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Diurnal rhythm of chilling sensitivity of cucumbers in light
Scientia Horticulturae, 38 (1989) 231-237 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
231
Diurnal Rhythm of Chilling Sensitivity of Cucumbers in Light E. RIETZE and H.-J. WIEBE
Institut fi~r Gemiisebau, Universiti~t Hannover, Herrenh~user Str. 2, 3000 Hannover 21 (F.R.G.) (Accepted for publication 19 August 1988)
ABSTRACT
Rietze, E. and Wiebe, H.-J., 1989. Diurnal rhythm of chilling sensitivity of cucumbers in light. Scientia Hortic., 38: 231-237. Cucumbers were grown at 20°C and chilled in light (4 h at 2°C and 49 W m-2). Resulting damage (photo-oxidation) was inhibited in an oxygen-free atmosphere and decreased by a subsequent dark period. It was increased by increasing light intensity during chilling. Plants were most sensitive to chilling in light at the beginning of the day (12-h light). During the night they were less sensitive. This rhythm disappeared in continuous light, but was induced again by 12 or 6 h of darkness. The rhythm seemed to be endogenously controlled. Keywords: chilling; cucumber; diurnal rhythm; photo-oxidation.
INTRODUCTION
Cucumbers are chilling-sensitive plants. They are severely damaged at temperatures just above 0 ° C. Chilling in the light generally intensifies the chilling injury (Oequist, 1983). Van Hasselt (1972) showed a faster pigment degradation of leaf discs from cucumbers chilled in light. Symptoms on leaves after chilling in light are different from those in dark (Rietze and Wiebe, 1987). In the light, plants exhibited necrosis especially on the youngest leaves. A daily rhythm in the chilling sensitivity in the dark has been observed. King et al. (1982) showed that the chilling sensitivity of tomato seedlings strongly increased during the last hours of the night period. They also generalized that this phenomenon occurred in other species. The question of a possible rhythm of sensitivity to chilling in light is investigated in this study. 0304-4238/89/$03.50
© 1989 Elsevier Science Publishers B.V.
232 MATERIAL AND METHODS
C u l t i v a t i o n a n d chilling t r e a t m e n t . - Cucumbers ('Corona') were sown in growth chambers (24°C). Light (49 W m -2) was given from 0800 to 2000 h with fluorescent lamps "Cool White" and incandescent lamps. The saturation deficit was 700 Pa. Plants were pricked off after a week into 12-cm plastic pots. The temperature was then lowered to constant 20 °C (other factors as above). The chilling treatment of 4 h at 2°C with light (49 W m -2) and a saturation deficit of 30 Pa was imposed 3 weeks later. The plants were moved directly from the climate room of 20 °C to the climate room with chilling conditions. Therefore the temperature decreased within a few minutes. The plants were placed in continuous light for 3 days after the treatment. Final leaf area of the youngest leaf, a good measurement for the damage (Fig. 1), was assessed 3 weeks after the treatment. O x y g e n - f r e e a t m o s p h e r e . - In order to test whether the damage is dependent on photo-oxidation, in one experiment the youngest leaf was enclosed in a glass box through which nitrogen was passed. The leaf of a control plant was also put in a glass box but treated with normal air in the same way. The plants were chilled during the first 4 hours of the day (08.00-12.00 h).
Fig. 1. Cucumbers chilled at 4 days (every day 4 h from 08.00 to 12.00 h) at 2°C (youngest leaf is above ). Left: chilled in light (10 klx); right: chilled in darkness.
