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 L., Helianthus annuus L. and Gossypium hirsutum L. as 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 air space 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 vapour gradients within the leaf.