Structure of plasma membrane in radicles from cotton seeds

Summary Cortical cells in the apical region of excised radicles from cotton (Gossypium hirsutum L. var. M-8) seeds of differing moisture content (6, 12, or 16%) were examined in thin sections of vapor-fixed tissue and in freeze-etched replicas of unfixed tissue with transmission electron microscopy (TEM). The structure of the plasma membrane, as revealed in chemically-fixed tissue, had the tripartite feature characteristic of unit membranes. Regions of the plasma membrane appearing as discontinuities or breaks were shown by stereo tilting to represent an optical artifact caused by image superposition of curved regions of the plasma membrane contained within the thickness of a thin section. In freeze-etched preparations two complementary-type images of the plasma membrane were consistently observed. The asymmetric distribution of membrane-associated particles (MAPS) in the P and E fracture faces suggests that the structure of plasma membranes in cells of dry seeds resembles that of plasma membranes in germinating seeds. The physiological significance of these findings is that leaching of ions and small molecules during seed imbibition does not appear to involve passage through disorganized membranes in the radicle.