measurement of pb, cd, cu binding with mucilage exudates from maize

8/19/2019 Measurement of Pb, Cd, Cu binding with mucilage exudates from maize

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Biol Fertil S oils (1986) 2: 29 - 34

Biologyand Fertih ty

of S o i l s

© Springer-Verlag 1986

Me asu rem ent of Pb 2 , Cu 2 and Cd 2 binding with

mucilage exudates from maize Zea mays L .) roots

J.L. Morel, M. Mench, and A. Guckert

Departme nt of Plant Science, Ecole Nationale Sup6rieure d Agro nomie et des Industries Alimentaires, 2, Avenu e de la For6t de Haye,

F-54500 Vandoeuvre, France

Summary.

The pectic nature of root mucilages sug-

gests a hypothetical act ion of these substances on

heavy metal flux into the root . In this study the

existence of relat ions betwe en heavy metals and root

mucilages were verified and quantified. In order to

obtain substantial amounts of pure root mucilages,

two m ethod s of collect ion were developed, using: (1)

maize plants grown in the field and (2) hydroponic

axenic cultures. T he study of mucilage-me tal binding

was con ducte d using the dialysis me thod , which was

develo ped in a previous work. Results show that root

mucilages are able to bind metals. The imp ortance of

the binding depends on the nature of the cat ion,

fol lowing the order Pb > Cu > Cd. These reactions

could be due to exc hange processes involving muci-

lage ca tions (Ca 2+, Mg 2+) and he avy m etals. T he role

of mucilages on the reten tion of heavy metals in the

rhizosphere is also discussed.

Key words: Root mucilages - Maize - Lead - C opper

- Cad miu m

Evidence that roots of numerous plant species are

embedded in a mucilaginous layer is supported by

severa l studies. This ma terial is rele ased into the soil

by outer root cap cel ls (Mo llenhauer et al . 961;

Rougier 1971) and is also derived from epidermal

cells located behind the t ip (Floyd and Ohlrogge

Offprint requests to J.L. Morel

1970; Foster 1982). Transmission electron microsco-

py studies have shown tha t the r oot m ucilages exhibit

a double granular and fibri l lar structure (Jenny and

Grossenbacher 1963; Rougier 1971; Guckert et al .

1975; Foster 1982). Mucilages are co mpo sed mainly

of high molecu lar weight polysaccharides, > 15.10

(Cortez and Billes 1982), containing fucose, galacto-

se, glucose, arabinose, xylose and mannose units

(Floyd and Ohlrogge 1970; Paull et al . 1975; Cha-

boud 1983). Especial ly a notable proport ion of ur-

onic acids (galacturonic acid) has been observed

(Jones an d Mo rre 1967; Floyd and O hlrogge 1970;

Wr ight and N orth cot e 1976; Paull et al. 1975; Cor tez

and Bil les 1982). These c omp ounds are sup posed to

display som e prope rt ies o f the pectin gels as well as to

have hydrophil ic characters and functions as cat ion

exchangers. This hypothesis has important implica-

t ions for plant nutri t ion. B ecause of their molecular

weight , the m ucilages are suppose d not to b e diffusi-

ble in soil or if so very slowly (Rovira 1969). Thus,

mucilages could play a major role in the r etention of

metal l ic cat ions in the rhizosphere (Brams 1969;

Lepp ard a nd Ra ma mo orth y 1975; Horst et al. 1982).

Over the past 20 years, concern has grown over the

accumu lation of heavy m etals in soils and their t rans-

fer to plants. H owev er, very l i tt le is known abo ut the

reactions involved in uptake of heavy metals from the

rh izosphere . The present work was conducted in

ord er to evaluate the role of mucilage-metal interac-

t ions on the influx of heavy metals. The aim was to

provide information about the mucilage-binding ca-

pacity for metal ions. Such work needs sufficient root

mucilages. This was achieved by two methods of



30 J L Morel et al : Binding of heavy metals with root mucilages

collect ion without al tering the material . The abil i ty

of mucilages to bind three heav y metals, Pb, Cu and

Cd, was m easured in combinat ion wi th a technique

based on dialysis and descr ibed previously (Morel et

al. 1983).

each culture tube. Aliquots of nutrient solutions were

transf erred to plates with nutrient agar and incubated

for 2 days at 28°C. The contaminated tubes were

discarded . The co l lec t ion was made by vacuum pump-

ing and the product ion es t imated to be 8 mg dry

mat ter muci lages per gram of dry roo ts.

aterials and metho ds

Collection of root mucilages.

