MIMEOGRAPH SERms #1631

DORMANCY IN APPLE TREE BUDS

B. C. Allen and H. Wann

Biomathematics Series No. 13Institute of Statistics Mimeo Series No. 1631North Carolina State UniversityRaleigh, North Carolina

September 1983

i

ACKNOWLEDGMENTS

This work was supported in part by USDA grant (Apple CIMP National

Project No. 71-59-2481-1-2-039-1). Ann Ethridge typed this manuscript with

efficiency and dispatch.

it

TABLE OF CONTENTS

...............................................ACKNOWLEDGMENTS ••••••••••••••••

LIST OF FIGURES

................................Page

i

iii

Hormonal Control of Dormancy •••••••.••••••••••••••••••••••

Phases of Dormancy •••••.••••••

Factors Controlling Dormancy

Origin of Endogenous Factors •...••••.••••••.•••.••••••

1

1

2

2

3

5

6

6

7

9

10

13

13

15

17

19

20

24

32

...........................

. .

. .

. .

. .

. .. .

.............................................

...............................................

. .

Balance and Interaction of Inhibitors and Promotors •..

Mechanism of Control and Nature of Dormant States

Environmental Factors: Photoperiod and Temperature

Interaction of Environmental and Endogenous Factors

General Remarks ••••••••

A Note on Nomenclature

Induction of Dormant States

Effects of Temperature on Various Phases of Dormancy

Growth and Development in Dormant Period

Physiological Changes •••••••••..••.•••.•••••••••••••••

Growth Inhibitors

Growth Promotors

Bibliography

A.

B.

A.

B.

A.

B.

C.

D.

E.

A.

B.

C.

D.

Introduction

Conclusions

II.

I.

III.

V.

VI.

IV.

iii

LIST OF FIGURES

Fig. 1. Diagrammatic representation of changes in temperature range forbud break at times of change in growth activity.

A. Narrowing and widening of the temperature range due tochanges in the maximum temperature at which bud break canoccur. (Adapted from Vegis (122».

B. Narrowing and widening of the temperature range due tochanges in the minimum temperature at which bud break canoccur.

Fig. 2. A schematic representationinvolving three classes ofcytokinin and inhibitors.(or ineffective) levels ofKhan (70».

of seed dormancy and germinationphytohormones, gibberellins,n+n (or "-") indicates effectivethe hormone. (Adapted from

Fig. 3. Possible pathway of action leading to bud break. (Adapted fromChandhry, Broyer and Young (30».

Fig. 4. A postulated scheme for the action of GA. (Adapted from Jarvis,Frankland and Cherry (67».

Fig. 5. Possible biochemical pathways in the termination of seeddormancy. (Adapted from Amen (5».

I. Introduction

This report summarizes the results of a part of coordinated efforts

led by Dr. H. J. Gold to develop apple orchard ecosystem models (144).

One essential component of the system is apple tree buds. To integrate

this component to the overall system, we must quantify the effects of

environmental factors on bud growth and development. The difficulties

in construction of such a model are mainly due to the complexity of

the underlying biological mechanism and lack of detailed data. As a

preliminary step of model development, we made a survey of current

literatures on bud physiology emphasizing environmental effects.

A. General Remarks

In the summer, apple tree buds enter dormant period during which

growth (dry matter accumulation) is sharply reduced. Normal growth will

not resume until the next spring. It is known that changes in

structure and internal function of the bud in dormant period can have

profound effects not only on timing of bud break but also on bud

growth (111).

The initiation, maintenance and release of dormancy may be viewed

as a continuous process consisting of a sequence of stages (111). The

morphological, physiological and molecular changes that occur during

dormant period are thought to be under genetic control but may be

modified by exogenous factors (57). From this point of view, the basic

mechanism responsible for dormancy can only be revealed by simultaneous

studies of changes in internal function of the bud and its relation to

environmental conditions.

2

We report here a survey of literature on apple bud dormancy and,

to a certain extent, some general aspects of dormancy in other fruit

trees. Its purpose is only to draw together some basic knowledge in

such a fashion that fundamental parts of this knowledge can be used to

develop a framework for quantitative experimental studies into the bud

development in relation to temperature and other environmental factors.

B. A Note on Nomenclature

In this survey, we will generally follow the nomenclature of

Romberger (104).. Thus, "dormancy" will refer to a period of time, or

the state of the bud, during which that bud will show no extensive

growth, for whatever reason. By "innate dormancy" or "rest" we mean

that portion of the dormant period in which the cause of the lack of

growth is due to endogenous factors; i.e., it is not imposed by the

environment. The reduction or stoppage of growth due to environmental

factors is called "quiescence" or "imposed dormancy". It must be

remembered that the state of dormancy, its imposition and release, is a

continuous process and that these terms are used only because they

are sometimes useful. However, the distinctions are not comp~etely

adequate because they separate the internal and external influences in

an artificial manner.

II. Factors Controlling Dormancy

It has long been noted that environmental factors influence the

development, maintenance, and break of dormancy. Factors most studied

are temperature and photoperiod (or light intensity) and to a lesser

extent nutrient and water stress. Early theories of dormancy suggested

that the resting state in buds results from a lack of nutrients in the

3

buds themselves. Now it is generally believed that dormancy is under

the control of plant-produced chemicals, i.e., growth promoters and

inhibitors (90). This does not imply that nutrient stress is not a

factor in the induction of rest; it means merely that this stress acts,

as do the other factors mentioned above, through the actions of hormones.

A. Environmental Factors: Photoperiod and Temperature

Changes in photoperiod have long been seen as a naturally occurring

cyclic process that could account for the alternation between dormant

and growth states (90, 122, 131). For the most part, short days are

thought to induce dormancy; long days to end it. We will not comment

upon this extensively since it has been shown (89, 128) that photoperiod

changes do not induce dormancy in apples. Suffice it to say that, as

in peaches (55), the intensity of apple bud break may be sensitive to

light (56).

Temperature (i.e., air temperature) is another matter however.

Cool temperatures (12.5°C) were found to be sufficient to halt growth

of apple trees (89). Of course, the well-documented "chilling

requirement" of many species is shown to be present in apples as

well (44). There are indications, nonetheless, that the effect of

temperature may be more complicated than simply a cool temperature

induction and a chilling temperature break of dormancy.

Weinberger (134) suggests and Erez and his colleagues (51, 52) attempt

to show that ~cling temperatures (alternately high and low) affect the

progression of dormancy in peaches. The important variables seem to be

cycle length and maximum temperature. The longer the cycle (six or

•4

nine days), the less the negative effect on rest progression. More

interesting is the effect of maximum temperature: in a daily cycle

(eight hours high, sixteen hours low) a high temperature of 21°C or

higher completely negated the prior chilling. However, an alternation

between 6° and 15°C ~nd to a lesser extent between 6°C and 18°)•

enhanced the chilling process; i.e., bud break was advanced.

Thompson, et al. (115) noted the same trends in apple. Borkowska (22)

showed that although chilling may be insufficient for bud burst it may

be enough to promote bud elongation. Generally, he suggests that there

may be two processes at work within the bud, one controlling burst and

the other controlling cell elongation, and that each has its own

environmentally optimum requirements.

Sparks (112) demonstrates that in pecan, subfreezing temperatures

may also advance bud break. He attributes this to tissue injury or to

prevention of the attainment of the deepest resting state. Indeed,

it is generally believed that the optimum chilling temperatures are

somewhat above freezing, roughly 2°C to 7°C (54, 100, 115), and that

freezing retards bud development and dormancy break (25). A number of

studies have suggested that the time of chilling, early or late in the

winter, will affect both the number of buds which break and their

subsequent growth (1, 72, 115). The rate of bud development in the

spring was found to depend most heavily on the cold conditions of the

preceding autumn or'early winter (1). That is, warmer autumn conditions

promote further bud development (delayed onset of quiescence), which

facilitates renewal and completion of growth once dormancy has ended

(1, 72). On the other hand, Abbott (1) suggests that warm temperatures

in late winter or spring may allow for earlier bud break (i.e., earlier

)

5

removal of imposed dormancy). Abbott also hypothesizes that the

temperature above which bud growth can resume in spring (i.e., above

which dormancy is no longer imposed) is not constant; it depends upon

the extent to which dormancy has been broken by cold treatment. Thus,

those buds which receive more chilling can leave the quiescent state

at lower temperatures than those which get less chilling. Eggert (44)

suggests that there is a correlation, within the apple species, between

the date of bloom and the chilling requirement: those varieties which

bloom late need more chilling during winter.

The general effect of temperature on dormancy may be summarized

as follows. Cooling air temperature promotes dormancy, which perhaps

begins as quiescence and may therefore be reversed by subsequent

warmer temperatures. Once the cooling has induced rest, that state is

maintained until chilling brings it to an end. That the process is more

complicated than this cursory summation allows is indicated by the

facts noted above for temperature cycling and times of cold imposition.

