Rhythmicity in Ethylene Production in Cotton Seedlings

Abstract

ABSTRACr Cotyledons of cotton (Gossypium hirsutum L.) seedlings grown under a photoperiod of 12 hour darkness and 12 hour light showed daily oscillations in ethylene evolution. The rate of ethylene evolution began to increase toward the end of the dark period and reached a maximum rate during the first third ofthe light period, then it declined and remained low until shortly before the end of the dark period. The oscillations in ethylene evolution occurred in young, mature, and old cotyledons (7 to 21 day old). These oscillations in ethylene evolution seemed to be endogenously controlled since they continued even when the photoperiod was inverted. Moreover, in continuous light the oscillations in ethylene evolution persisted, but with shorter intervals between the maximal points of ethylene evolution. In continuous darkness the oscillations in ethylene evolution disappeared. The conversion of 13,4-'4Cmethionine into 14C] ethylene followed the oscillations in ethylene evolution in the regular as well as the inverted photoperiod. On the other hand, the conversion of applied 1-aminocyclopropane-1-carboxylic acid into ethylene did not follow the oscillations in ethylene evolution, but was affected directly by the light conditions. Always, light decreased and darkness increased the conversion of applied 1-aminocyclopropane-1-carboxylic acid into ethyl-ene. It is concluded that in the biosynthetic pathway of ethylene the conversion of 1-aminocyclopropane-1-carboxylic acid into ethylene is directly affected by light while an earlier step is controlled by an endog-enous rhythm. Many plant processes are controlled by endogenous rhythms (12). There are several reports of daily changes in the level of plant hormones, e.g. cytokinin levels in poplar leaves (11), ABA levels in sorghum and pearl millet leaves (10, 13), and phaseic acid in sorghum leaves (13). The daily changes in ABA levels were only partially related to the leaf water potential. Recently, Lecoq et al. (16) showed that in soybean leaves the oscillations in ABA level persisted in continuous light, indicating that such ABA levels were endogenously controlled. Daily changes in ethylene evolution were found in leaves of several species (7, 15) and in cotton fruits (17). El-Beltagy et al. (7) showed that these oscillations were not related to changes in the leaf water saturation deficit or stomatal aperture but were endogenously controlled , since they occurred also in continuous light or darkness. The Volcani Center, Israel. in ethylene evolution in cotton seedlings mainly in relation to its biosynthesis. MATERUILS AND METHODS Cotton (Gossypium hirsutum L.) seeds were germinated and grown for 7 d in plastic pots (12.5 cm in diameter, 12 cm in height) filled with peat, and irrigated with water. The seedlings were grown in growth chambers at 29°C under photoperiods which were specified for each experiment. The light (200 sE m-2 s ') source was a combination of regular incandescent light and fluorescent light (F48T1 8-CW-VHO, Sylvania). The production of ethylene was measured in detached cotyle-dons (4 g) that were enclosed immediately in flasks (63 cm3) for 2 h. The flasks were kept in light conditions similar to those the seedlings would be in if they were not harvested. In order to prevent dehydration of the shoots, a filter paper moistened with 0.6 ml of distilled H20 was placed on the bottom of each flask. Ethylene was determined by GC as described by Aharoni et al. (3). AVG3 (0.1 mM) was applied by spraying the seedlings until runoff. ACC (1 mM) was applied by floating 6 discs (7 mm in diameter) on 1 ml of ACC solution in 25 ml Erlenmeyer flasks. The flasks were sealed with rubber serum stoppers before ethyl-ene production was determined. For tracer studies, 6 discs (7 mm in diameter) were floated on 1 ml of 0.95 ,uCi L-[3,4-14C]methionine (53 mCi/mmol) in 25-ml Erlenmeyer flasks. The flasks were sealed for 1 h before labeled ethylene from methionine was collected and determined as described by Aharoni et al. (2). Each experiment was repeated at least 3 times. The data presented are of a typical experiment with at least 3 replicates in each treatment. RESULTS