Cadmium accumulation in the shoots and roots of 93 weed species

Abstract

The present study was conducted to find useful weed species for cadmium (Cd) phytoremediation. Ninety-three weed species and eight crop species were grown for 2 months in pots containing sandy loam soil with 3 mg Cd kg⁻¹ dry weight (DW). The Cd concentrations in the shoots and roots of all species were determined by inductively coupled plasma optical emission spectroscopy and atomic absorption spectrometry. Shoot Cd concentrations (mg kg⁻¹ DW) of Cichorium intybus (77.0) and Matricaria chamomilla (64.4) were higher than that of Polygonum thunbergii (56.2), which is a recognized hyperaccumulator. Root Cd concentrations (mg kg⁻¹ DW) were relatively high (≥ 100) in 11 species, for example, Oenothera biennis (171.9), Calystegia sepium var. americana (122.6) and Cassia obtusifolia (122.2). Shoot–root ratios (ratio of shoot and root Cd concentrations) were higher in Compositae species, for example, Cichorium intybus (3.56) and Bidens frondosa (3.30), than in Gramineae species, for example, Oenothera biennis (0.01), Oryza sativa cv. Milyang 42 (0.02) and Coix lacryma-jobi (0.03). Twelve species, for example, Cyperus brevifolius var. leiolepis (50.0), Polygonum thunbergii (49.7) and Bidens frondosa (40.5), had relatively high plant contents of Cd (≥ 20.0 µg plant⁻¹). These results suggest that: (1) Cichorium intybus and Matricaria chamomilla accumulate high shoot Cd concentrations, (2) Oenothera biennis, Calystegia sepium and Cassia obtusifolia accumulate high root Cd concentrations, (3) Compositae species are better able than Gramineae species to translocate Cd from roots to shoots. As both plant biomass and Cd concentration are related to Cd content, it was concluded that Bidens frondosa, Bidens pilosa and Amaranthus viridis, which not only have a high Cd accumulation ability but also a large biomass, are useful species for Cd phytoremediation.

Key words:

• cadmium
• Compositae
• Gramineae
• phytoremediation
• weed

INTRODUCTION

Cadmium (Cd), which inhibits several plant physiological processes such as oxidative reactions and nitrogen metabolism, is well known to be harmful to plants (Attila et al. 2001; Laspina et al. 2005; Toppi and Gabbrielli 1999). Because Cd is also toxic to humans, Cd-polluted soil in agricultural land, especially paddy fields, caused by irrigation water containing Cd running out from mines has become a serious problem. Until now, soil dressing and soil washing have improved Cd-polluted soil; however, because these civil engineering techniques need large quantities of unpolluted replacement soil and are also costly they are deemed impracticable for the remediation of large areas. Therefore, phytoremediation, a new technique for rehabilitating contaminated soil, has recently received considerable attention (Elizabeth 2005).

Phytoremediation consists of three techniques, phytoextraction, phytostabilization and phytovolatilization, in which polluted substances are taken up by plant roots, fixed around and/or in the roots and volatilized during transpiration from the leaf surface to the atmosphere. Studies on phytoextraction have so far been limited to crop plants, such as Brassica napus (Grispan et al. 2006), Brassica juncea (Qudir et al. 2004) and Oryza sativa (Ishikawa et al. 2006), because crop plants are generally superior in seed supply and are more adapted to cultivation than weed species. However, weeds have several useful characteristics for phytoremediation, including vigorous growth, the production of large numbers of seeds and an ability to grow in poor habitats. For example, Chenopodium album and Amaranthus lividus, major cosmopolitan upland weeds, often grow up to 2 m or more in height and Conyza sumatrensis, a representative weed of wasteland, produces more than 100,000 seeds per plant. Atriplex subcordata and Panicum virgatum have been reported to be tolerant of saline soil and dry conditions (Ichizen et al. 1993; Takematsu and Ichizen 1987, 1993). Abe et al. (2006) reported that several weed species, including Portulaca oleracea, Phytolacca americana and Anthemis cotula, can survive even at Cd concentrations as high as > 30 mg kg⁻¹. However, although there are some exceptions, such as Thalaspi caerulescens (Schwartz et al. 2003), Polygonum thunbergii (Kim et al. 2003; Shinmachi et al. 2003), Solidago altissima and Athyrium yokoscense (Yamada et al. 1975), the use of weeds as Cd phytoremediators has not been thoroughly investigated.

