No. 71 1975

FLUSHING RESPONSE OF DOUGLAS FIR BUDS

TO CHILLING AND TO DIFFERENT

AIR TEMPERATURES AFTER CHILLING

b)!

R. van den Driessclhe

i

BRITISH COLUMBIA FOREST SERVICE Research Division

Victoria, B.C.

FLUSHING RESPONSE OF DOUGLAS F I R

BUDS TO C H I L L I N G AND TO DIFFERENT

A I R TEMPERATURES AFTER C H I L L I N G

by

R. van den D r i e s s c h e

E.P. 701

ABSTRACT -

Two-year old Douglas f i r p'l ants were exposed t o varying amounts of chi l l ing under ar t i f ic ia l condi t ions a t about 2 O C , and under natural conditions. Duration of exposure t o a l l temperatures below 4.4OC., w i t h i n the range .encountered under natural conditions, seemed a sat isfactory measurement of chilling as i t was comparable w i t h duration of chi l l ing under ar t i f ic ia l condi t ions. C h i l l i n g requirements of three Douglas f i r provenances were all reasonably well sa t i s f ied by 2,000 hours a t temperatures below 4.4OC, as measured by time taken for buds t o f lush af ter chi l l ing. There was clear indication that provenances d i f f e r i n their exact chi l l ing requirements, and the response of terminal buds and la te ra l buds t o chi l l ing d i d not show the same differences i n each provenance.

Evidence was obtained that interruption of chilling conditions w i t h periods of h i g h temperature (24OC) could offset the dormancy breaking effect of chi l l ing.

P1 ants which had received 1,250 hours of chil 1 i n g below 4.4OC flushed quite rapidly a t 24OC, b u t s lowly'at a mean dai ly temperature of 13OC. F l u s h i n g was rapid, however, a t a mean dai ly temperature of 12.8OC af ter p lants had received 2,070 hours chil l i n g .

Higher mean daily temperature increased the speed of flushing af te r ch i l l ing , and i t appeared that flushing of Douglas f i r buds was also influenced by the range of daily temperature.

INTRODUCTION

METHODS

RESULTS

DISCUSSION

ACKNOWLEDGEMENTS

LITERATURE CITED

APPENDICES

TABLE O F CONTENTS

P a g e

1

2

5

15

19

19

21

INTRODUCTION

Many woody perennials w i 11 only break dormancy and continue normal growth a f t e r exposure t o chi l l ing (Samish 1954, Vegi s 1963 , 1964) . This necessity for chi 11 ing has a1 s? beer, specif ical ly demonstrated for certain conifers (Romberger 1963, Nienstaedt 1966 , 1967 , Nagata 1967, Worrall and Mergen 1967) . According t o Lavender and Hermann (1 970) , Wommack , in 1964, showed t h a t Douglas f i r (Pseudotsuga menziesii (Mirb.) Franco) had a chilling requirement which bould be sa t i s f i ed by 8-12 weeks a t a temperature between 3 and 6 C . Under natural conditions, of course, plants which have a chilling requirement usually have th i s need fu l ly sa t i s f ied by the time spring arr-ives (c.f. Lavender et aZ. 1973). Now t h a t forest tree seedlings are frequently removed from natural conditions d u r i n g f a l l and winter, however, e i the r by bei ng placed in cold storage , or grown in containers inside green- houses , i t i s necessary t o pay attention t o whether the i r chi 1 I ing requirements are adequately met. This may be especially important i n re la t ion t o present practice since Lavender and Wareing (1972) have suggested t h a t chi l l ing may n o t only overcome bud dormancy in Douglas f i r , b u t also improve ab i l i t y of seedlings t o surviv'e cold storage.

There is also circumstantial evidence which suggests adequats chilling is necessary for satisfactory cold storage ( a t 2-5 C ) of Douglas f i r , because i t has been found that storage prior t o mid-November i s unsuccessful in Oregon (Hermann e t aZ. 1972) , and in Holland l i f t ing for s torage i s n o t recommended u n t i 1 mid- January (01 denkamp e t a2. 1969) .

The work reported here was confined t o measuring chi l l ing requirements necessary t o overcome bud dormancy in Douglas f i r , and examining the effect of temperature conditions on f lushing after ch i l l ing .

