J. Plant Physiol. 159. 1197-1204 (2002)  Urban & Fischer Verlag http://www.urbanfischer.de/journals/jpp On the evidence of a diffusion barrier in the outer cortex apoplast of cress-roots ( Lepidium sativum ), demonstrated by analytical electron microscopy Melanie Schröder, Ulrike Kunz, Ralf Stelzer, Heiner Lehmann* Institut für Tierökologie und Zellbiologie,Arbeitsgruppe Elektronenmikroskopie,Tierärztliche Hochschule Hannover, Bünteweg 17d, D-30559 Hannover Received February 27, 2002 · Accepted May 10, 2002 Summary To mark the apoplastic pathway of ions in the root of the dicotyledonous plant Lepidium sativum we used the heavy element lanthanum, which can be identified by analytical electron microscopy (EELS and ESI). In the front root tip, the primary walls of all meristematic cells contained lanthanum. 10– 15 mm behind the root apex, lanthanum was found in the cortex cell walls up to the endodermis, but not in the stele. 20–25 mm from the tip, lanthanum was accumulated in the radial cell walls of the hypodermis, which, however, is not a complete diffusion barrier for ions, so that traces of lanthanum also were found in the cortex cell walls up to the endodermis. This study provides evidence for the presence of two apolastic diffusion barriers in the region of highest water uptake in cress roots. Key words: Lepidium sativum – apoplast – EELS – ESI – ion transport – lanthanum Abbreviations: C = calyptra. – CS = casparian strip. – E = endodermis. – EELS = electron energy loss spectroscopy. – EFTEM = energy-filtering transmission electron microscopy. – ESI = electron spectro- scopic imaging. – H = hypodermis. – I = intercellular space. – LM = light microscopy. – PZ = pericy- cle. – R = root cortex. – RH = rhizodermis. – RoH = root hair. – TEM = transmission electron micro- scopy. – V = vesicle. – Z = central cylinder Introduction The transport of water and minerals from the soil solution ac- ross the cortex into the stele of the root follows the apoplastic as well as the symplastic pathways. Whereas the symplast of the plant is a continuum, the apoplastic pathway is blocked by the endodermis. At this border, the apoplastic transport has to convert to the symplastic, which means all substances have to pass the plasma membrane at least twice, i.e. when * E-mail corresponding author: lehmann@zellbiologie.tiho-hannover.de entering and leaving their symplastic route. There is some controversy in published discussions concerning whether peripheral cell layers of the cortex function as an apoplastic barrier for water and ions alongside the endodermis. In this respect, the cell layer below the rhizodermis (the so called hypodermis) plays an important role. In mono- and dicotyledonous plant species, suberin is de- posited in the radial walls of the hypodermis during develop- ment of the roots; after which this cell layer is called «exoder- mis» (Perumalla 1986). Whereas Perumalla (1986) described the suberised radial wall as a «Casparian band», Guttenberg (1943) maintained that these cell walls are not comparable 0176-1617/02/159/11-1197 $ 15.00/0