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; 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 ; CO₂ 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 (δ¹³ 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 δ¹³ C_leaf and WUE_plant in a C₄ species has not been well established. Therefore, to determine if there is a genetic relationship in a C₄ plant between δ¹³ 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 C₄ grasses Setaria viridis and S. italica. Three quantitative trait loci (QTL) for δ¹³ C_leaf were found and co-localized with transpiration, biomass accumulation, and WUE_plant . Additionally, WUE_plant for each of the δ¹³ C_leaf QTL allele classes was negatively correlated with δ¹³ C_leaf , as would be predicted when WUE_i influences WUE_plant . These results demonstrate that δ¹³ 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 C₄ species.