- 1 -

METHODS

Experiment 1

Two-year o l d n u r s e r y grown Douglas fir, o f t h r e e provenances (Table l ) , w e r e p o t t e d i n t o a 1 peat : 1 nursery s o i 1 m i x t u r e i n e a r l y September 1971. Three blocks , each c o n t a i n i n g 80 p l a n t s o f each provenance, were arranged i n the open , away from any source o f a r t i f i c i a 1 l i g h t . Provenances were randomly arranged i n rows o f 20 p l a n t s w i t h i n a b l o c k . A Stevenson screen containing a thermograph and thermometer, was p l a c e d a d j a c e n t t o t h e b l o c k s a t g r o u n d l e v e l t o o b t a i n a cont inuous temperature record.

On each o f f o u r 1 i f t i n g d a t e s , ( 1 ) 4 October , 1971 (2) 1 November , 1971 , (3 ) 6 December , 1971 , and (4 ) 6 February , 1972 60 p l a n t s o f each provenance were removed from the three blocks and groups o f 15 were subjected to one o f f o u r t r e a t m e n t s : ( 1 ) no c h i l l i n g , ( 2 ) two weeks c h i l l i n g , ( 3 ) f o u r weeks c h i l l i n g , and ( 4 ) e i g h t weeks c h i l d i n g . C h i l l i n g c o n s i s t e d o f e x p o s u r e t o a t e m p e r a p r e o f a b o u t 2 C, bu t occas iona l ex t reme va r ia t i on between 0 C and 10 C occurred. Temperature was cont inuous ly recorded i n t h e c h i l l i n g room. E igh t -hou r pho toper iods o f 3,745 l u x were main ta ined w i th incandescent lamps d u r i n g c h i l l i n g . On c o m p l e t i o n o f t h e c h i l l i n g t r e a t m e n t p l a n t s w e r e o p l a c e d i n t o a greenhouse where the temperature was c l o s e t o 24 C , and a 16-. hour photoper iod was imposed. Condi t ion of terminal bud and condi - t i o n o f l a t e r a l buds were determined when p l a n t s w e r e p l a c e d i n the greenhouse and subsequent ly a t two-week i n t e r v a l s f o r 18 weeks. Terminal buds were recorded ei ther as n o t f l u s h e d , o r f l u s h e d , and p lan ts were g iven a score of 1, 2, o r 3, acco rd ing t o whe the r t he re were no l a t e r a l buds f l u s h e d , some l a t e r a l buds f l u s h e d , o r a1 1 l a t e r a l buds had f lushed.

Experiment 2

F i f teen two-year o ld Doug las fir, from each o f t h r e e provenances (Table 1 ) , w e r e t r a n s p l a n t e d i n t o wooden boxes , measuring 45 x 30 x 15 cm, i n a known random sequence i n August. Ten such boxes were a l l o c a t e d i n p a i r s t o f i v e c h i l l i n g t r e a t m e n t s so t h a t 30 p l a n t s o f each provenance were subjected t o each t reatment

- 2 -

Table 1 Sources of provenances used i n experiments 1 and 2

Name Seed l o t P1 ace E l evati on Lat. Long.

number m

Experiment 1

Great Central 51 0 Block 769 457 49'20 125'1 5 I

Lake

Pemberton 1261 Pool e Creek 457 50°28' 122'40 ' Go 1 den 1707 Hospital Creek 915 51'19' 116'58'

Experiment 2

Sechel t 1284 Hal fmoon Bay 609 49'30 I 123'55'

Pemberton 1009 Green Lake 76% 50'1 2 122'55'

Chehalis River 1215 Vaughn Creek 762 49'21 ' 121'58'

- 3 -

from the beginning of OctobEr. Treatments consisted of imposing e i ther low temperature of 2 C under an 8 hour photoperiod of 3,20OO1ux intensi ty ( referred t o as " C l l ) , or h i g h temperature of 24 C under 8-10 hour photoperiods of daylight in a greenhouse (referred t o as " G " ) , i n various combinations. These combinations were ( 1 ) 8 weeks G , ( 2 ) 4 weeks G/4 weeks C y (3 ) 2 weeks G/6 weeks ( 4 ) 8 weeks C y (5) 2 weeks G/2 weeks C/2 weeks G/2 weeks C. A t the end of the 8 week treatment period treesowere placed in a greenhouse under 16 hour photoperiods, a t 24 C y and a record of terminal bud flushing was kept a t weekly intervals over the following 11 weeks. The length of new terminal shoot extension was also measured a f t e r 11 weeks.

