The role of mesophyll conductance in the estimation of Vcmax and Jmax in four European oaks under moderate drought and warming

Photosynthesis is one of the most sensitive processes to temperature and reduced water availability, strongly influencing forest productivity under present and future conditions. Accurate prediction of plant responses to these drivers requires accurate parameterisation of the maximum rate of RubisCo carboxylation (V cmax ) and maximum rate of electron transport (J max ). In the present study, we evaluated the response of photosynthesis to moderate water deficit and elevated temperature in four Quercus species: Q. robur , Q. cerris , Q. pubescens , and Q. pyrenaica . Leaf gas exchange and chlorophyll fluorescence measurements were used to derive photosynthetic parameters. Three estimations of V cmax and J max were compared, considering: (E1) finite mesophyll conductance (gₘ), (E2) infinite gₘ using C c -based (chloroplast CO2 concentration) compensation point (Γ *), and (E3) infinite gₘ with C i -based (substomatal CO2 concentration) compensation point (C i *). Net CO2 assimilation (A N ) and gₘ showed moderate, species-specific responses to water deficit and elevated temperature. Infinite gₘ produced lower V cmax and higher J max values compared to finite g m , although differences disappeared under moderate drought or elevated temperature. The strong coupling between V cmax and J max was maintained across all treatments, suggesting a conserved coordination between carboxylation and electron transport processes, while the statistical strength of this relationship depended on the gₘ estimation approach. These findings highlight the importance of methodological considerations related to mesophyll conductance in ensuring realistic photosynthetic characterisation and improving the robustness of vegetation models.