Heavy Metals in Soils, Vegetation Development and Heavy Metal Tolerance in Plant Populations from Metalliferous Areas Author(s): Eric Simon Source: New Phytologist, Vol. 81, No. 1 (Jul., 1978), pp. 175-188 Published by: Wiley on behalf of the New Phytologist Trust Stable URL: http://www.jstor.org/stable/2431706 . Accessed: 17/06/2014 16:39 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. . Wiley and New Phytologist Trust are collaborating with JSTOR to digitize, preserve and extend access to New Phytologist. http://www.jstor.org This content downloaded from 194.29.185.209 on Tue, 17 Jun 2014 16:39:59 PM All use subject to JSTOR Terms and Conditions
Transcript
Heavy Metals in Soils, Vegetation Development and Heavy Metal Tolerance in Plant Populationsfrom Metalliferous AreasAuthor(s): Eric SimonSource: New Phytologist, Vol. 81, No. 1 (Jul., 1978), pp. 175-188Published by: Wiley on behalf of the New Phytologist TrustStable URL: http://www.jstor.org/stable/2431706 .
Accessed: 17/06/2014 16:39
Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp
.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].
.
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HEAVY METALS IN SOILS, VEGETATION DEVELOPMENT AND HEAVY METAL TOLERANCE IN PLANT POPULATIONS FROM
METALLIFEROUS AREAS
By ERIC SIMON* Laboratoire de Genetique Ecologique et de Biosystematique, UniversitM Libre de Bruxelles
(Received 5 December 19 77)
SUMMARY
The development of vegetation (mainly the Violetum calaminariae Schwick.) in heavy metal- contaminated areas depends on the metals mobility in soils and on metal availability for plants. Moreover, the ability of plant populations to evolve metal tolerance is one of the most important characters which determines the structure, density and development of the vegetation in such areas. In this work, metal mobility in soils and availability to plants in both calcareous and non-calcareous situations were investigated in relation to the develop- ment of genetical heavy metal tolerance in plant populations.
In soils, exchangeable metals amounts are linearly related to total amounts. Availability of metals for plants depends on soil pH and on organic matter contents. High calcium con- tent in soils reduced lead toxicity more than zinc toxicity and generally reduced metal uptake but some exceptions were found. The structure and the density of the vegetation colonizing calcareous and non-calcareous places is related to the interaction between lead, zinc and exchangeable non-toxic cations. A relationship between exchangeable Pb++/Ca++ in soils and the lead tolerance level of plant populations was found. The relation between exchangeable Zn++/Ca++ and zinc tolerance level was not satisfactory.
INTRODUCTION
Ecological and pedological characteristics of metalliferous areas have been extensively studied (Duvigneaud, 1958; Duvigneaud and Denaeyer-De Smet, 1960; Baumeister, 1967; Ernst, 1974). The problems related to heavy metals and plant populations has been reviewed by Antonovics, Bradshaw and Turner (1971). Amounts of heavy metals in metalliferous soils are so high that normal development of the vegetation would be prevented. However, specialized plants have colonized the greatest part of the metalliferous areas, associated in typical plant communities. In Western Europe, the most widely distributed of those plant associations is the Violetum calaminariae Schwick., 1930. In fact, plants growing on these metalliferous substrates contain high amounts of heavy metals and resist contamination by being genetically tolerant to the metals present in the soil (copper, lead, zinc, nickel and cadmium). This was shown for example by Bradshaw (1952), Wilkins (1957), Gregory and Bradshaw (1965), Lefebvre (1975), Coughtrey and Martin (1977) and Simon (1977).
* Universit6 Libre de Bruxelles, Laboratoire de G6n6tique Ecologique et de Biosyst6matique, Jardin Experimental Jean Massart, 1850, Chausste de Wavre, 1160 Bruxelles, Belgium.
The development of metallicolous vegetation depends on the interaction between numer- ous factors; pedological and genecological. The experiences of Wilkins (1960) showed the importance of the interaction between metallic ions and other cations present in the soil as calcium, magnesium and potassium, in determining the soil toxicity. Similarly, the action of organic matter in locking up heavy metals must be taken into account (Dykeman and De Sousa, 1966).