Experiment 3

On 26 January four groups of 36 plants were selected for uniformity of height and appearance from 500 potted two-year old Douglas f i r of coastal provenance (Goldstream, Vancouver Is land) . Each group of plants was p u t in to one o f four growth chambers which a l l had a 12 hour photoperiod providing l igh t in tens i ty of 29,000 lux a t plant tops. A different temperature regime was applied i n each growth ' chamber and these regimes wered f o r J2-hour da6 ando12- hour nightoperiods respeStivedy: ( 1 ) 18.5 C/7.5 C y ( 2 ) 13 C/13 C (3) 24 C/2 C y and ( 4 ) 24 C/24 C . Consequently over e$ch 24 hour period the mean temperature in regimes 1 t o 3 w ~ s 13 C y and the mean temperature in regime 4 was, of course, 24 C .

A record of bud flushing was made three times a week during the 84 days the plants were in the growth chambers. Terminal buds were recorded as flushed or n o t flushed according t o whether needles were exposed from the bud scales or n o t . The number o f l a te ra l buds flushing was recorded up t o a total of 5 , and beyond t h a t the plant was given an arbitrary score of 8 when a l l buds had flushed. Consequently the la te ra l bud data are distorted, t h o u g h n o t unduly, because once a few la te ra l buds had flushed the remainder did so i n a relatively short t ime.

A t the end of the experiment total length of main stem and length of new main stem growth were measured, and then dry weights of old shoot, new shoot growth and roots were determined.

Exposure t o temperatures below 4.4OC experienced by these plants between 1 October and placement i n t o growth chambers was determined from thermograph records.

- 4 -

Experiment 4

An experiment o f similar design t o the previous one (experiment 3) was done. The plants used were of the same Goldstream provenance , and , on 24 Apri 1 , were placed under the fol lowing 24-hour temperature regimes: (1 ) 6 hours a t 18.3OC and 113 hours a t lO.90Cy ( 2 ) 14 hours a t 18.3OC and 10 hours a t 5,00C, (3) 6 hours a t 18.3OC and 18 hours a t 14.6OCY and (4) 14 hours a t 18.30C hnd 10 hours a t 11.7oC The mean temperature over each 24 hours period was 12.8% f o r regimes 1 and 2 , and 15.5OC fo r regimes 3 and 4. Photoperiods were of 1 2 hours and the middle o f the h i g h temperature period coincided w i t h the middle of the photoperiod. Measurements were made as described for experiment 3 , and th i s experiment was terminated after 52 days.

RESULTS

Experiment 1

flush was determined for each treatment from the raw da ta . Length of time d u r i n g which plants were exposed t o temperatures below 4.4OC, as we1 1 as degree hours below 4 . 4 O C t o which plants were exposed, was also estimated for each treatment from thermograph records. The time t o flush was then regressed over each of the two measurements of low temperature exposure for both terminal and la te ra l buds of each provenance. A s t r a igh t l i ne and a second degree polynom$al were f i t t e d t o each s e t of data by multiple regression and i t was found t h a t i n a l l s e t s , except t h a t of la te ra l buds i n provenance 1 the polynomial was a s ign i f i can t ly be t t e r f i t ( F i g . 1 , 2 , Appendix I > , In some instances these fitted curves turn upwards a f t e r having reached a minimum. This does n o t imply t h a t time required t o flush will increase again after a large amount o f chil ' ling, b u t merely that the polynomial model employed i s u n r e a l i s t i c a t this p o i n t .

The average time taken for terminal and la te ra l buds t o

Correlation coefficients were high for the relationships between time f o r buds t o flush and both measures of low temperature exposure (Table 2 ) . Use of hours exposure below 4 . 4 O C as the independent variable resulted in slightly higher correlation coeffictents than use of degree hours below 4 . 4 O C . Furthermore, p l o t t i n g time taken f o r flushing over degree hours below 4.4OC showed a discontinuity in the trend between the fou r th l i f t i n g date and the remainder of the data. For these reasons, and because duration of exposure t o temperature below 4 . 4 O C was the simplest o f the two low temperature exposure measurements to make, th i s independent variable seemed the most su i tab le for determining chilling received by Douglas f i r p l a n t s .

- 5 -

WEEKS TO TERMINAL FLUSHING

0 a 0 II L z

w a -I 0 0

0

- 0

0 rJ -0

- 0 0

(Y -0

0 -0

0

-0

z 0 I- LL

w Y

A a J

a I- 2

V W

I-

W

(3

a

a a

- 0 0

- 0 0

0 -0

R

0 0

-2

-0

- 6 -

WEEKS T O FLUSHING OF LATERAL BUDS COMPLETE0

z c 0 cr al w 5 W 0-

W Y

-J

-I

a 2 t-

W u t- w (3

a

a

a a

cn - 0 n

0

- 7 -

Table 2. Correlation coefficients for time required to flush regressed

on two measures of low temperature exposure.

