To elucidate the interaction between tropospheric O
3 and CO
2 in paddy rice, combined applications of O
3 (0–0.3 cm
3 m
−3) and CO
2 (380, 800 cm
3 m
−3) were performed at the vegetative stage or flowering to early maturing stage of rice plants using environmentally controlled chambers: O
3 decreases growth and yield, although CO
2 exhibits the opposite action. At the vegetative stage, elevated O
3 decreased the relative growth rate (RGR) and net assimilation rate (NAR) but elevated CO
2 ameliorated those effects. At the flowering to early ripening stage, elevated O
3 decreased dry matter production and yield with lowering of the percent maturity of grains, but the elevated CO
2 tended to offset these declines. The results indicate that elevated O
3 primarily inhibits photosynthesis-related process of paddy rice. Elevated CO
2, which is predicted for the future, interactively ameliorates that inhibition.
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