We later attempt to tie these observations to theories about the

action of growth hormones.

B. Interaction of Environmental and Endogenous Factors

As mentioned above, there is some difficulty in a notion of

dormancy which divides the dormant state into a rest (endogenous control)

phase and a quiescent (environmental control) phase. For one thing, we

have seen above that, even during rest, the environment (e.g.,

temperature) exerts an influence, shortening or prolonging rest.

Furthermore, with a hormonal mechanism postulated for the control of

6

growth, and knowing that the environment can affect hormone levels in

many ways, it is misleading to try to separate in time environmental

and endogenous effects. Rather, we would think, as does Tyurina (119),

of these effects as a dual control. This complements his notion that

growth and dormancy are just parts of a continuous process and that

in the course of one, the other is prepared for, actively. Thus there

is an endogenous rhythm upon which environmental stimuli can operate.

He states that if for some reason (e.g. because of a late growth start

or under unfavorable grOWing conditions) the endogenous rhythm is not

conducive to the imposition of rest, the environmental signals that

the plant receives can compensate and initiate rest itself. This

could also account for the fact that buds in a resting state will

eventually leave that state, even if they do not receive their

chilling "requirement." If they do get chilling, though, the endogenous

and environmental factors work in harmony and vegetative growth can

recommence much sooner.

III. Phases of Dormancy

A. Induction of Dormant States

There remains a question about the ability of plants to enter

dormancy. Specifically, will the bud enter dormancy whenever a

prolonged environmental stress is encountered, or does the bud have to

be at a certain stage of development before dormancy is induced?

There appears to be no resolution of this question as yet. Romberger (104)

reports that cells which do not contain significant amount of lipids

are not able to enter dormancy. In particular, o. Abbott (2) cites

7

Simon (1919), stating that buds will not enter a resting state until a

certain degree of development has been attained. On the other hand,

D. L. Abbott (1) supports the idea that mild autumns promote further

growth and development, harsher falls inhibit this, but buds at any

stage of development can enter a normal dormant state. Landsberg (72)

confirms this. Brown and Abi-Fadel (26) state that buds at earlier

developmental stages do not require more chilling to end rest, i.e.,

the bud developmental state seems to be no index of the chilling

requirement. These reports are not conclusive; the requisite degree of

development may be attained early in autumn or even in summer. The

various stages of morphogenesis reported by these authors may

correspond to development beyond the required minimum. More to the

point is an investigation by Chandler et. ale (29) who state that apple

leaf buds are quick to enter rest in summer if ever their shoots stop

growing. Moreover, these authors found that a water stress of a few

weeks duration, even in early summer, was enough to cause a resting

state so deep that no growth was possible for the remainder of that

summer.

B. Effects of Temperature on Various Phases of Dormancy

The realization that the onset of rest is not an instantaneous, but

rather a gradual process probably led Vegis (122) to propose a classi­

fication scheme for dormancy consisting of three phases: predormancy,

innate dormancy, and post-dormancy. During the predormant period, the

temperatures at which the plant can grow become more restricted, until

the plant can no longer grow at any temperature, the point which marks

8

the passage to innate dormancy. After some period of innate dormancy

the plant becomes capable of growing in some small temperature range;

as post-dormancy progresses this range becomes increasingly wider until

it reaches the widest limits for vegetative growth. Vegis states that

for fruit trees in the Rosaceae group the diagram would look like that

shown in Fig. lAo However, Tyurina (119) mentions a lowering of the

growth temperature minimum as dormancy comes to an end (for apples)

(Fig. 18). Vegis further states that a high temperature early in post­

dormancy can actually induce a secondary dormancy. Thus he visualizes

the widening of the range not as a monotonic process, but as one that

can be reversed. His view of this widening phenomenum is supported by

Rom and Arrington (103) who report that a stepwise increase in

temperature produces a better bud break than a sudden jump to the final

warm temperature.

This approach is more satisfactory for it does not try to separate

endogenous and environmental factors. The new characterization of the

process of dormancy uses forcing experiments to determine a depth of

the dormant state. This generally results in a bell-shaped curve

(49, 60) with the mode coinciding with innate dormancy. Such

observations are consistent with the division of the dormancy period

proposed by Samish (107). It consists of the following sequence:

quiescence -+ preliminary rest -+ mid rest -+ after rest -+ quiescence.

Here quiescence is a state of growth stoppage reversable by favorable

conditions. Preliminary rest and after rest are characterized by a lack

of growth, but an ability to be forced. Midrest corresponds roughly to

innate dormancy, the state in which forcing is difficult or impossible.

9

This scheme, it seems, can be made to correspond to that of Vegis. The

two characterizations may merely divide the continuous process of dormancy

into different discrete portions. A probable correspondence is

illustrated in Fig. lA. We will see later if any such scheme is

compatible with the internal mechanisms at work during dormancy.

c. Growth and Development in Dormant Period

It should not be thought that growth and development do not occur

during the dormant period. Tyurina (119) and Romberger (104) state that

the apical meristem cells remain active; the subapical cells are

inhibited in their gorwth. Both growth and development in dormant buds

proceed at slow but continuous rates (13, 27, 28, 134, 141). Flower

buds not only increase in weight (33), they show an increase in flower

clusters within the bud (27). Similarly, leaf buds demonstate an

increase in the number of primordia as dormancy progresses (104, 141).

Even though Brown and Kotob (27) state that Zeller reports slow

development of apple flower buds even during the coldest part of winter,

it is generally believed that subfreezing temperatures may retard bud

development (25, 119). Both Young(14l) and Romberger (104) state that

the bud parts developed during dormancy have_a morphology different from

those in vegetatively growing buds. Moreover, Young (141) has shown

that the increase in size and in number of leaf primordia is more

marked in March through June in buds with prolonged dormancy (given

insufficient chilling, i.e., no temperatures below 15°C). This

corresponds to the time when bud burst and rapid growth would occur had

dormancy ended as usual. Perhaps there are two mechanisms, i.e.,

10

internal systems, one controlling growth and development within the bud,

the other controlling the processes associated with bud burst. Indeed,

Hewitt and Wareing (64) suggest that development of the bud during

dormancy is mediated by regulators and that subsequent growth in spring

results from a "well-defined metabolic pathway." The growth and

development of apple leaf and flower buds during dormancy has been

reported by Zlobina (143), Tyurina (119), Brown and Kotob (27).

Since growth and development do occur during dormancy, albeit

slowly, how is dormancy different from vegetative growth? As mentioned

above, Tyurina (119) and Romberger (104) state that growth inhibition

occurs in the subapical meristem cells, and the former author stresses

that it is the transition to dormancy that initiates apical meristem

cell activity. Thus, there is no shoot elongation. But there are more

fundamental and important distinctions between dormant and non-dormant

states. Tyurina (120) hypothesizes that the advantage to temperate­

climate plants of a dormant phase is that such a phase allows the plant

to separate the energy-requiring processes of growLh and of

preparation for cold hardiness. This indeed appears to be the major

function of dormancy: reducing susceptability to environmental stresses.

D. Physiological Changes

Whatever is responsible for the maintenance of dormancy, the

external differences that are seen must reflect cellular changes

occurring in the buds. These changes not only result in the dormant

state but also prepare the cells for the subsequent phases of their

development (120). The internal changes are a direct result of the

11

mechanisms of dormancy control. Thus an examination of the changes may

reveal the nature of those mechanisms.

During the latter part of summer, perhaps after the onset of

quiescence, photosynthesis builds up a reserve of carbohydrates and

other compounds (2, 119, 120). Also noted is the synthesis of high­

molecular-weight, acid-insoluble phosphorus compounds. As dormancy

progresses, this ceases as acid-soluble phosphorus compounds

(apparently for storage) are manufactured (119). Phosphorus uptake

(109), and, correspondingly, phosphorus metabolism (119) cease almost

entirely during the deepest part of rest. The DNA-RNA-protein system

has been working actively in autumn (119) and by the time rest passes to

imposed dormancy the metabolic activity is completed through DNA

transcription. The fact that RNA and protein synthesis occurs during

dormancy is confirmed by many investigators (13, 37). Eke1und (46) notes

the slow pace of RNA and protein synthesis, rRNA being the primary

nucleic acid produced. However, Yeh Feng et. a1. (140) note that later

in dormancy (January to March) changes in the nucleus occur and more

protein and RNA are produced. Although transcription may be complete

by end of rest, the mRNA cannot enter the cytoplasm because of a double

membrane around the nucleus and the absence of nuc1eopores (140).

Other changes associated with the onset of dormancy include a decrease

in amino acids (48), perhaps due to active protein synthesis and increase

in the protein content of the ribosomal and mitochondrial fractions (119).

