|SUN, YING - University Of Texas
|GU, LIANHONG - Oak Ridge National Laboratory
|DICKINSON, ROBERT - University Of Texas
|PALLARDY, STEPHEN - University Of Missouri
|CAO, YONGHUI - Chinese Academy Of Forestry
|DAMATTA, FABIO - Federal University - Brazil
|DONG, XUEJUN - North Dakota State University
|ELLSWORTH, DAVID - Western Sydney University
|VAN GOETHEM, DAVINA - University Of Antwerp
|JENSEN, ANNA - Oak Ridge National Laboratory
|LAW, BEVERLY - Oregon State University
|LOOS, RODOLFO - Technological Center For Agricultural Research
|MARTINS, SAMUEL - Federal University - Brazil
|NORBY, RICHARD - Oak Ridge National Laboratory
|WESTON, DAVID - Oak Ridge National Laboratory
|WINTER, KLAUS - Smithsonian Tropical Research
Submitted to: Plant Cell and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/6/2013
Publication Date: 11/8/2013
Publication URL: http://handle.nal.usda.gov/10113/59542
Citation: Sun, Y., Gu, L., Dickinson, R., Pallardy, S., Baker, J.M., Cao, Y., Damatta, F., Dong, X., Ellsworth, D., Van Goethem, D., Jensen, A.M., Law, B.E., Loos, R., Martins, S.C., Norby, R.J., Weston, D., Winter, K. 2013. Asymmetrical effects of mesophyll conductance on fundamental photosynthetic parameters and their relationships estimated from leaf gas exchange measurements. Plant Cell and Environment. 37:978-994.
Interpretive Summary: Leaf-level photosynthesis measurements are commonly made in order to compare the performance of different varieties or to assess the impact of chemicals or environmental factors on plant performance. Typically this involves measuring photosynthesis rate at a range of intercellular CO2 concentrations to create what is known as an A-Ci curve, which can be analyzed to determine the key photosynthetic parameters: the maximum carboxylation rate Vcmax, the maximu eectron transport rate Jmax, and the triose phosphate utilization rate Tu. In this analysis it has always been assumed that the concentration of CO2 at the chloroplast is the same as that in the substomatal cavity, i.e.- that mesophyll conductance is negligible. We show that this leads to systematic underestimation of the photosynthetic parameters across a broad variety of plant species, and we have developed a general nonlinear function to correct past measurements if an estimate of mesophyll conductance is available. If this approach is used in future analyses of leaf gas exchange measurements it should result in improved understanding of the photosynthetic process.
Technical Abstract: 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.