Provenance Hours Q, 4.4OC O C hours 4 4.4OC

Terminal buds

Great Central Lake 0.97*

Pemberton 0.93

Go 1 den 0.95

0.96

0.92

0.94

Lateral buds

Great Central Lake 0.91

Pemberton 0.96

Go1 den 0.94

0.84

0.93

0.93

* All coef f ic ien ts a re s ign i f icant a t p = 0.01

- 8 -

Lapse of time, between the beginning of the experiment and placing plants into conditions suitable for flushing in the greenhouse, increased in much the same way as exposure t o low temperature. I t seemed desirable therefore t o see i f 1 apse of time , and so perhaps some endogenous process, could account for increased rates of flushing as the experiment ran i t s course. Plotting weeks required for terminal buds t o flush over lapse of time from s t a r t of the experiment until placement i n the greenhouse (Fig. 3) , showed t h a t weeks required for flushing was n o t related to lapse of time for the f i r s t th ree l i f t ing da tes . The steady decrease in weeks required for flushing between . l if t ing dates 3 and 4 could, however, have been partly related t o lapse of time. In general , t h o u g h , th i s examigation of the data did n o t invalidate duration of exposure below 4.4 C as a means of measuring chi1 ling.

.Based on exposure t o temperatures below 4 . 4 O C , chi 11 i ng requirments for terminal bud flushing of the Great Central Lake and Golden provenances were completely sa t i s f i ed by a b o u t 2,500 hours, whereas the Pemberton provenance required a b o u t 3,000 hours. Flushing was re la t ive ly r ap id i n a l l provenances,.however, a f t e r about 2,000 hours chi 1 l ing.

The relationship between time required for complete flushing of la te ra l buds and chi 11 i ng exposure was c lear ly different between provenances ( F i g . 2 ) . Decrease i n time for la te ra l bud flushing of the Great Central Lake provenance was proportional t o increase i n ch i l l ing , b u t in the Pemberton provenance time required t o flush decreased less rapidly with more ch i l l ing . In the Golden provenance the re1 ati onship was clearly curvil inear and showed l i t t l e d i f f e rences from the relationship for terminal buds.

Experiment 2

A m i n i m u m time of two weeks under long photoperiods, a t 2 4 O C in the greenhouse , was required for terminal buds of plants , which had received 8 weeks (1,344 hours) chi 11 ing , t o start flushing (Fig. 4) . Rate of flushing decreased as the weeks of exposure t o chilling decreased, with the exception t h a t 4 weeks ch i l l ing , preceded by 2 weeks i n the greenhouse and broken i n the middle by two weeks in the greenhouse, resulted in a lower r a t e of flushing than any other treatment.

This experiment fa i led t o demonstrate significant differ- ences between provenances , a1 t h o u g h means for the experiment ( F i g . 4 ) , indicated the Chehalis provenance flushed faster than the other two provenances. There was no indication of any interaction between provenance and treatment. Not a l l terminal buds had f lushed after 11 weeks in the greenhouse when plants had received 8 weeks ch i l l ing . The final flushing values were: Sechelt 7 2 % , Chehalis loo%, and Pemberton 63%, and these values had remained constant for about 3 weeks indicating no more flushing could be expected.

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WEEKS TO TERMINAL FLUSHING

1) cn 3

z W 0 J 0

-

0 - N

n

z 0 I-

w a m

n E

W

U Y

-I

J

a U I- z W 0

d - 10 -

3 -

- 0 N

-0

~

-0

L mcn

CnQ

-aJ s x %

e n o a w

M

aJ L 3 tT,

v) 3

2 60 aO1 % ! = i m

i < 4 0 7 Z I

0

I L S D

””” . 0

/‘ /

/

5 1 2 3

ac

2G/6C

4G/4C

8G

TIME FROM PLACEMENT INTO GREENHOUSE, WEEKS

4 5 1 2 3 4 5 f /

PROVENANCE SECHELT F’EMBE RTON CHEHALIS

Figure 4. Time taken for terminal buds t o f lush i s shown f o r a l l provenances by treatments in upper g raph . Labels re fer t o number of weeks in chi 11 i ng conditions (C) or in the greenhouse ( G ) .

Bel ow i s shown the final percentage o f terminal buds flushed by provenance and treatment. Treatment 1 = 8 6 , 2 = 4G/4C, 3 = ZG/6C, 4 = 8C, 5= 2G/ZC/2G/ZC.

