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Asymmetrical effects of mesophyll conductance on fundamental photosynthetic parameters and their relationships estimated from leaf gas exchange measurements

Ying Sun, Lianhong Gu, Robert E. Dickinson, Stephen G. Pallardy, John Baker, Yonghui Cao, Fabio Murilo DaMatta, Xuejun Dong, David Ellsworth, Davina Van Goethem, Anna M. Jensen, Beverly E. Law, Rodolfo Loos, Samuel C. Vitor Martins, Richard J. Norby, Jeffrey Warren, David Weston, Klaus Winter
Plant, cell and environment 2014 v.37 no.4 pp. 978-994
C3 plants, biogeochemical cycles, carbon dioxide, carboxylation, chloroplasts, electron transfer, gas exchange, leaves, mesophyll, photosynthesis, simulation models
Most previous analyses of leaf gas exchange measurements assumed an infinite value of mesophyll conductance (gm) and thus equaled CO2 partial pressures in the substomatal cavity and chloroplast. Yet an increasing number of studies have recognized that gm is finite and there is a drawdown of CO2 partial pressure from the substomatal cavity to chloroplast. Using measurements of over 100 species from multiple countries as well as model simulations, we demonstrate that gm critically affects not only the values of key photosynthetic parameters estimated from leaf gas exchange measurements but also the functional relationships among them. Assuming an infinite gm leads to underestimation of the maximum carboxylation rate Vcmax, the maximum electron transport rate Jmax, and the triose phosphate utilization rate Tu, with the degree of this underestimation depending on the magnitude of gm. Furthermore, these parameters show varying sensitivities to the variation of gm: Vcmax the most sensitive and Tu the least with Jmax in between. Consequently, the ratios of Jmax to Vcmax, Tu to Vcmax, and Tu to Jmax are all overestimated under the infinite gm assumption. Also, assuming infinite gm limits the freedom of variation of the estimated parameters, which artificially constrains the functional relationships among these parameters to tighter shapes. Finally we show that a nonlinear function can be used to convert the parameters estimated under the infinite gm assumption to proper values if estimated gm is available. We suggest that future analyses of leaf gas exchange measurements should explicitly consider gm for improved process understanding.