Leaf mass per area predicts palisade structural properties linked to mesophyll conductance in balsam poplar (Populus balsamifera L.)

Modifications to leaf structural components that drive variation in leaf mass per area (LMA) may substantially impact leaf physiology by changing how easily CO2 diffuses through intercellular air space to carboxylation sites in mesophyll tissues. Mesophyll conductance (gm) is inversely proportional to the total pathway length for CO2, including the structural resistances encountered. In balsam poplar (Populus balsamifera L.), gm increases with latitude, paralleled by an increase in LMA. We investigated a family of P. balsamifera (K4×C) with high variation in LMA for different characteristics (tissues, nitrogen content, ultrastructural attributes). We interpreted trait variability using a developmental scale quantified by the leaf plastochron index (LPI). Developmental age significantly affected LMA, but those effects were lost at LPI ≥ 6. We outlined contributions of anatomical components to LMA and found palisade mesophyll properties were the primary drivers of variation in LMA within mature leaves (LPI ≥ 6). Using anatomical data, we derived components corresponding to structural resistances for gm. Perimeters of palisade cells and surface area of palisade exposed to intercellular air space, which may strongly influence CO2 diffusion, were correlated to LMA. Variation in LMA is positively related to differences in structural features expected to increase the conductance to CO2 diffusion within palisade mesophyll.