A genetic link between leaf carbon isotope composition and whole-plant water use efficiency in the C4 grass Setaria

Genetic selection for whole-plant water use efficiency (yield per transpiration; WUEplant) in any crop-breeding programme requires high-throughput phenotyping of component traits of WUEplant such as intrinsic water use efficiency (WUEi; CO2 assimilation rate per stomatal conductance). Measuring WUEi by gas exchange measurements is laborious and time consuming and may not reflect an integrated WUEi over the life of the leaf. Alternatively, leaf carbon stable isotope composition (delta C-13(leaf)) has been suggested as a potential time-integrated proxy for WUEi that may provide a tool to screen for WUEplant. However, a genetic link between delta C-13(leaf) and WUEplant in a C-4 species has not been well established. Therefore, to determine if there is a genetic relationship in a C-4 plant between delta C-13(leaf) and WUEplant under well watered and water-limited growth conditions, a high-throughput phenotyping facility was used to measure WUEplant in a recombinant inbred line (RIL) population created between the C-4 grasses Setaria viridis and S. italica. Three quantitative trait loci (QTL) for delta C-13(leaf) were found and co-localized with transpiration, biomass accumulation, and WUEplant. Additionally, WUEplant for each of the delta C-13(leaf) QTL allele classes was negatively correlated with delta C-13(leaf), as would be predicted when WUEi influences WUEplant. These results demonstrate that delta C-13(leaf) is genetically linked to WUEplant, likely to be through their relationship with WUEi, and can be used as a high-throughput proxy to screen for WUEplant in these C-4 species.