In waterlogged soils, the volume occupied by air is reduced to less than 10% of the total volume of the soil. A low availability of oxygen in the root zone also can be caused by high temperatures, organic matter, or a combination of these factors. The presence of excess water greatly modifies the soil profile, altering its chemical and biological properties. This same physical change in the rooting medium, characterized by the complete disappearance of a gaseous stage, could prove to be the most damaging element for plant survival. In this situation the absorption of water by roots is limited, producing stress symptoms similar to those of drought, although the causes and mechanisms are distinct . Figure 5.38 outlines the processes leading to reductions in plant growth and crop yields due to excessive soil water.
The wilting of aerial organs is one of the first signs of the effects of excess water. In anaerobiosis, even during a short period of time, roots become less permeable to water, therefore increasing resistance to absorption. Water lost through transpiration exceeds the water absorbed by the roots, causing changes in the water status of plants, drop in
Figure 5.38. Processes leading to reductions in plant growth and grain yields due to excessive soil water.
leaf water potential, stomatal closing, reduction in stomatal conductance, and decrease in the rate of photosynthesis . Nevertheless, many studies show that flooding provokes stomatal closure while maintaining leaf water potential .
Several biochemical reactions that are required for plant nutrition and growth depend directly on respiratory metabolism. The absorption of nutrients is relatively constant as long as the concentration of oxygen does not descend below 10% of the total volume of the soil . When oxygen is lacking, a differential rate of absorption of elements is observed. The order of sensitivity isK > N > P > Ca > Mg . Sodium, on the other hand, accumulates in plants when aeration is insufficient.
Other effects are chlorosis and lower enzymatic activity (carboxylase), epinasty, abscission of leaves, hypertrophy of shoots, adventitious root formation, lenticel formation, decreased dry-weight accumulation, reduced root and root hair formation, root death, greater susceptibility to pathogen attacks, and finally plant death. In some species, flooding causes formation of aerenchyma in stems and roots or, less commonly, promotes petiole or shoot elongation. These symptoms are very similar to the effects of ethylene. Oxygen deficiency or hypoxia in the soil also may influence root and shoot hormone links, which affects plant response to waterlogging. In general, prolonged or intense periods of rainfall combined with poor soil drainage often increases ABA content and ethylene and indoleacetic acid in the aerial organs, whereas gibberellins and cytokinins at the root apex diminish. Also, proline content in the leaves increases and the nitrate-reductase enzyme is reduced.
Some species are able to adapt to soils with an excess of water, thanks to modifications in structure and functioning. The roots can lignify and develop tissues containing air pockets. On the other hand, new roots appear rapidly in resistant species, whereas they develop much more slowly in sensitive species.
Until recently, studies of flooding tolerance have been focused specifically on aerenchyma development  and the terminal products of anaerobic respiration  rather than on the role of hormones, such as ethylene production. However ethylene apparently plays an important role in improving root survival and thus survival of whole plants under anaerobic conditions of prolonged waterlogging . Enhanced endogenous ethylene levels following hypoxia induced the breakdown of critical cells and thus promoted lysigeneous aerenchyma in roots of maize .
However, the role of ethylene in the promotion of lysigeneous aerenchyma in cortical cells of rice is apparently cultivar dependent . Ethylene also stimulates development of adventitious roots and influences root extension rate.
The amount of injury depends on the crop species, the cultivar, the stage of plant development, the soil and air temperatures, and on the duration of waterlogging. Intermittent flooding is less damaging than continuous flooding, and the longer the duration of flooding, the greater the damage. There is a strong interaction between crop yield and soil temperature during inundation or waterlogging. High soil temperature conditions lead to oxygen deficiency in the soil profile because oxygen is depleted at a faster rate than it can be replenished from the atmosphere. Flooding during different growth stages has different effects on crop growth and final yield. Flooding during the early growth stage has the greatest impact. In grain crops, oxygen deficits provoke a yellowing of the plant and premature aging of the oldest leaves. Tilling, flowering, and grain filling are the growth stages most sensitive to water excess in cereals.
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