Cadmium accumulation and its effects on metal uptake in maize (Zea mays L.)

Bioresource Technology 98 (2007) 82–880960-8524/$ - see front matter © 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.biortech.2005.11.028Cadmium accumulation and its e V ects on metal uptakein maize (Zea mays L.)Min Wang, Jinhua Zou, Xuchuan Duan, Wusheng Jiang, Donghua Liu¤Department of Biology, College of Chemistry and Life Sciences, Tianjin Normal University, Tianjin 300074, ChinaReceived 15 June 2004; received in revised form 17 November 2005; accepted 19 November 2005Available online 19 January 2006AbstractThe e V ects of di V erent concentrations of Cd on growth of maize (Zea mays L.) and metal uptake were investigated. Cd accumulations in roots and shoots and the interactions among other metals (Mn, Fe, Cu and Zn) were analyzed using inductively coupled plasma atomic emission spectrometry (ICP-AES). The concentrations of cadmium chloride (CdCl 2·2.5H 2O) used ranged from 10¡4M to 10¡6M. Cd had stimulatory e V ects during the W rst 5days on root length of Nongda No. 108 at 10¡6M and 10¡5M Cd concentrations. Seedlings exposed to 10¡4M Cd solution exhibited substantial growth reduction, and root growth even stopped. Root growth of Liyu No. 6 was stimulated at concentrations of 10¡5M and 10¡6M Cd during the entire experiment (15days). Cadmium inhibited root growth of Liyu No. 6 at 10¡4M Cd after 10days of treatment. The Cd accumulation in roots and shoots of the two cultivars increased signi W cantly (P <0.05) with increasing Cd concentration and duration of treatment. Cadmium concentrated mainly in the roots, and small amounts were transferred to shoots. The proportion of Cd in the roots of Nongda No. 108 decreased with increases in Cd concentrations and dura-tion of treatment, except for the group exposed to 10¡4M Cd. In Liyu No. 6, the proportion of Cd in the root decreased progressively with an increase in Cd concentrations. Liyu No. 6 has a greater ability to remove Cd from solution and accumulate it when compared with Nongda No. 108. Liyu No. 6 can be considered a Cd-hyperaccumulator, according to the current accepted shoot concentration that de W -nes hyperaccumulation as 0.01% (w/w) for cadmium. This cultivar, producing many roots and a high biomass and with great ability to accumulate Cd can play an important role in the treatment of soils stressed by Cd.© 2005 Elsevier Ltd. All rights reserved.Keywords:Zea mays L.; Accumulation; Cd; Mn; Fe; Cu; Zn1. IntroductionCadmium concentrations of uncontaminated soils are usually below 0.5mg/kg but can reach up to 3mg/kg depending on the soil parent material (Schachtschabel et al., 1984). Cadmium is a particularly hazardous pollutant due to its high toxicity and solubility in water (Lockwood,1976). At low concentrations Cd is not toxic to plants, but at higher concentrations it is toxic and characteristically inhibits root growth and cell division (Liu et al., 2001, 2003/2004; Jiang et al., 2001). Cadmium toxicity symptomsare similar to Fe chlorosis and also Cd induces necrosis and wilting. There is a greater emphasis towards the prob-lems of Cd pollution with the development of modern industry and agriculture. Most conventional remediation approaches do not provide acceptable solution to toxicity caused by metal pollution. The development of phytoreme-diation technology for site decontamination has spurred recent interest in the mechanisms by which metals are accu-mulated in plants (Pollard et al., 2002). Phytoremediation is an emerging technique that o V ers the bene W ts of working in situ, being low cost