Adaptive strategies

There are a number of strategies to ameliorate the effects of drought and heat stress.

Agronomic strategies

Agronomic strategies include: (i) modifying planting time such that critical growth stages do not coincide with stressful conditions (McMaster et al., 2005); (ii) resource conserving technologies that help available growth inputs, especially water, to be supplied as optimally as possible to the crop (Hobbs and Govaerts, Chapter 10, this volume); and (iii) good husbandry to avoid weeds, pests and diseases from further exacerbating stress.

Trait-based strategies

The most effective genetic strategy has been to change the phenological pattern of the crop so that critical growth stages do not coincide with stressful conditions or simply to finish the life cycle early before severe stress conditions occur (Ludlow and Muchow, 1990). Another is to minimize the occurrence of stress through development of a good root system, which in the case of drought permits water to be accessed deeper in the soil (Lopes and Reynolds, 2010) and in the case of heat permits transpiration rates that better match evaporative demand (Amani et al., 1996), thereby permitting maximal carbon fixation with the benefits of canopy cooling. In environments where 'extra' water is not available to mitigate stress, other stress-adaptive strategies include a range of leaf canopy traits such as epicuticular wax, pigment composition, leaf angle and rolling, etc. that influence radiation load and photosynthetic response, while increased transpiration efficiency permits available water to be used more effectively (Richards, 2006). Maintaining foliar and root health through genetic resistance to pest and diseases is usually considered prerequisite. Such effects can be cumulatively significant - and will interact with other environmental and agronomic effects such as irrigation and tillage systems (Table 5.1). Examples of their application are discussed subsequently in the context of specific breeding efforts, as well as in various books (see Ribaut, 2006; Jenks et al., 2007).

Cellular and molecular strategies

It is expected that the growing understanding of the cellular and molecular basis of adaption to heat and drought stress will have significant impact in breeding for climate change in future decades. For example, it is established that plant response to drought involves multiple mechanisms associated with water relations, chemical signals and membranes (Chaves et al., 2003). In maize, part of the effect of drought on floret abortion - a trait which has a disproportionate effect on harvest index compared with its effect on water-use efficiency - has been traced to several genes involved in sucrose metabolism (Boyer and McLaughlin, 2007). Gene expression studies have confirmed that soluble starch synthase is a rate-limiting step for grain filling in wheat when exposed to high temperature (Hurkman et al., 2003), while surprisingly no clear role for heat shock proteins has been identified in cereals despite a well-established role in acclimation to heat stress in Arabidopsis (Barnabas et al., 2008). Favourable water relations are a crucial aspect of adaptation to both drought and heat stress so further understanding of the role of aquaporins, which show a high degree of diversity, in maintaining plant function under stress may lead to useful genetic modifications (Kaldenhoff et al., 2008). When combining heat and drought stress, novel metabolic responses have been demonstrated compared to when stresses are experienced in isolation (Mittler, 2006). Readers are referred to comprehensive reviews of genomic approaches to determine the mechanistic basis of adaptation to heat and drought stresses, which shed light on candidate genes for crop improvement, by Chaves et al. (2003), Shinozaki and Yamaguchi-Shinozaki (2007) and Barnabas et al. (2008) and references therein.

Part of the molecular basis for heat susceptibility in wheat seems to be related to ethylene levels. In a comparison of heat-susceptible versus -tolerant winter wheat cultivars, an increase in ethylene was shown to be directly responsible for regulating the heat-induced grain abortion and reduction in kernel weight (Hays et al., 2007a). Ethylene may be playing a fundamental role in stress signalling, given that the ethylene receptors share significant homology with two-component histidine kinase receptors in prokaryotes that have been shown to act as heat sensors. However, ethylene-induced kernel abortion and premature maturation in response to heat stress, while possibly being a useful survival trait (to temper progeny load in warm, dry climates), is clearly detrimental to productivity, and these studies have led to markers for selection against its expression.

However, the general current understanding of the complex interaction of cellular/molecular mechanisms with whole-plant adaptation to contrasting environments does not yet permit its reliable application in cultivar selection. Nevertheless, a few ambitious projects exist, such as the C4 rice initiative which aims to identify all of the genes necessary to introduce Kranz anatomy and CO2-concentrating mechanisms into C3 species (Hibberd et al., 2008), and genetic modifications associated with increasing CO2 fixation rate by Rubisco (Parry and Hawkesford, Chapter 8, this volume). If successful, these would lead the way to substantial increases in heat adaptation in C3 crops, as well as adaptation to moderate levels of moisture stress, though C4 photosynthesis is possibly more sensitive to dehydration stress than is C3 photosynthesis (Ghannoum, 2009). On the other hand, empirical studies involving genetically mapped populations have identified quantitative trait loci (QTLs) associated with adaptation to drought and heat; the potential of these QTLs to achieve genetic gains in yield is discussed later.

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