Plant productivity is fundamentally affected by abiotic stress. Climate and soil determine many adaptations in plants and the ecogeographical distribution of species and ecotypes demonstrate differences in physiology and developmental patterns that provide good evidence of adaptive mechanisms. Plants respond to environmental change as individuals through phenotypic plasticity and in populations through selection and associated evolutionary processes. It is not always easy to determine the genetics underlying adaptive processes because environmental factors may be complex or not clearly defined. However, extreme environmental pressures such as heavy metal contamination of soils (Humphreys and Bradshaw, 1977) or severe winter conditions (Helgadóttir et al., 2001) may produce detectable genetic shifts. Several genes may be responsible for a response to a given factor or the same gene(s) may be involved in different adaptive responses. Specific gene interactions may be in a state of flux or become “fixed,” restricting opportunities for further evolution. Phenotypic plasticity serves as a buffer to avoid excessive genetic flux in response to short-term changes. Traditional methods of evaluating phenotypic plasticity in terms of G × E (genotype × environment) interactions (e.g., Finlay and Wilkinson, 1963) equate “environment” to the mean performance of all genotypes in response to a particular site, management, or year. Yield of individual varieties is plotted against the measure of environment and yield potential may be defined as the highest yield in the best environment. Yield stability of a particular genotype should be measured across environments and may be equated to stress resistance (Thomas, 1997). Traits desirable in temperate European grasslands for resistance to periods of summer drought and cool winters are outlined in Table 2.1 and can be seen to differ among areas where grassland

agriculture is practiced in Europe. Blum (1988) reviewed general aspects of crop improvement for stress environments. This chapter aims to build on this and give an overview of the principles underlying breeding for abiotic stress in an era of rapidly advancing genetic information and technical development.