End-of-season drought or “terminal drought,” which occurs after flowering, is considered the most significant abiotic stress affecting crop yields. Wheat crop production in Mediterranean-type environments is often exposed to terminal drought due to decreasing rainfall and rapid increases in temperature and evapotranspiration during spring when wheat crops enter the reproductive stage. Under such conditions, every millimeter of extra soil water extracted by the roots benefits grain filling and yield and improves water use efficiency (WUE). When terminal drought develops, soil dries from the top, exposing the top part of the root system to dry soil while the bottom part is in contact with available soil water. Plant roots sense the drying soil and produce signals, which on transmission to shoots trigger stomatal closure to regulate crop water use through transpiration. However, transpiration is linked to crop growth and productivity and limiting transpiration may reduce potential yield. While an early and high degree of stomatal closure affects photosynthesis and hence biomass production, a late and low degree of stomatal closure exhausts available soil water rapidly which results in yield losses through a reduction in post-anthesis water use. The plant hormone abscisic acid (ABA) is considered the major chemical signal involved in stomatal regulation. Wheat genotypes differ in their ability to produce ABA under drought and also in their stomatal sensitivity to ABA. In this viewpoint article we discuss the possibilities of exploiting genotypic differences in ABA response to soil drying in regulating the use of water under terminal drought. Root density distribution in the upper drying layers of the soil profile is identified as a candidate trait that can affect ABA accumulation and subsequent stomatal closure. We also examine whether leaf ABA can be designated as a surrogate characteristic for improved WUE in wheat to sustain grain yield under terminal drought. Ease of collecting leaf samples to quantify ABA compared to extracting xylem sap will facilitate rapid screening of a large number of germplasm for drought tolerance.

Oilseed Brassica species are vulnerable to heat and drought stress, especially in the early reproductive stage. We evaluated plant imaging of whole plant and flower tissue, leaf stomatal conductance, leaf and bud temperature, photochemical reflectance index, quantum yield of photosynthesis, and leaf gas exchange for their suitability to detect tolerance to heat (H) and/or drought (D) stress treatments in 12 Brassica genotypes (G). A replicated factorial experiment was set up with 7 d of stress treatment from the beginning of anthesis with various levels of three factors H, D, and G. Most phenomics tools detected plant stress as indicated by significant main effects of H, D, and H×D. Whole plant volume was highly correlated with fresh weight changes, suggesting that whole plant imaging may be a useful surrogate for fresh weight in future studies. Vcmax, the maximum carboxylation rate of photosynthesis, increased rapidly on day 1 in H and H+D treatments, and there were significant interactions of G×H and G×D. Vcmax of genotypes on day 1 in H and H+D treatments was positively correlated with their harvested seed yield. Vcmax on day 1 and day 3 were clustered with seed yield in H and H+D treatments as shown in the heatmaps of genotypic correlations. TPU, the rate of triose phosphate use, also showed significant positive genotypic correlations with seed yield in H+D treatments. Flower volume showed significant interactions of G×H and G×D on day 7, and flower volume of genotypes on day 7 in H was positively correlated with their harvested seed yield. There were few interactions of G×H or G×D for leaf stomatal conductance, leaf and bud temperature, photochemical reflectance index, and quantum yield of photosynthesis. Vcmax, TPU, and volume of flowers are potential nondestructive phenomic traits for heat or combined heat and drought stress tolerance screening in Brassica germplasm.

Terminal drought is a common abiotic stress affecting wheat yield in Mediterranean-type environments. As terminal drought develops, top layers of the soil profile dry, exposing the upper part of the root system to soil water deficit while deeper roots can still access soil water. Since open stomata rapidly exhausts available soil water, reducing stomatal conductance to prolong availability of soil water during grain filling may improve wheat yields in water-limited environments. It was hypothesised that genotypes with more root biomass in the drying upper layer of the soil profile accumulate more abscisic acid in the leaf and initiate stomatal closure to regulate water use under terminal drought. The wheat cultivar Drysdale and the breeding line IGW-3262 were grown in pots horizontally split into two segments by a wax-coated layer that hydraulically isolated the top and bottom segments, but allowed roots to grow into the bottom segment. Terminal drought was induced from anthesis by withholding water from (i) the top segment only (DW) and (ii) the top and bottom segments (DD) while both segments in well-watered pots (WW) were maintained at 90% pot soil water capacity. Drysdale, initiated stomatal closure earlier than IGW-3262, possibly due to higher signal strength generated in its relatively larger proportion of roots in the drying top segment. The relationship between leaf ABA and stomatal conductance was strong in Drysdale but weak in IGW-3262. Analysis of ABA metabolites suggests possible differences in ABA metabolism between these two genotypes. A higher capability of deeper roots to extract available water is also important in reducing the gap between actual and potential yield.

