Mesophyll largely contributes to the historical increase in isotope discrimination of C3 plants and implications for water use efficiency

Much information of photosynthesis and carbon cycling was embedded in the stable carbon isotope, as plants discriminate against heavier carbon isotope. The process of isotope discrimination has been implemented in most terrestrial biosphere models (TBMs), mainly following the standard equation by Farquhar et al. With National Center for Atmospheric Research (NCAR) Community Land Model (CLM5), we found this standard equation cannot reproduce the historical long-term increase of isotope discrimination as deduced from atmospheric 13C/12C measurements. We attributed such a mismatch to the missed representation of photorespiration and mesophyll diffusion. Updating the discrimination equation by leveraging a mechanistic mesophyll diffusion model developed by Sun et al. (2014), we reproduce the trend towards a larger discrimination under higher CO2 levels: globally the trend is 0.013‰ ppm−1, consistent with atmospheric measurements. Mesophyll effects significantly contribute to this global trend, with the largest contribution in natural ecosystems. Moreover, we found that an explicit consideration of mesophyll conductance can lead to a higher response of historical water use efficiency to climate and environmental changes. Our results have implications for advanced modeling of isotopic discrimination and therefore for a better understanding of the coupled carbon-water cycle under changing climate.   References: Farquhar, G. D., Ehleringer, J. R., and Hubick, K. T.: Carbon isotope discrimination and photosynthesis, Annu. Rev. Plant Phys., 40, 503–537, (1989). Sun, Y. et al. Impact of mesophyll diffusion on estimated global land CO2 fertilization. Proc. Natl. Acad. Sci. U. S. A. 111, 15774–15779 (2014).