Also reported is a rapid hydor1ysis of polysaccharides and the appearance

of large quantities of oligosaccharides (118). Samish (107) reports

that the protop1asts in the cell contract and assume a convex shape;

12

they withdraw from the cell wall, rupturing the plasmodesmata, and

become covered with a lipoid layer. This is most evident in apple

and pear. Tyurina (119) states that upon entry into dormancy there is

a coincident increase in nuclear dimensions and protoplasmic content.

During the course of the dormant period, various changes in cell

activity occur. There is an increased metabolic activity of cell wall

formation and lipid droplets are at a maximum in early December; these

comprise the source for membranes of new organelles formed later (140).

Ekelund (46) notes the increase of ribonuclease activity from autumn

to spring, with much greater increase in March and April. An early

article by Abbott (2) reports that hydrogen ion concentration decreases

through winter in apple bud cells, a fact that he claims affects the

activity of certain plant enzymes. Bachelard and Wightman (10) note

that ~ absorption increases through dormancy; however Hatch and

Walker (60) saw no significant difference in respiration rates until

late in the dormant period, February or March.

As dormancy is coming to a close, many of the changes seen at the

inception of dormancy are reversed. Amino acid content increases, due

to the rapid breakdown of the protein found in large quantities up to

this time (10, 48). Nucleopores reappear, allowing mRNA to enter the

cytoplasm (140). Carbohydrates are markedly decreased (10), especially

starch content in apples (65). Organic acid content increases just

prior to break (48). There is a net loss of nitrogenous material (10)

and phosphorus uptake is resumed (109). Bachelard and Wightman (10)

report that the respiratory quotient decreases from about 3 (indicating

anaerobiosis) to between 1 and 1.5. They state that metabolic activity

13

changes from catabolism to anabolism about two weeks prior to renewed

growth. Before this occurs, the carpel size of the Golgi bodies

increases, an important event because of the extra cell surface area

provided. At about the same time, the number of mitochondria increases,

indicating that more energy is needed for cell reactions. Also smooth

endoplasmic reticulum (ER) is transformed into rough ER in preparation

for synthesis of proteins to be transported (140). Samish (107) reports

that the protoplasts swell and reestablish plasmodesmatic connections.

Total sugars increase significantly about two weeks after the completion

of rest (48), particularly the monosaccharides glucose and fructose (10).

A number of investigators (10, 114, 128) report the increased water

content of cells as they end dormancy. Taylor and Dumbroff (114) cite

a change from a low of 3810 of total fresh weight to 7110 at burst in

sugar maple buds.

IV. Hormonal Control of Dormancy

A. Origin of Endogenous Factors

We have seen that environment is one influence on the dormancy

status of woody plants. It remains to be considered by what internal

mechanisms the changes in this status are affected. As mentioned

previously, a hormonal control system is generally accepted (see

Wareing and Sanders (131) for a review). In particular, a balance

between inhibitor(s) and promoter(s) is hypothesized (131). An early

work (41) showed that dormancy is a property of the buds alone, i.e.,

no other tissues are dormant. In fact Corgan and Martin (33) suggest

that in peach flower buds rest is localized within the floral cup.

14

Semin and Madis (109) confirmed that the roots do not become dormant by

measuring uptake of labeled phosphorus. It is not at all clear however

that the place of origin of the inhibitors or promoters is within the

bud. Mielke and Dennis (86, 87) suggest that the senescing leaves may

be responsible for abscisic acid accumulation in the bud in autumn.

Mauget's work (84) would support such a conclusion. Davison and Young

(38) report a quick rise of abscisic acid (ABA) concentration in xylem

sap after leaf fall. Along the same lines, Corgan and Peyton (34) and

Chandler and Tufts (28) state that later leaf retention corresponds to

later bloom. On the other hand, Hodgson (65) reports no relationship

between leaf retention and rest. Promoters too have been associated

with the leaves: Eady and Eaton (43) note that in cranberry, gibberellic

acid is transported from leaves to terminal buds prior to elongation.

Luckwill and Whyte (80) suppose that cytokinins, another group of

promoters, may be translocated from the roots to the buds in spring since

the authors could detect none of these substances in the xylem sap from

September to January. However, Borokowska (21) found that in apples,

the distal buds had consistently high cytokinin activity throughtout

dormancy. Hewitt and Wareing (64) claim that the increase is

independent of the roots, but dependent on the sap. Romberger (104)

considers any root influence on growth hormone levels unlikely, an

opinion based primarily on the fact that break of buds can occur

normally in excised shoots.

The possibility of influences on the buds by other tissues seems to

be supported by a number of investigators who note that the dormant

state of buds can vary depending on their position on a shoot. In

15

peaches, a difference was found in the optimal chilling temperatures (54);

terminal buds had the highest optimum (were easiest to break), lateral

buds on lateral shoots had the lowest. Eggert (44) suggests that

lateral buds lag behind terminal buds and spur buds because of a

resumption of apical dominance. Weinberger (132) notes one result of

prolonged dormancy in peaches to be bloom only near the tips of shoots,

perhaps followed sometime later by basal bud bloom. This would seem to

corroborate Borkowska and Powell's hypothesis (21, 22) of a "rest

influence" that moves from base to apex as dormancy commences; thus

distal buds tend to be less dormant. In a somewhat related experiment,

Robitaille (102) reports that on shoots placed in a horizontal position,

the buds on the lower side exhibited dormant characteristics longer.

He attributes this to an auxin effect. In contrast, a group of French

authors report strikingly different results. Mauget (84), working

with almond trees, found that the basal ends of shoots are normally

least dormant. Arias and Crabbe (8) and Barnola et al. (12) confirm

this and go on to speak of a dormancy resisting (growth stimulating)

factor at the basal end, an "inertia" (growth inhibiting) factor at the

distal end. Mauget (84) also describes the effect of mechanical

defoliation, i.e., reduction of the dormant state as being strongest on

the distal buds.

B. Growth Inhibitors

Repeated reference has been made to the importance of inhibitors

and/or promoters in the control of dormancy. Many investigators have

reported correlations between endogenous levels of these substances and

the onset or lift of dormancy. For instance, El-Mansy and Walker (49)

· ,

16

have investigated the inhibitor naringenin in peach flower buds; they

found its levels to coincide with the bell-shaped rest intensity curve.

A substance of particular interest has been abscisic acid (ABA) found to

be a constituent of the a-inhibitor complex, very active in inhibiting

growth. Reports of its accumulation in late summer/early fall

(63, 87, 119), presence during dormancy (21, 33, 63, 69, 72, 77, 82, 98,

99) and diminution in spring (4, 63, 87, 88, 119) are plentiful. Some

researchers have found, however, that its levels do not correspond to

rest intensity (40, 86, 114). In particular, Dennis and Edgerton (40)

state that ABA levels correspond not to rest but to dormancy, a view

that appears to fit with the schemes of Vegis or Samish noted above,

i.e., the gradual development of rest, which perhaps then corresponds

to ABA above some threshold of irreversability. However, Mielke and

Dennis (87) report that in mechanically defoliated trees with low ABA

levels, the resting state is as intense as in buds with high ABA levels.

It has been suggested that the relative concentrations of a bound and a

free form of ABA are important in determining dormancy status: free ABA,

causing dormancy, decreases through winter with a concommitant increase

in bound ABA, an inactive form which allows bud burst (15, 77, 86).

Specifically, Lesham et a1. (77) working with almonds report that one

stereoisomeric form, trans-trans-ABA, is of overriding importance in the

inactivation, and that the binding is glycosidic in nature. Bardle and

Bulard (15) propose that the change from a free to a bound form is due

to chilling, but Mielke and Dennis (87) claim that chilling is not

necessary for a decrease in ABA activity in cherry tree buds. Corgan

and Martin (33) suggest that ABA breakdown is a secondary result due to

enzymatic changes brought about by chilling. A somehwat different finding

17

is reported by Kawase (69) for peach and Malus sy1vestris: dormancy

lasted as long as the inhibitor activity increased; as soon as it

started to decrease, the bud breaking process commensed, even though

activity was as high as levels which earlier maintained dormancy. It

appears (40, 82) that the inhibitor resides for the most part in the

bud scales, one report (40) stating that it did not enter the primordia

until after they have expanded. Mielke and Dennis (86) point out that

the concentration of ABA is highest in the primordia, especially in

November and early December, coincident with the deepest rest in some

years. This is consistent with the theory that ABA is transported from

leaves to scales (where it is store~, then to primordia. It is also

consistent with the thoughts of Corgan and Martin (33) that rest is a

property of the floral cup.