and environmentally sustainable (McGrath et al., 2002; Salt et al., 1998). There are two phy-toextraction strategies currently in use (Keller et al., 2003).One option (McGrath, 1998; Robinson et al., 1998) uses hyperaccumulators such as Thlaspi caerulescens or Alyssum*Corresponding author.E-mail address: donghua@ (D. Liu).M. Wang et al. / Bioresource Technology 98 (2007) 82–8883bertolonii to take up one or two metals. This option pro-duces low biomass compensated by very high metal con-centrations in the shoots (Baker and Brooks, 1989; Reeves and Baker, 2000). The other strategy involves the use of high biomass plants that are usually not metal-speci W c and contain low to average heavy metal concentrations which are compensated by high biomass (Keller et al., 2003). Maize is a common and important agricultural crop world-wide that has been used in many studies of elemental pollu-tion. The aim of this investigation was to study the e V ects of di V erent concentrations of Cd on growth and Cd accumu-lation in maize (Zea mays L.) and its e V ects on metal uptake in the plants using inductively coupled plasma atomic emission spectrometry (ICP-AES).2. MethodsSeeds of maize (Z. mays L.) cultivars “Nongda No. 108”and “Liyu No. 6”, were kindly provided by the Institute of Crops, Tianjin Academy of Agricultural Sciences, Tianjin, PR China. Healthy and equal-sized seeds were chosen from each variety, soaked in tap water for 24h and germinated in the dark (25°C). Following germination, 20 seedlings from each cultivar were chosen, W xed in cystose, and X oated in 50% modi W ed Hoagland’s solution (Stephan and Proc-hazka, 1989) in plastic containers in a greenhouse equipped with supplementary lighting (14-h photoperiod; 24–26°C). The Hoagland’s solution consisted of 5mM Ca(NO3)2, 5mM KNO3, 1mM KH2PO4, 50 M H3BO3, 1mM MgSO4, 4.5 M MnCl2, 3.8 M ZnSO4, 0.3 M CuSO4, 0.1mM (NH4)6Mo7O24 and 10 M FeE DTA adjusted to a pH of 5.5. Three days after culture, the seedlings were treated with Cd. Cadmium concentrations ranging from 10¡4M to 10¡6M were added to each container of 2L Hoagland’s solution. The solutions were aerated by a pump. Cadmium was provided as cadmium chloride (CdCl2·2.5H2O), pre-pared in deionized H2O. The Cd solutions were changed every 4days.Ten plantlets from each treatment were harvested based on uniformity of size and colour (removing the greatest and the smallest plantlets and then selecting randomly) after 5, 10 and 15days of incubation. Roots were rinsed in deion-ized H2O to remove traces of nutrients and Cd from the surface. Macroscopic observations were made at the end of each time interval. The plantlets were divided into roots and shoots. The samples were dried for 3days at 45°C, fol-lowed by 3days at 80°C in an oven, measured for dry weight (DW), and ashed for 2h at 200°C and then 10h at 650°C. The contents of Cd, Mn, Cu, Zn and Fe were deter-mined with inductively coupled plasma atomic emission spectrometry (ICP-AE S) (Leeman Labs Inc., New Hamp-shire, USA) after dry-ashing (Duan, 2003).Analysis of variance (ANOVA) using Sigma statistical software (Jandel Scienti W c Corporation) was performed. Tests of equality of averages using a t-test was applied. Statistical signi W cance was set at the P<0.05 con W dence level.3. Results3.1. Macroscopic e V ects of Cd on root growthThe e V ects of Cd on root growth varied with the di V er-ent Cd concentrations used and with cultivar (Figs. 1 and 2). Cadmium had a stimulatory e V ect on root elongation of Nongda No. 108 at 10¡6M and 10¡5M Cd concentrations during the W rst 5days. Root length