Structural properties related to mesophyll conductance and underlying variation in leaf mass area of balsam poplar (Populus balsamifera L.)

In leaves, the ease with which CO₂ may diffuse from substomatal cavities to the sites of carboxylation (mesophyll conductance, gm) is inversely proportional to the pathway length in the gas phase and the structural resistances encountered in the liquid phase. Increased length of any pathway component should decrease gm, whereas increased area for diffusion should increase gm. There is evidence that within native balsam poplar (Populus balsamifera L.) populations, gm may increase with latitude of origin, as do leaf mass (LMA) and nitrogen per unit area. To investigate the internal structural characteristics that might limit maximum gm in leaves, a balsam poplar family (K4×C) known to have high variation in LMA was chosen. Several whole tissue properties (LMA, carbon-nitrogen ratio, chlorophyll concentration index), microanatomy traits (leaf thickness, intercellular air space, cell wall surface area, chloroplast counts), and ultrastructural attributes (cell wall thickness, thylakoid grana thickness, and cell wall area adjacent to chloroplasts) were measured. There were significant genotypic differences in chlorophyll concentration index, numbers of chloroplasts per mesophyll cell, leaf thickness (tleaf), and thicknesses and cross-sectional areas of the palisade and spongy mesophyll layers. There were also differences in the fraction of intercellular air space (fias), as well as total and exposed cell wall surface areas of the mesophyll and its component tissues. Although genotypic differences in LMA were not significant, LMA varied as a function of tleaf (r=0.515, p<0.05) and fias (r=-0.510, p<0.05), which together explained considerable variation in this trait. The single best correlate of LMA was the cell wall area of the palisade (r=0.813, p<0.001), which accounted for well over half of the total cell wall area of the mesophyll. The areas of mesophyll cell walls and chloroplasts exposed to intercellular air space, both of which should contribute to gm, also increased with LMA. Along the same lines, but not correlated with LMA, there was a decrease in nitrogen density per unit exposed mesophyll surface area associated with thicker leaves (r=-0.481, p