Hypoxia induces anoxia tolerance in completely submerged rice seedlings

Survival of four rice (Oryza sativa L.) cultivars was evaluated after submergence in water containing different oxygen concentrations. The presence of O-2 in the floodwater was critical for the survival of 14-day-old seedlings: there was no effect of 4 d submergence in air-saturated water (0.24 mol.m(-3).O-2), or floodwater containing O-2 concentrations down to 0.05 mol.m(-3),whereas when seedlings were exposed to an <<anoxic shock>> (0.00 mol.m(-3).O-2), death of all four cultivars occured within 24-42 h. Prior exposure of seedlings to a hypoxic pre-treatment (submergence with 0.05 mol.m(-3).O-2 for 24 h) greatly improved the tolerance to 24 h anoxia in all four cultivars. Two submergence tolerant cultivars (IR49830 and FR13A) showed much greater tolerance to anoxia than two intolerant cultivars (IR36 and IR42). Following the hypoxic pre-treatment, only the two tolerant cultivars could tolerate 42 h anoxia, and IR49830 seedlings could withstand up to 3 d of submergence in anoxia. The two enzymes of ethanolic fermentation alcohol dehydrogenase (ADH, EC 1.1.1.1) and pyruvate decarboxylase (PDC, EC 4.1.1.1) were induced by the hypoxic pre-treatment. The activity of ADH was correlated to cultivar differences in anoxia tolerance. However, this enzyme is unlikely to limit the rate of ethanol fermentation since its in vitro activity was two orders of magnitude greater than the observed ethanolic fermentation rates. The in vitro activity of PDC was an order of magnitude lower than that of ADH, and showed a dear correlation with ethanol fermentation rates but was not correlated with anoxia tolerance. Our results suggest that the complete absence of oxygen in floodwater will have a major adverse effect on rice plants during submergence, and that the rice plant's ability to adapt to anoxic conditions is an important component of submergence tolerance. These findings are discussed in relation to metabolic adaptations to alleviate the adverse effects of anoxia during submergence.