Cadmium induces hypodermal periderm formation in the roots of the monocotyledonous medicinal plant Merwilla plumbea

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1 Annls of Botny 17: , 211 doi:1.193/o/mcq24, ville online t Cdmium induces hypoderml periderm formtion in the roots of the monocotyledonous medicinl plnt Merwill plume Alexnder Lux 1,2, Mrek Vculík 1, Michl Mrtink 1, Desn Lišková 2, Mnoj G. Kulkrni 3, Wendy A. Stirk 3 nd Johnnes Vn Stden 3, * 1 Deprtment of Plnt Physiology, Fculty of Nturl Sciences, Comenius University in Brtislv, Mlynská dolin B-2, SK Brtislv, Slovk Repulic, 2 Deprtment of Glycoiotechnology, Institute of Chemistry, Slovk Acdemy of Sciences, Dúrvská cest 9, SK Brtislv, Slovk Repulic nd 3 Reserch Centre for Plnt Growth nd Development, School of Biologicl nd Conservtion Sciences, University of KwZulu-Ntl Pietermritzurg, Privte Bg X1, Scottsville 329, South Afric * For correspondence. E-mil rcpgd@ukzn.c.z Received: 1 Septemer 21 Returned for revision: 5 Octoer 21 Accepted: 5 Novemer 21 Pulished electroniclly: 29 Novemer 21 Bckground nd Aims Merwill plume is n importnt Africn medicinl plnt. As the plnts grow in soils contminted with metls from mining ctivities, the dnger of humn intoxiction exists. An experiment with plnts exposed to cdmium (Cd) ws performed to investigte the response of M. plume to this hevy metl, its uptke nd trnsloction to plnt orgns nd rection of root tissues. Methods Plnts grown from seeds were cultivted in controlled conditions. Hydroponic cultivtion is not suitle for this species s roots do not tolerte qutic conditions, nd dditionl stress y Cd tretment results in totl root growth inhiition nd deth. After cultivtion in perlite the plnts exposed to 1 nd 5 mg Cd L 21 in hlf-strength Hoglnd s solution were compred with control plnts. Growth prmeters were evluted, Cd content ws determined y inductively coupled plsm mss spectroscopy (ICP-MS) nd root structure ws investigted using vrious stining procedures, including the fluorescent stin Fluorol yellow 88 to detect suerin deposition in cell wlls. Key Results The plnts exposed to Cd were significntly reduced in growth. Most of the Cd tken up y plnts fter 4 weeks cultivtion ws retined in roots, nd only smll mount ws trnslocted to uls nd leves. In rection to higher Cd concentrtions, roots developed hypoderml periderm close to the root tip. Cells produced y cork cmium impregnte their cell wlls y suerin. Conclusions It is suggested tht the hypoderml periderm is developed in young root prts in rection to Cd toxicity to protect the root from rdil uptke of Cd ions. Secondry meristems re usully not present in monocotyledonous species. Another interprettion explining formtion of protective suerized lyers s result of periclinl divisions of the hypodermis is discussed. This process my represent n s yet unknown defence rection of roots when exposed to elementl stress. Key words: Cdmium, environmentl stress, medicinl plnts, Merwill plume, monocotyledonous plnts, periderm, plnt ntomy, root structure. INTRODUCTION South Afric hs lrge concentrtion of metl, minerl nd col-mining industries with high rtes of wste disposl (Cooke nd Johnson, 22). The gold nd col-mining industries re responsile for lowering ph levels of South Africn rivers y the phenomenon known s cid mine dringe (Coetzee, 1995). The wste from mining industries contins hevy metls such s led, cdmium (Cd), mercury, luminium, zinc, copper, mgnesium nd mngnese (Deprtment of Wter Affirs, 1986; Wren nd Stephenson, 1991). Compred with other metls or metlloids, Cd hs higher tendency to ccumulte in plnt tissues (Kt-Pendis nd Pendis, 1984) nd is considered s highly mutgenic nd crcinogenic y the Interntionl Agency for Reserch on Cncer (IARC, 1993; Filipic nd Hei, 24). Consumption of edile plnt mteril with high Cd content my cuse toxicity in humns (Jckson nd Allowy, 1992; FAO/WHO, 1995). Due to its