Isotopi del carbonio e dell’ossigeno: un utile strumento per analizzare i flussi di CO2 e H2O tra le piante e l’atmosfera

Carbon and oxygen isotopes: a useful tool for the analysis of carbon and water exchanges between plants and the atmosphere. Recent studies suggest that isotope fractionation occurring in plants during photosynthesis influence the δ18O signal in atmospheric CO2, providing useful information on the global carbon cycle. The estimate of δ18O depends from the accuracy of existing models used. Complications arise due to uncertainties associated with: i) leaf water heterogeneity; ii) calculation of the CO2 concentrations at the site of CO2-H2O equilibrium; iii) rate of catalytic activity of enzyme carbonic anhydrase (CA). This research aimed to investigate the applicability of two models (Péclet & Craig-Gordon) to predict the δ18O in leaf water and the extent to which the evaporative enrichment of δ18O in leaf water is reflected in the oxygen isotope ratio of CO2 passing over the leaf. The experiment was carried out on cotton plants grown from seeds in a temperature and humidity controlled glasshouse. Environmental conditions (vapor pressure deficit, leaf temperature, light intensity and oxygen composition) were altered to induce large variation in photosynthetic activity and evaporative conditions and therefore different values of Δc (Δ18O of chloroplast CO2) and Δe (Δ18O of H2O at the evaporating site) respectively. The results indicate that Péclet model can predict total leaf water enrichment more accurately than the Craig-Gordon model. We obtained the reasonable L value, an important parameter in the Péclet model, compared to those of the previous reports when we removed the unenriched vein water. The L was not influenced by the change in the environmental conditions. The results showed that Δc was highly significantly correlated with Δe. The proportional oxygen isotope equilibrium between CO2 and chloroplast water was calculated to be very near unity at leaf temperatures of 29°C, and approximately 0.8 at leaf temperatures of 20°C. This large discrepancy confirmed that leaf temperature has a large impact on CA activity and as a consequence on the equilibrium of CO2 with water inside the leaf. Such differences should be taken into consideration in carbon balance model at the ecosystem level.