The significance of light irradiation in neerosis formation by Magnaporthe grisea toxin(s) on rice cv. Sekiguchi‐asahi was investigated. The effective wave region for light‐dependent neerosis formation was 400 700 nm. An absorption band of the toxin(s) was restricted to the wave region shorter than 400 nm. Both of the phytosynthesis and necrosis formation were inhibited by photosynthetic inhibitors, and the inhibition of both activities was dependent on concentration of the inhibitors. The necrosis formation by the toxin(s) depended on light intensity. The toxin(s) induced the necrosis formation only on the cells with many chloroplasts, and the cells without chloroplasts did not form necrosis even under the light with sufficient intensity. The more the number of chloroplasts decreased, the more the size of necrotic spot decreased on the leaf sheath. From these results we concluded that the photosynthetic activity was involved in the necrosis formation by Magnaporthe grisea toxin(s) in rice cv. Sekiguchi‐asahi.

Effects of thermal and cycloheximide pre‐treatments on light dependent‐necrosis formation by Pyricularia oryzae toxin(s) on cv. Sekiguchi‐asahi were investigated. Thermal treatments of rice leaves at 55 °C for 10 and 15 sec significantly suppressed leaf necrosis formation. Such heat‐induced insensitivity of rice leaves to toxin(s) was greater at a thermal treatment of 55 °C for 15 sec than that of 55 °C for 10 sec. Cycloheximide pre‐treatment also suppressed leaf necrosis formation under light.