Zinc and cadmium accumulation and tolerance in populations of Sedum alfredii

Environmental Pollution 147 (2007) 381e386 www.elsevier.com/locate/envpol

Zinc and cadmium accumulation and tolerance in populations of Sedum alfredii D.M. Deng a, W.S. Shu a, J. Zhang a, H.L. Zou a, Z. Lin a, Z.H. Ye a,*, M.H. Wong b,** a b

State Key Laboratory for Bio-Control, and School of Life Sciences, Zhongshan (Sun Yat-sen) University, Guangzhou 510275, PR China Croucher Institute for Environmental Sciences, and Department of Biology, Hong Kong Baptist University, Hong Kong SAR, PR China Received 19 May 2006; accepted 24 May 2006

Marked variation of Zn and Cd tolerance and accumulation exist in populations of Sedum alfredii, a Zn/Cd hyperaccumulator. Abstract To investigate the variation of Zn and Cd accumulation and tolerance of Sedum alfredii (a newly reported Zn/Cd hyperaccumulator), field surveys and hydroponic experiments were conducted among three populations of this species: two originating from old Pb/Zn mines in Zhejiang (ZJ) and Hunan (HN) Provinces and one from a ‘‘clean’’ site in Guangdong (GD) Province, China. Under field conditions, up to 12,524 and 12,253 mg kg1 Zn, and 1400 and 97 mg kg1 Cd in shoots of ZJ and HN plants were recorded respectively. Under hydroponic conditions, ZJ and HN plants accumulated significantly higher Zn and Cd in their leaves and stems, and possessed significantly higher Zn and Cd tolerance than GD plants. Among the two contaminated populations, ZJ plants showed higher Cd tolerance and accumulation (in leaves) than HN plants. The present results indicate that significant differences in Zn and Cd accumulation and tolerance exist in populations of S. alfredii. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Cadmium; Hyperaccumulator; Populations; Sedum alfredii Hance; Zinc

1. Introduction Metal hyperaccumulating plants can accumulate high concentrations of metals and metalloids such as nickel (Ni), zinc (Zn), and selenium (Se) in aboveground tissues (Kra¨mer et al., 2000). The phenomenon of heavy metal hyperaccumulation by some plant species has attracted considerable scientific attention for ecological and evolutionary studies. More recently, their ability to extract metals from soil and to concentrate them in their shoots, has given rise to the idea of practical applications, such as phytomining and phytoremediation (McGrath, 1998; Salt et al., 1998). The studies of metal

* Corresponding author: Tel.: þ86 20 8411 2958; fax: þ86 20 8411 3652. ** Corresponding author. Tel.: þ852 2339 7050; fax: þ852 2339 5995. E-mail addresses: [email protected] (Z.H. Ye), [email protected] (M.H. Wong). 0269-7491/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2006.05.024

hyperaccumulators have become a growing and exciting field of research. Metal hyperaccumulators seem to have a constitutive trait at species level. For example, it has been commonly reported that different populations of Pteris vittata are able to hyperaccumulate arsenic (As) in their fronds (Ma et al., 2001; Chen et al., 2002; Wang et al., 2006). On the other hand, hyperaccumulation seems to be metal specific (Taylor and Macnair, 2006); for example, Zn hyperaccumualtion in Thlaspi caerulescens seems to be a constitutive trait at species level (Meerts and van Isacker, 1997; Escarre´ et al., 2000); while cadmium (Cd) seems to be much less consistently hyperaccumulated than Zn in this species (Reeves et al., 2001). Thus different populations in the same species possess significant differences in their ability to accumulate metals, and therefore provide suitable materials for studying the mechanisms of metal hyperaccumulation. To date, intraspecies investigations under field and controlled conditions have been conducted on only

