摘要：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
2O 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.
