Sugar levels in the apoplast of assimilate exporting leaves were studied in two groups of plant species with contrasting structures of companion cells in minor veins. These species are termed either “symplastic” (with intermediary cells) or “apoplastic” (with transfer or ordinary cells). Sugars were measured in intercellular washing fluid after extracting the apoplast by an infiltration‐centrifugation technique. During the course of a day, sugar contents in the apoplast were, in general, lower in species with intermediary cells than in species with transfer or ordinary cells. In “symplastic” species, apoplastic sucrose concentrations were between 0.3 and 1 mM. In “apoplastic” species with transfer cells, they ranged between 2 and 6 mM. Apoplastic hexose contents were between 0.3 and 1 mM irrespective of presumed transport mode. “Symplastic” and “apoplastic” plants differed markedly in their response to a'translocation block. In “symplastic” plants, inhibition of assimilate export left apoplastic concentrations of sucrose and hexoses unchanged, whereas in “apoplastic” plants sugar levels increased, the maximal increase being observed with sucrose. In these plants, concentrations of sucrose were two to six times higher in the apoplast under export inhibition than in control leaves. The data suggest a different role of the leaf apoplast in the compartmentation and export of assimilates in the two plant groups under study.

Leaf cell structures were studied in relation to assimilate export and storage in plant species with numerous plasmodesmata at the mesophyll/phloem interface (Coleus blumei, Cucurbita pepo) and in plants with a much reduced frequency of plasmodesmata (Pisum sativum, Helianthus annuus). Cold girdling of petioles and darkening of plants was used to induce changes in the levels of assimilates in the symplast and the apoplast of leaves. The volume of the endoplasmic reticulum in intermediary cells and of the vacuole in transfer cells showed changes corresponding to the experimental changes of photosynthate export. When export was blocked, the endoplasmic reticulum of intermediary cells collapsed in Coleus and Cucurbita, and the vacuoles of transfer cells enlarged in Pisum and Helianthus. These changes were accompanied by starch accumulation in the mesophyll cells of all species studied. Condensation of the cytosol in the transfer cells reminiscent of plasmolysis was observed under export blockage. Condensation of organellar matrixes in the intermediary cells was a usual (probably osmotic) response to cold girdling in the case of symplastic species. The data presented are discussed in relation to two basic pathways of assimilate flux in leaves of different groups of plants.