REGULATION OF BUD-BURST TIMING BY TEMPERATURE AND PHOTOREGIMEs. DURING DORMANCY 1/ Robert K. Campbell r' Hlttra.ot.tf»l

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1 /3/.8 ( i REGULATION OF BUD-BURST TIMING BY TEMPERATURE AND PHOTOREGIMEs. DURING DORMANCY 1/ Robert K. Campbell r' Hlttra.ot.tf»l Abstract,--In Douglas-fir, vegetative buds throughout most 1 or all of the "dormant" period have the potential ability for ex- tension groth in response to temperatures above freezing. In terms of actual groth or development, this ability can be measured as a rate for each temperature ithin the normal range. Thus, potential ability can be conceptualized as a temperature response curve. Data from several experiments suggest that potential abilit an be described by an equation of the form + g x 1/ y = K/ (l+e ); here, 1/ y is developmental rate and is temperature. Other parameters in the equation change continuously during dormancy in response to complex, interacting environmental cues provided by chilling temperature, duration, and time and by photoperiod. Bud-burst timing reflects the groth that accrues during inter and spring in response to temperature pattern and changing potential developmental rates. The model may be appropriate, in principle, for other perennial species. Additional keyords: Douglas-fir, Pseudotsuga menziesii, phenology, chilling, developmental-cycle, groth rhythm. INTRODUCTION., - -. "{:Ill" Many temperate region forests include sites ith groing seasons that are severely restricted by cold or moisture deficit or both. Such sites are often difficult to regenerate naturally or artificially--synchronization of the plant developmental cycle to the seasonal cycle appears to be a critical factor (Eiche 1966), Problems arising from poor synchronization usually can be minimized by the close matching of seed source to plantation site, a compelling reason for the development of seed-transfer rules, As one basis for rules, I have been investigating the annual developmental cycle of Douglas-fir (Pseudotsuga menziesii vat. menziesii [Mirb.] Franco), ith the help of A. I. Sugano. We are concentrating on the system by hich Douglas-fir uses temperature and photoperiod as environmental cues for optimizing timing of the developmental cycle. This paper presents my concept of the system for bud-burst timing, up-dated from a previous report (Campbell and Sugano 1975), but still provisional and incomplete. Supporting examples are dran from several experiments, mainly designed to study the genecology of reaction to environmental cues. Results have been or are being reported elsehere. In this paper, documentation of experimental and analytical procedures is consequently minimal.! / Principal Plant Geneticist, Forestry Sciences Laboratory, Pacific Northest Forest and Range Experiment Station, USDA Forest Service, Corvallis, Oreg.

