Nature of adaptive change

Biological organisms undergo adaptive change as they acclimatize to new environments. Many adaptive characteristics result from the prevailing environment influencing expression of genes related to adaptation. Genes are also subject to constant change; random changes or 'mutations' can occur through internal errors in DNA repair

© CAB International 2010. Climate Change and Crop Production (ed. M.P. Reynolds)

and replication or when subjected to external influences such as ultraviolet irradiation. These mutations can alter expression or activity of encoded proteins.

Within any population of plants there is genetic variation: natural or induced. Over many centuries, farmers have generated heterogeneous plant populations adapted to local climates and cultivation conditions. More recently, plant breeders have selected superior variants to generate genetically homogenous, highly adapted, 'elite' cultivars.

Intense cereal breeding has resulted in spectacular improvements in yield and quality but has narrowed genetic diversity. Continued genetic gain is becoming increasingly difficult (Feuillet et al., 2008). Using biotechnology, plant breeders have sought to identify and deploy new sources of variation by understanding the available genetic variation, the genetic control of adaptation, and the gene-by-environment interactions.

Plant responses to different environments are not thoroughly understood nor are their genetic bases. Studies of fundamental adaptation mechanisms have focused on single, rather than multiple, genotype-stress relationships. Many have also examined plant survival under extreme stress rather than more realistic, agronomically relevant, stress scenarios (Cushman and Bohnert, 2000; Bartels and Salamini, 2001; Araus et al., 2003).

Adapting to drought stress is a huge challenge in plant breeding because 'drought' means many different things. Stresses may be caused during grain filling, pre-flowering or may be continuous throughout a growing season. In Mediterranean-type dryland areas, grain-filling stress is common, in South America pre-flowering stress is more likely and continuous stress is frequent in non-irrigated parts of southern Asia where plants are often dependent upon water stored in the soil (Reynolds et al., 2005). Plants often face multiple stresses concurrently; under water-limiting conditions, there may also be high temperatures, increased irradiance and less permeable soil. These factors make the plant breeder's task very complex, and breeders must grapple with drought-related stresses most relevant to them.

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