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

Leaf mass per area (LMA) has been suggested to negatively affect the mesophyll conductance to CO2 (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.