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Compartmentation of Assimilate Fluxes in Leaves

Ultrastructural Responses of Mesophyll and Companion Cells to the Alteration of Assimilate Export


Department of Ecophysiology Komarov Botanical Institute Prof. Popov Street 2 197376 St.-Petersburg Russia E-mail:


Abstract: 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.