Effects of intraleaf variations in carbonic anhydrase activity and gas exchange on leaf (COO)-O-18 isoflux in Zea mays

Variation in the (COO)-O-18 content of atmospheric CO2 (delta O-18(a)) can be used to distinguish photosynthesis from soil respiration, which is based on carbonic anhydrase (CA)-catalyzed O-18 exchange between CO2 and O-18-enriched leaf water (delta O-18(w)). Here we tested the hypothesis that mean leaf delta O-18(w) and assimilation rates can used to estimate whole-leaf (COO)-O-18 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 delta O-18(w) (7 to 21 parts per thousand), delta O-18 and delta C-13 of leaf organic matter (25 to 30 parts per thousand and -12.8 to -13.2 parts per thousand, respectively), and substomatal CO2 concentrations (intercellular CO2 concentrations, c(i), at the leaf center were similar to 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 similar to 10%. Conversely, use of measured parameters from a leaf segment could produce large errors in predicting leaf-integrated (COO)-O-18 fluxes.