2 2 Spring timing of bud-burst is an important component of adaptation for many species in north temperate climates, and especially in estern Oregon, particularly in valleys or at high elevations. Plants in valleys are exposed regularly to severe summer droughts. At high elevations frost-free periods are short. In either case, by appropriate timing of bud burst, the risks of spring frost are optimally balanced ith the advantages of an early display of the ne needles--in the valleys, to make best use of spring soil moisture; at high elevations, to maximize the use of arm days for photosynthesis and groth. THE SETTING The bud-burst timing mechanism orks remarkably ell. Spring frost is seldom a problem in more than a small fraction of native populations, in spite of the highly heterogeneous habitat occupied by Coastal Douglas-fir. In alpine areas, the frost-free season may be less than 9 days. In southern Oregon valleys, the total groing-season moisture deficit may reach 45 em and may start to accumulate as early as May 1. Such sites can be found ithin a fe kilometers horizontally, or ithin 1 5 m vertically, of sites ith frostfree seasons of 2 days, or total sumer moisture deficits of only 5-1 em. The year-to-year variation at a site can be as significant as the variation beteen sites. For example, in the Pacific Northest, the standard deviation of the last spring frost of -2.2 C (25 F) ranges from 15 days at high elevations to 27 days in coastal valleys. In a sample of dates of last spring frosts (-2.2 C) obtained from 36 eather stations in estern Washington and Oregon, year-to-year variation accounted for 43% of total variation. Weather stations spanned coastal and alpine regions of 6 of latitude. The system for bud-burst timing in Douglas-fir apparently can preadapt populations to much of the region's heterogeneity. When population samples from different parts of the Pacific Northest are gron together at Corvallis, Oregon, in any given year they ill usually burst buds ithin 2 or 3 eeks of one another. Usually only the earliest individuals of the earliest samples ill be injured by frost. Apparently the system is based mainly on a common response to spring temperatures. Except in years ith highly unusual temperature patterns, photoperiod appears to have a comparatively minor role. For example, some of the population-samples in the illustration above ould have burst buds up to half a month earlier in their native habitats than in Corvallis and some as much as 2 months later. Thus, photoperiods at bud-burst in native habitats ould have been from 1/ 2 hour shorter to 2-1/ 2 hours longer than at bud-burst in Corvallis. Also, for any one of the samples, bud-burst dates in succeeding years at Corvallis may have been separated by a month; i.e., by 1-1/4 hours in photoperiod, Therefore, any effect of photoperiod is probably in the nature of a modification of the basic response. That such modifications exist is indicated by an interaction of bud-burst timing ith year of observation in nursery tests. The year-to-year differences among population-samples depend on the population involved. Since modifications are population-specific, they probably serve to adapt local populntions to deviations from the average temperature pattern.

3 21 The modifications are likely to be fairly complex. Populations are adapted to greatly different temperature distributions ithin the region. For example, if temperatures are eighted according to their probability of occurrence, near the Pacific Ocean more than 8% of temperatures in the period November 15 to April 15 ill fall above 5 C (roughly approximated from probabilities of maximum and minimum temperatures based on 3 years of record; from Wakefield 1969). For Corvallis, the estimate for similar conditions is 6%; in the high Cascades, 12%. Temperatures near the Pacific Coast seldom drop into the severe frost range except in January. Also, they seldom exceed 24 C. In the high mountains, temperatures in December and January rarely exceed 7 C, but in summer months they may reach 35 C and can fall to near C almost any night. THE SYSTEM My idea of the system is based on to postulates: 1) buds during dormancy ill respond to any temperatures above freezing ith continued morphological development, extension groth, or change in physiological activity (Romberger 1963, p ; Perry 1971, p. 32). 2) Environmental cues received by the bud during dormancy act to change the bud' s response to temperature. Dormancy in this paper is defined in the general sense, i.e., the temporary suspension of visible groth; in conifers, the superficially inactive, budscale-covered phase beteen bud-set and bud-burst. At any time during dormancy, a bud has a potential ability to respond to temperatures. In terms of actual development toard bud-bursting, this ability is expressed as a rate for each temperature ithin the range of temperatures normal for the species. Therefore, potential ability at any point in time can be conceptualized as a temperature response curve; quantitatively it is described by a response equation. Each change in potential ability means a change in potential bud-developmental rates throughout the temperature range- hence a change in the temperature response curve. Parameters in the equation describing the curve change continuously as the season progresses from bud-set to bud-burst, in response to cues provided by chilling and photoperiod. Temperature, therefore, influences bud-development in to, partly independent, quantitative ays: 1) In the range -2 C to +12 C, approximately (from Wommack 1964, Sarvas 1974, Campbell and Sugano 1975), temperature acts mainly as information. It induces incremental changes in potential developmental rates (a)--letters in parenthesis here and in the folloing discussion refer to schematic paths in figure 1.

