Drought resistance, water-use efficiency, and yield potential—are they compatible, dissonant, or mutually exclusive?

This presentation is a concept review paper dealing with a central dilemma in understanding, designing, and acting upon crop plant improvement programs for drought conditions. The association among yield potential (YP), drought resistance (DR), and water-use efficiency (WUE) is often misunderstood, which in turn can lead to conceptual oversight and wrong decisions in implementing breeding programs for drought-prone environments. Although high YP is the target of most crop breeding programs, it might not be compatible with superior DR. On the other hand, high YP can contribute to yield in moderate stress environments. Plant production in water-limited environments is very often affected by constitutive plant traits that allow maintenance of a high plant water status (dehydration avoidance). Osmotic adjustment (OA) is a major cellular stress adaptive response in certain crop plants that enhances dehydration avoidance and supports yield under stress. Despite past voiced speculations, there is no proof that OA entails a cost in terms of reduced YP. WUE for yield is often equated in a simplistic manner with DR. The large accumulation of knowledge on crop WUE as derived from research on carbon isotope discrimination allows some conclusions on the relations between WUE on the one hand, and DR and YP on the other, to be made. Briefly, apparent genotypic variations in WUE are normally expressed mainly due to variations in water use (WU; the denominator). Reduced WU, which is reflected in higher WUE, is generally achieved by plant traits and environmental responses that reduce YP. Improved WUE on the basis of reduced WU is expressed in improved yield under water-limited conditions only when there is need to balance crop water use against a limited and known soil moisture reserve. However, under most dryland situations where crops depend on unpredictable seasonal rainfall, the maximisation of soil moisture use is a crucial component of drought resistance (avoidance), which is generally expressed in lower WUE. It is concluded that the effect of a single ‘drought adaptive’ gene on crop performance in water-limited environments can be assessed only when the whole system is considered in terms of YP, DR, and WUE. Additional keywords: plant breeding, osmotic adjustment, drought tolerance, phenology. Introduction As conventional breeding and biotechnologymake headways into the development of drought-resistant cultivars, the conceptual framework of what actually constitutes a viable target for selection in this respect is not always clear. The drought-resistant ideotype is not always well defined. In workshops dealing with breeding for drought-prone environments there is a constant debate of ‘putative’ drought resistance mechanisms, ‘water-use efficiency’, and their interrelationship and associationswith yield potential.Wateruse efficiency (WUE) is often equatedwith drought resistance and the improvement of crop yield under stress, which is not necessarily the case. Drought resistance is sometimes considered as a penalty towards yield potential, which is not necessarily the case. Molecular biologists entering this discipline often report the effect of an exotic gene towards ‘drought tolerance’ and advertise its expected value in breeding, which is rarely the case. Yield potential is defined here as the maximum yield realised under non-stress conditions. Drought resistance in its physiological context is defined according to Levitt (1972) being determined by ‘dehydration avoidance’ and/or ‘dehydration tolerance’. WUE is mostly discussed in terms of plant production rather than gas exchange. Yield under water-limited conditions can be determined by the genetic factors controlling yield potential, and/or drought resistance, and/or WUE. This review is therefore offered as a pragmatic concept paper discussing some of the key factors in the relationships among yield potential, drought resistance and water-use © CSIRO 2005 10.1071/AR05069 0004-9409/05/111159 1160 Australian Journal of Agricultural Research A. Blum efficiency, which may have an effect on a breeding program designed to release better yielding cultivars for water-limited environments.