Therefore, to obtain fundamental information for the development of a Cd phytoremediation technique with weed species, we analyzed the shoot and root Cd concentrations and Cd content per plant of 101 pot-grown species comprising 93 weed species and eight crop species.

MATERIALS AND METHODS

Plant materials and Cd application

Cd(NO₃)₂ was mixed with a sandy loam soil (Fluvisol; particle diameter < 2 mm; pH(H₂O) 5.8; total-Cd 0.17 mg kg⁻¹; P₂O₅ 256 mg kg⁻¹; total N 1.6 mg kg⁻¹) at 3 mg Cd kg⁻¹ dry weight (DW); the soil contained compound fertilizer (NH⁺ ₄-N : P₂O₅ : K₂O : MgO = 8:8:8:2) at 1.4 g kg⁻¹ DW. Next, 1.0 kg DW of the Cd-treated soil was placed into ceramic pots (measuring 11 cm in diameter and 12 cm in depth) and then seeds of all the plants listed in Table 1 were sown individually at 10 and/or 20 seeds per pot. After emergence, seedlings were thinned to 5–10 plants per pot and were grown with watering as needed. Two months after sowing the plants were harvested and divided into shoots and roots and samples were analyzed for Cd. All experiments were conducted in a glasshouse under natural daylight and temperature conditions and were replicated three times.

Cadmium analysis

The shoots and roots of 101 species were washed with distilled water and then dried in an oven at 70°C for 3 days. The dried plant samples were then ground to a powder with a stainless-steel mill and an aliquot of 0.5 g of the powder was weighed in a glass test tube. The samples were digested with 10 mL of HNO₃ and made up to 25 mL with distilled water. The Cd concentrations in the test solutions were determined by inductively coupled plasma optical emission spectroscopy (SPS1200VR; Seiko Instruments, Chiba, Japan) and atomic absorption spectrometry (SAS760; Seiko Instruments). The Cd analysis was replicated three times per plant and the mean Cd concentration in the shoots and roots was calculated.

RESULTS

The Cd concentration in the shoots and roots, the shoot–root ratios and the Cd contents in the shoot are shown in Table 1. Cadmium concentrations (mg kg⁻¹ DW) in the shoots of all plants tested ranged from 0.8 (Oryza sativa cv. Milyang 42) to 77.0 (Cichorium intybus), with a mean value of 15.4, and in roots the concentrations ranged from 7.7 (Bidens pilosa) to 171.9 (Oenothera biennis), with a mean value of 51.6. Shoot–root ratios ranged from 3.56 (Cichorium intybus) to 0.01 (Oenothera biennis), with a mean value of 0.47. Shoot Cd contents (µg plant⁻¹) also varied greatly among species; the maximum, minimum and mean values were 50.0 (Cyperus brevifolius var. leiolepis), 0.2 (Trifolium repens) and 8.40, respectively.

The Cd concentrations of the shoots and roots, the shoot–root ratios and the plant Cd contents of representative plant species are shown in Tables 2, 3, 4 – 5. Twelve species, including Cichorium intybus (77.0), Matricaria chamomilla (64.4) and Polygonum thunbergii (56.2), had relatively high shoot Cd concentrations (≥ 30 mg kg⁻¹ DW). Sixteen species, including Oryza sativa cv. Milyang 42 (0.8), Panicum dichotomiflorum (1.0) and Dactylis glomerata (1.2), had relatively low shoot Cd concentrations (≤ 3.0 mg kg⁻¹ DW).