233 RESULTS
Chilling damage in light The damage and its inhibition in an oxygen-free atmosphere. - Three days after the chilling t r e a t m e n t visible damage developed, with the youngest leaves most affected. The symptoms ranged from slight necrosis along the veins to strong necrosis over the whole leaf area. The growth of the youngest leaf was also affected. Figure 1 shows a severely damaged plant (chilled 4 times for 4 h in light from 08.00 to 12.00 h) in comparison with a plant which received the same chilling t r e a t m e n t in the dark. The difference in leaf area of the youngest leaves is clearly shown. In each of 3 replications the leaf in an oxygen-free atmosphere remained z undamaged, while the leaf in normal air showed the typical symptoms. Influence of light intensity. - In this experiment the chilling treatment was applied for 4 days (every day for 4 h). The damage was reduced by decreasing light intensity (Table 1). At 49 W m -2 the leaf was destroyed and at 4.9 or 0.49 W m -2 the leaves were significantly more damaged than in the dark. Decreasing of the damage by following darkness. - The chilling treatment was applied during the first 4 h of the day period (08.00-12.00 h). After that the plants were given dark periods of different lengths at 20 °C before they were returned to 20°C in the light. Increasing the length of the dark period after chilling in light decreased the damage (Table 2 ). After at least I h in the dark the differences with the control were significant. Dtur na7 rhythm in the chilling sensitivity in light The chilling t r e a t m e n t in light was applied for 4 h at different times of day. In spite of the same chilling and post-chilling conditions the plants were most TABLE1 Influence of the light intensity during chilling on damage to cumumber (chilled four times for 4 h at 2°C) Treatment
Leaf area (am2)
Control (20°C) Dark 0.49 W m -2 4.9 W m -2 49 W m -2
118a 74b 55c 50¢ - (destroyed)
Differing superscripts indicate significant differences at 5% levelby the Tukey test.
234 TABLE 2 Effect of darkness after chilling in light (2 ° C ) on the damage to cucumber Duration of dark period (20°C) after chilling in light (08.00-12.00 h)
Leaf area (cm2)
0 min 30 min 1h 4h 8h 16 h
53a 109ab 159bc 138b 134b 154b¢
Control 20 oC
203c
Differing superscripts indicate significant differences at 5% level by the Tukey test. TABLE 3 Rhythm of chilling sensitivity in light of cucumbers. The plants from each treatment were taken out of the normal day period, light from 08.00 to 20.00 h and dark from 20.00 to 08.00 h Chilling period
Leaf area (cm~)
08.00-12.00 12.00-16.00 16.00-20.00 20.00-24.00 24.00-04.00 04.00-08.00
h h h h h h
20a 40a 86b 123c 180d 166d
Control 20 ° C
191d
Differing superscripts indicate significant differences at 5% level by the Tukey test. s e n s i t i v e to t h e t r e a t m e n t a t t h e b e g i n n i n g o f t h e d a y ( T a b l e 3). T h e m o s t d a m a g e a p p e a r e d d u r i n g t h e first 8 h o f t h e light p e r i o d a n d w a s less a t t h e b e g i n n i n g of t h e night.
Regulation of the diurnal rhythm of chilling sensitivity to light Behaviour of the rhythm in continuous light. - T a b l e 4 s h o w s t h a t t h e d a m a g e w a s (as e x p e c t e d f r o m T a b l e 3) m o s t e x t r e m e in t h e m o r n i n g o f t h e first day. T h e n t h e d a m a g e d e c r e a s e d , a l t h o u g h light w a s given b e f o r e ( i n s t e a d o f t h e n o r m a l n i g h t in T a b l e 3 ). F r o m 08.00 h o f t h e following d a y to 04.00 h o f t h e third day the damage remained the same.
