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 (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 artefactual 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 artefacts if fractionation of carboxylation or Ci is positively biased or Δ(13) negatively biased. A non-zero 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 artefact. Recommendations are made to take advantage of sensitivities of the variable J and isotopic methods for estimating gm .
Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA.
Lianhong
L
Gu
Ying
Y
Sun
D016477:Artifacts; D002245:Carbon Dioxide / Q000378:metabolism*; D002247:Carbon Isotopes; D005591:Chemical Fractionation; D002736:Chloroplasts / Q000378:metabolism / Q000528:radiation effects; D004579:Electron Transport / Q000528:radiation effects; D007553:Isotope Labeling / Q000379:methods*; D008027:Light; D058503:Mesophyll Cells / Q000378:metabolism* / Q000528:radiation effects*; D010313:Partial Pressure; D010788:Photosynthesis / Q000528:radiation effects; D012273:Ribulose-Bisphosphate Carboxylase / Q000378:metabolism
D002247:Carbon Isotopes; D002245:Carbon Dioxide; D012273:Ribulose-Bisphosphate Carboxylase
carbon isotope discrimination; chlorophyll fluorescence; mesophyll diffusion; photosynthesis