Some Considerations When Breeding Crops Other than Wheat

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We have primarily discussed the adaptation of crop cultivars to a changing climate in the context of wheat. While the principles of assessing and introgressing genetic variability apply across the crop species, regardless of their ploidy level, genome size and geographic distribution, the breeding strategies used to combine this variability will differ among the crop species largely based on their reproductive system. The breeding strategies used to improve the stress tolerance of self-pollinated crops such as wheat, rice and barley are similar, although there are some minor variations. Wheat and barley are more amenable to double haploid production than rice, whereas gene expression is more easily understood in diploids such as barley and rice. There is evidence that wheat hybrids have more stable yield in drier environments (Nehvi et al. 2000), although the cost of producing hybrid seed makes hybrid wheat less attractive than hybrid rice, which is already under cultivation on large areas in Vietnam, India and China (FAO 2005). Nevertheless, the expression of water and heat stress tolerance in hybrid combinations still needs to be assessed. Most hybrid work has focused on the more productive environments where economic returns are greater.

In contrast, significant improvement in the stress response of maize, an open pollinated species, has been achieved through recurrent selection under both prevailing and managed stresses (Banziger et al. 2004). These schemes increase the frequency of favorable alleles by repeated cycles of selection and intercrossing of superior individuals. Cross specific QTLs (quantitative trait loci) can also be accumulated through recurrent selection using linked molecular markers. In contrast, it is difficult to employ these population improvement techniques in self-pollinated species as it is too expensive to make the required intercrosses among selected progeny. Nevertheless, male sterility (genetic, cytoplasmic and chemically induced) does exist in many self-pollinated species and could be used to facilitate intercrossing and the establishment of recurrent selection schemes.

9.2.4 Conclusion

Plant breeders working in the world's rainfed environments have made steady incremental gains in yield under stress. Over the past 10 years the research investment in plant response to drought and heat has increased significantly, largely driven by improvements in technology, and an increasing awareness of the impending impacts of climate change and reduced water availability on agriculture. Much of this investment has been driven by the private sector in high value crops such as maize (Braun and Brettell 2009). Nevertheless, the investment in wheat and rice, while considerably smaller, has also increased. Improved understanding of the molecular basis of the plant stress response has gone hand-in-hand with improved understanding of the physiological response. International centers such as CIMMYT, the International Center for Agricultural Research in the Drier Areas (ICARDA) and the International Rice Research Institute (IRRI) have extensive breeding and research programs targeting improved water-use and/or tolerance to heat. Other initiatives such as the Generation Challenge Program (GCP) focus on using genetic diversity to improve drought tolerance in crops and provide a mechanism whereby advanced research institutes, national programs in developing countries and international centers such as those mentioned above can bring their skills and resources to bear on these most intractable of problems.

Significant genetic diversity for stress response has already been identified in the primary gene pools of most of the major crop species. However, the challenge of efficiently and effectively introducing this diversity into elite crop backgrounds remains a significant impediment. The vagaries of the production environment, incomplete understanding of the underlying physiological response and the complexity of inheritance of stress responses remain major challenges.

An additional complexity is the relationship between drought and high-temperatures in many production environments. High evapotranspiration rates often lead to increased moisture stress, particularly at lower latitudes. There is significant variation for response to high temperature in most crop gene pools and materials can be selected by simply delaying planting time to expose plants to terminal heat stress. The higher heritability of the selection environment for heat tolerance compared to drought stress should lead to greater gains in productivity under elevated temperature. However, there is evidence that traits important for one stress also influence the other (Table 9.1). Osmotic adjustment, phenology, water-use-efficiency

Table 9.1 Commonality between traits associated with drought and heat stress

Tolerance mechanism

Drought

Heat

Osmotic adjustment

Rapid growth/earliness

Concentration of organic solutes

Water use efficiency

Osmotic adjustment renders plants tolerant to drought stress (Munns 2002; Farooq and Farooq-E-Azam 2001; Blum and Pnuel 1990) Earliness favors drought tolerance (Blum and Pnuel 1990)

Increased concentration of organic solutes is observed under drought stress (Sakamoto and Murata 2002; Ashraf and Foolad 2007) Water use efficiency of the plant improves drought tolerance (Shannon 1997; Machado and Paulsen 2001; Nultsch 2001)

Osmotic adjustment renders plants tolerant to heat stress (Blum and Pnuel 1990)

Earliness favors tolerance to heat (Ehlers and Hall 1998; Blum and Pnuel 1990) Increased concentration of organic solutes is observed under heat stress (Sakamoto and Murata 2002; Ashraf and Foolad 2007) Water use efficiency of the plant influences heat tolerance (Machado and Paulsen 2001)

and solute concentrations can have an impact on both tolerance to heat and drought. These relationships, if confirmed, will allow the breeder to simultaneously improve both characters.

Increasing levels of CO2 and higher atmospheric temperatures associated with climate change may at the same time offer both impediments and opportunities (Wahid et al. 2007). A changed climate may favor the cultivation of crops with a C4 photosynthetic pathway rather than the less efficient C3 pathway, although all else constant C3 crops appear to benefit more from increasing levels of CO2. This has renewed interest in the challenge of converting C3 crops to the C4 photosynthetic pathway and a major project is underway at IRRI to produce C4 rice (http://www. eurekalert.org/pub_releases/2009-01/irri-nhr011909.php).

Clearly genetic diversity is vital to realizing improved crop responses to drought and heat. In some instances there may be insufficient diversity within the crop gene pool to achieve the required levels of improved adaptation. The introduction of transgenes that regulate the plant response to stress may in the future contribute to the overall goal of improved productivity under stress.

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