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Genotypic variations in the accumulation of Cd, Cu, Pb and Zn exhibited by six commonly grown vegetables
Environmental Pollution 144 (2006) 736e745 www.elsevier.com/locate/envpol
Genotypic variations in the accumulation of Cd, Cu, Pb and Zn exhibited by six commonly grown vegetables P.D. Alexander a,*, B.J. Alloway b, A.M. Dourado a,1 b
a Royal Horticultural Society’s Garden, Wisley, Woking, Surrey GU23 6QB, UK Department of Soil Science, School of Human and Environmental Sciences, The University of Reading, P.O. Box 233, Reading RG6 6DW, UK
Received 26 September 2005; received in revised form 31 January 2006; accepted 2 March 2006
Genotypic differences between cultivars of vegetable species can be important in determining the extent of accumulation of metals from contaminated soil. Abstract Metal contaminants in garden and allotment soils could possibly affect human health through a variety of pathways. This study focused on the potential pathway of consumption of vegetables grown on contaminated soil. Five cultivars each of six common vegetables were grown in a control and in a soil spiked with Cd, Cu, Pb and Zn. Highly significant differences in metal content were evident between cultivars of a number of vegetables for several of the contaminants. Carrot and pea cultivars exhibited significant differences in accumulated concentrations of Cd and Cu with carrot cultivars also exhibiting significant differences in Zn. Distinctive differences were also identified when comparing one vegetable to another, legumes (Leguminosae) tending to be low accumulators, root vegetables (Umbelliferae and Liliaceae) tending to be moderate accumulators and leafy vegetables (Compositae and Chenopodiaceae) being high accumulators. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Contaminated soil; Vegetable cultivars; Heavy metal accumulation; Cadmium; Copper; Lead; Zinc
1. Introduction Domestic gardens and allotments in many countries have been contaminated with heavy metals. Increased urbanisation has resulted in houses being constructed on brownfield sites and, before the introduction of strict soil quality standards, many of these will have been contaminated with metals. Common heavy metal contaminants include Cd, Cu, Pb and Zn, from a wide range of sources. Cadmium and Pb could possibly pose a risk to human health where the occupants of the houses or holders of the allotments consume relatively large quantities of vegetables grown on these soils and/or ingest soil either on unwashed vegetables, on their hands or, in the case of children,
* Corresponding author. Tel.: þ44 1483 224234; fax: þ44 1483 211750. E-mail address: [email protected] (P.D. Alexander). 1 Present address: Malvern, London Road East, Amersham HP7 9DL, UK. 0269-7491/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2006.03.001
intentionally eating garden soil. Copper and Zn are regarded as being more of a phytotoxicity hazard than a risk to human health (ICRCL, 1987). Increasing awareness of such contamination has resulted in many countries introducing legislation to identify problem areas and minimise the risk to human health (e.g. Under Part IIA of the Environmental Protection Act, 1990 in the UK). Sources of heavy metal contamination of soils and plants in gardens and allotments can include: previous use of site, atmospheric deposition, paint particles, bonfires, use of contaminated fill for levelling the site, runoff from metal surfaces, use of ash and mineral waste for constructing paths, burial of metal-containing wastes, composts and fertilisers and leisure activities such as air gun shooting (Alloway, 2004). Surveys of garden soils in several countries have shown wide ranges of concentrations of heavy metals. For example, Culbard et al. (1988) found concentrations of up to 14,100 mg Pb kg1 (mean 266 mg Pb kg1), and up to 14,700 mg
P.D. Alexander et al. / Environmental Pollution 144 (2006) 736e745
Zn kg1 (mean 278 mg Zn kg1) in more than 4000 samples from gardens in 50 cities, towns and villages in England, Scotland and Wales. These are higher maximum and mean values than those found for more than 3600 garden soils in Germany where Gro¨ßmann and Wu¨stermann (1992) found up to 627 mg Pb kg1 (mean 65 mg Pb kg1) and up to 1035 mg Zn kg1 (mean 138 mg Zn kg1). Plants growing on contaminated soils will reflect elevated concentrations of heavy metals in the soils to varying extents, depending on the soil total concentrations, soil physico-chemical conditions (especially pH) and the genotype of the plant. Thornton and Jones (1984) reported concentrations of up to 23 mg Pb kg1 (dry matter) in lettuces and radishes growing in a garden on soil containing 4100 mg Pb kg1 and up to 333 mg Zn kg1 in radishes on soil with a total content of 2182 mg Zn kg1. Alloway et al. (1988) reported concentrations of up to 11.7 mg Cd kg1 and 18.5 mg Pb kg1 in vegetables from gardens in Shipham, England, which had been contaminated with wastes from Zn and Pb mining. Elevated heavy metal intake by humans from contaminated gardens can be due to the consumption of crop plants grown on soils with relatively high plant-available metal concentrations, ingestion of contaminated soil, either accidentally or intentionally (pica), inhalation of soil particles, and drinking water with high soluble concentrations of metals. Although all of these pathways can be important, the soil-plant-human pathway is likely to affect more people overall with regard to vegetable gardens than the others. The amount of metal taken up by plants relative to the composition of the soil in which they are growing can be expressed by the soileplant concentration factor (CF), which is based on the metal in the plant divided by metal in the soil [CF ¼ M (plant)/M (soil)]. This CF value is a function of the properties of the metal itself, especially the degree to which it is adsorbed in an unavailable form, the pH and sorptive properties of the soil and the genotype of the plant. Differences between crop plant species in the accumulation of metals are widely recognised. Much work has been done on the accumulation of Cd by plants in view of its relatively high soil-plant concentration factor. Pettersson (1977) showed that Cd concentrations in the shoots of plants grown in solution culture decreased in the order: lettuce > carrot, tomato > rape, kale, radish > mustard, maize > cucumber, sunflower, peas, bean > wheat, oats. Lehoczky et al. (1998) identified lettuce, beetroot, radish and carrot as being high Cd accumulators while cauliflower, red cabbage and savoy cabbage were only moderate accumulators. Other work by Kuboi et al. (1986) classified plant families into three groups with regard to the extent to which they accumulated Cd. These were: (1) low Cd accumulators (Leguminosae), (2) moderate accumulators (Gramineae, Liliaceae, Cucurbitaceae and Umbelliferae), and (3) high accumulators (Chenopodiaceae, Cruciferae, Solanaceae and Compositae). Fleming and Parle (1977) examined the order of accumulation of Pb, Zn and Cu by selected species of vegetables in a pot study and found that lettuce was consistently among the high accumulators and beans were consistently among the low accumulators.
737
However, there is less information available on varietal differences in metal accumulation within species, especially vegetables. In South Australia, McLaughlin et al. (1994) compared the uptake of Cd by 14 commonly grown potato cultivars. They found significant differences between the cultivars at most sites with an average range of concentrations of 30e50 mg Cd kg1 (fresh weight). At some sites, individual cultivars had Cd concentrations which exceeded the Australian maximum permissible concentration of 50 mg Cd kg1. They suggested Cd concentrations in potato tubers on some soils could be reduced by more than 50% by selecting low accumulating cultivars. John and van Laerhoven (1976) found a 21-fold variation in the Cd content of the leaves of nine lettuce cultivars grown in culture solution for five weeks. Crews and Davies (1985) also examined the uptake of Cd, Cu, Pb and Zn by six cultivars of lettuce from contaminated soil. They were able to identify high and low accumulators of Cd and Zn. Xue and Harrison (1991) also examined six lettuce cultivars but just for Zn uptake; they identified high and low accumulating cultivars. In a literature review, Grant et al. (1999) reported papers dealing with genotypic differences in Cd concentrations in cultivars of a range of crops, including: maize, non-oilseed sunflower, soybean, flax, rice, lettuce and durum wheat. The study reported here was designed to investigate variations in the accumulation of Cd, Cu, Pb and Zn by cultivars of six commonly grown vegetables from a contaminated soil. 2. Materials and methods 2.1. Site and soil The experiments were conducted at the Deers Farm experimental site of the Royal Horticultural Society’s Garden at Wisley (Woking, Surrey, UK). The soil used was a sandy loam, which had been spiked with solutions of cadmium nitrate, copper sulphate, lead nitrate and zinc sulphate to give concentrations of metals that were relatively typical of highly contaminated gardens. It was decided to spike a soil with metals, rather than use a contaminated soil from a garden, for a number of reasons: the costs associated with sourcing, transporting and handling contaminated soil, the potential problems of other metal and organic contaminants affecting uptake of the elements of interest and the advantage of using a homogeneous sandy soil which is easy to mix thoroughly. It was also considered that combining the four metal additions in the same soil would mimic the situation which could occur in a multielement contaminated soil in a ‘‘real’’ garden situation. It is accepted that various antagonistic or synergistic interactions could occur and affect uptake after spiking but all plants would be exposed to the same effects. The uptake of metals from spiked soils was expected to be greater than in the field situation even after a relatively long incubation period, but uptake by plants in large pots out in the open-air would be expected to be less enhanced than in small pots in the greenhouse (De Vries and Tiller, 1978). The spiking was carried out by spraying a solution of each metal salt over relatively dry soil spread out on large trays. The soil was turned over and sprayed several times and then watered and left to equilibrate outdoors under a waterproof tarpaulin for about three months. This long period was chosen to allow time for natural equilibration of the various sorption mechanisms in the soil. It was decided that the spiking targets would be based on the old provisional ICRCL targets. For Cu and Zn, which are considered to be more of a phytotoxicity and ecotoxicological hazard than a risk to human health (ICRCL, 1987), the ICRCL trigger values (total) were used to avoid directly compromising plant performance. The ICRCL trigger values for Cd and Pb were increased, Cd was doubled and Pb was increased by 50%. By setting
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these targets the concentrations were considered to be representative of many heavily contaminated garden and agricultural soils, without being unrealistically high. However, they were intended to be high enough to allow differences between cultivars to be detectable. The mean total concentrations of the metals in 10 separate samples of the thoroughly mixed spiked and control soils (in mg kg1) and the target concentrations are presented in Table 1. In the case of the lettuces, the soil was re-spiked 30 months later to the same approximate concentrations because of the relatively long time period since the initial spiking. After 3 months, the spiked soils were once again thoroughly mixed and put into large (12-L) plastic plant pots which were placed in saucers to prevent cross-contamination from drainage water.
