Two-Source δO Method to Validate the COO-Photosynthetic Discrimination Model: Implications for Mesophyll Conductance.

Theoretical models of photosynthetic isotopic discrimination of CO (C and O) are commonly used to estimate mesophyll conductance ( ). This requires making simplifying assumptions and assigning parameter values so that can be solved for as the residual term. Uncertainties in estimation occur due to measurement noise and assumptions not holding, including parameter uncertainty and model parametrization. Uncertainties in the C model have been explored previously, but there has been little testing undertaken to determine the reliability of estimates from the O model ( ). In this study, we exploited the action of carbonic anhydrase in equilibrating CO with leaf water and manipulated the observed photosynthetic discrimination (ΔO) by changing the oxygen isotopic composition of the source gas CO and water vapor. We developed a two-source δO method, whereby two measurements of ΔO were obtained for a leaf with its gas-exchange characteristics otherwise unchanged. Measurements were performed in broad bean () and Algerian oak () in response to light and vapor pressure deficit. Despite manipulating the ΔO by over 100‰, in most cases we observed consistency in the calculated , providing confidence in the measurements and model theory. Where there were differences in estimates between source-gas measurements, we explored uncertainty associated with two model assumptions (the isotopic composition of water at the sites of CO-water exchange, and the humidity of the leaf internal airspace) and found evidence for both. Finally, we provide experimental guidelines to minimize the sensitivity of estimates to measurement errors. The two-source δO method offers a flexible tool for model parameterization and provides an opportunity to refine our understanding of leaf water and CO fluxes.