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 .