. Wareing (1949) reported that southern
species grown in northern latitudes were susceptible to
low temperature injury due to the effect of the longer
photoperiod. By reducing the natural daylength, Davidson
(1957) found that Hibiscus syriacus, Rhododendron catawb-
$3253, and Weigela florida became highly resistant to
freezing temperatures. Conversly, Abelia grown under long-
days was killed by low temperature (Kramer, 1937).
Flowering

The limited information concerning the effects of
photoperiod on flowering of woody plants has, for the most

part, been restricted to the determination of the photo-

11.
periods favorable for flowering, and they have been summar-

ized by Wareing (1956).

Propagation

Caspary in 1860 was the first to report on the benefi-
cial influence of light on seed germination (Borthwick and
Hendricks, 1960). But it was not until 1955 however, when
Black and Wareing (1955) reported their findings on Betula
pubescens, that the effect of photoperiod on the germination

 

of woody plant seeds was known. They found that germination
occured when exposed to long photoperiods. This eliminated
the chilling requirement and chilled seeds were found to be
unaffected by long-days. Shortly thereafter, Waxman (1957)
found that the germination of Sciadopitys verticillata was
similar to the flowering response of short-day plants.
Germination.was inhibitwd by continuous light, but when
exposed to short-days germination was earlier and at a
significantly higher percentage.

The rooting of cuttings is influenced by photoperiod
in two ways; (a) directly upon the cutting, and (b) indirect-
ly through the stock plant. In general, long-days provided
during the rooting period result in an increase in the
rate and extent of rooting. The effects of exposing stock
plants to different photoperiods on the rooting of cuttings

varies from.species to species (Nitsch, 19575).

 

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EFFECTS OF GIBBERELLIN ON WOODY PLANTS

The first report on the effects of gibberellin, the
generic name for nine distinct chemical structures, on woody
plants was published in 1956 (Earth gt 3;, 1956). This was
immediately followed by numerous papers concerning its
remarkable effects on wood, plant behavior, and there is
still much interest in this area.

Application of gibberellin results in increased length
of internodes and accelerated shoot elongation of most
deciduous fruit, shade and ornamental trees and of most
shrubs (Marth gtflgl, 1956; Nitsch, 1957a; McVey and'Wittwer,
1958; Bukovac and Davidson, 1959). However, most conifers
fail to exhibit this growth response (Nelson, 1957; McVey
and Wittwer, 1958).

When applied to woody plants, gibberellin can: stimulate
growth of physiological (Barton, 1956) and genetic (McVey and
Wittwer, 1958) dwarfs; delay the setting of terminal buds and
thereby delay dormancy (McVey, 1961); cause dormant buds to
resume growth (Bukovac and Davidson, 1959); replace the cold
treatment of dormant embryos(Marth.2t_gl, 1956); induce
Juvinility (McVey, 1961); and increase the rate of seed
germination (Benjamin and Snyder, 1958).

Gibberellin also exerts an effect on some photoperiodic
responses of woody plants. By replacing long—photoperiods,
it can overcome the photoinduced dormancy of Weigela (Bukovac
and Davidson, 1959) and Rhg§|typhina (Nitsch, 1957a).

12.

 

‘l' It'll! I I,

MALEIC HYDRAZIDE

The first report on the growth regulatory preperties
of maleic hydrazide (MH), or 6-hydroxy-3(2H)pyridazinone
(Miller and White, 1956; Weller gt 5;, 1957), was made in
1949 by Schoene and Hoffman (1949). Since that time many
different morphological and physiological effects of this
compound on plants have been published, and an extensive
literature summary has been compiled by Zukel (1957).

Primarily, maleic hydrazide induces dormancy (Crafts
23'51, 1957) and brings about a temporary cessation of
termdnal meristem activity due to reduced or inhibited cell
division (Naylor and Davis, 1950). Preliminary observations
(Currier and Crafts, 1950) indicated that it might be an
effective herbicide, but Crafts 22.21 (1957), after extensive
studies reported that its effectiveness as an herbicide
without supplimental oil sprays or tillage did not warrant
its use. However, maleic hydrazide can: cause abscission
of fruits of 2123 (Hartman, 1955); be used to prevent
sprouting in storage of radishes (Dewey and Wittwer, 1955),
potatoes,onions, sugar boots, and other vegetable root
crops (Wittwer and Paterson, 1951); induce male sterility
in $23 (Naylor and Davis, 1950) and acorn squash.(Hillyer,
1956); shift the critical photoperiod of Xanthium (Naylor,
1950) and somewhat inhibit the photoperiodic induction of
Biloxi soybean (Klein and Leopold, 1953); delay or inhibit
flowering and fruiting (Miller and Erskine, 1949); and

13.

