Submergence tolerance in rainfed lowland rice: physiological basis and prospects for cultivar improvement through marker-aided breeding

Two important factors influencing rice plant survival during submergence are limitations to gas diffusion under water, and reduced irradiance that impair photosynthesis and efficient utilization of carbohydrates. Thus, survival during submergence may largely depend on accumulation of high carbohydrate concentrations prior to submergence and a capacity for maintaining energy production through rapid alcoholic fermentation under oxygen shortage. During flash flooding, a third factor thought to affect survival is the aerobic shock during the post-submergence period when floodwaters recede. Changes in the level of antioxidants and enzymes such as superoxide dismutase (SOD) suggest that tolerant rice cultivars develop protective systems to air after exposure to hypoxic or anoxic environments. These responses are similar to other wetland plants. The capacity to survive submergence depends not only on specific environmental factors, but also on the strategy that plants have evolved for adoption to particular flood-prone environments. In rice the two main strategies are to elongate and escape, or not to elongate and conserve resources. For rainfed lowland rice exposed to flash flooding, elongation growth during complete submergence has major adverse effects on survival, presumably since this competes with maintenance processes which require carbohydrates and energy. Selection for minimal elongation during submergence is currently being exploited as a trait for submergence tolerance by tainted lowland rice breeders in south and southeast Asia. Gene mapping for submergence tolerance has been useful in identifying one prominent locus for submergence tolerance. Fine scale gene mapping and sequencing may facilitate further progress in the physiology and genetics of submergence tolerance. Recently published data demonstrate that improving submergence tolerance may be possible through up-regulation of genes for particular traits such as pyruvate decarboxylase (PDC) for alcoholic fermentation. Validation of appropriate mechanisms in other cultivars for target environments, and development and utilization of molecular markers to follow these traits in breeding programs, will therefore be high priorities for future work on submergence tolerance of rice. (C) 2002 Elsevier Science B.V All rights reserved.