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Post-anthesis water use is important for grain yield in wheat under drought because this water is immediately used for grain filling. The aim of this study was to determine whether root capacity for water uptake from deeper layers in the soil profile differed between two genotypes with contrasting stomatal behaviour under terminal drought. The wheat cultivar Drysdale and the breeding line IGW-3262 were grown in 1 m deep pots in a glasshouse under well-watered conditions until anthesis, when three watering treatments were imposed: (i) watering maintained at 90% pot soil water capacity (WW), (ii) watering withheld but supplementary watering supplied to the bottom 30 cm of the pot to keep this layer of the soil profile wet until physiological maturity (WB) and (iii) watering completely withheld (WS). Stomatal conductance, post-anthesis water use and water use efficiency, and grain yield were measured. Post-anthesis water use in Drysdale was similar in the WB and WW treatments, while in IGW-3262 it was 30% less in the WB treatment than in the WW treatment. In the WB treatment as the top soil dried, stomatal closure was faster in IGW-3262 than in Drysdale, which may have affected the capacity of roots to uptake available water at depth. The reduction in post-anthesis water use in IGW-3262 resulted in a decline in grain yield.

Common bean (Phaseolus vulgaris L.) is important in African diets for protein, iron (Fe), and zinc (Zn), but traditional cultivars have long cooking time (CKT), which increases the time, energy, and health costs of cooking. Genomic selection was used to predict genomic estimated breeding values (GEBV) for grain yield (GY), CKT, Fe, and Zn in an African bean panel of 358 genotypes in a two‐stage analysis. In Stage 1, best linear unbiased estimates (BLUE) for each trait were obtained from 898 genotypes across 33 field trials in East Africa. In Stage 2, BLUE in a training population of 141 genotypes were used in a multivariate genomic analysis with genome‐wide single nucleotide polymorphism data from the African bean panel. Moderate to high genomic heritability was found for GY (0.45 ± 0.10), CKT (0.50 ± 0.15), Fe (0.57 ± 0.12), and Zn (0.61 ± 0.13). There were significant favorable genetic correlations between Fe and Zn (0.91 ± 0.06), GY and Fe (0.66 ± 0.17), GY and Zn (0.44 ± 0.19), CKT and Fe (−0.57 ± 0.21), and CKT and Zn (−0.67 ± 0.20). Optimal contributions selection (OCS), based on economic index of weighted GEBV for each trait, was used to design crossing within four market groups relevant to East Africa. Progeny were predicted by OCS to increase in mean GY by 12.4%, decrease in mean CKT by 9.3%, and increase in mean Fe and Zn content by 6.9 and 4.6%, respectively, with low achieved coancestry of 0.032. Genomic selection with OCS will accelerate breeding of high‐yielding, biofortified, and rapid cooking African common bean cultivars.

Crop breeding must achieve higher rates of genetic gain in grain yield (GY) and yield stability to meet future food demands in a changing climate. Optimal contributions selection (OCS) based on an index of key economic traits should increase the rate of genetic gain while minimising population inbreeding. Here we apply OCS in a global spring oilseed rape (canola) breeding program during three cycles of S0,1 family selection in 2016, 2018, and 2020, with several field trials per cycle in Australia and Canada. Economic weights in the index promoted high GY, seed oil, protein in meal, and Phoma stem canker (blackleg) disease resistance while maintaining plant height, flowering time, oleic acid, and seed size and decreasing glucosinolate content. After factor analytic modelling of the genotype-by-environment interaction for the additive effects, the linear rate of genetic gain in GY across cycles was 0.059 or 0.087 t ha−1 y−1 (2.9% or 4.3% y−1) based on genotype scores for the first factor (f1) expressed in trait units or average predicted breeding values across environments, respectively. Both GY and yield stability, defined as the root-mean-square deviation from the regression line associated with f1, were predicted to improve in the next cycle with a low achieved mean parental coancestry (0.087). These methods achieved rapid genetic gain in GY and other traits and are predicted to improve yield stability across global spring canola environments.

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