14.
overcome apical dominance (Beach and LeOpold, 1953; Rai and
Hamner, 1955). The effects of maleic hydrazide applications
on unlike plant species is strikingly uniform.(Naylor and

DQ'iB , 1950) 0

Responses 9!; 119931 Plants 39. Haleic Hydrazide
Exogenous applications of maleic hydrazide to woody
plants generally results in the cessation of terminal
activity. This is often accompanied by the removal of
apical dominance and increased lateral growth (Freeman, 1952;
Rai and Hamner, 1955; Reisch and Chadwick, 1955). Retard-
ation of growth can be of practical significance, for
applications of maleic hydrazide can: (a) reduce the number
of clippings required for hedge plants (Knott, 1950); (b)
result in smaller, more compact, higher quality shrubs;
(c) enhance frost protection by delaying the opening of
buds in early spring and retarding growth late in.the season
(Pinney and Chadwick, 1955); and (d) inhibit formation of
undesirable fruits of Catalpa (Miller and Erskine, 1949).
Most investigators concerned with the effects of
maleic hydrazide on flowering and fruiting of woody plants
have reported that flower and fruit formation was either

delayed or inhibited by maleic hydrazide (Miller and Erskine,
1949; White, 1950; Griesel, 1954; Zulel, 1957). However,

Davidson and Bukovac (1961) recently reported that maleic
hydrazide (3000 p.p.m.) treated Weigela plants exposed to
long photoperiods produced an abundance of flowers twelve

weeks after treatment.

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Naylor (1950) reported that maleic hydrazide seemingly
modified the photoperiodic effect. Applications of the
chemical at 0.025% to cocklebur at the beginning of the
photoinduction period slightly altered the critical daylongth.

Klein and Leopold (1955) reported that 4 x 10'5

g maleic
hydrazide completely inhibited the formation of flower
primordia in Wintrex barley, a long-day plant, but only
partially supressed phot0periodic induction of soybean, a
short-day plant. They suggested that the compound inhibits
the production of flower primordia through its "inhibitory
effect on growth, rather than by any specific action against
the photoperiodic mechanism.itse1f".

Ambiguous reports on the response of plants to maleic
hydrazide can be attributed to the effect of relative
humidity on absorption through the leaf surface. The rate
of absorption is directly preportional to relative humidity,
and turgid plants absorb maleic hydrazide at higher rates
than do wilted plants. The rates of absorption by tomato
plants exposed to either 18 hours light or 18 hours darkness
are essentially the same (Smith, 1955). Once absorbed, it
is translocated both basipetally and acropetally (Smith gtigl,

1957; Stone, 1957). Radioactive 014

maleic hydrazide
applied to tobacco showed that the greatest quantity was
translocated to the axillary buds and young leaves, with
gradually decreasing amounts in older leaves and the least
amount in.the oldest leaves. The same distribution pattern

was observed when applied to either young or old leaves

 

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16.
(Stone, 1957).

Desson and Rollen (1951) reported that maleic hydrazide
inhibits cell division by preventing the cells from entering
into mitosis, but Darlington and McLeish (1951) showed that
concentrations of 0'0005.fl did not completely inhibit
mitosis, but caused breakage of chromosomes in.!igig. At
concentrations greater than this mitosis was completely
inhibited.

Maleic hydrazide has been classified as an anti-auxin
by Leapold and Klein (1951), and Hillyer (1956) considered
this anti-auxin property as a probable cause of its selective
inhibition of staminate flowers, while permitting simultaneous
development of pistillate flowers of acorn squash. It has
also been suggested that growth inhibition due to maleic
hydrazide applications resulted from excessive oxidation of

indoleacetic acid (Andreas and Andreas, 1953).

v!)

MATERIALS AND METHODS

Experiments to determine (a) the effects of photoperiod
and gibberellin and (b) the effects of photoperiod and
maleic hydrazide on the initiation and expression of flower-
ing of Weigela were conducted in the Plant Science Green-
house at Michigan State University, from January 1961 to
August 1961.

Plant Material and Culture

Plants of Weigela clone 'Vanicek' grown from rooted
cuttings were used in all experiments. All plants in each
of the experiments were exposed to comparable temperatures

of 21°C or greater for the duration of the experiments.

