Abstract:Rice is one of the most important food crops in the world. In recent years, extremely and constantly high temperature often appears in many parts of China in summer, and it leads to high frequency of the heat injury in rice. The hazard of high temperature is serious to the forming of grain quality in rice filling-period. To reveal the physiological adaptation mechanism of high temperature stress in rice, two rice cultivars (Oryza sativa L. indica), K30-insensitive to high temperature, and R21-sensitive to high temperature, were treated with high temperature 34.9℃ and normal temperature 28.0℃ in artificial climate chamber, and the photosynthesis, endogenous phytohormones and grain quality were measured during grain-filling period at 5 d, 10 d, 15 d, 20 d, 25 d and 30 d. The results showed that the net photosynthesis rate (Pn) of K30 increased during whole grain-filling period under high temperature stress, whereas Pn of R21 changed little during early stage of filling period (5 d, 10 d and 15 d) and dropped during late stage (20 d, 25 d and 30 d). High temperature stress intensified the falling speed of Pn of R21 in late stage of filling-period. Stomatal conductance (Gs), transpiration rate (Tr) and CO2 concentration in middle lamella (Ci) also increased in the two cultivars during whole filling-periods under high temperature stress. The drop of Pn was related to non-stomatic factors. Leaf and grain abscisic acid (ABA) content of K30 increased, whereas ABA content of R21 changed little during grain-filling period. Gibberellins (GA3) and indoleacetic acid (IAA) contents were also affected by high temperature stress to some degree. High temperature increased leaf GA3 content of K30 and decreased grain GA3 content. The proportion of GA3 distribution in leaf and grain differed in two cultivars. Leaf and grain IAA contents of two cultivars decreased in late stage of filling-period. Grain IAA content of K30 changed little under high temperature, whereas IAA content of R21 ascended. For the two cultivars, the total starch and amylose contents of grain were lower than those of control, whereas the protein contents and chalkiness were significantly higher than control. The grain quality debased under high temperature stress, but K30 was more insensitive to high temperature than R21. The present study demonstrated that the change of endogenous phytohormones metabolism, which regulates many physiological properties under high temperature stress, is the main reason for degeneration of grain quality, contraction of functional period of leaf and drop of photosynthetic efficiency. The physiological mechanism of endogenous phytohormones is very complicated due to the process of signal transduction from the binding of receptor and phytohormone to the physiological effect. The present study provides a base to further study the physiological mechanism of rice under high temperature stress and to breed heat-tolerance rice cultivar.