CHANGES IN ANTIOXIDANT LEVELS AND ACTIVITIES OF RELATED ENZYMES IN RICE LEAVES EXPOSED TO OZONE

Changes in the levels of antioxidants such as ascorbate and glutathione and activities of related enzymes in rice leaves exposed to ozone at high or low concentrations were investigated. In rice plants (Oryza sativa L. cv. Koshihikari) exposed to ozone at a high concentration (0.50 x 10(-6) m3 m-3 (0.50 ppm)) for 8 h, L-ascorbate was oxidized while the content of oxidized glutathione (GSSG) increased without the concomitant decrease of the content of reduced glutathione (GSH). In addition, the activities of ascorbate peroxidase (AP), dehydroascorbate reductase (DHA reductase), and superoxide dismutase (SOD) declined, though the glutathione reductase (GR) activity remained unchanged. A high concentration of ozone was considered to destroy the protection system against active oxygen species, resulting in the injury of the cell structure and functions. On the other hand, in the rice leaves exposed for 5 weeks to ozone at low concentrations (0.05 X 10(-6) and 0.10 X 10(-6) m3 m-3), the amount of L-ascorbate and total ascorbate increased significantly by exposure to 0.05 X 10(-6) m3 m-3 O3 while the content of total glutathione (GSH + GSSG) decreased by exposure to 0.10 X 10(-6) m3 m-3 O3. Activities of AP, GR, and SOD increased by the exposure to ozone at low concentrations. These reactions may be interpreted as protective responses to prevent or alleviate the damage of rice plants induced by ozone.