Abstract: The cold resistant CR-4 and homobrassinolide BR-120 were used by soaking seeds of rice and then spraying on the leaves at the diplophyllous stage, respectively. After chilling stress (2 ±1℃ ) in dark for two days, the root system of the rice seedlings was more flourishing, the dry weight of root was heavier and the root length was longer than the untreated control sample. The dry weight of stem plus leaf increased and the activities of superoxide dismutase (SOD) and peroxidase (POX), and the content of glutathine (GHS) increased notably. At chilling stress, the effect of scavenging activated oxygen in the variety with strong cold resistance was better. When the test samples regained room temperature for 1~ 5 days (25±1℃, 3000 1x, l2 h / d), the activi-ties of SOD and POX, and the content of GSH and ascorbic acid (ASA) in leaves of two varieties showing strong or weak cold resistance increased continuously. The content of molondialdehyde (MDA) (a product of lipid peroxidation of membrane) decreased. But in the period of regaining temperature, the effect of scavenging activated oxygen in the variety with weak cold resistance was better and the increases of activity of POX and content of GSH were more obvious.

Key words: Cold resistant, Homobrassinolide, Cold resistance