RESPONSE OF OXIDATIVE STRESS DEFENSE SYSTEMS IN RICE (ORYZA-SATIVA) LEAVES WITH SUPPLEMENTAL UV-B RADIATION

The impact of elevated ultraviolet-B radiation (UV-B, 280-320 nm) on m embrane systems and lipid peroxidation, and possible involvement of ac tive oxygen radicals was investigated in leaves of two UV-B susceptibl e rice cultivars (Oryza sativa L. cvs IR74 and Dular). Rice seedlings were grown in a greenhouse for 10 days and then treated with biologica lly effective UV-B (UV-B-BE) radiation for 28 days. Oxidative stress e ffects were evaluated by measuring superoxide anion (O-2(.-)) generati on rate, hydrogen peroxide (H2O2) content, malondialdehyde (MDA) conce ntration and relative electrolyte conductivity (EC) for IR74 and Dular at 0 (control), 6 or 13 kJ m(-2) day(-1) UV-B-BE. Significant increas es in these parameters were found in rice plants grown at 13 vs 0 kJ m (-2) day(-1) UV-B-BE after 28 days; indicating that disruption of memb rane systems may be an eventual reason for UV-B-induced injury in rice plants. There was a positive correlation between O-2(.-) generation a nd increases in EC or MDA in leaves. Activities of enzymatic and nonen zymatic free radical scavengers were measured for IR74 after 7, 14, 21 and 28 days of exposure to 13 or 0 UV-B-BE to evaluate dynamics of th ese responses over time. Activities of catalase and superoxide dismuta se (but not ascorbate peroxidase) and concentrations of ascorbic acid and glutathione were enhanced by 13 vs 0 UV-B-BE after 14 days of UV-B exposure. Further exposure to 28 days of UV-B was associated with a d ecline in enzyme activities and ascorbic acid, but not glutathione. It is suggested that UV-B-induced injury may be associated with disturba nce of active oxygen metabolism through the destruction and alteration of both enzymatic and nonenzymatic defense systems in rice.