Improving the efficiency of water use by plant breeding and molecular biology

Plant breeding over the last century has indirectly increased water use efficiency of the major grain crops because yield has increased with no additional water use. These increases have been substantial in all major food crops. Improved varieties have come from conventional breeding programs where selection has been for yield. Although these increases are likely to continue, they may not occur at the same rate as before. Most of the increases have been due to improvements in harvest index which must now be approaching its theoretical limit in many of our major crops. There has been little increase in water use efficiency for biomass and this must be increased if improvements in water-use efficiency are to continue. Possibilities are numerous. They will depend on the crop and the environment in which it is grown. Modification of phenology to better suit the environment, which has been responsible for many of the increases in the past, will continue to be important in most crops. Genetic increases in water-use efficiency should also come from the manipulation of the crop canopy to reduce wasteful evaporation of water from the soil surface and from an improvement in transpiration efficiency. The most likely avenues for success are discussed.