Ultrastructural Modifications Associated with Imbibition of the Soybean Radicle

Ultrastructural studies of parenchyma cells of the radicle of soybean (Glycine max [L.] Merr. `Wayne') indicate that 20 min of imbibition result in extensive structural changes in the plasma membrane but only relatively minor changes in organelles. The plasma membrane, which in cells of dry seeds is disorganized and disrupted, becomes relatively uniform and continuous. Circular or whorled membranous structures commonly occur near the cell wall and in the cytoplasm. Specific plasma membrane-staining techniques indicate that these structures are, in fact, elaborations of the plasma membrane. The endoplasmic reticulum, which is rarely evident in dry tissue, appears in association with lipid and protein bodies. Mitochondria and proplastids do not change in structure in dry and imbibed tissue. The mitochondria are nearly structureless, with poorly defined external membranes and few narrow, fuzzy cristae. The proplastids contain starch and phytoferritin but no thylakoid membranes. These observations indicate that the initial imbibition of water into radicle cells results in extensive membrane changes.