Analytical electron microscopical investigations on the apoplastic pathways of lanthanum transport in barley roots

In transmission electron microscopy studies, lanthanum ions have been used as electron-opaque tracers to delineate the apoplastic pathways for ion transport in. barley (Hordeum vulgare L.) roots. To localize La(3+) on the subcellular level, e.g. in cell walls and on the surface of membranes, electron-energy-loss spectroscopy and electron-spectroscopic imaging were used. Seminal and nodal roots were exposed for 30 mill to mM LaCl(3) and 10 mM LaCl(3). respectively. In seminal roots, possessing no exodermis, La(3+) diffusion through the apoplast was stopped by the Casparian bands of the endodermis. In nodal roots with an exodermis, however, La(3+) diffusion through the cortical apoplast had already stopped at the right junctions of the exodermal cell walls resembling the Casparian bands of the endodermis. Therefore, we conclude that in some specialized roots such as the nodal roots of barley, the physiological role of the endodermis is largely performed by the exodermis.