C. Growth Promotors

Growth promoters have also received a great deal of attention. But

since vegetative growth was thought to be the normal state for plants,

promoters were at first studied only in relation to inhibitors or to

an inhibitor/promoter balance. Nevertheless, Taylor and Dumbroff (114)

now claim that it is variation in cytokinins, not ABA, that is

responsible for dormancy break in sugar maple. Gibbere11ins have been

studied extensively and high levels of their activity (perhaps with

simultaneous decrease in inhibitor) have often been found to promote

resumption of vegetative growth (4, 66, 111, 119, 138). In relation to

the dormancy period, Jarvis et a1. (67) propose that chilling

"potentiates" gibberellic acid (GA) synthesis, which then proceeds when

18

higher temperatures occur. Hull and Lewis (66) and Walker and

Donoho (125) claim that GA has no effect on apple tree growth or

dormancy. Auxins have been implicated in dormancy control less

frequently (4, 20). Blommaert (20) postulates that increase in auxin

levels by cold treatment functions to inactivate the inhibitor.

Eggert (45) has proposed a model for auxin control of apple spur bud

growth. As long as cells enlarge, auxin does not build up; it is present

in medium concentrations and so promotes growth. However, when

environmental factors slow or stop growth, auxin concentration increases

to inhibitory levels. This continues and buds enter "fore-rest".

Auxin accumulates further, unless there is some feedback or diminution

of produc~ion due to the senescence of the leaves.

Cytokinins are generally recognized as being important in dormancy

control (14, 21, 64, 98, 119), and are known to be active in apple

fruitlets and shoots (78). In fact, Khan (70) states that Piendazek

(1970) found cytokinins capable of reversing ABA inhibition in apple

explants, but not so for GA or auxins. Hewitt and Wareing (64)

emphasize the enhancement of cytokinin activity is dependent upon cold

treatment. The dynamics claimed by Borkowska (21), who proposed ABA

and cytokinins as the inhibitor/promoter pair in apples, are as follows:

in the period from December to February, cytokinin activity is low

(but detectable) with an increase in March and subsequent disappearance

just before burst. Hewitt and Wareing (64) corroborate this and add

that cytokinin activity again increases with shoot elongation.

Barthe (14), working with apple seed embryos, maintains that cytokinins

exist in free or bound form, similar to ABA, and that free-form

cytokinin increases during afterripening leading to germination.

19

D. Balance and Interaction of Inhibitors and Promotors

Corgan and Martin (33), noting the fluctuating levels of ABA during

dormancy, conclude that rest is a more complex phenomenon than a simple

accumulation and maintenance of an inhibitor at a concentration high

enough to stop growth. This view is supported by the findings of

Kawase (69), as stated above, and Ramsey and Martin (99), who show that

both the promoter and the inhibitor levels increase during dormancy.

They found however that the increase in promoter just prior to break was

much greater than that of inhibitor, so that the ratio of promoter to

inhibitor increased. These and other findings support the hypothesis

that the relative amounts (or concentrations) of promoter and inhibitor

are important in determining the dormancy status: a promoter-inhibitor

balance. The manner in which the effects of these substances are

balanced will be discussed later.

In an investigation on apple buds, Borkowska and Powell (22)

suggest that the processes of bud burst and shoot elongation are under

the control of different hormones, probably cytokinins and gibberellins

respectively. Khan (70) too, suggests different roles for these hormones,

as specified in his model,for see dormancy. Noting that ABA levels

increase and cytokinin levels decrease during stress, and that cytokinins

can modify the effects of other hormones without having a marked effect

by themselves (49), he proposes that gibberellin, cytokinin, and ABA

play primary, permissive, and preventive roles respectively. That is,

gibberellins generally promote seed germination whereas cytokinins and

ABA act, and interact with each other, to either allow or prevent

gibberelin from carrying out its role. A schematic representation is

20

shown in Fig. 2. Khan also proposed a similar scheme for bud dormancy,

but allowing for auxins to playa role, perhaps even prior to or in

conjunction with gibberellin. This later suggestion is consistent with

the enhancing capabilities of IAA on GA found by Ahmed and Mathew (4).

If it is allowed that varying concentrations of all four hormones have

effects more subtle than an effective-ineffective dichotomy, the picture

of bud doemancy becomes quite complicated. Moreover, Corgan and

Peyton (34) suggest possible interactions with inhibitors other than ABA.

Some such model may be appropriate. It can go a long way toward

explaining certain anomolous results. For example, Mielke and Dennis

(85, 87) note that temperature had no effect on ABA levels. The model

might then propose that the chilling treatment affected the cytokinin

levels in this species (cherry), negating ABA, and producing normal

break of dormancy.

E. Mechanism of Control and Nature of Dormant States

The most important question still remaining concerns the mode of

operation of the inhibitors and the promoters. It is still a largely

unanswered question, various investigators proposing various mechanisms.

It is now generally accepted that dormancy does not represent a total

inhibition of cellular activity, but rather a "change in direction" of the

growth process (19). Young et. al. (41) speak of "certain morphological

manifestations inherently induced by the environment." Thus, strictly

speaking, the terms "inhibitor" and "promoter" are inaccurate. They

derive from the earlier belief that the inhibitor acts as a hormone

which stops all growth and development. We have seen though that the

cells still respire, grow, and develop.

21

Further evidence that dormancy is not a simple shutdown of cellular

activity is provided by Esashi and Leiopold (57). In fact, they show

that dormancy is a programmed part of the plant's life cycle. When a

total gene repressor is applied before dormancy, a dormant state is

not attained. This result is not to be expected if dormancy is due to a

lack of essential elements in the metabolic activity of the cell. It

suggests that active transcription of genetic material is necessary for

the initiation of rest.

Given that the onset of dormancy is an active process, how is it

accomplished? There are two general categories into which auggestions

fall. First, the inhibitor/promoter pair affects the availability of

substances necessary for growth. Alternately, the pair is hypothesized

to interact with substances already present. Note, however, that if we

assume ABA to be the inhibitor, then, as Wareing et al. (130) suggest,

its simple structure may enable it to act in more than one enzyme

system. No single mechanism need be hypothesized, even for a single

inhibitor.

The inhibitor/promoter pair may act as hormones or as participants

in intermediate metabolic reactions (104). Such theories suggest that

ABA, as the inhibitor, interferes with the metabolic reactions of growth

or that promoter is necessary for such reactions to continue. On the

other hand the promoter may interfere with dormant metabolism, ABA may

enhance it. Specific suggestions include

(a) allosteric binding of inhibitor and promoter on a key growth

enzyme (91, 130).

22

(b) enzyme activation roles of the inhibitor and promoter

(65, 75) and

(c) interaction with cell membranes, controlling the availability

of enzymes, substrates, etc. (75).

Any of these means alone does not seem capable of explaining the "change

in direction" of metabolic activity seen upon entry to dormancy.

Zelniker (142) proposed that the lack of growth is due to a

deficiency in high energy phosphate bonds. Whether this is due to a

speculated effect of the e complex (i.e., the uncoupling of phosphorylation

and the electron transport system) (81) or due to the utilization of

the tyrosinase pathway rather than the cytochrome oxidase system

(Todd, 1953, in (30», the result is less ATP production. Chaudhry

et. al. (30) suggest that high temperature variation in spring causes

renewed production of the high energy bonds. Lack of energy may be

sufficient to explain the cessation of visible growth and may thus occur

in the subapical meristem cells, but it cannot account for the active

metabolism of the apical meristem cells. Some other mechanism(s) must

be involved.

For the reasons mentioned above, many authors have focused

attention on promoter/inhibitor control of protein synthesis. Control

of protein synthesis can arise from action at the transcription or

translation stages or after translation into polypeptides. Some

investigations (3, 31, 126) indicate that control may occur after

transcription, probably at the translation stage. Most, however,

concentrate on the block to transcription hypothesized as a function of

the inhibitor. Some possibilities include:

23

(a) action of the inhibitor/promoter as an inactivator/activator

pair for DNA or RNA polymerase (91, 128), or

(b) effect of the inhibitor on the availability of ribonucleoside

triphosphates (117).

But again, to reflect the selective change in metabolic activity, most

investigators have hypothesized that ABA and the promoter function only

on specific genes or RNA's (3, 31, 67, 119, 121, 123, 128). Chaudhry

et al. (30) have reviewed a number of proposals, specifically for

Pyrus Communis, and have concluded that the primary factor in dormancy

control is that identified by Tuan and Bonner (117), repression of DNA

and consequent lack of protein synthesis. Such repression may be due

to histones (67) or to other phosphoproteins (30) (see Fig. 3). In

neither of these cases is it clear how an inhibitor fits in. On the other

hand, Jarvis (67) postulates an operon system (Fig. 4).

Indeed, the manner in which the inhibitor-promoter pair interact is

a matter of some speculation. Apart from the opposing effects of

allosteric binding mentioned above, there have been a number of

suggestions. We have already touched upon the ideas of Khan (70). He

states that studies indicate that ABA and cytokinins act at the same

level (RNA transcription, for example) but that this is not known for

certain. It has been suggested (130) that ABA and a promoter (perhaps

cytokinins in the case of apples) affect the biosynthesis of one

another. It has even been proposed that GA and ABA arise from the same

precursor (3). A fact about cytokinins that may be important is that

many tRNA's actually have cytokinins incorporated into them (58). Any

interaction between ABA and cytokinins would then manifest itself in

24

blockage of tRNA formation and subsequent lack of protein synthesis.