- 11 -

Length of new shoot growth in cm, measured a f t e r 11 weeks in the greenhouse, was, for treatments 1 t o 5 respectively: 0.43 a b , 1.34 bc, 1.58 cy 2.54 d , and 0.13 a. (The same l e t t e r following a value indicates lack of difference (p=0.05) from other values followed by the 1 e t t e r ) .

Exper i men t 3

All plants had received 1,250 hours chi l l ing below 4.4OC on placement into the growth chambers , and flushing of terminal buds commenced a f t e r 14 days a t 2 4 O C in regime 4 (Fig. 5). Tn the three treatments w i t h a mean temperature o f 13OC flushing of terminal buds did not occur u n t i l a f t e r 35 days i n the growth chamber. The manner in which high and 1 ow temperatures were dis t r ibuted between day and night also affected the rate of flushing in these three treatments. The uniform dis t r ibut ion of 13OC d u r i n g day and n i g h t (regime 2 ) resul ted in the la tes t s tar t of flushing, A 2 4 O C day temperature combjned with a 2 O C night temperature resulted in an e a r l i e r start to flushing. Plants in the 18.5OC day and 7.5OC n i g h t showed the most rapid flushing of those in the three tempenlture regimes which averaged 13OC.

Flushing of la te ra l buds started sooner t h a n flushing of terminal buds: a f t e r 9 days i n regime 4 , and a f t e r 23 days i n the other three regimes ( F i g . 5 ) . Lateral buds showed the same general relationship between regimes as did terminal buds, b u t the difference between regimes 1 and 3 was n o t so pronounced in the lateral bud d a t a .

The mean length o f old main stems did n o t vary between regimes, as might be expected, and was 30.9 cm. Length growth of main stems d u r i n g the experiment was related t o temperature regimes, however, with greatest length growth occurring in regime 1 , and l ea s t in regimes 3 and 4 (Tab1 e 3) Increase i n plant dry weight was n o t affected by temperature regimes in the same way as main stem length. Greatest shoot and root weight occurred i n tbe 2 4 O C / 2 4 O C regime. The next largest plants occurred in the 18.5 C/7.5OC regime, and growth was l ea s t i n regimes 2 and 3. New shoot dry weight growth expressed as a percentage o f old shoot dry weight was n o t d i f fe ren t between regimes 1 and 4 , b u t t h i s was par t ly due t o a significant increase in old shoot dry weight which occurred in regime 4.

- 1 2 -

0 20 40 TIME IN DAYS

60 80

0 20 40 60 80 TIME IN DAYS

Figure 5. Rates o f flushing of terminal and la te ra l buds in experiment 3 . Numbers in brackets are treatment numbers, and labels on terminal bud curves show day (0) and n i g h t ( N ) temperatures i n OC.

- 13 -

Table 3. Length and d ry we igh t measurements o f stems and r o o t s a t

t h e ends o f exper iments 3 and 4.

Measurement

1

Regime

2 3 4

Day/night temperatures i n OC f o r expe r imen t 3

18.5/7.5 13/14 24/2 24/24

New stem length 35.2 a* 29.5 b 20.6 c 23.6 c as % o l d

New shoot weight 66.7 a 36.6 b 43.1 b 66.8 a as % o l d

Old shoot weight , g 4.01 a 3.54 a 3.61 a 5.41 b

To ta l shoot weight 6.67 a 4.86 b 5.18 b 9.01 c ( o l d & new), g

Root weight, g 3.26 a 2.61 ab 2.47 b 5.98 c

Hours x day/night temperatures i n OC fo r exper iment 4

6x1 8 .3 14~18.3 6x18.3 14~18.3 /18x10.9 /10x5.0 /18x14.6 /10x11.7

New s tem length 29.0 ab 25.0 b 36.2 a 29.2 ab as % o l d

New shoot weight 43.7 a 48.5 a 72.2 b 78.6 b as % o l d

Tota l shoot weight 5 .23 a 5.40 a 6.71 ab 7.62 b ( o l d & new), g

* va lues fo l lowed by the same l e t t e r n o t s i g n i f i c a n t l y d i f f e r e n t a t p=0.05

- 14 -

Experiment 4

Plants had received 2,070 hours exposure t o temperatures below 4 . 4 O C a t the time of placement under the four different temperature, regimes and flushing was rapid. Flushing of terminal buds commenced a f t e r 5 to 10 days , depending on temperature regime, and flushing of 1 a teral buds commenced a f t e r 3 t o 5 days (Fig . 6b. Regimes 3 and 4 , w i t h the higher mean daily temperature of 15.5 C y resulted in more rapid flushing of terminal buds t h a n the other two regimes where the mean d a i ly temperature was 12.8OC. W i t h i n each pair of regimes with the same mean daily temperature, however, the longer period of h i g h temperature, followed by a shorter period of re1 atively low temperature , promoted flushing more t h a n the other treatment. The only clear difference between rates of flushing of la te ra l buds was t h a t plants in regime 1 flushed more slowly than plants i n the other regimes.

