Abstract:To understand the physiological reasons for poor yield of the second rice crop in southern China challenged by low temperature. The authors investigated the effects of chilling temperature on photosynthesis and the activity of fructose 1, 6-diphosphatase (FBPase) of flag leaves at milky stage using two hybrid rice varieties with different cold tolerance. The results indicated that chilling temperature caused decreases of photosynthetic efficiency and FBPase activity in detached flag leaves. The decline of photosynthetic efficiency and FBPase activity was greater in Shanyou-63 which is as cold sensitive as its parents than in Xiuyou-57 which is cold tolerant like its parents. The milky stage is the period of the fastest grain filling. The decreased the yield of the second rice crop caused by natural low temperature was associated with decline of photosynthesis and the abilities of adaption and adjustment of FBPase in attached flag leaves. The relationship between the effects of chilling temperature on photosynthesis and photosynthate transport and the yield formed by grain was discussed.