In this work the development of the vegetation has been investigated in two situations showing marked differences in calcium content of soils. Relationships between zinc and lead tolerance in plant populations and edaphic factors are discussed.
MATERIALS AND METHODS
Two metalliferous areas were principally studied: Plombieres (Belgium) with soils poor in calcium; and Breinig (Germany), with a soil saturated in calcium from a calcareous bedrock. Soil characteristics are described in the text. The area of Mausbach (Germany) was not so extensively studied but a Festuca ovina L. population was collected in order to have a com- parison with the population from Breinig. In Mausbach, the sandy soil is rich in calcium carbonate (pH 7.2), zinc and lead.
Soil analysis From each site, soil samples were analysed for chemical components in three different
ways. (1) Total heavy metals: extraction with concentrated nitric acid at 250C, (10 g soil sample dried at 1050C in contact with 25 cc HNO3 for 48 h). (2) Exchangeable cations: extraction with ammonium actate 1 M at pH 7, (10 g soil sample dried at 1050C in contact with ammonium acetate during 12 h. Percolation with 250 cc ammonium acetate during 3 h). (3) Water-soluble elements: extraction with distilled water at 250C. Samples were shaken, (10 g soil sample, dried at 1050?C, in 25 cc H20 for 48 h).
After extraction, zinc, cadmium, and lead amounts were determined by atomic absorp- tion (Perkin-Elmer 303, in flame air-C2H2 for Zn, X = 21.33 A, for Pb, X = 28.33 A, for Cd, X = 22.88 A). For total heavy metal analysis, standards with 10 ml HNO3 (65%) were used. Exchangeable metals were analysed with standards in CH3COO-NH4 with 10 ml HNO3 (65%)/l. The soil extracts were identically acidified. Soluble metal extracts were acidified (10 ml HNO3 (65%)/l) and analysed with similar standards. In all cases, no interaction be- tween lead and calcium was found. Magnesium was also analysed by atomic absorption while sodium, calcium and potassium were determined by flame photometry (Eppendorf). The amounts of organic matter in soils of Plombieres were determined by loss on ignition with the soil fraction sifted at 2 mm, (10 g, dried at 1050C were carbonized at 4500C during 12 h). Organic matter contents in calcareous soils of Breinig were determined by the method of Anne (1945), (0.2 g of soil sifted at 0.4 mm, dried at 1050C, were analysed with K2Cr2O7, 10%. Titration of K2Cr2O7 after oxidation of soil carbon). Calcium carbonate was determined with the calcimeter of Schleiber-Dietrich.
Plant analysis Plant samples (aerial green parts) were collected in different localities. They were washed
with distilled water and dried at 1050C and reduced to powder. 1 g of powder was digested by a nitric-perchloric acid mixture. After filtration the samples were analysed by atomic absorption (Perkin-Elmer 303), for zinc, lead and cadmium.
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Method of testing for tolerance Heavy metal tolerance was investigated on populations of Festuca ovina L., from heavy
metal contaminated and from normal situations. Populations were transplanted for 2 years in the experimental garden 'Jardin Jean Massart' of the University of Brussels. Each popula- tion consisted of a minimum of twenty-five individuals. Heavy metal tolerance was deter- mined by a method similar to that of Wilkins (1957) based on comparative root growth in toxic and non-toxic solutions.
In each population twenty-five individuals were tested. For each individual, tillers were rooted in distilled water. When roots reached a constant rate of growth, the longest root of each tiller was measured. Tillers of each individual were then transferred to zinc and lead solutions while a tiller of each individual was maintained in distilled water. In toxic solu- tions, zinc was added as sulfate while lead was added as nitrate. After 5 days, the longest root was measured again. The growth increments of tillers in toxic and in control solutions were calculated. The ratio:
100 mean of longest root increment of individuals in toxic solution mean of longest root increment of individuals in non-toxic solution
is known as the 'tolerance index' of the population for a given metal used at a specific con- centration. Standard errors on the tolerance index are calculated by the method of Bliss (1967).
