Assessing the CO 2 concentration at the surface of photosynthetic mesophyll cells.

We present a robust estimation of the CO 2 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 CO 2 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 CO 2 concentrations. Accounting for c w improves the information that can be extracted from gas exchange experiments, allowing a more detailed description of the CO 2 and water vapor gradients within the leaf.