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

Abstract

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.

Drought Adaptation Mechanisms Should Guide Experimental Design

“…Other excellent reviews provide examples of drought adaptation [2,[9][10][11], but in practice little link is generally made between the proposed adaptation and the experimental manipulation.…”

Survival strategies of Centrosema molle and C. macrocarpum in response to drought

“…Drought evasion is related to the ability to sustain initial plant water status for a longer time by increasing access to deep soil water or minimizing water losses through transpiration (Blum 2005), and plasticity in leaf expansion, transpiration control and root proliferation are essential for drought resistance (Turner and Begg 1981). During the first dry period, small variations in LAI within legumes were the result of very small variations in SLA and leaf biomass among treatments.…”

“…In addition, turgor loss and tissue death can be delayed/avoided by other plant traits associated with efficient use of soil water, e.g. deep and extensive root systems, low or high root-stem hydraulic conductivity, reduced leaf and canopy area and leaf movements, until air evaporative demand can no longer be balanced by soil water uptake (Blum 2005;Bacelar et al 2012). Plants with low sensitivity to soil desiccation can "tolerate" progressive drought stress mainly by physiological mechanisms.…”

Prospects for Genetic Improvement to Increase Lowland Rice Yields with Less Water and Nitrogen

“…Wheat varieties with a 2-23% increase in grain yield over check varieties have been successfully developed for water-limited conditions. Blum (2005) pointed out that high yield under water-limited conditions is generally associated with reduced WUE mainly because of high water use. Plant traits such as small plants (small leaves and reduced tillering) or short growth duration are associated with low yield potential and high WUE because of reduced water use.…”