RESULTS Heavy metals in soils
Plombieres The metalliferous area of Plombieres includes a mosaic of small zones where the soils
have different structure and chemical composition. Numerous surfaces are covered by blast furnace slags from zinc and lead ores. Other places show soils where some slags were mixed with earth deposits. One part of the site is covered with earth deposits mixed with old pieces of house foundations and where metal slags are rare.
The word 'substrate' is more convenient than the word 'soil' for describing this mass of slags mixed with earth because 'soil' implies that pedogenesis has taken place which it has not because these artificial deposits are of recent origin. The response of the vegetation to these variations in the substrate is extremely clear. No vegetation is growing on slag-rich substrates. When slags are mixed with small quantities of earth we see the development of a pioneer turf with Armeria maritima (Mill.) Willd., Festuca ovina L. and Viola calaminaria Lej . The earthy zones where slags are better incorporated in soil, are colonized by a dense turf, where Festuca ovina is dominant. In contrast with these turf zones, we observe the develop- ment of a forest with Betula verrucosa Ehrh. and Salix caprea L. where the very earthy soil contains house foundations. The map of the mine area of Plombieres shows the relative im- portance of these vegetation groups (Fig. 1).
Nitric acid extractions (Table 1) show the distribution of total heavy metals which are related to the types of vegetation already described. The amounts of lead are the most inter- esting because a relation is observed between total lead amounts in soils and the type of vegetation. Lead amounts are the lowest in soils from the forest and from the dense well developed turfs. Soils of the pioneer turf are richer in lead and the substrates without any vegetation show the highest level for this metal.
Zinc total amounts are similar in soils collected in turfs but they are lower in soils from
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Fig. 1. Schematic map of the metalliferous area of Plombi6res. Types of vegetation and values of the ratios: 'exchangeable lead/exchangeable calcium' are indicated.
Table 1. Amounts of total heavy metals in the soils of Plombieres and Breinig (nitric acid extraction). The samples were sifted with a 2-mm sieve. Number of soils tested and standard
errors are indicated pH Organic Lead Zinc Calcium
matter (mg/kg) (mg/kg) carbonate (%) (%)
Plombieres Soils collected in a surface without 5.8 9.6 35833 19376 -
Fig. 2. Relation between total and exchangeable lead in the soils from Plombi6res.
the forest. The pH values varied for soils covered by different types of vegetation. The vegetation was more dense where the soil pH values were higher. The soils collected in the forest showed free carbonates.
Exchangeable cation analyses (Table 2) indicated that a relation exists between lead amounts and the development of the vegetation. The density of the vegetation increases when amounts of exchangeable lead decrease. A relation between total and exchangeable lead quantities was observed (Fig. 2).
A good relation was found between soil pH values and exchangeable lead quantities. If solubility of lead in soils is expressed by the ratio 'total Pb/exchangeable Pb' one observes that it is independent from the pH when its values are under 6.0.
Above this pH value, lead released in soil is linearly related with the pH (Fig. 3).
8 -
0
7- o
6 - 6 --*-----*--- y: 0- 020x + 5 977
r=0.335
2-5 5.0 7.5 10-0 12-5
Pb totai/Pb+-
Fig. 3. Relation between the pH and the lead solubility in soils of Plombieres. Pb tot/Pb++: ratio total lead amounts/exchangeable lead amounts, which expressed the solubility in lead. White points are for soils which have a pH above 6.0 and black points are taken for soils which have a pH under 6.0;
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Zn total (ppm) Fig. 4. Relation between total and exchangeable zinc amounts in soils (Plombieres). The organic matter amounts are taken into account. 1, soils with a mean value of organic matter -9.6% (regression coefficient: 0.981). 2, soils with a mean value of organic matter = 14.5% (regression coefficient: 0.932). 3, soils with a mean value of organic matter = 20.9% (regression coefficient: 0.960).
Calcium contents in soil increased with the density of the vegetation. The opposite distri- bution was found for lead. As the calcium influence on the pH is well known and as lead availability for plants is affected by the pH of soils, we consider that the soil toxicity could be well described by the ratio 'exchangeable Pb/exchangeable Ca' (Table 2).
