C4 grasses adapted to low precipitation habitats show traits related to greater mesophyll conductance and lower leaf hydraulic conductance

In habitats with low water availability, a fundamental challenge for plants will be to maximize photosynthetic C‐gain while minimizing transpirational water‐loss. This trade‐off between C‐gain and water‐loss can in part be achieved through the coordination of leaf‐level photosynthetic and hydraulic traits. To test the relationship of photosynthetic C‐gain and transpirational water‐loss, we grew, under common growth conditions, 18 C4 grasses adapted to habitats with different mean annual precipitation (MAP) and measured leaf‐level structural and anatomical traits associated with mesophyll conductance (gm) and leaf hydraulic conductance (Kleaf). The C4 grasses adapted to lower MAP showed greater mesophyll surface area exposed to intercellular air spaces (Smes) and adaxial stomatal density (SDada) which supported greater gm. These grasses also showed greater leaf thickness and vein‐to‐epidermis distance, which may lead to lower Kleaf. Additionally, grasses with greater gm and lower Kleaf also showed greater photosynthetic rates (Anet) and leaf‐level water‐use efficiency (WUE). In summary, we identify a suite of leaf‐level traits that appear important for adaptation of C4 grasses to habitats with low MAP and may be useful to identify C4 species showing greater Anet and WUE in drier conditions. The coordination of mesophyll conductance and leaf hydraulic conductance is important for optimizing photosynthetic C‐gain and transpirational water‐loss but has remained unexplored in C4 species till date. For the first time, we show that C4 grasses from drier habitats show greater mesophyll conductance but lower leaf hydraulic conductance. These findings contrast the previous reports for C3 species and have important implications for modelling the CO2 and water fluxes of grass‐dominated ecosystems.