Abiotic Stress Tolerance and Cultivar Screening Tools

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There is a large variation in tolerance or susceptibility to abiotic stresses, particularly to different climate change factors including UV-B radiation (Reddy et al., 2005; Singh et al., 2008a), high temperature stress (Craufurd et al., 2003; Prasad et al., 2006b; Ristic et al., 2008), and water stress (Foulkes et al., 2002; Upadhyaya, 2005; Bakheit, 2008; Singh, 2008). This variation can provide an opportunity for genetic improvement of plant species through either traditional plant breeding techniques (selection and crossing) or modern molecular biology techniques, such as plant transformation. Screening wide germplasm from various locations and origins including native wild relatives of crop plants and landraces for single and multiple abiotic climate change factors may prove useful for identifying tolerant traits and in developing climate-ready species or cultivars for a given region.

The available genotypic variability of a species offers an opportunity for breeders to design and develop specific plant types to suit different agro-ecological environments. Effectiveness of selection for a trait depends on the magnitude of genetic and nongenetic causes in the expression of phenotypic differences among the genotypes in a population and is expressed as heritability of the trait (Thiaw and Hall, 2004). A thorough understanding of the physiological basis of differences in stress tolerance could be used to select or create new cultivars of crops that have increased productivity under such conditions (Wentworth et al., 2006). The genetic association of a trait with a higher level of physiological and/or developmental attributes facilitates adaptation of a crop to a stress condition and has proven useful for breeding purposes and developing improved lines of a crop species (Singh and Sharma, 1996). Several screening methods, such as cell membrane thermostability in soybean (Martineau et al., 1979; Blum et al., 2001) and cowpea (Ismail and Hall, 1999), in vitro pollen germination in canola (Singh et al., 2008b), cotton (Kakani et al., 2005), peppers (Capsicum spp.; Reddy and Kakani, 2007), and soybean (Koti et al., 2004; Salem et al., 2007), chlorophyll fluorescence in Arabidopsis (Barbagallo et al., 2003), photosynthesis and stomatal conductance in cotton (Lu et al., 1998), and intrinsic water use efficiency and associated gas exchange parameters in almond (Prunus dulcis L.), wheat, and cowpea (Brodribb, 1996; Condon et al., 2002; Singh, 2008), have been used at field and laboratory scales to identify tolerant traits and genotypes to abiotic stresses.

Abiotic stresses adversely affect various cellular functions, but photosynthesis is particularly sensitive to heat and drought stress (Berry and Bjorkman, 1980; Brodribb, 1996; Haldimann and Feller, 2005). Fluorescence parameters have been shown to relate directly to the photosynthetic rates of leaves (Genty et al., 1990; Edwards and Baker, 1993) and have been widely used to study leaf photosynthetic performance (Maxwell and Johnson, 2000). Consequently, any small perturbation in photosynthetic metabolism significantly modifies the fluorescence characteristics of plants. The sensitivity of chlorophyll fluorescence to the stress-induced perturbation in plant metabolism can potentially make it useful for screening genotypes with differential responses to abiotic factors (Brodribb, 1996; Barbagallo et al., 2003). Previous studies suggest significant changes in the photochemical activities of cowpea leaves subjected to heat (Costa et al., 2003; Costa et al., 2004), UV-B (Premkumar and Kulandaivelu, 1996; Lingakumar et al., 1999), and drought conditions (Lopez et al., 1987; Souza et al., 2004).

Hall (2004b) proposed a yield component model that can be incorporated for selection of most legumes, including cowpea cultivars in the high-temperature-limited production zones. Four yield components (number of flowers per unit area, number of pods per flower, number of seeds per pod, and weight of individual seeds) that contribute to yield reduction were recognized. In a simple screening approach for heat tolerance, Ismail and Hall (1999) found an association between reproductive-stage heat tolerance and higher cell membrane thermostability measured as electrolyte leakage from leaves subjected to high-temperature treatment. In an extremely hot field environment, negative correlations were observed between grain yield and electrolyte leakage (r = - 0.79, n = 9), and pod set and electrolyte leakage (r = - 0.89, n = 9) among nine cowpea breeding lines.