4 22 )9..--.v,... L CELL PHYSIOLOGIC AL ST ATE POTENTIAL DEVELOPMENTAL RATE DEVELOPMENT AND GROWT H Figure 1.--Model of the system by hich temperature and photoperiod during dormancy are used to regulate bud extension-groth and bud-burst timing. 2) Temperatures greater than approximately 12 C act mainly to release energy for groth (c). Temperatures belo 12 C also can release energy for groth, very sloly hen potential developmental rates are lo, and more rapidly after high potential rates have been induced by prolonged chilling. Cool temperatures change the cell physiological state (b) by processes (collectively termed as "induction") still imperfectly knon (Romberger 1963, Smith and Kefford 1964) and outside the scope of this paper. Photoperiod during the flushing period also influences the cell physiological state (d). Photo-effects are also manifested as a change in parameters of the temperature equation. The degree of change depends on the physiological state already induced by chilling. Thus, photoperiod effects are considered here as a modification, secondary to induction by chilling, but still influencing potential developmental rates (e). Some factors hich condition the influence of chilling on potential developmental rates are: 1) The temperature of chilling, 2) the timing of chilling, i. e., hether chilling occurs early or late in dormancy--this may reflect the state of bud development reached before chilling occurs (i) as ell as the passage of time beteen bud-set and chilling (Dormling, et al. 1968, Sugano 1971), and 3) the physiological state already induced by previous chilling (h and j). The photoperiod immediately preceding or during chilling may also influence induction (k) (Lavender and Wareing 1972). MATERIALS AND METHODS Several inter-relations ithin the system are described quantitatively in the next section to illustrate major features of the system and also to suggest its complexity. Relationships, hich are presented graphically, are based on data primarily from three experiments. As noted earlier, these experiments are being reported in detail elsehere. As genetic entries, the three experiments each used samples from the same 23 populations of Douglas-fir, Origins ranged from 42 to 49 N latitude, from

5 23 6 to 1 46 m, and from Pacific Coast to mountain passes on the Cascades crest, all in estern Washington and Oregon, Germinated seed from five open-pollinated, individual tree collections at each location ere planted in pots in March, gron in a greenhouse until May, then transferred to a lathhouse (SO% shade). On or about Oct 1, pots ere put in groth chambers held at a constant 16 C and 9-h daylength until scheduled chilling treatments began. Seedlings ere subjected to various chilling and flushing treatments in groth chambers, starting in November or December. Depending on the experimental objectives, chamber treatments ere factorial combinations of chilling duration and flushing temperatures, or chilling duration and photoregime during flushing, or flushing temperature and photoregime. A caution is necessary. Photoregimes are designated in illustrations by a daylength, e. g, 8 h. This represents the initial photoperiod of a regime in hich photoperiod as increased periodically and regularly. The rate varied from experiment to experiment, from 15 to 6 min/k. Treatment results ere assessed by their effect on length of the flushing period, measured in days to bud-burst from the time a plant entered the flushing treatment. To m1n1m1ze problems associated ith correlated inhibition, only terminal buds ere scored. Data ere analyzed by multiple regression. Then the equation resulting for each experiment as solved for a factorial set of selected values for chilling period and flushing temperature (e.g., chilling 11, 44, 77 days; flushing temperature, 1 14, 18, 22 C) and for provenance topographic vari ables (43, 45.5 and 48 N latitude; 1-, 6-, 1 1-m elevation; 6, 128, 18 km from the ocean). Solutions of equations provided several hundred expected values (e.g., 3 chilling periods x 4 flushing temperatures x 3 latitudes x 3 elevations x 3 distances = 324 values). Values for each chilling- and flushing-period combination ere then averaged over all provenance points in the system (e. g., 3 latitudes x 3 elevations x 3 distances) to provide average effects for chilling, flushing, and photoperiod treatments. Most of the relationships illustrated in this paper are based on calculations using these average effects. Thus, they describe an "average" response of coastal Douglasfir. Responses of individual population-samples deviated significantly from this average response. QUANTITATIVE RELATIONSHIPS With some restrictions that are discussed belo, it is proposed that the energy-release or groth effects of temperature on rates of development toard bud-burst (path c, figure 1) can be described by an empirical logistic equation of the form: a+bx 1/y = K/ (1 + e ); here, 1/y is the developmental rate and represents the average percentage development made by the buds per day at a given temperature x, -! represents the distance in units of developmental rate beteen the upper and loer asymptotes of the curve, represents the relative position of the curve's origin on the abscissa, quantifies the degree of increasing developmental rate ith increasing temperature ' and