Induction of the rhythm. - I t w a s t e s t e d w h e t h e r t h e r h y t h m could b e i n d u c e d
235 TABLE 4 The rhythm of chilling sensitivity of cucumbers in continuous light adapted to a 12-h day. Light was switched on at 08.00 h on the first day (leaf area in cm 2) Chilling period
Day 1
Day 2
Day 3
08.00-12.00 12.00-16.00 16.00-20.00 20.00-24.00 24.00-04.00 04.00-08.00
45 ~ 8~ 27 ~ 175 b 196 b 144 b
189 b 140 b 126 b 144 b 115 b 176 b
152 b 127 b 161 b 139 b 155 b 158 b
h h h h h h
Differing superscripts indicate significant differences at 5% level by the Tukey test. TABLE5 Induction of the rhythm of chilling sensitivity of cucumbers after a continuous light period (leaf area in cm 2) (a) Induction by 12 or 6 h dark and 144 h light as control (continuous light followed by dark as shown ) Chilling period
132 h light, 12 h dark
138 h light, 6 h dark
144 h light
08.00-12.00 12.00-16.00 16.00-20.00 20.00-24.00 24.00-04.00 04.00-08.00
122 ~ 42 b 67 ab 106 a 147 a 135 ~
139 ~ 74 b 65 b 122 ~ 152 ~ 132 a
124 a 115 a 137 a 148 ~ 114 ~ 123 ~
h h h h h h
(b) Induction by 12 h dark at the "wrong" time (continuous light followed by dark) Chilling period
144 h light, 12 h dark
20.00-24.00 24.00-04.00 04.00-08.00 08.00-12.00 12.00-16.00 16.00-20.00
113 a 20 b 61 b 146 ~c 168 c 142 ~c
h h h h h h
Differing superscripts indicate significant differences at 5% level by the Tukey test. after several days in continuous
light. Some plants served as controls and re-
c e i v e d 144 h c o n t i n u o u s l i g h t b e f o r e t h e c h i l l i n g t r e a t m e n t s . O t h e r p l a n t s r e c e i v e d 12 o r 6 h o f d a r k n e s s a f t e r 132 o r 1 3 8 h c o n t i n u o u s l i g h t , r e s p e c t i v e l y , b e f o r e t h e c h i l l i n g t r e a t m e n t s . S o m e o t h e r s a l s o r e c e i v e d 12 h d a r k b e f o r e t h e
236 chilling treatment, but after 144 h continuous light, which means at the "wrong" time. After 144 h continuous light the rhythm disappeared (Table 5a). Twelve and 6 h of darkness could induce the rhythm. The period resulting in the most extreme damage was 12.00-16.00 h. So this induced rhythm is something different from that in Table 3. The rhythm was also induced if darkness was applied at the "wrong" time (during the day from 08.00 to 20.00 h) (Table 5b). DISCUSSION The observed damage after chilling in light was caused by two processes: photo-inhibition and photo-oxidation (Powles, 1984 ). Photo-oxidation means bleaching of leaf pigments and follows a photo-inhibition. Photo-inhibition has an effect on the turnover of proteins in Photosystem II (Kyle and Ohad, 1985) and disturbs the flow of electrons. But there is also evidence of photoinhibitional events beside photosynthesis, e.g. photo-inhibition of photolabile enzymes in the peroxisomes (Feierabend and Engel, 1986). The peroxisomal enzymes have a relation to photo-oxidation, but the mechanism is not yet clear (Feierabend, 1983). Van Hasselt ( 1972 ), working with leaf discs of cucumber, reported an inhibition of chilling damage in light and an oxygen-free atmosphere. He concluded that the damage was a photo-oxidation. So the damage of whole plants observed here is caused by photo-oxidation because it was also inhibited by an oxygen-free atmosphere. Higher light intensities increased photo-inhibition and photo-oxidation. Tanczos (1974) showed, with cucumber leaf discs, that the damage was doubled by increasing light intensities from 29.4 to 58.8 W m -2. Our results ranged from destruction at 49 W m -2 to slight damage at 0.49 W m -2. The damage is decreased by following darkness. Lasley et al. (1979) also reported a faster recovery of photosynthesis after chilling in light if the plants were returned to darkness. It seems that a kind of recovery in darkness is possible. Perhaps the recovery is related to activities of photolabile enzymes which recover faster in darkness. The observed rhythm of chilling