decreased, however, with an increasing Cd concentration and duration of treat-ment (Fig.1). Seedlings exposed to 10¡4M Cd exhibited substantial growth reduction, and root growth even stopped. Roots appeared thinner and were sparsely branched. Root growth of Liyu No. 6 di V ered from that of Nongda No. 108 (Fig.2). In contrast to the control, root growth was stimulated at 10¡5M and 10¡6M Cd during the entire experiment. Cadmium inhibited root elongation at 10¡4M Cd after 10days.84M. Wang et al. / Bioresource Technology 98 (2007) 82–88Usually, the plantlets (4–5 leaves) of Nongda No. 108 in the 10¡6M–10¡4M Cd treatments did not grow as large,robust and healthy as the control during the experiment.With increasing Cd concentration, this phenomenon was progressively obvious. After 5days, the colour of the leaves was not green as the control and chlorosis was evident in the W rst and second leaves. After 10days, roots exposed to 10¡4M Cd appeared yellow-brown and some were broken.When compared to the control, the plantlets of Liyu No. 6exposed to 10¡6M and 10¡5M Cd grew stronger with more branch roots and leaf colour was dark green. The toxic e V ects of Cd on the group treated with 10¡4M Cd appeared after 5days of treatment. Leaf margins and leaf tips wilted.Roots were stunted, some of which turned black and were slightly decomposed.3.2. Cd uptake and accumulationCadmium uptake and accumulation in roots and shoots of Nongda No. 108 and Liyu No. 6 varied depending on Cd concentration and treatment time. The Cd contents in roots and shoots of the two cultivars increased signi W cantly (P <0.05) with increasing Cd concentration and duration of treatment (Table 1). Cadmium accumulated mainly in the roots, and small amounts were transferred to shoots (Table 2). The proportion of Cd in the roots of Nongda No. 108decreased with an increase in Cd concentration and dura-tion of treatment, except for the group exposed to 10¡4MCd, while the proportion of Cd in the shoot increased. As for Liyu No. 6, the proportion of Cd in the root decreased with an increase in Cd concentration; however, the amount of Cd in shoots increased. Liyu No. 6 has a greater ability to remove and accumulate Cd from solutions when com-pared with Nongda No. 108 (Table 1).3.3. E V ects of Cd on Mn, Fe, Cu and Zn levels in maize The e V ect of Cd on Mn, Fe, Cu and Zn uptake, accumu-lation and distribution in the cultivars were investigated. In the controls and cultivars treated with Cd, tissue Zn was highest, followed by Fe, Mn and Cu (Table 3).Manganese accumulation in Nongda No. 108 and Liyu No. 6 signi W cantly (P <0.05) decreased with Cd addition in nutrient solution. The Mn levels decreased with increasing Cd concentration and duration of treatment. Manganese content in the control was highest. The Mn mainly accumu-lated in the roots of both cultivars treated with 10¡6M Cd,and mainly occurred in the shoots exposed to 10¡5M Cd and 10¡4M Cd (Table 4). The Mn content in Liyu No. 6was higher than that in Nongda No. 108.The plantlets of the two cultivars accumulated signi W cant (P <0.05) iron after treatment with di V erent concentrations of Cd. Tissue Fe increased by prolonged treatment time (Table 5). All Fe levels of the treated groups were higher than the control, except for Nongda No. 108 treated with 10¡4M Cd, 10days. Small amounts of Fe were transferredTable 1Cadmium uptake and accumulation by Z. mays L.Values followed by same letters are not signi W cantly di V erent at (P <0.05). Means §SE, n D 3.Cultivars Treatment (M)Root ( g/g DW)Shoot ( g/g DW)5days 10days 15days 5days 10days 