6 24 e is the base of the natural logarithm. Davidson (1944) has used this equation-form to express the relationship beteen temperature and speed of development in insects. Sarvas' data (1972, table 5) on pollen-catkin developmental rates in Betula fit the curve form quite precisely (figure 2). 7r--r r Betula pubescens - 6 ;; / -. Y e3.!59-o. 2 ox - 5 R 2 = a:: _J z 3 :: a.. _J 2 > / y : X l+e' R2= TEMPERATURE (C) Figure 2.--Pollen-catkin developmental rates fitted to temperature response equations (rates derived from data by Sarvas 1972). I suggest that, over the normal range of autumn, inter, and spring temperatures, the same equation type describes the groth effects of temperature on bud-development in Douglas-fir. We have not tested this relationship by sampling temperatures as intensively as did Sarvas for lack of facilities; but at temperatures above 12 C, it satisfactorily describes our data. Belo 12 C, e have not yet experimentally separated chilling-induction effects from temperature groth-effects--belo 12 C the actual response to temperature has been greater than ould be expected from the logistic relationship. Partly for this reason, and partly because nonlinear equations are excessively expensive to analyze in complicated designs, e have used a simpler type of equation in published reports. Nevertheless, the logistic equation is probably the correct form; it is used for illustrations in this paper.

7 :: 25 Chilling has a general effect on the ability of buds to respond to flushing temperature; the longer the chilling, the steeper the temperature response curve. Consequently, parameters in the equations describing response to temperature are affected by chilling duration (table 1). Induced differences in developmental rate due to added chilling are more evident at higher temperatures (figure 3). Table 1.--Coefficients of equations used to prepare curves in figures 3 and 6 Curve a/ Parameters- K a b Figure 3 Chilling Figure 6 Photoregime a/ K a b distance in developmental units beteen upper and loer asymptotes. relative position of curve origin on abscissa. degree of change in slope >. ' >. 4=- I- U) :: II I- :::> CD 3 DAYS _j :::> CD OF CHILLING 2 I- U)... _j > 12 TEMPERATURE Figure 3.--The influence of chilling duration (4 C) on potential developmental rates and on days to bud-burst in Douglas-fir (derived from data by Campbell and Sugano, In press).

8 26 It is, hoever, timing of bud-burst, rather than developmental rate, that is the critical factor in natural selection. In this respect, chilling duration has its most important influence on bud-development at loer flushing temperatures. For example, consider Douglas-fir seedlings that have been chilled for 11 days at 4 C. In a constant temperature of 1 C, they are expected to flush in 156 days (figure 3). If chilled for 12 days rather than 11, the expected number of days to bud-burst is decreased by 1.38 days (figure 4). If chilled 11 days and flushed at 2 C, buds are expected to burst in 37 days (figure 3). And, in this case, an added day of chilling hastens bud-burst by only.52 days (figure 4). Even after 77 days of chilling, the expected date of flushing is altered more drastically by additional chilling if buds are flushed in cool temperatures (figure 4). Over short periods, the development of relatively unchilled buds in lo temperatures is almost imperceptible. From 1 to 28 days are required for flushing in temperatures of 1 C and 8 C, respectively, even if buds are flushed in long, 14-h daylengths (figure 3). FLUSHING TEMPERATURE CUMULATIVE DAYS OF CHILLING TO WHICH. THE ADDITIONAL DAY IS ADDED Figure 4.--Decrease in days to bud-burst due to one added day of chilling (4 C) as influenced by length of previous chilling and by flushing temperature. The change in the temperature response curve induced by chilling is influenced by chilling te peratures as ell as by duration. Sarvas (1974) proposed that effects of different temperatures could be described quantitatively by an "optimal" curve, ith a high, median peak and steeply sloping sides. For Betula, the optimum is about 3.5 C. Above and belo 3.5 C, effectiveness falls off sharply so that temperatures of -3 C and +1 C are only about one-tenth as effective as the optimum. Our experience indicates that the actual relationship is far more complex than is implied by the optimal curve. Sarvas' curve form may, hoever, be adequate for some tissues or under some conditions. For example, it may describe the chilling-temperature differences induced by initial stages of chilling. In Douglas-fir, hen a high-elevation provenance as chilled for 2 days starting in October (Campbell and Sugano 1975), potential developmental rates ere

