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 mu mol mol(-1). Differences between adaxial and abaxial c(i) ranged from a few mu mol mol(-1) to almost 50 mu mol 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 mu mol 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.

Automated summary

We propose a reliable method to estimate the CO2 concentration at the surface of photosynthetic mesophyll cells (c(w)) using independent adaxial and abaxial gas exchange measurements and accounting for mesophyll airspace resistances. Our experiments using model plants confirm that c(w) is lower than adaxial and abaxial estimated intercellular CO2 concentrations (c(i)), with differences usually larger than 10 mu mol mol(-1). Accounting for c(w) improves the information that can be extracted from gas exchange experiments, providing a more detailed description of the CO2 and water vapor gradients within the leaf.