Differential sensitivities of transpiration to evaporative demand and soil water deficit among wheat elite cultivars indicate different strategies for drought tolerance

In order to satisfy increasing wheat demands, scaling up wheat production will require boosting yield in suboptimal, drought-prone areas. Under rain-fed environments, one promising option is the identification of traits allowing for soil water conservation until the next rain episode. This can be achieved either by limiting transpiration rate (TR) of the crop to a maximum level when atmospheric drought (or VPD for vapor pressure deficit) is too high or by decreasing stomata conductance earlier in the soil drying cycle. Although promising, those strategies were never explored in wheat. A first objective of this study was to investigate the extent of the genetic variability of TR sensitivity to both VPD and soil water deficit among a group of eight elite wheat lines, which are cultivated under south Australian conditions. Those consisted of seven differentially drought-adapted lines and one check cultivar. TR responses to VPD were highly variable among genotypes, with six lines displaying a breakpoint in their TR response to VPD that ranged from 2.4 to 3.9 kPa, while two others had their TR increasing linearly as VPD increased. Transpiration response to a progressively decreasing fraction of transpirable soil water (FTSW) was investigated in those lines. A significant genetic variability in the responses among genotypes was observed. They revealed different FTSW thresholds at which transpiration started to decrease at levels ranging from 0.43 to 0.52, and different slopes for the decrease. A second objective was to investigate the existence of phenotypic correlations between the parameters characterizing transpiration sensitivities to both sources of water deficit (i.e., VPD and FTSW). Significant correlations were observed revealing that genotypes with conservative water use in their response to high VPD were also conservative in response to decreasing FTSW and that the drought tolerance of other lines might stem from an apposite strategy, invoking decreased sensitivity of TR to both sources of drought. Those findings provide new options for breeding drought tolerant lines based on this germplasm.