Elevated CO2 Decouples the Short-Term Thermal Responses of Mesophyll and Stomatal Conductance, Enhancing Carbon Gain and Water Use at Moderate Heat but Failing at Extreme Temperatures

Rising atmospheric CO2 concentration and global warming jointly affect plant photosynthesis and water use, but how elevated CO2 regulates photosynthetic short-term thermal response and its implications for leaf carbon-water balance remain poorly understood. We investigated the temperature responses (25–45 °C) of net photosynthetic rate (An), intrinsic water use efficiency (iWUE), stomatal conductance (gs), and mesophyll conductance (gm) in Liriodendron tulipifera L., a fast-growing and economically important hardwood species sensitive to heat stress, under five CO2 concentrations (150–800 μmol mol−1), and quantified the partitioning of stomatal, mesophyll, and biochemical limitations. Elevated CO2 increased the optimal temperature (Topt) of An, iWUE, and gm, but decreased that of gs, creating a divergent thermal response pattern that decoupled carbon acquisition from water loss. The upward shift in the Topt of gm enhanced CO2 supply to chloroplast carboxylation sites at high temperatures, sustaining higher carboxylation rates, indicating that improved mesophyll diffusion, rather than biochemical capacity, drove photosynthetic short-term thermal response. The downward shift in Topt of gs aligned with optimal stomatal behavior theory, promoting earlier stomatal closure to conserve water, which together with enhanced mesophyll diffusion resulted in a net improvement in water use efficiency below 40 °C. At 45 °C, however, mesophyll limitation (lm) became dominant (~70% of total limitation), indicating that elevated CO2 cannot prevent photosynthetic breakdown under extreme heat, and the water-saving benefit was lost. We suggest that terrestrial biosphere models incorporating separate temperature dependencies of gm and gs would improve predictions of tree carbon-water balance under future climates.