# Leaf-scale quantification of the effect of photosynthetic gas exchange on Δ 17O of atmospheric CO 2

Understanding the processes that affect the triple oxygen isotope composition of atmospheric inline-formulaCO2 during 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 inline-formulaCO2 with different inline-formulaΔ17O. We directly quantify the effect of photosynthesis on inline-formulaΔ17O of atmospheric inline-formulaCO2 for the first time. Our results demonstrate the established theory for inline-formulaδ18O is applicable to inline-formulaΔ17O(CO2) at leaf level, and we confirm that the following two key factors determine the effect of photosynthetic gas exchange on the inline-formulaΔ17O of atmospheric inline-formulaCO2. The relative difference between inline-formulaΔ17O of the inline-formulaCO2 entering the leaf and the inline-formulaCO2 in equilibrium with leaf water and the back-diffusion flux of inline-formulaCO2 from the leaf to the atmosphere, which can be quantified by the inline-formulacmca ratio, where inline-formulaca is the inline-formulaCO2 mole fraction in the surrounding air and inline-formulacm is the one at the site of oxygen isotope exchange between inline-formulaCO2 and inline-formulaH2O. At low inline-formulacmca ratios the discrimination is governed mainly by diffusion into the leaf, and at high inline-formulacmca ratios it is governed by back-diffusion of inline-formulaCO2 that has equilibrated with the leaf water. Plants with a higher inline-formulacmca ratio modify the inline-formulaΔ17O of atmospheric inline-formulaCO2 more strongly than plants with a lower inline-formulacmca ratio. Based on the leaf cuvette experiments, the global value for discrimination against inline-formulaΔ17O of atmospheric inline-formulaCO2 during photosynthetic gas exchange is estimated to be inline-formula $M33inlinescrollmathml-normal 0.57±normal 0.14$ 64pt10ptsvg-formulamathimgca37baaffa40a84cf7796ab0b502fe57 bg-17-3903-2020-ie00001.svg64pt10ptbg-17-3903-2020-ie00001.png  ‰ using inline-formulacmca values of 0.3 and 0.7 for Cinline-formula4 and Cinline-formula3 plants, respectively. The main uncertainties in this global estimate arise from variation in inline-formulacmca ratios among plants and growth conditions.

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Adnew, Getachew Agmuas / Pons, Thijs L. / Koren, Gerbrand / et al: Leaf-scale quantification of the effect of photosynthetic gas exchange on Δ17O of atmospheric CO2. 2020. Copernicus Publications.

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