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Modification of chilling susceptibility in seedlings of cucumber and zucchini squash by the bioregulator paclobutrazol (PP333)
Scientia Horticulturae, 26 (1985) 293--298 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
293
M O D I F I C A T I O N O F C H I L L I N G S U S C E P T I B I L I T Y IN S E E D L I N G S O F C U C U M B E R A N D Z U C C H I N I S Q U A S H BY T H E B I O R E G U L A T O R P A C L O B U T R A Z O L (PP333)
CHIEN "krlWANG Horticultural Crops Quality Laboratory, Agricultural Research Service, U.S.D.A., Beltsville, MD 20705 (U.S.A.) (Accepted for publication 27 March 1985)
ABSTRACT Wang, C',.Y., 1985. Modification of chilling susceptibility in seedlings of cucumber and zucctdni squash by the bioregulator paclobutrazol (PP333). Scientia Hortic., 26: 293--298. Application of paclobutrazol at 1, 5 or 10 mg 1-~ by soil drench inhibited growth and increased chlorophyll content of leaf discs of cucumber and zucchini squash. The treatments also significantly postponed symptoms of chilling injury of seedlings at 5° C. The degree of protection from injury sustained at low temperature increased with increasing duration of the paclobutrazol treatment. Cucumber seedlings were more sensitive to chilling injury and were more responsive to the paclobutrazol treatment than seedlings of zucchini squash. Keywords: chilling injury; chlorophyll; Cucumis sativus L.; Cucurbita pepo L.; growth inhibitor; paclobutrazol. Abbreviation: paclobutrazol = PP333, (2RS, 3RS)-l-(4-chlorophenyl)-4, 4-dimethyl-2 (1, 2, 4-triazol-l-yl)pentan-3-ol.
INTRODUCTION Seedlings o f c u c u m b e r ( C u c u m i s sativus L.) and zucchini squash (Cucurbita p e p o L.) are sensitive t o chilling injury. E x p o s u r e o f these seedlings t o chilling t e m p e r a t u r e s results in wilting, desiccation and necrosis o f leaves or d e a t h o f the w h o l e seedling (Wolk and Herner, 1982). D e v e l o p m e n t o f s o m e applied t e c h n i q u e s t o alleviate chilling i n j u r y is n e e d e d so t h a t these crops can be cultivated over w i d e r growing regions and f o r longer growing periods. M o d i f i c a t i o n o f chilling susceptibility b y chemicals is o n e o f t h e feasible ways t;o r e d u c e injury (Wang, 1982). Abscisic acid has b e e n s h o w n t o ameliorate chilling injury in c u c u m b e r seedlings (Rikin and R i c h m o n d , 1976). P r o t e c t i o n o f severely chilled c u c u m b e r plants by m e f l u i d i d e has also b e e n d e m o n s t r a t e d (Tseng and Li, 1984). 0304-4238/85/$03.30
© 1985 Elsevier Science Publishers B.V.
