Cell‐level anatomical characteristics explain high mesophyll conductance and photosynthetic capacity in sclerophyllous Mediterranean oaks

Summary Leaf mass per area ( LMA ) has been suggested to negatively affect the mesophyll conductance to CO 2 ( g m ), which is the most limiting factor for area‐based photosynthesis ( A N ) in many Mediterranean sclerophyll species. However, despite their high LMA , these species have similar A N to plants from other biomes. Variations in other leaf anatomical traits, such as mesophyll and chloroplast surface area exposed to intercellular air space ( S m / S and S c / S ), may offset the restrictions imposed by high LMA in g m and A N in these species. Seven sclerophyllous Mediterranean oaks from Europe/North Africa and North America with contrasting LMA were compared in terms of morphological, anatomical and photosynthetic traits. Mediterranean oaks showed specific differences in A N that go beyond the common morphological leaf traits reported for these species (reduced leaf area and thick leaves). These variations resulted mainly from the differences in g m , the most limiting factor for carbon assimilation in these species. Species with higher A N showed increased S c / S , which implies increased g m without changes in stomatal conductance. The occurrence of this anatomical adaptation at the cell level allowed evergreen oaks to reach A N values comparable to congeneric deciduous species despite their higher LMA .