Root Cd concentration was highest in Oenothera biennis (171.9) followed by Calystegia sepium var. americana (122.6) and Cassia obtusifolia (122.2), and eight other species had relatively high root Cd concentrations (≥ 100 mg kg⁻¹ DW). Eleven species, for example, Bidens pilosa (7.7), Bidens frondosa (9.4) and Lactuca indica (11.4), had relatively low root Cd concentrations (≤ 20 mg kg⁻¹ DW).

Four species, for example, Cichorium intybus (3.56) and Bidens frondosa (3.30), had relatively high shoot–root ratios of Cd concentration (≥ 2.0) and 15 species, for example, Oenothera biennis (0.01), Plantago virginica (0.02) and Oryza sativa (cv. Milyang 42) (0.02), had relatively low values (≤ 0.05).

The 101 species were classified as high (≥ 20.0 µg plant⁻¹) or low (≤ 1.0 µg plant⁻¹) Cd accumulators, respectively. Twelve species, for example, Cyperus brevifolius var. leiolepis (50.0), Polygonum thunbergii (49.7) and Bidens frondosa (40.5), were high Cd accumulators and 13 species, for example, Trifolium repens (0.2), Dactylis glomerata (0.3) and Trifolium arvense (0.4), were low Cd accumulators.

Table 1 Cadmium concentrations in the shoots and roots, the shoot–root ratio of Cd and the shoot Cd content of 101 plant species grown in a sandy loam soil containing 3 mg Cd kg−1 dry weight for 2 months

