Assessing the CO₂ concentration at the surface of photosynthetic mesophyll cells

We present a robust estimation of the CO₂ concentration at the surface of photosynthetic mesophyll cells (c_w ), applicable under reasonable assumptions of assimilation distribution within the leaf. We used Capsicum annuum, Helianthus annuus and Gossypium hirsutumas model plants for our experiments. We introduce calculations to estimate c_w using independent adaxial and abaxial gas exchange measurements, and accounting for the mesophyll airspace resistances. The c_w was lower than adaxial and abaxial estimated intercellular CO₂ concentrations (c_i ). Differences between c_w and the c_i of each surface were usually larger than 10 μmol mol^-1 . Differences between adaxial and abaxial c_i ranged from a few μmol mol^-1 to almost 50 μmol mol^-1 , where the largest differences were found at high air saturation deficits (ASD). Differences between adaxial and abaxial c_i and the c_i estimated by mixing both fluxes ranged from -30 to +20 μmol mol^-1 , where the largest differences were found under high ASD or high ambient CO₂ concentrations. Accounting for c_w improves the information that can be extracted from gas exchange experiments, allowing a more detailed description of the CO₂ and water vapor gradients within the leaf.