Dissimilarity of ascorbate-glutathione (AsA-GSH) cycle mechanism in two rice (Oryza sativa L.) cultivars under experimental free-air ozone exposure

Rising tropospheric ozone concentrations ([O-3]) can lead to considerable damage to agricultural crops. Experimental data have shown that the relationship between O-3 injury and yield loss differs with crop species and that O-3 sensitivity differs among cultivars of the same species. To explore a breeding strategy to adapt crops to high [O-3], it is necessary to investigate the different mechanisms for cultivar resistance to O-3 stress. Although several chamber-based studies have examined antioxidant defence differences in rice (Oryza sativa L.) cultivars in response to elevated [O-3], such as the utilisation of the ascorbate-glutathione (AsA-GSH) cycle to eliminate reactive oxygen species (ROS), little research has been conducted under free-air O-3 enrichment (O-3-FACE) to address the different AsA-GSH cycle responses of rice cultivars. In this experiment, O-3-FACE was used to investigate the AsA-GSH cycle in two rice cultivars, SY63 (O-3-sensitive) and WXJ14 (O-3-resistant). The results indicated that, compared with the ambient [O-3], elevated [O-3] (1.5 x ambient [O-3]) induced increases in the superoxide anion (O-2(center dot-)) production rate, hydrogen peroxide (H2O2) content, malondialdehyde (MDA) content and relative electrical conductivity; increases that were greater in SY63 than in WXJ14. Continuous O-3 stress also resulted in a less efficient metabolism of the AsA-GSH cycle in SY63 compared to WXJ14. The results indicated that in SY63, elevated [O-3] accelerated ROS metabolism rates, and the antioxidant system could not prevent oxidative damage, thus increasing membrane lipid peroxidation. In contrast, in WXJ14, there was transient damage in response to elevated [O-3] early in the sampling period, which triggered the ROS to stimulate the antioxidant system to avoid O-3 stress. Our results suggested that ROS detoxification by the AsA-GSH cycle under long-term exposure to an O-3-enriched atmosphere plays a more important role in an O-3-resistant rice cultivar than in an O-3-sensitive rice cultivar and that a better understanding of antioxidant system mechanisms is essential to the assessment of different rice cultivars' responses to future tropospheric [O-3]. (C) 2012 Elsevier B.V. All rights reserved.