Leaf-scale quantification of the effect of photosynthetic gas exchange on Δ17O of atmospheric CO2

 

 

Understanding the processes affecting the triple oxygen isotope composition of atmospheric CO₂ during photosynthesis can help to constrain the interaction and fluxes between the atmosphere and the biosphere. We conducted leaf cuvette experiments under controlled conditions, using sunflower (Helianthus annuus), an annual C₃ species with high photosynthetic capacity and stomatal conductance for CO₂, an evergreen C₃ species, ivy (Hedera hybernica) with lower values for these traits, and a C₄ species maize (Zea mays) that has a high photosynthetic capacity and low stomatal conductance. The experiments were conducted at different light intensities and using CO₂ with different ¹⁷O- excess. Our results demonstrate that two key factors determine the effect of photosynthetic gas exchange on Δ¹⁷O of atmospheric CO₂: The relative difference in Δ¹⁷O of the CO₂ entering the leaf and Δ¹⁷O of leaf water, and the back-diffusion flux from the leaf to the atmosphere, which can be quantified by the c_m/c_a ratio.  At low c_m/c_a the discrimination is governed by diffusion into the leaf, and at high c_m/c_a by back-diffusion of CO₂ that has equilibrated with the leaf water. Plants with a higher c_m/c_a ratio modify the Δ¹⁷O of atmospheric CO₂ more strongly than plants with lower c_m/c_a. 

Based on the leaf cuvette experiments using both C₄ and C₃ plants, the global discrimination in ¹⁷O-excess of atmospheric CO₂ due to assimilation is estimated to be -0.6±0.2‰. The main uncertainty in the global estimation is due to the uncertainty in the c_m/c_a ratio.