Studies on the Leaf of Populus Deltoides (Salicaceae): Ultrastructure, Plasmodesmatal Frequency, and Solute Concentrations

The minor veins and contiguous tissues of mature leaves of Populus deltoides Bartr. ex Marsh. were examined with the electron microscope to determine the ultrastructural characteristics of the component cells and to determine the structure, distribution, and frequency of plasmodesmata between the various cell types. In addition, plasmolytic studies were carried out to determine the solute concentrations of the various cell types of the minor veins and contiguous tissues. The cells comprising the mesophyll and bundle sheath contain all the components typical of photosynthetic cells. Paraveinal mesophyll cells and bundle‐sheath cells have fewer microbodies and smaller chloroplasts than do palisade parenchyma cells. Vascular parenchyma and companion cells tend to intergrade with one another structurally but can be distinguished from one another by their characteristic plastids. The mature, enucleate sieve‐tube member is lined by a parietal layer of cytoplasm consisting of plasmalemma, endoplasmic reticulum, mitochondria, plastids, and P‐protein. Plasmodesmata occur along all possible routes from the palisade parenchyma cells to the sieve tubes of the minor veins, and their frequency increases with increasing proximity to the sieve‐tube members. Plasmolytic studies revealed that the paraveinal mesophyll cells had a higher C 50 (estimated mannitol concentration plasmolyzing, on the average, 50% of a given cell type) than any other cell type of the leaf. Concentration gradients existed along the palisade cell/bundle‐sheath cell/companion cell (or vascular parenchyma cell) route as well as along the paraveinal mesophyll cell/bundle‐sheath cell/companion cell (or vascular parenchyma cell) route. Considering the frequency of plasmodesmata along these routes, it is conceivable that photosynthate diffuses from palisade cells to the companion cells along concentration gradients. Within the minor veins, the C 50 was higher for sieve‐tube members than for either companion cells or vascular parenchyma cells, indicating that loading of the sieve tubes is an active, energy‐dependent process.