Daily variations in the sensitivity of soybean seedlings to low temperature

Every 4 hr during a time span of 32 hr and of 76 hr a different group of soybean seedlings [Glycine max (L.) Merr., cv. Corsoy] maintained under a regime of 16 hr of light followed by 8 hr of darkness was exposed to -10 degrees C for 4 min. Extent of sensitivity to low temperature was evaluated approximately 10 days after exposure to freezing temperature by determining the weight of the plants and chlorophyll content of the cotyledons. The amount of sensitivity to low temperature was related to the time of exposure and displayed a significant 24-hr oscillation. Plants appeared to be least sensitive to cold injury during the late portions of the light span. Plants subjected to water stress were less sensitive to cold and no significant 24-hr oscillation in response to low temperature could be detected by the cosinor method of analysis.