Abstract
Climatic variability, typified by erratic heavy-rainfall events, causes waterlogging in intensively irrigated crops and is exacerbated under warm temperature regimes on soils with poor internal drainage. Irrigated cotton is often grown in precisely these conditions, exposing it to waterlogging-induced yield losses after substantial summer rainfall. This calls for a deeper understanding of mechanisms of waterlogging tolerance and its relevance to cotton. Hence this review suggests possible causes of waterlogging-induced yield loss in cotton and approaches to improvement of waterlogging tolerance, drawing upon the slight body of published data in cotton and principles from other species. The yield penalty depends on soil type, phenological stage and cumulative period of root exposure to air-filled porosities below 10 %. Events in the soil include O2 deficiency in the root zone that changes the redox state of nutrients, making them unavailable (e.g. nitrogen) or potentially toxic for plants. Furthermore, root-derived hormones that are transported in the xylem have long been associated with oxygen deficits. These belowground effects (impaired root growth, nutrient uptake and transport, hormonal signalling) affect the shoots, interfering with canopy development, photosynthesis and radiation-use efficiency. Compared with the more waterlogging-tolerant cereals, cotton does not have identified adaptations to waterlogging in the root zone, forming no conspicuous root aerenchyma and having low fermentative activity. We speculate that these factors contribute substantially to the sensitivity of cotton to sustained periods of waterlogging. We discuss the impact of these belowground factors on shoot performance, photosynthesis and yield components. Management practices, i.e. soil aeration, scheduling irrigation and fertilizer application, can reduce waterlogging-induced damage. Limiting ethylene biosynthesis using anti-ethylene agents and down-regulating expression of genes controlling ethylene biosynthesis are strong candidates to minimize yield losses in waterlogged cotton crops. Other key pathways of anoxia tolerance are also cited as potential tools towards waterlogging-tolerant cotton genotypes.
Highlights
Waterlogging is a worldwide phenomenon that strongly influences the distribution of plant species and crop production
This review aims to provide information on the possible mechanisms through which waterlogging damages cotton crops and suggests remediation pathways
This review draws on our knowledge of the physiological and biochemical responses of plants to O2 limitation in order to understand how these processes affect growth and yield in cotton (Fig. 5)
Summary
Waterlogging is a worldwide phenomenon that strongly influences the distribution of plant species and crop production. In a microarray study of waterlogged cotton roots and leaves, Christianson et al (2010b) observed up-regulation of genes controlling biochemical processes such as glycolysis, fermentation and mitochondrial electron transport pathways, again underlining the role of ethanolic fermentation and residual respiratory activity in plant survival under hypoxia. Radicle growth was significantly reduced at these relatively low fermentation rates compared with tolerant plants (Table 2), indicating that cotton seeds generate insufficient energy from fermentation under waterlogging or that acetaldehyde toxicity impedes growth This does not preclude engineering a higher level of fermentation in root apices or other tissues during waterlogging events. Hypoxia-induced cotton growth and yield reduction could be the result of: (i) nutrient deficiency (Bange et al 2004); (ii) increased ethylene accumulation (Christianson et al 2010b) and/or (iii) impaired photosynthesis and net carbon fixation per unit of leaf area (RUE). Improvements in weather forecasting signalling major rainfall events would assist in identifying the need to apply foliar fertilizers and hormones
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