Exchangeable zinc in soil had a different distribution than lead. It increased with the density of the vegetation (turfs). A linear relation between total and exchangeable zinc was found for soils having similar organic matter content. This showed a positive correlation between exchangeable zinc and organic matter (Fig. 4). The ratio 'exchangeable zinc/ exchangeable calcium' decreased with the density of the vegetation (Table 2) but not so strongly as the ratio exchangeable lead/exchangeable calcium.
Amounts of soluble elements are indicated in Table 3. Soluble zinc amounts were higher
Table 3. Amounts of soluble cations in the soils of Plombieres and Breinig (distilled water extraction). The samples were sifted with a 2-mm sieve. Number of soils tested and standard
than soluble lead amounts with respect to their own solubility (ratio: soluble amounts/ total amounts).
Breinig The metalliferous hills of Breinig in Germany enclose metallic veins of zinc and lead ores
which are contaminating the calcareous soil. The vegetation in Breinig is a well-developed turf with a great number of species of which Festuca ovina is dominant. The plant associa- tion is the Violetum calaminariae mixed with some elements of the Mesobrometum erecti. The non-contaminated places observed in Breinig are colonized by the Gentiiano- Koelerietum (Knapp, 1942) R. Tx. 1955 (Ernst, 1976).
Table 4. Cadmium in soils of Plombieres and Breinig (total, exchangeable and soluble amounts). For the different situations, minimal and maximal values are given (in mg/kg of
dried soil). Exceptional values in parenthesis Total amounts Exchangeable Soluble amounts
amounts Min. Max. Min. Max. Min. Max.
Plombieres Soils collected in a surface without 19.0 45.0 0.5 5.4 0.0 0.25 vegetation Soils collected in a pioneer 18.5 99.9 4.0 53.3 0.2 0.35 (1.0) turf (20) Soils collected in dense turfs (26) 14.4 111.0 8.9 49.4 0.2 0.40 (1.0) Soils collected in a forest 33.0 70.0 8.5 13.5 0.0 with Salix caprea (6) Breinig Soils collected in a dense turf (20) 52.0 233.0 13.7 46.4 0.0
Nitric acid soil extractions show high amounts of lead and zinc. Zinc is higher than in the soils from Plombieres (Table 1).
Exchangeable amounts of metals (Table 2) are comparatively lower than in Plombieres. The calcareous substrate is responsible for this. The very low ratios exchangeable Pb/ex- changeable Ca and exchangeable Zn/exchangeable Ca indicate a low toxicity but they are still higher than the ratios calculated for soils pertaining to forest in Plombieres.
Exchangeable zinc was positively correlated with total amounts (r= 0.823 for twenty- four soils). The same relation was found with lead but not so closely (r = 0.455 for twenty- four soils). Water extractions (Table 3) confirm the very low solubility of metals when the pH is high.
Cadmium contamination of soils The areas of Plombieres and Breinig are contaminated with large amounts of cadmium.
Table 4 gives total, exchangeable and soluble cadmium contents of soils. In Plombieres and Breinig, relations between vegetation development and cadmium availability for plants can- not be given precisely at the present time. Moreover, cadmium tolerance in plant popula- tions has just been shown (Simon, 1977; Coughtrey and Martin, 1977) and the mechanisms of this genetical tolerance are not yet explained.
Cadmium is distributed rather uniformly in all soils of Plombieres and Breinig and a linear relation between total zinc and total cadmium soil contents was found (r = 0.607 for thirty
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soils of Plombieres and r 0.952 for twenty-one soils of Breinig). Exchangeable cadmium amounts were relatively high and indicated the great mobility of this very toxic metal in soils and its availability for plants. In Plombieres, exchangeable cadmium was linearly re- lated to total cadmium, (r = 0.885 for soils) while this correlation did not appear for soils of Breinig. No relation was observed between exchangeable zinc and exchangeable cadmium in either situation. Soluble amounts in soils were rather low but the solubility of cadmium is very high when compared with zinc and lead solubility. This explains partly the high mobility of cadmium in soil and its high availability for plants. In this way, cadmium tolerance in plant populations appears as a necessary advantage for growing in such situations.
