Photoassimilate Pathway(s) and Phloem Loading in the Leaf of Moricandia arvensis (L.) DC. (Brassicaceae)

Mesophyll and bundle-sheath cells of mature leaves of Moricandia arvensis, a C3-C4 intermediate, contain all the ultrastructural components typical of photosynthesizing cells, as do vascular parenchyma cells and companion cells. Plasmodesmata occur between all possible cell combinations of the leaf, with the possible exception of the bundle-sheath cell/sieve-tube member interface, where plasmodesmata are either rare or absent. Within the contiguous tissues, the greatest plasmodesmatal frequencies occur at the anticlinal interfaces between palisade parenchyma cells and those mesophyll cells that lie in the plane of the minor venation and the interfaces between palisade parenchyma cells and bundle-sheath cells. Of the interfaces involving cell types of the minor veins, the greatest frequencies occur at bundle-sheath/phloem parenchyma and phloem parenchyma/phloem parenchyma or phloem parenchyma/companion cell interfaces. Plasmodesmata between companion cells and other parenchymatous cells of the vascular bundle and between companion cells and bundle-sheath cells are modified on the companion-cell side of the wall, where most lack discernible desmotubules and appear to be occluded with a uniformly dense, amorphous material. Concentration gradients exist along the mesophyll cell/bundle-sheath cell/vascular parenchyma cell and phloem parenchyma cell routes. Neither sieve-tube members nor companion cells plasmolyzed at 1.4 M of osmoticum, the highest molarity used. On the basis of plasmodesmatal distribution and concentration gradient, we conclude that photoassimilates move symplastically to the region of the sieve tube-companion cell complexes and that phloem loading in the leaf of M. arvensis occurs from the apoplast and is an active process.