294
Recently, a plant bioregulator [(2RS, 3RS)-l-(4-chlorophenyl)-4, 4dimethyl-2-(1, 2, 4-triazol-l-yl) pentan-3~l] (paclobutrazol; PP333) has been found to effectively retard plant growth and interfere with gibberellic acid (GA3) biosynthesis (Couture, 1982; Graebe, 1982; Quinlan and Richardson, 1983; Steffens et al., 1983; Williams, 1983). It can also regulate various metabolic processes in apple seedlings (Wang et al., 1985). The present study was initiated to evaluate the effectiveness of paclobutrazol on the reduction of chilling injury in cucumber and zucchini squash seedlings. MATERIALS AND METHODS
Cucumber and zucchini squash seedlings were grown in l l - c m diameter pots with a perlite:peat moss:compost (1:1:1, v/v/v) mixture. Greenhouse temperatures were approximately 25°C during the day and 20°C at night. The plants were watered daily. Fertilizer was apphed 10 days after planting with Peter's water-soluble fertilizer (N :P :K, 20:20: 20). Seedlings were treated with paclobutrazol when 2 true leaves were showing (14 days after planting). A 100-ml aliquot of aqueous paclobutrazol solution at concentrations of 0, 1, 5 or 10 mg 1-1 was added to each pot. Seven days after treatment, measurements were taken from 8 plants in each treatment for plant height, length of stem and length of the longest leaf. Five 8-mm diameter leaf discs were taken from each of 3 plants in a treatm e n t for chlorophyll determination. Leaf discs were extracted with 80% ethanol at 70°C for 10 min and chlorophyll content was measured spectrophotometrically (Bruinsma, 1961). Forty-eight seedlings were transferred from the greenhouse to a 5°C r o o m 7 and 14 days following the paclobutrazol treatment. Three pots from each treatment were transferred daffy from 5 to 25°C for evaluation o f chilling injury. Chilling injury in cucumber seedlings was characterized by the water-soaked appearance, wilting and dehydration of the leaves. The symptoms o f chilling injury in zucchini squash seedlings included wilting, marginal necrosis, and inward curl of the leaves. The severity of the injury was rated on a scale of 1--5, ranging from normal to severe. RESULTS
Paclobutrazol treatment suppressed plant height, length of the stem and length of the longest leaf in both cucumber and zucchini squash seedlings (Table I). The reduction was significant in all concentrations used. In zucchini squash, the inhibition of growth by 1 mg 1-~ paclobutrazol was comparable to that by 5 or 10 mg 1-1 , whereas in cucumber, it appeared that the higher the concentration the higher the inhibition. The greatest difference between the control and the treated samples existed in the height of the plants. Chlorophyll contents in the leaves were increased in the treated samples
295 TABLE I Effect of paclobutrazol on the growth and chlorophyll content of cucumber and zucchini squash seedlings. Measurements were made 7 days after the treatment. Mean separation in columns by Duncan's multiple range test, 5% level
Treatment
Plantheight
Stem length
Leaf length
Chlorophyll
(rag 1-~)
(cm)
(cm)
(cm)
(OD at 665 nm)
13.8a 9.2b 8.4bc 7.6c
10.7a 8.1b 6.4c 6.0c
0.41a 0.63b 0.82c 0.98d
6.9a 3.8b 4.1b 3.7b
7.8a 6.5b 6.5b 6.2b
0.34a 0.61b 0.68b 0.72b
Cucumberseedlings 0 18.2a 1 13.9b 5 12.5c 10 ll.4d Zucc~nisqua~seedlings 0 19.4a 1 ll.7b 5 ll.3b 10 lI.6b
in b o t h cucumber and zucchini squash seedlings (Table I). Similar increases in chlorophyll contents were found in zucchini squash treated with different concentrations, b u t in cucumber seedlings the contents increased with the concentrations used. More than twice as much chlorophyll was found in the 10 mg 1-~-treated cucumber seedlings than in the untreated samples. Chilling injury to cucumber seedlings initially appeared as water-soaked spots on the leaves. This s y m p t o m was observed while the seedlings were still in the chilling temperature (5°C). This was followed b y the wilting of the lemres and the collapse of the whole seedlings. The wilting and the collapse of the plants usually did not take place until after transfer to 25°C. I:[ the injury was slight, the seedlings could regain their vigor within 5 hour.,, at 25°C. However, if the damage was severe, wilting persisted and the seedlings eventually desiccated. The control cucumber seedlings developed symptoms of chilling injury within '.2 days of exposure to 5°C (Fig. 1). The wilting of the leaves became irreversible after 4 days at 5°C. Treated seedlings did not show any sign o f clamage until after 3 days at 5° C. Seedlings treated with 1 and 5 mg 1-~ of paclobutrazol wilted after 4 and 5 days at 5°C following transfer to 25°C, but t h e y recovered and resumed growth. The injury became irreversible after 6 days at 5°C for 1 and 5 mg 1-1 treatments and after 7 days for the 10 mg 1-~ treatment. Initial s y m p t o m s of chilling injury at 5°C in zucchini squash seedlings w e r e similar to those observed in cucumber seedlings. However, the leaves o f zucchini squash did not wilt as readily as those of cucumber seedlings. Instead, desiccation of the margins of leaves occurred and this was followed by discoloration and inward curl of the leaves. In severe cases, necrotic spots appeared along the vein and the entire leaf eventually dehydrated.
