Abstract: After 30 days growing in greenhouse and field in winter, the winter wheat (Triticum aestivum) plants were exposed to naturally rhythmical change of diurnal temperature and to extremely high or low temperature, with the dynamic changes in the leaf antioxidant enzyme activities and osmoregulation substance contents analyzed, aimed to understand the physiological mechanisms of winter wheat to adapt freezethaw stress. The plants from the greenhouse (average temperature 11 ℃) at jointing stage and from the field (average temperature 1 ℃) at tillering stage all had the same patterns of leaf antioxidant enzyme activities and proline content in response to naturally rhythmical diurnal temperature change, i.e., increased with sunrise, peaked at 13:00, and decreased with sunset. When the plants from the field were exposed to high temperature (18 ℃) for 2 days, the leaf antioxidant enzyme activities and proline content increased, but it was in adverse when the plants were exposed to low temperature (-2.5 ℃). After freezethaw stress, the plants grew well. When the plants from the greenhouse were shortly exposed to -6 ℃, the leaves were frozen, their antioxidant enzyme activities and proline content decreased, but increased after exposure to 18 ℃. After short exposure to freezethaw stress, the plants from the greenhouse died. However, the plants from the field had lower net photosynthesis rate (P_n) but higher activities of antioxidant enzymes than those from the greenhouse. It was suggested that the rapid responses of the leaf antioxidant enzyme system and proline to the diurnal temperature and light intensity to scavenge oxygen freeradical and keep water balance in cell would play important role in the adaptation of winter wheat to diurnal freezethaw stress, and the higher P_n and lower antioxidant enzyme activities in the leaves of winter wheat growing in warming winter could be the reason why winter wheat died in late spring cold of next year.

Key words: tomato, sodium naphthalene acetate, suboptimum temperature and light intensity, vigorous seedling index.