This study investigated the effects of elevated O ₃ level on photosynthetic traits and antioxidant systems of various rice genotypes under fully open-air conditions. Eight modern rice cultivars ( Oryza sativa L.), including 4 hybrids and 4 inbreds, were exposed to 2 O ₃ treatments: the ambient atmosphere (A-O ₃ ) and the 1.5 times ambient O ₃ concentration (E-O ₃ ) throughout the reproductive period. The measurements of gas exchange, chlorophyll a fluorescence, photosynthetic pigments, lipid oxidation, antioxidant content, and antioxidant enzyme activity were conducted on the flag leaf during the early and late grain-filling periods. The results indicated that E-O ₃ had differential effects on the photosynthesis and antioxidant indicators of rice genotypes. In the hybrid rice, the reduction in the light-saturated photosynthetic rate ( A _sat ) was primarily due to the nonstomatal factors, such as the changes in antioxidants, particularly ascorbate and carotenoids. In contrast, the reduction in A _sat levels in inbred rice was mainly due to the stomatal factors. Additionally, the hybrid rice was more sensitive to O ₃ than the inbred rice, with the sensitivity determinants varying between types. The sensitivity of hybrid rice to O ₃ was influenced by the magnitude of its antioxidant capacity, whereas the inbred rice sensitivity was determined by the stomatal conductance ( g _s ) and the antioxidant enzyme activity. The rice cultivars with higher antioxidant enzyme activities could help mitigate the damage caused by O ₃ pollution. This study could provide valuable insights for screening the O ₃ -tolerant rice cultivars and promoting the food security sustainability.