Plant breeding and drought in C3 cereals: what should we breed for?

Abstract

Drought is the main abiotic constraint on cereal yield. Analysing physiological determinants of yield responses to water may help in breeding for higher yield and stability under drought conditions. The traits to select (either for stress escape, avoidance or tolerance) and the framework where breeding for drought stress is addressed will depend on the level and timing of stress in the targeted area. If the stress is severe, breeding under stress-free conditions may be unsuccessful and traits that confer survival may become a priority. However, selecting for yield itself under stress-alleviated conditions appears to produce superior cultivars, not only for optimum environments, but also for those characterized by frequent mild and moderate stress conditions. This implies that broad avoidance/tolerance to mild-moderate stresses is given by constitutive traits also expressed under stress-free conditions. In this paper, we focus on physiological traits that contribute to improved productivity under mild-moderate drought. Increased crop performance may be achieved through improvements in water use, water-use efficiency and harvest index. The first factor is relevant when soil water remains available at maturity or when deep-rooted genotypes access water in the soil profile that is not normally available; the two latter conditions become more important when all available water is exhausted by the end of the crop cycle. Independent of the mechanism operating, a canopy able to use more water than another would have more open stomata and therefore higher canopy temperature depression, and 13C discrimination (delta13C) in plant matter. The same traits would also seem to be relevant when breeding for hot, irrigated environments. Where additional water is not available to the crop, higher water-use efficiency (WUE) appears to be an alternative strategy to improve crop performance. In this context delta13C constitutes a simple but reliable measure of WUE. However, in contrast to lines performing better because of increased access to water, lines producing greater biomass due to superior WUE will have lower delta13C values. WUE may be modified not only through a decrease in stomatal conductance, but also through an increase in photosynthetic capacity. Harvest index is strongly reduced by terminal drought (i.e. drought during grain filling). Thus, phenological traits increasing the relative amount of water used during grain filling, or adjusting the crop cycle to the seasonal pattern of rainfall may be useful. Augmenting the contribution of carbohydrate reserves accumulated during vegetative growth to grain filling may also be worthwhile in harsh environmcnts. Alternatively, extending the duration of stem elongation without changing the timing of anthesis would increase the number of grains per spike and the harvest index without changing the amount of water utilized by the crop.

Fig. 1. Pattern of increases in mean wheat and barley (inset) yields during the 20th century in countries characterized by good water conditions or prone to water deficits. Data derived from FAO’s web‐site (www.fao.org).

Fig. 2. Relative increases in wheat yield due to breeding under favourable (UK) and drought (Australia) conditions.

Fig. 3. A, Several scenarios derived from Passioura’s equation. B, Schematic diagram showing a common negative G × E interaction, implying a crossover at relatively low levels of environmental index (arrow), when cultivars were selected for yield per se in near optimum conditions (1) or under severe stress (2). C and D, Comparative performance of old (closed circles) and new (open circles) wheat cultivars in favourable (C) and drought (D) environments.

Fig. 4. Retrospective studies comparing the pattern of seasonal rainfall and the changes in earliness of wheat cultivars released in western Australia and Rolling Pampas, Argentina.

Fig. 5. Changes in pre‐anthesis and post‐anthesis water use as well as in the post/pre‐anthesis ratio due to the earlier anthesis time in the most recent cultivars of wheat. All lines were fitted by regression (r = 0·65–0·85; P < 0·05). Data from Siddique et al. (1990).

Fig. 6. Relationship between the ratio of post‐ to pre‐anthesis water use and harvest index. The year of release of each wheat cultivar is shown in parentheses. Data from Siddique et al. (1990).

Fig. 7. Relationship between yield of eight CIMMYT wheat cultivars differing in yield potential and average flag leaf photosynthetic rate at light saturation (A_max, A and B), stomatal conductance (g_s, A) and canopy temperature depression (CTD, B). Adapted from Fischer et al. (1998).

Fig. 8. Typical variation in the spectra due to differences in water regimes. The three lines correspond to plots of wheat measured around mid‐grain filling at ICARDA. a, Rainfed in a very dry environment; b, rainfed in a moderately dry environment; and c, irrigated. The main differences are: the magnitude of the increase in reflectance at around 700 nm, which indicates differences in biomass; the pattern in the PAR region, which indicates differences in pigment composition; and the decrease in reflectance beyond 930 nm which indicates differences in water content.

Fig. 9. Prediction of grain yield based on the combination of three different spectroradiometrical indices calculated from the spectra reflected by the canopy. Measurements were made (in early June 1998) at mid‐grain filling in a set of 74 genotypes of durum wheat corresponding to the WANADIN collection developed by the CIMMYT/ICARDA breeding programme. The three canopy reflectance indices used were SAVI = (R₉₀₀ – R₆₈₀)/(R₉₀₀ + R₆₈₀ + L) × (1 + L) with L = 0·5 for most crops; SIPI = (R₈₀₀ – R₄₃₅)/(R₄₁₅ + R₄₃₅); and NPQI = (R₄₁₅ – R₄₃₅)/(R₄₁₅ + R₄₃₅). The function of prediction was grain yield = 3616 – 4297SAVI – 1370SIPI – 3240NPQI. Its performance seems to be based on the differences across genotypes in the date of maturity. Plants were grown at Tel‐Hadya (headquarters of ICARDA), NW Syria (Casadesús, Araus and Nachit, unpubl. res.).