Environ. Sci. Techno/. 1995, 29, 1232- 1238
n: The Use of TO
Introduction
Is
hS d s P. B. A. NANDA KUMAR,’ VIATCHESLAV DUSHENKOV,I HARRY MOTTO,’ A N D ILYA RASKIN*lt AgBiotech Center and Department of Environmental Sciences, Rutgers University, Cook College, P.O. Box 231, New Brunswick, New jersey 08903-0231
A small number of wild plants that grow on metalcontaminated soil accumulate large amounts of heavy metals in their roots and shoots. This property may be exploited for soil reclamation if an easily cultivated, high biomass crop plant able to accumulate heavy metals is identified. Therefore, the ability of various crop plants to accumulate Pb in shoots and roots was compared. While all crop Brassicas tested accumulated Pb, some cultivars of Brassica juncea (L.) Czern. showed a strong ability to accumulate Pb in roots and to transport Pb to the shoots (108.3 mg of Pb/g DW in the roots and 34.5 mg of Pb/g DW in the shoots). B. juncea was also able to concentrate Cr“, Cd, Ni, Zn, and Cu in the shoots 58-, 52-, 31-,17-, and 7-fold, respectively,from a substrate containing sulfates and phosphates as fertilizers. The high metal accumulation by some cultivars of B. juncea suggests that these plants may be used to clean up toxic metal-contaminated sites in a process termed phytoextraction .
The notion that the elemental composition of plants is very different from that of the soil in which they grow is taken for granted. Most of these differences can be attributed to a plant’s ability to fix carbon from the air and to absorb essential macro- and micronutrients from the soil, which include heavy metals. The root morphology of terrestrial plants is a good example of how plants are able to produce a large surface to volume ratio in order to maximize the uptake of various elements and compounds from soil. The total length of roots (including root hairs) of a single potgrown rye plant is about 387 mi (I) and can be even larger for field-grown plants. For a long time, the ability of plants to accumulate metals was considered a detrimental trait. Being at the bottom of many natural food chains, metal-accumulating plants are directly or indirectly responsible for a large proportion of the dietary uptake of toxic heavy metals by humans and animals (2). While some heavy metals are required for life, their excessive accumulation in living organisms is always toxic. The danger of heavy metals is aggravated by their almost indefinite persistence in the environment. For example, lead (Pb), which is one of the more persistent metals, was estimated to have a soil retention time from 150 to 5000 years (3). Heavy metals are present in soils as natural components or as a result of human activity. Metal-rich mine tailings, metal smelting, electroplating, gas exhausts, energy and fuel production, downwash from power lines, intensive agriculture, and sludge dumping are the most important human activitiesthat contaminate soilswith large quantities of toxic metals (4). In spite of the ever-growingnumber of toxic metal-contaminated sites, the most commonly used methods of dealingwith heavy metal pollution are still either the extremely costly process of removal and burial or simply isolation of the contaminated sites. In addition to sites contaminated by human activity, natural mineral deposits containing particularly large quantities of heavy metals are present in many regions of the globe. These areas often support characteristic plant species that thrive in these metal-enriched environments. Some of these species can accumulate unusually high concentrations of toxic metals to levels which far exceed the soil levels (5).Accumulators of nickel (61,cobalt and copper (7,manganese (81,lead and zinc (91, and selenium (10)have been reported. As a result of their association with specific ore deposits, many metallophyte plants are used in prospecting for mineral deposits (11, 12). Only recently has the value of metal-accumulatingplants for environmental remediation has been fully realized (13181, giving birth to a new technology termed “phytoextraction”. The process of phytoextractiongenerallyrequires the translocation of heavy metals to the easily harvestable shoots. In some cases, roots can be harvested as well. In the proposed phytoextraction process, several sequential crops of hyperaccumulating plants may be used to reduce * To whom correspondence should be addressed: Telephone: (908) 932-8734; Fax: (908) 932-6535; e-mail address:
