Abstract. 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+ onthesubcellularlevel,e.g.incellwalls and on the surface of membranes, electron-energy-loss spectroscopy and electron-spectroscopic imaging were used.Seminalandnodalrootswereexposedfor30minto 1mMLaCl 3 and10mMLaCl 3 ,respectively.Inseminal roots,possessingnoexodermis,La 3+ diusionthrough theapoplastwasstoppedbytheCasparianbandsofthe endodermis.Innodalrootswithanexodermis,however, La 3+ diusionthroughthecorticalapoplasthadalready stoppedatthetightjunctionsoftheexodermalcellwalls resembling the Casparian bands of the endodermis. Therefore, we conclude that in some specialized roots suchasthenodalrootsofbarley,thephysiologicalroleof theendodermisislargelyperformedbytheexodermis. Key words: Casparianband±Endodermis± Exodermis± Hordeum La 3+ transport)± Lanthanumlocalization,translocation)±Root Introduction The exodermis and endodermis are both considered to restrictphysiologicallyuncontrollediondiusionviathe apoplastintothestele.Whereasthistypeofphysiological function has long been recognized for the endodermis LaÈuchli 1976; Clarkson 1988; Marschner 1995), the physiological role of the exodermis is largely unknown Clarkson 1996). A possible reason for this is the fact that,incontrasttotheendodermis,theexodermisisnot present in the roots of all investigated plant species Perumalla et al. 1990; Peterson and Perumalla 1990). Barleyplants,forexample,formtwodierenttypesof roots:the®rstgroup,belongingtothe®rstgenerationof adventitious roots, has no exodermis but the second group of adventitious roots, the ``nodal roots'', exhibit exodermaltissuesapproximately3.5cmbehindtheapex Gierth et al. 1999). Thus the root systems of barley species are very suitable for studying the physiological roleoftheexodermisandoftheendodermis,respectively. Therare-earthelementlanthanumisnormallyabsent fromtheplantgrowthmedia.Itsnumerousinteractions with®xednegativechargesofcellwallsandmembrane surfacesmayberesponsibleforthesuccessfulexclusionof lanthanumfromsymplasticuptake.Onlyafteralongtime of La 3+ exposure6±18h)waslanthanumalsofoundin pinocytotic vesicles, vacuoles and in the endoplasmic reticulum Peterson et al. 1986; Lazzaro and Thomson 1992).Suchpropertiesincombinationwithitshighatomic massz=57)allowlanthanumtobeusedintransmission electronmicroscopyTEM)studiesasanelectron-opaque tracer to delineate apoplastic pathways. Initially, the ultrastructural tracing of lanthanum was applied to animal tissues Revel and Karnovsky 1967) and subse- quentlyalsotoplantsforprobingapoplasticiondiusion Thomsonetal.1973;RobardsandRobb1974;Halletal. 1977; Maier-Maercker 1980; Drennan et al. 1987). Be- causeofitshighanitytocalcium-bindingsitesHanzely andHarmet1982;PictonandSteer1985)lanthanumhas also been used as a tracer for calcium Kramer and Lehmann1986;Lehmannetal.1990;Moyssetetal.1994; ApitiusandLehmann1995).Inthisstudylanthanumin combination with energy-loss spectroscopic imaging ESI)wasusedtotestwhethertheexodermisofbarley rootsisabarrieragainstapoplasticiontransport. Materials and methods Plant material BarleyHordeum vulgare L.cv.Alexis)plantswerehydroponically grown for 4 weeks in a growth cabinet under controlled diurnal Abbreviations:EELS=electron-energy-lossspectroscopy;ESI  electron-spectroscopic imaging; TEM = transmission electron microscopy Correspondence to:H.Lehmann; E-mail: lehmann@zellbiologie.tiho-hannover.de; Fax:+49-511-9538584 Planta2000)211:816±822 Analytical electron microscopical investigations on the apoplastic pathways of lanthanum transport in barley roots Heiner Lehmann, Ralf Stelzer, Stephan Holzamer, Ulrike Kunz, Markus Gierth InstitutfuÈrTieroÈkologieundZellbiologie,TieraÈrztlicheHochschuleHannover,BuÈnteweg17d,30559Hannover,Germany Received:28July1999/Accepted:24February2000