Activation of rape (Brassica napus L.) embryo during seed germination. III. Ultrastructure of dry embryo axis

Abstract

Mature dry winter rape (Brassica napus L.), var. oleifera, cv. Górczański embryos were studied in the light and the electron microscope. Considerable modifications and regression of the cell ultrastructure were noted in the resting embryo as compared with the metabolically active cells. The degree of regression of the ultrastructure differed in the particular organs and tissues of the embryo. Of most regressed character are the cells of the storage organs - the hypocotyl and cotyledones. They are almost completely filled with protein and lipid bodies. The small spaces between them are filled with dense cytoplasm with a lobular nucleus and not numerous, difficult to identify, plastids and mitochondria. The cells of the shoot primordium and radicle, particularly of the protoderm at the boundary of the hypocotyl and root and columella of root cap have a less regressed ultrastructure. They contain less storage material, a less dense cytoplasm and nearly all cell organelles with a normal appearance. The mitochondria are quite numerous with rather large cristae. Plastids are large with characteristic infolds filled with cytoplasm and some lamellae and a few agglomerations of plastoglobules. The nucleus is lobular with distinctly double and porous nuclear envelope and uniformly dense nucleolus. These cells do not contain dictyosomes and the ER is reduced to short, mostly rough cisternae and vesicles. Cells within the columella itself are also differentiated. The least regression of ultrastructure is seen in the cells of external layers containing the most numerous and most active looking mitochondria and more ER structures. The promeristem cells are similar to those of the deeper columella layers but their mitochondria are more regressed. The cells of the lateral parts of the cap and radicle cells, distant from the promeristem are more similar to the hypocotyl cells.