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.

叶绿体中羧化位点处的CO ₂分压取决于对从细胞间空域到羧化位点的CO ₂扩散的电导，称为叶肉电导（g _m）。我们通过结合使用可调二极管激光光谱仪进行的气体交换和碳同位素测量，研究了g _m随叶片年龄和通过烟草（Nicotiana tabacum）冠层的变化。我们将这些测量结果与叶片的解剖特征结合在一起。CO ₂同化率A和g _m随着叶片的衰老而降低，并且在冠层中降低，并且线性相关。这伴随着大的解剖变化，包括叶片厚度的增加。在冠层中，暴露于细胞间空域的每单位叶面积（S _c）的叶绿体表面积也降低了。较老的叶子具有较厚的叶肉细胞壁，g _m与细胞壁厚度成反比。我们得出的结论是，冠层下部较低的老叶的g _m减少与S _c的减少和叶肉细胞壁的增厚有关。