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a few hyperaccumulators occurring in both metalliferous and non-metalliferous soils. The study of gene expression in T. caerulescens was operated on the populations that differ greatly in their ability to accumulate and tolerate Cd, and the results indicated that Cd uptake in at least one population of this species is probably mediated by a high-affinity Cd transport system (Lombi et al., 2002; Zhao et al., 2002a). However, similar studies concerning other Zn and Cd hyperaccumulators are not available. Found on both metalliferous and non-metalliferous soils, Sedum alfredii has been identified as a Zn and Cd hyperaccumulator native to China (Yang et al., 2001, 2004). Although Zn/Cd accumulation, translocation, and compartmentation of this species have been studied (Long et al., 2002; Ni and Wei, 2003; Ye et al., 2003), the variation on tolerance, accumulation and distribution of Zn and Cd in different populations within this species grown on metalliferous and non-metalliferous soils have not been clearly illustrated. In this project, a field investigation was conducted to assess the variation in Zn and Cd accumulation among three populations of S. alfredii grown at two Pb/Zn mine sites and an uncontaminated site. Pot experiments were further conducted using young plants of similar size of the three populations of this species to study their accumulation and tolerance of Zn and Cd under hydroponic conditions. 2. Materials and methods 2.1. Sample collection S. alfredii grew very well and was a dominant species in the two mine areas. Mature individual samples of this plants and associated soil samples (around the roots of the plants at 0e20 cm depth) were collected for analysis from two Pb/Zn mine sites in Zhejiang and Hunan Provinces (ZJ and HN populations) (12 from each sites) and a ‘‘clean’’ site in Guangdong Province (GD population) (four), China respectively.

2.2. Plant and soil analysis Plant samples were washed thoroughly with deionised water, and the roots of intact plants were immersed in 20 mmol l1 Na2-EDTA for 15 min to remove the Cd2þ and Zn2þ adhered to the root surfaces (Yang et al., 1996). The plants were then separated into roots and shoots, oven dried (70  C) to a constant weight, grounded and passed into powder for Cd and Zn analysis. Grounded plant materials were digested with a mixture of conc. HNO3 and conc. HClO4 (4:1, v/v) (Allen, 1989). The soil samples were air-dried at room temperature for 2 weeks, ground into fine powder, passed through a 0.5 mm nylon sieve, and digested with conc. HCl and HClO4 (4:1, v/v) (McGrath and Cunliffe, 1985). Concentrations of Cd or Zn in digests of plants or soils were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES). For extractable metals, soil samples were extracted with DTPA solution, and the concentrations of Zn and Cd in filtered solution were also determined by ICP-AES (Page et al., 1982). Blanks, plant (tomato) standard material (GBW-08505), and soil standard material (GBW-08303) purchased from the China Standard Materials Research Center, Beijing, PR China, were used for quality control, and the recovery rates were within 90  10%.

2.3. Hydroponic experiments Mature plants collected from the three sites were grown on ‘clean’ soil and propagated vegetatively in a glasshouse, and healthy new shoots of the plants were selected for the hydroponic test. New shoots without roots of each population were cut and cleaned with tap water, and grew in 1/10 Hoagland

solution (Hoagland and Arnon, 1950) for 2 weeks for initiation of new roots. The composition of all hydroponic solutions was as follows: Ca(NO3)2$4H2O 2.00 mmol l1, KH2PO4 0.10 mmol l1, MgSO4$7H2O 0.50 mmol l1, KCl 0.10 mmol l1, K2SO4 0.70 mmol l1, H3BO3 0.01 mmol l1, MnSO4$H2O 0.50  103 mmol l1, ZnSO4$7H2O 0.50  103 mmol l1, CuSO4$5H2O 0.20  103 m mol l1, (NH4)6Mo7O24 0.01  103 mmol l1, Fe-EDTA 0.10 mmol l1. In Zn treatments, Zn (ZnCl2) was added at the concentrations of 2.5, 5, 10, 20, 40 mg l1, and in Cd treatments, Cd (CdCl2) was added at the concentrations of 0.25, 0.5, 1, 2 mg l1 (1/10 Hoagland solution). There were five replicates for each treatment, and each replicate consisted of five plants. During the period of metal treatment, solutions were adjusted daily with HCl and NaOH to pH 5.8, aerated every day and renewed once every 3 days.