[email protected]. edu. AgBiotech Center. * Department of Environmental Sciences. +
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@ 1995 American Chemical Society
soil concentrations of heavy metals to environmentally acceptable levels (17). Dried, ashed, or composted plant residues highly enriched in heavy metals may be isolated as hazardous waste or recycled as metal ore. While the most heavily contaminated soils do not support plant growth, sites with light to moderate toxic metal contamination could be remediated by growing metal-accumulating plants. While phytoextraction research is still in the early development stage, small-scalefield trials with wild metal accumulators collected from naturally contaminated soils have demonstrated the feasibility of the phytoextraction approach (15). There are many advantagesin using metal-accumulating plants for the removal of metals from contaminated soils: lower costs, generation of a recyclable metal-rich plant residue, applicability to a range of toxic metals and radionuclides; minimal environmental disturbance, elimination of secondary air or water-borne wastes, and public acceptance. Unfortunately, most of the metal-accumulating plants identified so far are slow growing, small, and/or weedyplants that produce low biomass and have undefined growth requirements and characteristics. Therefore, our efforts to develop phytoextraction concentrated on identifying crop and crop-related species that can accumulate heavy metals while producing high biomass in response to established agriculturalpractices. Particular emphasis was placed on the crop-related members of the Brassicaceae family that are related to many of the known wild metal accumulators. Lead was used in most of the initial studies because of its importance as an environmental pollutant. In addition, Pb is tightly bound in most soils (2),a property which makes it a particularly challenging metal for phytoextraction. Subsequent workwas extended to other heavy metals that pose significantenvironmentalhazards. In this paper, we report the identification of several cultivars of Indian mustard (Brassicajuncea),ahigh biomass crop plant, that efficiently accumulates Pb and other heavy metals. We have also characterized the biological parameters of metal accumulation in roots and shoots of this plant and evaluated the potential uses of B. juncea in the phytoextraction process.
Materials and Methods Plant Material. Brassica species (B. nigra, B. oleracea, B. campestris, B. carinata, B. juncea, and B. napus) were obtained from the Crucifer Genetics Cooperative,Madison, Wisconsin. Additional Brassica species and cultivars were provided by USDA/ARS Plant Introduction Station of Iowa State University. Seeds of synthetic B. juncea with B. campestris (cultivarTobin) and B. nigru (accession R1819) cytoplasms were obtained from Agriculture Canada Research Station, Saskatoon, Canada. The seeds of other plants were purchased from local seed markets. Experimental Design. Seedlings were grown in a greenhouse equipped with supplementary lighting (16-h photoperiod; 24-28 "C). Unless mentioned otherwise, seedlings were grown for 10 days in acid-washed coarse sand and fertilized every 2 days either with full-strength Hoagland's solution or with 1g/L of Hydrosol supplemented with 0.6 g/L Ca(NO3I2.Ten-day-old seedlings were transplanted (in sets of two) into 150 g DW of acid-washed 1:l (vlv) mixture of coarse sand and coarse Perlite placed in 3.5-in. round plastic pots. Transplanted seedlings were well-watered and fertilized with KN03solution (0.404 g/L) for 7 days before the metal application. Phosphates and
