Abstract
Studies with the variable J method have reported that mesophyll conductance (gm) rapidly decreases with increasing intercellular CO2 partial pressures (Ci) 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 gm is constant, the variable J method can produce an artifactual dependence of gm on Ci 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; Ci 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 artifacts if fractionation of carboxylation or Ci are positively biased or 13 negatively biased. A nonzero chloroplastic resistance to CO2 movement results in a qualitatively different dependence of gm on Ci or irradiance and this dependence is only sensitive at low Ci. We thus cannot rule out the possibility that previously reported dependence of gm on Ci or irradiance is a methodological artifact. Recommendations are made to take advantage of sensitivities of the variable J and isotopic methods for estimating gm.