The observation (114) that cytokinin levels, not ABA levels, are crucial

to break of dormancy is perhaps tied to this point.

Other observations about inhibitor-promoter interactions are

numerous. Chrispeels and Varner (31) suggest that GA and ABA act

antagonistically (but neither competitively nor noncompetitively).

They postulate the promotive effect of GA on the transcription of

specific mRNA's and the inhibitory effect of ABA to stop that synthesis.

Kawase (69) notes that application of GA decreases inhibitor levels

in vivo in Malus sylvestris. This may be accomplished via hydrolytic

and proteolytic enzymes (which are affected by GA (138», increasing

the permeability of cell membranes. Increased water content dilutes

the ABA. Zelniker (142) noted that for rapid synthesis of nucleic

acids, the cell must reach certain levels of hydration: at 65%

hydration RNA content increases (coinciding with initiation of growth),

at 71% DNA content increases (coinciding with growth maximum).

In a note of caution, Lan~ (75) and Chrispeels and Varner (31)

state that all the results indicating action by inhibitors or promoters

on the protein-synthesizing system are open to reinterpretation. These

authors recognize the fact that the most that can safely be said now is

that nucleic acid and protein synthesis must proceed at certain rates

for the hormones to exert an influence.

v. Conclusions

Given our present knowledge, and the lack of conclusive evidence for

any particular theory of dormancy regulation, it is certainly not

25

possible to propose a general correspondence between the phases of

dormancy as espoused by Vegis (122) or Samish (107) and the endogenous

mechanisms of control. It is possible that observations of empirical

behavior will allow one to decide between likely proposals. That is,

any promising candidate among models of internal, biochemical action

must at least be amenable to such tests.

Little work has been done to try to reconcile environmental

influences and endogenous controls. Those that have are sketchy~ For

instance, Moustafa (88), working with peach seeds, states that cold

(stratification) inactivates auxin oxidase, causing an increasee in

auxin concentration, more RNA synthesis and consequent de~ synthesis

of lipase. Also hypothesized is a rather more vague effect of cold on

the removal of an inhibitor of a-oxidation of fatty acids. As cited

above Jarvis (67) suggests that chilling "potentiates" GA synthesis,

which proceeds later with increased temperatures. More generally,

El-Mansy and Walker (48) state that cold inactivates some inhibitors

and stimulates synthesis and release of enzymes necessary for the

degradation of metabolites. Much more work needs to be done in this

area. Any modeling attempt will necessarily be quite speculative on the

point of environmental-endogenous control interaction.

Several empirical models have been proposed (72, 100, 101). They

deal with the action of chilling in the break of dormancy and subsequent

growth. Amen (5) has proposed a model of seed dormancy; it deals only

superficially with external influences. If his ideas can be fleshed

out, a useful basis for a model of bud dormancy may be available. His

basic scheme is as pictured in Fig. 5.

26

In closing, we would like to point out those areas or ideas that

we believe to be most worth further investigation. The modeling attempt

by Amen (5) should be expanded and experimental results obtained to

corroborate his ideas. The work of Khan (70) on the interaction of

three or more hormones all involved in dormancy control appears to have

much promise. A point that does not seem to be adequately emphasized is

the distinction between apical and subapical meristem cells. It may

turn out that these two areas have distinct hormonal control mechanisms.

The major weak point of course is the absence of a link between

environment and biochemical workings. Work here is needed.

A

TEMPERATURE

MAX

MIN

27

B

......... ',J'" ....,.

QUIESCENCE PRE-LIMINARY

REST

'INNATE AFTER QUIE ENCEREST REST

TIME

TEMPERATURE

Fig. 1.

MIN

TIME

Fig. 2

GIBBERELLIN

+

+

+

+

CYTOKININ

+

+

+

+

INHIBITOR

+

+

+

+

28

GERMINATION

Pi' SUGAR, AMINO ACIDS, ETC.

TEMPERATURE

. -a4....-- (CRITICAL

VARIATION) rATP

DNA.PHOSPHOPROTEIN

COMPLEX

DNA + Protein

29

ATP

ATP -

ATP_

Fig. 3

RENE.WED

PLANT GROWTH &

DEVElOPMENT

- e

mRNA's

REGULATORI REPRESSIO~--:lTOR U !TRulL GENts

- ..

Fig. 4

DEREPRESSIONPROK>TER

OPERON

Co.)

o

e e e

germinationinhibitor

glutenin boundpolymerase

releasingenzyme

proteinase activation(trypsin)

protein hydrolysis

~proteinases

/

· I ~ inhibitor inactivation~

Phy tochrome-L gibberell in ~ pro teolytic enzymes\(trigger agent) (germination ag~ (releasing enzymes) ~.--iir-------rlr.,_~_~~~,r..~_~~~,r._~_~\\""""-------------'i

germination -../ ., /inhibitor I

cellulase

~wall hydrolysis

~Germina tion -< structural

~starch hydrolysis

1~~ / derepressing

(activation) 1 i id h d 1 i enzymenuc e c at y ro ya a ~ Jrepressed

a-amylase a-amylas~ mRNA DNA~DNA

(synthesis stimulation)1 I histone, , ~ ribonuclease actinomycin-D

Fig. 5 w....

.'32

VI. BIBLIOGRAPHY

1. Abbott, D. L., 1962, "The Effect of Four Controlled Winter Tempera­tures on the Flowering and Fruiting of the Apple", J. Hort.ScL 37:272.

2. Abbott, 0., 1923. "Chemical Changes at Beginning and Ending of RestPeriod in Apple and Peach", Bot. Gaz. 76:767.

3. Addicott, F. T. and J. L. Lyon. "Physiology of Abscisic Acid andRelated Substances", Ann. Rev. Plant Physio1. 20: 139.

4. Ahmad, S. M. and P. Mathew, 1977. "Interaction Between Indole-3-AceticAcid and Gibberellic Acid on Overcoming Bud Dormancy of Mulberry",Ind. J. Plant Physiol. !!:296.

5. Amen, R. D., 1968. "A Model of Seed Dormancy", Bot~ Rev. 34:1.

6. Anderson, J. L., G. L. Ashcroft, E. A. Richardson, J. F. Alfaro,R. E. Griffin, G. R. Hanson and • Keller. "Effects of EvaporativeCooling on Temperature and Development of Apple Buds", J. Am.Soc. Hort. Sci. 100:229.

7. Anstey, T. H., 1966. "Prediction of Full Bloom Date for Apple, Pear,Cherry, Peach and Apricot from Air Temperature Data;, Prqc. Am.Soc. Hort. Sci. 88:57.

8. Arias, o. and J. Crabbe, 1975. "Alterations de L'etat de DormanceUlterieur des Bourgrons Obtenues par Diverses Modalites deDecapitation Estival, Realisees sur de Jeunes Plants de PrunusAvium L.", C. R. Acad. Sci. Paris 280:2449.

9. Arias, O. and J. Crabbe, 1975.Rameau d'un an des VegetauxPhys. Veg. 13(1):69-82.

"Les Gradients Morphogenetiques deLigneux, en Repos Apparent",

10. Bachelard, E. P. and F. Wightman 1973. "Biochemical and PhysiologicalStudies on Dormancy Release in Tree Buds. I. Changes in Degreeof Dormancy, Respiratory Capacity, and Major Cell Constituentsin Overwintering Vegetative Buds of Populus Balsamifera",Can. J. Bot. 51:2315.

11. Bagni, N., G. Marino, P. Torrigiani and S. Audisio 1977. "Levelsand Aggregation of Ribosomes During Dormancy and Dormancy Break ofPeach Flower Buds", Physio1. Plant. 39: 165-168.

12. Barnola, P., P. Champagnat, and S. Lavarenee, 1976. "Taille en Vertdes Rameaux et Dormance des Bourgeons Chez le Noisetier", C. R.Acadeomie d'Agriculture de France 62:1163.

33

13. Barskaya, E. N. and E. Z. Oknina, 1959. "The Role of Nucleic Acidsin the Processes of Growth and Bud Development of Fruit Crops",Sov. Plant Physiol. ~:470.

14. Barthe, Ph., 1979. "Bound and Free Cytokinin Levels in Dormant andAfter-Ripened Embryos of Pyrus Malus L. cv. Golden Delicious",A. Pflanzenphysiologie 95(2):771.

15. Barthe, Ph. and C. Bulard, 1978. "Bound and Free Abscisic AcidLevels in Dormant and After-Rip'ened Embryos of Pyrus MulusL. cv. Golden Delicious", Z. fur Pflanzenphysiologie 90:201.