2.2. Vegetable cultivars and planting The vegetable cultivars (cv) chosen were selected from the UK National Catalogue of Accepted Cultivars (1998) list to ensure widespread use and availability (presented in Table 2). The seeds were sown directly into the pots of spiked and control soils, respectively. The spinach, carrot and French bean cultivars were grown simultaneously (all sown on 9 June, 1999). The onions (sown 8 September, 1999) and peas (sown 26 October, 1999) were also grown in the same season. The lettuce cultivars were grown separately, later, in May 2002, in re-spiked soil. Sufficient seed was sown to guarantee healthy germination (plants were thinned after germination). Each vegetable species was grown in different pots outdoors using overhead drip irrigation. All vegetables were grown to maturity and the pots were replicated five times in a fully randomised block design. The nature of French beans enabled two harvests to be taken (results are presented for both). By growing the vegetables outdoors through their normal planting season, it was hoped to more accurately reflect the growing practices of gardeners and allotment holders (with the exception of the use of large pots). However, this does make comparing plants grown under slightly variable environmental conditions more difficult.
2.3. Analytical methods For the soil, metals extractable by aqua regia were measured. Soil samples (1.5 g) were weighed into duplicate 100 ml digestion tubes, conc. HNO3 (3 ml) and conc. HCl (10.5 ml) added, and allowed to stand overnight. They were then heated for 1 h at 50 C followed by 3 h at 140 C. After cooling, the ingests were filtered and made to volume by the method for plant digests except that 0.5 M HNO3 was used instead of water. Analytical quality assurance was addressed by the systematic use of blanks, replicates and analysis of a Certified Reference Soil (CRM 143R) and an in-house reference material (sludge-amended soil). For all crops, only the edible portions were sampled as the work was focused on the soil-plant-human pathway of trace metals (with particular regard to amateur gardeners and allotment holders). These edible portions were thoroughly washed (not peeled), oven dried (48 C for 48 h) and ground in a mill. Samples of plant material (0.5 g) were weighed into 100 ml block digestion tubes, concentrated nitric acid (10 ml) added and allowed to stand overnight. They were then heated for 3 h at 60 C, followed by 6 h at 110 C. After cooling, the digests were passed through a pre-washed filter (Whatman No. 540), the digestion tubes were rinsed three times, passing the washings through the filter and the filtrates made up to 100 ml volume using
Table 1 Mean total concentrations (mg kg1) of the metals in thoroughly mixed spiked and control soils (n ¼ 10)
Spiked soil SE Control soil SE Target
Cadmium
Copper
Lead
Zinc
4.2 0.1 0.3 0.03 6
87.9 2.12 16.4 0.19 130
744.5 15.84 40.5 1.41 750
237.0 4.43 83.8 1.28 300
Table 2 Species of vegetable and cultivars grown Species
Cultivars grown
Spinach Carrot French bean Peaa Onion Lettuce
Bloomsdale, Giant Thick Leaved, Grodane, Mediana, Spartacus Amsterdam Forcing, Early Nantes, Ingot, Mokum, Nairobi Cropper Teepee, Masterpiece, Prince, Sprite, Tendergreen Douce Provence, Feltham, Fortune, Meteor, Pilot Buffalo, Express, Keepwell, Shenshyu, Toughball Corsair, Little Gem, Lobjoits, Paris Island, Pinokkio
a
All peas are normal garden peas, not sugar snap, mange tout etc.
ultrapure water. Trace metal concentrations were measured in duplicate by ICP-OES spectrometry (Perkin Elmer Optima 3000) using a certified multielement standard (Merck, Darmstadt, Germany). A certified reference material (bush leaf material) GBW07603, Qunghai Province, China (supplied via the Laboratory of the Government Chemist, UK) was used to monitor the recovery of metals from the plant samples. Statistical analysis was undertaken using GENSTAT (Analysis of Variance) with support from the Statistical Services Department of The University of Reading.
3. Results 3.1. Differences in metal accumulation between cultivars 3.1.1. Carrot (Daucus carota L.; Umbelliferae) Highly significant differences ( p < 0.001) in Cd concentrations were found between carrot cultivars grown on the metalspiked soil (see Table 3a); cv. Nairobi showed the lowest Cd concentration and cv. Amsterdam the highest. Significant differences ( p ¼ 0.034) in Cu concentrations were found between carrot cultivars on the metal-spiked soil; cv. Nairobi showed the lowest and cv. Amsterdam the highest mean concentrations. No significant differences ( p ¼ 0.457) were found in Pb concentrations between carrot cultivars when grown on the spiked soil. However, certain trends were evident with the cultivars Nantes, Nairobi and Mokum showing the lowest Pb concentrations and Ingot and Amsterdam the highest. Significant differences ( p ¼ 0.006) were found in Zn concentrations between carrot cultivars grown on the spiked soil. The cultivars Nairobi and Ingot showed the lowest metal contents and cv. Amsterdam and cv. Mokum the highest. 3.1.2. Spinach (Spinacia oleracea L.; Chenopodiaceae) No significant differences ( p ¼ 0.095) were found in Cd, Cu, Pb or Zn concentrations between spinach cultivars growing on the metal-spiked soil (see Table 3b). Nevertheless, trends were evident, with cv. Bloomsdale showing the lowest Cd concentration and Grodane, Mediana and Spartacus all having similar, higher Cd contents. In the case of Cu, the cultivars Mediana and Bloomsdale had the lowest concentrations and cv. Spartacus the highest mean concentration. No significant differences in either Pb or Zn concentrations ( p ¼ 0.97 and p ¼ 0.243, respectively) were found between spinach cultivars when grown on the spiked soil and no trends were evident.
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Table 3 Mean metal (Cd, Cu, Pb and Zn) concentrations (mg /kg-1 dry matter) of (a) carrot, (b) spinach, (c) pea, (d) French beandfirst harvest, (e) French beandsecond harvest, (f) onion and (g) lettuce cultivars grown in control and spiked soil (n ¼ 5) Cadmium
(a) Carrot
Amsterdam Nantes Ingot Mokum Nairobi F pr. isd
Copper
Treated
Control
2.521 c 1.812 b 2.298 c 2.133 bc 1.215 a <0.001 0.3977
0.068 a 0.054 a 0.074 a 0.05 a 0.06 a
Treated 5.77 b 4.43 a 4.43 a 5.45 b 3.85 a 0.034 0.913
Lead Control
6.31 a 5.01 a 7.23 a 5.18 a 5.04 a 0.457 1.931
0.34 0.17 0.42 0.29 0.34
Treated
a a a a a
52.59 c 42.54 b 33.38 a 54.09 c 32.55 a 0.006 8.855
Cadmium
(b) Spinach
Treated Bloomsdale G. Thick Leaved Grodane Mediana Spartacus F pr. isd
4.24 a 5.13 ab 6.94 c 6.48 bc 6.14 bc 0.095 1.4700
0.36 0.25 0.45 0.33 0.32
Treated
a a a a a
9.01 a 9.56 ab 10.08 b 8.97 a 10.7 b 0.09 1.086
6.29 6.05 6.44 7.31 7.7c
Control
Treated
Control
0.62 0.49 0.69 0.51 0.76
334.9 a 341.8 a 380.2 a 343.2 a 361.7 a 0.243 62.68
246.7 218.6 209 a 256.4 274.9
0.4282 0.2615 0.3355 0.2337 0.1785 0.033 0.128
Treated 1.399 a 0.547 a 0.731 a 0.724 a 0.508 a 0.406 0.8194
a a a a a
Control
1.74 a 1.68 a 1.78 a 2a 1.89 a 0.97 0.937
a a a a a
ab a ab bc
Control c ab bc ab a
a a a a
Copper
0.1054 0.1055 0.1441 0.2088 0.0532
Lead
Douce P Feltham Fortune Meteor Pilot F pr. isd
19.68 19.02 18.43 19.72 16.27
Treated
Treated Douce P Feltham Fortune Meteor Pilot F pr. isd
Control
Zinc
Cadmium
(c) Pea
a a a a a
Copper Control
Lead
Bloomsdale G. Thick Leaved Grodane Mediana Spartacus F pr. isd
2.78 2.42 2.81 2.59 2.62
Zinc
Treated Amsterdam Nantes Ingot Mokum Nairobi F pr. isd
Control
Treated ab ab ab b a
5.163 a 5.727 ab 6.617 b 4.72 a 6.722 b 0.008 1.0548
Control 4.117 4.189 4.572 4.595 3.772
a a a a a
Zinc Control 0.412 0.322 0.692 0.546 0.542
a a a a a
Treated 47.49 a 47.8 a 54.27 a 51.53 a 59.9 a 0.26 5.286
Control 32.36 a 36.83 a 36.68 a 33.2 a 42.08 a
(continued on next page)
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P.D. Alexander et al. / Environmental Pollution 144 (2006) 736e745
Table 3 (continued) Cadmium
(d) French bean (Hv 1)
Treated Cropper Teepee Masterpiece Prince Sprite Tendergreen F pr. isd
Copper Control
0.0356 a 0.0156 a 0.0596 a 0.0388 a 0.051 a 0.566 0.0377
Treated
0.0248 a 0.004 a 0.032 a 0.0048 a 0.002 a
Lead Control
0.278 a 0.245 a 0.244 a 0.342 a 0.18 a 0.818 0.2546
0.272 0.155 0.135 0.138 0.152
Treated
a a a a a
48.46 a 42.12 a 42.54 a 43.2 a 45.28 a 0.194 6.233
Cadmium
(e) French bean (Hv 2)
Treated Cropper Teepee Masterpiece Prince Sprite Tendergreen F pr. isd
0.0726 0.1086 0.1218 0.0908 0.1344 0.868 0.0872
a a a a a
Treated
0.0098 a 0.04 a 0.1098 a 0.026 a 0.0962 a
0.439 a 0.532 a 0.378 a 0.445 a 0.356 a 0.65 0.2149
Treated 4.0229 3.5566 3.3498 3.7283 3.4464 0.501 0.546
Control
Treated 39.77 a 39.3 a 32 a 40.24 a 35.18 a 0.265 5.58
Treated 6.486 a 8.745 a 6.519 a 7.77 a 7.772 a 0.659 2.6985
45.1 a 42.9 a 35.66 a 38.76 a 35.68 a
Control 5.19 a 3.91 a 3.23 a 3.7 a 3.96 a
Control a a a a a
Control 36.63 31.03 22.59 29.13 24.76
a a a a a
Copper
0.1928 0.2237 0.1107 0.2089 0.1987
Lead
Buffalo Express Keepwell Shenshyu Toughball F pr. isd
5.93 a 5.71 a 4.28 a 5.47 a 5.32 a 0.418 0.933
0.284 a 0.216 a 0.243 a 0.17 a 0.208 a
Cadmium
Buffalo Express Keepwell Shenshyu Toughball F pr. isd
Control
Zinc
Treated
(f) Onion
a a a a a
Copper Control
Lead
Cropper Teepee Masterpiece Prince Sprite Tendergreen F pr. isd
5.45 4.54 3.27 3.92 4.67
Zinc
Treated Cropper Teepee Masterpiece Prince Sprite Tendergreen F pr. isd
6.35 a 5.11 a 4.43 a 4.85 a 5.08 a 0.802 0.944
Control
Treated a a a a a
2.374 a 3.078 a 2.878 a 2.693 a 3.055 a 0.856 1.1817
Control 2.307 2.755 2.342 2.985 2.611
Zinc Control 1.689 1.216 1.125 1.678 1.384
a a a a a
Treated 50.45 a 60.94 a 66.97 a 65.09 a 78.34 a 0.291 21.861
Control 23.75 32.28 16.25 31 a 25.82
a a a a
a a a a a
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Table 3 (continued ) Cadmium
(g) Lettuce
Copper
Treated Corsair Little Gem Lobjoits Paris island Pinokkio F pr. isd
Corsair Little Gem Lobjoits Paris island Pinokkio F pr. isd
9.033 8.173 9.083 7.948 8.817 0.339 1.017 Lead
a a a a a
Control
Treated
0.144 0.233 0.084 0.157 0.405
12.55 a 9.97 a 8.1 a 6.73 a 7.43 a 0.27 3.524 Zinc
a a a a a
Treated
Control
Treated
11.73 a 19.68 a 12.95 a 9.97 a 18.51 a 0.093 5.651
0.06 a 2.9 a 1.02 a 2.51 a 1.11 a
160.69 a 172.17 a 163.88 a 142.57 a 162.35 a 0.56 24.519
Control 5.67 7.07 5.35 5.21 4.19
a a a a a
Control 51.24 70.59 58.15 57.72 51.47
a a a a a
Cultivars in same column followed by same letter do not differ at stated significance level.