Root muci lages were

obta ined from maize p lants Zea mays L., var LG 11)

grown u nde r non-steri le and steri le conditions.

Non-sterile conditions. Maize has seminal roots and

nodal roots. Un der a high hygro metr ic level nodal

roots are em bed ded in a large mucilage coating. This

property prev iously used by Floyd and Ohlrogge

(1970) is of grea t interest in the study of root mucila-

ges. Durin g the first stage of developm ent the nodal

roots a re aer ia l s and thus are no t contaminated by

soil particles (e.g. , clays). Furthe r, im portant quanti-

t ies of mucilages can be collected because nodal roots

are part icu lary large. M aize plants grown in the field

and having a crow n of young nodal roo ts were harvest-

ed . A eria l par t s and lower roo ts were d iscarded . The

pieces of stalk we re then imm ersed in dist il led water.

The layer of muci lage which f i rmly adhered to the

root was collected by suction using a narrow plast ic

tube l inked to a collect ion glass tube und er vacuum .

Two harvests made on the same p lan t p roduced

about 6- to 8-mg dry matter mucilages.

Axenic conditions.

Steri le hydroponic cultures were

used in ord er to obtain mucilages of plant origin not

contaminated by microorganisms or so i l compo-

nents. The system consisted of a series of 35 x

480-mm glass tubes obturated at the bottom by a

si l icified rubber cap and at the top by cotton wool.

Each tube conta ined 50 ml of a Hoagland p lan t

nutrient solution. A hypodermic neddle was driven

into the ru bber cap and co nnec ted to an air fi l t ration

system (0.2-/ ,m porosi ty) by a polycarbonate tube.

Thus, the solution culture could be aerate d, a condi-

t ion which is essential to the p roduction of mucilages

by the roots (Trollden ier and Hech t-Buch holtz 1984).

The series of tubes were steri l ized (120°C for 30

min). Maize seeds w ere surface steri l ized by washing

with 95 ethano l and imme rsing for 45 min in a 10

H202 solution. E ach tub e con tained six seeds which

were introduced under axenic condit ions. The cul-

ture system was kept in the darkness for the germina-

t ion period (3 days). For the nex t stage the condit ions

were as fol lows: i l lumination, 14 h; temperature:

day, 25°C; night, 22°C; duration, 12 days. The day

before the collect ion, a steri l i ty check was made on

Preparation and analysis of mucilages. The crude

mucilages were centrifuged for 30 min at 5000 g to

rem ove debris and most of the sloughed-off cells. T he

super natan t was then dialysed 3 t imes against deion-

ized water (Visking Tubing 12 000- to 14 000-MW

membrane). For steri le mucilages the purificat ion

was cond ucted in the presence of a cat ionic exchang e

resin in the H + form (IR 120). Th e ma terial was then

lyophil ized and stored un der vacu um bef ore analysis.

It was analysed quanti tat ively for C, N (Carlo-Erba

CHN appara tus) , po lysaccharides (phenol reagent ;

Dubois et al . 1956), pr oteins (Folin phenol reage nt;

Lowry et al. 1951), uronic acids (carbazole reagent;

Bitter and Muir 1962) and qualitatively for sugars

(gas chromatogra phy of the hydro lysed product ) In-

frared spectra observations wer e made from lyophilis-

ed m ucilage-KB r pellets.

Study of the interactions between mucilages and me-

ta/s. This part of the work was conducted by the

equil ibrium dialysis procedure (Zunino and Martin

1977; Blaser et al. 1980). With th e help o f a sem iper-

meable membrane the separa t ion of an organome-

tall ic system in two co mpa rtme nts is obtained which

makes the dist inct ion possible between the free and

the bound metal . This method was described in a

previous paper (Morel et al. 1983).

A first experim ent was designed to test the abil ity

of the meth od to me asure m uci lage-meta l b ind ing.