15days Nongda No. 108Control 9.5§0.12a 19.2§0.04a 15.6§0.50a 4.7§0.007a 3.0§0.04a 0.2§0.05a 10¡6130.5§0.74b 193.2§0.29b 280.9§0.17b 15.2§0.05b 41.8§0.03b 82.9§0.39b 10¡5259.8§0.64c 441.5§1.25c 550.6§3.00c 44.1§0.24c 101.9§0.43c 205.1§0.30c 10¡4502.2§1.57d 962.6§2.47d 1948.7§15.56d 353.3§1.41d 403.7§1.42d 364.5§1.27d Liyu No. 6Control 1.8§0.06a 4.6§0.06a 36.1§1.35a 0.9§0.07a 0.9§0.03a 6.7§0.12a 10¡6264.0§0.64b 486.6§1.52b 768.8§2.18b 42.2§0.10b 49.1§0.07b 126.0§0.63b 10¡5434.0§1.93c 1042.4§0.91c 1017.8§1.26c 115.4§0.44c 210.7§0.75c 213.5§0.65c 10¡4832.1§1.03d1543.5§5.22d1960.4§8.79d254.4§0.59d460.9§1.44d443.5§0.84dTable 2Distributive changes of cadmium in roots and shoots of cultivars after treatment with di V erent Cd concentrations and treatment time Cultivars Treatment (M)Total amount ( g/g DW)Root ( g/g DW) (%)Shoot ( g/g DW) (%)5days 10days 15days 5days 10days 15days 5days 10days 15days Nongda No. 108Control 14.222.215.866.986.598.733.113.5 1.310¡6145.7235.0363.889.682.277.210.417.822.810¡5303.9543.4755.185.581.272.914.518.827.110¡4855.51366.32313.258.770.584.241.329.515.8Liyu No. 6Control 2.7 5.542.866.783.684.333.316.415.710¡6306.2535.7894.886.290.885.913.89.214.110¡5549.41253.11231.379.083.282.721.016.817.310¡41086.52004.42403.376.677.081.623.423.018.4M. Wang et al. / Bioresource Technology 98 (2007) 82–8885to the shoots, but they accumulated mainly in the roots (Table 5). With prolonged duration of treatment, the Fe level increased in the roots and decreased in shoots.Copper levels of Nongda No. 108 increased with an increase in Cd concentration, except for the group treated with 10¡4M Cd (Table 6). The Cu content in the control was higher than that in Nongda No. 108 treated with Cd. The Cu concentration in Liyu No. 6 exposed to 10¡4M Cd was lowest. bThe cultivars accumulated Cu mainly in the roots,and small amounts were transferred to shoots (Table 6).Zinc accumulation in Nongda No. 108 and Liyu No. 6decreased signi W cantly (P <0.05) with di V erent concentrationsTable 3Concentrations of elements in Nongda No. 108 and Liyu No. 6 exposed to di V erent Cd concentrations Values followed by same letters are not signi W cantly di V erent at (P <0.05).aMeans §SE, n D 3.Elements Cultivars Treatment (M)Root a ( g/g DW)Shoot a ( g/g DW)5days 10days 15days 5days 10days 15days MnNongda No. 108Control 140.3§0.59a 114.7§0.83a 323.5§2.84a 73.4§0.27a 60.3§0.08a 150.6§0.44a 10¡692.0§0.08b 66.8§0.25b 59.0§0.12b 53.5§0.21b 80.5§0.15b 76.9§0.21b 10¡515.3§0.04c 13.3§0.02c 13.4§0.04c 46.3§0.30c 57.6§0.27c 34.5§0.05c 10¡412.1§0.03d 7.7§0.03d 9.3§0.01d 47.4§0.14c 41.6§0.05d 12.9§0.06d Liyu No. 6Control 123.2§1.35a 101.8§1.07a 154.6§0.41a 84.0§0.91a 163.1§0.45a 121.2§0.09a 10¡6150.1§0.51b 145.4§0.37b 141.8§0.13b 80.7§0.08a 80.4§0.08b 112.8§0.55a 10¡519.2§0.03c 18.4§0.06c 17.3§0.04c 45.8§0.11b 64.0§0.13c 44.6§0.14b 10¡418.7§0.10c 10.7§0.06d 10.7§0.09d 37.2§0.05c 38.1§0.11d 12.2§0.01c Fe Nongda No. 108Control 109.8§1.30a 239.4§1.31a 506.1§0.83a 56.7§0.28a 49.1§0.16a 71.0§0.40a 10¡6242.1§0.65b 390.4§1.12b 689.9§3.61b 94.9§0.07b 67.8§0.19b 31.2§0.18b 10¡5296.8§1.01c 536.5§2.01c 934.4§1.07c 56.8§0.09c 46.4§0.21c 45.1§0.26c 10¡4310.9§0.62d 239.0§0.37d 719.6§2.77d 98.6§0.50d 30.8§0.18d 60.4§0.74d Liyu No. 6Control 160.7§1.75a 264.3§2.57a 585.8§2.29a 93.7§0.27a 68.3§0.45a 39.0§0.12a 10¡6267.0§0.53b 336.0§0.35b 749.1§2.39b 71.1§0.25b 87.9§0.29b 42.8§0.08b 10¡5218.9§0.75c 505.4§2.74c 1037.7§6.21c 86.8§0.27c 95.1§0.52b 48.3§0.22c 10¡4250.4§0.77d 733.3§3.34d 890.6§4.42d 127.5§0.51d 77.0§0.24c 38.5§0.17d Cu Nongda No. 108Control 49.2§0.35a 91.6§1.15a 71.1§0.48a 14.7§0.21a 