2.4. Plant harvest and analysis Four weeks after Zn or Cd treatments, plants were harvested and washed with tap water and then deionised water. The plants were then separated into root, stem and leaf, and oven dried (70  C) to a constant weight. The dry weight of each part of the plants was determined and digested for Cd and Zn analysis using the methods described above. Tolerance index (TI) was calculated by the following equation (Wilkins, 1978): Tolerance indexð%Þ ¼

growth in solution þ metal  100 growth in solution  metal

2.5. Statistical analysis Data on metal concentrations in plant tissues, plant biomass, and metal tolerance index were analyzed using one-way ANOVA followed by the LSD test as available in the SPSS statistical package. The translocation factor (TF) for metals within a plant was expressed by the ratio of [metal]Leaf/[metal]Root which showed the metal translocation properties from roots to aboveground parts (Stoltz and Greger, 2002).

3. Results 3.1. Concentrations of Zn and Cd in plants and their associated soils under field conditions Among the 12 samples of S. alfredii collected from the ZJ site, 11 of them accumulated over 10,000 mg Zn kg1 in their shoots, with an average of 11,172 mg kg1. Moreover, all the 12 samples from this site accumulated over 400 mg Cd kg1 in their shoots, with an average of 1051 mg kg1 (Table 1). Five of the 12 samples of the same species collected from the HN site accumulated over 10,000 mg Zn kg1 (8020e 12,253 mg g1) in their shoots, while only up to 97 mg Cd kg1 (average 57 mg kg1) was recorded in shoot samples from this site. Unlike the samples collected from the ZJ and HN sites, all four samples of S. alfredii collected from the GD site accumulated less than 160 mg Zn kg1 and less than 1.3 mg Cd kg1 in their shoots (Table 1). Concentrations of total and DTPA-extractable Zn and Cd in soil samples collected from the ZJ and HN sites were similar, and were significantly higher than those from the GD site (Table 1). 3.2. Dry biomass and tolerance of ZJ, HN and GD plants of S. alfredii exposed to different metal concentrations The biomass of ZJ plants increased at 2.5, 5, and 10 mg l1 Zn treatments, but decreased at 20 and 40 mg l1 Zn

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Table 1 The concentrations of Zn and Cd (mg kg1, dry wt.) in shoots and roots of Sedum alfredii and associated soils collected from two Pb/Zn mines in Zhejiang and Hunan Provinces and a ‘clean’ soil in Guangdong Province, China (mean  SE, and range, n ¼ 12, 12 and 4 in ZJ, HN and GD populations, respectively) Site

Soil

Plant

Total

ZJ HN GD

DTPA extractable

Shoot

Root

Zn

Cd

Zn

Cd

Zn

Cd

Zn

Cd

3705  436a 1217e6943 3804  329a 1944e6525 469  29b 382e531

31  4.6a 12e79 58  33a 24e146 39  3.1a 29e45

175  17a 70e277 133  40a 80e195 113  11a 84e147

5.6  1.2a 1.2e17 5.9  3.7a 2.1e16 4.6  0.43a 3.2e5.4

11732  238a 9737e12619 9818  352b 8020e12253 138  5.9 c 128e157

1051  104a 422e1600 57  5.5b 32e97 1.1  0.64b 0.9e1.2

10017  912a 5031e16187 9952  744a 5972e15209 809  39b 687e904

400  53a 85e626 58  13b 27e305 4.8  0.37b 4.2e6.0

Different letters in the same column indicate a significant difference among the ZJ, HN and GD populations at the 5% level according to the LSD test.

treatments, while the biomass of HN plants slightly increased at 5, 10, and 20 mg l1 Zn treatments, but did not change significantly at 2.5 and 40 mg l1 Zn treatments, when compared with those growing in the control solution. The biomass of GD plants decreased with the increase of Zn (Table 2). The biomass of ZJ plants increased, and GD and HN plants decreased with the increase of Cd, when compared with the plants grown in control treatment (Table 2). At each Zn and Cd treatment, TIs to Zn of ZJ and HN populations were always significantly higher than those of the GD population (P < 0.05). Although there was no significant difference in TIs between the HN and ZJ populations (P > 0.05) at Zn treatments, the TIs of the ZJ population were significantly higher than those of HN population (P < 0.05) at Cd treatments (Table 3).

treatments. No significant difference (P > 0.05) was observed on Zn concentrations between ZJ and HN plants at 40 mg l1 Zn treatment in leaves and 2.5, 10, 40 mg l1 Zn treatments in stems. Concentrations of Cd were similar in stems between ZJ and HN plants but always significantly higher in leaves of ZJ plants than those of HN plants ((Figs. 1 and 2). The ratios of S/R and L/R of ZJ and HN plants were also remarkably higher than those of GD plants at all Zn or Cd treatments and were similar between ZJ and HN plants at higher Zn treatments (20 and 40 mg l1). Under Cd treatment, although the ratios of S/R in ZJ and HN plants were similar, the ratios of L/R in ZJ plants were remarkably higher than those in HN plants (Table 4).

