Raising the water-use efficiency of both irrigated and rain-fed crop production is an urgent imperative (Hamdy et al., 2003
). Of the world's allocatable water resource, c. 80% is currently consumed by irrigated agriculture. This level of consumption by agriculture is not sustainable into the future. Projected population growth (another 2 billion people within 2–3 decades) will require that more of the available water resource be used for domestic, municipal, industrial, and environmental needs. The most realistic solution to the increased demand for water will be reallocation to these other purposes of some of the water currently used by agriculture. Even a modest reallocation, reducing agriculture's share to 70%, would increase the amount of water available for other purposes by up to 50% (Hamdy et al., 2003). However, the expanded population will not only need more water to satisfy these other purposes, it will also need to be fed and clothed. This will require substantially more efficient production from a smaller irrigation water resource. It will also require substantially higher water-use efficiency from rain-fed agriculture, which remains the primary means of food production in most countries and for most farmers. Several strategies will be required to improve the productivity of water use in irrigated and rain-fed agriculture (Wang et al., 2002). Breeding crop varieties that are more efficient in their water use is one such strategy. Others include better management of the water resource and changes in crop management. None of these strategies should be seen as operating in isolation. Rather, it is likely that the greatest gains will be obtained through complementary approaches involving each of them.
In this article, recent progress in breeding for high water-use efficiency will be reviewed and some possible avenues for making further advances will be outlined. As a starting point, the article will first establish a conceptual framework for considering ways by which crop water-use efficiency might be improved through breeding. The prospects for improving crop water-use efficiency by changing the water-use efficiency of leaf gas exchange will be considered in some detail. Other likely avenues will also be considered briefly where substantial gains in crop water-use efficiency could be made. The review will concentrate on cereals and particularly on wheat, a dominant food crop which is grown in irrigated and rain-fed production systems over a wide range of latitudes. Wheat is also a crop that has been the focus of a long-standing breeding effort for higher water-use efficiency (Richards et al., 2002). This effort has culminated in the recent release in Australia of two new commercial wheat varieties, ‘Drysdale’ and ‘Rees’, bred specifically for higher water-use efficiency.
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