Estimating Mesophyll Conductance from Measurements of C18OO Photosynthetic Discrimination and Carbonic Anhydrase Activity

Carbonic anhydrase (CA) activity in leaves catalyzes the 18 O exchange between CO 2 and water during photosynthesis. This feature has been used to estimate the mesophyll conductance to CO 2 ( g m ) from measurements of online C 18 OO photosynthetic discrimination (∆ 18 O). Based on CA assays on leaf extracts, it has been argued that CO 2 in mesophyll cells should be in isotopic equilibrium with water in most C 3 species as well as many C 4 dicot species. However, this seems incompatible with ∆ 18 O data that would indicate a much lower degree of equilibration, especially in C 4 plants under high light intensity. This apparent contradiction is resolved here using a new model of C 3 and C 4 photosynthetic discrimination that includes competition between CO 2 hydration and carboxylation and the contribution of respiratory fluxes. This new modeling framework is used to revisit previously published data sets on C 3 and C 4 species, including CA-deficient plants. We conclude that (1) newly ∆ 18 O-derived g m values are usually close but significantly higher (typically 20% and up to 50%) than those derived assuming full equilibration and (2) despite the uncertainty associated with the respiration rate in light, or the water isotope gradient between mesophyll and bundle sheath cells, robust estimates of ∆ 18 O-derived g m can be achieved in both C 3 and C 4 plants.