Abstract: Effects of low temperature on content of photosynthetic pigments and activity of antioxidant enzymes in flag leaves of two varieties of rice at the heading stage were analyzed.Of the two varieties,one is a newly-developed high-yielding hybrid rice（Oryza sativa L.） "Liangyoupeijiu" and the other is a traditional hybrid rice "Shanyou 63".The crops were put under two different temperatures,20 and 15 ℃,separately for 2 and 4 days.Results show that under low temperature,the contents of photosynthetic pigments（chlorophyll and carotenoid） in flag leaves of the plants declined with increase of duration,and so did the activity of CAT.Variation of the activity of SOD and POD,however,differed between the two varieties.In Liangyoupeijiu,the activity of SOD increased while the activity of POD declined after 4 days of low temperature treatment.The case of Shanyou 63 was the opposite.Unbalanced changes in the activity of antioxidases led to the accumulation of O^-₂,which could cause membrane peroxidation as indicated by increased MDA content.These changes caused decline of the net photosynthetic rate of flag leaves.The lower changing rate of photosynthetic pigments,the higher antioxidase activity and the lower POD activity,which could alleviate the production of reactive oxygen,and might be responsible for Liangyoupeijiu being more tolerant to low temperature than Shanyou 63.

Key words: low temperature, super-high yield hybrid rice(Oryza sativa L.), photosynthetic pigment, antioxidase, heading stage