Plant species	Cd concentration (mg kg^−1 DW)		Shoot–root ratio	Shoot Cd content (µg plant^−1)
	Shoot	Root
Oenothera biennis	1.8	171.9	0.01	3.7
Malvaceae
Abutilon theophrasti	14.9	35.8	0.42	13.4
Hibiscus cannabinus	22.1	18.3	1.21	21.4
Malva sylvestris	33.5	37.9	0.89	12.7
Sida rhombifolia	23.8	111.9	0.21	4.2
Sida spinosa	8.7	46.0	0.19	6.8
Euphorbiaceae
Euphorbia esula	12.5	46.3	0.27	1.0
Euphorbia helioscopia	23.2	57.3	0.41	1.0
Euphorbia peplus	21.6	75.2	0.29	1.7
Lineceae
Linum usitatissimum (cv. Blue earth)	17.5	50.4	0.35	4.3
Leguminosae
Cassia obtusifolia	7.3	122.2	0.06	3.7
Medicago sativa	9.2	19.0	0.49	0.9
Melilotus officinalis	8.8	38.2	0.23	0.5
Phseolus aureus	3.5	48.6	0.07	2.5
Sesbania cannabia	4.4	41.0	0.11	2.5
Sesbania exaltata	2.1	48.1	0.04	1.7
Trifolium arvense	8.0	56.8	0.14	0.4
Trifolium repense	2.7	23.9	0.11	0.2
Cruciferae
Brassica juncea	14.0	18.8	0.74	7.6
Camelina sativa	9.6	15.9	0.60	6.2
Diplotaxis tenuifolia	7.3	43.2	0.17	3.8
Eruca sativa	11.0	52.3	0.21	4.4
Sinapis alba	25.6	45.4	0.56	21.4
Sisymbrium altissimum	48.2	98.2	0.49	21.4
Sisymbrium orientale	48.2	70.3	0.69	7.2
Papaveraceae
Argemone mexicana	8.6	86.1	0.10	1.5
Caryophyllaceae
Arenaria serpyllifolia	15.5	71.7	0.22	2.9
Silene armeria	8.7	57.4	0.15	2.5
Silene dioica	4.9	113.0	0.04	2.1
Silene noctiflora	21.4	88.4	0.24	21.1
Portulaceae
Portulaca oleracea var. sativa	10.9	61.3	0.18	5.7
Portulaca oleracea	4.4	55.6	0.08	3.9
Phytolaceaceae
Pytolacca americana	4.4	82.6	0.05	3.8
Amaranthaceae
Amaranthus hybridus	23.7	73.9	0.32	20.7
Amaranthus rudis	17.3	68.1	0.25	15.1
Amaranthus spinosus	24.5	50.8	0.48	7.5
Amaranthus viridis	24.8	61.4	0.40	22.5
Chenopodiaceae
Atriplex subcordata	12.3	30.3	0.41	5.3
Chenopodium album	5.4	40.7	0.13	4.7
Chenopodium album var. centrorubrum	13.4	46.3	0.29	12.2
Chenopodium ambroisioides	17.6	68.3	0.26	8.6
Polygonaceae
Polygonum thunbergii	56.2	91.5	0.61	49.7
Rumex conglomeratus	9.5	29.3	0.32	3.5
Rumex crispus subsp. japonicus	11.6	12.2	0.96	1.7
Rumex obtusifolius	23.6	20.9	1.13	3.9
Compositae
Achillea millefolium	17.1	22.3	0.77	7.7
Ambrosia trifida	22.7	28.3	0.80	15.6
Anthemis cotula	23.2	37.6	0.62	6.0
Bidens frondosa	31.0	9.4	3.30	40.5
Bidens pilosa	17.0	7.7	2.21	24.1
Compositae
Cichorium intybus	77.0	21.6	3.56	38.8
Galinsoga eiliata	17.2	28.5	0.60	5.6
Hypochoeris radicata	16.5	16.3	1.01	6.2
Lactuca indica	25.9	11.4	2.20	11.6
Matricaria chamomilla	64.4	50.4	1.28	8.4
Matricaria inodora	19.3	28.1	0.69	4.6
Onopordum acanthium	30.0	37.7	0.80	10.0
Picris echinoides	41.0	43.8	0.94	5.1
Sonchus asper	37.0	20.7	1.79	14.0
Sonchus oleraceus	25.4	14.2	1.78	12.1
Plantagineaceae
Plantago lanceolata	5.5	62.2	0.09	1.5
Plantago major	3.2	46.0	0.07	2.6
Plantago virginica	1.4	73.1	0.02	0.9
Solanaceae
Nicandra physaloides	11.3	50.7	0.22	9.7
Solanum luteum	28.6	20.7	1.38	16.9
Solanum nigrum	6.6	52.4	0.13	3.5
Solanum sodomeum	14.8	36.7	0.40	7.3
Labiatae
Salvia plebeia	9.7	116.6	0.08	5.4
Boraginaceae
Amsinckia barbata	13.4	43.4	0.31	2.6
Convolvulaceae
Calystegia sepium var. americana	5.7	122.6	0.05	2.9
Ipomoea coccinea	6.8	101.8	0.07	7.6
Ipomoea lacunosa	5.3	47.0	0.11	3.4
Gramineae
Arthraxon hispidus	12.5	13.2	0.95	10.1
Avena fatua	4.7	21.5	0.22	2.5
Avena sativa	14.8	26.2	0.56	8.5
Coix lacryma-jobi	2.2	71.3	0.03	3.2
Dactylis glomerata	1.2	35.7	0.03	0.3
Gramineae
Digitaria ciliaris	1.9	22.8	0.08	2.7
Echinochloa crus-galli var. praticola	2.1	47.9	0.04	2.5
Echinochloa crus-galli var. crus-galli	9.0	63.6	0.14	13.2
Eragrostis curvula	8.4	26.0	0.32	3.3
Festuca arundinacea	5.3	115.4	0.05	1.3
Lolium multiflorum	7.2	39.8	0.18	3.7
Lolium perenne	4.4	105.8	0.04	1.1
Oryza sativa (cv. Koshihikari)	13.1	102.2	0.13	18.8
Oryza sativa (cv. Milyang 23)	11.8	81.8	0.14	13.2
Oryza sativa (cv. Milyang 25)	15.6	117.5	0.13	16.0
Oryza sativa (cv. Milyang 42)	0.8	44.0	0.02	0.7
Panicum bisulcatum	1.5	58.1	0.03	0.8
Panicum dichotomiflorum	1.0	29.0	0.03	0.9
Panicum virgatum	2.2	22.6	0.10	0.8
Paspalum dilatatum	2.2	20.1	0.11	1.4
Pennisetum alopecuroides	5.5	61.9	0.09	0.8
Poa pratensis	2.7	84.2	0.03	1.0
Setaria faberi	3.0	25.7	0.12	3.5
Setaria glauca	4.0	27.2	0.15	3.6
Setaria viridis	12.3	29.3	0.31	2.8
Sorghu bicolor (cv. Hybrid sorghum)	2.2	22.0	0.10	3.7
Cyperaceae
Cyperus brevifolius var. leiolepis	48.3	35.6	1.35	50.0
Cyperus globosus	39.6	43.3	0.91	30.3
Cyperus sanguinolentus	25.6	55.2	0.46	19.8
Mean value	15.4	51.6	0.47	8.40
DW, dry weight.