Temperature regimes affected main stem length and shoot dry weight growth during the experiment (Table 3 ) . Shoot length growth i n regime 3 was s ignif icant ly greater than in regime 2 , and new s h o o t dry weight growth was greater in regimes 3 and 4 than in regimes 1 and 2 .

DISCUSSION

Chilling requirements of Douglas f i r p lan ts a re sa id to be sa t i s f i ed by 8 t o 12 weeks (1,344 t o 3,016 hours) a t a temperature between 3 and 6 O C (Lavender and Hermann 1970). This i s 1 argely confirmed by the results of the present experiments. In experiment 1 increase i n chi 11 i ng beyond 2,000 hours resulted in very 1 i t t l e reduction in time necessary for terminal buds to flush, except perhaps i n the Pemberton provenance. On the other hand , the regressions suggested t h a t l a te ra l buds of the Great Central Lake and Pemberton provenances responded t o chi l l ing of up to 3,000 hours. In experiment 2 , plants which had received 1,344 hours of chil l ing f lushed after two weeks , although n o t a l l terminal buds had flushed after 9 weeks, showing t h a t chilling requirements for some of the plants were n o t fu l ly s a t i s f i ed .

Terminal buds of plants , which had received 1,250 hours chil l ing in experiment 3 , commenced flushing after 14 days a t an average daily temperature of 2 4 O C , b u t there was a further delay of 21 days before flushing commenced in the regimes with an average daily temperature of.130C. On the other hand, i n experiment 4 , terminal buds of plants which had received 2,O;TO hours chil l ing f lushed after 10

-. 15 -

100 -

S 80-

z (3

v) I

3 i 60- a m 3

40

20

0 0 20 40

TIME IN DAYS

8

7

6

5

4

3

2

1

0

0 2lO

TIME IN DAYS

Figure 6. Rates of flushing o f terminal and la te ra l buds i n experiment 4. Numbers in brackets are treatment numbers, and labels on terminal bud curves show day ( D ) and n i g h t ( N ) temperatures i n OC.

- 1.6 -

days even when the average daily temperature was only 1 2 . 8 O C . Allowance should, perhaps, be made for the difference i n h igh and low temperature combinations between the regimes, b u t i t seemed tha t ch i l l ing for 2,070 hours allowed p lan ts t o flush a t l e a s t as rapidly a t a daily average of 12.8OC as those chilled for 1,250 hours could flush a t a daily average of 2 4 O C . In other words, more ch i l l ing allowed flushing t o occur rapidly a t a lower daily average temperature.

Bearing in mind t h a t coastal , transition", and i n t e r io r provenances were used in experiment 1 , flushing of many provenances o f dormant Douglas f i r can be expected over a range of temperature condi t ions af ter exposure t o temperatures below 4.4oC f o r about 2,000 hours. W i t h less chi l l ing, say 8 weeks, the range of temperature, and perhaps other conditions, over which rapid flushing occurs would be less . This is i n accordance with the generally accepted view on dormancy (Romberger 1963, Vegis 1963) in which i t i s considered t h a t as breaking of: dormancy becomes more complete, due to continued ch i l l i ng , growth becomes possible over a wider range o f temperature condi t i ons .

Tt has been suggested t h a t long days can " 'subst i tute ful ly for the cold requirement i n breaking bud dormancy" in Douglas f i r (Roberts e t aZ, 1974). The da ta supporttng this claim are n o t convincing, since some of the plants d i d not ach-ieve more t h a n 25% bud break under long day conditions. Results obtained in experiments 1 and 2, in the present work, a l so conf l ic t w i t h this claim in t h a t bud flushing of plants placed under long photoperiods, i n the green- house, showed a c l ea r e f f ec t of previous chill-lng treatment. Flushing would have occurred on a l l p l an t s a f t e r an equal delay i f long days really substi tuted fully for chi1 1 i n g requirement. On the other hand, long photoperiods may eventually permit flushing, particularly of par t ia l ly ch i l led Douglas f i r p l a n t s , as they do i n some other woody species (.e. g. Worrall and Mergen 1967) .