9 ..... increased more by a median temperature (4.4 C) than by higher temperatures (7.4 C and 1. C). The optimal curve may also be appropriate in circumstances here induced rates reflect a summation of the chilling effects of several eeks (Wommack 1964). A less consistent picture emerges hen the total influence of chilling is broken into its incremental daily contributions. In our experiment after the initial 2 days of chilling, 1 C as more effective per added day of chilling than as 4.4 c--an added day of chilling at 1 C decreased days to bud-burst by.74 d3ys compared to.52 days if chilled at 4.4 C (figure 5, b). Plants chilled at 4.4 C ere at higher developmental rates after 2 days, and added chilling may have been less effective for that reason only. Nevertheless, on any given day in fall or inter, chilling at 1 C can be more effective than chilling at 4 C, probably depending on previous conditions a. LOW-ELEVATION b. HIGH- ELEVATION.. <. (JZ CHILLING O::.. 1. TEMPERATURE =>...., "\ co- ' (C) :r "\ cu,.., ::LL...,, ' co ' o '.8,....., 4.4 ' g... ' ~ (Jc.....,4.4 '... ' ow '1. "" 1. zc 1. -<{ Wo:: (J <ta. CHILLING STARTED OCTOBER 4.4 :::(1 u ----CHILLING STARTED DECEMBER 5 c_ e CUMUL ATIVE DAYS OF CHIL LING TO WHICH ADDI TIONAL DA Y IS ADDED Figure 5.--Decrease in days to bud-burst by one added day of chilling at 4.4 C vs. 1. C. The amount of decrease is influenced by the date chilling is started, the duration of previous chilling and the elevation of the populations being sampled (derived from data by Campbell and Sugano 1975). There is some other evidence that the changes induced by chilling are influenced by conditions at the time of chilling, i.e., by the time of season at hich chilling is experienced. In the above experiment, if chilling as started December 5 rather than October 22, 4 C as more effective than 1 C, even after the initial 2 days (figure 5, b), Thus, the relative chilling effectiveness of 4 C and 1 C depended on time of chilling, perhaps because the chilling periods started at different cell physiological-states. It almost certainly started at different stages of bud-development. In Douglasfir, bud morphogenesis continues after bud-set, in some population-samples until mid-december (Allen and Oens 1972). The high-elevation population

