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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. Vc max , 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 . Vc max of genotypes on day 1 in H and H+D treatments was positively correlated with their harvested seed yield. Vc max 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. Vc max , TPU, and volume of flowers are potential nondestructive phenomic traits for heat or combined heat and drought stress tolerance screening in Brassica germplasm.

Wheat grown in the Mediterranean-type environments of southern Australia is often exposed to end-of-season drought (terminal drought). During the development of terminal drought, soil dries from the top of the profile exposing the upper part of the root system to water stress while deeper roots may still be able to access deeper soil water for grain filling. It is hypothesised that the part of the root system exposed to drying soil signals abscisic acid (ABA) production and the corresponding rise in ABA concentration in leaves causes partial stomatal closure, regulating the extraction of available water at depth. In the first step to test this hypothesis, a glasshouse experiment was conducted to identify contrasting stomatal response to terminal drought and production of ABA using four wheat genotypes adapted to different soil moisture environments. Terminal drought was induced by withholding water from anthesis in one half (WD) or both halves (DD) of the root system using split pots. Stomatal conductance decreased in all four genotypes in WD plants, but leaf ABA concentration and leaf water status differed. The cultivar Drysdale had higher leaf ABA concentrations and lower stomatal conductance, but leaf water status decreased. Leaf ABA concentration did not increase in WD plants of the breeding line IGW-3262, but stomatal conductance decreased and leaf water status was unchanged. All the genotypes behaved similarly under DD conditions, with increased leaf ABA concentration, lower stomatal conductance and severely dehydrated leaves. The possible causes of the above differences in leaf ABA concentration despite similar stomatal behaviour in wheat genotypes are discussed.

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.

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.