high toxicity, the mximum permissile limit of Cd in medicinl plnts set y the World Helth Orgniztion (WHO) is.3 p.p.m. In recent investigtion y Street et l. (28), Cd exceeded the limits recommended y the WHO (1998) in ul nd tuer smples of South Africn medicinl plnts otined from street mrkets. In nother study, Cd ccumultion in uls of smll nd medium-sized plnts of Tulghi species used medicinlly incresed with incresing Cd concentrtion (Street et l., 21). Buls of mture Merwill plume plnts ccumulted 24-fold more Cd thn the WHO guidelines when irrigted with 2 mg Cd L 21. At the sme time, the ul extrcts showed incresed nticteril ctivity (Street et l., 29). Cdmium hd detrimentl effect on the root growth of Bowie voluilis, which is nother importnt ulous medicinl plnt in South Afric (Street et l., 27). In most of these studies, it ws indicted tht uls of medicinl plnts hve tendency to ccumulte Cd. However, the uptke mechnism of Cd is still uncler. Downloded from t University of KwZulu Ntl, Medicl Lirry on Jnury 3, 213 # The Author 21. Pulished y Oxford University Press on ehlf of the Annls of Botny Compny. All rights reserved. For Permissions, plese emil: journls.permissions@oup.com

2 286 Lux et l. Cdmium nd hypoderml periderm formtion in roots As roots of seedlings represent the primry contct zone of plnts in soils, ntomicl ltertions occurring during Cd uptke my hve n effect on ccumultion processes nd vegettive growth of the plnts when they re exposed to this metl. The root rection of vrious plnt species ws recently reviewed y Lux et l. (211). Inhiition of root growth nd rnching s result of the inhiitory effect of Cd on cell division ws oserved y Fusconi et l. (26) nd M et l. (21). Species-specific rections nd chnges in root tissue orgniztion nd development occur fter Cd tretment (Lunáčková et l., 23). Chnges in the development of root poplsmic rriers were shown to e relted to Cd uptke nd trnsloction (Lux et l., 24; Mrtink nd Lux, 24; Vculík et l., 29). Considering these fcts, elevted concentrtions of Cd in the rhizosphere my influence root growth nd development of M. plume plnts. In the present contriution, two types of cultivtion of n extensively used medicinl plnt in South Afric, M. plume, were tested for the evlution of uptke nd trnsloction of Cd. An ntomicl study ws performed to investigte the possile effect of Cd tretment on root structure. MATERIALS AND METHODS Growth conditions nd experimentl design Seeds of Merwill plume (Lindl.) Spet, otined from the Botnicl Grden of the University of KwZulu-Ntl in Pietermritzurg, South Afric, were germinted on wet filter pper in Petri dish for 1 d. Therefter, the young seedlings were trnsferred to perlite nd irrigted with tp wter for 4 weeks nd then y hlf-strength Hoglnd s solution for 8 weeks in the greenhouse t Comenius University in Brtislv, Slovki. Two different types of cultivtion were pplied: (1) cultivtion of experimentl plnts directly in hydroponic solution (hydroponics) nd (2) cultivtion of plnts in perlite, which ws irrigted with the sme solutions s used for hydroponics. For hydroponics, the first set of prepred young plnts with lredy developed uls ws trnsferred to 5 L continers filled with hlf-strength Hoglnd s solution. Another set of plnts for perlite ws trnsferred to 1 L pots filled with perlite nd plced in growth chmer. The plnts in oth experiments were grown for 4 weeks t 25 8C, 6 % humidity, 16/8 h light/drk photoperiod, PAR 15 mmol m 22 s 21. For oth experiments, three different tretments were used: (1) control irrigted with hlf-strength Hoglnd s solution; (2) Cd 1 irrigted with hlf-strength Hoglnd s solution contining 1 mg Cd L 21 ; nd (3) Cd 5 irrigted with hlfstrength Hoglnd s solution contining 5 mg Cd L 21. Cdmium ws pplied in the form of Cd(NO 3 ) 2.4H 2 O. The hydroponic solution ws chnged weekly. The pots contining perlite were irrigted with 2 ml of solution on the