Table 2 Plant species that had relatively high (≥ 30.0 mg kg−1) and low (≤ 3.0 mg kg−1) Cd concentrations in their shoots

Scientific name	Family name	Cd concentration (mg kg^−1 DW)
Plant species with relatively high Cd concentrations
Cichorium intybus	Compositae	77.0
Matricaria chamomilla	Compositae	64.4
Polygonum thunbergii	Polygonaceae	56.2
Cyperus brevifolius var. leiolepis	Cyperaceae	48.3
Sisymbrium orientale	Cruciferae	48.2
Sisymbrium altissimum	Cruciferae	48.2
Picris echinoides	Compositae	41.0
Cyperus globosus	Cyperaceae	39.6
Sonchus asper	Compositae	37.0
Malva sylvestris	Malvaceae	33.5
Bidens frondosa	Compositae	31.0
Onopordum acanthium	Compositae	30.0
Plant species with relatively low Cd concentrations
Oryza sativa (cv. Milyang 42)	Gramineae	0.8
Panicum dichotomiflorum	Gramineae	1.0
Dactylis glomerata	Gramineae	1.2
Plantago virginica	Plantagineaceae	1.4
Panicum bisulcatum	Gramineae	1.5
Oenothera biennis	Onagraceae	1.8
Digitaria ciliaris	Gramineae	1.9
Sesbannia exaltata	Leguminosae	2.1
Echinochloa crus-galli var. prachicola	Gramineae	2.1
Coix lacryma-jobi	Gramineae	2.2
Sorgum bicolor (cv. Hybrid sorghum)	Gramineae	2.2
Panicum virgatum	Gramineae	2.2
Panicum dilatatum	Gramineae	2.2
Trifolium repens	Leguminosae	2.7
Poa pratensis	Gramineae	2.7
Setaria faberi	Gramineae	3.0
DW, dry weight.

Table 3 Plant species that had relatively high (≥ 100.0 mg kg−1) and low (≤ 20.0 mg kg−1) Cd concentrations in their roots

Scientific name	Family name	Cd concentration (mg kg^−1 DW)
Plant species with relatively high Cd concentrations
Oenothera biennis	Onagraceae	171.9
Calystegia sepium var. americana	Convolvulaceae	122.6
Cassia obtusifolia	Leguminosae	122.2
Oryza sativa (cv. Milyang 25)	Gramieae	117.5
Salvia plebeia	Lbiatae	116.6
Festuca arundinacea	Gramineae	115.4
Silene dioica	Caryophyllaceae	113.0
Sida rhombifolia	Malvaceae	111.9
Lolium perenne	Gramineae	105.8
Oryza sativa (cv. Koshihikari)	Gramineae	102.2
Ipomoea cocconea	Convolvulaceae	101.8
Plant species with relatively low Cd concentrations
Bidens pilosa	Compositae	7.7
Bidens frondosa	Compositae	9.4
Lactuca indica	Compositae	11.4
Rumex crispus subsp. japonicus	Polygonaceae	12.2
Arthraxon hispidus	Gramineae	13.2
Sonchus oleraceus	Compositae	14.2
Camelina sativa	Cruciferae	15.9
Hypochoeris rsdicata	Compositae	16.3
Hibiscus cannabinus	Malvaceae	18.3
Brassica juncea	Cruciferae	18.8
Medicago sativa	Leguminosae	19.0
DW, dry weight.

