There is now growing evidence that targeting specific traitsin a breeding programme may lead to higher agronomic water-useefficiency. It is also clear that the effects of any one traitmust be considered in the context of the environment in whichthe crop is to be grown. A particular trait, such as high A/T,may be associated with higher yield in one type of environmentbut may have no effect or even be detrimental in other environments.Breeding for high A/T using low-13C measured in the leaves ofunstressed wheat plants has resulted in the release of new,higher-yielding varieties for eastern Australia. The new varietiesresult from a backcrossing programme targeting environmentswhere stored soil moisture needs to be metered out from relativelyearly in the cropping season so as to maximize seed set andsustain seed growth. By contrast, for cereals growing in Mediterranean-typecropping regions and irrigated environments, higher yield appearsto be associated with high-13C of grain. For Mediterranean-typeregions this association may, in part, be a result of an associationbetween high-13C and fast crop growth rate. Fast crop growthrate is reflected in the vigorous development of leaf area toshade the soil surface, a key trait for cropping environmentswhere the soil surface is frequently rewetted and large gainsin T/ET can be made. Effective selection protocols for fasterleaf area growth have been devised and shown to be successfulfor yield improvement in Mediterranean-type environments. Itis possible that even greater yield gains may be achievablein these and other rain-fed environments if fast crop growthrate can be combined with high A/T. Breeding has been initiatedto combine these two traits in bread and durum wheat, but thismay prove difficult if an association between high vigour andhigh-13C cannot be broken.
A strategy that sets out deliberately to target a combinationof high early vigour and high A/T in wheat may not be necessaryin other species. It will depend strongly on the extent andtiming of any water limitation in relation to developmentalphases critical for yield determination, and whether there isan association between high A/T and slow crop growth rate. Suchan association is likely to depend on whether variation in A/Tis due to variation in stomatal conductance or photosyntheticcapacity and the effects of these two components on crop growthrate. Relatively simple techniques are now available for characterizingvariation in stomatal conductance and photosynthetic capacity.Other interactions may also come into play. In groundnut, A/Tis positively associated with crop biomass production, but negativelyassociated with HI. Breeding progress is being made in thisspecies by applying concurrent selection pressure for high A/Tand high HI. Progress may be more rapid if the reasons for theassociation between A/T and HI were better understood.