16. Bauer, M. et al. 1976. '~ffects of Evaporative Cooling DuringDormancy on 'Redhaven' Peach Wood and Fruit Bud Hardiness",J. Am. Soc. Hort. Sci. 101(4):452-454.

17. Bell, H. P., 1940, "Winter Growth in the Vegetative Buds of theWagener Apple", Can. J. Res. 18:585.

18. Beriashvili, 1. G., 1979. "Spunk Formation of the Walnut Tree",Soobshch. Akad. Nauk Grnz. SSR 93(9):681.

19. Blommaert, K. L. J. 1955. "The Significance of Auxin and GrowthInhibiting Substances in Relation to Winter Dormancy of thePeach Tree", U. of S. Afr. Dept. Agr. Sci. Bull. 368.

20. Blommaert, K. L. J. 1959. "Winter Terperature in Relation toDormancy and the Auxina nd Growth Inhibitor Content of PeachBuds", S. Afr. J. Agr. Res. 2:507.

21. Borkowska, B. 1976. "Variations in Activity of Cytokinin-likeCompounds in Apple Buds During Release from Dormancy",Biochem. Physiol. Pflanzen 170:153.

22. Borkowska, B. and L. E. Powell, 1979. "The Dormancy Status of AppleBuds as Determined by an In Vitro Culture System ': J. Amer.Soc. Hort. Sci. 104:796.

23. Bradford, F. C., 1922. "Relation of Temperature to Blossoming inthe Apple and the Peach", Mo. Agr. Expt. Stn. Res. Bull. 53.

24. Broome, o. and R. H. Zimmerman, 1976. "Breaking Bud Dormancy inTea Crabapple with Cytokinins", J. Am. Soc. Hort. Sci. 101:28.

25. Brown, D. S., 1965. "The Relation of Temperature to the Growth ofApricot Flower Buds", Proc. Am. Soc. Hort. Sci. 75:138.

26. Brown, D. S. and J. F. Abi-Fadel, 1953. "The Stage of Development ofApricot Flower Buds in Relation to Their Chilling Requirement",Proc. Am. Soc. HOrt. Sci. 61:770.

34

27. Brown, D. S. and F. A. Kotob, 1957. "Growth of Flower Buds ofApricot, Peach" and Pear During the Rest Period", Proc. Am.Soc. Hort. Sci. 69:158.

28. Chandler, W. H. and W. P. Tufts, 1933. "Influence of the Rest Periodon Opening of Buds of Fruit Trees in Spring and on Developmentof Flower Buds of Peach Trees", Proc. Am. Soc. Hort. Sci.30:180.

29. Chandler, W. H., M. H. Kimball, G. I. Philp, W. P. Tufts, and G. P.Weldon, 1937. "Chilling Requirements for Opening of Budson Deciduous Orchard Trees and Some Other Plants inCalifornia", Calif. Agr. Expt. Stn. Bull. 611:1.

30. Chaudhry, W. M., T. C. Broyer, and L. C. T. Young, 1970. "ChemicalChanges Associated with the Breaking of the Rest Period inVegetative Buds of Pyrus Communis", Physiol. Plant. 23:1157.

31. Chrispeels, M. J. and J. E. Varner, 1967. "Hormonal Control ofEnzyme Synthesis: On the Mode of Action of Gibberellic Acidand Abscisic in Aleurone Layers of Barley", Plant Physiol.42:1008.

32. Chvojka, L., M. Travn!tek and M. Zakou¥i1ova, 1962. "The Influenceof Stimulating Doses of 6-Benzylaminopurine", BiologiaPlantarium 4:203.

33. Corgan, J. N. and G. C. Martin, 1971. "Abscisic Acid Levels inPeach Floral Cups", HortScience 6:405.

34. Corgan, J. N. and C. Peyton 1970. "Abscisic Acid Levels in DormantPeach Flower Buds", J. Am. Soc. Hort. Sci. 95:770.

35. Couvillon, G. A. and G. H. Hendershott, 1974. "A Characterization ofthe 'After-Rest' Period of Flower Buds of Two Peach Cultivarsof Different Chilling Requirements", J. Amer. Soc. Hort.Sci. 99:23.

36. Crabbe, J. and H. Lakhoua, 1978. "Shoot Bending and Gravimorphismin Apple Tree Demonstration of Gravimorphic Effects onIsolated Buds, After Their Induction in Various Conditions",Annales des Sciences Naturilles, Botantque, (Ser. 12) 19:125.

37. Craker, L. E., L. V. Gusta and C. J. Weiser, 1969. "SolubleProteins and Cold Hardiness of Two Woody Species", Can. J.Plant Sci. 49:279.

38. Davison, R. M. and H. Young, 1974. "Seasonal Changes in the Levelsof Abscisic Acid in Xylem Sap of Peach", Plant ScienceLetters 2:79.

35

39. Dawidowicz-Grzegorzewska and Zarska-Maciejewska, 1979. "Anatomy,Histochemistry, and cytology of Dormant and Stratified AppleEmbryos", New Phytol. 83(2):385.

40. Dennis, F. G. and L. J. Edgerton, 1961. "The Relationship Betweenan Inhibitor and Rest in Peach Flower Buds", Proc. Am. Soc.Hort. Sci. 77:107.

41. Denny, F. E. and E. N. Stanton, 1928. "Localization of Response ofWoody Tissues to Chemical Treatments that Break the Rest Period",Am. J. Bot. 15:337.

42. Dhanvantari, B. N., 1978. "Cold Predisposition of Dormant PeachTwigs to Nodal Cankers Caused by Levcostoma SPP",Phytophathology 68: 1779-1783.

43. Eady, F. C. and G. W. Eaton, 1972. "The Role of Gibberellic Acidand Gibberellin-like Substances in the Dormancy of theCranberry", Can. J. Plant Sci. 52:263.

44. Eggert, F. P., 1951.and Other FruitSoc. Hort. Sci.

"A Study of Rest in Several Varieties of AppleSpecies Grown in New York State", Proc. Am.57:169.

45. Eggert, F. P., 1953. "The Auxin Content of Spur Buds of Apple asRelated to the Rest Period", Proc. Am. Soc. Hort. Sci. 62:191.

46. Ekelund, R. 1977. "Changes in Ribonuclease Activity and Content ofRNA During Dormancy of Growth of Bird-Cherry Buds", Physiol.Plant 40:244-249.

47. Ekelund, R. and L. Persson, 1977. "Change in ElectrophoreticPattern of Ribonuclease During Dormancy and Growth of Bird­Cherry Buds", Physio1. Plant. 40:280.

48. El-Mansy, H. I. and D. R. Walker, 1969. "Seasonal Fluctuations ofAmino Acids, Organic Acids, and Simple Sugars in 'Elberta'Peach and 'Chinese' Apricot Flower Buds During and After Rest",J. Amer. Soc. Hort. Sci 94:184.

49. El-Mansy, H. 1., and D. R. Walker, 1969. "Seasonal Fluctuation ofFlavanones in 'Elberta' Peach Flower Buds During and Afterthe Termination of Rest", J. Am. Soc. Hort. Sci. 94:298.

50. Erez, A. 1979. "The Effect of Temperature on the Activity of Oil +DNOC Sprays to Break the Rest of Apple Buds", Hortscience14(2):141-142.

36

51. Erez, A., G. A. Couvillon and C. H. Hendershoot, 1979. "QuantitativeChilling Enhancement and Negation in Peach Buds by HighTemperatures in a Daily Cycle", J. Amer~ Soc. Hort. Sci.104:536.

52. Erez, A., G. A. Couvillon, and C. H. Hendershoot, 1979. "The Effectof Cycle Length on Chilling Negation by High Temperatures inDormant Peach Leaf Buds", J. Amer. Soc. Hort. Sci. 104:573.

53. Erez, A., G. A. Couvillon, and S. J. Kays, 1980. "The Effect ofOxygen Concentration on the Release of Peach Leaf Buds fromRest", Hort. Science 15:39.

54. Erez, A. and S. Lavee, 1971. "The Effect of Climatic Conditions onDormancy Development of Peach Buds. I. Temperature", J~ Amer.Soc. Hort. Sci. 96:711.

55. Erez, A., S. Lavee, and R. M. Samish, 1968. "The Effect ofLimitation in Light During the Rest Period on Leaf Bud Break ofthe Peach", phys. Plant. 21:759.

56. Erez, A., R. M. Samish, and S. Lavee, 1966. "The Role of Light inLeaf and Flower Bud Break of the Peach", Physio1. Plant 19: 650.

57. Esashi, Y., and A. C. Leopold, 1969. "Regulation of the Onset ofDormancy in Tubers of Begonia Evansiana", Plant Physiol.44:1200.