3.1.3. Pea (Pisum sativum L.; Leguminosae) Significant differences ( p ¼ 0.033) were found between pea cultivars when grown on the metal-spiked soil (see Table 3c). The cv. Pilot showed the lowest accumulation of Cd and cv. Douce Provence the highest. Interestingly, a significant difference ( p ¼ 0.033) was detected between the cultivars Meteor (high Cd accumulation) and Pilot (low Cd accumulation) in the control soil. This would suggest that cv. Pilot is a low Cd accumulator regardless of the Cd concentration in the soil. For Cu in the spiked soil, the pea cultivar Meteor showed the lowest concentration and cv. Pilot the highest. In the case of Pb there were no significant differences ( p ¼ 0.406) between Pb concentrations in pea cultivars on the metal-spiked and control soils and between cultivars on the spiked soils. No significant differences ( p ¼ 0.26) were found in Zn concentrations between pea cultivars growing on the spiked soil. However, the results for Zn did identify an interesting trend with cv. Pilot accumulating the highest Zn concentrations in both the treated and control soils while cv. Douce Provence accumulated the lowest concentrations in both soils.
3.1.4. French bean (first harvestd56 days after sowing) (Phaseolus vulgaris L.; Leguminosae) No significant differences ( p ¼ 0.566) were found in Cd concentrations between cultivars grown on the metal-spiked soil (see Table 3d). The accumulated concentrations were very low (lowest of all species for Cd) and no trends were evident. Although no significant differences ( p ¼ 0.802) were found for Cu concentrations between French bean cultivars on the spiked soil, cv. Prince showed an interesting trend of accumulating low Cu concentrations in both the treated and control soils while cv. Cropper Teepee accumulated high concentrations in both soils.
For both Pb and Zn, there were no significant differences ( p ¼ 0.818 and p ¼ 0.194, respectively) between cultivars grown on the spiked soil. No trends were evident for Pb as the cultivar differences were negligible, but for Zn, cv. Cropper Teepee exhibited a tendency to accumulating high concentrations in both the spiked and control soils. 3.1.5. French bean (second harvestd71 days after sowing) No significant differences were found in Cd, Cu, Pb or Zn concentrations between cultivars grown on the metal-spiked soil (see Table 3e). The accumulated concentrations of Cd, Pb and Zn were very low and no trends were evident. However, in the case of Cu, a similar trend was observed that in both the first and second harvests where cv. Prince accumulated low Cu concentrations in both the treated and control soils and cv. Cropper Teepee accumulated high concentrations in both soils. For Zn, there was also a visible trend for cv. Prince accumulating low concentrations in both the treated and control soils. 3.1.6. Onion (Allium cepa L.; Liliaceae) Apart from accumulating significantly higher concentrations of Cd, Pb and Zn on the spiked soil (see Fig. 1), onions showed no significant differences between cultivars on the spiked soil for any element and no trends were evident as the cultivar differences were small (see Table 3f). 3.1.7. Lettuce (Lactuca sativa L.; Compositae) The lettuce cultivars were grown two and a half years after the other vegetable species but the soil had been re-spiked and the same system of growing outdoors was used. This crop accumulated higher mean Cd concentrations than any other species, but no significant differences ( p ¼ 0.339) in Cd contents were found between cultivars on the spiked soil
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(a)
Cadmium
Cd (mg/kg dry matter)
10
a
Cd treated
8
a
control
6
a
4
a 2
b
b
b
b
0 Lettuce P<0.001
Spinach P<0.001
Carrot P<0.001
Onion P<0.001
a b
a b
a b
Pea P<0.001
F Bean H1 P<0.05
F Bean H2 P<0.05
While no significant differences ( p ¼ 0.093) were evident in mean Pb concentrations between cultivars on the spiked soil, low and high accumulators were observed. The cultivar Paris Island showed the lowest and cv. Little Gem the highest concentrations. No significant differences ( p ¼ 0.56) in Zn concentrations were found between cultivars when grown on the spiked soil. No trends were evident as the differences in concentrations were negligible.
Vegetable
(b)
3.2. Differences in metal accumulation between vegetable crops
Copper
Cu (mg/kg dry matter)
12 10
a
a
Cu treated
b
8
b
control
a
a
6
b
4
a a
a b
a
b
b
2 0 Lettuce P<0.001
Spinach P<0.001
Carrot P<0.001
Onion P=0.413
Pea P<0.001
F Bean H1 P<0.001
F Bean H2 P<0.001
Vegetable
(c) Pb (mg/kg dry matter)
16
Lead
a
Pb treated 12
control
a 8
a
4
b
a
b
b
b
a a
a a
a b
Pea P=0.031
F Bean H1 P=0.128
F Bean H2 P<0.001
0 Lettuce P<0.001
Spinach P<0.001
Carrot P<0.001
Onion P<0.001
Vegetable
(d)
Zinc 400
a
Zn (mg/kg dry matter)
Zn treated 300 200 100
control
b a b
a
b
a
b
a b
a b
Pea P<0.001
F Bean H1 P=0.002
a
b
0 Lettuce P<0.001
Spinach P<0.001
Carrot P<0.001
Onion P<0.001
F Bean H2 P<0.001
Vegetable
Fig. 1. Differences in mean vegetable accumulation (edible portion) of (a) cadmium, (b) copper, (c) lead and (d) zinc of plants grown in spiked and control soils (SE). Initial soil concentrations Cd, Cu, Pb and Zn: spiked ¼ 4.2, 87.9, 744.5 and 237; control ¼ 0.3, 16.4, 40.5 and 83.8 mg kg1. Significant differences in mean vegetable uptake when comparing treated to control are indicated by different letters above the column ( p value stated on graph).
(see Table 3g). No trends were evident as the differences between cultivars were negligible. No significant differences ( p ¼ 0.27) in Cu concentrations were found between lettuce cultivars when grown on the spiked soil. However, it was observed that cv. Corsair accumulated the highest Cu concentration and cv. Paris Island the lowest.
The differences in mean metal accumulation between vegetable cultivars grown in spiked and control soils are shown in Fig. 1. Five of the vegetables showed significantly higher concentrations (carrots, spinach, pea and lettuce all at p < 0.001 and French bean at p < 0.05) of Cd, Cu and Zn when grown in the metal-spiked soils compared with those in the untreated control soils. Onion exhibited the pattern of significance ( p < 0.001) but only for Cd, Pb and Zn. The only element where this trend was not followed was Pb in peas and French beans. In peas, three cultivars showed a markedly higher Pb content on the spiked soil but two showed no obvious difference. This resulted in there being no overall significant differences ( p ¼ 0.131) between plants on spiked and control soils. In French bean there was a trend for the plants on the spiked soil to accumulate more Pb than on the control in all cultivars at the first harvest (not significant; p ¼ 0.128), while the difference in concentrations accumulated in the second harvest was significant ( p < 0.001). All the other vegetables showed a significant difference between Pb concentrations in plants on the spiked and control soils ( p < 0.001). Marked differences were exhibited between vegetables with regard to the mean concentrations of metals accumulated in all the cultivars combined on the metal-spiked soil. For Cd the order of accumulation was (crop mean concentrations in parenthesis in mg kg1 dry matter): Lettuce (8.6 Cd) > Spinach (5.8 Cd) > Onion (3.6 Cd) > Carrot (2.0 Cd) > Pea (0.29 Cd) > French bean (0.07 Cd). For Cu, the order of accumulation was: Spinach (9.7 Cu) > Lettuce (9.0 Cu) > Pea (5.8 Cu) > French bean (5.3 Cu) > Carrot (4.8 Cu) > Onion (2.8 Cu). Lettuce accumulated by far the highest concentration of Pb, which was almost twice as high as that in onions, the second largest mean value. The order was: Lettuce (14.6 Pb) > Onion (7.5 Pb) > Carrot (5.8 Pb) > Spinach (1.8 Pb) > Pea (0.78 Pb) > French bean (0.34 Pb). In the case of Zn, spinach accumulated more than twice as much as the next highest crop, lettuce. The order was: Spinach (352.4 Zn) > Lettuce (160.4 Zn) > Onions (64.4 Zn) > Pea (52.2 Zn) > Carrot (43.0 Zn) > French bean (40.7 Zn). Statistical analysis of the differences between vegetables was not valid because they were not all grown at the same time, nor in the same soil. However, direct (non-statistical) comparisons can be made between the data for spinach, carrot and French bean as they were grown simultaneously in the
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same soil. Similarly, direct comparisons can be drawn between the onion and pea results as they were also grown in the same season and in the same soil. Lettuces, although grown independently at a later date, were grown in soil comparable to that used for the spinach, carrot and French bean. Fig. 1 illustrates the differences in metal accumulation between the vegetables and also shows the relatively small errors associated with the data. When considering the mean accumulation of Cd by all the vegetable species, spinach clearly accumulated greater concentrations than both the carrot and French bean, while the carrot also accumulated significantly more Cd than the French bean. A comparison of onions and peas clearly showed that onions accumulated more Cd. While the lettuce were grown independently, their uptake of Cd was greater than in all the other vegetables. With regard to Pb, carrots accumulated greater concentrations than both the spinach and French bean (the latter accumulating slightly lower Pb of the two). When comparing the onions and peas, the onions accumulated more Pb. Lettuce accumulated much greater concentrations of Pb than all the other vegetables. In the case of Cu, spinach accumulated greater amounts than either the carrot or French bean (which showed similar concentrations). A comparison of onions and peas showed that the pea that accumulated greater Cu (interestingly, with a similar concentration to the French bean and carrot). Lettuce accumulated concentrations similar to those in spinach. Zinc concentrations in carrots, French beans, onions and peas were all similar and relatively low. Spinach showed by far the highest Zn concentrations, which were twice as high as those in lettuce, the second highest accumulator. Interestingly the spinach also accumulated high concentrations in the control soil suggesting that it is a major accumulator of Zn. 4. Discussion From the data presented, certain important conclusions can be drawn with regard to the suitability of vegetable crops and cultivars for growing on soils contaminated with one or more of the elements investigated in this study. In the vegetable species that showed differences between cultivars, the lower accumulators would be most suitable for growing on contaminated soil. However, this would depend on the total concentration of the respective metals being below the CLEA soil guideline values for Cd and Pb. Of the metals investigated, only these two elements are considered to be of a high potential risk to human health. Although not statistically testable, some marked differences between species were apparent for these elements. Cadmium accumulated to the greatest extent in spinach and lettuce, and Pb accumulated in lettuce and onion. Unfortunately, no significant differences in the uptake of these two metals by the cultivars of these four vegetables were identified. However, carrot, which had an intermediate level of Cd accumulation, did show significant differences between cultivars (cv. Nairobi being a low accumulator). Peas are generally low accumulators of Cd but cv.
