Mesophyll CO conductance and leakiness are not responsive to short- and long-term soil water limitations in the C plant Sorghum bicolor.

Breeding economically important C crops for enhanced whole-plant water-use efficiency (WUE ) is needed for sustainable agriculture. WUE is a complex trait and an efficient phenotyping method that reports on components of WUE , such as intrinsic water-use efficiency (WUE , the rate of leaf CO assimilation relative to water loss via stomatal conductance), is needed. In C plants, theoretical models suggest that leaf carbon isotope composition (δ C), when the efficiency of the CO -concentrating mechanism (leakiness, ϕ) remains constant, can be used to screen for WUE . The limited information about how ϕ responds to water limitations confines the application of δ C for WUE screening of C crops. The current research aimed to test the response of ϕ to short- or long-term moderate water limitations, and the relationship of δ C with WUE and WUE , by addressing potential mesophyll CO conductance (g ) and biochemical limitations in the C plant Sorghum bicolor. We demonstrate that g and ϕ are not responsive to short- or long-term water limitations. Additionally, δ C was not correlated with gas-exchange estimates of WUE under short- and long-term water limitations, but showed a significant negative relationship with WUE . The observed association between the δ C and WUE suggests an intrinsic link of δ C with WUE in this C plant, and can potentially be used as a screening tool for WUE in sorghum.