Journal of Environmental Quality 31:1649-1656 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORTS
Organic Compounds in the Environment
Soil-to-Root Transfer and Translocation of Polycyclic Aromatic Hydrocarbons by Vegetables Grown on Industrial Contaminated Soils
Joëlle Fismesa,
Corinne Perrin-Ganiera,
Pascal Empereur-Bissonnetb and
Jean Louis Morel*,a
a Laboratoire Sols et Environnement, UMR 1120, ENSAIA-INPL/INRA, 2 avenue de la Forêt de Haye, BP 172, 54 505 Vandoeuvre lès Nancy cedex, France
b Service des Etudes Médicales, EDF-GDF, 22-28 rue Joubert, 75 009 Paris, France
* Corresponding author (jean-louis.morel{at}ensaia.inpl-nancy.fr)
Received for publication July 31, 2001.
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ABSTRACT
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Polycyclic aromatic hydrocarbons (PAHs) are possible contaminants in some former industrial sites, representing a potential risk to human health if these sites are converted to residential areas. This work was conducted to determine whether PAHs present in contaminated soils are transferred to edible parts of selected vegetables. Soils were sampled from a former gasworks and a private garden, exhibiting a range of PAH concentrations (4 to 53 to 172 to 1263 and 2526 mg PAHs kg-1 of dry soil), and pot experiments were conducted in a greenhouse with lettuce (Lactuca sativa L. var. Reine de Mai), potato (Solanum tuberosum L. var. Belle de Fontenay), and carrot (Daucus carota L. var. Nantaise). At harvest, above- and belowground biomass were determined and the PAH concentrations in soil were measured. In parallel, plates were placed in the greenhouse to estimate the average PAH-dust deposition. Results showed that the presence of PAHs in soils had no detrimental effect on plant growth. Polycyclic aromatic hydrocarbons were detected in all plants grown in contaminated soils. However, their concentration was low compared with the initial soil concentration, and the bioconcentration factors were low (i.e., ranging from 13.4 x 10-4 in potato and carrot pulp to 2 x 10-2 in potato and carrot leaves). Except in peeled potatoes, the PAH concentration in vegetables increased with the PAH concentration in soils. The PAH distribution profiles in plant tissues and in soils suggested that root uptake was the main pathway for high molecular weight PAHs. On the opposite, lower molecular weight PAHs were probably taken up from the atmosphere through the leaves as well as by roots.
Abbreviations: PAH, polycyclic aromatic hydrocarbon
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INTRODUCTION
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POLYCYCLIC AROMATIC HYDROCARBONS (PAHs) are ubiquitous soil contaminants originating from natural and anthropogenic sources (Edwards et al., 1982). In general, these nonpolar and hydrophobic molecules with two or more benzene rings persist in the environment. Several PAHs have been shown to be carcinogenic and/or mutagenic (USEPA, 1985; Edwards, 1988), and human exposure to PAHs can occur through different environmental pathways, including internal absorption through food and water consumption (Edwards et al., 1982; Wild and Jones, 1992; Empereur-Bissonnet, 1996). Former industrial sites, such as coking plants and gasworks sites, may contain variable concentrations of PAHs. These sites represent a potential source for food chain contamination if they are converted to residential uses where gardening occurs.
