Leaf scale quantification of the effect of photosynthetic gas exchange on Δ47 of CO2

Abstract

The clumped isotope composition (Δ₄₇, the anomaly of the mass 47 isotopologue relative to the abundance expected from a random isotope distribution) of CO₂ has been suggested as an additional tracer for gross CO₂ fluxes. However, the effect of photosynthetic gas exchange on Δ₄₇ has not been directly determined and two indirect/conceptual studies reported contradicting results. In this study, we quantify the effect of photosynthetic gas exchange on Δ₄₇ of CO₂ using leaf cuvette experiments with one C₄ and two C₃ plants. The experimental results are supported by calculations with a leaf cuvette model. Our results demonstrate the important roles of the Δ₄₇ value of CO₂ entering the leaf, kinetic fractionation as CO₂ diffuses into, and out of the leaf and CO₂-H₂O isotope exchange with leaf water. We experimentally confirm the previously suggested dependence of Δ₄₇ of CO₂ in the air surrounding a leaf on the stomatal conductance and back-diffusion flux. Gas exchange can enrich or deplete the Δ₄₇ of CO₂ depending on the Δ₄₇ of CO₂ entering the leaf and the fraction of CO₂ exchanged with leaf water and diffused back to the atmosphere, but under typical ambient conditions, it will lead to a decrease in Δ₄₇.