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.