| Heat stress is causing an increasing injury on the crops, expeicailly on the rice in the southern region of china, as the worldwide temperature incrases. The experiments were conducted at Fuyang experimental station of China National Rice Research Institute (CNRRI) with 14 rice maintainer lines, 26 rice restorer lines and 28 rice near-isogenic lines as materials. These tested materials were subjected in heat stress during the flowering stage for 15d to evaluate their heat stress tolerance by the heat stress index. Several heat tolerance and sensitive material were selected and two of them (V20B, Heat tolerance and Zhong9B, heat sensitive) were chosen to study the physiological characteristic of rice differing in heat tolerance. Base on this, the role of ABA,SA,GA3 and BR foliagesprayed 2d before taselling or full heading on grain yield and yield components of rice were studied in this experiment. The results showed that:1. Damages caused by heat stress during the flowering stage on the grain yield of rice was to significantly increase the spikelet sterility of rice. The results of the tested materials evaluated under heat stress indicated that heat stress mainly dameged the spikelet fertility of rice (P<0.01), and then decreased the kernel weight of rice. Base on the heat stress index, the tested materials used in this experiments were ranked into four levels regarding to heat tolerance: heat tolerance, heat semi-tolerance, heat semi-sensitive and heat sensitive.2. Higher physiological adaptation was showed in the heat tolerance culticars than the heat sensitive ones. The results in this experiment indicated heat stress was harmful to the yield components of rice. The seed-setting rate of both cultivars decreased with the heat stress time increased, but in which the significant difference were observed between the superior grains and inferior grains. The seed-setting rate of superior grains of both cultivars decreased when the heat stress time was prolonged, while that of inferior grains of V20B was higher than CK. In addition, higher photosynthetic rate,chlorophyll content,chlorophyll fluorescence,SOD activity of leaves and water potential of spikelets as well as the decreased range in temperature of flag leaves, spikelets and anthers and pollen activities compared to control were observed in V20B than those of Zhong9B.3. SA could protect rice against the heat stress during flowering stage. The role of ABA,GA3,SA and BR foliagesprayed at 2d before tasseling or full heading on the grain yield and yield components of elite japonica-hybrid rice Changyou 1 were studied in this experiment. The results showed that SA increased the grain yield related to enhancing both the seed-setting rate and kernel weight at 2d before tasseling and full heading stage of rice, while that of BR were only found in suitable concentration. A minor decrease in grain yield of rice was showed in the ABA treatments and the GA3 treatments have no significant effect on the grain yield of rice in this experiment. The results also indicated higher grain filling rate and Q-enzymes activities of superior and inferior grains were showed in SA200 treatment(200mg/L) than those of SA100(100mg/L) and CK(0mg/L), especially during the late grain filling stage. In addition, the photosynthetic rate, protein contents and chlorophyll contents of flag leaves in SA200 treatment were also much higher than CK at the late filling stage of rice. So we considered that SA200 increased the grain yield was related to promoting grain filling and delaying the leaves senescence of rice in this experiment.
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