sulfates were not used in order to prevent precipitation of Pb and other heavy metals, except in the experiments in Figure 5. Control plants were watered with K N 0 3 solution calculated to contain the same amount of NO3- as in Pb(N0312. Approximately10 mLof metal-containingsolutions were applied to the surface of the growth medium once at the beginning of the treatment period. In experiments comparing the uptake of differentmetals by B. juncea, seedlings were started in 3.5-in. plastic pots with approximately 150 g of sand-Perlite mixture fertilized with Hydrosol solution supplementedwith 0.6 glL Ca(N03)2 and grown for 17 days without transplanting. Before metal treatment, the pots containing seedlings were flushed with water. The metals used were Cd [2 mglL, supplied as Cd(N03)2*4H201,C P 150 mglL, supplied as Cr(N03)39H20], C P (3.5mg/L, supplied as K2Cr207),Cu [ 10 mg/L, supplied as C U ( N O ~ ) ~ - ~ HNi~ O (100 I , mg/L, supplied as Ni(N03)26H201,Pb [500 mg/L, supplied as Pb(N03)21,and Zn [lo0 mg/L, supplied as Zn(N03)2.GH20].The concentrations at which the metals were used correlatedwiththe water quality standards in the State of New Jersey and were below the levels toxic to B. juncea. Excess soil moisture was trapped in 5.5-in. plastic saucers placed below each pot to prevent leaching from pots. No fertilizer was given during the metal treatment. Every other day, the plants were foliarly fertilized with Miracle Gro solution until most of the leaves were wet. Each pot contained two seedlings. At least four replicates for each metal concentration were used. Liquid Cultures. Plants were hydroponically grown in 400 mL of 1 g/L Hydrosol supplemented with 0.6 glL Ca(NO& for 17 days. Hydroponic solutionswere continuously aerated and replaced every 5 days. Nutritionally starved plants were removed from the nutrient solution and their roots were immersed in deionized water from day 10 to day 17, while control plants remained in the nutrient solution. Seventeen-day-old seedlings were treated with different concentrations of Pb, supplied as Pb(N03)2dissolved in deionized water. Heavy Metal Analysis. Roots and shoots of metaltreated and control plants were harvested and washed thoroughly with running tap water. The dry matter accumulation, metal-related phytotoxicity symptoms and root and shoot weight of treated and control plants were determined. Plant tissue was cut into small pieces with scissors, dried for 2 day at 80 "C, and ashed in a muffle furnace at 500 "C for 6 h. The ash was dissolved in a mixture of 2 M HCl and 1 M HN03. The metal content of the acid extract was determined with a Fisons direct current plasma spectrometer, Model SS-7.
Experimental Results Metal Uptake by Different Species and Cultivars. Table 1 compares the ability of 12 species of crop plants grown in a sandlperlite mixture containing 625 pg of Pb/g DW to accumulate Pb in shoots and roots. All tested species concentrated Pb in their roots. The phytoextraction coefficient (the ratio between pg of metallg DW of tissue and pg of metallg DW of substrate) for the roots of B. cannatawas 174.2 and 13.1forsorghum bicolor. Onaverage Brassica specieshad higher root phytoextractioncoefficients than non-Brassicaspecies. The levels of Pb in roots ranged from 0.82 to 10.9% DW. W i l e the phytoextraction coefficients for shoots were lower than those for roots, Brassica species were also superior to other species in their ability VOL. 29, NO. 5,1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
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TABLE 1
Shoot
lead CoRtent of Roots and Shoots of Crop Brassicas and Other Plants 30
mg of Pblg dry weight f SE
plant species'
shoot
root
Brassica juncea (L.) Czern. 6. nigra (L.)Koch 6. campestris L. 6. carinata A. Br. 6. napus L. B. oleracea L. Helianthus annuus L. Nicotiana tabacum L. Sorghum bicolor L. Amaranthus hybridus L. A. paniculata L. Zea mays L.
