Genetic and cultural improvement of soybean for waterlogged conditions in Asia

Abstract Soybean ( Glycine max (L.) Merr.) is vulnerable to waterlogging, which threatens soybean productivity in humid regions of Asia. Waterlogging can happen during any growth stage of soybean, but flooding stress before, at or after germination causes severe seed and seedling damage, resulting in substantial reduction of grain yield at maturity. Under waterlogged conditions, seeds imbibe rapidly, destroying seed tissues due to abrupt swelling of the cells. Anatomical observations during water absorption of seeds revealed that the aleurone layer blocks abrupt water penetration into the embryo regardless of genotype. Based on these observations, genetic analyses of waterlogging-tolerant genotypes are currently in progress and attempts to incorporate the genes responsible for such tolerance are being made. Among cultural measures to alleviate waterlogging-induced damage during germination, pre-hydrating seeds prior to sowing proved to be most effective. After emergence, waterlogging impairs root function, primarily due to hypoxia, and thereby the capacity for nutrient uptake and growth. Molecular characterization of enzymes expressed in growing plants subjected to waterlogging treatment has identified several genes involved in anaerobic respiration. In addition, some soybean cultivars were found to form aerenchyma, which transports oxygen from aboveground tissues to the root system, in response to waterlogging. These phenomena should be exploited for breeding waterlogging-tolerant cultivars. Cultural methods including partial tillage and adjustment of the water table also effectively mitigate damage caused by waterlogging. Integration of these approaches should lead to stable soybean production in humid agricultural areas of Asia.