3.3. Concentrations of Zn and Cd in ZJ, HN and GD plants of S. alfredii exposed to different metal concentrations

4.1. Metal accumulation in ZJ, HN and GD populations of S. alfredii

Concentrations of Zn and Cd in the leaves and stems of ZJ and HN plants were significantly higher than those of GD plants (P < 0.05) at Zn and Cd treatments respectively (Figs. 1 and 2). However, the concentrations of Zn or Cd in root tissue were similar among the three plants at Zn or Cd

4. Discussion

Metal hyperaccumulation seemed to be a constitutive trait at species level (Bert et al., 2002). In the case of the As hyperaccumulator, P. vittata, plants collected from contaminated and uncontaminated soils show a similar accumulation ability (Zhao et al., 2002b; Wang et al., 2006). However, the present study indicated that the ZJ, HN and GD populations of

Table 2 The dry biomasses (mean  SE, n ¼ 5) of the GD, ZJ, and HN populations of Sedum alfredii subjected to different concentrations (mg l1) of Zn and Cd GD

ZJ

HN

Zn2þ in solution 0.0 2.5 5.0 10.0 20.0 40.0

0.20  0.03 0.13  0.01 0.10  0.01 0.09  0.01 0.08  0.01 0.08  0.01

a*-a# b-b b-bc c-c b-c a-c

0.17  0.02 0.23  0.03 0.27  0.03 0.23  0.01 0.12  0.01 0.12  0.01

ab-b a-a a-a a-a a-b a-b

0.12  0.01 0.12  0.01 0.13  0.01 0.15  0.02 0.12  0.01 0.13  0.01

b-a b-a b-a b-a a-a a-a

Cd2þ in solution 0.00 0.25 0.50 1.0 2.0

0.23  0.03 0.15  0.02 0.11  0.01 0.09  0.01 0.06  0.01

a-a b-b b-bc b-c b-c

0.24  0.03 0.30  0.03 0.35  0.02 0.41  0.03 0.37  0.03

a-c a-bc a-ab a-a a-ab

0.19  0.02 0.15  0.01 0.14  0.02 0.10  0.01 0.08  0.01

a-a b-ab b-b b-bc b-c

An asterisk indicates the difference in the same row. # indicates the difference in the same column within the same metal. Different letters in the same row or in the same column indicate significant difference at the 5% level according to the LSD test.

D.M. Deng et al. / Environmental Pollution 147 (2007) 381e386

384

Table 3 The tolerance indices of (TI) (mean  SE, n ¼ 5, %) of GD, ZJ, and HN populations of Sedum alfredii subjected to different concentrations (mg l1) of Zn and Cd ZJ

Zn2þ in solution 2.5 38  1.2 b*-a# 5.0 38  0.80 b-a 10 37  1.1 b-a 20 31  3.9 c-b 40 31  0.80 b-b Cd2þ in solution 0.25 38  3.0 c-a 0.50 34  4.3 c-a 1 31  4.5 c-a 2 25  3.8 c-a

123  5.4 a-d 198  5.8 a-b 220  7.3 a-a 178  11 a-c

89  2.8 79  2.3 66  2.0 62  2.0

b-a b-b b-c b-c

An asterisk indicates the difference in the same row. # indicates the difference in the same column within the same metal. Different letters in the same row or in the same column indicate significant difference at the 5% level according to the LSD test.