58. Fox, J. E. and C. M. Chen, 1968. In Biochemistry and Physiology ofPlant Growth Substances, p. 777. Wightman, F. and G. Setterfield,Eds., The Runge Press Ltd., Ottawa, Canada.

59. Fulford, R. M., 1965. "The Morphogenesis of Apple Buds 1. TheActivity of the Apical Meristem", Ann. Bot., 29:167.

60. Hatch, A. H. and D. R. Walker, 1969. "Rest Intensity of DormantPeach and Apricot Leaf Buds as Influenced by Temperature,Cold Hardiness and Respiration", J. Amer. Soc. Hort. Sci. 94:304.

61. Hemberg, T., 1949. "Growth-Inhibiting Substances in Terminal Budsof Fraxinus", Phys. Plant. ~:37.

62. Hendershott, C. H. and L. F. Bailey, 1955. "Growth InhibitingSubstance in Extracts of Dormant Flower Buds of Peach",Proc. Am. Soc. Hort. Sci. 65:85.

63. Hendershott, C. H. and Walker, D. R., 1959. "Seasonal Fluctuation inQuantity of Growth Substances and Resting Peach Flower Buds",Proc. Am. Soc. HOrt. Sci. 74:121.

37

64. Hewett, E. W. and P. F. Wareing, 1973. "Cytokinins in Populus xRobusta: Changes During Chilling and Bud Burst", Physiol.Plantarium 28:393.

65. Hodgson, F. R., 1924. "Observation on the Rest Period ofDeciduous Fruit Trees in a Mild Climate", Proc. Am. Soc.Hort. Sci. 20:151.

66. Hull, J. and L. N. Lewis, 1959. "Response of One-Year-Old Cherryand Mature Bearing Cherry, Peach, and Apple Trees toGibberellin", Proc. Am. Soc. Hort. Sci. 74: 93.

67. Jarvis, B. C., B. Frankland, and J. H. Cherry, 1968. "IncreasedNucleic-Acid Synthesis in Relation to the Breaking of Dormancyof Hazel Seed by Gibberellic Acid", Planta 83:257.

68. Jonkers, H., 1979. "Bud Dormancy of Apple and Pear in Relation tothe Temperature During the Growth Period", ScientiaHorticulturae 10:149.

69. Kawase, M. 1966. "Growth-Inhibiting Substance and Bud Dormancy inWoody Plants", Proc. Am. Soc. Hort. Sci. 89:752.

70. Khan, A. A., 1975. "Primary, Preventive and Permissive Roles ofHormones in Plant Systems", The Botanical Review 41:391.

71. Lake, J. V. 1956. "The Effect of Microclimatic Temperature Uponthe Date of Flowering of the Apple", J. Hort. Sci. 31:244.

72. Landsberg, J. J., 1974. "Apple Fruit Bud Development and Growth;Analysis and an Empirical Model", Ann. Bot. 38:1013.

73. Landsberg, J. J., D. R. Butler and M. R. Thorpe, 1974. "AppleBud and Blossom Temperatures", J. Hort. Sci. 49:227.

74. Lane, R. E. C., H. O. Jackson and F. D. Stroud, 1977. "Influence ofPeach Seedling Rootstocks on Defoliation and Cold Hardiness ofPeach Cultivare", J. Am. Soc. Hort. Soc. lQ.2(l):89-92.

75. Lang, A. 1967. "Plant Growth Regulation - Report of a Meeting",Science 157:589.

76. Lenton, J. R., V. M. Perry and P. F. Saunders, 1972. '~ndogenous

Abscisic Acid in Relation to Photoperiodically Induced BudDormancy", Planta 106:13.

77. Leshem, Y., S. Philosoph, and j. Wurzburger, 1974. "Glycosylation ofFree Trans-Trans Abscisic Acid as a Contributing Factor in BudDormancy Break", Biochem. and Biophys. Res. Comm. 57:526.

38

78. Letham, D. S. and M. W. Williams, 1967. "Regulations of Cell Divisionin Plant Tissues VIII. The Cytokinins of the Apple Fruit",Physiologia Planatarum 22:925.

79. Li. P. H. and C. J. Weiser, 1967. "Evaluation of Extraction andAssay Methods for Nucleic Acids from Red-Osier Dogwood andRNA, DNA, and Protein Changes During Cold Acclimation",Proc. Am. Soc. Hort. Sci. 91:716.

80. Luckwill, L. C. and P. Whyte, 1968. "Hormones in the Xylem Sap ofApple Trees", S. C. 1. Monograph 3l:87~

81. Marinos, N. G. and T. Hemberg, 1960. "Observations on a PossibleMechanism of Action of the Inhib.itor-a Complex," Physiol.Plant 13:571.

82. Mathur, S. N., N. K. Saah, and V. Gupta, 1979. "Break of Winter Bud­Dormancy with Gibberellic Acid and Surgical Treatment in SomeDeciduous Trees of Kathmandu", J. Ind. Bot. Soc. 58:229.

83. Mauget, J. C. 1976. "Sur la Dormance des Bougeons Vegetatifs deNoyer", C. R. Acad. Sc. Paris, Ser D 283:399.

84. Mauget, J. C. 1978. "Influence d'une Ablation Totale du Feuillagesur L'entree en Dormance des Bourgeons du Noyer", C. R. Acad.Sci. Paris 286:745.

85. Mielke, E. A. and F. G. Dennis, 1975. "Hormonal Control of FlowerBud Dormancy in Sour Cherry. I. Identification of AbscisicAcid", J. Am. Soc. Hort. Sci. 100(3):285-287.

86. Mielke, E. A. and F. G. Dennis 1975. "Hormonal Control of FlowerBud Dormancy in Sour Cherry. II. Levels of Abscisic Acid andits Water Soluble Complex", J. Amer. Soc. Hort. Sci. 100:287.

87. Mielke, E. A. and F. G. Dennis, 1978. "Hormonal Control of FlowerBud Dormancy in Sour Cherry. III. Effects of Leaves,Defoliation and Temperature on Levels of Abscisic Acid inFlower Primordia", J. Am. Soc. Hort. Sci. 103:446.

88. Moustafa, S. M. 1979. "The Effect of Chilling on the ActivityLevels of Auxin Oxidase, Lipase, and Fatty Acids of DormantPeach Seeds", J. Ind. Bot. Soc. 58:50.

89. Nichols, D. G., D. L. Jones and W. K. Thompson, 1974. "Effects ofAutumn on the Induction of Dormancy in Apple and Peach Seedlingsand Their Subsequent Regrowth in Spring", Aust. J. Agric.Res. 25:899.

39

90. Nooden, L. D. and J. A. Weber, 1978. "EnvirQnmental and HormonalControl of Dormancy in Terminal Buds of Plants" in Dormancy andDevelopment Arrest, M. E~ Clutter, ed., pp. 22l-268~ Acad.Press, New York.

91. van Overbeek, J., J. E. Loeffler, M. 1. R. Mason, 1967. "Dormin(Abscisin II), Inhibitor of plant DNA Synthesis?", Science156:1497.

92. Overcash, J. P. and J. A. Campbell, 1955. "The Effects of IntermittentWarm and Cold Periods on Breaking the Rest Period of Peach LeafBuds", Proc. Am. Soc. Hort. Sci. 66:87.

93. Perry, T. o. 1971, "Dormancy of Trees in Winter", Science 171:29.

94. Philips, 1. D. J., 1962. "Some Interactions of Gibberellic Acidwith Naringenin in the Control of Dormancy of Growth in Plants",J. Expt. Bot. 13 213.

95. Proebsting, E. L. and H. H. Mills, 1967. "A Standardized Temperature­Survival Curve for Dormant Elberta Peach Fruit Buds", Proc. Am.Soc. Hort. Sci., 91:85.

96. Proebsting, E. L. and H. A. Mills, 1978. "A synoptic Analysis ofPeach and Cherry Flower Bud Hardiness", J. Am. Soc. Hort.Sci. 103(6):842.

97. Proebsting, E. L. and H. H. Mills, 1978. "Low Temperature Resistanceof Developing Flower Buds of 6 Deciduous Fruit Species", J. Am.Soc. Hort. Sci. 103(2):192.

98. Rageau, R., 1978. "Croissance et Debourrement des BourgeonsVegetatifs de Pecher au cours d'un Test Classique de Dormance",C. R. Acad. Sc. Paris, 287:1119.

99. Ramsey, J. and G. C. Martin, 1970. "Seasonal Changes in GrowthPromoters and Inhibitors in Buds of Apricot", J. Am~ Soc. Hort.Sci. 95: 569.

100. Richardson, E. A., Schyuler D. Seely, and D. R. Walker, 1974. "A Modelfor Estimating the Completion of Rest for 'Redhaven' and 'Elberta'Peach Trees", Hort. Sci. 9:331.