10 28 sample used in th s illustrat on had completed bud-set by August 1. In the same experiment, a lo-elevation population-sample had completed bud-set much later, by Septellber 22, Patterns of chilling temperature effectiveness ere quite different for the to samples (compare figures 5, a and 5, b), The shapes of temperature response curves change ith lengthening photoperiods just as they do ith longer chilling periods. This occurs even after buds have been exposed to 14 continuous days of chilling at 4 C (table 1). Also, just as ith chilling effects, photoperiod-effects are more pronounced, in respect to bud-burst timing, hen buds flush in loer temperatures (figure 6). In a photoregime starting at 9 hours, an increase in flushing temperature from 1 C to 11 C decreased days to bud-burst by 5.3 days (figure 7). If the regime started at 13 hours, the same increase in flushing temperature decreased days to bud-burst by only 3.8 days. 5 >. '- 7 a: Q. 6 - (l a: I- 5 i< o Ill 1- (f 4 :: :: 1- :: 12 3 C/u CD _j >- <(W a o 3 :: a: z Z<! CD - o 2 (llr 2 Q_ <(W (f > a :>a: Q. _j a:(/ U)- W<( a PHOTOREGIME 9 II 13 2 TEMPERATURE TO WHICH AN ADDITIONAL DEGREE (C) IS ADDED Figure 6.--The influence of photoregime on developmental rates and on days to bud-burst in Douglas-fir (derived from data by Campbell and Sugano, In prep.). Response of seedlings that had been chilled 14 days at 4 C. Figure 7.--Decrease in day to bud-burst due to an added degree (C) of temperature during flushing, as influenced by photoregime and previous temperature. Had the chilling duration been shorter than 14 days, the differences among photoregimes in figure 6 ould have been greater. The longer the chilling period, the less the spread among photoregimes (figure 8). Moreover, potential developmental rates are augmented more by an added day of chilling if the preceding chilling period has been short and if buds are flushed in shorter daylengths, The longer the photoperiods in the photoregime, the less the influence of added chilling (figure 9). Longer photoperiods tend to compensate for chilling, as has been noted previously for several species (Jensen and Gatherum 1965, Nienstaedt 1966, Worrall and Mergen 1967, Farmer 1968 ).

11 29 2 l-<9 (fz 1-16 ::- :J :::J 1.1 (/ :: :J CD_ o :r :J u 1.3 PHOTOREGI ME (D CDLL 9 :J 12 >:.9 <: (D PHOTOREGI ME (fo 1- o zo.7 (/ 8 -<J II 9 Wa:: (/ II <!a_ 13 :: (/ 4 u Wo --13 II 44 DAYS OF CHI LLI NG CUMULATIVE DAYS OF CHILLI NG TO WHI CH THE ADDI TIONAL DAY IS ADDED Figure 8.--The influence of chilling duration (4 C) and photoregime on days to bud-burst. Seedlings flushed at 15.5 C (derived from data by Campbell and Sugano, In prep.). Figure 9.--Decrease in days to bud-burst due to one added day of chilling, as influenced by length of previous chilling and photoregime in hich seedlings are flushed. In Douglas-fir, there is question hether photoperiod and chilling ever completely compensate for one another. After 8 days of chilling, an added day decreased days to bud-burst by about one-half day, if plants ere flushed in a 13-h photoregime at 15 C (figure 9). Even after 14 days (15 eeks) of continuous chilling at 4 C, plants that ere flushed in 13-h photoregimes at 15 C burst their buds 8 days before plants in a 9-h photoregime (figure 6). DISCUSSION Our experience indicates that responses to stimuli during dormancy can be measured quite precisely by their effects on developmental rate. Thus, complicated interrelations become susceptible of study. It is these interrelations that are emphasized in this paper, in support of a suggested quantitative model for the bud-burst timing mechanism. The proposed system appears to suffice as a preliminary model for Douglas-fir. It also may be appropriate, in principle, for describing responses in other perennial, temperate-region species. Our results agree, qualitatively, ith results in many previous reports. Chilling effects, first reported by Coville (192), and effects of a substitution of photoperiod for chilling as reported by Gustafson (1938 ) and others cited earlier in this paper, usually are similar to those e found. As in Douglas-fir, experimental data in these other species, hardoods as ell as conifers, appear to be

