Circadian rhythm of heat resistance in cotton seedlings: synthesis of heat-shock proteins.

Cotton (Gossypium hirsutum L. cv. Deltapine 50) seedlings grown under light-dark cycles of 12:12 h at 33 degrees C showed rhythmic changes in their resistance to heat shock of 53 degrees C for 40 min. The resistance was maximal at the middle of the light period and declined toward the end of the light period. One more peak of resistance developed in the middle of the dark period and declined toward the end of the dark period. Rhythmic changes in heat resistance persisted under continuous light for 3 cycles, indicating a circadian control. Under continuous light only one phase of resistance developed, lasting from the middle of the subjective night to the middle of the subjective day. The major heat shock proteins (HSPs) synthesized upon 30-min exposure to 40 degrees C, 49 degrees C or 53 degrees C were of 115, 89, 73, and 19 kDa. Their rate of synthesis depended on the inducing temperature, on previous exposure to high temperature and on the time in the light-dark cycle. The time dependency of the induction of certain HSPs persisted under continuous light, indicating a circadian control. No positive correlations was found between the rhythmic changes in heat resistance and the rhythmic changes in the synthesis of any HSP.