Review of prospects for germplasm improvement for waterlogging tolerance in wheat, barley and oats
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A review is presented for prospects of germplasm improvement for waterlogging tolerance in wheat, barley and oats using a mechanistic approach based on adaptive physiological traits. In `The waterlogged environments for crop production' section, the extent of waterlogging is reviewed commencing with determination of environmental factors which may limit plant growth and development in waterlogging prone regions. This highlights that different types of waterlogging may exist, there may be large spatial and temporal variation in waterlogging, and that waterlogging may be confounded in field experiments with additional environmental factors. Environmental characterisation is therefore a key step to using mechanistic approaches for germplasm improvement for target environments, for extrapolation to other environments, and for development of screening protocols under controlled conditions that accurately reflect the field environment. In the `Information on key components required for germplasm improvement' section, the genetic diversity in wheat, barley and oats for waterlogging tolerance is confirmed. Physiological mechanisms for waterlogging tolerance are diverse and can be grouped into adaptive traits relating to (1) phenology, (2) morphology and anatomy, (3) nutrition, (4) metabolism including anaerobic catabolism and anoxia tolerance, and (5) post anoxic damage and recovery. For wheat and barley, there is some genetic diversity for waterlogging tolerance at the germination stage, however the full potential seems yet to be exploited. Varietal differences in tolerance at the germination stage often differ from tolerance at later stages of development, and this supports the view that different mechanisms of tolerance exist at the whole plant and tissue level. Limited work from genetic studies indicates a high heritability for waterlogging tolerance. It is concluded that the best opportunities for germplasm improvement are for further exploration and utilisation of genetic diversity by improving selection criteria including the use of marker assisted selection. Additional opportunities are described for increasing genetic diversity using wide hybridisations and development of transgenic plants.
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