first dy t the strt of the experiment nd fterwrds with n dditionl 1 ml of solution every seventh dy for 4 weeks. Growth prmeter determintion The plnts were hrvested nd fresh weights of roots, uls nd leves were determined. The totl root length ws recorded nd expressed s cumultive root length per plnt (CRL). The length of individul roots ws clculted from the CRL divided y the numer of roots on ech plnt. The sme mesurements nd clcultions were done with the leves. Fresh roots, uls nd leves were dried t 7 8C for 3 d nd fter tht the dry weight of these orgns ws determined. Determintion of Cd concentrtion Dried smples of roots, uls nd leves were nlysed for Cd y inductively coupled plsm mss spectroscopy (ICP-MS). The smples were dissolved in HNO 3 nd the Cd concentrtion ws determined y n Eln 6 spectrometer (Pe Sciex, Cnd). Antomicl oservtions Series of hnd sections of roots were prepred t 1 mm intervls from the root pex to the se. For suerin visuliztion in fluorescence microscopy, the free hnd sections were stined y.1 % Fluorol yellow 88 dissolved in lctic cid for 3 min nd wshed in distilled wter ccording to Lux et l. (25). The smples were plced into drop of.1 % FeCl 3 dissolved in 5 % glycerin prior to oservtion. The sections for right-field oservtions were stined with toluidine lue. The sections were oserved under n Zeiss Axioskop 2 plus epifluorescence microscope (Jen, Germny) nd documented y n Olympus DP 72 digitl cmer. Sttisticl nlyses The experimentl dt shown in the figures re mens of 12 replictes + s.e. from independent experiments repeted three times. The significnce ws compred t the.5 level using Student s t-test (Sttgrphics Centurion XV, v , SttPoint, Inc.). RESULTS AND DISCUSSION Growth of the plnts The growth of M. plume plnts in hydroponics ws poor, nd only in control conditions did some new dventitious roots form. These exhiited limited growth. The roots of plnts treted with Cd were stunted nd no new root formtion occurred. The mjority of roots in oth Cd tretments grdully died (Fig. 1). The growth prmeters of plnts from this experiment re shown in Tle 1. It ws cler tht cultivtion in hydroponics is not fesile for this species. The growth of plnts cultivted in perlite under control conditions ws dequte nd plnts exhiited increses in oth root nd shoot growth (Figs 2 nd 3). Use of solid sustrte is necessry for this species. Street et l. (29) used sterilized cid-wshed qurtz snd wetted with Hoglnd s solution for cultivtion of this species. In our experiments the growth of M. plume plnts ws retrded fter Cd ppliction; the root system ws reduced in comprison with the control nd leves were shorter nd nrrower, nd yellowish compred with those of control plnts (Fig. 1). Shorter leves of M. plume were oserved fter CdCl 2 tretment, ut Downloded from t University of KwZulu Ntl, Medicl Lirry on Jnury 3, 213

3 Lux et l. Cdmium nd hypoderml periderm formtion in roots 287 A B Control 1 mg Cd L 1 5 mg Cd L 1 FIG. 1. Effect of cdmium on 17-week-old Merwill plume plnts grown for 4 weeks in (A) hydroponics nd (B) perlite. Three different tretments were used: control (C); 1 mg Cd L 21 (Cd1); nd 5 mg Cd L 21 (Cd5). Scle rs ¼ 1 cm. TABLE 1. Growth prmeters of Merwill plume plnts grown for 4 weeks in hydroponics Tretment Growth prmeter Control Cd1 Cd5 Root f. wt (g) Root d. wt (g) Lef f. wt (g) Lef d. wt (g) Bul f. wt (g) Bul d. wt (g) CRL (cm) ARL (cm) Downloded from t University of KwZulu Ntl, Medicl Lirry on Jnury 3, 213 Three different tretments were used: control, hlf-strength Hoglnd s solution; Cd1, hlf-strength Hoglnd s solution contining 1 mg kg 21 Cd; nd Cd5, hlf-strength Hoglnd s solution contining 5 mg kg 21 Cd. ARL, verge root length; CRL, cumultive root length (mens + s.e., different letters represent significnt differences t P,.5). without sttisticlly significnt differences etween control nd treted plnts (Street et l., 29). However, Cd