15.3.5 Drought stress and adaptive responses

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Table 15.4

The sheer diversity of plants that grow in arid and semi-arid regions implies that there is no single gene for drought resistance. Rather, vascular plants use an array of mechanisms to withstand drought, depending on environment and species. Mechanisms of drought resistance observed in plants and implications for productivity are categorised in Table 15.4.

Despite the complexity of drought resistance, plant breeders in semi-arid and arid regions of the world have improved the performance of crops and pastures for these regions. While this has usually been achieved by breeding for improved yield or quality under conditions of water shortage, plant physiologists have assisted in this process by identifying mechanisms underlying resistance mechanisms and assisting in the development of selection techniques (Whan et al. 1993).

Drought resistance is a broad term and applied loosely to both native species and crop plants with adaptive features that enable them to cope with soil moisture shortage. In functional terms, a qualitative distinction exists between those plants that escape drought via opportunistic growth and reproduction cycles such as desert ephemerals, and those that tolerate drought by alleviating adverse impacts of soil moisture shortage on growth and reproductive development (features widely sought in crop plants). A third category of plants resist extreme and prolonged drought by enduring tissue desiccation (proto-plasmic dehydration in Table 15.4; see Feature essay 15.1).

Two features of these drought-resistance characteristics need to be recognised. First, a trait which in nature may enable an individual merely to survive may not be useful for a crop species where yield is important. Consequently many characteristics that occur in desert species are not beneficial in crops. Second, environmental conditions must be specified. For example, drought escape by rapid phenological develop-ment will be beneficial where terminal drought prevails but will not be beneficial where rainfall follows a bimodal distribution.


Table 15.5

Wheat provides a good example of the benefits of breeding for improved drought resistance. Studies in Western Australia have shown that plant breeding has resulted in steady improvement in yield under water-limited conditions. Yields of 28 wheat cultivars grown under identical conditions at Merredin near the eastern margin for agriculture in Western Australia (mean growing season rainfall of 210 mm) summarised in relation to their year of release show a genetic gain of 5.8 kg ha–1 year–1 or 0.6% per year on average since the first wheat cultivars were released in the 1860s. Similar rates of increase have been observed elsewhere in Australia and in water-limited environments in the USA, but contrast with genetic gains observed in Britain where water limitations are minimal (Table 15.5).


Figure 15.19 Yield of wheat grown under identical conditions is highly dependent upon the year of release of the cultivar. Yield has increased by approximately 0.6% per year partly because of an improved match between phenology and seasonal conditions. As each season progresses, rainfall declines, temperatures increase and soil water content decreases. Consequently cultivars selected for rapid development can escape the worst of the developing soil water deficit and yield improves. Selection for increased ear: stem ratio has contributed further to increased yields of wheat over the past 120 years (Based on data compiled from 28 cultivars by N.C.Turner)

One major reason for improvement in yield from modern cultivars has been a better matching of growth and water use to the seasonal pattern of rainfall (Figure 15.19). Such selection for drought escape by rapid phenological develop-ment in modern cultivars (Table 15.5; Figure 15.19) ensures that ear emergence and grain filling occur before water deficits and high temperatures induce premature senescence and pinched grain. Despite those gains, improved phenological development and drought escape actually limit yields in years with plentiful rainfall, particularly if phenological plasticity is limited.

Studies conducted on historical as well as modern wheats show that drought escape is not the only reason for improved yields in modern cultivars. Modern cultivars also have a greater number of fertile florets per spikelet, a longer duration of grain growth and a high ear:stem dry mass ratio at anthesis compared to historical cultivars (Loss et al. 1989). These characteristics increase the harvest index (HI) of wheat under water-limited conditions. HI can be selected and improved further in specific breeding programs, as has been done for the ear:stem ratio in wheat (Siddique and Whan 1994).


Figure 15.20 Early vigour of crops is an important characteristic for high-yielding plants in Mediterranean—type climate regions such as southwestern Western Australia. Biomass accumulated by the five- to six-leaf stage is positively correlated with grain yield on sandy soils where water—holding capacity is low and rapid early development allows access to soil moisture ahead of losses due to deep drainage or evaporation (Based on unpublished data compiled by N.C.Turner)

Early vigour was similarly identified as an important characteristic that improved wheat yields under Mediterranean climatic conditions (Turner and Nicolas 1987). Biomass pro-duction at the five- to six-leaf stage was positively correlated with grain yield (Figure 15.20) on deep sandy soils with poor water-holding capacity. By developing early biomass, available moisture can be utilised ahead of loss due to deep drainage and soil evaporation.

Osmotic adjustment is an adaptive feature of many plants in water-limited environments that enables a plant to maintain turgor by solute accumulation as water deficits develop (Turner and Jones 1980). Studies in controlled environ-ments showed that wheat genotypes varied in their capacity to osmotically adjust (Morgan 1980) and field studies showed that the lines with a high degree of osmotic adjust-ment yielded up to 60% more than genotypes without this characteristic (Morgan 1983; Morgan et al. 1986). Plants that show strong osmotic adjustment generate a substantially higher osmotic pressure in their tissues and are able to extract more water from the soil. This capability appears to be controlled by only one or two genes and is simply inherited (Morgan and Condon 1986).

Finally, theoretical considerations suggest that restricting the diameter of the major xylem vessel in the seminal roots of wheat will reduce water use during vegetative growth, thereby providing more water during grain filling (Richards and Passioura 1981). This decreased hydraulic conductance is a drought resistance characteristic that should be beneficial where wheat grows primarily on stored soil moisture. Screening for decreased vessel diameter identified a land race from Turkey with a vessel diameter of about 50 µm compared with vessel diameters of 60–65 µm in cultivars in use in Australia. The land race with the narrow vessels was back-crossed to two locally adapted cultivars to give lines of wheat similar to the parent, but with a reduced diameter of the main vessel in the xylem of the seminal roots. The yield of these lines was 3–11% better than the unselected cultivars at dry sites and in dry seasons, but as expected these lines showed no yield reduction at wet sites and in wet seasons (Richards and Passioura 1989).

Other characteristics that have been identified and selected in wheat are high assimilate transfer to the grain (Turner and Nicolas 1987; Whan et al. 1993) and rapid grain growth (Whan et al. 1993). They have been shown to contribute to improved yields under water-limited conditions.

Drought resistance is not synonymous with water use efficiency, which can be defined in various ways according to level of organisation. At a leaf level water use efficiency can be regarded as a ratio of net assimilation to transpiration (moles of carbon fixed per mole of water transpired), also termed instantaneous transpiration efficiency. At a community level water use efficiency can be: (1) the ratio of net assimilation rate to evapotranspiration (where evapotranspiration = crop transpiration plus soil evaporation), (2) the ratio of dry matter production to cumulative water use, or (3) the ratio of grain yield to cumulative water use.

Genetic variation with respect to instantaneous transpiration efficiency has been demonstrated in a wide range of species, using the carbon isotope signature (D) of dried plant material as a surrogate for physical measurement of assimilation and transpiration (Case study 15.3). Selection for low D lines has been confirmed as a valid approach to improved water use efficiency and is likely to underpin much future genetic improvement for dryland cereals.