Tw enty mil l il i tres of a 1-g 1 nodal root m ucilage

suspension was intr oduce d into a dialysis bag which

was imm ersed in a metal l ic solution adjusted to pH 6

and containing sufficient KNO3 to maintain a con-

stant ionic strength in the system (/z = 0.01). Af ter

equil ibrium the concentrat ion of metal in the exter-

na l med ium was measured . I t was made possib le to

calculate the quanti ty of metal bound to the muci-

lages by comparison with results obtained from a

check system where muci lages were rep laced by the

e lec t ro ly te ad justed to pH 6 . Three meta ls were

studied (Pb, Cu and Cd) using, for each, the same

init ial concentrat ion .

The second b inding experiment was perform ed to

determine the abil i ty of mucilages to bind heavy

metals and the energy of the associat ion. A 20-ml

mucilage suspension (250-mg 1 ~ mucilages issued

fro m noda l ro ots, 100-mg 1-1 sterile mu cilages) was

introduced into a dialysis bag and immersed in a



J .L. Morel et al . : Bindin g of heavy metals with root muci lages 31

200-ml metall ic solution adjusted to pH 6 and 0.01

ionic strength. After a 24-h equilibrium period the

f ree meta l concen tra t ion was measured in the ex ter-

nal solution and the metall ic solution subsequently

rep laced by de ion ized water in o rder to rem ove f ree

metal ions. Three dialyses against water were suff i-

cient to obta in a negligible diffusible metal concentra-

t ion in the system. This way , the meta l con ten t o f the

internal solution cou ld be considered as bou nd metal .

I t s concen tra t ion w as measured by a tomic absorp t ion

(Varian AA6) after acidif ication with pure HNO3

(100 tzl/10 ml) . The experiment was repeated for

increasing concentratio ns o f metal . A function simi-

lar to that o f Langm uir (1) was used for the treatm ent

of data collected (Morel et al. 1983):

( Mr) -

M t ) a , 1 )

M b ) K . M B A M B A

where (My ) = free m etal c oncentratio n, Mb) =

bound meta l concen tra t ion , M B A - maximum bind-

ing abil i ty (Zun ino and M artin 1977), K - overall

stabil i ty constant of the association and a _ num ber

of si tes per unit of l igand.

This equation was solved from the distr ibutions of

boun d m eta l concen tra t ions as a funct ion of f ree

metal concentrations. I t was then possible to calcu-

late the two parameters describing the binding of

meta l to the muci lages: MBA and K.

R e s u l t s

Analys is o f mucilages. The mucilages are of a polysac-

charidic nature as show n in Table 1. The m aterial also

contains protein s and a high proportio n of uronic

acids. The consti tutive sugars found by gas chrom ato-

graphy of the hydro lysed mater ia l w ere fucose , r ibo-

Conccn~t aCd.tm of I~ta2

in ~ o ~ t

without ~il age s

4

3

2

I

Pb Iz) Cu { I I ) Cd I I)

able

1. Analysis of muci lages ( of dry mat te r)

C N Poly- Proteins Uronie

sacchar ides acids

Nodal root 38 .9 0 .5 95.0 4 .0 34.0

muci lages

Ster i le 36.9 3.5 94.4 3.6 21.9

muci lages

| I I I I I

4000 3000 2000 1600 1400 1000 600 200

Fig. 1. Infrared spectra of nodal root muci lages

se, galactose, glucose, mannose and xylose. The

infrared spectra showed absorption bands at 1610

and 1725 cm 1, w hich are characterist ic o f carboxyl

groups as salt and acid forms (Fig. 1).

Evidence of heavy m etal binding to root mucilages. In

the f irst dialysis experiment the organometall ic mix-

tu re was par t i t ioned in two compar tm ents , one con-

taining the metal l inked to the l igand and the free

metal , the other the free metal only. Thus, the

decrease o f meta l concen tra t ion in the compar tm ent

without mucilages indicated the binding of some of

the init ial free metal to the mucilages (Fig. 2) . The

quant i ties o f meta l bound to the roo t m uci lages vary

with thena ture o f the ca tion , fo l lowingtheorder : Pb >

C u > C d .