Photoperiods

Photoperiods employed in these experiments were provided
by extending or reducing the duration of the prevailing
natural daylongth. The high intensity light phase consisted
of nine hours of natural irradiation daily (8am to 5pm).
Long photoperiods (16 hours) were obtained by supplimentary
low intensity irradiation provided by 60 watt incandescent
bulbs for a period of seven hours (5pm to 12pm) following
the high intensity light phase. Short photoperiods (9 hours)
were obtained by excluding light from the plants for fifteen
hours daily (5pm to 8am) by covering with black greenhouse
shade cloth.

17

 

To

18.

Bud Analysis

Representative samples of buds were removed for micro-
scopic examination from alternate nodes of all stems exposed
to short-days in the phot0period-gibberellin experiment,
and from like nodes of all plants of the photOperiod-maleic
hydrazide experiment. Buds removed were killed and fixed
in FAA (50 parts 95% ethyl a1c0hol, 5 parts glacial acetic
acid, 10 parts formaldehyde, 35 parts water); dehydrated in
a series of ethanol-tertiary butyl a1c0hol solutions of
increasing TBA concentrations from 50 to 100%; imbedded in
paraffin; longitudinally sectioned 10 microns thick; and
microscopically observed to determine the presence or

absence of floral primordia.

 

 

 

 

PHOTOPERIOD-GIBBERELLIN EXPERIMENT

Six 2-stem and six 3-stem.p1ants of Weigela c1.'Vanicek'
in 6 inch clay poys were selected for uniformity of size
and vigor, removed from long-day exposure, and 3 plants
were subjected to each of the following treatments:
A. 3-stem.p1ants: 1 stem exposed to long-days; 2 stems
and pot exposed to short-days
B. 3-stem plants: 1 stem exposed to short-days; 2 stems
and pot exposed to long-days
C. 2-stem.p1ants: 1 stem exposed to long-days; 1 stem
and pot exposed to short-days
D. 2-stem.p1ants: l stem exposed to short-days; 1 stem
and pot exposed to long-days
Each of the four treatments was replicated 3 times.
Providing unlike photoperiods to a single plant was
accomplished by the use of double thickness black greenhouse
shade cloth mounted on a wooden frame. Once the stems had
been inserted through the light barrier, the openings were
sealed so as to be light-tight. All leaves of stems placed
through the light barrier, arising from nodes below the
barrier, were removed, and any new leaves arising from.these
nodes were removed when first observed.
Shoot elongation was measured at 7-day intervals for
7 weeks following treatment and the results are presented
in Figure 1.
Seventy-two days after being exposed to the above
treatments, all plants were removed and re-staked. Stems

previously exposed to long-days were pinched below the

 

.l‘lIlflllll [I'll Ill I

29.

terminal pair of expanding leaves, and a foliar spray of
1000 p.p.m. (acid equivalent) potassium gibberellate was
applied to all plants. After treatment with gibberellin,
one half of each of the above treatments was subjected to
long-day exposure, and the remaining plants were exposed
to short-days.

Observations on the eXpression of flowering were made
periodically for 16 weeks, at which time final measurements
were recorded and representative samples of buds were

removed for microscopic examination.

PHOTOPERIOD-MALEIC HYDRAZIDE EXPERIMENT

Thirty completely vegetative one-stem plants of Weigela
c1. 'Vanicek' previously grown from rooted cuttings were
removed from.1ong-day exposure and divided into 3 groups
of 10 plants each. One group was exposed to short-days
(9 hours), another to long-days (16 hours), and the third
group received a foliar spray to run-off of maleic hydra-
zide, supplied as 3000 pap.m. (acid equivalent) of the
diethanolamine salt of 6-hydroxy-3(2H)pyridazinone (MR),
and then exposed to long-days (16 hours).

Increases in stem.length and incidence of flowering
were recorded at 7-day intervals for 10 weeks (Figure 7).
At the end of this 10 week period, representative samples

of buds were removed for microscopic analysis.

21.

 

 

.I 'l‘ll.

 

Illilllll

I ll l‘lrlllulll

RESULTS

PHOTOPERIOD-GIBBERELLIN EXPERIMENT

Vegetative Responses

Growth rates of Weigela for the first seven weeks
after photOperiodic treatment were determined and are
presented in Figure 1. Growth rates, as measured by shoot
elongation, of stems eXposed to long photoperiods increased
in a linear fashion with time, whereas stems under short
photOperiodic regimes ceased growing by the third week
following light treatment. Providing short-days to one
portion of the plant and long-days to the other had no effect
on growth (Figures 2 and 3).