Plant uptake of pollutants may occur through various pathways, including root uptake and atmospheric deposition from gaseous or particulate forms (Edwards, 1988; Simonich and Hites, 1995). It is generally reported that PAHs are transferred to plants from particle-phase deposition on the waxy leaf cuticle or by uptake in the gas phase through the stomata (Larsson and Sahlberg, 1981; Kipopoulou et al., 1999). However, as PAHs are lipophilic molecules, they are able to pass through the cuticle by solubilization in waxes (Keymeulen et al., 1991; Kipopoulou et al., 1999), but they are also strongly kept by Van der Waals or covalent bonds. Because of this, low molecular weight PAHs are easier than high molecular weight to penetrate waxy leaf cuticle (Bauer et al., 1997). On the other hand, there are conflicting reports in the literature concerning the extent of root uptake and translocation of PAHs to shoots (Gunther et al., 1967; Harms, 1975; Ellwardt, 1977; Edwards, 1988; Preusser et al., 1993; Simonich and Hites, 1995; Chaîneau et al., 1997). In fact, recovered PAHs in plants are either adsorbed on root suberine cortical zones (lipophilic constituants) or absorbed by root cells and subsequently transferred to the aerial parts (Briggs et al., 1982; Edwards, 1988; Sims and Overcash, 1983). Root uptake of non-ionized chemicals (described by the root concentration factor, RCF) is generally well correlated with lipophilicity (log Kow, the water–octanol partition coefficient), with RCF being the maximum at log Kow > 3 (Briggs et al., 1982, 1983; Ryan et al., 1988). However, most of these molecules are adsorbed on but not absorbed by roots. Only low molecular weight PAHs were able to migrate to shoots when high molecular weight PAHs were strongly adsorbed on the root epidermis (Wild and Jones, 1992; Larsson and Sahlberg, 1981; Kipopoulou et al., 1999). The transfer in the xylem is also correlated with the lipophilicity of PAHs (Paterson et al., 1990; Goodman et al., 1992). Finally, a relationship exists between the transpiration stream concentration factor (TSCF) and the log Kow, with TSCF being very low (<0.4) at log Kow > 3 (Briggs et al., 1982; Ryan et al., 1988; Burken and Schnoor, 1998).
This work was undertaken to determine whether the PAHs resulting from historical contamination due to former industrial activities (i.e., gasworks) can be subject to a soil-to-root transfer and subsequent translocation to edible organs. A pot experiment was set up where three vegetables (lettuce, potato, and carrot) were grown and soils and plant parts were analyzed for the 16 USEPA PAHs listed in Table 1. To discriminate between root uptake and other contamination pathways (dust-associated PAHs and gaseous PAHs), the average PAH deposition with dust particles was quantified, and the distribution of the 16 USEPA PAHs in plants was compared with that in the soil where the plants were grown.
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Table 1. Concentrations of the 16 USEPA polycyclic aromatic hydrocarbons (PAHs) in the five soils used in this study.
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MATERIALS AND METHODS
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Soil Samples and Analysis
Soil samples were collected from a former gasworks site and a private garden, both located in eastern France and presenting comparable physical and chemical characteristics. Preliminary sampling and analysis were performed to select a set of soil samples showing a gradient of PAH concentrations. Soils were sampled in the upper horizon (0–20 cm), air-dried, and sieved (5 mm). Finally, five soils presenting a gradient of pollution were sampled: soil PAH1 with 4 mg 16 USEPA PAHs kg-1 dry soil, soil PAH2 with 53 mg 16 USEPA PAHs kg-1 dry soil, soil PAH3 with 172 mg 16 USEPA PAHs kg-1 dry soil, soil PAH4 with 1263 mg 16 USEPA PAHs kg-1 dry soil, and soil PAH5 with 2526 mg 16 USEPA PAHs kg-1 dry soil (the concentrations of the individual 16 USEPA PAHs are given in Table 1). Soils were analyzed for water holding capacity (saturated soil compression at 0.01 MPa for 24 h), particle-size distribution (pipette method; NF [French norm] X31-107) (AFNOR, 1994), pH (1:2.5 in H2O; NF X31-103), total CaCO3 (Drouineau–Gallet method; NF X31-106), total organic carbon (Anne method, by sulfochromic oxidation; NF X31-109), total nitrogen (Kjeldahl method; NF X31-111), extractable phosphorus (Olsen method), and cation exchange capacity (Metson method; NF X31-130) (Table 2). Total Cd, Cu, Ni, Pb, and Zn (by acid oxidation; NF X31-151), and ammonium acetate–extractable Cu and Zn (ammonium acetate–EDTA extraction; NF X31-120) also were measured. Soils were calcareous, and exhibited large differences in organic C according to the PAH concentration (2.18% in PAH1 to 9.45% in PAH5). All nutrients were at sufficient levels for plant growth except Mg, which showed an unbalanced content in comparison with K (competition between the two elements for plant absorption) in PAH1, PAH2, and PAH3, and P, which was at low concentration in PAH4 and PAH5. Metals (i.e., Zn, Ni, Pb, and Cd) were present at concentrations frequently recorded in garden soils in Europe (Morel and Schwartz, 1999; Schwartz et al., 2000).