S. alfredii exhibited a population-specific and metal-specific variation in Zn and Cd accumulation. For the ZJ population, 9737e12,524 mg kg1 Zn (average 11,172 mg kg1) and 422e1400 mg kg1 Cd (1051 mg kg1)

10

c

1

10000

b

a b

c

0

0.25

0.5

B

1000

c

c

b

b

a a

100

b

b

b

100

119  2.2 a-bc 125  7.0 a-b 144  16 ab-a 132  11 b-ab 86  4.1 ab-c

a

a a

a

HN

111  3.4 a-c 148  20 a-b 158  19 a-a 103  25 a-a 91  17 a-d

A

1000

Cd concentration (mg kg-1)

GD

10000

1

2 a b

a a

a

b a c

c

c

b

b

10 1

10000

0

0.25

0.5

1

2

0.25

0.5

1

2

C

1000 100

10000

Aa

a

a

b

b a

a b

b

1000 c 100

a a

a

c

b

c

c

1

b

GD population

Zn concentration (mg kg-1)

1

0

2.5

B

10000

b

a a

a

5

10

20

40

a b

a a

a b

a a

c

b

1000

b

b

c

c 100 10 1

10000

0

2.5

5

10

20

40

2.5

5

10

20

40

C

1000 100 10 1

0

Cd concentration in solution (mg l-1)

10

100000

10

0

Zn concentration in solution (mg l-1) GD population

ZJ population

HN population

Fig. 1. Tissue concentrations of Zn (mg kg1, dry wt.) in leaf (A), stem (B), and root (C) of GD, ZJ, and HN populations of Sedum alfredii subjected to different concentrations (mg l1) of Zn2þ (values are means plus SE, n ¼ 5).

ZJ population

HN population

Fig. 2. Tissue concentrations of Cd (mg kg1, dry wt.) in leaf (A), stem (B), and root (C) of GD, ZJ, and HN populations of Sedum alfredii subjected to different concentrations (mg l1) of Cd2þ (values are means plus SE, n ¼ 5).

(Table 1) were accumulated in shoot tissues under field conditions. Under hydroponic conditions, the maximum concentrations of Zn and Cd of the ZJ population reached 7793 (10 mg l1 Zn) and 5032 mg kg1 (2 mg l1 Cd) in leaves and 10,136 (20 mg l1 Zn) and 5195 mg kg1 (2 mg l1Cd) in stems, respectively (Fig. 2). The criterion for the Zn hyperaccumulator suggested by Baker and Brookes (1989) is 10,000 mg kg1 in shoots. However, this criterion was later considered too strict and was modified to 3000 mg Zn kg1 in shoots (Reeves and Baker, 2000). A cadmium hyperaccumulator is defined as a plant species capable of accumulating more than 100 mg kg1 (dry wt.) in the shoots (Baker et al., 2000). Thus, the results presented here show that S. alfredii grown at the ZJ mine site was a Zn and Cd hyperaccumulator, which confirmed the results reported by Yang et al. (2001, 2004). Similar to ZJ plants, HN plants could accumulate up to 12,253 mg Zn kg1 in shoots under field conditions, and 8156 and 9157 mg Zn kg1 (20 mg l1 Zn) in leaves and stems respectively under hydroponic conditions (Fig. 1, Table 1). However, only 32e97 mg kg1 Cd (average 57 mg kg1) (Table 1) in shoots tissues were recorded under field conditions, which were lower than those of ZJ plants. Thus,

D.M. Deng et al. / Environmental Pollution 147 (2007) 381e386 Table 4 Ratio of concentrations of Zn or Cd in leaf, stem and root of GD, ZJ, and HN populations of Sedum alfredii subjected to different concentrations (mg l1) of Zn and Cd GD

ZJ

HN

S/R

L/R

S/R

L/R

S/R

L/R

Zn2þ in solution 2.5 0.60 5.0 0.23 10 0.22 20 0.16 40 0.26

0.10 0.03 0.04 0.03 0.05

6.9 3.9 2.6 1.5 1.8

5. 5 3.1 2.0 0.94 0.98

2.2 2.4 1.8 1.2 2.0

1.2 1.4 1.1 1.0 1.2

Cd2þ in solution 0.25 1.6 0.50 0.61 1.0 0.16 2.0 0.07

0.17 0.11 0.02 0.02

3.4 4.4 4.8 2.3

4.2 6.0 6.3 2.3

3.8 2.8 2.9 3.1

0.7 0.6 0.7 0.8

S/R represents stem metal concentration/root metal concentration, L/R represents leaf metal concentration /root metal concentration.