17.0§0.23a 13.1§0.10a 10¡624.5§0.12b 25.9§0.11b 43.9§0.08b 10.0§0.05b 19.7§0.08b 12.2§0.08b 10¡534.6§0.13c 58.0§0.22c 62.2§0.14c 9.3§0.11c 10.5§0.08c 12.7§0.07b 10¡426.2§0.23d 38.9§0.10d 67.8§0.35d 10.4§0.02d 10.5§0.07c 37.1§0.20c Liyu No. 6Control 50.5§0.33a 82.4§0.51a 35.9§0.24a 15.3§0.26a 18.2§0.04a 8.6§0.09a 10¡643.7§0.08b 55.3§0.26b 101.2§0.12b 11.6§0.03b 11.0§0.06b 11.7§0.06b 10¡550.8§0.14c 103.9§0.17c 78.1§0.29c 12.3§0.06c 12.9§0.05c 11.8§0.10b 10¡429.8§0.07d 23.7§0.19d 25.7§0.24d 7.6§0.03d 8.2§0.09d 8.6§0.04c Zn Nongda No. 108Control 439.6§4.91a 795.8§7.18a 771.5§3.98a 283.9§1.57a 396.3§2.66a 331.5§3.32a 10¡6586.6§1.07b 486.7§1.36b 578.7§2.87b 282.1§2.38a 373.9§1.47b 373.8§2.65b 10¡5376.3§1.46c 384.3§1.01c 348.9§1.22c 280.2§0.12a 207.0§0.41c 491.0§1.07c 10¡4329.2§1.34d 254.9§1.48d 160.3§0.75d 270.0§0.98b 218.8§2.56c 239.3§3.08d Liyu No. 6Control 360.6§2.47a 563.9§3.73a 720.4§4.32a 325.4§2.52a 486.2§1.61a 331.4§1.19a 10¡6529.1§1.15b 552.1§3.89a 654.2§2.79b 316.8§1.70a 434.0§4.08b 531.0§0.95b 10¡5288.8§0.77c 493.6§3.39b 487.2§1.03c 288.6§0.43b 504.5§1.07c 663.0§1.62c 10¡4270.7§1.28d248.8§0.88c184.8§0.52d240.4§0.92c362.0§1.49d308.4§1.51dTable 4Relative amount changes of Mn in roots and shoots of cultivars after treatment with di V erent Cd concentrations and treatment time Cultivars Treatment (M)Total amount ( g/g DW)Root ( g/g DW) (%)Shoot ( g/g DW) (%)5days 10days 15days 5days 10days 15days 5days 10days 15days 10¡6145.5147.3135.963.245.343.436.854.756.610¡561.670.947.924.818.828.075.281.272.010¡459.549.322.220.315.641.979.784.458.1Liyu No. 6Control 207.2264.9275.859.538.456.140.561.643.910¡6230.8225.8254.665.064.455.735.035.644.310¡565.082.461.929.522.327.970.577.772.110¡455.948.822.933.521.946.766.578.153.386M. Wang et al. / Bioresource Technology 98 (2007) 82–88of Cd in nutrient solution (Table 3). In addition, tissue Zn levels decreased with duration of treatment (10¡4M Cd; Nongda No. 108). Tissue Zn was highest in the control. The Zn accumulated more in the roots of the cultivars compared with shoots (Table 7).4. DiscussionCadmium is a toxic element without any known physio-logical function in plants. There are several de W nitions of metal hyperaccumulation (Baker and Brooks, 1989; Baker et al., 2000; Baker and Whiting, 2002). Most recognize standard criteria based on metal concentrations in above-ground tissue of plant material sampled from its natural habitat (Pollard et al., 2002). According to the currently accepted shoot concentration de W ning hyperaccumulation being 0.01% (w/w) for cadmium (Baker et al., 2000) and data from Table 1, the Liyu No. 6 cultivar can be con-sidered a metal-hyperaccumulator. This cultivar having a dense root system, a high biomass and considerable ability to accumulate Cd can play an important role in treating Cd-contaminated soil. Although Cd concentrations in the shoots of Nongda No. 108 were 205.1 g/g DW (at 10¡5M Cd) and 364.5 g/g DW (10¡4M Cd) respectively, root growth nearly stooped after 10days treatment. These results demonstrate that Liyu No. 6 has an advantage in uptake and accumulate of Cd when compared to Nongda No. 108.The results of the present investigation are similar to those of Kumar et al. (1995), Dushenkov et al. (1995) and Jiang et al. (2001) for the use of plants to remove heavy metals from an aquatic habitat and soils. The reportedTable 5Relative amount changes of Fe in roots and shoots of cultivars after treatment with di V erent Cd concentrations and treatment timeCultivars Treatment (M)Total amount ( g/g DW)Root ( g/g DW) (%)Shoot ( g/g DW) (%)5days10days15days5days10days15days5days10days15days