Plants
Three plant species commonly cultivated in private gardens and representing various types of edible parts (roots, leaves, and tubers) were chosen: lettuce, potato, and carrot.
Pot Experiments
An amount of 5300 g dry soil was introduced in 10-L plastic pots containing a 3-cm gravel layer. Then, 300 g of composite soil were sampled from each pot and PAH measurement was performed on each soil sample individually. Five lettuce seedlings pre-germinated on compost material were transplanted at the five- or six-leaf stage, and arranged in line in each pot. The soil surface was then covered with a metal grid to avoid contact with aerial plant parts. Carrot seeds were sown directly in pots at a 0.5- to 2-cm depth at a rate of about 50 seeds per pot. After germination, excess seedlings were removed to keep five plants per pot. Two pre-germinated potato tubers were planted per pot at a 6- to 7-cm depth. Soils were watered at 80% of the water holding capacity with nutrient solutions which brought 11 mg N kg-1, 8 mg P2O5 kg-1, 41 mg K2O kg-1, and 3 mg CaO kg-1 to lettuce; 19 mg N kg-1, 9 mg P2O5 kg-1, and 6 mg K2O kg-1 to potatoes; and 5 mg N kg-1, 28 mg P2O5 kg-1, and 46 mg K2O kg-1 to carrots. Five replicates were prepared per each treatment, and pots were arranged in split-plot blocks in a greenhouse (22°C minimum temperature; 12 h of light). Soils were irrigated daily to maintain the moisture content at 80% of the field moisture capacity. Two applications of 10 mg N kg-1 were made on carrots on Day 55 and Day 77. Also, 9 mg MgO kg-1 were added to potatoes at 37 d. Plant height (potato and carrot), number of leaves (lettuce and carrot), length of the main leaf (potato and carrot), and leaf area (lettuce) were measured periodically. Plants were harvested on Day 25 for lettuce, Day 60 for potato, and Day 90 for carrot. Shoots were separated from roots and tubers, and all plant parts were washed with tap water to remove adherent soil particles. Fresh biomass was measured. All carrots and half of the total potatoes were peeled with a kitchen peeler (0.2-mm-thick peel) in order to quantify PAHs only in edible pulp of carrots and in both edible pulp and edible peel of potatoes (some consumers eat unpeeled potatoes). The various plant parts were dried at 75°C for 12 h, weighed, and mixed prior to PAH analysis. Polycyclic aromatic hydrocarbons were measured in the intact potatoes and in peeled potatoes, and PAH contents in peels were obtained by difference because the weight of potato peels was too low to allow direct PAH measurements. Composite soil samples were collected from each pot after harvest and air-dried. Polycyclic aromatic hydrocarbon analysis was performed on each soil sample. Bioconcentration factors (BCF = PAH concentration in fresh parts of plant/PAH concentration in dry soil) were calculated for each pot at harvest.
Estimation of Dust Deposition of Polycyclic Aromatic Hydrocarbons
Four pairs of stainless steel plates (50 cm2) covered with a silicone gel were placed horizontally on poles at the start of the pot experiment. They were placed at the same level as lettuce leaves, and along the series of pots bearing the lettuce culture. They were collected at the same time as lettuce harvest (25 d). Dust was removed with a dichloromethane–hexane (50/50 v/v) solvent and its content in PAHs was determined. In parallel, the length of the five major lettuce leaves was measured at planting, after 10 d, and at harvest, and the leaf surface was calculated assuming that leaves were (i) ellipses from planting to the seven-leaf stage and (ii) circles to the end of the experiment.
