Leaf mesophyll diffusion conductance in 35 Australian sclerophylls covering a broad range of foliage structural and physiological variation

Foliage structure, chemistry, photosynthetic potentials (Vcmax and Jmax), and mesophyll diffusion conductance (gm) were quantified for 35 broad-leaved species from four sites with contrasting rainfall and soil fertility in eastern Australia. The aim of the study was to estimate the extent to which gm and related leaf properties limited photosynthesis (A), focusing on highly sclerophyllous species typical of the 'slow-return' end of the leaf economics spectrum. Leaf dry mass per unit area (MA) varied ∼5-fold, leaf life span (LL) and N (NM) and P (PM) contents per dry mass ∼8-fold, and various characteristics of foliage photosynthetic machinery 6- to 12-fold across the data set. As is characteristic of the 'leaf economics spectrum', more robust leaves with greater MA and longevity were associated with lower nutrient contents and lower foliage photosynthetic potentials. gm was positively correlated with Vcmax and Jmax, and these correlations were stronger on a mass basis. Only gm/mass was negatively associated with MA. CO2 drawdown from substomatal cavities to chloroplasts (Ci-CC) characterizing mesophyll CO2 diffusion limitations was larger in leaves with greater MA, lower gm/mass, and lower photosynthetic potentials. Relative limitation of A due to finite mesophyll diffusion conductance, i.e. 1-A(infinite gm)/A(actual gm), was always >0.2 and up to 0.5 in leaves with most robust leaf structure, demonstrating the profound effect of finite gm on realized photosynthesis rates. Data from different sites were overlapping in bivariate relationships, and the variability of average values between the sites was less than among the species within the sites. Nevertheless, photosynthesis was more strongly limited by gm in low rain/high nutrient and high rain/low nutrient sites that supported vegetation with more sclerophyllous foliage. These data collectively highlight a strong relationship between leaf structure and gm, and demonstrate that realized photosynthesis rates are strongly limited by gm in this highly sclerophyllous flora.