743
Pilot was shown to accumulate the least Cd of the pea cultivars examined. While Cu and Zn are considered a much lower risk to human health than Cd and Pb, some significant cultivar differences were established. For carrot, spinach and pea, significant differences between cultivars for Cu were identified. Carrots cvs. Nairobi, Nantes and Ingot, spinach cvs. Mediana and Bloomsdale and pea cvs. Meteor and Douce Province were all identified as low accumulators of Cu. In the case of Zn, the only significant differences identified between cultivars were in carrots, where cvs. Nairobi and Ingot were lower accumulators. In general, both spinach and lettuce appear to be relatively high accumulators of Cd and thus it could be argued that these species should not be grown on soils with significantly elevated Cd concentrations. On the basis of the soil guideline values used in England and Wales, this would be at concentrations in soils above 1 mg Cd kg1 at pH 6, 2 mg Cd kg1 at pH 7 and 8 mg Cd kg1 at pH 8 (Environment Agency, 2002a). On the other hand, French bean and pea appear to be relatively low accumulators of Cd and so would be more suitable species to grow. Carrots tend to accumulate less Cd than lettuce, spinach or onions but the significant differences between cultivars indicate that cv. Nairobi is the most suitable for minimising Cd intake via this vegetable on Cd contaminated soils. It also accumulates lower levels of Cu, Pb and Zn on soil contaminated with these metals. Interestingly, the relative order of Cd accumulation by the six crop species investigated follows exactly the classification by Kuboi et al. (1986) mentioned earlier. French bean and pea, which had the lowest Cd contents are members of the Leguminosae, which were classed by Kuboi et al. (1986) as ‘‘low accumulators’’. Carrot and onion, had intermediate concentrations of Cd and are members of the Umbelliferae and Liliaceae families, respectively, which were classed as ‘‘moderate accumulators’’. The highest Cd concentrations found in this study were in lettuce (Compositae) and spinach (Chenopodiaceae), which were classed these as ‘‘high accumulators’’. Many garden soils have been contaminated with Pb, especially in older urban areas. Atmospheric deposition of Pb onto plants has often been considered to be of greater importance than root uptake and translocation to edible portions of plants. However, Pb has an index of accumulation of 101e1.0, which is in the same range as Cu and Zn (Kabata-Pendias and Pendias, 1992). Therefore genotypic differences in accumulation between cultivars are still important. In this study, all crops were grown at the same location so it is expected that the deposition would have been relatively similar but the edible portions analysed were thoroughly scrubbed to remove soil and any other deposits. In view of the relatively high accumulation of Pb shown by lettuce, this species would probably not be suitable where Pb concentrations exceed the 450 mg kg1 soil guideline value in the Contaminated Land Exposure Assessment (CLEA) recommendations (Environment Agency, 2002b). Onion also exhibited relatively high accumulation concentrations and should probably be avoided on Pb-contaminated soils. In general it
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would appear that spinach, French bean and peas would probably be suitable for growing on account of their relatively low accumulation of Pb. A comparison of the mean concentrations of Cd and Pb in the vegetables with the European Union maximum permitted concentrations in foodstuffs (EC, 2001) shows some interesting results. For Cd there are three limits for different types of vegetables: ‘‘leafy’’ vegetables 0.2 mg Cd kg1 wet weight, ‘‘root’’ vegetables 0.1 mg Cd kg1 wet weight and ‘‘all other’’ vegetables 0.05 mg Cd kg1 wet weight. In order to convert to wet weight, the median value of the moisture content ranges of common vegetables presented by Duckworth (1966) were used. On the spiked soils, lettuce and spinach appear to exceed the ‘‘leafy vegetable’’ limit and the carrot and onion exceed the ‘‘root vegetable’’ limit, pea exceed the ‘‘all other vegetables’’ while French beans appear to fall within the Cd limit. However, on the unspiked soil none of the vegetables exceeded the limit. In the case of the Pb spiked soil, the lettuce appeared to exceed the limits of 0.3 mg Pb kg1 for ‘‘leafy’’ vegetables while the onion, carrot and pea all appeared to exceed the limits of 0.1 mg Pb kg1 for ‘‘all other’’ vegetables, only spinach and French bean appear to fall within the limits. When examining the vegetables grown on the unspiked soil, all fall within the statutory limits for Pb with the exception of the onion. It must be recognised that the spiking and the use of pots, instead of field soil, probably gave rise to enhanced uptake. However, the relatively low values in this European directive help to emphasise the importance of selecting cultivars with the lowest uptake of Cd and Pb. Elevated Cu concentrations in soils are more of a phytotoxicity and ecotoxicological hazard than a risk to human health (ICRCL, 1987). In general terms, spinach and lettuce were found to be the highest accumulators of Cu and onion the lowest in this study. Zinc, like Cu, constitutes a greater risk of phytotoxicity than a hazard to human health but spinach accumulated far higher concentrations of Zn than any of the other vegetable in this research. Considering that approximately 20.5% of the world’s population is estimated to be at risk of inadequate Zn intake (Wuehler et al., 2005) it may be worth considering accumulator species, such as spinach, as being a convenient way of increasing the dietary intake of Zn. However, the concentrations of potentially harmful elements, such as Cd, Pb and Hg in spinach would also need to be considered. These results indicate that it would be worthwhile extending this type of screening study to other elements and a wider range of cultivars of other vegetables in order to be able to advise gardeners on how best to minimise their intake of potentially harmful elements through the consumption of home-grown vegetables. In addition, further research comparing the accumulation of metals by different vegetables grown at the same time would be of great interest. Acknowledgements Thanks are due to Lynne Gregory, Emily Reid and Ian Waghorn of RHS Wisley for undertaking much of the field
work associated with the project, and also to Chris Prior of RHS Wisley for advice at many stages of the project, Geoffrey Warren of The University of Reading Department of Soil Science for analysis of soils and plants, and finally Eleanor Allan of Statistical Services Centre (The University of Reading) for statistical advice.
References Alloway, B.J., 2004. Contamination of soils in domestic gardens and allotments: a brief review. Land Contamination and Reclamation 12 (3), 179e187. Alloway, B.J., Thornton, I., Smart, G.A., Sherlock, J., Quinn, M.J., 1988. Metal availability (in ‘‘The Shipham Report’’). Science of the Total Environment 75, 41e69. Crews, H.M., Davies, B.E., 1985. Heavy metal uptake from contaminated soils by six varieties of lettuce (Lactuca sativa L.). Journal of Agricultural Science (Cambridge) 105, 591e595. Culbard, E.B., Thornton, I., Watt, J., Wheatley, M., Moorcroft, S., Thompson, M., 1988. Metal contamination in British suburban dusts and soils. Journal of Environmental Quality 17, 226e234. De Vries, M.P.C., Tiller, K.F.G., 1978. Sewage sludge as a soil amendment, with special reference to Cd, Cu, Mn, Ni, Pb and Zndcomparison of results from experiments conducted inside and outside a glasshouse. Environmental Pollution 16 (3), 231e240. Duckworth, R.B., 1966. Fruit and Vegetables. Pergamon Press, Oxford, UK. EC (European Community), 2001. Commission Regulation (EC) 466/2001 Setting maximum levels for certain contaminants in foodstuffs. Official Journal of the European Communities L 77, 1e25. 16 March 2001. Environment Agency, 2002a. Soil Guideline Values for Cadmium. Environment Agency, R&D Publication SGV 3. Environment Agency, Bristol. Environment Agency, 2002b. Soil Guideline Values for Lead. Environment Agency, R&D Publication SGV 10. Environment Agency, Bristol. Environmental Protection Act, 1990. Part IIA Contaminated Land. HMSO, Her Majesty’s Stationary Office. C43. Fleming, G.A., Parle, P.J., 1977. Heavy metals in soils, herbage and vegetables from an industrialised area west of Dublin city. Irish Journal of Agricultural Research 16, 35e48. Grant, C.A., Bailey, L.D., McLaughlin, M.J., Singh, B.R., 1999. Management factors which influence cadmium concentrations in crops. In: McLaughlin, M.J., Singh, B.R. (Eds.), Cadmium in Soils and Plants. Kluwer Academic Publishers, Dordrecht, pp. 151e198. Gro¨ßmann, G., Wu¨stermann, M., 1992. Belastungen in Haus- und kleingarten durch anorganische und organische Stoffe mit Schadstoffpotential. Forschungsbericht 1160868. Umweltforschungsplan des Bundesministers fu¨r Umwelt, Naturschutz und Reaktorsicherheit. ICRCL, 1987. Interdepartmental Committee for the Redevelopment of Contaminated Land: Guidance on the Assessment and Redevelopment of Contaminated Land. Department of the Environment, London. Guidance Note 59/83. John, M.K., van Laerhoven, C.J., 1976. Differential effects of cadmium on lettuce varieties. Environmental Pollution 10, 163e173. Kabata-Pendias, A., Pendias, H., 1992. Trace Elements in Soils and Plants, second ed. CRC Press, Boca Raton. Kuboi, T., Noguchi, A., Yazaki, J., 1986. Family-dependent cadmium accumulation in higher plants. Plant and Soil 92, 405e415. Lehoczky, E´., Szabo´, L., Horva´th, Sz., 1998. Communications in Soil Science and Plant Analysis. Cadmium uptake by lettuce in different soils 29 (11-14), 1903e1912. McLaughlin, M.J., Williams, C.M.J., McKay, A., Kirkham, R., Gunton, J., Jackson, K.J., Thompson, R., Dowling, B., Partington, D., 1994. Effect of cultivar on uptake of cadmium by potato tubers. Australian Journal of Agricultural Research 45, 1483e1495. Pettersson, O., 1977. Differences in cadmium uptake between plant species and cultivars. Swedish Journal of Agricultural Research 7, 21e24.
