Increased adaxial stomatal density is associated with greater mesophyll surface area exposed to intercellular air spaces and mesophyll conductance in diverse C 4 grasses.

Mesophyll conductance (g _m ) is the diffusion of CO ₂ from intercellular air spaces (IAS) to the first site of carboxylation in the mesophyll cells. In C ₃ species, g _m is influenced by diverse leaf structural and anatomical traits; however, little is known about traits affecting g _m in C ₄ species. To address this knowledge gap, we used online oxygen isotope discrimination measurements to estimate g _m and microscopy techniques to measure leaf structural and anatomical traits potentially related to g _m in 18 C ₄ grasses. In this study, g _m scaled positively with photosynthesis and intrinsic water-use efficiency (TE _i ), but not with stomatal conductance. Also, g _m was not determined by a single trait but was positively correlated with adaxial stomatal densities (SD _ada ), stomatal ratio (SR), mesophyll surface area exposed to IAS (S _mes ) and leaf thickness. However, g _m was not related to abaxial stomatal densities (SD _aba ) and mesophyll cell wall thickness (T _CW ). Our study suggests that greater SD _ada and SR increased g _m by increasing S _mes and creating additional parallel pathways for CO ₂ diffusion inside mesophyll cells. Thus, SD _ada , SR and S _mes are important determinants of C ₄ -g _m and could be the target traits selected or modified for achieving greater g _m and TE _i in C ₄ species.
(© 2019 The Authors. New Phytologist © 2019 New Phytologist Trust.)