Application of carbon isotope discrimination for improving crop performance in water-limited and saline environments

Full text: The work of a breeding program aimed at improving the water-use efficiency and yield of wheat varieties grown in Australia's rainfed cropping systems is described. The work could be divided into three main themes covering traits associated with: (1) Pushing more of the available water through the crop rather than have it wasted as evaporation from the soil surface or drainage beyond the root-zone. (2) Acquiring more carbon (biomass) in exchange for the water that is transpired by the crop. (3) Partitioning more of the achieved biomass into the desired product, marketable grain. To increase the amount of water transpired by the crop, research is focussed on developing wheats with a faster rate of leaf area growth. This can be achieved by breeding wheats with longer coleoptiles, to ensure a dense plant stand, and by breeding wheats with greater early vigour, to restrict loss of water by direct evaporation from the soil surface. Another important component of crop water-use efficiency is transpiration efficiency (TE), i.e. the amount of biomass produced per unit water transpired. High crop TE is favoured by vigorous growth during those periods of the growing season when evaporative demand is low. High crop TE may also be favoured by selecting genotypes with high leaf-level TE. Carbon isotope discrimination (Δ) provides a measure of relative variation in leaf-level TE among genotypes of C3 species. This variation in TE may contribute to greater water-use efficiency and yield of C3 crop species grown in water-limited environments. A summary is provided of studies investigating the relationship between grain yield and Δ in bread wheat in Australia. These studies have revealed that there is large variation in Δ among Australian bread wheat cultivars. When measured on leaves sampled from well-watered plants at full tillering, variation in Δ among Australian wheats exceeds 2 per mille. Among wheat genotypes, variation in . is negatively correlated with water-use efficiency of pot-grown plants. In field studies the relationships between yield and Δ have been highly variable, ranging from strongly positive to strongly negative. The reasons for this variability are discussed and opportunities to exploit variation in TE most effectively are considered. Details are provided of a backcross breeding program to improve the TE and yield of wheats for Australia's eastern wheatbelt. The first commercial wheat variety produced by this breeding program, Drysdale, will be released in Australia in late 2001 by CSIRO and partner agencies. This new variety has been bred by backcrossing high TE into the widely-grown variety Hartog. The use of Δ for improving crop performance in saline environments is also discussed briefly. It is concluded that there may be limited opportunity to use Δ for this purpose. Much greater progress would be made by identifying crop genotypes which are effective at either excluding salt at the roots or sequestering salt in vacuoles in the leaves. (author)