12 3 quantitatively interpretable as resulting from changes in developmental rates. Vegis (1963) clearly recognized the quantitative interaction of photoperiod and temperature, and a "idening of the temperature response" ith passage through dormancy. This latter can also be interpreted quantitat i vely as a change in the temperature response curve induced by chilling--in early dormancy it is only the loer temperatures that can effectively increase actual developmental rates, by first increasing the potential developmental rates. Sarvas (1974) added to Vegis' conceptual frameork by providing a semi-quantitative model. His model predicted bud-development as the passage in time of the developmental cycle through three succeeding categories of dormancy. Sarvas did not conceptually differentiate beteen potential and actual development rates, and he did not account for effects of photoperiod. For some species, a division of dormancy into phases of rest (Samish 1954, Vegis 1964) or stages of dormancy, hatever they may be called (Doorenbos 1953, Sarvas 1974), may be both unnecessary and undesirable. Categories of dormancy imply steady states and boundaries. Categorical definitions ithin dormancy are apparently difficult to apply and seemingly inadequate (Romberger 1963, Sarvas 1974). Romberger advised more detailed physiological studies to provide a more satisfactory nomenclature. But, for some species, it may be that it is the concept of a partitioned dormancy rather than the definition of categories that is at fault. In these species, e.g. Douglas-fir, e do not appear to be dealing ith steady states, or even ith steady states separated by transitional phases as proposed by Smith and Kefford (1964). In Douglas-fir, dormancy is apparently constantly transitional, ith potential developmental rates changing continuously in response to cool-season environmental stimuli. Also, buds apparently can gro continuously during dormancy, either by cell production or by cell elongation, the degree depending on potential developmental-rate and temperature. In this case, categories such as predormancy, true dormancy, and postdormancy have little functional meaning (Sarvas 1973). They tend to confuse by providing an apparent rationale for delineating boundaries, or for seeking qualitative differences beteen steady states hen such differences do not exist. The system as proposed here is still incomplete in several ays. In deciduous trees, continuous chilling has been reported to be more effective than alternating arm and cold periods (Overcash and Campbell 1955). Also, arm periods can reverse the effects of previous cold periods (Bennett 195). These phenomena have not been investigated for Douglas-fir, or, to my knoledge, for any other conifer. The model presented here assumes strict additivity of development toard bud burst--each day's actual development is added to previous development. Also, the model does not make provision for reversals in potential developmental rates. Whether these are serious deficiencies, or are even applicable to conifers, ill require further experimentation. The structure of the temperature response curve also needs refinement. In Douglas-fir, diurnally fluctuating temperatures give rise to developmental rates that are larger than ould be predicted from rates in constant temperatures (Campbell and Sugano 1975). In the cited study, night temperatures ere lo enough to have contributed to chilling. Thus, increased rates may have been partly due to higher potential rates induced by chilling. Still, fluctuating temperatures ere so much more effective than constant temperatures that

13 31 some otl1er stimulus may have been involved. fn my experiments I h ave taken the vie that details in the dormancy system that are not directly related to developmental-cycle timing are not acted on by natural selection and therefore cannot be treated as meaningful parts of the system. Xn other ords, for differences exposed in groth cham bers or nurseries to have an accepted place in the system, they must have functional relevance ithin the genetic structure of the species. The parts of the system e have. so far studied apparently satisfy this requirement. Not only did different population-samples respond differently to chilling, photoperiod, and temperature, but many of these differences could be related clinally to postulated selecting agents at population origin. I have proposed that these differential responses to stimuli are responses to environmental cues for adjusting bud-burst to its seasonal cycle. Elsehere, e (Campbell and Sugano, In press) suggest that the degree of response to any particular cue is specified by the conditional probabilities of future selecting events such as frost and drought, at the population origin, given that particular cue. LITERATURE CITED Allen, G. S., and Oens, J. N The life history of Douglas-fir. Inf. Can., Can. For. Serv., Ottaa, 139 p. Bennett, J. P Temperature and bud rest period. Effect of temperature and exposure on the rest period of deciduous plant leaf buds investigated. Calif. Agric. 4(1): 11, 13, 15, 16. Campbell, R. K., and Sugano, A. I Phenology of bud burst in Douglasfir related to provenance, photoperiod, chilling and flushing temperature. Bot. Gaz. 136: (In press). Genecology of bud-burst phenology in Douglas-fir--response to flushing temperature and chilling. Bot. Gaz. Coville, F. V The influence of cold in stimulating the groth of plants. J. Agric. Res. 2: , Davidson, J On the relationship beteen temperature and rate of development of insects at constant temperatures. J. Animal Ecol. 13: Doorenbos, J, Revie of the literature on dormancy in buds of oody plants. (Wageningen) Landbou. hoogeseh. Meded. 53: Dormling, I. Gustafsson,., and von Wettstein, D The experimental control of the life cycle in Picea abies (L.) Karst. I. Some basic experiments on the vegetative cycle. Silvae Genet. 17: Eiche, V Cold damage and plant mortality in experimental provenance plantations ith Scots pine in northern Seden. Stud. For. Suec. 36:

