Light and CO₂ do not affect the mesophyll conductance to CO ₂ diffusion in wheat leaves

In C3 plants, diffusion of CO2 into leaves is restricted by stomata and subsequently by the intercellular airspaces and liquid phase into chloroplasts. While considerable information exists on the effect of environmental conditions on stomatal conductance (gs), little is known on whether the mesophyll conductance to CO2 diffusion (gm) changes with respect to photon flux density (PFD) and CO2 partial pressure (pCO2). In this study, the effects of PFD and/or pCO2 on gm were examined in wheat leaves by combining gas exchange with carbon isotope discrimination measurements using a membrane inlet mass spectrometer. Measurements were made in 2% O2 to reduce the fractionation associated with photorespiration. The magnitude of gm was estimated using the observed carbon isotope discrimination (Δ), ambient and intercellular pCO2, CO2 assimilation and respiration rates, either from an individual measurement made under one environmental condition or from a global fit to multiple measurements where PFD was varied. It was found that respiration made a significant and variable contribution to the observed discrimination, which associated with the difference in isotopic composition between CO2 in the greenhouse and that used for gas exchange measurements. In wheat, gm was independent of PFD between 200 and 1500 μmol m^-2 s^-1 and was independent of pi between 80 and 500 μbar.