Foliage photosynthetic and structural traits were studied in 15 species with a wide range of foliage anatomies to gain insight into the importance of key anatomical traits in the limitation of diffusion of CO2 from substomatal cavities to chloroplasts. The relative importance of different anatomical traits in constraining CO2 diffusion was evaluated using a quantitative model. Mesophyll conductance (g m) was most strongly correlated with chloroplast exposed surface to leaf area ratio (S c/S) and cell wall thickness (T cw), but, depending on foliage structure, the overall importance of g m in constraining photosynthesis and the importance of different anatomical traits in the restriction of CO2 diffusion varied. In species with mesophytic leaves, membrane permeabilities and cytosol and stromal conductance dominated the variation in g m. However, in species with sclerophytic leaves, g m was mostly limited by T cw. These results demonstrate the major role of anatomy in constraining mesophyll diffusion conductance and, consequently, in determining the variability in photosynthetic capacity among species.

Mesophyll diffusion conductance to CO(2) is a key photosynthetic trait that has been studied intensively in the past years. The intention of the present review is to update knowledge of g(m), and highlight the important unknown and controversial aspects that require future work. The photosynthetic limitation imposed by mesophyll conductance is large, and under certain conditions can be the most significant photosynthetic limitation. New evidence shows that anatomical traits, such as cell wall thickness and chloroplast distribution are amongst the stronger determinants of mesophyll conductance, although rapid variations in response to environmental changes might be regulated by other factors such as aquaporin conductance. Gaps in knowledge that should be research priorities for the near future include: how different is mesophyll conductance among phylogenetically distant groups and how has it evolved? Can mesophyll conductance be uncoupled from regulation of the water path? What are the main drivers of mesophyll conductance? The need for mechanistic and phenomenological models of mesophyll conductance and its incorporation in process-based photosynthesis models is also highlighted.

The leaf economics spectrum (LES) represents a suite of intercorrelated leaf traits concerning construction costs per unit leaf area, nutrient concentrations, and rates of carbon fixation and tissue turnover. Although broad trade-offs among leaf structural and physiological traits have been demonstrated, we still do not have a comprehensive view of the fundamental constraints underlying the LES trade-offs. Here, we investigated physiological and structural mechanisms underpinning the LES by analysing a novel data compilation incorporating rarely considered traits such as the dry mass fraction in cell walls, nitrogen allocation, mesophyll CO diffusion and associated anatomical traits for hundreds of species covering major growth forms. The analysis demonstrates that cell wall constituents are major components of leaf dry mass (18-70%), especially in leaves with high leaf mass per unit area (LMA) and long lifespan. A greater fraction of leaf mass in cell walls is typically associated with a lower fraction of leaf nitrogen (N) invested in photosynthetic proteins; and lower within-leaf CO diffusion rates, as a result of thicker mesophyll cell walls. The costs associated with greater investments in cell walls underpin the LES: long leaf lifespans are achieved via higher LMA and in turn by higher cell wall mass fraction, but this inevitably reduces the efficiency of photosynthesis.

Finite mesophyll diffusion conductance (gm) significantly constrains net assimilation rate (An), but gm variations and variation sources in response to environmental stresses during leaf development are imperfectly known. The combined effects of light and water limitations on gm and diffusion limitations of photosynthesis were studied in saplings of Populus tremula L. An one-dimensional diffusion model was used to gain insight into the importance of key anatomical traits in determining gm. Leaf development was associated with increases in dry mass per unit area, thickness, density, exposed mesophyll (Smes/S) and chloroplast (Sc/S) to leaf area ratio, internal air space (fias), cell wall thickness and chloroplast dimensions. Development of Smes/S and Sc/S was delayed under low light. Reduction in light availability was associated with lower Sc/S, but with larger fias and chloroplast thickness. Water stress reduced Sc/S and increased cell wall thickness under high light. In all treatments, gm and An increased and CO2 drawdown because of gm, Ci-Cc, decreased with increasing leaf age. Low light and drought resulted in reduced gm and An and increased Ci-Cc. These results emphasize the importance of gm and its components in determining An variations during leaf development and in response to stress.

Leaf age-dependent changes in structure, nitrogen content, internal mesophyll diffusion conductance ( g m ), the capacity for photosynthetic electron transport ( J max ) and the maximum carboxylase activity of Rubisco ( V cmax ) 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 ( M A ), larger leaf dry to fresh mass ratio, and lower nitrogen contents per dry mass ( N M ) in all species, and lower nitrogen contents per area ( N A ) in L. nobilis and Q. ilex . Older leaves had lower g m , J max and V cmax . Due to the age-dependent increase in M A , mass-based g m , J max and V cmax 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 CO 2 concentration ( C c / C i ) and larger drawdown of CO 2 from leaf internal air space to chloroplasts ( D D D D C ) in older leaves with lower g m . Overall the agedependent decreases in photosynthetic potentials were associated with decreases in N M and in the fraction of N in photosynthetic proteins, whereas decreases in g m were associated with increases in M A and the fraction of cell walls. These age-dependent modifications altered the functional scaling of foliage photosynthetic potentials with M A , N M , and N A . 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 g m 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.