101. Richardson, E. A., S. D. Seely, D. R. Walker, J. L. Anderson, andG. L. Ashcroft, 1975. "Pheno-Climatography of Spring PeachBud Development", Hort. Sci. 10:236.

102. Robitaille, H. A., 1975. "Stress Ethylene Production in Apple Shoots",J. Am. Soc. Hort. Sci. 100:524.

, ..40

103. Rom, R. C. and E. H. Arrington,. 1966. "The Effect of VaryingTemperature Regimes on Degree-Days to Bloom in the ElbertaPeach", Proc. Am. Soc. Hort. Sci. 88: 239 ~

104. Romberger, J. A., 1963. Meristems, Growth,· and ·Development in WoodyPlants U.S.D.A. Tech. Bull. No. 1293.

105. Rye, M. and Lewak, S., 1980. "Role of Abscisic-Acid in Regulationof Some Photosynthetic Enzyme Activities in Apple Seedlings inRelation to Embryonal Dormancy", Zeitschrift fUrPflanzenphysiologie 96(3):195.

106. Saks, Y., M. Megbi and 1. Ilan, 1980. "Involvement of NativeAbscisic Acid in the Regulation of Onset of Dormancy inSpirodela Phlyrrhiza", Aust. J. Plant Plysiol. z: 73-9.

107. Samish, R. M., 1954. "Dormancy in Woody Plants", Ann. Rev. PlantPhysiol. 5:183.

108. Schmid, P. P. S. and Feucht, W. 1980. "Isoelectric Focusing ofProtein and Some Enzymes from Secondary Phloem of CherryGrafting - Combinations. III. Enzymes, .EspeciallyPeroxidases", Scientia Horticulturae 12:69~

109. Semin, V. S. and V. 1. Madis, 1964. "On the Dormant Period in Plants",Sov. Plant. Physiol. 11:243.

110. Siminovitch, D., J. Singh, and 1. A. de la Roche, 1975. "Studieson Membranes in Plant Cells Resistant to Extreme Freezing.I. Augmentation of Phospholipids and Membrance Substancewithout Changes in Unsaturation of Fatty Acids During Hardeningof Black Locust Bark", Cyrobiology 12:144.

111. Smith, H. and N. P. Kefford, 1964. "Tlle Chemical Regulation of theDormancy Phases of Bud Development". Am. J. BoL 51: 1002.

112. Sparks, D. 1976. "Effect of Sub-freezing Temperatures on BudBreak of Pecan", HortScience .!!.:4l5~

113. Spotts, R. A. and D. C. Ferree, 1979. "Effect of a DormantApplication of Surfactants on Bud Development and DiseaseControl in Selected Deciduous Fruit Plants". HortScience14(1):38-39.

114. Taylor, J. S. and E. B. Dumbroff, 1975. "Bud, Root, and Growth­Regulator Activity in Acer Saccharum During the DormantSeason", Can. J. Bot. 53:321.

115. Thompson, W. K., D. L. Jones and D. G. Nichols, 1975.Dormancy Factors on the Growth of Vegetative BudsApple Trees", Aust. J. Agric. Res. 26:989.

"Effects ofof Young

• I" 41

116. Tinklon, 1. G. and W. W. Schuuabe, 1970. "Lateral Bud Dormancy in theBlackcurrent", Ann. Bot. 34: 691.

117. Tuan, D. Y. H. and J. Bonner, 1964. "Dormancy Associated withRepression of Genetic Activity", Plant Physio1.", 39: 768.

118. Tumanov, 1. 1., G. V. Kuzina and L. D. Karnikova, 1973. "The Periodof Dormancy and Ability of Woody Plants to be Hardened by LowTemperatures", Sov. Plant Physio1. 20: 1.

119. Tyurina, M. M., 1979. "Development of Ideas about the State ofDormancy in Woody Plants", Sov. Plant Physio1. 26:729.

120. Tyurina, M. M., G. A. Gogo leva , A. S. Jegurasdova and T. G.Bulatova, 1978. "Interaction Between Development of FrostResistance and Dormancy in Plants", Acta Horticulturae 81:51.

121. Varner, J. E., G. R. Chandra and M. J. Chrispeels, 1965."Gibberellic Acid-Controlled Synthesis of a-Amylase in BarleyEndosperm", J. Cell. and Compo Physio1. 66 (Supplement}:55.

122. Vegis, A. 1964. "Dormancy in Higher Plants", Ann. Rev. PlantPhysio1. 15: 185.

123. Villiers, T. A., 1968. "An Autoradiographic Study of the Effect ofthe Plant Hormone Abscisic Acid on Nucleic Acid and ProteinMetabolism", Planta 82:342.

124. Walker, D. R., 1970. "Growth Substances in Dormant Fruit Buds andSeeds", HortScience 1,:414.

125. Walker, D. R. and C. W. Donoho, 1959. "Further Studies on the Effectof Gibberellic Acid on Breaking the Rest Period of Young Peachand Apple Trees", Proc. Am. Soc. Hort. ScL, 74:87.

126. Walton, D. C., G. S. Soo£1, and E. Sondheimer. 1970. "The Effectsof Abscisic Acid on Growth and Nucleic Acid Synthesis inExcised Embryonic Bean Axes", Plant Physio1. 45:37.

127. Wareing, P. F., 1953. "Growth Studies in Woody Species V.Photoperiodism in Dormant Buds of Fagus Sylvatica L."Physio1. Plant. ~:692.

128. Wareing, P. F., 1954. "The Control of Bud Dormancy in Seed Plants"in Dormancy and Survival, H. W. Woolhouse, ed., pp. 241-262.Symp. Soc. Exp. BioI. No. XXIII. Cambridge Univ. Press,London.

129. Wareing, P. F., 1954. "Growth Studies in Woody Species VI. TheLocus of Photoperiodic Perception in Relation to Dormancy",Physiol. Plant. 2:261.

4 •.,.

42

130. Wareing, P. F., J. Good and J._Manue1, 1968. "Some PossiblePhysiological Roles of Abscisic Acid" in Biochemistry andPhysiology of Plant Growth Substances, F. Wightman andG. Satterfield, Eds., pp. 1561-1579~ The Runge Press, Ltd.,Ottawa, Canada.

131. Wareing, P. F., and P. F. Saunders 1971. "Hormones and Dormancy",Ann. Rev. Plant Physio1. 22:261.

132. Weinberger, J. H. 1950. "Chilling Requirements of Peach Varieties"and "Prolonged Dormancy of Peaches", Proc. Am. Soc. Hort.Sci. 56:122-133.

133.Weinberger, J. H. 1956. "Prolonged Dormancy Trouble in Peaches inthe Southeast in Relation to Winter Temperatures", Proc. Am.Soc. Hort. Sci. 67:107.

134. Weinberger, J. H. 1967. "Some Temperature Relations in NaturalBreaking of the Rest of Peach Flower Buds in the San JoaquinValley" California", Proc. Am. Soc. Hort. ScL 91:84.

135. Williams, M. W. and E. A. Stahly. 1968. "Effect of Cytokinins onApple Shoot Development from Axillary Buds", HortScience 3:68.

136. Williams, R. R., G. R. Edwards and B. G. Coombe, 1979. "Determinationof the Pattern of Winter Dormancy in Lateral Buds of Apples",Ann. Bot. 44:575.

137. Wolak, R. J. and Couvillon, G. A. 1976. "Time of Thiourea-KN03Application on the Rest Requirement and Bud Development in'Loring' Peach", HortScience .!!(4):400-402.

138. Wurzburger, J. and B-Z Farkash. 1976. "Endogenous Gibberellin LevelDuring Bud Dormancy Break of Diospyros virginiana and itsPossible Effect on Water Content", Plant Science Letters ~:1.

139. Yanke1evich, B. B. and Niko1aeva, M. G., 1975. "Changes of EnzymaticActivity in Apple Seeds under the Influence of Stratification",Sov. Plant. Physio1. 22:535.

140. Yeh, Feng, W. F. Campbell and D. R. Walker, 1974. "UltrastructuralChanges in Peach Flower Buds During Rest", J. Amer. Soc. Hort.ScL 99: 427.

141. Young, L. C. T., J. T. Winneberger and J. P. Bennett. 1974. "Growthof Resting Buds", J. Amer. Soc. Hort. ScL 99:146.

142. Ze1niker, Y. L. 1963. "The Relation Between the Annual GrowthCycle of Arboreal Shoots, Nucleic Acid Contents and WaterRelations of the Growing Points", Sov. Plant Physio1. 10:279.

c ., .....

...

43

143. Zlobina, 1974. "Growth of Buds of Trees and Bushes During the Autumn­Winter-Spring Period," Sov. Plant Physio. 21:712.

144. Gold, H. J., P. Sha,ffer, M. Wann and E. Johnson. 1983. "A modelingstrategy for the orchard ecosystem," N. C. State ARSBulletin (in press).