Metabolic changes in rice seedlings with different submergence tolerance after desubmergence

When flash flood intolerant rice cultivars are submerged, they show greater morphological changes such as elongation and chlorosis than tolerant cultivars. These morphological responses are caused by ethylene produced during submergence, however, a visible damage of intolerant cultivars is markedly developing after desubmergence rather than during submergence, which is probably due to oxidative damage. We studied the effect of ethylene produced during submergence on antioxidant content and oxidative damage after desubmergence. When rice (Oryza sativa) was submerged for 8 days, both tolerant cultivar (BKNFR) and intolerant cultivars (Mashuri and IR42) showed a decrease in ascorbate concentration during submergence. After 3 days of desubmergence, the tolerant cultivar showed a rapid recovery of total ascorbate and ascorbic acid, whereas intolerant cultivars showed a slow recovery of them, an increase in malondialdehyde formation, and low survival rate (about 30%). However, applying 200 mg l^?1 of AgNO₃ as an ethylene antagonist to intolerant cultivars suppressed the decrease in ascorbate and the increase in malondialdehyde formation after desubmergence, and improved survival rate to about 60%. Ascorbic acid supply to leaf discs from submerged IR42 suppressed increase in malondialdehyde formation by incubation under the light for 24 h. In addition, strong negative correlations were observed between malondialdehyde formation with ascorbate concentration (r=?0.93) and with percentage of survival (r=?0.98). Our results indicate that the accumulated ethylene during submergence adversely affected antioxidant mechanism in intolerant rice cultivars after desubmergence, and ascorbic acid was an important antioxidant in vivo for the recovery of submerged rice seedlings.