Leaf internal diffusion conductance limits photosynthesis more strongly in older leaves of Mediterranean evergreen broad-leaved species

Leaf age-dependent changes in structure, nitrogen content, internal mesophyll diffusion conductance (gm), the capacity for photosynthetic electron transport (Jmax) and the maximum carboxylase activity of Rubisco (Vcmax) were investigated in mature non-senescent leaves of Laurus nobilis L., Olea europea L. and Quercus ilex L. to test the hypothesis that the relative significance of biochemical and diffusion limitations of photosynthesis changes with leaf age. The leaf life-span was up to 3 years in L. nobilis and O. europea and 6 years in Q. ilex. Increases in leaf age resulted in enhanced leaf dry mass per unit area (MA), larger leaf dry to fresh mass ratio, and lower nitrogen contents per dry mass (NM) in all species, and lower nitrogen contents per area (NA) in L. nobilis and Q. ilex. Older leaves had lower gm, Jmax and Vcmax. Due to the age-dependent increase in MA, mass-based gm, Jmax and Vcmax declined more strongly (7- to 10-fold) with age than area-based (5- to 7-fold) characteristics. Diffusion conductance was positively associated with foliage photosynthetic potentials. However, this correlation was curvilinear, leading to lower ratio of chloroplastic to internal CO2 concentration ( Cc/Ci) and larger drawdown of CO2 from leaf internal air space to chloroplasts (DC) in older leaves with lower gm. Overall the agedependent decreases in photosynthetic potentials were associated with decreases in NM and in the fraction of N in photosynthetic proteins, whereas decreases in gm were associated with increases in MA and the fraction of cell walls. These age-dependent modifications altered the functional scaling of foliage photosynthetic potentials with MA, NM, and NA. The species primarily differed in the rate of agedependent modifications in foliage structural and functional characteristics, but also in the degree of agedependent changes in various variables. Stomatal openness was weakly associated with leaf age, but due to species differences in stomatal openness, the distribution of total diffusion limitation between stomata and mesophyll varied among species. These data collectively demonstrate that in Mediterranean evergreens, structural limitations of photosynthesis strongly interact with biochemical limitations. Age-dependent changes in gm and photosynthetic capacities do not occur in a co-ordinated manner in these species such that mesophyll diffusion constraints curb photosynthesis more in older than in younger leaves.