To obtain basic information concerning the evaluation of the effects of O3 on the yield of Japanese rice (Oryza sativa L.), we investigated the effects of O3 on the growth, yield and leaf gas exchange rates of rice. Two Japanese cultivars of rice (Koshihikari and Kinuhikari) were grown in plastic boxes filled with flooded Andisol in 9 chambers, and were daily exposed to charcoal-filtered air or O3 at 60 or 100 nl l 1 (The whole-plant dry mass and grain yield per plant of the two cultivars at

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... final harvest were significantly reduced by the exposure to O3. The reduction rates of the grain yield by the exposure to O3 at 60 and 100 ppb were 3 and 23 in Koshihikari, respectively, and 18 and 34 in Kinuhikari, respectively. The sensitivity of growth and yield to O3 was not significantly different between Koshihikari and Kinuhikari. The exposure to O3 significantly reduced the panicle number per plant, and the dry mass per panicle and filled grain number per panicle due to the increase in sterility, but did not reduce 1000 grain weight. The O3-induced reduction in the yield of the two cultivars was mainly attributed to that in the whole-plant growth during the early vegetative period and the biomass allocation to panicle during the reproductive period. During the early vegetative period, the exposure to O3 significantly reduced net photosynthetic rate and stomatal diffusive conductance, regardless of the leaf position. Therefore, the O3-induced reduction in the whole-plant growth of the two cultivars during the early vegetative period was mainly due to that in net photosynthetic rate, partially caused by the stomatal closure. Furthermore, the sensitivity to O3 of the yield components, growth parameters and leaf gas exchange rate were not significantly different between Koshihikari and Kinuhikari.