Compartmentation of Assimilate Fluxes in Leaves

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.

Scaling of phloem hydraulic resistance in stems and leaves of the understory angiosperm shrub Illicium parviflorum

“…The massive presence of sieve areas in the lateral walls of sieve tube elements from minor veins is in line with the idea of a division of function between the minor veins, which mainly work as sugar loaders, and major veins, where directional transport occurs within leaves (Russin and Evert, ; Turgeon, ; Carvalho et al., , ). A high number of symplasmic connections typically associate with passive sugar loading in the minor veins (van Bel et al., ; Turgeon, ; Gamalei et al., ; Rennie and Turgeon, ; Turgeon, ; Davidson et al., ; Zhang et al., ), but the heterogeneity of the species evaluated leave this question still unresolved (Slewinski et al., ). So far, sugar radiolabeling appears as the most reliable measure of loading type, which has yet to be applied to Illicium parviflorum leaves.…”

Cytology of the minor-vein phloem in 320 species from the subclass Asteridae suggests a high diversity of phloem-loading modes†

“…Another well-known structural trait of ICs is the presence of many small vacuoles instead of one large vacuole; these unusual vacuoles were found in all ICs of stachyose-transporting plants leading to the speculation that they might represent the compartment for RFO synthesis (Gamalei, 1990; Voitsekhovskaja, 2002). However, we did not include this feature in the present classification because, in contrast to other features, the presence and the number of these vacuoles strongly depend on the physiological condition of the plant (Gamalei et al, 2000). In the present study, we classified the companion cells only on the basis of their stable structural features.…”

Phloem structure and development in Illicium parviflorum, a basal angiosperm shrub

“…This is in line with the idea of a division of function between the minor veins, which mainly work as sugar loaders, and major veins, where directional transport occurs within leaves (Russin & Evert, 1985; Turgeon, 2006; Carvalho et al ., 2017a, 2018). A high number of symplasmic connections typically associate with passive sugar loading in the minor veins (van Bel et al ., 1992; Turgeon, 1996; Gamalei et al , 2000; Rennie & Turgeon, 2009; Turgeon, 2010; Davidson et al ., 2011; Zhang et al ., 2014), but the heterogeneity of the species evaluated leave this question still unresolved (Slewinski et al ., 2013). So far, sugar (radio) labeling appears as the most reliable measure of loading type, which has yet to be applied to Illicium parviflorum leaves.…”