t 2 mg L 21 resulted in significnt reduction of fresh weight of leves, uls nd roots, when compred with the control (Street et l., 29). Similrly in our experiment in the plnts treted with Cd(NO 3 ) 2.4H 2 O, the fresh nd dry weight of roots were significntly lower,.5 %, fter cultivtion in hydroponics with 1 mg Cd L 21 when compred with the control plnts. This difference ws even higher when the higher Cd concentrtion (5 mg Cd L 21 ) ws used. However, the difference etween 1 nd 5 mg Cd L 21 ws not significnt. The sme trend ws oserved for the leves. The min differences were oserved with uls; the fresh nd dry weights of uls of plnts treted with 1 mg Cd L 21 were not different

4 288 Lux et l. Cdmium nd hypoderml periderm formtion in roots Fresh weight (g) Root C Cd1 Cd5 from those of the controls. After higher Cd ppliction (5 mg Cd L 21 ), the fresh nd dry weights of uls were lower, out 5 %, when compred with the control nd 1 mg Cd L 21 -treted plnts (Figs 2 nd 3). The numer of dventitious roots ws lower in plnts cultivted with Cd; the men numers for control, 1 mg Cd L 21 nd 5 mg Cd L 21 were 6.5, 6. nd 4.9, respectively. CRL s well s the verge length of roots (ARL) ws not different etween control nd 1 mg Cd L 21 -treted plnts. However, oth CRL nd ARL were significntly different etween the control nd 5 mg Cd L 21 tretments nd lso etween oth Cd tretments (Figs 4 nd 5). The inhiition of root growth ws proly cused y Cd-induced depolymeriztion of microtuules of the cell cytoskeleton nd chromosome errtions, which resulted in lower mitotic ctivity of meristemtic cells (Fusconi et l., 26; Seth et l., 28), nd thus inhiition of root elongtion. Cumultive lef length per plnt nd lso the verge length of leves were significntly different etween the control nd oth tretments. The growth of roots of crrot nd rdish seedlings ws inhiited t 2 mg Cd L 21 nd this inhiitory effect incresed with incresing concentrtions of Cd in oth liquid culture nd pot experiments (Chen et l., 23). In the present study, the inhiitory effect ws more pronounced t 5 mg Cd L 21, suggesting tht Lef Plnt prt Bul F IG. 2. Effect of cdmium on fresh weight of 17-week-old Merwill plume plnts grown for 4 weeks in perlite. Three different tretments were used: control (C); 1 mg Cd L 21 (Cd1); nd 5 mg Cd L 21 (Cd5). Mens ( + s.e.) with different letters re significntly different (P,.5). Dry weight (g) C Cd1 Cd5 Root Lef Bul Plnt prt FIG. 3. Effect of cdmium on dry weight of 17-week-old Merwill plume plnts grown for 4 weeks in perlite. Three different tretments were used: control (C); 1 mg Cd L 21 (Cd1); nd 5 mg Cd L 21 (Cd5). Mens ( + s.e.) with different letters re significntly different (P,.5). A Cumultive lef length (cm) B Cumultive root length (cm) M. plume seedlings re more sensitive to Cd thn the seedlings of crrot nd rdish. Recently, Street et l. (29) showed tht 2 mg Cd L 21 negtively ffected the fresh weight of leves, uls nd roots of mture plnts of M. plume exhiiting high ccumultion of Cd in the roots. This suggests tht inhiition of root growth t high Cd concentrtion my ffect nutrient nd wter uptke, resulting in inhiition of shoot elongtion. However, there is not enough informtion on the mechnism of Cd toxicity in the root system, which is the min orgn for uptke in plnts (Chen et l., 23). Cdmium concentrtion Lrge differences were found in ccumultion of Cd y different plnt orgns of M. plume (Fig. 6). Roots ccumulted considerly higher mounts of Cd when compred with the uls nd leves. This phenomenon is well known for mny plnt species. It is ssumed tht this is one of the min mechnisms to protect the ove-ground prts of plnts ginst toxic Cd effects (Lux et l., 211). The concentrtion of Cd significntly incresed in roots treted with 1 mg Cd L 21 nd even more Cd ws found in roots when grown t higher Cd stress (5 mg Cd L 21 ). The sme tendency ws oserved in ulous species cultivted t 5 nd 25 mm Cd (Soudek et l., 29). Similrly to the roots, the Cd concentrtions in the other plnt orgns were elevted correspondingly with incresing concentrtion of Cd in the medi. However, the C Cd1 Cd5 Tretment F IG. 4. Effect of cdmium on cumultive lef nd root length of 17-week-old Merwill plume plnt grown for 4 weeks in perlite. Three different tretments were used: control (C); 1 mg Cd L 21 (Cd1); nd 5 mg Cd L 21 (Cd5). Mens ( + s.e.) with different letters re significntly different (P,.5). c Downloded from t University of KwZulu Ntl, Medicl Lirry on Jnury 3, 213