Stems treated with gibberellin, which were previously
under short-day exposure and which had not been growing for
seven weeks, resumed growth. This occured when.these
treated, dormant stems were placed under either long- or
short-days (Figure 4: A,b; B,d). Resumption and extent of
growth was of a greater magnitude under long photoperiods.

Shoots from.the node nearest the pinch on stems formerly
exposed to long photoperiods began to elongate by the
second week following pinching and treatment with gibbere-
llin. Stems which remained under long-day exposure showed
an average elongation of 70.4 cm. sixteen weeks after treat-
ment as compared to 10.8 cm. for those placed under short-
days following treatment (Figure 4: A,a; B,c).

22.

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III" III Illa-I.

 

7’"

25.

Figure 1.

Growth rates of Weigela grown under long- and short-
day exposure. Points are the mean elongation values of
long- or short-day stems of the four treatments. Curves
were fitted to these means by apprOpriate orthogonal
polynomials (Anderson and Houseman, 1942; Dedolph, 1960)

or regression methods (Snedecor, 1956).

_. ‘r—v —»——. ———.— l

 

24,

 

 

40.

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25.

Figure 2.

Effect of long- and short-days 0n serarate stems of
the same plant. (Photographed 10 weeks after placement
into light treatments.)

Figure 3.

Three-stem which received photoperiodic treatments.

Left: One stem exposed to short-days, and two stems

and pot exposed to long-days.

Right: One stem exposed to long-days, and two stems

and pot exposed to short-days.
Figure 4.
Effect of pinching, gibberellin, and photoperiod on

the growth of Weigela.

A. Two stems previously exposed to long-days, and
one stem exposed to short-days, which after
pinching and treatment with gibberellin were '
exposed to long photoperiodic regimes.
a. Point of pinching
b. Point where growth ceased under short-days
B. Same as A, but following pinching and treatment
with gibberellin, was exposed to short-days.
a. Point of pinching
b. Point where growth ceased under short-days

Figure 5.
Responses of Weigela after three weeks exposure to
long-days, short-days, and long-days plus 3000 p.p.m.

maleic hydrazide. (Left to right; SD, LD, LD + NH)

 

 

27.

Floral Responses

 

Simultaneous exposure of stems of a single plant to
different phot0periods had no effect on flowering.

No flowers were expressed at any time from stems which
were maintained under short-days both before and after
gibberellin treatment; nor were any flowers expressed from
stems exposed to long-days prior to pinching and gibberellin
treatment, and short-days thereafter. However, buds removed
and analyzed at the end of the 26 week period (16 weeks
after pinching and gibberellin treatment) revealed that
87.5 percent of the stems continually under short-days,
and 62.5 percent of the stems exposed to short—days prior
to, and long-days following treatment with gibberellin
possessed well defined flower primordia (Figure 6).

Stems which resumed growth after treatment and exposure
to long-days expressed flowers, on the average, from 23.3
percent of the nodes produced after treatment. These were
invariably from the 3 to 5 most terminal nodes. An occasional
flower was observed arising from nodes Just below the point
from which growth resumed.

New shoots arising from the pinch of plants continu-
ally exposed to long phot0periods expressed flowers, on the
average, from 36.6 percent of the nodes. Here again,

flowering was from the more terminal nodes.

28.

Figure 6.
Photomicrographs of longitudinal sections of buds

of Weigela.

A. Typical vegetative bud developed under short-day
exposure (150 X).

B. Bud from.p1ant treated with.3000 p.p.m. maleic
hydrazide and grown under long-days (45 X).

0. Well defined floral primordia contained in.hud
from.stem.which was exposed to-short-days
throughout the experiment (45 X).

D. Well defined floral primordia contained in.hud
from stem which was exposed to short-days for
10 weeks, followed by 16 weeks under long-
days (45 X).

 

PHOTOPERIOD-MALEIC HYDRAZIDE EXPERIMENT

Vegetative Responses

Growth rates of the Weigela plants used in this experi-
ment are presented in figure 7. Shoot elongation of plants
exposed to long photoperiods increased in a curvilinear
faahion.with time. Those which were treated with maleic
hydrazide and grown under long-days increased in a linear
fashion with.time. However, the growth rate was but approx,
imately 50 percent of that of the non-treated long-day
plants. By the fourth week after exposure to short photo-
periods, the plants in this treatment ceased growing and
remained in a dormant state thereafter.

