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.

Abstract

Mesophyll conductance (gm) 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 gm 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, gm data estimated with online 13C 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 gm was observed when the CO2 mole fraction was increased from 300 μmol mol−1 to 900 μmol mol−1, and a 60% increase when irradiance was increased from 200 μmol mol−1 to 1100 μmol mol−1 photosynthetic photon flux density (PPFD). The relative contribution of respiration and photorespiration to overall 13C discrimination was also estimated. Not taking this contribution into account may lead to a 50% underestimation of gm but had little effect on the CO2- and irradiance-induced changes. In conclusion, (i) the observed responses of gm to CO2 and irradiance were not artefactual; (ii) the respiratory term is important to assess absolute values of gm 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 gm in Eucalyptus seedlings.