Mesophyll conductance to CO2, assessed from online TDL-AS records of 13CO2 discrimination, displays small but significant short-term responses to CO2 and irradiance in Eucalyptus seedlings

Mesophyll conductance (g(m)) is now recognized as an important limiting process for photosynthesis, as it results in a significant decrease of CO2 diffusion from substomatal cavities where water evaporation occurs, to chloroplast stroma. Over the past decade, an increasing number of studies proposed that g(m) can vary in the short term (e. g. minutes), but these variations are still controversial, especially those potentially induced by changing CO2 and irradiance. In this study, g(m) data estimated with online C-13 discrimination recorded with a tunable diode laser absorption spectrometer (TDL-AS) during leaf gas exchange measurements, and based on the single point method, are presented. The data were obtained with three Eucalyptus species. A 50% decrease in g(m) was observed when the CO2 mole fraction was increased from 300 mu mol mol(-1) to 900 mmol mol(-1), and a 60% increase when irradiance was increased from 200 mu mol mol(-1) to 1100 mu mol mol(-1) photosynthetic photon flux density (PPFD). The relative contribution of respiration and photorespiration to overall C-13 discrimination was also estimated. Not taking this contribution into account may lead to a 50% underestimation of g(m) but had little effect on the CO2- and irradiance-induced changes. In conclusion, (i) the observed responses of g(m) to CO2 and irradiance were not artefactual; (ii) the respiratory term is important to assess absolute values of g(m) but has no impact on the responses to CO2 and PPFD; and (iii) increasing irradiance and reducing the CO2 mole fraction results in rapid increases in g(m) in Eucalyptus seedlings.