Leaf scale quantification of the effect of photosynthetic gas exchange on Δ(47) of CO(2).

The clumped isotope composition (Δ(47), the anomaly of the mass 47 isotopologue relative to the abundance expected from a random isotope distribution) of CO(2) has been suggested as an additional tracer for gross CO(2) fluxes. However, the effect of photosynthetic gas exchange on Δ(47) 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 Δ(47) of CO(2) using leaf cuvette experiments with one C(4) and two C(3) plants. The experimental results are supported by calculations with a leaf cuvette model. Our results demonstrate the important roles of the Δ(47) value of CO(2) entering the leaf, kinetic fractionation as CO(2) diffuses into, and out of the leaf and CO(2)-H(2)O isotope exchange with leaf water. We experimentally confirm the previously suggested dependence of Δ(47) of CO(2) in the air surrounding a leaf on the stomatal conductance and back-diffusion flux. Gas exchange can enrich or deplete the Δ(47) of CO(2) depending on the Δ(47) of CO(2) entering the leaf and the fraction of CO(2) exchanged with leaf water and diffused back to the atmosphere, but under typical ambient conditions, it will lead to a decrease in Δ(47).