Table 4 Plant species that had relatively high (≥ 2.0) and low (≤ 0.05) shoot–root ratios

Scientific name	Family name	Shoot–root ratio
Plant species with relatively high shoot–root ratio
Cichorium intybus	Compositae	3.56
Bidens frondosa	Compositae	3.30
Lactuca indica	Compositae	2.27
Bidens pilosa	Compositae	2.21
Plant species with relatively low shoot–root ratio
Oenothera biennis	Onagraceae	0.01
Plantago virginica	Plantaginaceae	0.02
Oryza sativa (cv. Milyang 42)	Gramineae	0.02
Dactylis glomerata	Gramineae	0.03
Coix lacryma-jobi	Gramineae	0.03
Panicum bisulcatatum	Gramineae	0.03
Panicum dichotomiflorum	Gramineae	0.03
Poa pratensis	Gramineae	0.03
Lolium perenne	Gramineae	0.04
Echinochloa crus-galli var. praticola	Gramineae	0.04
Silene dioica	Caryophyllaceae	0.04
Sesbania exaltata	Leguminosae	0.04
Calystegia sepium var. americana	Convolvulaceae	0.05
Festuca arundinacea	Gramineae	0.05
Phyrolacca americana	Phytolaccaceae	0.05

Table 5 Plant species that had relatively high (≥ 20 µg) and low (≤ 1.0 µg) shoot Cd contents

Scientific name	Family name	Shoot Cd content (µg plant^−1)
Plant species with relatively high Cd content
Cyperus brevifolius var. leiolepis	Cyperaceae	50.0
Polygonum thunbergii	Polygonaceae	49.7
Bidens frondosa	Compositae	40.5
Cichorium intybus	Compositae	38.8
Cyperus globosus	Cyperaceae	30.3
Bidens pilosa	Compositae	24.1
Amaranthus viridis	Amaranthaceae	22.5
Hibiscus cannabinus	Malvaceae	21.4
Sinapis alba	Cruciferae	21.4
Sisymbrium altissimum	Cruciferae	21.4
Silene notiflora	Caryophyllaceae	21.1
Amaranthas hybridus	Amaranthaceae	20.7
Plant species with relatively low Cd content
Trifolium repens	Leguminosae	0.2
Dactylis glomerata	Gramineae	0.3
Trifolium arvense	Leguminosae	0.4
Melilotus officinalis	Leguminosae	0.5
Oryza sativa (cv. Milyang 42)	Gramineae	0.7
Panicum virgatum	Gramineae	0.8
Pennisetum alopecuroides	Gramineae	0.8
Panicum bisulcatum	Gramineae	0.8
Medicago sativa	Leguminosae	0.9
Plantago virginica	Planatagineaceae	0.9
Euphorbia esula	Euphorbiaceae	1.0
Euphorbia helioscopia	Euphorbiaceae	1.0
Poa pratensis	Gramineae	1.0