P.D. Alexander et al. / Environmental Pollution 144 (2006) 736e745 Thornton, I., Jones, T.H., 1984. Sources of lead and associated metals in vegetables grown in British urban soils: uptake from soil versus air deposition. In: Hemphill, D.D. (Ed.), Trace Substances and Environmental Health. XVIII. University of Columbia, Missouri, pp. 303e310. Wuehler, S.E., Peerson, J.M., Brown, K.H., 2005. Use of national food balance data to estimate the adequacy of zinc in national food supplies:
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methodology and regional estimates. Public Health Nutrition 8 (7), 812e819. Xue, Q., Harrison, H.C., 1991. Effect of soil zinc, pH, and cultivar on cadmium uptake in leaf lettuce (Lactuca sativa L. var. crispa). Communications in Soil Science and Plant Analysis 22 (9, 10), 975e991.
Genotypic variations in the accumulation of Cd, Cu, Pb and Zn exhibited by six commonly grown vegetables P.D. Alexander a,*, B.J. Alloway b, A.M. Dourado a,1 b
a Royal Horticultural Society’s Garden, Wisley, Woking, Surrey GU23 6QB, UK Department of Soil Science, School of Human and Environmental Sciences, The University of Reading, P.O. Box 233, Reading RG6 6DW, UK
Received 26 September 2005; received in revised form 31 January 2006; accepted 2 March 2006
Genotypic differences between cultivars of vegetable species can be important in determining the extent of accumulation of metals from contaminated soil. Abstract Metal contaminants in garden and allotment soils could possibly affect human health through a variety of pathways. This study focused on the potential pathway of consumption of vegetables grown on contaminated soil. Five cultivars each of six common vegetables were grown in a control and in a soil spiked with Cd, Cu, Pb and Zn. Highly significant differences in metal content were evident between cultivars of a number of vegetables for several of the contaminants. Carrot and pea cultivars exhibited significant differences in accumulated concentrations of Cd and Cu with carrot cultivars also exhibiting significant differences in Zn. Distinctive differences were also identified when comparing one vegetable to another, legumes (Leguminosae) tending to be low accumulators, root vegetables (Umbelliferae and Liliaceae) tending to be moderate accumulators and leafy vegetables (Compositae and Chenopodiaceae) being high accumulators. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Contaminated soil; Vegetable cultivars; Heavy metal accumulation; Cadmium; Copper; Lead; Zinc
1. Introduction Domestic gardens and allotments in many countries have been contaminated with heavy metals. Increased urbanisation has resulted in houses being constructed on brownfield sites and, before the introduction of strict soil quality standards, many of these will have been contaminated with metals. Common heavy metal contaminants include Cd, Cu, Pb and Zn, from a wide range of sources. Cadmium and Pb could possibly pose a risk to human health where the occupants of the houses or holders of the allotments consume relatively large quantities of vegetables grown on these soils and/or ingest soil either on unwashed vegetables, on their hands or, in the case of children,
* Corresponding author. Tel.: þ44 1483 224234; fax: þ44 1483 211750. E-mail address: [email protected] (P.D. Alexander). 1 Present address: Malvern, London Road East, Amersham HP7 9DL, UK. 0269-7491/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2006.03.001
intentionally eating garden soil. Copper and Zn are regarded as being more of a phytotoxicity hazard than a risk to human health (ICRCL, 1987). Increasing awareness of such contamination has resulted in many countries introducing legislation to identify problem areas and minimise the risk to human health (e.g. Under Part IIA of the Environmental Protection Act, 1990 in the UK). Sources of heavy metal contamination of soils and plants in gardens and allotments can include: previous use of site, atmospheric deposition, paint particles, bonfires, use of contaminated fill for levelling the site, runoff from metal surfaces, use of ash and mineral waste for constructing paths, burial of metal-containing wastes, composts and fertilisers and leisure activities such as air gun shooting (Alloway, 2004). Surveys of garden soils in several countries have shown wide ranges of concentrations of heavy metals. For example, Culbard et al. (1988) found concentrations of up to 14,100 mg Pb kg1 (mean 266 mg Pb kg1), and up to 14,700 mg
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Zn kg1 (mean 278 mg Zn kg1) in more than 4000 samples from gardens in 50 cities, towns and villages in England, Scotland and Wales. These are higher maximum and mean values than those found for more than 3600 garden soils in Germany where Gro¨ßmann and Wu¨stermann (1992) found up to 627 mg Pb kg1 (mean 65 mg Pb kg1) and up to 1035 mg Zn kg1 (mean 138 mg Zn kg1). Plants growing on contaminated soils will reflect elevated concentrations of heavy metals in the soils to varying extents, depending on the soil total concentrations, soil physico-chemical conditions (especially pH) and the genotype of the plant. Thornton and Jones (1984) reported concentrations of up to 23 mg Pb kg1 (dry matter) in lettuces and radishes growing in a garden on soil containing 4100 mg Pb kg1 and up to 333 mg Zn kg1 in radishes on soil with a total content of 2182 mg Zn kg1. Alloway et al. (1988) reported concentrations of up to 11.7 mg Cd kg1 and 18.5 mg Pb kg1 in vegetables from gardens in Shipham, England, which had been contaminated with wastes from Zn and Pb mining. Elevated heavy metal intake by humans from contaminated gardens can be due to the consumption of crop plants grown on soils with relatively high plant-available metal concentrations, ingestion of contaminated soil, either accidentally or intentionally (pica), inhalation of soil particles, and drinking water with high soluble concentrations of metals. Although all of these pathways can be important, the soil-plant-human pathway is likely to affect more people overall with regard to vegetable gardens than the others. The amount of metal taken up by plants relative to the composition of the soil in which they are growing can be expressed by the soileplant concentration factor (CF), which is based on the metal in the plant divided by metal in the soil [CF ¼ M (plant)/M (soil)]. This CF value is a function of the properties of the metal itself, especially the degree to which it is adsorbed in an unavailable form, the pH and sorptive properties of the soil and the genotype of the plant. Differences between crop plant species in the accumulation of metals are widely recognised. Much work has been done on the accumulation of Cd by plants in view of its relatively high soil-plant concentration factor. Pettersson (1977) showed that Cd concentrations in the shoots of plants grown in solution culture decreased in the order: lettuce > carrot, tomato > rape, kale, radish > mustard, maize > cucumber, sunflower, peas, bean > wheat, oats. Lehoczky et al. (1998) identified lettuce, beetroot, radish and carrot as being high Cd accumulators while cauliflower, red cabbage and savoy cabbage were only moderate accumulators. Other work by Kuboi et al. (1986) classified plant families into three groups with regard to the extent to which they accumulated Cd. These were: (1) low Cd accumulators (Leguminosae), (2) moderate accumulators (Gramineae, Liliaceae, Cucurbitaceae and Umbelliferae), and (3) high accumulators (Chenopodiaceae, Cruciferae, Solanaceae and Compositae). Fleming and Parle (1977) examined the order of accumulation of Pb, Zn and Cu by selected species of vegetables in a pot study and found that lettuce was consistently among the high accumulators and beans were consistently among the low accumulators.
