Leaf anatomy does not explain apparent short-term responses of mesophyll conductance to light and CO2 in tobacco

Mesophyll conductance to CO2 (g(m)), a key photosynthetic trait, is strongly constrained by leaf anatomy. Leaf anatomical parameters such as cell wall thickness and chloroplast area exposed to the mesophyll intercellular airspace have been demonstrated to determine g(m) in species with diverging phylogeny, leaf structure and ontogeny. However, the potential implication of leaf anatomy, especially chloroplast movement, on the short-term response of g(m) to rapid changes (i.e. seconds to minutes) under different environmental conditions (CO2, light or temperature) has not been examined. The aim of this study was to determine whether the observed rapid variations of g(m) in response to variations of light and CO2 could be explained by changes in any leaf anatomical arrangements. When compared to high light and ambient CO2, the values of g(m) estimated by chlorophyll fluorescence decreased under high CO2 and increased at low CO2, while it decreased with decreasing light. Nevertheless, no changes in anatomical parameters, including chloroplast distribution, were found. Hence, the g(m) estimated by analytical models based on anatomical parameters was constant under varying light and CO2. Considering this discrepancy between anatomy and chlorophyll fluorescence estimates, it is concluded that apparent fast g(m) variations should be due to artefacts in its estimation and/or to changes in the biochemical components acting on diffusional properties of the leaf (e.g. aquaporins and carbonic anhydrase).