DISCUSSION

The results indicate that shoot and root Cd concentrations vary greatly among species: differences in shoot and root Cd concentrations among the 101 species showed variations of up to 96.3-fold and 22.3-fold, respectively. The Cd concentrations in the shoots were relatively high (≥ 30 mg kg⁻¹ DW) in 12 species, for example, Cichorium intybus and Matricaria chamomilla, and it was supposed that these 12 species can accumulate high levels of Cd in their shoots. Shoot Cd concentrations of Cichorium intybus and Matricaria chamomilla were higher than those of Polygonum thunbergii, which has been recognized as a Cd hyperaccumulator (Shinmachi et al. 2003); thus, both weeds are considered to be promising plants for phytoextraction of Cd-polluted soil. Moreover, although the numbers of Compositae (15 species) and Gramineae (26 species) plants tested were much higher than the numbers from other plant families, the results suggest that Compositae species can accumulate large amounts of Cd in their shoots because 6 of the 12 species with high shoot Cd concentrations (≥ 30 mg kg⁻¹ DW) belonged to Compositae. Harada and Hatanaka (2000) also reported that a large number of Compositae species had high shoot Cd concentrations. Therefore, it is supposed that Compositae species can accumulate Cd specifically in their shoots. This trait of Compositae is considered to be associated with certain physiological characteristics, such as Cd detoxification and Cd translocation from roots to shoots. In general, it is assumed that Cd absorbed by roots is concentrated and detoxified in roots and then translocated in the xylem sap to the shoots via the stem (Shinmachi et al. 2003). For example, Grill et al. (1987) reported that certain plants form phytochelatin, Cd-binding peptides, and Shinmachi et al. (2003) suggested that glutamine and fructose may play an important role in Cd detoxification and translocation in plants.

In contrast, 12 of 16 species that contained relatively low concentrations of Cd (≤ 3.0 mg kg⁻¹ DW) in shoots belonged to Gramineae, suggesting that Gramineae species accumulate low shoot Cd concentrations compared with species from the other families tested, especially Compositae. However, 4 of 11 species with high root Cd concentrations (≥ 100 mg kg⁻¹ DW) belonged to Gramineae, but none of these 11 root Cd accumulators were Compositae species. This result suggests that Gramineae species can accumulate more Cd in their roots than Compositae species, or that Cd is difficult to translocate from roots to shoot in Gramineae species. In addition, Gramineae weed species, such as Echinochloa crus-galli var. crus-galli and Setaria faberi, have also been reported to be Cd tolerant (Abe et al. 2006); therefore, Gramineae weed species could be useful for Cd phytostabilization.

In the present study, the shoot–root ratio of Cd concentration was determined to understand the translocation of Cd from roots to shoot in weed species. The shoot–root ratio value was relatively high (≥ 2.0) in Cichorium intybus, Bidens frondosa, Lactuca indica and Bidens pilosa, and it was considered that these weeds have a high ability to translocate Cd from roots to shoot because an increase in the shoot–root ratio value implies enhanced Cd translocation.

In addition to the shoot and root Cd concentrations and shoot–root ratio, shoot Cd content was calculated for all species to evaluate the ability of weeds to act as Cd phytoremediators. Because both plant biomass and plant Cd concentration are related to shoot Cd content, it is supposed that plant species that possess not only the ability to accumulate high levels of Cd but are also of large biomass are suitable for Cd phytoremediation. Therefore, Polygonum thunbergii, Bidens frondosa, Bidens pilosa and Amaranthus viridis are deemed to be more useful species for Cd phytoremediation than Cyperus brevifolius var. leiolepis and Cichorium intybus.

In conclusion, we found that: (1) several weed species, for example, Cichorium intybus, Matricaria chamomilla and Polygonum thunbergii, accumulate a high shoot Cd concentration, (2) most Compositae species accumulate a higher Cd concentration in shoots than in roots, but Gramineae species have the reverse tendency, (3) several weed species, for example, Bidens frondosa, Bidens pilosa and Amaranthus viridis, major cosmopolitan weeds in paddy and upland fields, contain high levels of plant Cd.

Although these experiments with weed species have yielded useful information on Cd phytoremediation, further study is needed in Cd-contaminated fields where growth and soil conditions, such as the form and level of soil Cd, and soil type may differ from the conditions used in this glasshouse experiment.

ACKNOWLEDGMENTS

We thank T. Namatame, S. Momma, H. Kanai, Y. Kita, A. Sekine, and M. Minegishi for their assistance with the Cd analysis.