sensitivity in light has an endogenous character. Severe damage occurred at the end of the night period (at 08.00 h) in spite of the fact that these plants had not received the change from dark to light before (Table 3). The damage was decreased after 12 h even if the light remained on (Table 4). Also the induction of the rhythm with only 6 h and with 12 h at the "wrong" time (Table 5) confirms the endogenous character. It is possible that the endogenous rhythm was no longer noticeable on the second day because the light afterwards included far red. Far red is known to decrease endogenous rhythms (Buenning, 1977). Many processes which are related to photo-inhibition and photo-oxidation
237 follow diurnal variations, b u t o f t e n it is n o t clear w h e t h e r t h e y are e n d o g e n o u s or exogenous. F u r t h e r m o r e , it is possible t h a t one o f t h e m or several t o g e t h e r are responsible for t h e o b s e r v e d r h y t h m . F o r example, t h e o x y g e n c o n c e n t r a t i o n varies in a c i r c a d i a n r h y t h m as s h o w n with t h e algae A c e t a b u l a r i a (van den Driessche, 1984). T h e f o r m a t i o n of some p r o t e i n s of P h o t o s y s t e m II also shows a circadian r h y t h m ( K l o p p s t e c h , 1985). L e a f fluorescence i n d u c t i o n kinetics v a r y d i u r n a l l y ( E v e r s o n et al., 1983 ). T h i s m e a n s t h a t the state of the t h y l a k o i d m e m b r a n e s is c h a n g i n g d u r i n g the day. W i t h t h e s e e x p e r i m e n t s , it was n o t possible to deduce t h e m e c h a n i s m of the r h y t h m . N e v e r t h e l e s s t h e e x i s t e n c e of such a r h y t h m m a y be i n t e r e s t i n g for f u r t h e r physiological r e s e a r c h a n d for strategies of climate c o n t r o l in the glasshouse. ACKNOWLEDGEMENT We t h a n k t h e D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t for financial assistance. REFERENCES Buenning, E., 1977. Die Physiologische Uhr. Springer Verlag, Berlin-Heidelberg-New York, 3rd edn., 186 pp. Everson, G., Chen, S.-S. and Black, C.C., 1983. Diurnal variations in leaf fluorescence induction kinetics. Plant Physiol., 72: 455-460. Feierabend, J., 1983. Mode of interference of chlorosis-inducing herbicides with peroxisomal enzyme activities. Physiol. Plant., 57: 346-351. Feierabend, J. and Engel, S., 1986. Photoinactivation of catalase in vitro and in leaves. Arch. Biochem. Biophys., 251: 567-576. King, A.J., Reid, M.S. and Patterson, B.D., 1982. Diurnal changes in the chilling sensitivity of seedlings. Plant Physiol., 70:211-214. Kloppstech, K., 1985. Diurnal and circadian rhythmicity in the expression of light-induced plant nuclear messenger RNAs. Planta, 165: 502-506. Kyle, D.J. and Ohad, I., 1985. The mechanism of photoinhibition in higher plants and green algae. Encyclopaedia of Plant Physiology, Vol. 19, Springer Verlag, Berlin-Heidelberg-New York, pp. 468-475. Lasley, S.E., Garber, M.P. and Hodges, C.F., 1979. After effects of light and chilling temperature on photosynthesis in excised cucumber cotyledons. J. Am. Soc. Hortic. Sci., 104: 477-480. Oequist, G., 1983. Effects of low temperature on photosynthesis. Plant Cell Environ., 6: 281-300. Powles, S.B., 1984. Photoinhibition of photosynthesis induced by visible light. Annu. Rev. Plant Physiol., 35: 15-44. Rietze, E. and Wiebe, H.-J., 1987. K~ilte im Licht m6gen Pflanzen nicht. Deutsche Gartenbauges., 41: 2436-2437. Tanczos, O.G., 1974. Invloed van lage temperatuur op de bladeren van cucumis sativus. Proefschrift, Rijksuniversiteit Groningen, The Netherlands, pp. 89. Van den Driessche, T., 1984. Photosynthesis, circadian rhythms and herbicides. In: C. Sybesma (Editor), Advances in Photosynthesis Research, Vol. IV. Nijhoff and Junk, The Hague, pp. 301-304. Van Hasselt, P.R., 1972. Photooxidation of leaf pigments in cucumis leaf discs during chilling. Acta Bot. Neerl., 21: 539-548.