Comparison of the two methods of estimating chilling showed duration of exposure t o temperatures below 4.4% was most sat isfactory. Chilling measured in degree hours below 4,4OC was n o t an improvement, and i t must be assumed t h a t the level of temperature below the threshold i s r e l a t ive ly unimportant. T t is l ikely that temperatures below a t l ea s t 7 O C are effective for chil l ing (Nienstaedt 1967, Samish 1954), b u t presumably under natural conditions, i n experiment 1 , exposure to effective chilling temperatures was proportional t o duration of exposure below 4.4%.

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Duration of exposure t o chi l l ing was calculated from 1 October, since l i t t l e c h i l l i n g i s l i k e l y t o occur before this date; perhaps 50 hours, or l e s s , below 4 . 4 O C a t Victoria. After this date chil l ing in the open seems t o have been cumulative since i t was comparable with ch i l l ing under a r t i f ic ia l condi t ions , as judged from the good f i t obtained i n the correlations of experiment 1 . On the other hand, whether short ,periods of ch i l l ing , d u r i n g cool nights, are really Cumulative, has been questioned (Romberger 1963, p. 159) . High temperatures between chilling periods may reverse the effect of ch i l l ing . This seems t o .have occurred in treatment 5 of experiment 2 when plants were exposed t o a temperature of about 2 4 O C between two periods of ch i l l ing . I t must be assumed, however, tha t day time temperatures experienced by plants of experiment 1 d u r i n g the fa l l were n o t h i g h enough, or o f suff ic ient durat ion, t o offset the cumulative e f fec t of previous chilling. Possibly the reason why cold spells occurring befone the 1 October can be ignored i n terms of ch i l l ing , i s because they are usually followed by suf f ic ien t ly h i g h temperatures to cancel their e f fec t .

Over 11 weeks mean shoot height growth appeared related to chilling treatment th rough ra te of terminal bud flushing in experiment 2. Normal, rapid shoot growth would presumably be dependen t on a p l a n t h a v i n g r e c e i v e d a d e q u a t e c h i l l i n g d u r i n g the previous period of dormancy.

Higher mean daily temperature increased the rate of flushing i n experiments 3 and 4. Furthermore, differences i n ra tes of flushing occurred among regimes with the same mean daily temperature , depending on how temperature levels were apportioned during the 24 hour period. I t must be concluded t h a t flushing of Douglas f i r buds showed a thermoperiodic response as well as a dependence on mean daily temperature. These temperatures were measured in ambient air of the growth chambers , and the soil in which the plants were rooted would have had similar temperatures. Different results might have been obtained, however, i f soil temperatures had been dissimilar t o a i r temperatures because there i s evidence that f lushing of Douglas f i r i s pa r t ly dependent on a suf f ic ien t ly h i g h soil temperature (Lavender e t aZ. 1973).

As background information, t o ass i s t in te rpre ta t ion of the previous experiments, an indication of ch i l l ing exposure attained a t f ive nurseries i n various parts of Brit ish Columbia i s included (Appendix 11). The data are 1 imi ted, b u t i t seems less than 2,000 hours chi 11 ing i s often received a t Surrey and Green Timbers d u r i n g a winter. A t Campbell River 2;OOO hours chil l ing is received by the end of February. A t bo th Skimikin and Red Rock nurser ies this amount of c h i l l i n g i s accumulated some time i n mid-January.

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ACKNOWLEDGEMENTS

The author gratefully acknowledges s ta t i s t i ca l advice and data processing assistance from Mr. A. R . Fraser and Mr. M . Kovats, and technical assistance from Mr. M. B . Balderston, who a re a l l members of the British Columbia Forest Service Research Divi sion. Data of Appendix I1 were prepared by Dr. M . C. Coligado of the Bri t i s h Col umbi a ELUC Secre t a r i a t .

LITERATURE CITED

Hermann, R . K . , D . P . Lavender, and J . B. Zaerr. 1972. Lifting and storing western conifer seedlings. Oregon State University Forest Res. Lab. Res. Pap. 17. pp. 8.

Lavender, D . P . , and R . K . Hermann. 1970. Regulation of the growth potential of Douglas f i r seedlings d u r i n g dormancy. New Phytol. , 69:675-694.

Lavender, D . P . , G . B . Sweet, J.B. Zaerr, and R . K . Hermann. 1973. S p r i n g shoot growth i n Douglas f i r may be in i t i a t ed by gibberellins exported from the roots. Science 182 (4114): 838.

Lavender, D.P. , and P .F . Wareing. 1972. Effects of daylength and chi 11 i n g .on the responses of Doug1 as f i r (Pseudotsuga rnenziesii (Mi rb) Franco) seedlings to root damage and storage. New Phytol. 71 :1055-1067.