S. alfredii grown at the HN mine site could only be called a Zn hyperaccumulator. By contrast, Zn and Cd concentrations in shoots of GD plants were significantly lower than those of ZJ and HN plants and did not reach the threshold of a Zn and Cd hyperaccumulator under field conditions. The present work is the first attempt to provide information on the variation of Zn and Cd accumulation in natural populations of S. alfredii. For Cd, metallicous populations (ZJ and HN populations) showed a significantly higher ability to accumulate Cd than non-metallicous population (GD population). Moreover, a significant difference in Cd accumulation also existed between metallicous populations. Lombi et al. (2002) found a similar intraspecific variation among four populations in T. caerulescens. However, one non-metallicous population of T. caerulescens shows this ability to hyperaccumulate Cd in the study conducted by Roosens et al. (2003). Bert et al. (2002) reported great variations on Cd accumulation in both metallicous and non-metallicous populations of Arabidopsis halleri, as concentrations of Cd in shoots of 13 metallicous populations ranged from 2 to 354 mg kg1, whereas those of 20 non-metallicolous populations ranged from 3 to 89 mg kg1. They also found that 17% of the plant individuals of metallicous populations and 10% of the plant individuals of non-metallicous populations could accumulate more than 100 mg Cd kg1 in their shoots. A number of studies (Lombi et al., 2002; Assunc¸~ao et al., 2003; Roosens et al., 2003; Taylor and Macnair, 2006) considered that Zn hyperaccumulation is a constitutive trait at species level in A. halleri and T. caerulescens. Assunc¸~ao et al. (2003) suggested a high-affinity transporter system with high Zn preference in populations of T. caerulescens from different soil types. By contrast, the present study showed that only metallicous populations of S. alfredii own the trait of Zn hyperaccumulation. The distinct intraspecific variations of S. alfredii provide potential material for further genetic and physiological dissection of the hyperaccumulation trait.

385

4.2. Metal tolerance in the three populations of S. alfredii Most experimental studies on heavy metal tolerance confirm the fundamental tenet that populations surviving in metal contaminated habitats are differentiated from ‘normal’ populations of the same species by possessing genetically based tolerances (Antonovics et al., 1971). The results of dry biomass (Table 2) and tolerance index (TI) (Table 3) derived from the hydroponic experiments, especially the values of TIs, further demonstrated that the ‘clean’ population (GD population) was sensitive to Zn and Cd, while the metal-contaminated populations (ZJ and HN populations) had evolved with a high ability in Zn and Cd tolerance. It was reported that a large uncorrelated variation of tolerance and accumulation of Zn, Cd or Ni existed in populations of T. caerulescens (Assunc¸~ao et al., 2003), and A. halleri (Bert et al., 2002). However, the ZJ population showed a higher tolerance to Cd than the HN population (Table 1), which was in line with the pattern of metal accumulation. The relation between metal tolerance and accumulation in hyperaccumulators is not well understood. 4.3. Metal translocation and distribution in the three populations of S. alfredii ZJ and HN plants translocated more Zn and Cd into the aboveground tissues (Table 4) than GD plants. Besides, ZJ plants could accumulate and translocate more Cd in shoots than HN plants (Figs. 1 and 2, Tables 1 and 4) under field and hydroponic conditions. The values of L/R and S/R (Table 4) in Cd treatments suggested that ZJ plants possessed a stronger ability to translocate Cd from roots to stems/leaves. The values presented in Fig. 2 indicated that ZJ plants could also distribute higher percentages of total accumulated Cd in leaves than HN plants. Transporters were reported to mediate metal translocation from stems to leaves molecularly; but the factors governing differential metal accumulation and storage are unknown (Clemens et al., 2002). Metal chelators, such as metallothioneins and phytochelatins, were suggested to play a role in metal detoxification (Baker et al., 2000); while their function in S. alfredii was still unclear and further study is needed. 4.4. Potential for phytoremediation of the three populations of S. alfredii The ideal plant to be used in phytoextraction should be tolerant to high levels of the metal, and accumulate high levels of the metal in its harvestable parts (Salt et al., 1998). The phytoremediation efficiency is determined by the amount of metal transported to the aboveground tissues, and the aboveground biomass of the hyperaccumulating plant. The results presented here show that ZJ plants could extract the largest amount of Zn or Cd in their aboveground part among the three populations (Figs. 1 and 2, Table 1), and the ZJ population, therefore, was more efficient than the other two populations for the remediation of Zn and Cd contaminated soils. HN plants could

