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

ABSTRACT Internal conductances to CO 2 transfer from the stomatal cavity to sites of carboxylation (g i ) in hypostomatous sun‐and shade‐grown leaves of citrus, peach and Macadamia trees (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 chloro‐plast 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 chloro‐plast 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 ( r 2 =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 CO 2 diffusion in the mesophylls of these hypostomatous species.