Vegetation stmcture in relation with heavy metals availability for plants Considering the interaction between metals and nutrients, a satisfactory model of the vegeta- tion structure in the metalliferous areas can be elaborated with a ternary relation between lead, zinc and plant useful exchangeable cations (Fig. 5). For drawing this Figure all values
o ioo
0
'3
100 0 0 Zn 100
Fig. 5. Structure of the vegetation in Plombi4res and Breinig in relation with the interactions existing between the metallic and other exchangeable cations in soils. Each point represents a soil and its position in the graph is bounded to the relative abundance of the different ex- changeable elements in a total = 100%. The original values used for building the graph are the amounts of cations expressed in meq/100 g dried soil. 1, Plombieres: zones without vegetation; 2, Plombi6res: Pioneer turfs; 3, Plombi6res: Dense and well-developed turfs; 4, Plombi&res: Forest with Salix and Betula; 5, Breinig (white points): dense turf on cal- careous soil.
of exchangeable cations were expressed in mEq/100 g of dried soil. In this way the impor- tance of valence was considered. All the vegetation groups are clearly individualized. The vegetation from Breinig has a peculiar situation near the lowest toxic zones where we found a forest development in Plombieres. This structure of the vegetation is accorded with the ecological groups described by Simon (1975). The metallophytes* group is well represented on soils where the ratio Pb++/Ca"+ is situated between 5.0 and 17.0 (pioneer turfs) and is well developed on soils where this ratio is situated between 0.5 and 5.0 in association with the mttallovagues indifferentst group. The mktallovagues accidentelst group only develops
* Metallophytes: Taxa growing only on metals contaminated substrates. t Mtallovagues: Taxa found in association with the mhtallophytes in contaminated areas but growing
also outside the metalliferous zone in the same region. They can be 'indiffbrents' growing as well on toxic soils as on normal soils, or 'accidentels' rarely present in metalliferous area and growing better in normal situations.
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where the ratio is under 1.0. This is found where the forest is growing (Fig. 1). A similar observation can be made with the ratio Zn++/Ca"+ but the differences between ecological groups are not so strong (Table 2).
Heavy metals contents in plants and in soils Plant species analyses (Table 5) show specific differences in heavy metals contents. Metals (mainly zinc) are generally more abundant in the dicotyledons than in the monocotyledons. Zinc content of plants is always higher than lead even when total lead in soil is higher than zinc (pioneer turfs). This was found too by Aucquier and De Leval (1974). Lead in plants growing in well-developed turfs (Plombieres and Breinig) is clearly lower than in plants from pioneer turfs. Festuca from Breinig contains the same amount of lead as Festuca from Plombieres though the pH is higher and the ratio Pb++/Ca"+ much lower in Breinig.
Table 5. Lead, zinc and cadmium amounts in different plant species (aerial green parts) col- lected in Plombieres and Breinig. Values given in mg/kg of dried weight
Sites and species Well-developed turfs Pioneer turfs Lead Zinc Cadmium Lead Zinc Cadmium
Differences in plant zinc contents are found for the samples collected in both pioneer and well-developed turfs in Plombieres. Zinc amounts appeared higher in plants from pioneer turfs (except Festuca) even though total and exchangeable zinc in soils are lower than in dense turfs. An interaction zinc-organic matter which limits zinc availability for plants could be taken into account. Plant samples from Breinig show in general lower zinc quantities. This is well observed in Festuca which has zinc content lower than in Plombieres. These results indicate that the ratio Zn++/Ca++ in soil is one of the ecological factors determining the absorption of zinc by plants growing in metalliferous areas. This effect of calcium was described by Warren and Delavault (1949).
Cadmium seems to follow a peculiar distribution. Results (Table 5) show specific differ- ences for cadmium amounts in plants growing in both pioneer and developed turfs in Plombieres.
The plant analysis of species collected in Breinig indicate that plants growing on cal- careous soil are as rich in cadmium as plants developing in neutral conditions. The values obtained for Euphrasia stricta Host. and Hieracium pilosella L. are exceptionally high and demonstrate the high availability of cadmium for plants though the solubility of this metal in soil at high pH is not detectable (Table 4).