5 Lux et l. Cdmium nd hypoderml periderm formtion in roots 289 A 1 Averge lef length (cm) B 1 Averge root length (cm) C Cd1 Tretment Cd5 FIG. 5. Effect of cdmium on verge lef nd root length of 17-week-old Merwill plume plnts grown for 4 weeks in perlite. Three different tretments were used: control (C); 1 mg Cd L 21 (Cd1); nd 5 mg Cd L 21 (Cd5). Mens ( + s.e.) with different letters re significntly different (P,.5). Cd concentrtion (mg kg 1 d. wt) Root Bul Lef concentrtion of Cd ws more thn eight times higher in the roots thn in the leves nd uls when plnts were grown in 1 mg Cd L 21. The differences etween plnt prts were considerly igger when the higher Cd concentrtion (5 mg Cd L 21 ) ws used. Comprle results were otined with C Cd1 Cd5 Tretment FIG. 6. Effect of cdmium ppliction on cdmium concentrtion in roots, uls nd leves of 17-week-old Merwill plume plnts grown for 4 weeks in perlite. Three different tretments were used: control (C); 1 mg Cd L 21 (Cd1); nd 5 mg Cd L 21 (Cd5). Mens ( + s.e.) with different letters re significntly different (P,.5). c c d CdCl 2 tretment of the sme species y Street et l. (29). The concentrtion of Cd in the roots ws pprox. 15 times higher compred with the uls nd out 3 times higher when compred with the leves. The concentrtion of Cd ws the sme in the leves nd the uls when lower Cd concentrtion (1 mg Cd L 21 ) ws used, nd out twice s high in the uls thn in the leves t higher Cd tretment (5 mg Cd L 21 ) (Fig. 6). Soudek et l. (29) hve oserved tht experimentlly tested ulous plnts retined on verge 75 % of ccumulted Cd in the roots, nd only 18 % nd 7 % in the uls nd leves, respectively. Despite the trnsloction fctor eing quite low in M. plume, the Cd ccumulted in the uls exceeded, t ll experimentl vrints, the limit for mximum Cd content (.3 p.p.m.) set y the WHO for medicinl plnts. Another medicinl plnt species, Tulghi violce, ccumulted Cd in its uls in the rnge of mg g 21 d. wt depending on the size of test plnts fter 6 weeks tretment with 2 mg L 21 Cd. Tulghi violce ccumulted Cd in the leves t the level of nd mg g 21 t 2 nd 5 mg L 21, respectively, depending on plnt ge the older the plnt, the less ccumultion (Street et l., 21). Severl plnt species, such s Thlspi cerulescens, Aridopsis hlleri nd Sedum lfredii, cn ccumulte Cd in concentrtions exceeding even the level set for hyperccumultors (which is 1 mg Cdg 21 d. wt) (Slt et l., 1995; Zho et l., 26; Deng et l., 27; Bnsov et l., 28). Usully importnt crop plnts, such s mize, rice, whet nd sunflower, do not rech this hyperccumultion limit when treted with Cd concentrtions, 1 mg kg 21 of soil (Greger nd Lnderg, 28; Fessler et l., 21; Liu et l., 21). Monocotyledonous ulous plnt species, such s Allium cep, Allium stivum, Allium schoenoprsum nd Allium porrum, ehve similrly, ut in some cses they cn exceed the hyperccumultion limits when grown under specific conditions, mostly in hydroponics nd t high Cd concentrtions in the medi (Soudek et l., 29). Allium schoenoprsum ws le to ccumulte this metl to.17 nd 2 mg g 21 in roots nd leves, respectively, during 28 d of 5 mm Cd tretment (Brzni et l., 24). Root ntomy The dventitious roots of M. plume seedlings show the typicl structure of monocotyledonous root (Fig. 7A). One lyer of rhizodermis is formed y smll cells s the outermost tissue. The cortex is reltively rod, with one-lyered hypodermis. Hypoderml cells re rdilly extended nd develop Csprin nds nd suerin lmelle 1 15 mm from the root pex (Fig. 7B, C); this lyer cn e clssified s exodermis (Peterson nd Peruml, 199). Corticl prenchym is multilyered nd cells re lterntively rrnged with smll intercellulr spces. The single-lyered endodermis develops Csprin nds close to the root pex, t distnce of pprox. 