Analysis of Polycyclic Aromatic Hydrocarbons in Soils and Plant Tissues
Five grams of soil and plant samples were extracted with a 50-mL mixture of 50% hexane and 50% dichloromethane (v/v) in a Dionex (Sunnyvale, CA) Model 200 accelerated solvent extractor at a pressure of 13.6 MPa and a temperature of 100°C during 8 min. After each extraction, the extraction cells were automatically rinsed with the same mixture. Solvent extracts were then evaporated to dryness and dissolved in 1 mL acetonitrile. All extracts were filtered prior to analysis with cellulose filter units and then analyzed for the 16 USEPA PAHs by high performance liquid chromatography (HPLC) with ultraviolet (UV) and fluorescence detection. The system consisted of an automatic injector, a high-pressure pump, and a C18 Vydac (Hesperia, CA) column. A programmable fluorescence detector and a UV detector were used. Chromatography was performed at 30°C. Replicate analyses gave an error in the ±5 to 10% range.
All PAH analyses were run with quality assurance procedures at the IRH-Environnement laboratory (Nancy, France). Growth parameters and PAH contents in soils and in plant tissues were subject to variance analysis and statistically compared according to the Tukey's test at the 0.05 probability level.
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RESULTS
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Plant Growth and Biomass Production
No phytotoxicity symptoms were observed for plants regardless of the PAH concentration in soil, except Mg deficiency in potatoes, which was alleviated by the addition of MgO on Day 40, on all potato soils. Leaf emergence of the three species was linear during plant development. The leaf area and the aerial biomass of lettuce increased between 10 and 25 d after replanting. Shoot heights of carrots and potatoes increased between 20 and 45 d, then reached plateaus. The PAH2 and PAH3 soils were more favorable to the development of lettuce leaves than other soils (PAH1, PAH4, and PAH5) with a production of 2.76 g dry matter per plant in PAH2 and 2.97 g dry matter per plant in PAH3 against 2.09 g dry matter in PAH1, 1.88 g dry matter in PAH4, and 2.28 g dry matter in PAH5. Also, a reduction in lettuce root biomass was recorded in PAH4 (0.36 g dry matter per plant) and PAH5 (0.39 g dry matter per plant) against 0.49 g dry matter in PAH1, 0.45 g dry matter in PAH2, and 0.47 g dry matter in PAH3. High PAH concentrations in soil (superior than 1200 mg kg-1) corresponded with greater vegetative development of potatoes (4.6 g dry matter per plant in PAH4 and 4.0 g dry matter per plant in PAH5, against 2.8 g dry matter in PAH1, 3.9 g dry matter in PAH2, and 3.4 g dry matter in PAH3). They corresponded also to greater development of both above- and belowground organs of carrots (3.1 g dry matter leaves per plant in PAH4 and from 1.7 to 2.3 g dry matter in other soils; 2.7 g dry matter roots in PAH4 and 3.5 g dry matter roots in PAH5 against 1.4 to 2.1 g dry matter in other three soils).