10.3 i 2.9 9.4 f 2.5 7.2 f 2.2 4.6 f 2.6 3.4 f 1.0 0.6 i 0.2 5.6 i 1.3 0.8 f 0.3 0.3 i 0.0 0.3 i 0.04 0.4 f 0.04 0.2 f 0.1
103.5 f 12.3 106.6 i 10.7 103.4 f 7.7 108.9 f 13.9 61.2 f 11.9 52.7 f 3.8 61.6 f 3.3 24.9 f 7.8 8.2 i 0.6 8.7 i 0.7 8.9 f 0.3 14.7 i 0.9
Plants were grown for 14-20 days in a substrate containing 625 u g of Pb2+/gDW supplied as Pb(NO& ( n = 4). a
to concentrate Pb in shoots. The phytoextraction coefficient for B. juncea cultivar Rcb shoots was 16.5,with a total shoot accumulation of 1.03%Pb on a DW basis. B. nigra and B. campestris also effectively transported and concentrated Pb from the substrate to their shoots. Non-Brassica plants, with the exception of sunflower (Helianthus annuus) and tobacco (Nicotiana tabacum), had phytoextraction coefficients of less than unity. All Pb-treated plants showed stunted growth and reduced leaf expansion. At 625 pg/g DW of substrate, Pb inhibited shoot growth by at least 50% compared to the controls (data not shown). Lead-treated Brassicasalso showed anthocyanin pigmentation of leaves and stems, which developed 8-10 days after treatment. Root growth of all the Pb-treated plants was retarded compared to the controls. Based on the observation that B. juncea accumulated large amounts of Pb in its shoots, we screened 106 different cultivars of B. juncea for their ability to accumulate Pb in roots and to transport it to the shoots. The concentration of Pb in roots and shoots varied greatly among different B. juncea cultivars, suggesting that this species may have a large geneticvariability in its ability to accumulate Pb. Figure 1A shows the levels of Pb in the shoots of six best and four worst B.juncea cultivarsrated for their abilityto accumulate Pb. The most efficientshoot accumulator (cultivar426308) contained 34.5 mg of Pb/g DW (or 3.5% on a DW basis) while the least efficient (cultivar 184290) contained only 0.4mgofPb/gDW (or 0.04%),equivalent to phytoextraction coefficients of 55.2 and 0.70, respectively. Root Pb concentrations showed much lower variability than shoot concentrations and ranged from 7 to 19%on the DW basis [Figure 1B). When the tissue distribution of the Pb in the shoots of selected cultivars of B. juncea was investigated, almost 90% of the Pb was found in stems, and only a small proportion (5-1076) was detected in leaves (data not shown). We chose cultivar 182921 for further study of Pb uptake in B. juncea because enough seeds of this cultivar were available to complete all planned experiments. Other Brassica amphidiploids, namely, B. carinata and B. napus, also showed substantial genetic variability in shoot Pb accumulation among cultivars (data not shown). No significant correlation between the degree of Pb phytotoxicity and the ability to accumulate Pb in shoots or roots was detected in tested cultivars of B. juncea, B. carinata, and B. napus. 1234
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10
B
n M 2 0
Root B
B. juncea cultivars FIGURE 1. Lead content of shoots (A) and roots (B) of B. juncea cultivars grown in sandPerlite mixture containing 625 p g of Pblg DW for 14 days. Vertical bars denote SE ( n = 4).
Synthetic B. juncea with B. campestris or B. nigra cytoplasms, obtained from reciprocal crosses between B. campestris (cultivar Tobin) and B. nigra (accession R1819) were used to test the influence of the cytoplasmic background of B. juncea on Pb uptake and accumulation. Pb uptake into roots and shoots of both the synthetic B. juncea combinations remained the same after 14 days (Figure21, indicating that cytoplasmic factors do not influence the Pb-accumulating ability of B. juncea. B. juncea cultivar 182921 was included in the experiment for comparison. Dose-Response and Time Course Studies. Dose dependence of Pb accumulation in B. juncea cultivar 182921 was studied in plants grown in the liquid hydroponic medium. Roots of 17-day-old seedlings were exposed to 400 mL of water containing 0, 6, 22, 47, 94, or 188 mg of Pb/L, and the plants were harvested 14 days after treatment. The Pb concentration in the roots of B. juncea increased with increasing solution concentration, although the rate of increase declined at higher Pb concentrations (Figure 3B). Root Pbuptakesaturatedatabout lOOmg/gDWwhen the Pb levels in solution exceeded 188 mg/L (data not shown). Lead concentration in the shoots did not increase significantly until the solution concentration reached 100 mg/L (Figure3A). At these levels, more than 50%of the Pb
80
Shoot A
T
60
40
20
B n PD
\
E f
0 200
Root B
T
T
150 100
plants contained similar amounts of Pb. At the beginning of the experiment, shoots of nutritionally starved plants were significantly smaller than the shoots of normally grown plants. At the end of the experiment, dried shoots and roots of the nutritionally starved plants weighed 0.277 and 0.035 g, respectively, while shoots and roots of normally grown plants weighed 0.503 and 0.034 g, respectively. Uptake of Different Metals. To test whether B. junceu accumulatesother toxic metals beside Pb, 17-day-oldplants were exposed for 14 day to either CF+,Cd2+,Ni2+,Zn2+, Cu2+,Pb2+or CI3+ applied to the sand/Perlite substrate (Figure 5). In contrast to previous experiments, the seedlings were not transplanted to a fertilizer-freemedium before metal application. Nutrient application was stopped immediately prior to metal treatment. Under these conditions, CI6+ had the highest phytoextractioncoefficient (581, followed by Cd2+and Ni2+. Lead and Cr3+had the lowest phytoextraction coefficients, 1.7 and 0.1, respectively. Plants treated with Cu, Cd, C P , and Pb did not show any visible symptoms of phytotoxicity or growth retardation compared to the controls. C F , Ni, and Zn produced mild leaf chlorosis, while Zn increased anthocyanin pigmentation.