296 s F cucuM.ER
7
.......
,~
I
I
J [ ] 1 rng/i paclob~Jtrazol •
~4
0
1"15
"
-
R]lo
-
.
1
I
•
IBm1
•
i~l
I~ /
4
3
2
3
4
5
6
7
Days at 5 ° C ,
Fig. 1. Severity o f chilling i n j u r y in c u c u m b e r seedlings t r e a t e d w i t h p a c l o b u t r a z o l . Seedlings were t r a n s f e r r e d f r o m t h e g r e e n h o u s e t o 5° C a t 7 days a f t e r t h e p a c l o b u t r a z o l t r e a t m e n t s . Chilling i n j u r y was e v a l u a t e d 24 h a f t e r t r a n s f e r o f seedlings f r o m 5 t o 25°C. T h e s e v e r i t y o f i n j u r y was r a t e d o n a scale o f 1--5 w i t h 1 = n o r m a l , 2 = trace, 3 = slight, 4 = m o d e r a t e , a n d 5 -- severe injury.
ilc°r° "° 5 j
ZUCCHINI
4
la 1
SQUASH
mg/I pscJobutrazol
r'15
3
•
•
"
"
L
~
- 5
PJl
" 4
- 3
2
" 2
1
1
0
3
4
5
6
7
8
D a y s at 5 ° C .
Fig. 2. S e v e r i t y o f chilling i n j u r y in z u c c h i n i s q u a s h seedlings t r e a t e d w i t h p a c l o b u t r a z o l .
Details as in Fig. 1. The symptoms of chilling injury could proceed slowly if the seedlings were kept at the chilling temperature, but the development of s y m p t o m s was hastened after transfer to 25 ° C. Zucchini squash seedlings were less sensitive to chilling injury than cucumber seedlings. Control seedlings of zucchini squash did not show any symptoms of chilling injury until after 4 days of exposure to 50C, and it t o o k 7 days of exposure at 50C for the control plants to reach the severe injury stage (Fig. 2). Paclobutrazol treatment also effectively delayed the progress of symptoms of chilling damage, although the beneficial effect was not as dramatic as that in cucumber seedlings. Chilling damage of treated seedlings became irreversible after 7 days at 5° C. Chilling injury symptoms of control seedlings of cucumber and zucchini squash transferred from the greenhouse to 50C at 14 days after the paclobutrazol treatment generally followed the same pattern as those transferred 7 days after treatment. In the treated seedlings, however, the development of chilling injury symptoms in the 14-day samples was further delayed for 1 or 2 days longer (data not shown). It appeared that the longer the duration of the paclobutrazol treatment, the more effective was the protection.
297 DISCUSSION
The marked inhibition of growth of seedlings of cucumber and zucchini squash by paclobutrazol in this study parallels the results found in sunflowers (Wample and Culver, 1983), rice seedlings (Jung and Rademacher, 1983} and apple and pear seedlings and trees (Quinlan and Richardson, 1983; Raese and Burts, 1983; Steffens et al., 1983; Williams, 1983). The morphological alterations induced by paclobutrazol appear to correlate to the increase of chilling resistance. Seedlings of cucumber and zucchini squash treated with paclobutrazol exhibited less wilting and desiccation following chilling exposure. The mechanism of the reduction of chilling injury by paclobutrazol is not known. Increase in resistance to low temperatures has been associated with a large decrease in GA content (Reid et ah, 1974) and an increase in ABA/GA ratio (Waldman et al., 1975). Since paclobutrazol inhibits GA biosynthesis (Couture, 1982; Graebe, 1982), it is possible that paclobutrazol may also enhance chilling resistance through modification of the hormonal balance. Chemical modification of chilling susceptibility reported previously has been achieved mostly through the regulation of membrane lipid synthesis and the alteration of the degree of fatty acid unsaturation (St. John and Christiansen, 1976; Waring et al., 1976; Willemot, 1977; Wang and Baker, 1979). The paclobutrazol treatment may also change the chilling sensitivity of seedlings by affecting the composition of cellular membranes. Further studies are desirable to determine if the increased chilling resistance is also related to membrane alterations induced by paclobutrazol. ACKNOWLEDGEMENTS
The author wishes to express his gratitude to Noreen Krause for her technic~J assistance and to ICI Americas Inc. for providing the paclobutrazol. Use of a company or product name by the U.S. Department of Agriculture does not imply approval or recommendation of the product to the exclusion of others which may also be suitable.
