On the relationship between leaf anatomy and CO2 diffusion through the mesophyll of hypostomatous leaves.

Internal conductances to CO₂ transfer from the stomatal cavity to sites of carboxylation (g_i in hypostomatous sun-and shade-grown leaves of citrus, peach and macadamiatrees (Lloyd et al. 1992) were related to anatomical characteristics of mesophyll tissues. There was a consistent relationship between absorptance of photosynthetically active radiation and chlorophyll concentration (mmol m^-2) for all leaves, including sclerophyllous Macadamia whose transmittance was high despite its relatively thick leaves. In thin peach leaves, which had high g_i, the chloroplast volume and mesophyll surface area exposed to intercellular air spaces (ias) per unit leaf area were similar to those in the thicker leaves of the evergreen species. Peach leaves, however, had the lowest leaf dry weight per area (D/a),the lowest tissue density (T_d) and the highest chloroplast surface area (S_c) exposed to ias. There were negative correlations between g_i and leaf thickness or D/a, but positive correlations between g_i and S_c or S_c/T_d. We developed a one-dimensional diffusion model which partitioned g_i into a gaseous diffusion conductance through the ias (g_ias) plus a liquid-phase conductance through mesophyll cell walls (g_cw). The model accounted for a significant amount of variation (r²=0.80) in measured g_i by incorporating both components. The g_ias component was related to the one-dimensional path-length for diffusion across the mesophyll and so was greater in thinner peach leaves than in leaves of evergreen species. The g_cw component was related to tissue density and to the degree of chloroplast exposure to the ias. Thus the negative correlations between g_i and leaf thickness or D/a related to g_ias, whereas positive correlations between g_i and S_c or S_c/T_d related to g_cw. The g_cw was consistently lower than g_ias, and thus represented a greater constraint on CO₂ diffusion in the mesopylls of these hypostomatous species. [ABSTRACT FROM AUTHOR]