G. J. Rebetzke

Wheat yields globally will depend increasingly on good management to conserve rainfall and new varieties that use water efficiently for grain production. Here we propose an approach for developing new varieties to make better use of deep stored water. We focus on water-limited wheat production in the summer-dominant rainfall regions of India and Australia, but the approach is generally applicable to other environments and root-based constraints. Use of stored deep water is valuable because it is more predictable than variable in-season rainfall and can be measured prior to sowing. Further, this moisture is converted into grain with twice the efficiently of in-season rainfall since it is taken up later in crop growth during the grain-filling period when the roots reach deeper layers. We propose that wheat varieties with a deeper root system, a redistribution of branch root density from the surface to depth, and with greater radial hydraulic conductivity at depth would have higher yields in rainfed systems where crops rely on deep water for grain fill. Developing selection systems for mature root system traits is challenging as there are limited high-throughput phenotyping methods for roots in the field, and there is a risk that traits selected in the lab on young plants will not translate into mature root system traits in the field. We give an example of a breeding programme that combines laboratory and field phenotyping with proof of concept evaluation of the trait at the beginning of the selection programme. This would greatly enhance confidence in a high-throughput laboratory or field screen, and avoid investment in screens without yield value. This approach requires careful selection of field sites and years that allow expression of deep roots and increased yield. It also requires careful selection and crossing of germplasm to allow comparison of root expression among genotypes that are similar for other traits, especially flowering time and disease and toxicity resistances. Such a programme with field and laboratory evaluation at the outset will speed up delivery of varieties with improved root systems for higher yield.

Genetic advances in grain yield under rainfed conditions have been achieved by empirical breeding methods. Progress is slowed, however, by large genotype x season and genotype x location interactions arising from unpredictable rainfall, which is a feature of dry environments. A good understanding of factors limiting and/or regulating yield now provides us with an opportunity to identify and then select for physiological and morphological traits that increase the efficiency of water use and yield under rainfed conditions. The incorporation of these traits into breeders' populations should broaden their genetic base. It also may lead to faster selection methods and selection for the traits may result in correlated gains in yield. Here, we undertake a review of factors that limit yield in rainfed environments and discuss genetic opportunities and genetic progress in overcoming them. The examples given are for wheat (Triticum aestivum L.), but the principles apply to all cereal crops grown in dry environments.

Greater yield per unit rainfall is one of the most important challenges in dryland agriculture. Improving intrinsic water-use efficiency (W(T)), the ratio of CO(2) assimilation rate to transpiration rate at the stomata, may be one means of achieving this goal. Carbon isotope discrimination (Delta(13)C) is recognized as a reliable surrogate for W(T) and there have now been numerous studies which have examined the relationship between crop yield and W(T) (measured as Delta(13)C). These studies have shown the relationship between yield and W(T) to be highly variable. The impact on crop yield of genotypic variation in W(T) will depend on three factors: (i) the impact of variation in W(T) on crop growth rate, (ii) the impact of variation in W(T) on the rate of crop water use, and (iii) how growth and water use interact over the crop's duration to produce grain yield. The relative importance of these three factors will differ depending on the crop species being grown and the nature of the cropping environment. Here we consider these interactions using (i) the results of field trials with bread wheat (Triticum aestivum L.), durum wheat (T. turgidum L.), and barley (Hordeum vulgare L.) that have examined the association between yield and Delta(13)C and (ii) computer simulations with the SIMTAG wheat crop growth model. We present details of progress in breeding to improve W(T) and yield of wheat for Australian environments where crop growth is strongly dependent on subsoil moisture stored from out-of-season rains and assess other opportunities to improve crop yield using W(T).

Genetic gain is characteristically slow when selecting directly for increased grain yield under water‐limited conditions. Genetic increases in grain yield may be achieved through increases in aerial biomass following selection for greater transpiration efficiency (TE as aerial biomass/water transpired). Strong negative correlations between TE and carbon isotope discrimination (Δ) in wheat ( Triticum aestivum L.) suggest that selection of progeny with low Δ may increase TE and aerial biomass under water‐limited conditions. This study investigated how early generation, divergent selection for Δ affected aerial biomass and grain yield among 30 low‐ and 30 high‐Δ, ‘Hartog’‐like, BC 2 F 4:6 progeny and the recurrent, high‐Δ parent Hartog. Lines were evaluated in nine environments varying for seasonal rainfall (235–437 mm) and hence grain yield (1.3–6.2 Mg/ha). Selection for low Δ in early generation progeny was associated with significantly ( P < 0.01) smaller Δ, higher grain yield (+5.8%), aerial biomass (+2.7%), harvest index (+3.3%), and kernel size (+4.8%) in tested lines. Kernel number was the same for low‐ and high‐Δ selected groups. Grain yield advantage of the low Δ group increased with reductions in environment mean yield ( r = −0.89, P < 0.01) and total seasonal rainfall ( r = −0.85, P < 0.01) indicating the benefit of low Δ, and therefore high TE for genetic improvement of grain yield in lower rainfall environments. Narrow‐sense heritability on a single‐plot basis was much greater for Δ ( h 2 = 0.63 ± 0.10) than for either aerial biomass (0.06 ± 0.05) or grain yield (0.14 ± 0.04). Strong genetic correlations between Δ and both aerial biomass ( r g = −0.61 ± 0.14) and grain yield (−0.58 ± 0.12) suggest Δ could be used for indirect selection of these traits in early generations. Selection of low Δ (high TE) families for the advanced stages of multiple‐environment testing should increase the probability of recovering higher‐yielding wheat families for water‐limited environments.