231
Diurnal Rhythm of Chilling Sensitivity of Cucumbers in Light E. RIETZE and H.-J. WIEBE
Institut fi~r Gemiisebau, Universiti~t Hannover, Herrenh~user Str. 2, 3000 Hannover 21 (F.R.G.) (Accepted for publication 19 August 1988)
ABSTRACT
Rietze, E. and Wiebe, H.-J., 1989. Diurnal rhythm of chilling sensitivity of cucumbers in light. Scientia Hortic., 38: 231-237. Cucumbers were grown at 20°C and chilled in light (4 h at 2°C and 49 W m-2). Resulting damage (photo-oxidation) was inhibited in an oxygen-free atmosphere and decreased by a subsequent dark period. It was increased by increasing light intensity during chilling. Plants were most sensitive to chilling in light at the beginning of the day (12-h light). During the night they were less sensitive. This rhythm disappeared in continuous light, but was induced again by 12 or 6 h of darkness. The rhythm seemed to be endogenously controlled. Keywords: chilling; cucumber; diurnal rhythm; photo-oxidation.
INTRODUCTION
Cucumbers are chilling-sensitive plants. They are severely damaged at temperatures just above 0 ° C. Chilling in the light generally intensifies the chilling injury (Oequist, 1983). Van Hasselt (1972) showed a faster pigment degradation of leaf discs from cucumbers chilled in light. Symptoms on leaves after chilling in light are different from those in dark (Rietze and Wiebe, 1987). In the light, plants exhibited necrosis especially on the youngest leaves. A daily rhythm in the chilling sensitivity in the dark has been observed. King et al. (1982) showed that the chilling sensitivity of tomato seedlings strongly increased during the last hours of the night period. They also generalized that this phenomenon occurred in other species. The question of a possible rhythm of sensitivity to chilling in light is investigated in this study. 0304-4238/89/$03.50
© 1989 Elsevier Science Publishers B.V.
232 MATERIAL AND METHODS
C u l t i v a t i o n a n d chilling t r e a t m e n t . - Cucumbers ('Corona') were sown in growth chambers (24°C). Light (49 W m -2) was given from 0800 to 2000 h with fluorescent lamps "Cool White" and incandescent lamps. The saturation deficit was 700 Pa. Plants were pricked off after a week into 12-cm plastic pots. The temperature was then lowered to constant 20 °C (other factors as above). The chilling treatment of 4 h at 2°C with light (49 W m -2) and a saturation deficit of 30 Pa was imposed 3 weeks later. The plants were moved directly from the climate room of 20 °C to the climate room with chilling conditions. Therefore the temperature decreased within a few minutes. The plants were placed in continuous light for 3 days after the treatment. Final leaf area of the youngest leaf, a good measurement for the damage (Fig. 1), was assessed 3 weeks after the treatment. O x y g e n - f r e e a t m o s p h e r e . - In order to test whether the damage is dependent on photo-oxidation, in one experiment the youngest leaf was enclosed in a glass box through which nitrogen was passed. The leaf of a control plant was also put in a glass box but treated with normal air in the same way. The plants were chilled during the first 4 hours of the day (08.00-12.00 h).
Fig. 1. Cucumbers chilled at 4 days (every day 4 h from 08.00 to 12.00 h) at 2°C (youngest leaf is above ). Left: chilled in light (10 klx); right: chilled in darkness.
