Mesophyll CO 2 conductance and leakiness are not responsive to short‐ and long‐term soil water limitations in the C 4 plant Sorghum bicolor

Summary Breeding economically important C 4 crops for enhanced whole‐plant water‐use efficiency ( WUE plant ) is needed for sustainable agriculture. WUE plant is a complex trait and an efficient phenotyping method that reports on components of WUE plant , such as intrinsic water‐use efficiency ( WUE i , the rate of leaf CO 2 assimilation relative to water loss via stomatal conductance), is needed. In C 4 plants, theoretical models suggest that leaf carbon isotope composition (δ 13 C), when the efficiency of the CO 2 ‐concentrating mechanism (leakiness, ϕ) remains constant, can be used to screen for WUE i . The limited information about how ϕ responds to water limitations confines the application of δ 13 C for WUE i screening of C 4 crops. The current research aimed to test the response of ϕ to short‐ or long‐term moderate water limitations, and the relationship of δ 13 C with WUE i and WUE plant , by addressing potential mesophyll CO 2 conductance ( g m ) and biochemical limitations in the C 4 plant Sorghum bicolor . We demonstrate that g m and ϕ are not responsive to short‐ or long‐term water limitations. Additionally, δ 13 C was not correlated with gas‐exchange estimates of WUE i under short‐ and long‐term water limitations, but showed a significant negative relationship with WUE plant . The observed association between the δ 13 C and WUE plant suggests an intrinsic link of δ 13 C with WUE i in this C 4 plant, and can potentially be used as a screening tool for WUE plant in sorghum.