Effects of intraleaf variations in carbonic anhydrase activity and gas exchange on leaf C¹⁸OO isoflux in Zea mays

• Variation in the C¹⁸OO content of atmospheric CO ₂ (δ¹⁸O_a) 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 (δ¹⁸O_w). • Here we tested the hypothesis that mean leaf δ¹⁸O_w 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 δ¹⁸O_w (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, c_i, 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.