233 RESULTS
Chilling damage in light The damage and its inhibition in an oxygen-free atmosphere. - Three days after the chilling t r e a t m e n t visible damage developed, with the youngest leaves most affected. The symptoms ranged from slight necrosis along the veins to strong necrosis over the whole leaf area. The growth of the youngest leaf was also affected. Figure 1 shows a severely damaged plant (chilled 4 times for 4 h in light from 08.00 to 12.00 h) in comparison with a plant which received the same chilling t r e a t m e n t in the dark. The difference in leaf area of the youngest leaves is clearly shown. In each of 3 replications the leaf in an oxygen-free atmosphere remained z undamaged, while the leaf in normal air showed the typical symptoms. Influence of light intensity. - In this experiment the chilling treatment was applied for 4 days (every day for 4 h). The damage was reduced by decreasing light intensity (Table 1). At 49 W m -2 the leaf was destroyed and at 4.9 or 0.49 W m -2 the leaves were significantly more damaged than in the dark. Decreasing of the damage by following darkness. - The chilling treatment was applied during the first 4 h of the day period (08.00-12.00 h). After that the plants were given dark periods of different lengths at 20 °C before they were returned to 20°C in the light. Increasing the length of the dark period after chilling in light decreased the damage (Table 2 ). After at least I h in the dark the differences with the control were significant. Dtur na7 rhythm in the chilling sensitivity in light The chilling t r e a t m e n t in light was applied for 4 h at different times of day. In spite of the same chilling and post-chilling conditions the plants were most TABLE1 Influence of the light intensity during chilling on damage to cumumber (chilled four times for 4 h at 2°C) Treatment
Leaf area (am2)
Control (20°C) Dark 0.49 W m -2 4.9 W m -2 49 W m -2
118a 74b 55c 50¢ - (destroyed)
Differing superscripts indicate significant differences at 5% levelby the Tukey test.
234 TABLE 2 Effect of darkness after chilling in light (2 ° C ) on the damage to cucumber Duration of dark period (20°C) after chilling in light (08.00-12.00 h)
Leaf area (cm2)
0 min 30 min 1h 4h 8h 16 h
53a 109ab 159bc 138b 134b 154b¢
Control 20 oC
203c
Differing superscripts indicate significant differences at 5% level by the Tukey test. TABLE 3 Rhythm of chilling sensitivity in light of cucumbers. The plants from each treatment were taken out of the normal day period, light from 08.00 to 20.00 h and dark from 20.00 to 08.00 h Chilling period
Leaf area (cm~)
08.00-12.00 12.00-16.00 16.00-20.00 20.00-24.00 24.00-04.00 04.00-08.00
h h h h h h
20a 40a 86b 123c 180d 166d
Control 20 ° C
191d
Differing superscripts indicate significant differences at 5% level by the Tukey test. s e n s i t i v e to t h e t r e a t m e n t a t t h e b e g i n n i n g o f t h e d a y ( T a b l e 3). T h e m o s t d a m a g e a p p e a r e d d u r i n g t h e first 8 h o f t h e light p e r i o d a n d w a s less a t t h e b e g i n n i n g of t h e night.
Regulation of the diurnal rhythm of chilling sensitivity to light Behaviour of the rhythm in continuous light. - T a b l e 4 s h o w s t h a t t h e d a m a g e w a s (as e x p e c t e d f r o m T a b l e 3) m o s t e x t r e m e in t h e m o r n i n g o f t h e first day. T h e n t h e d a m a g e d e c r e a s e d , a l t h o u g h light w a s given b e f o r e ( i n s t e a d o f t h e n o r m a l n i g h t in T a b l e 3 ). F r o m 08.00 h o f t h e following d a y to 04.00 h o f t h e third day the damage remained the same.