U YaacLln~

Fig. 2. Effect of nodal root muci lages on the

concentrat ion of f ree metal in a dialys is exper i -

m en t



32 J .L. Morel et al . : Binding of heavy metals with root muci lages

The rem oval of the free m etal by successive dialysis

of the organometa l l ic mix ture against water a l lowed

the measurement of the meta l bound more speci f i -

cal ly to the l igand destroying low-e nergy linkages

whos e role is imp ortant only at high concentrat ions of

metal . If we suppose that th e m ore specific si tes were

sa tura ted by the meta l , we could es t imate the re ten-

tion capacity, which varied from 48 to 71 /xmol g-1

mucilages or as expressed in terms of root exchange

capacity, from 10 to 14 mEq 100 g-l.

Determination of M BA and K for the binding o f Pb2+,

Cu2+ and Cd2+ with the mucilag es. Whe n the concen-

t ra t ion of bound meta l

Mb)

is plot ted against concen-

trat ion of the free metal ion (Mr) in equil ibrium,

isotherm curves are obtained (Figs. 3, 4). These are

similar in shape to Langmuir isotherms. For each

meta l over the range of meta l concent ra t ion used

here , the binding of m etals with root mucilages is well

represe nted as in s imple Langmuir i so therms. Thus,

in order to ca lcu la te the b inding parameters , MBA

A{, ~t}

| He l n t ~ am i ~

mas gension

5 ~ • Cugves ~ ~ . tO

I ~_ar ~

l s o t ~ m

I b

10 A

~,

Cd

~}

25 5n ~

Fig. 3. Bindin g of Pb 2+, Cu 2+ and Cd 2+ with n odal root muci lages

250 mg 1-1: dis t r ibut ion isoth erms of bou nd and f ree m etal pH 6, ,~

= o.ol)

,~ Zat~ raue l la~ . Carves coz-~sl~axl to

~ l o n

llnelr ~ isothen~s

2 5.

. ~ C d

b ~ ) ~

10 2

Fig. 4. Bindin g of Pb 2+, Cu 2+ and Cd 2+ with no dal s ter i le muci lages

10 0 m g 1-~:di s t r ibut ion i sotherms of bound and f r ee meta l pH 6 , p .

= 0.01)

Table

2. Maximum b inding abi l i ty MBA) and overa l l s t abi l i t y

cons t an t ca l cula t ed wi th the Linear Langmui r Equat ion

Metals Pb Cu Cd

Mucilages 1) 2) 1) 2) 1) 2)

r 0 .97 0 .99 0 .90 0 .99 0 .95 0 .98

MBA

23.7 16.3

15.2 9.2 9.9 4.4

me q 100 g4 )

log K 4 .17 5 .6 4 .14 5 .4 4 .17 5 .3

1) , nod al root muci lages; 2) , ster i le muci lages; r , correlat ion co-

ef f i c i ent for t he funct ion Mf) / Mb ) = M.D

and K, the l inear form of the Langmuir equat ion

could be applied. T he c orrelat ion coefficients of the

Mf)/ Mb) = f Mf) function were highly significant.

However, this function fi t ted much better to the

experimental data obtained with steri le mucilages

than to those with the mucilages collected under

non-steri le condit ions (Table 2). The calculat ion

gave MB A and logK values somewhat d i fferen t and

lower than those obtained for metal-polygalacturonic

acid complexes (Morel et al. 1983). Values also

depended on the method of muci lage co l lec t ion .

Stabil i ty constants were similar from one metal to

another while the maximum binding abil i ty values

were rather different , exhibit ing the fol lowing se-

quence: Pb > Cu > Cd. This orde r reflects the differ-

ences be tween the i so therms. W hen expressed in the

same uni t s used for exchange capaci ty , MB A showed

values quite similar to that of the root i tself (Table 2).

D i s c u s s i o n

The mucilages collected by the two procedure s adopt-

ed in this work can be considered as the material

exuded by maize roots during their l i fe in the soil .

The mucilages are mainly composed of polysaccha-

rides. Th e proteins could be of microbial origin (no-

dal root mucilages), but could also be derived from

the plant i tself (Wright and N orthco te 1976; Cha-

boud 1983). The high content of uronic acids confirm

the pectic nature of maize root mucilages. From a

physicochem ical point o f view, mucilages are highly

hydra ted mater ia l s which improve the contac t be-

tween roots an d soil part icles. This int imate co ntact

faci l itates exchange processes by ma intaining a l iquid

cont inu i ty be tween the roo t surface and the ad jacent

soil especial ly under dry condit ions ( Nam biar 1976).

