Anatomical Features of Foliar Vasculature as Flux-Capacity Proxies

We present a procedure for preparing leaf tissue for minor-vein analysis using light and transmission electron microscopy that facilitates quantification of phloem and xylem features that correlate with photosynthetic capacity or transpiration rates, respectively, and thus provide proxies for foliar sugar- and/or water-transport capacity. We suggest here that the numbers and individual sizes of phloem cells of foliar minor veins (sometimes normalized by vein density) and transfer-cell wall ingrowths provide quantitative proxies for the capacity of the leaf to load and export photosynthates. Among apoplastic loaders, for which sugar loading into phloem depends on cell membrane-spanning transport proteins, photosynthetic capacity correlated positively with phloem-cell number and cross-sectional area (within a species or normalized by minor-vein density among different species) and level of cell-wall ingrowths (where present in transfer cells). Among active symplastic loaders, for which sugar loading into the phloem depends on cytosolic enzymes, photosynthetic capacity was positively correlated with individual phloem-cell size and vein density. All these anatomical features and photosynthetic capacity were subject to acclimatory adjustment to environmental conditions, with increased levels of these features associated with higher rates of photosynthesis. The numbers and cross-sectional areas of tracheary elements likewise exhibited acclimation to growth conditions and a strong, positive correlation with transpiration rate, which was greatest when normalized by vein density.