Induction of the rhythm. - I t w a s t e s t e d w h e t h e r t h e r h y t h m could b e i n d u c e d
235 TABLE 4 The rhythm of chilling sensitivity of cucumbers in continuous light adapted to a 12-h day. Light was switched on at 08.00 h on the first day (leaf area in cm 2) Chilling period
Day 1
Day 2
Day 3
08.00-12.00 12.00-16.00 16.00-20.00 20.00-24.00 24.00-04.00 04.00-08.00
45 ~ 8~ 27 ~ 175 b 196 b 144 b
189 b 140 b 126 b 144 b 115 b 176 b
152 b 127 b 161 b 139 b 155 b 158 b
h h h h h h
Differing superscripts indicate significant differences at 5% level by the Tukey test. TABLE5 Induction of the rhythm of chilling sensitivity of cucumbers after a continuous light period (leaf area in cm 2) (a) Induction by 12 or 6 h dark and 144 h light as control (continuous light followed by dark as shown ) Chilling period
132 h light, 12 h dark
138 h light, 6 h dark
144 h light
08.00-12.00 12.00-16.00 16.00-20.00 20.00-24.00 24.00-04.00 04.00-08.00
122 ~ 42 b 67 ab 106 a 147 a 135 ~
139 ~ 74 b 65 b 122 ~ 152 ~ 132 a
124 a 115 a 137 a 148 ~ 114 ~ 123 ~
h h h h h h
(b) Induction by 12 h dark at the "wrong" time (continuous light followed by dark) Chilling period
144 h light, 12 h dark
20.00-24.00 24.00-04.00 04.00-08.00 08.00-12.00 12.00-16.00 16.00-20.00
113 a 20 b 61 b 146 ~c 168 c 142 ~c
h h h h h h
Differing superscripts indicate significant differences at 5% level by the Tukey test. after several days in continuous
light. Some plants served as controls and re-
c e i v e d 144 h c o n t i n u o u s l i g h t b e f o r e t h e c h i l l i n g t r e a t m e n t s . O t h e r p l a n t s r e c e i v e d 12 o r 6 h o f d a r k n e s s a f t e r 132 o r 1 3 8 h c o n t i n u o u s l i g h t , r e s p e c t i v e l y , b e f o r e t h e c h i l l i n g t r e a t m e n t s . S o m e o t h e r s a l s o r e c e i v e d 12 h d a r k b e f o r e t h e
236 chilling treatment, but after 144 h continuous light, which means at the "wrong" time. After 144 h continuous light the rhythm disappeared (Table 5a). Twelve and 6 h of darkness could induce the rhythm. The period resulting in the most extreme damage was 12.00-16.00 h. So this induced rhythm is something different from that in Table 3. The rhythm was also induced if darkness was applied at the "wrong" time (during the day from 08.00 to 20.00 h) (Table 5b). DISCUSSION The observed damage after chilling in light was caused by two processes: photo-inhibition and photo-oxidation (Powles, 1984 ). Photo-oxidation means bleaching of leaf pigments and follows a photo-inhibition. Photo-inhibition has an effect on the turnover of proteins in Photosystem II (Kyle and Ohad, 1985) and disturbs the flow of electrons. But there is also evidence of photoinhibitional events beside photosynthesis, e.g. photo-inhibition of photolabile enzymes in the peroxisomes (Feierabend and Engel, 1986). The peroxisomal enzymes have a relation to photo-oxidation, but the mechanism is not yet clear (Feierabend, 1983). Van Hasselt ( 1972 ), working with leaf discs of cucumber, reported an inhibition of chilling damage in light and an oxygen-free atmosphere. He concluded that the damage was a photo-oxidation. So the damage of whole plants observed here is caused by photo-oxidation because it was also inhibited by an oxygen-free atmosphere. Higher light intensities increased photo-inhibition and photo-oxidation. Tanczos (1974) showed, with cucumber leaf discs, that the damage was doubled by increasing light intensities from 29.4 to 58.8 W m -2. Our results ranged from destruction at 49 W m -2 to slight damage at 0.49 W m -2. The damage is decreased by following darkness. Lasley et al. (1979) also reported a faster recovery of photosynthesis after chilling in light if the plants were returned to darkness. It seems that a kind of recovery in darkness is possible. Perhaps the recovery is related to activities of photolabile enzymes which recover faster in darkness. The observed rhythm of chilling