Polycyclic Aromatic Hydrocarbons in Vegetables
The 16 USEPA PAHs were present in above- and belowground parts of the three vegetables cultivated on all soils (Fig. 1)
. However, total concentrations of PAHs in leaves were low (from 0.2 to 2.7 mg kg-1 dry matter) and showed no significant difference among the three vegetables for a given soil. Concentrations in plants tended to increase with PAH concentration in soils over the range of concentrations tested. The increase was significant in leaves of lettuce and potatoes grown on soils containing more than 1200 mg PAHs kg-1 dry soil. The concentration in lettuce roots was greater than in leaves (up to 22 mg PAHs kg-1 dry matter). Very low concentrations were found in peeled potatoes, with a maximum of 0.35 mg kg-1 dry matter, but higher values were recorded in the peels, ranging between 0.44 and 0.61 mg kg-1 dry matter. However, the PAH concentration in peeled and intact potatoes was independent of the PAH concentration in soils. The PAH concentration in peeled carrots significantly increased from 0.03 to 0.1 mg kg-1 dry matter with the degree of soil contamination. Bioconcentration factors showed very low values and were inversely proportional to the total PAH concentration in soil (Table 3). It varied from 0.06 x 10-3 (PAH5) to 3.3 x 10-3 (PAH1) in lettuce leaves and from 0.08 x 10-2 (PAH5) to 1.4 x 10-2 (PAH1) in lettuce roots, from 0.01 x 10-2 (PAH5) to about 2 x 10-2 (PAH1) in potato and carrot leaves, and from about 0.01 x 10-4 (PAH5) to 13 x 10-4 (PAH1) in pulp of potatoes and carrots; in potato peels, bioconcentration factor increased from 0.02 x 10-3 (PAH5) to 13.7 x 10-3 (PAH1).
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Fig. 1. Concentration of the 16 USEPA PAHs in (A) aerial parts of lettuce, potato, and carrot, (B) roots of lettuce, and (C) storage organs of potato and carrot, after growth in soils showing a gradient of PAH concentration (PAH1 to PAH5). Bars are mean values of five replicates. Bars affected by the same letter are not significantly different at the 5% probability level (Tukey's test).
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Table 3. Bioconcentration factors (BCF) in lettuce, carrot, and potato. AP, aerial parts; RO, roots; PAH, polycyclic aromatic hydrocarbon; PPu, potato pulp; PPe, potato peels.
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Distribution of Polycyclic Aromatic Hydrocarbons in Soils and Vegetables
The 16 USEPA PAHs were grouped according to the number of aromatic rings (i.e., two, three, four, and five or six rings) (Fig. 2) . Their distribution in the soils was independent of the origin and the history of the soils collected from the garden or the industrial site. The PAH profiles in the lettuce root samples were fairly similar to those of the soil. High molecular weight PAHs dominated in both soils and lettuce roots. However, profiles were different between soils and lettuce leaves. The low molecular weight PAHs in lettuce leaves in the slightly polluted soils (PAH1, PAH2, and PAH3) were in higher proportion than in the corresponding soils, and the percentage of low molecular weight PAHs decreased with increasing soil contamination. Proportion of compounds with three and four aromatic rings increased while proportion of compounds with five aromatic rings decreased in pulp of potatoes and carrots as compared with soils and potato peels. The distribution of PAHs was similar in potato peels and in soils, with a predominance of high molecular weight PAHs, while low molecular weight PAHs were dominant in potato pulp.
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Fig. 2. Distribution of the different PAH groups (number of aromatic rings) in (A) lettuce, (B) carrot , and (C) potato. S, soil; AP, aerial parts; RO, roots; PPu, potato pulp; PPe, potato peels.
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Deposition of Polycyclic Aromatic Hydrocarbons on Plates
Mass of particles deposited on plates during 25 d reached 75 mg m-2 d-1 (Table 4). The estimation of the surface of lettuce aboveground parts allowed the calculation of the dust deposition during the experiment, assuming a similar deposition to that on plates. The average surface of lettuce was 80 cm2 at planting and 700 cm2 at harvest. Hence, deposition evolved from 0.6 (0.008 m2 x 75 mg m-2 d-1) to 5.25 mg d-1 (0.07 m2 x 75 mg m-2 d-1), or a total average deposition of 73 mg dry particles per lettuce [(0.6 mg d-1 + 5.25 mg d-1)/2 x 25 d]. The average PAH concentration in dust was 8.38 mg kg-1 dry deposit. The average total deposition was 0.61 µg PAHs per lettuce (8.38 mg kg-1 x 73 x 10-6 kg). Therefore, the aerial PAH supply was negligible compared with the PAH concentration in lettuce, which varied from 0.4 (PAH1) to 10.4 (PAH5) mg PAHs per plant. Despite a higher contribution of PAH aerial deposition for lettuce cultivated on the slightly polluted soils (1.6
in PAH1) than on highly polluted soils (0.06 in PAH5), this contamination pathway remained negligible.
