| The physiochemical response and its mechanism of rice exposed to enhanced ultraviolet-B(UV-B) radiation was discussed through the studies on plant height,leaf area,dry matter accumulation , active oxygen metabolism, chloroplast ultrastructure, photosynthetic system,UVB-absorbing flavoid and the change of plant growth based on previous researches. The results showed that UV-B radiation stress significantly decreased the activities of protective enzymes such as superoxide dismutase(SOD), ascorbate peroxidase(ASP) and catalase(CAT) as well as the concentration of carotenoid (car), consequently increased the contents of hydrogen peroxide (H202) and malondialhyde(MDA),a product of lipid peroxidation, destroyed chloroplast ultrastructure, and reduced the chlorophyll concentration, chlalchlb ratio and the fluorescenece induction kinetics parameters of chlorophyll a.Therefore, the photosynthesis was weakened, and the growth of rice seedlings concerned was inhibited. Meanwhile, effects of UV-B radiation on the three tested rice cultivars were different beacause of different genotypes.It therefore could be infered in three possible ways of adaptive responses to UV-B stress from the results:( I )through the change of polyamine metabolism,the contents of polyamines (PAs),especially putrescine(Put) increased. (2)through the regulation of phytohormone metabolism, IAA and GA decreased but ABA increased, which resulting in unbalanced phytohormone system,(3) through the induction of new protein ,and the activation of phenylalanine ammomia-lyase(PAL) that caused the accumulation of UVB-absorbing substance. Additionally, the results and the existed problems were further discussed in this paper.
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