Mesophyll conductance plays a central role in leaf functioning of Oleaceae species exposed to contrasting sunlight irradiance

The ability to modify mesophyll conductance (gm) in response to changes in irradiance may be a component of the acclimation of plants to shade-sun transitions, thus influencing species-specific distributions along light-gradients, and the ecological niches for the different species. To test this hypothesis we grew three woody species of the Oleaceae family, the evergreen Phillyrea latifolia (sun- requiring), the deciduous Fraxinus ornus (facultative sun-requiring) and the hemi-deciduous Ligustrum vulgare (shade-tolerant) at 30 or 100% sunlight irradiance. We show that neither mesophyll conductance calculated with combined gas exchange and chlorophyll fluorescence techniques (gm) nor CO2 assimilation significantly varied in F. ornus because of sunlight irradiance. This corroborates previous suggestions that species with high plasticity for light requirements, do not need to undertake extensive reorganization of leaf conductances to CO2 diffusion to adapt to different light environments. On the other hand, gm steeply declined in L. vulgare and increased in P. latifolia exposed to full sun conditions. In these two species, leaf anatomical traits are in part responsible for light-driven changes in gm, as revealed by the correlation between gm and mesophyll conductance estimated by anatomical parameters (gmA). Nonetheless, gm was greatly over-estimated by gmA when leaf metabolism was impaired because of severe light stress. We show that gm is maximum at the light intensity at which plant species have evolved and we conclude that gm actually plays a key role in the sun and shade adaptation of Mediterranean species. The limits of gmA in predicting mesophyll conductance are also highlighted.