REFERENCES Bruinsma, J., 1961. A c o m m e n t on the spectrophotometric determination of chlorophyll. Biochim. Biophys. Acta, 52: 576--578. Couture, R.M., 1982. PP333: A new experimental plant growth regulator from ICI. Proc. Plant Growth Regulator Soc. Am., 9: 59. Graebe, J.E., 1982. Gihherellin biosynthesis in cell-free systems from higher plants. In: P.F. Wareing (Editor), Plant Growth Substances. Academic Press, London, pp. 71--80. Jung, J. and Rademacher, W., 1983. Plant growth regulating chemicals - - cereal grains. I~n: L.G. Nickell (Editor), Plant Growth Regulating Chemicals. Vol. 1. CRC Press, Boca Ratan, FL, pp. 253--271.
298 Quinlan, J.D. and Richardson, P.J., 1983. Effect of paclobutrazol (PP333) on apple shoot growth. Acta Hortic., 146: 96--102. Raese, J.T. and Burrs, E.C., 1983. Increased yield and suppression o f shoot growth and mite populations of ' d ' A n j o u ' pear trees with nitrogen and paclobutrazol. HortScience, 18:212-214. Reid, D.M., Pharis, R.P. and Roberts, D.W.A., 1974. Effects of four temperature regimes on the gibberellin content of winter wheat cv. Kharkov. Physiol. Plant., 30: 53--57. Rikin, A. and Richmond, A.E., 1976. Amelioration of chilling injuries in cucumber seedlings by abscisic acid. Physiol. Plant., 38: 95--97. Steffens, G.L., Wang, S.Y., Steffens, C.L. and Brennan, T., 1983. Influence of paclobutrazol (PP333) on apple seedling growth and physiology. Proc. Plant Growth Regulator Soc. Am., 10: 195--205. St. John, J.B. and Christiansen, M.N., 1976. Inhibition of linolenic acid synthesis and modification of chilling resistance in cotton seedlings. Plant Physiol., 57: 257--259. Tseng, M.J. and Li, P.H., 1984. Mefluidide protection of severely chilled crop plants. Plant Physiol., 75 : 249--250. Waldman, M., Rikin, A., Dovrat, A. and Richmond, A.E., 1975. Hormonal regulation of morphogenesis and cold-resistance. J. Exp. Bot., 26: 853--859. Wample, R.L. and Culver, E.B., 1983. The influence of paclobutrazol, a new growth regulator, on sunflowers. J. Am. Soc. Hortic. Sci., 108: 122--125. Wang, C.Y., 1982. Physiological and biochemical responses of plants to chilling stress. HortScience, 17 : 173--186. Wang, C.Y. and Baker, J.E., 1979. Effects of two free radical scavengers and intermittent warming on chilling injury and polar lipid composition of cucumber and sweet pepper fruits. Plant Cell Physiol., 20 : 243--251. Wang, S.Y., Byun, J.K. and Steffens, G.L., 1985. Controlling plant growth via the gibberellin biosynthesis system. II. Biochemical and physiological alterations in apple seedlings. Physiol. Plant., 63: 169--175. Waring, A.J., Breidenbach, R.W. and Lyons, J.M., 1976. In vivo modification of plant membrane phospholipid composition. Biochim. Biophys. Acta, 443: 157--168. Willemot, C., 1977. Simultaneous inhibition of linolenic acid synthesis in winter wheat roots and frost hardening by BASF 13-338, a derivative of pyridazinone. Plant Physiol., 60: 1--4. Williams, M.W., 1983. Use of bioregulators to control vegetative growth of fruit trees and improve fruiting efficiency. Acta Hortic., 146 : 88--95. Wolk, W.D. and Herner, R.C., 1982. Chilling injury o f germinating seeds and seedlings. HortScience, 17: 169--173.