Nagata, H . 1967. Studies on the photoperiodism i n the dormant bud of Pinus d e n s i f b r a Sieb. e t Zucc. I1 Effects of temperature a n d photoperiod on the breaking of w,inter dormancy of f i r s t year seedlings. 3. Jap. For. SOC. , 49 (12 ) : 415-420.

Nienstaedt, H . 1966. Dormancy and dormancy release i n white spruce. For. Sci. , 12 (3):374-384.

Nienstaedt, H. 1967. Chilling requirements i n seven Picea species. S i lv . Gen., 16 (2):65-68.

Oldenkamp, L . , H . Blok, and B. C . M . van Elk. 1969. Opslagperiode en bewaarmethode van zaailingen van bosplantsoen. Ned. Bosb. T i jdschr . , 41 : 23-29.

Roberts, A . N . , B.J. Tomasovic, and L . H . Fuchigami. 1974. Intensity o f bud dormancy i n Douglas f i r and . i t s re lat ion t o scale removal and rooting abil i ty. Physiol. Plant. 31: 211-216.

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Romberger, J.A. 1963. Meristems, growth and development i n woody plants , U.S. Dept. Agric. , Forest Serv. Bull. 1293 pp . 214.

Samish, R . M . 1954. Dormancy i n woody plants. A n n . Rev. Plant Physiol. 5~183-204.

Vegis , A. 1963. Climatic contro dormancy pp. 265-287. growth. Ed. Evans. L London.

1 of germination , bud break , and

.TrAcademic Press. New York and In Environmental control of p l a n t

Vegis, A. 1964. Dormancy i n higher plants . Ann. Rev. Plant Physiol. 15 : 185-224.

Worrall, J . , and F. Mergen. 1967. Environmental and genetic control of dormancy i n Picea abies. Physiol. Plant. 20 : 733-745.

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Appendix I Equations for regressions of time required t o flush a t

24OC on duration of exposure t o temperatures of less

than 4.4OC, and F r a t io s fo r 1 i near and quadratic

components o f each regression.

F Provenance Equation Linear Quadrati c

Great Central y* = 17.98 - (1.0368 x 10-2)x 85.94' 34.28

Pemberton y = 18.67 - (9.4888 x 10-3)x 21.07 4.89

Lake + (1.9075 x 10-6)x2

+ (1.3312 x 10-6)x2 Go1 den y = 21.21 - (1 .:3493 x lo-z)x 46.48 17.00

+ (2.3775 x 10-6)x2

Lateral buds

Great Central y = 9.87 - (2.1095 x 1OW3)x 64.55 (a )

Pemberton y = 11.03 - (4.8045 x 10-3)x 39.24 9.00 Lake

i- (6.7030 x 1 0 - 7 ) ~ ~ Go1 den y = 18.59 - (1.1854 x 42.21 16.25

+ (2.1428 x 10-6)x2

* y = time required to flush i n weeks x = duration of exposure t o temperatures below 4.4OC i n hours

degrees of freedom appropriate for the F r a t io s were 2 and ' 13 w i t h the exception o f ( a ) , where 1 and 14 were appropriate.

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Appendix 11. Accumulation of chilling (hours <4.4OC) by month for f ive nurseries i n British Columbia .t.

Winter Month

SeP oc t Nov Dec Jan Feb Mar

Whal ley [for Green Timbers)

1971 -72 2 100 195 842 1,422 1,730 1,879

1972-73 11 53 207 595 984 1,202 1,344

Surrey ~ ~~ ~~

1973-74 .. 8* 420 695 1,151 1,422 c

~

Campbell River

1972-73 27 545 781 1,326 1,934 2,362 2,705

Ski m i k i n

1972-73 100 576 1,255 1,963 2,702 3,350 3,934

1973-74 32 322 1,035 1,777 2,472 3,135 3,761

1974-75 35 31 8 855 1,675 - c -

Red Rock

1969-70 49 42 3 891 1,585 2,308 2,876 3,349

1970-71 81 366 1,058 1,802 2,524 3,188 3,906

1971 -72 132 564 1,205 1,948 2,692 3,386 3,835

1972-73 184 535 1,225 1,941 2,593 3,164 3,723

t Data prepared from maximum and minimum temperature, measured a t 1.22 m (4 f t ) above ground, by the technique of R . E . Neild (Monthly Weather Review, Vol 95 (8):583-584, 1967).

* from 18 Oct. only. - 22 -