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DISCUSSION
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It has been shown that plants growing on PAH-contaminated soils may contain PAHs in their tissues (Edwards et al., 1982; Wild and Jones, 1992; Kipopoulou et al., 1999) which may originate from volatile compound absorption by leaves in the surrounding air, deposition of contaminated soil particles (splash), and dust on leaves, followed by retention in cuticle or penetration through it, and soil-to-root transfer followed by subsequent translocation by the transpiration stream. The soil-to-root transfer of PAHs has been demonstrated from hydroponic experiments (Durmishidze et al., 1974; Edwards et al., 1982) and spiked soils (Edwards et al., 1982; Edwards, 1988). However, the contribution of root uptake to PAH content in plants remains unclear. In this work, we have demonstrated that PAHs in soils contaminated by former industrial activities can be detected at significant amounts in both above- and belowground parts of vegetables. The PAH concentrations in plants were similar to values provided by the literature (e.g., Larsson and Sahlberg, 1981) and, except in potatoes, they tended to increase with increasing PAH concentration in soil. Also, evidence is given that most of the PAHs recovered in the upper plant organs resulted from a soil-to-root transfer and subsequent root-to-shoot translocation. Indeed, in the greenhouse, all plants were in contact with a similar atmosphere (i.e., a similar source of volatile and dust-bearing PAHs). If this was the only PAH source, plants would exhibit a similar PAH concentration, as the leaf surface presented only slight variation within a given species. Also, using the plate system, we have shown that only a small portion of the PAHs recovered in leaves could be attributed to an aerial deposition of PAH-bearing dust. In general, the accessibility and availability of organic pollutants as PAHs may become more difficult during aging (Chung and Alexander, 1998). In this study, sieving the soil broke aggregates and may have increased the availability of PAHs. However, the bioconcentration factor calculated for all plant parts was very low, varying from 10-4 to 10-2, compared with values obtained in carrots and lettuce grown on industrial areas (Northern Greece) (from 0.13 to 3.2 in carrot cores and from 0.11 to 8.3 in lettuce leaves) (Kipopoulou et al., 1999). However, dry leaf–air bioconcentration factors (m/m) recorded in azalea leaves submitted to different organic pesticide vapors exhibited higher values, ranging from 1.8 (hexachlorobenzene) to 1.9 x 105 (DDT) (Bacci et al., 1990).
No detrimental effect on plant growth was recorded, probably because of the low concentration of volatile compounds. In fact, Chaîneau et al. (1997) and Sims and Overcash (1983) indicated that toxicity of PAHs decreased with time because of the evaporation of naphtalene, which is 20 times more toxic than heavier PAHs. The soil drying and sieving favored the volatilization of volatile compounds (PAHs with two or three rings presenting a low Henry constant) and probably contributed to reduced soil toxicity. A reduction in tuber biomass in some soils was recorded, possibly due to an unfavorable soil structure (PAH1 presented a high apparent density with limited gaseous fraction) or a competition for photoassimilates between vegetative growth and tuberization (PAH4 and PAH5 favored aerial biomass production). On the contrary, greater growth (potatoes, carrots) was observed at concentrations in soil greater than 1200 mg PAH kg-1, possibly due to a stimulating effect that has been already noticed (e.g., increase in algae cell height [Gräf, 1965] and increase in yield of cabbage [+20%], tobacco [+100%], and rice [+300%] in the presence of benzo(a)pyrene [Gräf and Nowak, 1966]). The higher the molecular weight, the stronger the stimulation, and high molecular weight PAHs, which exhibit a structure close to that of gibberelline, were supposed to act as growth-promoting substances (Gräf and Nowak, 1966).
