Comparative Ultrastructure of Developing Seed Coats of "Hard-Seeded" and "Soft-Seeded" Varieties of Soybean, Glycine max (L.) Merr.

Abstract

The seed coat structure of Glycine max is similar to that of other legumes in having a cuticle-covered outer macrosclereid layer underlaid by an osteosclereid layer. The remainder of the testa is composed of a parenchymatous region containing scattered vascular bundles. The outer layer of the endosperm is often appressed to this parenchyma zone. Developing macrosclereids have a dense cytoplasm with organelles typical of actively metabolic cells. The osteosclereids develop massive central walls, but the walls near the tips remain thin, allowing the terminal portions to expand and become bulbous. The endosperm cells typically contain stacks of rough endoplasmic reticulum. The macrosclereids of the hard-seeded Brachett variety stain intensely with FeSO₄, indicating tannins. These phenolic substances are represented by electron-dense deposits when viewed with transmission electron microscopy. Scanning electron microscopy reveals pores in seed coats of the soft-seeded Hardee but not in the hard-seeded Brachett variety. Also, the light line is more pronounced in Brachett. The impermeability of hard-seeded soybean seeds may involve the lack of pores, the phenolic layer, the more prominent light line, and the cutin in the hilum region.