A genetic link between leaf carbon isotope composition and whole‐plant water use efficiency in the C 4 grass Setaria

Summary Genetic selection for whole‐plant water use efficiency (yield per transpiration; WUE plant ) in any crop‐breeding programme requires high‐throughput phenotyping of component traits of WUE plant such as intrinsic water use efficiency (WUE i ; CO 2 assimilation rate per stomatal conductance). Measuring WUE i by gas exchange measurements is laborious and time consuming and may not reflect an integrated WUE i over the life of the leaf. Alternatively, leaf carbon stable isotope composition (δ 13 C leaf ) has been suggested as a potential time‐integrated proxy for WUE i that may provide a tool to screen for WUE plant . However, a genetic link between δ 13 C leaf and WUE plant 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 δ 13 C leaf and WUE plant under well watered and water‐limited growth conditions, a high‐throughput phenotyping facility was used to measure WUE plant in a recombinant inbred line (RIL) population created between the C 4 grasses Setaria viridis and S. italica . Three quantitative trait loci (QTL) for δ 13 C leaf were found and co‐localized with transpiration, biomass accumulation, and WUE plant . Additionally, WUE plant for each of the δ 13 C leaf QTL allele classes was negatively correlated with δ 13 C leaf , as would be predicted when WUE i influences WUE plant . These results demonstrate that δ 13 C leaf is genetically linked to WUE plant , likely to be through their relationship with WUE i , and can be used as a high‐throughput proxy to screen for WUE plant in these C 4 species.