Nongda No. 108Control166.5288.5577.165.983.087.734.117.012.2 10¡6337.0458.2721.171.885.295.728.214.8 4.310¡5353.6582.9979.583.992.095.416.18.0 4.610¡4409.5269.8780.075.988.692.324.111.47.7 Liyu No. 6Control254.4332.6624.863.279.593.836.820.5 6.2 10¡6338.1423.9791.979.079.394.621.020.7 5.410¡5305.7600.51086.071.684.295.628.415.8 4.410¡4377.9810.3929.166.390.595.933.79.5 4.1 Table 6Distributive changes of Cu in roots and shoots of cultivars after treatment with di V erent Cd concentrations and treatment timeCultivars Treatment (M)Total amount ( g/g DW)Root ( g/g DW) (%)Shoot ( g/g DW) (%)5days10days15days5days10days15days5days10days15days Nongda No. 108Control63.9108.684.277.084.384.423.015.715.6 10¡634.545.656.171.056.878.329.043.221.710¡543.968.574.978.884.783.021.215.317.010¡436.649.4104.971.678.764.628.421.335.4 Liyu No. 6Control65.8100.644.576.781.980.723.318.119.3 10¡655.366.3112.979.083.489.621.016.610.410¡563.1116.889.980.589.086.919.511.013.110¡437.431.934.379.774.374.920.325.725.1 Table 7Distributive changes of Zn in roots and shoots of cultivars after treatment with di V erent Cd concentrations and treatment timeCultivars Treatment (M)Total amount ( g/g DW)Root ( g/g DW) (%)Shoot ( g/g DW) (%)5days10days15days5days10days15days5days10days15days Nongda No. 108Control723.51192.11103.060.866.869.939.233.230.1 10¡6868.7860.6952.567.556.660.832.543.439.210¡5656.5591.3839.957.365.041.542.735.058.510¡4599.2473.7399.654.953.840.145.146.259.9 Liyu No. 6Control686.01050.11051.852.653.768.547.446.331.5 10¡6845.9986.11185.2562.556.055.237.544.044.810¡5577.4998.11150.250.049.542.450.050.657.610¡4511.1610.8493.253.040.737.547.059.362.5M. Wang et al. / Bioresource Technology 98 (2007) 82–8887di V erences in root uptake and shoot accumulation might be explained by the fact that one of the normal functions of roots is to selectively acquire ions from the soil solution (Salt et al., 1997). Cadmium-tolerant plants must be able to prevent the absorption of excess Cd or detoxify the Cd after it has been absorbed.Bowler et al. (1992) indicated that Mn is associated with some Mn-metalloproteins. The results from this investiga-tion showed that Mn uptake and accumulation were reduced signi W cantly (P<0.05) in the cultivars treated with di V erent Cd concentrations and progressively decreased with an increase in Cd concentration, which is in agreement with reports by Yang et al. (1996) and Hernández et al. (1998). The relative distribution of Mn between root and shoot of the control showed that Mn mainly accumulated in the roots. These results are in agreement with previous observations, where pea, maize and lettuce plants were treated with Cd (Hernández et al., 1998, 1996). Hagemeyer (1999) indicated that one Mn de W ciency symptom is inter-veinal chlorosis, occurring W rst in young leaves. Maynard (1979) found that Mn de W cient dicots often show small yellow spots on the leaves and interveinal chlorosis. Chlo-rosis that was noted on leaves of Nongda No. 10 may be explained by the fact that the Mn level decreased signi W-cantly (P<0.05) after treatment with Cd, suggesting that this was one factor for better growth in Liyu No. 6 than Nongda No. 108 at Cd concentrations ranging from 10¡6M to 10¡5M.Iron uptake in the groups treated with Cd was greater than in control plants in the present investigation. This is in agreement with the W ndings of Hernández et al. (1998), where Fe in pea plants was higher than in the control plants after treatment with 50 M Cd. Hernández et al. (1998) stated that Fe in pea plants exposed to Cd was absorbed