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However, there is less information available on varietal differences in metal accumulation within species, especially vegetables. In South Australia, McLaughlin et al. (1994) compared the uptake of Cd by 14 commonly grown potato cultivars. They found significant differences between the cultivars at most sites with an average range of concentrations of 30e50 mg Cd kg1 (fresh weight). At some sites, individual cultivars had Cd concentrations which exceeded the Australian maximum permissible concentration of 50 mg Cd kg1. They suggested Cd concentrations in potato tubers on some soils could be reduced by more than 50% by selecting low accumulating cultivars. John and van Laerhoven (1976) found a 21-fold variation in the Cd content of the leaves of nine lettuce cultivars grown in culture solution for five weeks. Crews and Davies (1985) also examined the uptake of Cd, Cu, Pb and Zn by six cultivars of lettuce from contaminated soil. They were able to identify high and low accumulators of Cd and Zn. Xue and Harrison (1991) also examined six lettuce cultivars but just for Zn uptake; they identified high and low accumulating cultivars. In a literature review, Grant et al. (1999) reported papers dealing with genotypic differences in Cd concentrations in cultivars of a range of crops, including: maize, non-oilseed sunflower, soybean, flax, rice, lettuce and durum wheat. The study reported here was designed to investigate variations in the accumulation of Cd, Cu, Pb and Zn by cultivars of six commonly grown vegetables from a contaminated soil. 2. Materials and methods 2.1. Site and soil The experiments were conducted at the Deers Farm experimental site of the Royal Horticultural Society’s Garden at Wisley (Woking, Surrey, UK). The soil used was a sandy loam, which had been spiked with solutions of cadmium nitrate, copper sulphate, lead nitrate and zinc sulphate to give concentrations of metals that were relatively typical of highly contaminated gardens. It was decided to spike a soil with metals, rather than use a contaminated soil from a garden, for a number of reasons: the costs associated with sourcing, transporting and handling contaminated soil, the potential problems of other metal and organic contaminants affecting uptake of the elements of interest and the advantage of using a homogeneous sandy soil which is easy to mix thoroughly. It was also considered that combining the four metal additions in the same soil would mimic the situation which could occur in a multielement contaminated soil in a ‘‘real’’ garden situation. It is accepted that various antagonistic or synergistic interactions could occur and affect uptake after spiking but all plants would be exposed to the same effects. The uptake of metals from spiked soils was expected to be greater than in the field situation even after a relatively long incubation period, but uptake by plants in large pots out in the open-air would be expected to be less enhanced than in small pots in the greenhouse (De Vries and Tiller, 1978). The spiking was carried out by spraying a solution of each metal salt over relatively dry soil spread out on large trays. The soil was turned over and sprayed several times and then watered and left to equilibrate outdoors under a waterproof tarpaulin for about three months. This long period was chosen to allow time for natural equilibration of the various sorption mechanisms in the soil. It was decided that the spiking targets would be based on the old provisional ICRCL targets. For Cu and Zn, which are considered to be more of a phytotoxicity and ecotoxicological hazard than a risk to human health (ICRCL, 1987), the ICRCL trigger values (total) were used to avoid directly compromising plant performance. The ICRCL trigger values for Cd and Pb were increased, Cd was doubled and Pb was increased by 50%. By setting
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these targets the concentrations were considered to be representative of many heavily contaminated garden and agricultural soils, without being unrealistically high. However, they were intended to be high enough to allow differences between cultivars to be detectable. The mean total concentrations of the metals in 10 separate samples of the thoroughly mixed spiked and control soils (in mg kg1) and the target concentrations are presented in Table 1. In the case of the lettuces, the soil was re-spiked 30 months later to the same approximate concentrations because of the relatively long time period since the initial spiking. After 3 months, the spiked soils were once again thoroughly mixed and put into large (12-L) plastic plant pots which were placed in saucers to prevent cross-contamination from drainage water.
2.2. Vegetable cultivars and planting The vegetable cultivars (cv) chosen were selected from the UK National Catalogue of Accepted Cultivars (1998) list to ensure widespread use and availability (presented in Table 2). The seeds were sown directly into the pots of spiked and control soils, respectively. The spinach, carrot and French bean cultivars were grown simultaneously (all sown on 9 June, 1999). The onions (sown 8 September, 1999) and peas (sown 26 October, 1999) were also grown in the same season. The lettuce cultivars were grown separately, later, in May 2002, in re-spiked soil. Sufficient seed was sown to guarantee healthy germination (plants were thinned after germination). Each vegetable species was grown in different pots outdoors using overhead drip irrigation. All vegetables were grown to maturity and the pots were replicated five times in a fully randomised block design. The nature of French beans enabled two harvests to be taken (results are presented for both). By growing the vegetables outdoors through their normal planting season, it was hoped to more accurately reflect the growing practices of gardeners and allotment holders (with the exception of the use of large pots). However, this does make comparing plants grown under slightly variable environmental conditions more difficult.
2.3. Analytical methods For the soil, metals extractable by aqua regia were measured. Soil samples (1.5 g) were weighed into duplicate 100 ml digestion tubes, conc. HNO3 (3 ml) and conc. HCl (10.5 ml) added, and allowed to stand overnight. They were then heated for 1 h at 50 C followed by 3 h at 140 C. After cooling, the ingests were filtered and made to volume by the method for plant digests except that 0.5 M HNO3 was used instead of water. Analytical quality assurance was addressed by the systematic use of blanks, replicates and analysis of a Certified Reference Soil (CRM 143R) and an in-house reference material (sludge-amended soil). For all crops, only the edible portions were sampled as the work was focused on the soil-plant-human pathway of trace metals (with particular regard to amateur gardeners and allotment holders). These edible portions were thoroughly washed (not peeled), oven dried (48 C for 48 h) and ground in a mill. Samples of plant material (0.5 g) were weighed into 100 ml block digestion tubes, concentrated nitric acid (10 ml) added and allowed to stand overnight. They were then heated for 3 h at 60 C, followed by 6 h at 110 C. After cooling, the digests were passed through a pre-washed filter (Whatman No. 540), the digestion tubes were rinsed three times, passing the washings through the filter and the filtrates made up to 100 ml volume using
Table 1 Mean total concentrations (mg kg1) of the metals in thoroughly mixed spiked and control soils (n ¼ 10)
Spiked soil SE Control soil SE Target
Cadmium
Copper
Lead
Zinc
4.2 0.1 0.3 0.03 6
87.9 2.12 16.4 0.19 130
744.5 15.84 40.5 1.41 750
237.0 4.43 83.8 1.28 300
Table 2 Species of vegetable and cultivars grown Species
Cultivars grown
Spinach Carrot French bean Peaa Onion Lettuce
Bloomsdale, Giant Thick Leaved, Grodane, Mediana, Spartacus Amsterdam Forcing, Early Nantes, Ingot, Mokum, Nairobi Cropper Teepee, Masterpiece, Prince, Sprite, Tendergreen Douce Provence, Feltham, Fortune, Meteor, Pilot Buffalo, Express, Keepwell, Shenshyu, Toughball Corsair, Little Gem, Lobjoits, Paris Island, Pinokkio
a
All peas are normal garden peas, not sugar snap, mange tout etc.
ultrapure water. Trace metal concentrations were measured in duplicate by ICP-OES spectrometry (Perkin Elmer Optima 3000) using a certified multielement standard (Merck, Darmstadt, Germany). A certified reference material (bush leaf material) GBW07603, Qunghai Province, China (supplied via the Laboratory of the Government Chemist, UK) was used to monitor the recovery of metals from the plant samples. Statistical analysis was undertaken using GENSTAT (Analysis of Variance) with support from the Statistical Services Department of The University of Reading.
3. Results 3.1. Differences in metal accumulation between cultivars 3.1.1. Carrot (Daucus carota L.; Umbelliferae) Highly significant differences ( p < 0.001) in Cd concentrations were found between carrot cultivars grown on the metalspiked soil (see Table 3a); cv. Nairobi showed the lowest Cd concentration and cv. Amsterdam the highest. Significant differences ( p ¼ 0.034) in Cu concentrations were found between carrot cultivars on the metal-spiked soil; cv. Nairobi showed the lowest and cv. Amsterdam the highest mean concentrations. No significant differences ( p ¼ 0.457) were found in Pb concentrations between carrot cultivars when grown on the spiked soil. However, certain trends were evident with the cultivars Nantes, Nairobi and Mokum showing the lowest Pb concentrations and Ingot and Amsterdam the highest. Significant differences ( p ¼ 0.006) were found in Zn concentrations between carrot cultivars grown on the spiked soil. The cultivars Nairobi and Ingot showed the lowest metal contents and cv. Amsterdam and cv. Mokum the highest. 3.1.2. Spinach (Spinacia oleracea L.; Chenopodiaceae) No significant differences ( p ¼ 0.095) were found in Cd, Cu, Pb or Zn concentrations between spinach cultivars growing on the metal-spiked soil (see Table 3b). Nevertheless, trends were evident, with cv. Bloomsdale showing the lowest Cd concentration and Grodane, Mediana and Spartacus all having similar, higher Cd contents. In the case of Cu, the cultivars Mediana and Bloomsdale had the lowest concentrations and cv. Spartacus the highest mean concentration. No significant differences in either Pb or Zn concentrations ( p ¼ 0.97 and p ¼ 0.243, respectively) were found between spinach cultivars when grown on the spiked soil and no trends were evident.