Discussion
50
0 AABB
BBAA
B. juncea
182921
type
FIGURE 2. Influence of cytoplasmic genome on Pb accumulation in B. junceashoots (A) and roots (B). B. campesfriswas the cytoplasmic donor parent for AABB type of 8. juncea, while B. nigra was the cytoplasmic donor in BBAA. B. juncea cultivar 182921 was used as a standard. Lead was applied at 188 mg/L for 14 days. Vertical bars denote SE ( n = 4).
uptake capacity of B. junceu roots was already saturated (Figure 3B). At the highest concentration (188 mg of Pb/ L), Pb levels in the shoot reached 1.6%. Shoot growth was only slightly affected by exposure to Pb over the concentration range used; shoot DW of plants treated with 6 (the lowest concentration used) and 188mg of PblL (the highest concentration used) were 0.390 and 0.352 g compared to 0.389 g for the control plants (Figure 3C). Root growth was more sensitive to Pb. At 6 and 188 mg of Pb/L, root DW was 0.097 and 0.064 g, respectively, compared to 0.104 g for the control (Figure 3D). For the hydroponic time course studies, nutritionally starved and normally fed plants were exposed to 188 mg of Pb/L for 0,4,8, and 14 d (Figure 4). At day 0, plants were sampled 1 h after the metal treatment. The Pb levels in roots of nutritionally starved plants showed a dramatic increase for the first 8 days and remained relatively constant for the next 6 days. In normally fertilized plants, Pb levels in roots continued to increase linearly for the duration of the experiment. In contrast, Pb levels in the shoots of nutritionally starved plants were significantly lower than those of normally fed plants after 8 days of Pb treatment. However, at harvest time, 14 days after Pb treatment, both roots and shoots of normally fed and nutritionally starved
Our results demonstrate that crop and crop-related Brassica species have an unusual ability to take up heavy metals from solid substrates and to transport and concentrate these metals in their shoots (Table 1). B. junceu and B. nigru had the highest metal-accumulating ability among the species tested. This abilitymay be inherited from some of the wild species in the Brassicaceae, which grow in metal-rich soils and accumulate large amounts of heavy metals in their roots and shoots (5, 9, 19-22). We still know very little about the biological and evolutionarysignificance of metal accumulation in plants adapted to soils naturally rich in heavy metals. Present evidence is strongest for a function of accumulated metals in defense against herbivores (13, 23). In spite of their limitedvalue for phytoextraction, wild metal accumulatorscan be useful as model systems to study the mechanism of metal accumulationand metal tolerance. These mechanisms, which are thought to involve extra and intracellular metal chelation, precipitation, compartmentalization, and translocation in the vascular system are poorlyunderstood and need further study (17). In addition, wild metal accumulators may serve as a valuable reservoir of genes for metal translocation and accumulation. Our attempts to exploit the genetic variability of B. junceu in order to identify easily cultivated, high biomass plants that can be used for soil remediationled to the identification of several Pb-accumulating cultivars. Cultivar 426308 was the most efficient shoot accumulator of Pb (3.5%on a DW basis, phytoextraction coefficient of 55.2) (Figure 1). If plants are ashed, the extracted metals could be concentrated at least another 10-fold. In addition to being a Pb accumulator, B. junceu produces 18 t/ha of biomass (24) and can be easily adapted to cultivation in various climatic conditions using existing agricultural practices (25).It may be calculated that cultivar426308,with 3.5%Pb in its shoots, can extract 630 kg of Pb/ha with a single harvest of the above-ground biomass. If roots are also harvested, the extraction of the Pb could be much larger. However, unless special agricultural practices that make soil metals more available to plants are discovered and employed, the phytoextraction coefficients for field-grown plants are VOL. 29, NO, 5, 1995 /ENVIRONMENTAL SCIENCE & TECHNOLOGY
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20