from the nutrient solution at a lower extent than Mn, which disagreed with the present results, where Fe uptake and accumulation is much higher than Mn. It is because the plants were di V erent. There are many reports related to changes of Fe concentrations in the presence of Cd. Iron concentrations in tomato (Moral et al., 1994), bean (Chaoui et al., 1997), maize (Hernández et al., 1996) and lettuce plants (Gárate et al., 1992) were not well correlated with levels of Cd supplied; the experimental conditions were quite di V erent.Phytochelatins (PCs) have been shown to bind Cd and Cu directly (Murasugi et al., 1981; Reese et al., 1988). Cad-mium has a stronger response to PC synthesis than does Cu (Ste V ens, 1990). PCs (Rauser, 1990, 1995) are believed to function as detoxifying agents for Cd by virtue of their abil-ity to bind Cd and thereby prevent it from reacting with the sulfydryl groups of vital enzymes and proteins (Grant et al., 1998). Various interactions occur when plants are exposed to excessive concentrations of more than one trace element. The results from this investigation indicated that the Cu total contents of two cultivars increased with prolonged treatment time, and the plants concentrated Cu in the roots more than above-ground parts after treatments with di V er-ent Cd concentrations. This disagrees with the W ndings of Liu et al. (2001) where Z. mays was treated with Cu without Cd added and the results showed that a large amount of Cu was accumulated in the shoots (10¡4M and 10¡5M Cd).Hagemeyer (1999) stated that signi W cant inhibition of root elongation was observed with Zn concentrations> 1000 M in soil. Lasat et al. (2000) indicated that genes of the Zn transporter-ZNT1(T. caerulescens) were cloned, suggesting that ZNT1 could enhance Cd translocation in plants. Cadmium uptake can be increased (Girling and Peterson, 1981) or decreased (Cataldo et al., 1983) by additional Zn supply to a soil, depending on the resulting Zn/Cd ratio in the soil solution. Jarvis et al. (1976) and Cataldo et al. (1983) indicated that Zn reduced tissue Cd uptake more than Cd reduced Zn uptake. Cadmium was found to be most e V ective in promoting PC production, followed by copper, zinc, silver and mercury (Zenk, 1996; Maitani et al., 1996). It was observed that root tips of maize exposed to Zn could produce PCs (Tukendorf, 1996). The results from this investigation indicated that the Cd:Zn ratio in tissue (Tables 2 and 7) can obviously not reach the ratio of 1 g Cd:>100 g Zn. Zinc accumulation in the culti-vars decreased signi W cantly (P<0.05) with increasing Cd concentration. Questions on the ratio of Cd:Zn and inhibi-tory e V ects of Cd on Zn uptake and vice versa remain to be answered.E Y ciency of phytoextraction is relative to the ability of the plant to grow on polluted soils and produce substantial biomass with high concentrations of target metal in the above-ground parts (Schwartz et al., 2001). In the present investigation, the cultivars can take up and accumulate sub-stantial amounts of Cd at an external concentration of 10¡4M Cd but the growth of the seedlings was severely reduced. Therefore, it is important for researchers to select a suitable phytoremediation species with high Cd uptake and accumulation capabilities without the plant undergo-ing severe damage.AcknowledgementsThis project was supported by the National Natural Sci-ence Foundation of China. The authors wish to express their appreciation to the reviewers for this paper.ReferencesBaker, A.J.M., Brooks, R.R., 1989. 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