3 mm. A secondry stte of endoderml development (suerin lmelle development) occurs pprox. 3 mm from the root pex. The centrl cylinder is nrrow; it consists of one-lyered pericycle, 6 12 rdilly rrnged poles of xylem nd phloem elements, nd centrl prenchymtous region. Downloded from t University of KwZulu Ntl, Medicl Lirry on Jnury 3, 213

6 29 Lux et l. Cdmium nd hypoderml periderm formtion in roots A D B C F IG. 7. Cross-sections of dventitious roots of Merwill plume. Plnts were grown in control conditions in perlite (A C) nd treted with 5 mg kg 21 Cd (D F). Roots in control conditions exhiit the typicl structure of monocotyledonous roots formed y single-lyered epidermis nd single-lyered exodermis (A, B); exoderml cells develop suerin lmelle close to the root pex, s shown in C fter Fluorol yellow 88 stining in fluorescence microscopy. After cdmium tretment the hypoderml periderm is formed in the peripherl corticl zone close to the root pex (D F). Cork cmium produces cells y periclinl division (E); these ecome impregnted y suerin, s shown in F fter Fluorol yellow 88 stining in fluorescence microscopy. The distnces of sections from the root pex re 5 mm (A, B, D, E) nd 3 mm (C, F). Arevitions: ep, epidermis; ex, exodermis; me, mesodermis (mid-corticl lyers); en, endodermis; cc, centrl cylinder; sterisks indicte periclinl divisions in cork cmium. Scle rs: (A, D) ¼ 1 mm, (B, C, E, F) ¼ 5 mm. E F Downloded from t University of KwZulu Ntl, Medicl Lirry on Jnury 3, 213 The oldest prt of M. plume root, close to the ul, is chrcterized y the presence of contrctile zone. Contrctile roots pull uls down into the soil. The ssocition etween contrctile roots, wter uptke nd hitt ridity ws investigted for gves, yuccs nd loes y North et l. (28). Cdmium ppliction in our experiments t higher concentrtion (5 mg Cd L 21 ) cused chnges in the root tissue orgniztion nd development. Exposure to Cd induced the formtion of cork cmium in corticl tissues internlly, djcent to the exodermis (Fig. 7D). Cells divided periclinlly nd the derivtives impregnted their cell wlls with suerin (Fig. 7E, F). Hypoderml periderm formtion occurred in the young, su-picl prt of the root. The first periclinl divisions were present pprox. 5 mm from the root pex. The root of most monocotyledons is covered y the originl primry epidermis during their whole life; under some conditions it ecomes lignified or even suerized. These cell wll modifictions my reduce the growth of plnt orgns nd cn ffect the trnsport of nutrients nd wter. Only few studies of periderm formtion in monocotyledons hve een completed, mostly focused on rorescent monocotyledons (Dickinson, 2). In ulous plnts, wound periderm formtion ws oserved in roots fter infection cused y

7 Lux et l. Cdmium nd hypoderml periderm formtion in roots 291 FIG. 8. Cross-section of dventitious roots of n dult Merwill plnt growing in soil (Botnicl Grden of University of KwZulu-Ntl, Pietermritzurg, South Afric) stined with Fluorol yellow 88 in UV light. The formtion of multiserite cell lyers with suerized cell wlls (*) resemling typicl periderm occurs fter injury of the root surfce. Scle r ¼ 1 mm. vrious species of Penicillium (Sltink, 1971). Exceptionl periderm is sometimes present elow the exodermis in some species of Asprgles (Kuff et l., 2). We suggest tht formtion of hypoderml periderm in young su-picl prts of Merwill roots represents defence rection of roots exposed to toxic metl tretment. Periderm, with impermele cell wlls, impregnted