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Relations between heavy metal tolerance in plant populations and edaphic factors The amounts of heavy metals in soil represent only one aspect conditioning the distribution of plants in the metal contaminated areas. The ability to evolve tolerance is an essential condition for plant populations to colonize metalliferous areas (Antonovics et al., 1971). There is tolerance to lead, zinc and cadmium in populations of Festuca ovina, Armeria maritima and Agrostis tenuis growing in Plombieres, Breinig and Mausbach. For zinc and lead, Armeria was always more tolerant than Festuca and Festuca was more tolerant than Agrostis (Simon and Lefebvre, 1977). This illustrates that metal tolerance level is a specific character. Simon (unpublished) found that Plantago lanceolata L. and Achillea millefolium L. were tolerant to lead and zinc but much less than Agrostis. All these results must be ap- plied to the ecological groups described previously. It means that the metallophytes (Ar- meria, Festuca) are more tolerant than the pseudometallophytes (Agrostis). The metallo- vagues indiff6rents (Plantago, Achillea) are still less tolerant.
(a) (b)
100 1
801
*E: 60
cu 40 0
203 2
1-0 2-0 5.0 10.0 5-010-0 30.0 50-0
Pb (ppm)
Fig. 6. Comparison between lead tolerance indexes of three Festuca ovina populations. 1, population from PlombiEres; 2, population from Breinig; 3, test population from Frasnes (non-comtaminated situation). (a) Tolerance indexes measured in Pb(NO3)2 solutions; (b) tolerance indexes measured in Pb(NO3)2 + Ca(NO,)2 (0.5 g/l) solutions. Tolerance indexes (It) are expressed in %. Standard errors (%) are indicated.
Concerning relations between metal tolerance and edaphic factors Simon and Lef6bvre (1977) found that there was no direct relation between the level of tolerance in these plant populations and the amounts of total or exchangeable metals in soil.
In this paper, the study of interaction between metal tolerance and edaphic factors has been restricted to the species Festuca ovina, which has a great ability to evolve tolerance. The edaphic factors studied here are the ratios Pb++/Ca++ and Zn++/Ca++.
Wilkins (1957) showed that the lead tolerance index of plant populations was increased when calcium was added to the culture solution. This was observed too with Festuca popu- lations from Plombieres and Breinig (Fig. 6). Calcium and other cations moderate the toxicity of lead and other metals. In this way, the ratio Pb++/Ca++ will be an important fac- tor for evaluating the selective pressure exerted by the substrate which maintains a minimum level of lead tolerance in plant populations. Our results for Festuca populations and sub- populations (Fig. 7) show strong differences between their respective lead tolerance level. Tolerance increases with the ratio Pb++/Ca++ of soils. The addition of nutrients in the culture
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Fig. 7. Relation between lead tolerance index in Festuca populations and the ratio 'ex- changeable lead/exchangeable calcium' of soils where the populations were collected. Festuca populations were tested on a solution of Pb(N03)2 with a Pb concentration of 1 ppm. Br = Population from Breinig. Ma = Population from Mausbach. P1 1, P1 2 and P1 3 -Sub-populations from Plombieres. White columns are the tolerance indexes. Black col- umns are the ratios Pb++/Ca++ of soils. Standard errors on the tolerance indexes (%) are indicated.
solution reduces the toxicity of zinc but this moderation is not as important as for lead. Moreover, populations of Agrostis tenuis Sibth. (collected on both calcareous and non- calcareous polluted soils near a smelting complex in Prayon, Belgium) previously tested for zinc tolerance showed the same tolerance index for both populations (Simon and Lefebvre, 1977).
We found significant differences between zinc tolerance indexes of Festuca from Breinig and Plombieres. Although calcium soil contents in Breinig are much higher than in Plom- bieres, the tolerance index difference cannot be related accurately to the ratio Zn++/Ca++ as seen with results obtained for other populations (Fig. 8). This indicates that the ratio Zn++/Ca++ cannot be considered as a direct measure of the selective pressure maintaining the level of zinc tolerance in Festuca populations. However, this does not mean that calcium is not of importance for moderating Zn toxicity. Further work must be devoted to this problem.