However , the experiments carr ied out in th i s work

showed that maize root mucilages are able to bind

heavy metals. The est imation of the binding abil i ty

indicated values approaching the root cat ion ex-

change capaci ty . Such a property may bed ue to the



J.L. Morel et al . : Binding of heavy metals with root mucilages

33

carboxyl groups of the uronic acids located on the

polysaccharide chains. Proteins or glucoproteins

could also be involved in the binding of metals, as has

already been shown for the soluble fract ion of maize

roo t exu date s Me nch et al. 1985). So far, there is

l i t t le information about the structure of mucilage

molecules. Apical mucilages consist of a cellulosic

central core coated with polysaccharides containing

glucose, fucose and galacturonic acid Wright and

Northcote 1976). These units are probably arranged

in blocks as in the pectin cell wall Jarvis 1984). The

presence of uronic acids at the periphery of the

molecule could a l low the form at ion of h igh molecular

weight gels in mediu ms rich in cat ions such as calcar-

eous soils and cat ions act ing as bridges between

consecutive uronic chains Cortez an d Bil les 1982).

Thus, meta ls a re probably bound by two forms to the

root m ucilages depending on the status of their bind-

ing si tes. On one side such ions may be integrated

into the gel by acting as a bridge, while on the oth er

side they could be l inked by more labile forms to

isolated uron ic units.

The h ierarchy observed between the th ree s tud ied

metals suggests that binding results from an exch ange

reaction. Indeed, in the mucilages, carboxyl groups

seem to occu r largely unde r salt forms Nye 1981) as

shown by the infrared spectra. Thus, the introduction

of metal ions into the mucilages would induce an

exchan ge of th e ma jor cat ions, Ca 2+ and Mg 2+, which

are know n to offer less affini ty to the pectin-binding

sites than he avy metals Ma krido u 1977; Mo rel 1985).

From the observed h ierarchy i t may be assumed

that lead wo uld be mor e efficient in desorbing majo r

cations from mucilage si tes than c opper or especially

cadmium . It wou ld be strongly retaine d in the mucila-

ges. Thus, in the rhizosphere, on e can expect a large

retention of metals having a high affini ty for the

binding si tes. Moreover, in the root environment

ma ny othe r com pone nts clays, microbial products,

etc.) which combin e with the root m ucilages forming

a mixed product cal led mucigel Jenny and Grossen-

bacher 1963) can also bind heavy metals. Mucigel

would appear to be an accumulation si te for heavy

metals. S uch an a ccumu lation is of considerable im-

porta nce since heavy metals can alter biodegradation

of mucilages More l 1985).

Fro m a plant nutri t ion point of view, mucilages and

therefore mucigel could modify the f lux of heavy

cations to the root . Due to the viscosi ty of the

medium, the d i ffus ion of any e lement i s p robably

lower than in the soil solut ion. M oreov er, because of

the binding reactions, the flux of high-affinity metals

for the root mucilages Pb, Cu) would decrease in the

rh izosphere , whi le o ther meta ls Cd) would be more

easi ly transferred from the soil to the root surface.

Thus, mucilages could supposedly act as select ive

fi l ters for the heavy cat ions, completing the role of

cell walls from which they are derived .

These results may be related to field experiments

which are concerned with the response of plants to

excess metals in the soil. S ome elemen ts Cd) are

known to be easi ly transferred to the aerial parts of

the plants while others Pb) are retained in the root

system. Without making causal relat ionship, i t is

remark able to no te tha t e lem ents concerned with a

rapid translocation to maize leaves correspond with

those which offer a relat ively low affini ty for the

mucilage substances. It could be assumed that the

readily transferred metals are those which can be

kept under soluble forms in the rhizosphere. More-

over , m ucilages as well as cell walls could be involved

in the select ive uptake of heavy metals by different

plants.

In conclusion, the determination of the risks of

heavy m etal t ransfer in the soil plant system needs to

take into account rhizosphere condit ions and espe-

cially those which govern th e solubil ity of the elem ent

in the roo t envi ronment .

Acknowledgments

The authors gratefully acknowledge the sup-

port for this research by the French Ministry of the Environm ent.

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measurement of pb, cd, cu binding with mucilage exudates from maize

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