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

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Abstract
Summary •V ariation in the C 18 OO content of atmospheric CO 2 ( δ 18 O a ) can be used to dis- tinguish photosynthesis from soil respiration, which is based on carbonic anhydrase (CA)-catalyzed 18 O exchange between CO 2 and 18 O-enriched leaf water ( δ 18 O w ). • Here we tested the hypothesis that mean leaf δ 18 O w and assimilation rates can be used to estimate whole-leaf C 18 OO flux (isoflux), ignoring intraleaf variations in CA activity and gas exchange parameters. •W e 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 δ 18 O w (7 to 21‰), δ 18 O and δ 13 C of leaf organic matter (25 to 30‰ and − 12.8 to − 13.2‰, respectively), and substomatal CO 2 concentrations (intercellular CO 2 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 C 18 OO fluxes.