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Table 3 Mean metal (Cd, Cu, Pb and Zn) concentrations (mg /kg-1 dry matter) of (a) carrot, (b) spinach, (c) pea, (d) French beandfirst harvest, (e) French beandsecond harvest, (f) onion and (g) lettuce cultivars grown in control and spiked soil (n ¼ 5) Cadmium
(a) Carrot
Amsterdam Nantes Ingot Mokum Nairobi F pr. isd
Copper
Treated
Control
2.521 c 1.812 b 2.298 c 2.133 bc 1.215 a <0.001 0.3977
0.068 a 0.054 a 0.074 a 0.05 a 0.06 a
Treated 5.77 b 4.43 a 4.43 a 5.45 b 3.85 a 0.034 0.913
Lead Control
6.31 a 5.01 a 7.23 a 5.18 a 5.04 a 0.457 1.931
0.34 0.17 0.42 0.29 0.34
Treated
a a a a a
52.59 c 42.54 b 33.38 a 54.09 c 32.55 a 0.006 8.855
Cadmium
(b) Spinach
Treated Bloomsdale G. Thick Leaved Grodane Mediana Spartacus F pr. isd
4.24 a 5.13 ab 6.94 c 6.48 bc 6.14 bc 0.095 1.4700
0.36 0.25 0.45 0.33 0.32
Treated
a a a a a
9.01 a 9.56 ab 10.08 b 8.97 a 10.7 b 0.09 1.086
6.29 6.05 6.44 7.31 7.7c
Control
Treated
Control
0.62 0.49 0.69 0.51 0.76
334.9 a 341.8 a 380.2 a 343.2 a 361.7 a 0.243 62.68
246.7 218.6 209 a 256.4 274.9
0.4282 0.2615 0.3355 0.2337 0.1785 0.033 0.128
Treated 1.399 a 0.547 a 0.731 a 0.724 a 0.508 a 0.406 0.8194
a a a a a
Control
1.74 a 1.68 a 1.78 a 2a 1.89 a 0.97 0.937
a a a a a
ab a ab bc
Control c ab bc ab a
a a a a
Copper
0.1054 0.1055 0.1441 0.2088 0.0532
Lead
Douce P Feltham Fortune Meteor Pilot F pr. isd
19.68 19.02 18.43 19.72 16.27
Treated
Treated Douce P Feltham Fortune Meteor Pilot F pr. isd
Control
Zinc
Cadmium
(c) Pea
a a a a a
Copper Control
Lead
Bloomsdale G. Thick Leaved Grodane Mediana Spartacus F pr. isd
2.78 2.42 2.81 2.59 2.62
Zinc
Treated Amsterdam Nantes Ingot Mokum Nairobi F pr. isd
Control
Treated ab ab ab b a
5.163 a 5.727 ab 6.617 b 4.72 a 6.722 b 0.008 1.0548
Control 4.117 4.189 4.572 4.595 3.772
a a a a a
Zinc Control 0.412 0.322 0.692 0.546 0.542
a a a a a
Treated 47.49 a 47.8 a 54.27 a 51.53 a 59.9 a 0.26 5.286
Control 32.36 a 36.83 a 36.68 a 33.2 a 42.08 a
(continued on next page)
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P.D. Alexander et al. / Environmental Pollution 144 (2006) 736e745
Table 3 (continued) Cadmium
(d) French bean (Hv 1)
Treated Cropper Teepee Masterpiece Prince Sprite Tendergreen F pr. isd
Copper Control
0.0356 a 0.0156 a 0.0596 a 0.0388 a 0.051 a 0.566 0.0377
Treated
0.0248 a 0.004 a 0.032 a 0.0048 a 0.002 a
Lead Control
0.278 a 0.245 a 0.244 a 0.342 a 0.18 a 0.818 0.2546
0.272 0.155 0.135 0.138 0.152
Treated
a a a a a
48.46 a 42.12 a 42.54 a 43.2 a 45.28 a 0.194 6.233
Cadmium
(e) French bean (Hv 2)
Treated Cropper Teepee Masterpiece Prince Sprite Tendergreen F pr. isd
0.0726 0.1086 0.1218 0.0908 0.1344 0.868 0.0872
a a a a a
Treated
0.0098 a 0.04 a 0.1098 a 0.026 a 0.0962 a
0.439 a 0.532 a 0.378 a 0.445 a 0.356 a 0.65 0.2149
Treated 4.0229 3.5566 3.3498 3.7283 3.4464 0.501 0.546
Control
Treated 39.77 a 39.3 a 32 a 40.24 a 35.18 a 0.265 5.58
Treated 6.486 a 8.745 a 6.519 a 7.77 a 7.772 a 0.659 2.6985
45.1 a 42.9 a 35.66 a 38.76 a 35.68 a
Control 5.19 a 3.91 a 3.23 a 3.7 a 3.96 a
Control a a a a a
Control 36.63 31.03 22.59 29.13 24.76
a a a a a
Copper
0.1928 0.2237 0.1107 0.2089 0.1987
Lead
Buffalo Express Keepwell Shenshyu Toughball F pr. isd
5.93 a 5.71 a 4.28 a 5.47 a 5.32 a 0.418 0.933
0.284 a 0.216 a 0.243 a 0.17 a 0.208 a
Cadmium
Buffalo Express Keepwell Shenshyu Toughball F pr. isd
Control
Zinc
Treated
(f) Onion
a a a a a
Copper Control
Lead
Cropper Teepee Masterpiece Prince Sprite Tendergreen F pr. isd
5.45 4.54 3.27 3.92 4.67
Zinc
Treated Cropper Teepee Masterpiece Prince Sprite Tendergreen F pr. isd
6.35 a 5.11 a 4.43 a 4.85 a 5.08 a 0.802 0.944
Control
Treated a a a a a
2.374 a 3.078 a 2.878 a 2.693 a 3.055 a 0.856 1.1817
Control 2.307 2.755 2.342 2.985 2.611
Zinc Control 1.689 1.216 1.125 1.678 1.384
a a a a a
Treated 50.45 a 60.94 a 66.97 a 65.09 a 78.34 a 0.291 21.861
Control 23.75 32.28 16.25 31 a 25.82
a a a a
a a a a a
P.D. Alexander et al. / Environmental Pollution 144 (2006) 736e745
741
Table 3 (continued ) Cadmium
(g) Lettuce
Copper
Treated Corsair Little Gem Lobjoits Paris island Pinokkio F pr. isd
Corsair Little Gem Lobjoits Paris island Pinokkio F pr. isd
9.033 8.173 9.083 7.948 8.817 0.339 1.017 Lead
a a a a a
Control
Treated
0.144 0.233 0.084 0.157 0.405
12.55 a 9.97 a 8.1 a 6.73 a 7.43 a 0.27 3.524 Zinc
a a a a a
Treated
Control
Treated
11.73 a 19.68 a 12.95 a 9.97 a 18.51 a 0.093 5.651
0.06 a 2.9 a 1.02 a 2.51 a 1.11 a
160.69 a 172.17 a 163.88 a 142.57 a 162.35 a 0.56 24.519
Control 5.67 7.07 5.35 5.21 4.19
a a a a a
Control 51.24 70.59 58.15 57.72 51.47
a a a a a
Cultivars in same column followed by same letter do not differ at stated significance level.
3.1.3. Pea (Pisum sativum L.; Leguminosae) Significant differences ( p ¼ 0.033) were found between pea cultivars when grown on the metal-spiked soil (see Table 3c). The cv. Pilot showed the lowest accumulation of Cd and cv. Douce Provence the highest. Interestingly, a significant difference ( p ¼ 0.033) was detected between the cultivars Meteor (high Cd accumulation) and Pilot (low Cd accumulation) in the control soil. This would suggest that cv. Pilot is a low Cd accumulator regardless of the Cd concentration in the soil. For Cu in the spiked soil, the pea cultivar Meteor showed the lowest concentration and cv. Pilot the highest. In the case of Pb there were no significant differences ( p ¼ 0.406) between Pb concentrations in pea cultivars on the metal-spiked and control soils and between cultivars on the spiked soils. No significant differences ( p ¼ 0.26) were found in Zn concentrations between pea cultivars growing on the spiked soil. However, the results for Zn did identify an interesting trend with cv. Pilot accumulating the highest Zn concentrations in both the treated and control soils while cv. Douce Provence accumulated the lowest concentrations in both soils.
3.1.4. French bean (first harvestd56 days after sowing) (Phaseolus vulgaris L.; Leguminosae) No significant differences ( p ¼ 0.566) were found in Cd concentrations between cultivars grown on the metal-spiked soil (see Table 3d). The accumulated concentrations were very low (lowest of all species for Cd) and no trends were evident. Although no significant differences ( p ¼ 0.802) were found for Cu concentrations between French bean cultivars on the spiked soil, cv. Prince showed an interesting trend of accumulating low Cu concentrations in both the treated and control soils while cv. Cropper Teepee accumulated high concentrations in both soils.
For both Pb and Zn, there were no significant differences ( p ¼ 0.818 and p ¼ 0.194, respectively) between cultivars grown on the spiked soil. No trends were evident for Pb as the cultivar differences were negligible, but for Zn, cv. Cropper Teepee exhibited a tendency to accumulating high concentrations in both the spiked and control soils. 3.1.5. French bean (second harvestd71 days after sowing) No significant differences were found in Cd, Cu, Pb or Zn concentrations between cultivars grown on the metal-spiked soil (see Table 3e). The accumulated concentrations of Cd, Pb and Zn were very low and no trends were evident. However, in the case of Cu, a similar trend was observed that in both the first and second harvests where cv. Prince accumulated low Cu concentrations in both the treated and control soils and cv. Cropper Teepee accumulated high concentrations in both soils. For Zn, there was also a visible trend for cv. Prince accumulating low concentrations in both the treated and control soils. 3.1.6. Onion (Allium cepa L.; Liliaceae) Apart from accumulating significantly higher concentrations of Cd, Pb and Zn on the spiked soil (see Fig. 1), onions showed no significant differences between cultivars on the spiked soil for any element and no trends were evident as the cultivar differences were small (see Table 3f). 3.1.7. Lettuce (Lactuca sativa L.; Compositae) The lettuce cultivars were grown two and a half years after the other vegetable species but the soil had been re-spiked and the same system of growing outdoors was used. This crop accumulated higher mean Cd concentrations than any other species, but no significant differences ( p ¼ 0.339) in Cd contents were found between cultivars on the spiked soil
P.D. Alexander et al. / Environmental Pollution 144 (2006) 736e745
742
(a)
Cadmium
Cd (mg/kg dry matter)
10
a
Cd treated
8
a
control
6
a
4
a 2
b
b
b
b
0 Lettuce P<0.001
Spinach P<0.001
Carrot P<0.001
Onion P<0.001
a b
a b
a b
Pea P<0.001
F Bean H1 P<0.05
F Bean H2 P<0.05
While no significant differences ( p ¼ 0.093) were evident in mean Pb concentrations between cultivars on the spiked soil, low and high accumulators were observed. The cultivar Paris Island showed the lowest and cv. Little Gem the highest concentrations. No significant differences ( p ¼ 0.56) in Zn concentrations were found between cultivars when grown on the spiked soil. No trends were evident as the differences in concentrations were negligible.
Vegetable
(b)
3.2. Differences in metal accumulation between vegetable crops
Copper
Cu (mg/kg dry matter)
12 10
a
a
Cu treated
b
8
b
control
a
a
6
b
4
a a
a b
a
b
b
2 0 Lettuce P<0.001
Spinach P<0.001
Carrot P<0.001
Onion P=0.413
Pea P<0.001
F Bean H1 P<0.001
F Bean H2 P<0.001
Vegetable
(c) Pb (mg/kg dry matter)
16
Lead
a
Pb treated 12
control
a 8
a
4
b
a
b
b
b
a a
a a
a b
Pea P=0.031
F Bean H1 P=0.128
F Bean H2 P<0.001
0 Lettuce P<0.001
Spinach P<0.001
Carrot P<0.001
Onion P<0.001
Vegetable
(d)
Zinc 400
a
Zn (mg/kg dry matter)
Zn treated 300 200 100
control
b a b
a
b
a
b
a b
a b
Pea P<0.001
F Bean H1 P=0.002
a
b
0 Lettuce P<0.001
Spinach P<0.001
Carrot P<0.001
Onion P<0.001
F Bean H2 P<0.001
Vegetable
Fig. 1. Differences in mean vegetable accumulation (edible portion) of (a) cadmium, (b) copper, (c) lead and (d) zinc of plants grown in spiked and control soils (SE). Initial soil concentrations Cd, Cu, Pb and Zn: spiked ¼ 4.2, 87.9, 744.5 and 237; control ¼ 0.3, 16.4, 40.5 and 83.8 mg kg1. Significant differences in mean vegetable uptake when comparing treated to control are indicated by different letters above the column ( p value stated on graph).
(see Table 3g). No trends were evident as the differences between cultivars were negligible. No significant differences ( p ¼ 0.27) in Cu concentrations were found between lettuce cultivars when grown on the spiked soil. However, it was observed that cv. Corsair accumulated the highest Cu concentration and cv. Paris Island the lowest.