y suerin my reduce rdil uptke of Cd ions y roots. The formtion of protective su-epiderml lyers in response to Cd tretment is interpreted here s hypoderml periderm or wound periderm formtion. It cn proly lso e interpreted s multiserite hypodermis. This interprettion is sed on multiserite hypodermis shown in roots of some monocotyledonous species y Sego nd Mrsch (1989), Sego et l. (2) nd Heimsch nd Sego (28), nd in Iris y Meyer et l. (29). These studies showed developmentl schemes of multiserite exodermis nd hypodermis which cn e formed from the root picl meristem (RAM), in some cses mny millimetres ehind the tip. Exodermis sent in control conditions nd induced y slt stress ws found in roots of dicotyledonous plnt species (cotton) y Reinhrdt nd Rost (1995). These dt suggest the possiility of formtion of su-epiderml lyers oserved y us s result of meristemtic ctivity derived from the RAM. We cn support our opinion out the formtion of secondry meristem in su-epiderml root lyers of Merwill in response to stress y dditionl oservtions. Adventitious roots of dult plnts growing in soil (Botnicl Grden of University of KwZulu-Ntl, Pietermritzurg, South Afric) were investigted. In wounded or dmged res of older root prts, formtion of multiserite cell lyers with suerized cell wlls resemling typicl periderm (wound periderm) of dicotyledonous roots ws oserved (Fig. 8). Depending on interprettion, we cn cll these structures in monocotyledonous plnt species hypoderml periderm, or regrd their formtion s result of dditionl divisions of corticl cells. Otherwise we cn ccept the possiility of cork cmium formtion resulting in periderm lyers in monocotyledons. Conclusions Accumultion of toxic mounts of hevy metls in plnts represents helth thret for humns. Medicinl plnts used in trditionl medicine re little studied with respect to Cd intke. The cpcity of M. plume, n intensively trded medicinl plnt of South Afric, ws studied here. The dt show tht this plnt cn ccumulte considerle mounts of Cd. Plnts limit shoot Cd ccumultion y restricting Cd movement to the xylem through oth the symplsmic nd the poplsmic pthwys. Restriction of poplsmic movement of Cd ws found in vrious plnt species y root cell wll modifictions nd impregntions, ccelerted mturtion of poplsmic rriers exodermis nd endodermis nd lignifiction of peripherl root tissues. In roots of M. plume, erly formtion of hypoderml periderm close to the root pex occurs fter Cd tretment. Periderm is only rrely developed in roots of monocotyledonous plnts; therefore, nother interprettion my e dditionl periclinl division of hypoderml lyers. We hypothesize tht this process nd production of secondry tissues impregnted y suerin in M. plume treted y Cd hve functionl significnce in reducing the rdil trnsport of toxic Cd ions to the xylem nd susequently to the shoot. This my e novel defence rection of plnts exposed to Cd. ACKNOWLEDGEMENTS We thnk the Ntionl Reserch Foundtion RSA Slovk Collortive Project, South Afric nd the Slovk Reserch nd Development Agency (grnt APVV SK-ZA-7-7) for finncilly supporting the ilterl collortive project, nd the Slovk Grnt Agency VEGA (1/472/1). The uthors re grteful to oth referees for vlule nd stimulting comments nd suggestions. LITERATURE CITED Bnsov V, Hork O, Nduinsk M, Cimporov M, Lichtscheidl I. 28. Hevy metl content in Thlspi cerulescens J. et C. Presl growing on metlliferous nd non-metlliferous soils in Centrl Slovki. Interntionl Journl of Environment nd Pollution 33: Brzni O, Dudi N, Khdk UR, Goln-Goldhirsh A. 24. Cdmium ccumultion in Allium schoenoprsum L. grown in n queous solution. Chemosphere 57: Chen YX, He YF, Luo YM, Yu YL, Lin Q, Wong MH. 23. Physiologicl mechnism of plnt roots exposed to cdmium. Chemosphere 5: Coetzee MAS Wter pollution in South Afric: its impct on wetlnd iot. In: Cown G. ed. Wetlnds of South Afric. 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