293
M O D I F I C A T I O N O F C H I L L I N G S U S C E P T I B I L I T Y IN S E E D L I N G S O F C U C U M B E R A N D Z U C C H I N I S Q U A S H BY T H E B I O R E G U L A T O R P A C L O B U T R A Z O L (PP333)
CHIEN "krlWANG Horticultural Crops Quality Laboratory, Agricultural Research Service, U.S.D.A., Beltsville, MD 20705 (U.S.A.) (Accepted for publication 27 March 1985)
ABSTRACT Wang, C',.Y., 1985. Modification of chilling susceptibility in seedlings of cucumber and zucctdni squash by the bioregulator paclobutrazol (PP333). Scientia Hortic., 26: 293--298. Application of paclobutrazol at 1, 5 or 10 mg 1-~ by soil drench inhibited growth and increased chlorophyll content of leaf discs of cucumber and zucchini squash. The treatments also significantly postponed symptoms of chilling injury of seedlings at 5° C. The degree of protection from injury sustained at low temperature increased with increasing duration of the paclobutrazol treatment. Cucumber seedlings were more sensitive to chilling injury and were more responsive to the paclobutrazol treatment than seedlings of zucchini squash. Keywords: chilling injury; chlorophyll; Cucumis sativus L.; Cucurbita pepo L.; growth inhibitor; paclobutrazol. Abbreviation: paclobutrazol = PP333, (2RS, 3RS)-l-(4-chlorophenyl)-4, 4-dimethyl-2 (1, 2, 4-triazol-l-yl)pentan-3-ol.
INTRODUCTION Seedlings o f c u c u m b e r ( C u c u m i s sativus L.) and zucchini squash (Cucurbita p e p o L.) are sensitive t o chilling injury. E x p o s u r e o f these seedlings t o chilling t e m p e r a t u r e s results in wilting, desiccation and necrosis o f leaves or d e a t h o f the w h o l e seedling (Wolk and Herner, 1982). D e v e l o p m e n t o f s o m e applied t e c h n i q u e s t o alleviate chilling i n j u r y is n e e d e d so t h a t these crops can be cultivated over w i d e r growing regions and f o r longer growing periods. M o d i f i c a t i o n o f chilling susceptibility b y chemicals is o n e o f t h e feasible ways t;o r e d u c e injury (Wang, 1982). Abscisic acid has b e e n s h o w n t o ameliorate chilling injury in c u c u m b e r seedlings (Rikin and R i c h m o n d , 1976). P r o t e c t i o n o f severely chilled c u c u m b e r plants by m e f l u i d i d e has also b e e n d e m o n s t r a t e d (Tseng and Li, 1984). 0304-4238/85/$03.30
© 1985 Elsevier Science Publishers B.V.
294
Recently, a plant bioregulator [(2RS, 3RS)-l-(4-chlorophenyl)-4, 4dimethyl-2-(1, 2, 4-triazol-l-yl) pentan-3~l] (paclobutrazol; PP333) has been found to effectively retard plant growth and interfere with gibberellic acid (GA3) biosynthesis (Couture, 1982; Graebe, 1982; Quinlan and Richardson, 1983; Steffens et al., 1983; Williams, 1983). It can also regulate various metabolic processes in apple seedlings (Wang et al., 1985). The present study was initiated to evaluate the effectiveness of paclobutrazol on the reduction of chilling injury in cucumber and zucchini squash seedlings. MATERIALS AND METHODS
Cucumber and zucchini squash seedlings were grown in l l - c m diameter pots with a perlite:peat moss:compost (1:1:1, v/v/v) mixture. Greenhouse temperatures were approximately 25°C during the day and 20°C at night. The plants were watered daily. Fertilizer was apphed 10 days after planting with Peter's water-soluble fertilizer (N :P :K, 20:20: 20). Seedlings were treated with paclobutrazol when 2 true leaves were showing (14 days after planting). A 100-ml aliquot of aqueous paclobutrazol solution at concentrations of 0, 1, 5 or 10 mg 1-1 was added to each pot. Seven days after treatment, measurements were taken from 8 plants in each treatment for plant height, length of stem and length of the longest leaf. Five 8-mm diameter leaf discs were taken from each of 3 plants in a treatm e n t for chlorophyll determination. Leaf discs were extracted with 80% ethanol at 70°C for 