Response of photosynthetic 13C discrimination to vapour pressure deficit reflects changes in bundle-sheath leakiness in two C4 grasses

Combined measurements of photosynthetic 13C discrimination (Δ) and gas exchange can be used to estimate bundle-sheath leakiness (Φ), a key efficiency parameter for the CO2-concentrating mechanism of C4 species and their intrinsic water-use efficiency (iWUE). Yet, how vapour pressure deficit (VPD) influences the CO2-concentrating mechanism and Φ in low-leakiness species has been rarely explored. Here, we performed on-line 13C discrimination, CO2 and H2O gas exchange measurements and dry matter δ13C analysis to study the responses of Δ, Φ, and iWUE to VPD in two graminaceous C4 species (guinea grass and pearl millet). Growth at high VPD led to an average increase of photosynthetic 13C discrimination (∆A) by 1.4‰ and a concurrent increase of Φ by 0.08 for both species. ∆ of leaf dry matter (∆DM) showed similar responses to VPD, but was consistently higher than ∆A by 0.4‰. The increase of ∆A was associated with increased leaf nitrogen content at high VPD. ∆A was positively correlated with iWUE and leaf nitrogen content but was unrelated with specific leaf area. This study demonstrated that Φ is variable at different growth VPD, an effect that should be taken into account when using ∆ to estimate iWUE in C4 species.