Leaf-scale quantification of the effect of photosynthetic gas exchange on δ¹⁷O of atmospheric CO₂

Understanding the processes that affect the triple oxygen isotope composition of atmospheric CO₂during gas exchange can help constrain the interaction and fluxes between the atmosphere and the biosphere. We conducted leaf cuvette experiments under controlled conditions using three plant species. The experiments were conducted at two different light intensities and using CO₂with different δ¹⁷O. We directly quantify the effect of photosynthesis on δ¹⁷O of atmospheric CO₂for the first time. Our results demonstrate the established theory for δ¹⁸O is applicable to δ¹⁷O.CO₂/at leaf level, and we confirm that the following two key factors determine the effect of photosynthetic gas exchange on the δ¹⁷O of atmospheric CO₂. The relative difference between δ¹⁷O of the CO₂entering the leaf and the CO₂in equilibrium with leaf water and the back-diffusion flux of CO₂from the leaf to the atmosphere, which can be quantified by the cm=ca ratio, where ca is the CO₂mole fraction in the surrounding air and cm is the one at the site of oxygen isotope exchange between CO₂and H₂O. At low cm=ca ratios the discrimination is governed mainly by diffusion into the leaf, and at high cm=ca ratios it is governed by back-diffusion of CO₂that has equilibrated with the leaf water. Plants with a higher cm=ca ratio modify the 117O of atmospheric CO₂more strongly than plants with a lower cm=ca ratio. Based on the leaf cuvette experiments, the global value for discrimination against δ¹⁷O of atmospheric CO₂during photosynthetic gas exchange is estimated to be-0:57±0:14% using cm=ca values of 0.3 and 0.7 for C₄and C₃plants, respectively. The main uncertainties in this global estimate arise from variation in cm=ca ratios among plants and growth conditions.