Cell-level anatomy explains leaf age-dependent declines in mesophyll conductance and photosynthetic capacity in the evergreen Mediterranean oak Quercus ilex subsp. rotundifolia

Leaves of Mediterranean evergreen tree species experience a reduction in net CO assimilation (A) and mesophyll conductance to CO (g) during aging and senescence, which would be influenced by changes in leaf anatomical traits at cell level. Anatomical modifications can be accompanied by the dismantling of photosynthetic apparatus associated to leaf senescence, manifested through changes at the biochemical level (i.e., lower nitrogen investment in photosynthetic machinery). However, the role of changes in leaf anatomy at cell level and nitrogen content in g and A decline experienced by old non-senescent leaves of evergreen trees with long leaf lifespan is far from being elucidated. We evaluated age-dependent changes in morphological, anatomical, chemical and photosynthetic traits in Quercus ilex subsp. rotundifolia Lam., an evergreen oak with high leaf longevity. All photosynthetic traits decreased with increasing leaf age. The relative change in cell wall thickness (T) was less than in chloroplast surface area exposed to intercellular air space (S/S), and S/S was a key anatomical trait explaining variations in g and A among different age classes. The reduction of S/S was related to ultrastructural changes in chloroplasts associated to leaf aging, with a concomitant reduction in cytoplasmic nitrogen. Changes in leaf anatomy and biochemistry were responsible for the age-dependent modifications in g and A. These findings revealed a gradual physiological deterioration related to the dismantling of the photosynthetic apparatus in older leaves of Q. ilex subsp. rotundifolia.