Physiology and Genetics of Submergence Tolerance in Rice

We review a multidisciplinary approach to improve flooding tolerance in rice and, specifically, tolerance of complete submergence. Environmental characterizations in India and Thailand suggest that limited gas diffusion and, sometimes, low irradiance are the most important factors contributing to plant mortality. This supports the view that submergence tolerance of rice seedlings is related to maintenance of energy supply partly through fast rates of alcoholic fermentation which require high levels of carbohydrates. In germinating seeds, rates of coleoptile elongation during anoxia are highly correlated with rates of alcoholic fermentation and carbohydrate supply for energy production. In older seedlings, survival during submergence is highly correlated with carbohydrate supply. Optimization of growth vs. maintenance processes affects survival because elongation growth competes for energy and carbohydrate reserves essential for maintenance processes. This was demonstrated by experiments using: (a) cultivar comparisons, (b) growth regulators and (c) dwarf-mutants. Hence, submergence tolerance of 14-d-old rice seedlings can increase by up to 98 % during 10 d submergence when elongation growth is reduced in these three ways. This is consistent with the observation that submergence tolerance and elongation ability rarely occur in the same genotype. Plant breeding has produced elite lines with up to four-fold greater yields and submergence tolerance equal to the world's most tolerant cultivars, but successful introduction of these elite lines in the field is elusive. Recent production of double haploid populations differing in submergence tolerance permitted testing of the physiological and genetic linkage of traits, or genes, with submergence tolerance. Genetics research with segregating populations of 15- to 50d-old seedlings demonstrated (a) there is one dominant gene for submergence tolerance and (b) this gene is present in three out of four of the world's most tolerant rice cultivars. This suggests that a common factor related to tolerance of limited gas diffusion, (e.g. one of the enzymes of alcoholic fermentation) may be responsible for genotypic differences in submergence tolerance of rice. An alternative possibility is that a gene for a transcription factor is involved in the expression of a multiple gene cascade that confers submergence tolerance. C 1997 Annals of Botany Company