Relationships between circadian rhythm of chilling resistance and acclimation to chilling in cotton seedlings

Cotton (Gossypium hirsutum L. cv. Deltapine 50) seedlings grown under light-dark cycles of 12:12 h at 35° C showed rhythmic daily changes in chilling resistance. Chilling treatment (5° C, 48 h) started at the beginning or middle of the daily light period resulted in a substantial growth inhibition of the seedlings upon return to 35° C whereas when chilling was started at the beginning or middle of the dark period the subsequent growth of the seedlings was much less inhibited. This rhythm in chilling resistance persisted under continuous light for three 24-h periods, indicating that it is of an endogenous nature. Seedlings grown under continuous light from germination showed no daily changes in resistance, but a rhythm was initiated by introduction of a dark period of 6 h or longer. In 24-h cycles with different light and dark periods, maximal resistance was reached just before the start of dark period. Seedlings grown at 35° C could be acclimated to chilling by exposure to low, non-damaging temperatures (25—15° C). A short-term (6 h) exposure to 25° C started at the resistant phase resulted in a large increase in resistance during the following otherwise sensitive phase. The resistance induced by the low temperature matched or slightly exceeded the maximal resistance reached during the resistant phase of the daily rhythm of chilling. The low-temperature-induced resistance and the daily rhythmic increase in resistance were not additive, indicating a common mechanism for the two kinds of resistances. An adaptive advantage of a combination of a rapid temperature-induced acclimation and the daily rhythmic increase in resistance is suggested.