Spatial and age-dependent modifications of photosynthetic capacity in four Mediterranean oak species

Drought is one of the most important limitations of photosynthesis in Mediterranean climates. However, Mediterranean sclerophyllous species with long-lived leaves also support extensive and dynamic canopies, with potentially large spatial and age-dependent gradients. We studied within-canopy and temporal patterns in foliage structure, chemistry and photosynthesis in the evergreen species Quercus coccifera L., Q. ilex L. subsp. ballota (Desf.) Samp. in Bol. and Q. suber L. and in the semi-deciduous marcescent species Q. faginea Lam. to determine the role of within-canopy shading and leaf age on foliage functioning. There was a 2.5-fold within-canopy gradient in leaf dry mass per unit area (M_A) that was accompanied by a 3-fold range in area-based leaf nitrogen (N) content, the capacity for photosynthetic electron transport (J_max) and maximum Rubisco carboxylase activity (V_cmax), while the fractional investments of leaf nitrogen in electron transport (F_B) and in Rubisco (F_R) were relatively constant within the canopy. Leaf aging led to increased M_A, larger or constant mass-based N content, larger phosphorous (P) and structural carbon contents, but decreased movable cation contents. Age-dependent increases in M_A and N per dry mass meant that J_max and V_cmax per area were weakly related to leaf age, with a trend of decreasing values in older leaves. However, J_max and V_cmax per unit dry mass decreased 4-fold across the range of leaf age, primarily owing to decreases in apparent N investments in photosynthetic machinery. This decrease in apparent N investments in photosynthetic machinery was possibly the result of a larger fraction of N bound to cell walls, or of an enhanced CO₂ diffusion resistance from the outer surface of cell walls to the chloroplasts in older leaves with thicker and more lignified cell walls. The age-dependent variation in apparent fractional investments of N in photosynthetic machinery reduced the generality of leaf nitrogen v. photosynthesis relationships. Photosynthetic characteristics qualitatively fitted the same patterns with leaf age in all species, but at a common leaf age, area-based leaf photosynthetic potentials depended on species-specific values of M_A. These data collectively demonstrate important canopy and age-dependent controls on leaf structure, chemistry and photosynthetic potentials that should be included in larger-scale photosynthesis simulations in Mediterranean climates.