10 min and chlorophyll content was measured spectrophotometrically (Bruinsma, 1961). Forty-eight seedlings were transferred from the greenhouse to a 5°C r o o m 7 and 14 days following the paclobutrazol treatment. Three pots from each treatment were transferred daffy from 5 to 25°C for evaluation o f chilling injury. Chilling injury in cucumber seedlings was characterized by the water-soaked appearance, wilting and dehydration of the leaves. The symptoms o f chilling injury in zucchini squash seedlings included wilting, marginal necrosis, and inward curl of the leaves. The severity of the injury was rated on a scale of 1--5, ranging from normal to severe. RESULTS
Paclobutrazol treatment suppressed plant height, length of the stem and length of the longest leaf in both cucumber and zucchini squash seedlings (Table I). The reduction was significant in all concentrations used. In zucchini squash, the inhibition of growth by 1 mg 1-~ paclobutrazol was comparable to that by 5 or 10 mg 1-1 , whereas in cucumber, it appeared that the higher the concentration the higher the inhibition. The greatest difference between the control and the treated samples existed in the height of the plants. Chlorophyll contents in the leaves were increased in the treated samples
295 TABLE I Effect of paclobutrazol on the growth and chlorophyll content of cucumber and zucchini squash seedlings. Measurements were made 7 days after the treatment. Mean separation in columns by Duncan's multiple range test, 5% level
Treatment
Plantheight
Stem length
Leaf length
Chlorophyll
(rag 1-~)
(cm)
(cm)
(cm)
(OD at 665 nm)
13.8a 9.2b 8.4bc 7.6c
10.7a 8.1b 6.4c 6.0c
0.41a 0.63b 0.82c 0.98d
6.9a 3.8b 4.1b 3.7b
7.8a 6.5b 6.5b 6.2b
0.34a 0.61b 0.68b 0.72b
Cucumberseedlings 0 18.2a 1 13.9b 5 12.5c 10 ll.4d Zucc~nisqua~seedlings 0 19.4a 1 ll.7b 5 ll.3b 10 lI.6b
in b o t h cucumber and zucchini squash seedlings (Table I). Similar increases in chlorophyll contents were found in zucchini squash treated with different concentrations, b u t in cucumber seedlings the contents increased with the concentrations used. More than twice as much chlorophyll was found in the 10 mg 1-~-treated cucumber seedlings than in the untreated samples. Chilling injury to cucumber seedlings initially appeared as water-soaked spots on the leaves. This s y m p t o m was observed while the seedlings were still in the chilling temperature (5°C). This was followed b y the wilting of the lemres and the collapse of the whole seedlings. The wilting and the collapse of the plants usually did not take place until after transfer to 25°C. I:[ the injury was slight, the seedlings could regain their vigor within 5 hour.,, at 25°C. However, if the damage was severe, wilting persisted and the seedlings eventually desiccated. The control cucumber seedlings developed symptoms of chilling injury within '.2 days of exposure to 5°C (Fig. 1). The wilting of the leaves became irreversible after 4 days at 5°C. Treated seedlings did not show any sign o f clamage until after 3 days at 5° C. Seedlings treated with 1 and 5 mg 1-~ of paclobutrazol wilted after 4 and 5 days at 5°C following transfer to 25°C, but t h e y recovered and resumed growth. The injury became irreversible after 6 days at 5°C for 1 and 5 mg 1-1 treatments and after 7 days for the 10 mg 1-~ treatment. Initial s y m p t o m s of chilling injury at 5°C in zucchini squash seedlings w e r e similar to those observed in cucumber seedlings. However, the leaves o f zucchini squash did not wilt as readily as those of cucumber seedlings. Instead, desiccation of the margins of leaves occurred and this was followed by discoloration and inward curl of the leaves. In severe cases, necrotic spots appeared along the vein and the entire leaf eventually dehydrated.
296 s F cucuM.ER
7
.......
,~
I
I
J [ ] 1 rng/i paclob~Jtrazol •
~4
0
1"15
"
-
R]lo
-
.
