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Episodic waterlogging of the soil occurs when water enters soil faster than it can drain away under gravity. Waterlogging and flooding events are increasing in severity worldwide. One cause is rising sea levels, which particularly affect delta regions. The second is large-scale irrigation of farmland, and a third is change of land use, including urbanization.
In waterlogged soil, diffusion of gases through soil pores is so strongly inhibited by their water content that it fails to match the needs of growing roots. A slowing of oxygen influx is the principal cause of injury to roots, and the shoots they support. The maximum amount of oxygen dissolved in the floodwater in equilibrium with the air is a little over 3 % of that in a similar volume of air itself. This small amount is quickly consumed during the early stages of flooding by aerobic micro-organisms and roots. In addition to imposing oxygen shortage, flooding also impedes the diffusive escape or oxidative breakdown of gases such as ethylene or carbon dioxide that are produced by roots and soil micro-organisms.
The duration and severity of flooding or deeper submergence can be influenced not only by the rate of water input but also by the rate of water flow out from the rooting zone and by the water absorbing capacity of the soil. Topography plays an important part in determining the speed of lateral flow within and above the soil. Drainage through the soil profile is also strongly affected by soil structure, which is highly variable. Approximately half the soil volume is made of solid material that is permeated by spaces filled with water, with gas or with roots and other living organisms. The total volume of these pores, the size range of the pores, their interconnectivity, stability and the relative proportions of each size class all have a major impact on how much water is held by the soil and how readily it drains through the profile. Small pores hold water more strongly by capillary forces than do larger ones. Interconnected pores with a diameter range larger than 50 μm drain under gravity. Pores with diameters of 50 – 0.5 μm can hold water against the force of gravity but weakly enough for roots to extract it. Pores smaller that 0.2 μm hold water so strongly that neither gravity nor roots can extract their contents. These are, therefore, permanently filled with water. Drainage rates are also affected by a soil’s macro-structure.
Although reduced oxygen supply is the primary cause of plant injury in waterlogged soils, several associated factors are also potentially toxic, including high ethylene, CO2, and the increased availability of toxic heavy metals as the redox potential declines.
Protocols for screening plants for waterlogging tolerance therefore take into account the soil type, temperature, associated toxicities. Solution culture is used as a way of avoiding some of these issues.
The Impact of Flooding Stress on Plants and Crops. MB Jackson. University of Bristol, Woodland Road, Bristol UK, and Plant Ecophysiology, Faculty of Biology, University of Utrecht, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands.
Colmer TD, Voesenek LACJ (2009) Flooding tolerance: Suites of plant traits in variable environments. Functional Plant Biology 36, 665-681.