Abstract: With the purpose of selecting and breeding cold stress tolerance of hybrid rice cultivars, the effect of chilling on five hybrid rice combinations was investigated. The results indicated that the cold tolerance of different hybrid rice varieties were different. The order of the cold tolerance as the degrees of inhibition of maximum photosynthetic rate and of apparent quantum yield of flag leaves at primary heading stage by chilling was as follows: The cold tolerance of Japanica type Xiuyou-57 was the highest then lndica type Qing-Youzao followed by Shangyou-63, Shangyou-64 and Weiyou-64. The cold tolerance of Fx hybrid rice was similar to that of the maternal lines and not similar to the paternal lines. The cold tolerance of maintainer lines was similar to that of the male sterile lines. The tests on the survival rate of hybrid rice seedlings after chilling treatment also showed a similar re gular patterns. These indicated that the responses by the chilling temperature in these five hybrid combinations were exactly the same in the Seedling stage as well as in the late growing stage. The results of these experiments on the impact of selecting and breeding cold tolerance in hybrid rice were discussed.

Key words: Chilling stress, Hybrid rice, Maximum photosynthetic rate, Apparent quantum yield, Survival rate of seedlings