14 32 Farmer, R, E., Jr. 1968, Seetgum dormancy release: effects of chilling, photoperiod, and genotype. Physiol. Plant. 21: Gustafson, F. G Influence of the length of day on the dormancy of tree seedlings. Plant Physiol. 13: , Jensen, K. F., and Gatherum, G, E Effects of temperature, photoperiod, and provenance on groth and development of Scotch pine seedlings. For. Sci, 11: Lavender, D. P., and Wareing, P. F Effects of daylength and chilling on the responses of Douglas-fir (Pseudotsuga menziesii Phyto l (Mirb,) Franco) seedlings to root damage and storage. Ne. 71: Nienstaedt, H Dormancy and dormancy release in hite spruce. For. Sci. 12: Overcash, J. P., and Campbell, J. A The effects of intermittent arm and cold periods on breaking the rest period of peach leaf buds. Proc. Amer. Soc. Hort. Sci. 66: Perry, T., Dormancy of trees in inter. Science 171: Romberger, J. A Meristems, groth and development of oody plants. U.S.D.A. For. Serv., Tech. Bull. No, 1293, 214 p. Samish, R. M Dormancy in oody plants. Ann. Rev. Plant Physiol. 5: Sarvas, R Investigations on the annual cycle of development of forest trees--active period, Communications Instituti Forestalis Fenniae 76.3, 11 p. 1973, The annual developmental cycle in forest trees. IUFRO Working Party S2.1.4 Symposium on Dormancy in Trees. Kornik, Poland, Sept. 5-9, 17 p. 1974, Investigations on the annual cycle of development of forest trees. II. Autumn dormancy and inter dormancy. Communications Instituti Forestalis fenniae 84,1, 11 p. Smith, H., and Keiford, N. p, 1964, The chemical regulation of the dormancy phases of bud d,eyelopment. Amer. J. Bot. 51: Sugano, A. I The effects of lo temperatures on dormancy release in Douglas-fir from estern Oregon, Washington and California. MSc. Thesis, Oreg. State Univ,, Corvallis. Vegis, A Climatic control of germination, bud break, and dormancy. P , In Evans, L. T. Environmental Control of Plan t Groth. Academic Press, Ne York, 449 p.

15 33 Vegis, /\ Dormancy in higher plants. Ann, Rev. Plan t PhysioJ. 15: Wakefield, J. D., ed. 1969, Cl imatological Handbook, Columbia Basin States temperature. Vol. 1, pt. A. Pacific Northest River Basins Commission, Vancouver, Wash. Wommack, D. E Temperature effects on the groth of Douglas-fir. Ph.D. Thesis, Oregon St. Univ., Corvallis. Worrall, J., and Mergen, F Environmental and genetic control of dormancy in Picea abies. Physiol. Plant. 2: Reproduced from PROCEEDINGS: FIFTH NORTH AMERICAN FOREST BIOLOGY WORKSHOP 1978, compiled and edited by Charles A. Hollis and Anthony E. Squillace, by the FOREST SERVICE, U.S. Department of Agriculture, for official use GPO

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