Despite a very different vegetative system among the three species, the PAH concentration was similar in the leaves. Also, since the three vegetables produced a similar aerial dry biomass during their growth, we infer a passive transport of PAHs from soil to leaves driven by the transpiration flux.
Concentrations of PAHs in peeled potatoes were very low and were not correlated with the PAH concentration in soils. The PAH contents in whole tubers were greatly higher than those of peeled tubers. Chiou et al. (2001) and Kipopoulou et al. (1999) ascribed this to the fact that the peels have higher lipid contents than the pulp. Therefore, storage organs like potatoes filled by transfer of assimilates from leaves via the phloem vessels, and lipophilic organic pollutants, including PAHs, are barely transported by the phloem since it is water based (Simonich and Hites, 1995; Kipopoulou et al., 1999). In peeled carrots, PAH concentrations were directly related to the PAH contents in soil, as carrots are simultaneously roots and storage organs. Moreover, carrots have a high lipid content and oil channels in the roots, which have been reported to give greater potential for the uptake of nonpolar chemicals (Edwards, 1988; Ryan et al., 1988; Wild and Jones, 1992; Kipopoulou et al., 1999).
The comparison of the PAH distribution in both plants and soils showed a higher abundance of the high molecular weight PAHs, in contradiction with the literature (e.g., Larsson and Sahlberg, 1981; Wild and Jones, 1992; Kipopoulou et al., 1999). The low persistence of the low molecular weight PAHs in soil restricts their availability from uptake in the long term. As the lettuce roots were carefully washed, their high PAH content was not due to the contamination by adherent soil particles. Indeed, Larsson and Sahlberg (1981) demonstrated that washing vegetables had little effect on phenanthrene levels, but considerably reduced the levels of high molecular weight PAH compounds. However, it is possible that part of the PAHs measured in lettuce roots was due to their strong adsorption on the root epidermis. Similar profiles between soils and lettuce roots may be due to adsorbed PAHs. In fact, root peels are mainly made of suberin, a polyester with phenolic and aromatic functions presenting a lipophilic pole (Kolattukudy, 1980) able to strongly adsorb PAHs (Briggs et al., 1982; Schwab et al., 1998). The proportion of light PAHs in plant leaves decreased with the degree of soil contamination. Low molecular weight PAHs were simultaneously assimilated from the atmosphere through the leaves and from the soil through the roots, while high molecular weight PAHs were essentially taken up by roots and translocated to aerial parts. The different PAH distribution observed in soils and in storage organs suggested that only low molecular weight PAHs are transported in the phloem. In addition, Kipopoulou et al. (1999) indicated that the low molecular weight PAHs are more able to move from the peel tissues into the core of carrots.
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CONCLUSION
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Vegetables growing on a soil material contaminated by former industrial activities (i.e., historically contaminated soils) may contain PAHs in their tissues at significant concentrations. However, in the range of soil contamination tested, the bioconcentration factor was very low and probably overestimated as, in general, pot experiments exaggerate the availability of pollutants. The PAHs in plants originated from both the atmosphere and the soil, but the soil-to-root transfer was predominant in the range of concentrations tested. The leaves of the three plant species tested in this study responded similarly to the soil contamination, but the PAH translocation from leaves to storage organs (i.e., potato tubers and carrot roots) was negligible. Germination of seeds and growth of plants were not significantly affected by the presence of PAHs even at high concentrations in soil. Therefore, despite a significant soil-to-root transfer of PAHs, vegetables can grow in soils heavily contaminated without harmful effects on the biomass production or other signs of phytotoxicity.
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ACKNOWLEDGMENTS
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This study was funded by Gaz de France. All PAH analyses in soils, aerial dusts, and plant tissues were performed by the IRH-Environnement laboratory with an insurance quality control. The authors are debtful to Bernard Colin and Stéphane Colin of the Sols et Environnement ENSAIA-INRA laboratory for their contribution to the soil sampling.
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