1
I
•
IBm1
•
i~l
I~ /
4
3
2
3
4
5
6
7
Days at 5 ° C ,
Fig. 1. Severity o f chilling i n j u r y in c u c u m b e r seedlings t r e a t e d w i t h p a c l o b u t r a z o l . Seedlings were t r a n s f e r r e d f r o m t h e g r e e n h o u s e t o 5° C a t 7 days a f t e r t h e p a c l o b u t r a z o l t r e a t m e n t s . Chilling i n j u r y was e v a l u a t e d 24 h a f t e r t r a n s f e r o f seedlings f r o m 5 t o 25°C. T h e s e v e r i t y o f i n j u r y was r a t e d o n a scale o f 1--5 w i t h 1 = n o r m a l , 2 = trace, 3 = slight, 4 = m o d e r a t e , a n d 5 -- severe injury.
ilc°r° "° 5 j
ZUCCHINI
4
la 1
SQUASH
mg/I pscJobutrazol
r'15
3
•
•
"
"
L
~
- 5
PJl
" 4
- 3
2
" 2
1
1
0
3
4
5
6
7
8
D a y s at 5 ° C .
Fig. 2. S e v e r i t y o f chilling i n j u r y in z u c c h i n i s q u a s h seedlings t r e a t e d w i t h p a c l o b u t r a z o l .
Details as in Fig. 1. The symptoms of chilling injury could proceed slowly if the seedlings were kept at the chilling temperature, but the development of s y m p t o m s was hastened after transfer to 25 ° C. Zucchini squash seedlings were less sensitive to chilling injury than cucumber seedlings. Control seedlings of zucchini squash did not show any symptoms of chilling injury until after 4 days of exposure to 50C, and it t o o k 7 days of exposure at 50C for the control plants to reach the severe injury stage (Fig. 2). Paclobutrazol treatment also effectively delayed the progress of symptoms of chilling damage, although the beneficial effect was not as dramatic as that in cucumber seedlings. Chilling damage of treated seedlings became irreversible after 7 days at 5° C. Chilling injury symptoms of control seedlings of cucumber and zucchini squash transferred from the greenhouse to 50C at 14 days after the paclobutrazol treatment generally followed the same pattern as those transferred 7 days after treatment. In the treated seedlings, however, the development of chilling injury symptoms in the 14-day samples was further delayed for 1 or 2 days longer (data not shown). It appeared that the longer the duration of the paclobutrazol treatment, the more effective was the protection.
297 DISCUSSION
The marked inhibition of growth of seedlings of cucumber and zucchini squash by paclobutrazol in this study parallels the results found in sunflowers (Wample and Culver, 1983), rice seedlings (Jung and Rademacher, 1983} and apple and pear seedlings and trees (Quinlan and Richardson, 1983; Raese and Burts, 1983; Steffens et al., 1983; Williams, 1983). The morphological alterations induced by paclobutrazol appear to correlate to the increase of chilling resistance. Seedlings of cucumber and zucchini squash treated with paclobutrazol exhibited less wilting and desiccation following chilling exposure. The mechanism of the reduction of chilling injury by paclobutrazol is not known. Increase in resistance to low temperatures has been associated with a large decrease in GA content (Reid et ah, 1974) and an increase in ABA/GA ratio (Waldman et al., 1975). Since paclobutrazol inhibits GA biosynthesis (Couture, 1982; Graebe, 1982), it is possible that paclobutrazol may also enhance chilling resistance through modification of the hormonal balance. Chemical modification of chilling susceptibility reported previously has been achieved mostly through the regulation of membrane lipid synthesis and the alteration of the degree of fatty acid unsaturation (St. John and Christiansen, 1976; Waring et al., 1976; Willemot, 1977; Wang and Baker, 1979). The paclobutrazol treatment may also change the chilling sensitivity of seedlings by affecting the composition of cellular membranes. Further studies are desirable to determine if the increased chilling resistance is also related to membrane alterations induced by paclobutrazol. ACKNOWLEDGEMENTS
The author wishes to express his gratitude to Noreen Krause for her technic~J assistance and to ICI Americas Inc. for providing the paclobutrazol. Use of a company or product name by the U.S. Department of Agriculture does not imply approval or recommendation of the product to the exclusion of others which may also be suitable.
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