Stomatal response to VPD in C4 plants with different biochemical sub‐pathways.

C4 NAD‐malic enzyme (NAD‐ME) species occurs in drier regions and exhibit different drought responses compared to C4 NADP‐malic enzyme (NADP‐ME) species. However, a physiological mechanism explaining the geographical discrepancies remains uncertain. This study examined gas exchange patterns that might explain different distributions observed between two subtypes of C4 photosynthesis. We measured the response of leaf gas exchange to vapour pressure deficit (VPD) and CO2 in plants from six distinct C4 clades having closely related NAD‐ME and NADP‐ME species using a Li‐Cor 6400 gas exchange system. We found that NAD‐ME species exhibited greater relative reductions in stomatal conductance with increases in VPD than NADP‐ME species but observed no consistent subtype differences in C4 cycle activity as indicated by the initial slope of the A response to intercellular CO2 concentration. Based on these results, we hypothesise the greater response of gs to increasing VPD may enable NAD‐ME plants to outperform NADP‐ME plants in hot, dry environments where VPD is normally high. Summary statement: NAD‐malic enzyme (NAD‐ME) C4 species have more sensitive stomatal closures in response to increasing vapour pressure deficit than NADP‐malic enzyme C4 species which may explain why NAD‐ME grass species are found in drier areas than NADP‐ME species. This provides insight into the role of plant eco‐physiology in shaping biogeography. [ABSTRACT FROM AUTHOR]