It is noteworthy that of the plants under long-days,
those not treated with maleic hydrazide elongated about
twice as much as the treated plants, but the number of nodes
produced after light treatment varied but slightly. Long-
day plants produced an average of 11.8 nodes, whereas the
maleic hydrazide-treated plants produced an average of
9.8 nodes (Table I). The terminal 4 to 6 nodes of the,
chemically treated plants were extremely close together.

In general, by the end of the experiment the chemically
treated long-day plants were compact, the non-treated long-
day plants were tall, and the plants under short-day exposure
were short, but not as compact as the chemically treated
plants (Figure 5).

30.

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31.

Figure 7.

Growth rates of Weigela subjected to long-days,
short-days, and long-days plus 3000 p.p.m. maleic
hydrazide. Points are the mean elongation values for
10 plants. Curves were fitted to these values by appro-
priate orthogonal polynomials (Anderson and Houseman,

1942; Dedolph, 1960) or regression methods (Snedecor,

1956).

70¢

60”

T
O

50

40—

Shoor E Iongan‘an ( cm.)

20~

 

32.

A LD ([6 Hours)
0 SD (9 Hours)
a LDOMH (3000mm)

 

 

Weeks After Treatment

 

33

Table l. Comparisons of Plants EXposed to Long-days, Short-days, and Long-days

plus Maleic Hydrazide.2/

 

 

Long-days‘plus

 

Long-days Maleic Hydrazide Short-days

Shoot elongation (cm.) 57.7 2A.] 15.0
Nodes produced after

treatment ll.8 9.8 2.8
Nodes which expressed

flowers (%) 52.2 90.9 0.0
Nodes not expressing

flowers, but possess-

ing reproductive buds

(7.) 80.0 81.8 ”.8

 

2] Values for IO plants over the l0-week period.

34.
Floral Responses

Flower Expression

No flowers were expressed by plants receiving short
photoperiods in this experiment. Plants under long-days
produced a small amount of sporadic flowering from.10wer
nodes by the sixth week, and thereafter only flowered from
nodes produced following treatment. Long-day - maleic
hydrazide-treated plants were all flowering by the eleventh
week and incidence of flowering increased with time.

At no time did the nonptreated long-day plants express
flowers from terminal nodes. However, plants treated with
the chemical expressed flowers (in.the colored bud stage)
from.appr9ximately 30 percent of the nodes which were

terminal at the end of the experiment.

£993 91 Flowering

Differences in the flowering habit owing to exogenous
applications of maleic hydrazide, as indexed by the nodes
from.which flowers were produced, were striking (Figure 8).
Non-treated long—day plants expressed very few flowers
from nodes produced prior to light treatment, whereas treat-
ed plants produced an abundance of flowers from.correspond-
ing nodes. Floral expression from nodes produced after
treatment was similar for both treated and nonptreated
plants. However, the magnitude of flowering in the maleic

hydrazide treated plants was greater. As mentioned above,

uvyv

 

35.

Figure 8.

Percent incidence of flower expression by nodes of
Weigela exposed to long-days and long-days plus maleic
hydrazide (3000 p.p.m.).. Each percentage value based on

10 plants. (Node number 15 = node terminal at time of

treatment)

36.

100'-

 

100

 

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10

woos ms: (BASE tour)

37.
terminal flowers were observed in treated, but not in the
non-treated plants. No flowers were expressed from nodes

which were terminal at the time of treatment (eg. node 15,

Figure 8).

Floral Initiation

Upon microscopic examination, buds removed from plants
under long photoperiods at the end of the experiment showed
that of the buds at nodes which had not flowered, 80 percent
contained flower primordia. Only 11.8 percent of the buds
examined of the plants exposed to short photoperiods possessed

flower primordia (Figure 6; Table I).

DISCUSSION AND CONCLUSIONS

PHOTOPERIOD-GIBBERELLIN EXPERIMENT

Vegetative Effects

Growth rates for the first seven weeks of Weigela
plants in which different stems of the same plant were
exposed to unlike photoperiods that the effect of photo-
period on one stem.did not carry over and influence another.
As reported by Downs and Borthwick (1956a), short-days
inhibited, and long-days promoted growth of Weigela (Figure
1).

The most pronounced effect of treatmwnt with gibberellin
was its ability to cause a resumption in growth from the
dormant short-day stems, or the ability to break "photo-
induced dormancy" (Bukovac and Davidson, 1959).

