Response of antioxidative system in rice (Oryza sativa) leaves to simulated acid rain stress

To clarify tolerant mechanism in crops to acid rain, we investigated the response of simulated acid rain on accumulation of reactive oxygen species, membrane stability, antioxdative enzymes activities and their genes expression levels and thereafter recovery mechanism (without simulated acid rain). After being exposed to simulated acid rain (pH 5.5, 5.0, 4.5 or 4.0) for 5 days, malondialdehyde content and membrane permeability in rice leaves showed no differences from those of the control, and concentrations of superoxide anion and hydrogen peroxide and antioxidant enzymes (superoxide dismutase, catalase and peroxidase) activities did not differ from those of the control as well. Simulated acid rain at pH 3.5 increased activities of superoxide dismutase, catalase and peroxidase by up-regulating expression of Cu/Zn-SOD, CAT1 and POD1 at transcriptional level as well as ascorbic acid content to maintain the content of superoxide anion and hydrogen peroxide in cells closed to the control level, and then oxidative damage were not occurred (malondialdehyde content and membrane permeability were no different from the control) in rice leaves. However, simulated acid rain at pH 3.0 or pH 2.5 caused the increase in malondialdehyde and membrane permeability by increasing contents of superoxide anion and hydrogen which exceeded the scavenging ability of antioxidant enzymes regulated by expression of Cu/Zn SOD, CAT1 and POD1 and ascorbic acid. These results show that activities of SOD (Cu/Zn SOD), CAT (CAT1) and POD (POD1) and ASA could play predominant roles in tolerance of rice to oxidative damages induced by acid rain, and can be indicating indices for reflecting tolerance of plants to acid rain. Besides, the regulating effect of antioxidant system on acid-tolerance of plants was limited by intensity of acid rain.