SUMMARYClimate change is now unequivocal, particularly in terms of increasing temperature, increasing CO2 concentration, widespread melting of snow and ice and rising global average sea level, while the increase in the frequency of drought is very probable but not as certain.However, climate changes are not new and some of them have had dramatic impacts, such as the appearance of leaves about 400 million years ago as a response to a drastic decrease in CO2 concentration, the birth of agriculture due to the end of the last ice age about 11 000 years ago and the collapse of civilizations due to the late Holocene droughts between 5000 and 1000 years ago.The climate changes that are occurring at present will have – and are already having – an adverse effect on food production and food quality with the poorest farmers and the poorest countries most at risk. The adverse effect is a consequence of the expected or probable increased frequency of some abiotic stresses such as heat and drought, and of the increased frequency of biotic stresses (pests and diseases). In addition, climate change is also expected to cause losses of biodiversity, mainly in more marginal environments.Plant breeding has addressed both abiotic and biotic stresses. Strategies of adaptation to climate changes may include a more accurate matching of phenology to moisture availability using photoperiod-temperature response, increased access to a suite of varieties with different duration to escape or avoid predictable occurrences of stress at critical periods in crop life cycles, improved water use efficiency and a re-emphasis on population breeding in the form of evolutionary participatory plant breeding to provide a buffer against increasing unpredictability. ICARDA, in collaboration with scientists in Iran, Algeria, Jordan, Eritrea and Morocco, has recently started evolutionary participatory programmes for barley and durum wheat. These measures will go hand in hand with breeding for resistance to biotic stresses and with an efficient system of variety delivery to farmers.

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Genotype × environment (GE) interactions in general, and GE interactions of crossover type in particular, have a negative impact on the progress of breeding programmes, particularly when breeders try to avoid them by searching for widely adapted cultivars. However, breeders may exploit GE interactions by selecting for specific adaptation. This is particularly relevant when breeding for adaptation to unfavourable environments and subsistence agriculture. Throughout this chapter, unfavourable environments are defined as those where crop yields are commonly low due to the concomitant effects of several abiotic and biotic stresses. Because of the high probability of low yields and crop failures in unfavourable environments, the use of inputs such as fertilizers, pesticides and weed control is seen by farmers as risky. Therefore, the adoption of improved agronomic practices has been limited, and the only economic solution to increase crop yields in unfavourable environments is through breeding. However, empirical plant breeding for these environments has been historically much less successful than it has for more favourable or for high potential environments. This chapter describes a strategy to exploit GE interactions in national and international breeding programmes for such areas. Issues such as the choice of the selection environment, specific versus wide adaptation, and genetic uniformity versus genetic diversity are discussed, and some implications for the genetic base are noted.

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