sensitivity in light has an endogenous character. Severe damage occurred at the end of the night period (at 08.00 h) in spite of the fact that these plants had not received the change from dark to light before (Table 3). The damage was decreased after 12 h even if the light remained on (Table 4). Also the induction of the rhythm with only 6 h and with 12 h at the "wrong" time (Table 5) confirms the endogenous character. It is possible that the endogenous rhythm was no longer noticeable on the second day because the light afterwards included far red. Far red is known to decrease endogenous rhythms (Buenning, 1977). Many processes which are related to photo-inhibition and photo-oxidation
237 follow diurnal variations, b u t o f t e n it is n o t clear w h e t h e r t h e y are e n d o g e n o u s or exogenous. F u r t h e r m o r e , it is possible t h a t one o f t h e m or several t o g e t h e r are responsible for t h e o b s e r v e d r h y t h m . F o r example, t h e o x y g e n c o n c e n t r a t i o n varies in a c i r c a d i a n r h y t h m as s h o w n with t h e algae A c e t a b u l a r i a (van den Driessche, 1984). T h e f o r m a t i o n of some p r o t e i n s of P h o t o s y s t e m II also shows a circadian r h y t h m ( K l o p p s t e c h , 1985). L e a f fluorescence i n d u c t i o n kinetics v a r y d i u r n a l l y ( E v e r s o n et al., 1983 ). T h i s m e a n s t h a t the state of the t h y l a k o i d m e m b r a n e s is c h a n g i n g d u r i n g the day. W i t h t h e s e e x p e r i m e n t s , it was n o t possible to deduce t h e m e c h a n i s m of the r h y t h m . N e v e r t h e l e s s t h e e x i s t e n c e of such a r h y t h m m a y be i n t e r e s t i n g for f u r t h e r physiological r e s e a r c h a n d for strategies of climate c o n t r o l in the glasshouse. ACKNOWLEDGEMENT We t h a n k t h e D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t for financial assistance. REFERENCES Buenning, E., 1977. Die Physiologische Uhr. Springer Verlag, Berlin-Heidelberg-New York, 3rd edn., 186 pp. Everson, G., Chen, S.-S. and Black, C.C., 1983. Diurnal variations in leaf fluorescence induction kinetics. Plant Physiol., 72: 455-460. Feierabend, J., 1983. Mode of interference of chlorosis-inducing herbicides with peroxisomal enzyme activities. Physiol. Plant., 57: 346-351. Feierabend, J. and Engel, S., 1986. Photoinactivation of catalase in vitro and in leaves. Arch. Biochem. Biophys., 251: 567-576. King, A.J., Reid, M.S. and Patterson, B.D., 1982. Diurnal changes in the chilling sensitivity of seedlings. Plant Physiol., 70:211-214. Kloppstech, K., 1985. Diurnal and circadian rhythmicity in the expression of light-induced plant nuclear messenger RNAs. Planta, 165: 502-506. Kyle, D.J. and Ohad, I., 1985. The mechanism of photoinhibition in higher plants and green algae. Encyclopaedia of Plant Physiology, Vol. 19, Springer Verlag, Berlin-Heidelberg-New York, pp. 468-475. Lasley, S.E., Garber, M.P. and Hodges, C.F., 1979. After effects of light and chilling temperature on photosynthesis in excised cucumber cotyledons. J. Am. Soc. Hortic. Sci., 104: 477-480. Oequist, G., 1983. Effects of low temperature on photosynthesis. Plant Cell Environ., 6: 281-300. Powles, S.B., 1984. Photoinhibition of photosynthesis induced by visible light. Annu. Rev. Plant Physiol., 35: 15-44. Rietze, E. and Wiebe, H.-J., 1987. K~ilte im Licht m6gen Pflanzen nicht. Deutsche Gartenbauges., 41: 2436-2437. Tanczos, O.G., 1974. Invloed van lage temperatuur op de bladeren van cucumis sativus. Proefschrift, Rijksuniversiteit Groningen, The Netherlands, pp. 89. Van den Driessche, T., 1984. Photosynthesis, circadian rhythms and herbicides. In: C. Sybesma (Editor), Advances in Photosynthesis Research, Vol. IV. Nijhoff and Junk, The Hague, pp. 301-304. Van Hasselt, P.R., 1972. Photooxidation of leaf pigments in cucumis leaf discs during chilling. Acta Bot. Neerl., 21: 539-548.