15
10
5
3
n 2
0
& M
El
75
50
25
0
FIGURE 3. Effect of different Pb concentrations supplied in the hydroponic medium on Pb uptake by shoots (A) and roots (B) of 6. juncea cultivar 182921. Shoot and root dry weights of the same plants (C and D). Vertical bars denote SE ( n = 3).
expected to be significantly lower. Lead binding to clay and organic matter and its inclusion in insoluble precipitates make a significant fraction of Pb unavailable for root uptake by field-grown plants. Crop Brassicas include six crop species of the genus Brassica. Of these, B. nigra (BB genome), B. oleracea (CC genome),and B. cumpestris (AAgenome) are monogenomic diploids,while B. carinata (BBCC genome), B. junceu (AABB genome), and B. napus (AACC genome) are digenomic tetraploids or amphidiploids. All cultivated B. juncea cultivars carryB. campeszris cytoplasm and an amphidiploid nuclear genome of B. nigra and B. campestris. Our results indicate that Pb uptake and accumulation in B. juncea is not influenced by the cytoplasmic genome and that both AA (B.cumpestris)and BB (B. nigra) genomes actingtogether are responsible for high Pb accumulation in B. juncea (Figure 2). B. napus and B. carinata, which are also amphidiploid crop Brassicas, displayed lower levels of Pb accumulationwhen compared to B. juncea. Existing genetic variability in Pb accumulation suggests that selection and genetic manipulation may produce Brassica varieties with even greater phytoextraction coefficientsthan those already identified. Tight binding of Pb to soils and plant material at least partially explains the relatively low mobility of this metal in soils and in plants. While plants are known to concentrate Pb in the roots, Pb translocation to the shoots is normally very low (2, 9, 26-28). This may explain the observation that significant Pb translocation to the shoots of B. junceu 1236 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 5.1995
was observed only at relatively high concentrations of Pb in the hydroponic solution and after the Pb-bindingcapacity of roots was partially saturated (Figure 3). Nevertheless, these results demonstrate that B. junceu has an exceptional ability to transport Pb from the growth medium to the shoots. Thlaspi rotundifolium ssp. cepaeifolium, a known metal accumulator and a noncrop member of the Brassicaceae family, may be the only other Pb accumulator reported prior to this study. Pb levels as high as 8.2 mglg DW were measured in the leaves of this plant (9). The time of exposure to Pb is another factor that may affect Pb levels in plant roots (2,291. We have confirmed this observation for the roots of hydroponically grown B. juncea exposed to 188 mg of Pb/L. While the rate of Pb uptake into the roots slowed as a function of the exposure time, the rate of translocation to the shoots showed an increase with time (Figure 4). This may indicate that the physiological and biochemical mechanisms involved in root to shoot transport of Pb require some time to develop and to become functional. Normally fertilized plants were less effective in accumulatingpb in the roots and more effective in translocating Pb to the shoots for the first 8 days of Pb treatment when compared to plants deprived of nutrients for 7 days prior to Pb exposure (Figure 4). This difference disappeared 14 days after exposure to an aqueous solution of Pb(NO& when normally grown plants started to show symptoms of starvation, since their nutrient supply with the exception of N was also stopped during the Pb treatment. It is known that the mechanisms involvedinthe root uptake
20
Ishoot 202
T
104
-
FIGURE 5. Accumulation of different heavy metals in shoots of B. juneea cultivar 182921 grown for 14 days in a fertilized sandperlite mixture supplemented with a heavy metal as indicated by the symbol below each bar. The values in parentheses indicate the micrograms of metal/gram DW of soil. The values above the columns indicate micrograms of metal/gram of DW shoot tissue. Phytoextraction coefficients are the ratios between micrograms of metal/gram DW of shoot and microgram of metal/gram DW of soil. Vertical bars denote SE ( n = 4).
