Heavy metal contamination of agricultural soils, aquatic waters, and ground water can pose serious environmental and health concerns (45). Experimentation into the phyotoextraction of copper from soils is limited (46). However, approximately 24 copper-hyperaccumulating plant species have been reported, including members of Cyperaceae, Lamiaceae, Poaceae, and Scrophulariaceae families (46). Reportedly, the only true copper-accumulating plants are from the central African countries of Zaïre and Zambia (47,48). The political instability of these regions makes obtaining plant material for research experimentation difficult and has hindered the work in this area (47,48). Work by Morrison (49) with Zaïrian copper-tolerant plants showed mint species (Aeollanthus biformifolius De Wild) to accumulate 3920 µg Cu g^-1 dry weight; figwort species, bluehearts, (Buchnera metallorum L.) to accumulate 3520 µg g^-1 dry weight; gentian species (Faroa chalcophila P. Taylor) to accumulate 700 µg g^-1 dry weight; and mint species (Haumaniastrum robertii (Robyns) Duvign. & Plancke) to accumulate 489 µg g^-1 dry weight (47,48). Rhodegrass (Chloris gayana Kunth.), African bristlegrass or forage setaria (Setaria sphacelata Stapf. and C.E.Hubb), two indigenous grass species, and oat (Avena sativa L.) were evaluated for copper soil extraction in Ethiopian vegetable farms irrigated with wastewater from a textile factory, water from the Kebena and Akaki Rivers, and potable tap water. The maximum copper concentration of these plants was only 10.4 mg kg^-1 dry weight. However, soil copper levels for the experiments ranged from 2.5 to 3.5mg kg^-1, and these low values may indicate low copper delivery from these irrigation sources (50).

Some plants have shown a strong potential for hyperaccumulation of copper in their tissues. A population of aromatic madder (Elsholtzia splendens Nakai) collected on a copper-contaminated site in the Zhejiang providence of China demonstrated phytoremediation potential after the species was noted to accumulate 12,752 µg Cu g^-1 dry weight in roots and 3417 µg Cu g^-1 dry weight in shoots when cultured in nutrient solutions containing 1000 µM Cu²⁺ (55). Alfalfa shoots accumulated as much as 12,000 mg Cu kg^-1 (56). Roots of a willow species (Salix acmophylla Boiss.), an economically important tree which grows on the banks of water bodies, accumulated nearly 7 to 624 µg Cu g^-1 dry weight in response to increasing copper treatments in soil from 0 to 10,000mg kg^-1 (45). On three soils in Zambia, the roots of a grass species (Stereochlanea cameronii Clayton) accumulated 9 to 755 µg Cu g^-1 dry weight in response to a range from 0.2 to 203 µg Cu g^-1 in soil (57). Evidence suggests quantitative genetic variation in the ability to hyperaccumulate heavy metals between- and within-plant populations (58). Populations of knotgrass (Paspalum distichum L.) and bermudagrass (Cynodon dactylon Pers.) located around mine tailings in China contained 99 to 198 mg Cu kg^-1. These native grass populations were more tolerant to increasing CuSO₄ concentrations in solution culture than similar genotypes collected from sites containing much lower levels of copper in soil (2.55 mg Cu kg^-1) (59). Legumes, Lupinus bicolor Lindl. and Lotus purshianus Clem. & Clem., growing on a copper mine site (abandoned in 1955) in northern California showed greater tolerance to 0.2 mg Cu L^-1 in solution culture than genotypes growing in an adjacent meadow (60). Among ten Brassicaceae, only Indian mustard (Brassica juncea L.) and radish showed seed germination higher than 90% after 48 h exposure to copper concentrations ranging from 25 to 200 µM (18). As noted with other heavy metals, copper actually caused a slight increase in the degree of seed germination, possibly due to changes in osmotic potential that promote water flow into the seeds (18).



Copper toxicity limits have been established for grass species used to restore heavy metalcontaminated sites. Using sand culture, the lethal copper concentration for redtop (Agrostis gigantea Roth.) was 360 mg Cu L^-1, for slender wheatgrass (Elymus trachycaulus Gould ex Shiners) was 335 mg Cu L^-1, and for basin wildrye (Leymus cinereus A. Love) was 263 mg Cu L^-1, whereas tufted hairgrass (Deschampsia caespitosa Beauv.) and big bluegrass (Poa secunda J. Presl) displayed less than 50% mortality at the highest treatment level of 250 mg Cu L^-1 (61). Success has been shown with sodium-potassium polyacrylate polymers for copper remediation in solution and sand culture; however, the cost of application is often prohibitive. This polymer material at 0.07% dry mass in sand culture absorbed 47, 70, and 190 mg Cu g^-1 dry weight at 0.5µM, 1µM, 0.01 M Cu (as CuSO₄.5H₂O) in solution, respectively (62). In this experiment, the polyacrylate polymer increased the dry weight yield of the third and fourth cutting of perennial ryegrass (Lolium perenne L.) after 50 mg Cu kg^-1 was applied.