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extract a substantial amount of Zn (Fig. 1, Table 1), which shows that they could also be used in phytoremediation of Zn. 5. Conclusions The present results show that marked metal specific differences exist among the GD, ZJ and HN populations of S. alfredii in terms of metal accumulation, tolerance, translocation and distribution. The plant grown at the ZJ Pb/Zn mine site is a Zn and Cd hyperaccumulator, while the plant of the same species grown at the HN Pb/Zn mine is a Zn hyperaccumulator. The GD plant at the control site cannot hyperaccumulate Zn or Cd. The differences in metal tolerance, translocation, and distribution are in agreement with the differences in metal accumulation. S. alfredii has been proved to be a valuable material for studying the mechanisms of Zn and Cd hyperaccumulation. The ZJ population shows a high potential for Zn and Cd phytoremediation, while the HN population also shows a high potential for Zn phytoremediation. Acknowledgements This work was financially supported by the Research Grant Council of University Grant Committee, Hong Kong (HKBU 2181/03 M), and the National Natural Science Foundation of China (No. 30570345). References Allen, S.E., 1989. Chemical Analysis of Ecological Materials, second ed. Blackwell Scientific Publications, Oxford, UK. Antonovics, J., Bradshaw, A.D., Turner, R.G., 1971. Heavy metal tolerance in plants. In: Cragg, J.B. (Ed.), Advances in Ecological Research, vol. 7. Academic Press, New York, pp. 1e85. Assunc¸~ao, A.G.L., Bookum, W.M., Nelissen, H.J.M., Vooijs, R., Schat, H., Ernst, W.H.O., 2003. Differential metal-specific tolerance and accumulation patterns among Thlaspi caerulescens populations originating from different soil types. New Phytologist 159, 411e419. Baker, A.J.M., Brooks, R.R., 1989. Terrestrial higher plants which hyperaccumulate metallic elements e a review of their distribution, ecology and phytochemistry. Biorecovery 1, 81e126. Baker, A.J.M., McGrath, S.P., Reeves, R.D., Smith, J.A.C., 2000. Metal hyperaccumulator plants: a review of the ecology and physiology of a biological resource for phytoremediation of metal-polluted soils. In: Terry, N., Banuelos, G., Vangronsveld, J. (Eds.), Phytoremediation of Contaminated Soil and Water. Lewis Publisher, Boca Raton, FL, pp. 85e107. Bert, V., Bonnin, I., Saumitou-Laprade, P., Lague´rie, P., Petit, D., 2002. Do Arabidopsis halleri from nonmetallicolous populations accumulate zinc and cadmium more effectively than those from metallicolous populations? New Phytologist 155, 47e57. Chen, T.B., Wei, C.Y., Huang, Z.C., Huang, Q.F., Lu, Q.C., Fan, Z.L., 2002. Arsenic hyperaccumulator Pteris Vittata L. and its arsenic accumulation. Chinese Science Bulletin 47, 902e905. Clemens, S., Palmgren, M.G., Kra¨mer, U., 2002. A long way ahead: understanding and engineering plant metal accumulation. Trends in Plant Science 7, 309e315. Escarre´, J., Lefe`bvre, C., Gruber, W., Leblanc, M., Lepart, J., Rivie`re, Y., Delay, B., 2000. Zinc and cadmium hyperaccumulation by Thlaspi caerulescens from metalliferous and nonmetalliferous sites in the Mediterranean area: implications for phytoextraction. New Phytologist 145, 429e437. Hoagland, D.R., Arnon, D.I., 1950. The Water-culture Method for Growing Plants without Soil. California Agricultural Experiment Station Circular No. 347.

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