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 50-
d-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.