A similar approach could also be used to assess variable differences among species and cultivars of the same species under UV-B radiation.

An increased concern in regard to abiotic stress effects on crop plants has prompted screening for tolerance in crop populations (Hall, 2001). Many crops have been screened by using various abiotic stress response indices derived from the different stages of plant growth in response to single or multiple abiotic stresses (Dai et al., 1994; Saile-Mark and Tevini, 1997; Koti et al., 2004; Hubbard and Wu, 2005). Several crops, including rice (Dai et al., 1994), wheat (Yuan et al., 2000), bean (Saile-Mark and Tevini, 1997), and corn (Hubbard and Wu, 2005), have been screened by using several UV-B and drought response indices derived from plant growth responses under UV-B or drought conditions. In addition, multivariate analyses, such as principal component analyses and factor analysis, have been used for efficiently characterizing the stress responsiveness of a population under study and the associated plant attributes (Hofmann et al., 2001; Kaspar et al., 2004; Singh et al., 2008a).

Simultaneous occurrences of different abiotic stresses are common in natural plant habitats, which greatly modify the individual stress effect. This modification in the degree of response mechanisms could have been caused because of co-activation of different response pathways by simultaneous exposure of plants to different abiotic stresses leading to synergistic or antagonistic effects (Mittler, 2006). Developing a crop plant with enhanced tolerance to a stress combination, including UV-B radiation by either traditional breeding or genetic engineering, requires an understanding of the complex cross-communication between different signaling pathways and their direct or indirect effects on plant growth and metabolism (Hall, 2004a; Mittler, 2006).

Hall and Ziska (2000) recommended that plant breeders should consider possible climate change when developing a breeding strategy. Grain yield in legumes, such as cowpea, can be enhanced by selection of greater reproductive sinks under high temperatures, which will minimize the feedback effect that down regulates the photosynthetic mechanisms (Ahmed et al., 1993; Hall and Allen, 1993). However, yield has less importance in a trait-based breeding program particularly for high-temperature and drought tolerance. The yield reduction caused by abiotic stresses is a consequence of several first-order effects, such as photosynthetic performance (photosynthesis and fluorescence reduced water use efficiency), morphogenesis (differentiation and developmental rate), and production of defense compounds (phenolic compounds, and free amino acids and waxes) affecting overall vegetative growth and dry matter production. Therefore, survival capacity and maintenance of normal metabolic activity in the presence of stress conditions are key features for sustaining higher yields and should be considered an important component of breeding programs.

Molecular genetic mapping of the plant genome has facilitated identification of biomarkers that are closely linked to known resistance genes such that their isolation is clearly feasible in the future (Easterling et al., 2007). Temperature and drought stress resistance are especially relevant to climate change. Earlier studies have demonstrated genetic modifications to major crop species (e.g., maize and soybean) that increased their water deficit tolerance (Drennen et al., 1993; Kishor et al., 1995; Cheikh et al., 2000), although this may not extend to a wider range of crop plants. Little is known about how the desired traits achieved by genetic modification will perform under multiple abiotic conditions that commonly occur in the natural environment. The genomic approach offers new germplasm and understanding, but the emergent nature of yield from physiological processes demands that all components contributing to the yield be considered. It is important to understand the interactions of various regulatory pathways within plants, and between plants and environments, to understand key links between gene activity and crop yield (Sinclair and Purcell, 2005). Biotechnology is not expected to replace conventional agronomic breeding (Easterling et al., 2007); however, it will be a crucial adjunct to conventional breeding because both will be needed to meet future environmental challenges, including climate change (Cheikh et al., 2000; FAO, 2004).

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