Temperature-induced phase shifting of circadian rhythms in cotton seedlings as related to variations in chilling resistance

The relationship between the degree of chilling resistance and phase shifting caused by low-temperature pulses was examined in two circadian rhythms in cotton (Gossypium hirsutum L. cv. Deltapine 50) seedlings grown under light-dark cycles of 12∶12 h at 33° C. The seedlings showed a circadian rhythm of chilling resistance and of cotyledon movement. A pulse of 19° C for 12 h during the chilling-sensitive phase (light period) caused a phase delay of 6 h, while a similar temperature pulse during the chilling-resistant phase (dark period) did not cause any phase shift. Exposure to 19° C, 85% RH (relative humidity) for 12 h during the dark period induced chilling resistance in the following otherwise chilling-sensitive light period. In this light period a 12-h 19° C pulse did not cause a phase shift of chilling resistance. Pulses of low temperatures (5-19° C) were more effective in causing phase delays in the rhythm of cotyledon movement when given during the chilling-sensitive phase than when given during the chilling-resistant phase. A 12-h pulse of 5° C, 100% RH during the light period caused a phase delay of cotyledon movement of 12 h. However, when that pulse had been preceded by a chill-acclimating exposure to 19° C, 85% RH for 12 h during the dark period the phase delay was shortened to 6 h. The correlation between higher degree of chilling resistance and the prevention or shortening of the phase delay caused by low temperatures indicates that the mechanism that increases chilling resistance directly or indirectly confers greater ability for prevention of phase shifting by low temperatures in circadian rhythms.