- • Variation in the C18OO content of atmospheric CO2 (δ18Oa) can be used to distinguish photosynthesis from soil respiration, which is based on carbonic anhydrase (CA)-catalyzed 18O exchange between CO2 and 18O-enriched leaf water (δ18Ow).
- • Here we tested the hypothesis that mean leaf δ18Ow and assimilation rates can be used to estimate whole-leaf C18OO flux (isoflux), ignoring intraleaf variations in CA activity and gas exchange parameters.
- • We observed variations in CA activity along the leaf (> 30% decline from the leaf center toward the leaf ends), which were only partially correlated to those in δ18Ow (7 to 21‰), δ18O and δ13C of leaf organic matter (25 to 30‰ and −12.8 to −13.2‰, respectively), and substomatal CO2 concentrations (intercellular CO2 concentrations, ci, at the leaf center were ∼40% of those at the leaf tip).
- • The combined effect of these variations produced a leaf-integrated isoflux that was different from that predicted based on bulk leaf values. However, because of canceling effects among the influencing parameters, isoflux overestimations were only ∼10%. Conversely, use of measured parameters from a leaf segment could produce large errors in predicting leaf-integrated C18OO fluxes.