Artefactual responses of mesophyll conductance to CO2 and irradiance estimated with the variable J and online isotope discrimination methods

Studies with the variable J method have reported that mesophyll conductance (g(m)) rapidly decreases with increasing intercellular CO2 partial pressures (C-i) or decreasing irradiance. Similar responses have been suggested with the online isotope discrimination method, although with less consistency. Here we show that even when the true g(m) is constant, the variable J method can produce an artefactual dependence of g(m) on C-i or irradiance similar to those reported in previous studies for any of the following factors: day respiration and chloroplastic CO2 photocompensation point are estimated with Laisk method; C-i or electron transport rate is positively biased; net photosynthetic rate is negatively biased; insufficient NADPH is assumed while insufficient ATP limits RuBP regeneration. The isotopic method produces similar artefacts if fractionation of carboxylation or C-i is positively biased or Delta(13) negatively biased. A non-zero chloroplastic resistance to CO2 movement results in a qualitatively different dependence of g(m) on C-i or irradiance and this dependence is only sensitive at low C-i. We thus cannot rule out the possibility that previously reported dependence of g(m) on C-i or irradiance is a methodological artefact. Recommendations are made to take advantage of sensitivities of the variable J and isotopic methods for estimating g(m).