Floral Effects

Flower expression was enhanced by long photoperiods
and prevented by short photoperiods. However, flower
primordia were differentiated under short-day exposure, but
expression of these buds was supressed. No floral effects
can here be attributed to exogenous applications of gibb-
erellin, for all stems of all plants in the experiment were

treated.

38.

PHOTOPERIOD-MALEIC HYDRAZIDE EXPERIMENT

Vegetative Effects

Exogenous applications of 3000 p.p.m. maleic hydrazide
drastically reduced the growth rate of Weigela as indexed
by nonytreated plants under the same photoperiodic regime
(Figure 6).

Freeman (1952) and Reisch and Chadwick,(1955) reported
that the chemical resulted in the cessation of meristematic
activity in some woody species. But in this study, it was
observed that although the length of internodes was greatly
reduced, the rate of node formation was only slightly less
than corresponding nonptreated plants. These data suggest
that cell enlargement, rather than cell division is inhibited.
Furthermore, flowers were observed arising from nodes which
were terminal at the end of the 10 week period, indicating
that differentiation was not supressed by chemical treatment.
In order for maleic hydrazide to cause complete cessation of
terminal.meristematic activity in.Weigela, concentrations in

excess of 3000 p.p.m. would be required.

Flowering Effects
Prior to flowering, Weigela produces "flower shoots"
from axillary buds. These shoots consist of 3 to 5 pairs
of opposite leaves, and flowers are produced from.the axils
of the 2 or 3 most terminal leaf pairs of the "flower shoot".
This study has shown that completely differentiated flower
39.

40.
primordia exist in the axillary buds prior to the extension
of the "flower shoot" (Figure 6-C; 6-D).

Short-day Plants: The flower primordia observed in plants
exposed to short-days suggested that flowers may be differ-
entiated in 10 weeks under these conditions. However, this
has not been conclusively demonstrated, for the limited
amount of flower primordia observed (11.8%), were at nodes
which were present when the plants were removed from.10ng-
days. It is very possible that these had been initiated by
that time. A typical short-day vegetative bud is illustrated

in Figure 6-A.

Long-day Plants: In non-treated long-day plants, about

80 percent of the buds at nodes which had not expressed
flowers by the end of the 10 week period contained flower
primordia. The flowers expressed were produced, with but

a few exceptiond, from nodes laid down following placemnt
into light treatment. Expression of these floral primordia
was somehow inhibited.

Inhibition of this type did not occur when the plants
were given exogenous applications of maleic hydrazide.
Chemical treatment apparently overcame the inhibition of
these buds. These data are graphically illustrated in
Figure 8.

How maleic hydrazide negated this inhibition is not
clearly understood. One possible explanation, that chemical

treatment removed apical dominance resulting in the extension

\u:

41.
of "flower shoots", is probably not too valid. Complete
cessation of apical meristematic activity did not occur,
for nodes were produced at a rate only slightly less than
that of the non-treated long-day plants. If a partial loss
of apical dominance did occur, then it is doubtful that
such an effect would be expressed so far from the apex.

It appears that maleic hydrazide affected the buds
present at the time of treatment and caused them to flower.
Observed flower initiation and expression from growth
produced after treatment suggested that the long photo-
periods were more influencial than treatment with the chemical.

Thus, it was surmised that exogenous applications of
maleic hydrazide to Weigela resulted in flower expression
from buds present at the time of treatment, and in reduced
growth rates due primarily to its effect of cell enlargement.
The mode of flowering from nodes produced after chemical

treatment was not shown to be materially affected.

SUMMARY

To ascertain the mode of floral initiation and
expression, plants of Weigela clone 'Vanicek', grown under
long and short photoperiods, were given exogenous applica-
tions of gibberellin or maleic hydrazide.

Shoot elongation under long-days (16 hours) was cont-
inuous, and under short-days (9 hours) plants ceased growing.

Applications of gibberellin stimulated shoot growth in
general. When applied at 1000 p.p.m. to dormant stems under
short-days, a resumption of growth occured.

Under long-days, exogenous applications of maleic
hydrazide markedly inhibited shoot elongation, but the
number of nodes produced was only slightly reduced.
Cessation of meristematic activity did not occur. Rather,
growth inhibition in these experiments was apparently due
to reduced cell elongation and not to reduced cell division.

Flowering did not occur on.plants under short-days,
however, a limited amount of flower primordia were observed.

Evidence obtained indicated that applications of maleic
hydrazide at 3000 p.p.m. caused flowers to be expressed
from nodes present at the time of treatment, but flowering
from.nodes produced following chemical treatment was not

materially affected.

42.

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C
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o
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. .5
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