Responses of Antioxidant Enzymes to Chronic Free-Air Ozone Stress in Rice (Oryza sativa L.) Cultivars with Different Ozone-Sensitivities

Free-air O-3 enrichment was used to investigate the responses of different antioxidant mechanisms in different rice (Oryza sativa L.) cultivars - O-3-sensitive hybrid indica (O-3-S) cultivars and O-3-tolerant conventional japonica (O-3-T) cultivars across all growth stages. Elevated [O-3] induced increases in reactive oxygen species (ROS) production in O-3-S cultivars, which were more pronounced in the later growing stages. In O-3-S cultivars, continuous O-3 stress decreased catalase (CAT), peroxidase (POD) and glutathione peroxidase (GPX) activities, while in O-3-T cultivars, short-term O-3 stress decreased superoxide dismutase (SOD), CAT, POD and GPX activities. The same POD isozyme patterns were observed in both O-3-S and O-3-T cultivars, while SOD and APX isozymes varied by cultivar. The results suggest that O-3 tolerance might be improved at different rice development stages through regulating the responses of antioxidant mechanisms to O-3 stress.