100 - 5 -
e 80 4-
N X 0L CD 60 - 3 -
g: 40 2-
20 - I
Br PLI PL2 Ma
Fig. 8. Relation between zinc toleraiwe index in Festuca populations and the ratio 'ex- changeable zinc/exchangeable calcium' of soils where the populations were collected. Festuca populations were tested on a solution of ZnSO4 with a Zn concentration of 30 ppm. Br = Population from Breinig. P1 1 and P1 2 = Sub-populations from Plombieres. Ma = Population from Mausbach. White columns are the tolerance indexes (%) Standard errors (%) are indicated. Black columns are the ratios Zn++/Ca++ of soils.
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Soil and plant analysis performed in two metalliferous areas (calcareous and non-calcareous) gave the following conclusions. Though total amounts of metals in soils of Plombieres and Breinig appeared very high in both places, the exchangeable and soluble quantities were sig- nificantly different being lower in soils from the calcareous area. Exchangeability of metals depends on the pH and lead is the most affected metal by this factor. Organic matter had a stronger influence on zinc exchangeability than on lead exchangeability.
In neutral conditions (Plombieres) all exchangeable metal amounts were positively corre- lated with total amounts. In calcareous soils, only zinc was strongly correlated. Zinc was the most abundant water-soluble metal in soils. It was still soluble at very high pH though lead and cadmium were completely water-insoluble (Breinig). But, concerning their respective solubility (ratio; soluble amounts/total amounts), cadmium is the most soluble metal fol- lowed by zinc and lead.
Plant analysis showed specific differences in metal contents. In Plombieres, lead in plants appeared under the pH influence and more precisely under the effect of the ratio Pb++/ Ca', but other factors control lead uptake as it was well seen with Festuca from calcareous area which was as rich as Festuca growing in neutral conditions. Zinc contents are influenced by calcium amounts in soils and the ratio Zn++/Ca++ could be a measure of zinc availability for plants growing in metalliferous areas. Cadmium in plants showed a very peculiar distribu- tion being often higher in plants growing on calcareous soils although soluble and exchange- able cadmium was higher in neutral soils. Further studies concerning cadmium uptake by plants and cadmium mobility in soil should be made for explaining it.
The vegetation structure in the areas studied is related to the interaction between lead, zinc and other cations as calcium, magnesium and potassium. However, the most striking factor acting on the vegetation structure and development is the ratio Pb++/Ca++ which de- termines the toxicity of the substrate. Ecological groups can be distinguished in relation with this toxicity. Lead and zinc tolerance level in plant populations are not directly related to total, exchangeable or soluble amounts of metals in soil. For some populations tested here, lead tolerance level was in relation with the ratio Pb++/Ca++ of soils. This ratio ap- peared as a measure of the selective pressure exerted by the substrate on the Festuca popu- lations. On the contrary, zinc tolerance was independant from the ratio Zn++/Ca++. This indicates that different tolerance mechanisms occur for zinc and lead. In fact, the mechan- ism of zinc tolerance must be internal (Antonovics et al., 1971). Recently, Mathys (1977) described a mechanism for zinc tolerance based mainly on the action of malate which acts, in tolerant plants, as a complexing agent for zinc within the plasma. Lead tolerance could be due to an exclusion mechanism and this mechanism would be partly an external one (Anto- novics et al., 1971). Our results indicate that lead tolerance depends more on the availability of this metal in soil while zinc tolerance is more independent from this factor. This indicates that external mechanism such as lead tolerance is more sensitive to the selective pressure exerted by the available metal amounts in soil than the internal mechanism assuring zinc tolerance.
Further work will be devoted to the study of selective pressure due to edaphic factors and we propose to test other factors applied to a greater number of plant populations.
ACKNOWLEDGMENT
I wish to thank Dr C. Lefebvre for his kind advice and for reviewing the manuscript. The
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author is indebted to the 'Institut pour 1'Encouragement de la Recherche Scientifique dans l'Industrie et I'Agriculture' for a maintenance grant.
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