CO₂ diffusion in tobacco: a link between mesophyll conductance and leaf anatomy

The partial pressure of CO₂ at the sites of carboxylation within chloroplasts depends on the conductance to CO₂ diffusion from intercellular airspace to the sites of carboxylation, termed mesophyll conductance (g _m). We investigated how g _m varies with leaf age and through a tobacco (Nicotiana tabacum) canopy by combining gas exchange and carbon isotope measurements using tunable diode laser spectroscopy. We combined these measurements with the anatomical characterization of leaves. CO₂ assimilation rate, A, and g _m decreased as leaves aged and moved lower in the canopy and were linearly correlated. This was accompanied by large anatomical changes including an increase in leaf thickness. Chloroplast surface area exposed to the intercellular airspace per unit leaf area (S _c) also decreased lower in the canopy. Older leaves had thicker mesophyll cell walls and g _m was inversely proportional to cell wall thickness. We conclude that reduced g _m of older leaves lower in the canopy was associated with a reduction in S _c and a thickening of mesophyll cell walls.