The entry of ions and molecules into roots: an investigation using electron-opaque tracers.

Various solutions containing ions or molecules which may be visualized in the electron microscope have been presented to barley (Hordeum vulgare L.) roots. Large molecules such as gold sol particles (<6.0 nm) or ferritin (approx. 12.0 nm) were not found to be taken into the cytoplasm of any cells. Very dilute solutions of uranyl acetate or lanthanum nitrate resulted in the presence of typical electron-opaque crystals in the cortical apoplasm, as well as in cytoplasmic vesicles of cortical and some endodermal cells, but not in the cytoplasm or cell walls of stellar cells. Colloidal lanthanum hydroxide, however, while also impeded by the Casparian band, accumulated in vesicles in endodermal cells, and also penetrated into the stele.These results support the concept that different pathways exist for the movement of water and different ions across barley roots. They also indicate the relevance of the Casparian bands, the suberin lamellae, the formation of endocytotic vesicles, and the plasmodesmata, in studies on water and ion uptake.