Physiologia Plantarum 157: 54–68. 2016 © 2015 Scandinavian Plant Physiology Society, ISSN 0031-9317 Mesophyll conductance plays a central role in leaf functioning of Oleaceae species exposed to contrasting sunlight irradiance Alessio Fini a,* , Francesco Loreto b , Massimiliano Tattini c , Cristiana Giordano d , Francesco Ferrini a , Cecilia Brunetti a,e and Mauro Centritto e a Dipartimento di Scienze delle Produzioni Agroalimentari e dell’Ambiente, Università di Firenze, Viale delle Idee 30, I-50019 Sesto Fiorentino (FI), Italy b Dipartimento di Scienze Bio-Agroalimentari, Consiglio Nazionale delle Ricerche, P.le Aldo Moro 7, I-00185 Roma, Italy c Istituto per la Protezione Sostenibile delle Piante, Consiglio Nazionale delle Ricerche, Via Madonna del Piano 10, I-50019 Sesto Fiorentino, Firenze, Italy d Centro di Microscopie Elettroniche “Laura Bonzi”, Istituto dei Composti Organometallici, Consiglio Nazionale delle Ricerche, Via Madonna del Piano 10, I-50019 Sesto Fiorentino, Firenze, Italy e Istituto per la Valorizzazione del Legno e delle Specie Arboree, Consiglio Nazionale delle Ricerche, Via Madonna del Piano 10, I-50019, Sesto Fiorentino, Firenze, Italy Correspondence *Corresponding author, e-mail: alessio.fini@unifi.it Received 9 June 2015; revised 21 September 2015 doi:10.1111/ppl.12401 The ability to modify mesophyll conductance (g m ) in response to changes in irradiance may be a component of the acclimation of plants to shade-sun tran- sitions, 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 lat- ifolia (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 (g m ) nor CO 2 assimilation significantly varied in F. ornus because of sunlight irra- diance. This corroborates previous suggestions that species with high plasticity for light requirements, do not need to undertake extensive reorganization of leaf conductances to CO 2 diffusion to adapt to different light environments. On the other hand, g m steeply declined in L. vulgare and increased in P. lat- ifolia exposed to full-sun conditions. In these two species, leaf anatomical traits are in part responsible for light-driven changes in g m , as revealed by the correlation between g m and mesophyll conductance estimated by anatomical parameters (g mA ). Nonetheless, g m was greatly overestimated by g mA when leaf metabolism was impaired because of severe light stress. We show that g m is maximum at the light intensity at which plant species have evolved and we conclude that g m actually plays a key role in the sun and shade adaptation of Mediterranean species. The limits of g mA in predicting mesophyll conductance are also highlighted. Abbreviations – NPQ, non-photochemical quenching; PPFD, photosynthetic photon flux density; PSII, photosystem II; T, temperature; t, diffusion path tortuosity; WUE, water use efficiency. 54 Physiol. Plant. 157, 2016