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Ultrastructure, plasmodesmatal frequency, and solute concentration in green areas of variegated Coleus blumei Benth. leaves

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Abstract

The photosynthetic tissue of green portions of variegated Coleus blumei leaves consists primarily of palisade and spongy parenchyma cells as well as bundle-sheath cells. The moderate numbers of plasmodesmata connecting these cells may be sufficient to provide a symplastic pathway for assimilates moving toward the minor veins. The minor veins, however, are unusual in having two sets of phloem-loading cells which have little symplastic continuity with one another: one consisting of large, peripherally located intermediary cells, and a second set made up of smaller, usually more internal companion cells, both sets having their associated sieve-tube members. The intermediary cells are connected to vascular-parenchyma and bundle-sheath cells by unique branched plasmodesmata which are particularly abundant at the bundle-sheath interface. In addition, numerous plasmodesmata-pore connections occur between the intermediary cells and their associated sieve-tube members. Neither the intermediary cells nor their sieve-tube members plasmolyze when treated with 1.4 M mannitol, whereas mesophyll and vascular-parenchyma cells plasmolyze at 0.5 M and bundle-sheath cells at 0.6 M mannitol. By contrast, the companion cells and their associated sieve-tube members are symplastically isolated from the bundle-sheath cells and the sieve-tube-intermediary-cell complexes, and share few plasmodesmata with the vascular-parenchyma cells. Moreover, the companion cells plasmolyze at 1.1 M mannitol and their sieve tubes at 1.3 M. The intermediary-cell-sieve-tube complex thus appears to be structurally equipped to load assimilates entirely via the symplast, while the sieve-tube-companion-cell complex is probably loaded from the apoplast.

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Abbreviations

ER:

endoplasmic reticulum

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Fisher, D.G. Ultrastructure, plasmodesmatal frequency, and solute concentration in green areas of variegated Coleus blumei Benth. leaves. Planta 169, 141–152 (1986). https://doi.org/10.1007/BF00392308

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Key words

  • Coleus
  • Leaf ultrastructure
  • Phloem loading
  • Plasmodesmatal frequency