0
5
10
15
Days after treatment FIGURE 4. Pb accumulation in the shoots (A) and roots (B) of B. junceacultivar182921exposedfor14daysto188mg of Pb/Lsupplied in the hydroponic medium. Roots were either kept in complete nutrient solution immediately prior to Pb treatment (0)or immersed in deionized water for 7 days before Pb application (0).Vertical bars denote S. E. ( n = 3).
of such essential elements as Fe and Cu are activated during nutritional stress and that this activation leads to the increased uptake of other metals (30). It is tempting to suggest that nutritional stress increases Pb root uptake because, in metal accumulating plants, Pb and other nonessential heavy metals may use the same uptake mechanisms as the essential metallic elements (17). In addition to Pb, such heavy metals as Cr, Cd, Ni, Zn, and Cu are also major soil contaminants in the United States and throughout the world. B. juncea effectively removed at least some of these metals from the sandlperlite mixture under conditions where metal availabilityis at least partially limited by the presence of inorganic anions (mainlysulfates and phosphates) added to the substrate as part of a fertilizer treatment (Figure 5). The phytoextraction coefficients for these metals correlated with the solubilities of their respective phosphates and sulfates, suggesting that formation of insoluble inorganic complexes in soil and inside the plant can sipficantlyreduce the phytoextractionefficiency of B. juncea. This observation underscores the need for research directed toward the development of agricultural * practices and soil amendments (acidifiers and chelators) that will make such tightly bound and relatively immobile heavy metals as Cu, Pb, and Cr3+more available for plant uptake and translocation. The rudimentary state of our understanding of mechanisms of metal uptake may not
allow us to predict phytoextraction coefficients of fieldgrown plants. Therefore, caution should be exercised in using laboratory data to predict field performance.
Conclusions The bioconcentration of heavy metals by plants is a fascinating area of research that, in addition to important commercial applications, should provide answers to some of the fundamental questions of plant biochemistry, nutrition, and stress physiology. The unique “metal accumulation” and “metal resistance” genes of metalaccumulating plants may directly benefit world agriculture and the environment. Phytoremediation, although still in its infancy, may one day become an established environmental cleanup technology. Further development of phytoremediation requires an integrated multidisciplinary research effort that combines plant biology, soil chemistry, soil microbiology, and agricultural and environmental engineering. It is possible that plants that accumulate toxic metals may be grown and harvested economically, leaving the soil or water with a greatly reduced level of toxic metal contamination. Plants already give us food, energy, construction materials, natural fibers, and various chemicals. The use of plants in environmental cleanup may guarantee a greener and cleaner planet for all of us.
Acknowledgments This work was supported by the U.S. Department of Environment (Grant R818619), New JerseyCommission for Science and Technology Grant 93-240380-1, New Jersey Agricultural Experiment Station, Phytotech Inc., and Exxon Co. We thank Alan Baker, David Salt, and Burt Ensley for helpful discussions. P.B.A.N.K., V.D., and I.R. have equity in Phytotech Inc., which commercializes the use of plants for environmental remediation. VOL. 29. NO. 5 , 1 9 9 5 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
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Received for review July 25, 1994. Revised manuscript received December 27, 1994. Accepted January 12, 1995.@
ES9404611 @Abstractpublished in Advance ACSAbstracts, February 15, 1995.
