PHYSIOLOGY AND GENETICS OF SUBMERGENCE TOLERANCE IN RICE

We review a multidisciplinary approach to improve flooding tolerance i n rice and. specifically, tolerance of complete submergence. Environme ntal characterizations in India and Thailand suggest that limited gas diffusion and, sometimes, low irradiance are the most important factor s contributing to plant mortality. This supports the view that submerg ence tolerance of rice seedlings is related to maintenance of energy s upply partly through fast rates of alcoholic fermentation which requir e high levels of carbohydrates. In germinating seeds, rates of coleopt ile elongation during anoxia are highly correlated with rates of alcoh olic fermentation and carbohydrate supply for energy production. In ol der seedlings, survival during submergence is highly correlated with c arbohydrate supply. Optimization of growth vs. maintenance processes a ffects survival because elongation growth competes for energy and carb ohydrate reserves essential for maintenance processes. This was demons trated by experiments using: (a) cultivar comparisons, (b) growth regu lators 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 eq ual to the world's most tolerant cultivars, but successful introductio n of these elite lines in the field is elusive. Recent production of d ouble 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 popula tions of 15- to 50-d-old seedlings demonstrated (a) there is one domin ant gene for submergence tolerance and (b) this gene is present in thr ee out of four of the world's most tolerant rice cultivars. This sugge sts that a common factor related to tolerance of limited gas diffusion , (e.g. one of the enzymes of alcoholic fermentation) may be responsib le for genotypic differences in submergence tolerance of rice. An alte rnative possibility is that a gene for a transcription factor is invol ved in the expression of a multiple gene cascade that confers submerge nce tolerance. (C) 1997 Annals of Botany Company