The differences in mean metal accumulation between vegetable cultivars grown in spiked and control soils are shown in Fig. 1. Five of the vegetables showed significantly higher concentrations (carrots, spinach, pea and lettuce all at p < 0.001 and French bean at p < 0.05) of Cd, Cu and Zn when grown in the metal-spiked soils compared with those in the untreated control soils. Onion exhibited the pattern of significance ( p < 0.001) but only for Cd, Pb and Zn. The only element where this trend was not followed was Pb in peas and French beans. In peas, three cultivars showed a markedly higher Pb content on the spiked soil but two showed no obvious difference. This resulted in there being no overall significant differences ( p ¼ 0.131) between plants on spiked and control soils. In French bean there was a trend for the plants on the spiked soil to accumulate more Pb than on the control in all cultivars at the first harvest (not significant; p ¼ 0.128), while the difference in concentrations accumulated in the second harvest was significant ( p < 0.001). All the other vegetables showed a significant difference between Pb concentrations in plants on the spiked and control soils ( p < 0.001). Marked differences were exhibited between vegetables with regard to the mean concentrations of metals accumulated in all the cultivars combined on the metal-spiked soil. For Cd the order of accumulation was (crop mean concentrations in parenthesis in mg kg1 dry matter): Lettuce (8.6 Cd) > Spinach (5.8 Cd) > Onion (3.6 Cd) > Carrot (2.0 Cd) > Pea (0.29 Cd) > French bean (0.07 Cd). For Cu, the order of accumulation was: Spinach (9.7 Cu) > Lettuce (9.0 Cu) > Pea (5.8 Cu) > French bean (5.3 Cu) > Carrot (4.8 Cu) > Onion (2.8 Cu). Lettuce accumulated by far the highest concentration of Pb, which was almost twice as high as that in onions, the second largest mean value. The order was: Lettuce (14.6 Pb) > Onion (7.5 Pb) > Carrot (5.8 Pb) > Spinach (1.8 Pb) > Pea (0.78 Pb) > French bean (0.34 Pb). In the case of Zn, spinach accumulated more than twice as much as the next highest crop, lettuce. The order was: Spinach (352.4 Zn) > Lettuce (160.4 Zn) > Onions (64.4 Zn) > Pea (52.2 Zn) > Carrot (43.0 Zn) > French bean (40.7 Zn). Statistical analysis of the differences between vegetables was not valid because they were not all grown at the same time, nor in the same soil. However, direct (non-statistical) comparisons can be made between the data for spinach, carrot and French bean as they were grown simultaneously in the
P.D. Alexander et al. / Environmental Pollution 144 (2006) 736e745
same soil. Similarly, direct comparisons can be drawn between the onion and pea results as they were also grown in the same season and in the same soil. Lettuces, although grown independently at a later date, were grown in soil comparable to that used for the spinach, carrot and French bean. Fig. 1 illustrates the differences in metal accumulation between the vegetables and also shows the relatively small errors associated with the data. When considering the mean accumulation of Cd by all the vegetable species, spinach clearly accumulated greater concentrations than both the carrot and French bean, while the carrot also accumulated significantly more Cd than the French bean. A comparison of onions and peas clearly showed that onions accumulated more Cd. While the lettuce were grown independently, their uptake of Cd was greater than in all the other vegetables. With regard to Pb, carrots accumulated greater concentrations than both the spinach and French bean (the latter accumulating slightly lower Pb of the two). When comparing the onions and peas, the onions accumulated more Pb. Lettuce accumulated much greater concentrations of Pb than all the other vegetables. In the case of Cu, spinach accumulated greater amounts than either the carrot or French bean (which showed similar concentrations). A comparison of onions and peas showed that the pea that accumulated greater Cu (interestingly, with a similar concentration to the French bean and carrot). Lettuce accumulated concentrations similar to those in spinach. Zinc concentrations in carrots, French beans, onions and peas were all similar and relatively low. Spinach showed by far the highest Zn concentrations, which were twice as high as those in lettuce, the second highest accumulator. Interestingly the spinach also accumulated high concentrations in the control soil suggesting that it is a major accumulator of Zn. 4. Discussion From the data presented, certain important conclusions can be drawn with regard to the suitability of vegetable crops and cultivars for growing on soils contaminated with one or more of the elements investigated in this study. In the vegetable species that showed differences between cultivars, the lower accumulators would be most suitable for growing on contaminated soil. However, this would depend on the total concentration of the respective metals being below the CLEA soil guideline values for Cd and Pb. Of the metals investigated, only these two elements are considered to be of a high potential risk to human health. Although not statistically testable, some marked differences between species were apparent for these elements. Cadmium accumulated to the greatest extent in spinach and lettuce, and Pb accumulated in lettuce and onion. Unfortunately, no significant differences in the uptake of these two metals by the cultivars of these four vegetables were identified. However, carrot, which had an intermediate level of Cd accumulation, did show significant differences between cultivars (cv. Nairobi being a low accumulator). Peas are generally low accumulators of Cd but cv.
743
Pilot was shown to accumulate the least Cd of the pea cultivars examined. While Cu and Zn are considered a much lower risk to human health than Cd and Pb, some significant cultivar differences were established. For carrot, spinach and pea, significant differences between cultivars for Cu were identified. Carrots cvs. Nairobi, Nantes and Ingot, spinach cvs. Mediana and Bloomsdale and pea cvs. Meteor and Douce Province were all identified as low accumulators of Cu. In the case of Zn, the only significant differences identified between cultivars were in carrots, where cvs. Nairobi and Ingot were lower accumulators. In general, both spinach and lettuce appear to be relatively high accumulators of Cd and thus it could be argued that these species should not be grown on soils with significantly elevated Cd concentrations. On the basis of the soil guideline values used in England and Wales, this would be at concentrations in soils above 1 mg Cd kg1 at pH 6, 2 mg Cd kg1 at pH 7 and 8 mg Cd kg1 at pH 8 (Environment Agency, 2002a). On the other hand, French bean and pea appear to be relatively low accumulators of Cd and so would be more suitable species to grow. Carrots tend to accumulate less Cd than lettuce, spinach or onions but the significant differences between cultivars indicate that cv. Nairobi is the most suitable for minimising Cd intake via this vegetable on Cd contaminated soils. It also accumulates lower levels of Cu, Pb and Zn on soil contaminated with these metals. Interestingly, the relative order of Cd accumulation by the six crop species investigated follows exactly the classification by Kuboi et al. (1986) mentioned earlier. French bean and pea, which had the lowest Cd contents are members of the Leguminosae, which were classed by Kuboi et al. (1986) as ‘‘low accumulators’’. Carrot and onion, had intermediate concentrations of Cd and are members of the Umbelliferae and Liliaceae families, respectively, which were classed as ‘‘moderate accumulators’’. The highest Cd concentrations found in this study were in lettuce (Compositae) and spinach (Chenopodiaceae), which were classed these as ‘‘high accumulators’’. Many garden soils have been contaminated with Pb, especially in older urban areas. Atmospheric deposition of Pb onto plants has often been considered to be of greater importance than root uptake and translocation to edible portions of plants. However, Pb has an index of accumulation of 101e1.0, which is in the same range as Cu and Zn (Kabata-Pendias and Pendias, 1992). Therefore genotypic differences in accumulation between cultivars are still important. In this study, all crops were grown at the same location so it is expected that the deposition would have been relatively similar but the edible portions analysed were thoroughly scrubbed to remove soil and any other deposits. In view of the relatively high accumulation of Pb shown by lettuce, this species would probably not be suitable where Pb concentrations exceed the 450 mg kg1 soil guideline value in the Contaminated Land Exposure Assessment (CLEA) recommendations (Environment Agency, 2002b). Onion also exhibited relatively high accumulation concentrations and should probably be avoided on Pb-contaminated soils. In general it
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P.D. Alexander et al. / Environmental Pollution 144 (2006) 736e745
would appear that spinach, French bean and peas would probably be suitable for growing on account of their relatively low accumulation of Pb. A comparison of the mean concentrations of Cd and Pb in the vegetables with the European Union maximum permitted concentrations in foodstuffs (EC, 2001) shows some interesting results. For Cd there are three limits for different types of vegetables: ‘‘leafy’’ vegetables 0.2 mg Cd kg1 wet weight, ‘‘root’’ vegetables 0.1 mg Cd kg1 wet weight and ‘‘all other’’ vegetables 0.05 mg Cd kg1 wet weight. In order to convert to wet weight, the median value of the moisture content ranges of common vegetables presented by Duckworth (1966) were used. On the spiked soils, lettuce and spinach appear to exceed the ‘‘leafy vegetable’’ limit and the carrot and onion exceed the ‘‘root vegetable’’ limit, pea exceed the ‘‘all other vegetables’’ while French beans appear to fall within the Cd limit. However, on the unspiked soil none of the vegetables exceeded the limit. In the case of the Pb spiked soil, the lettuce appeared to exceed the limits of 0.3 mg Pb kg1 for ‘‘leafy’’ vegetables while the onion, carrot and pea all appeared to exceed the limits of 0.1 mg Pb kg1 for ‘‘all other’’ vegetables, only spinach and French bean appear to fall within the limits. When examining the vegetables grown on the unspiked soil, all fall within the statutory limits for Pb with the exception of the onion. It must be recognised that the spiking and the use of pots, instead of field soil, probably gave rise to enhanced uptake. However, the relatively low values in this European directive help to emphasise the importance of selecting cultivars with the lowest uptake of Cd and Pb. Elevated Cu concentrations in soils are more of a phytotoxicity and ecotoxicological hazard than a risk to human health (ICRCL, 1987). In general terms, spinach and lettuce were found to be the highest accumulators of Cu and onion the lowest in this study. Zinc, like Cu, constitutes a greater risk of phytotoxicity than a hazard to human health but spinach accumulated far higher concentrations of Zn than any of the other vegetable in this research. Considering that approximately 20.5% of the world’s population is estimated to be at risk of inadequate Zn intake (Wuehler et al., 2005) it may be worth considering accumulator species, such as spinach, as being a convenient way of increasing the dietary intake of Zn. However, the concentrations of potentially harmful elements, such as Cd, Pb and Hg in spinach would also need to be considered. These results indicate that it would be worthwhile extending this type of screening study to other elements and a wider range of cultivars of other vegetables in order to be able to advise gardeners on how best to minimise their intake of potentially harmful elements through the consumption of home-grown vegetables. In addition, further research comparing the accumulation of metals by different vegetables grown at the same time would be of great interest. Acknowledgements Thanks are due to Lynne Gregory, Emily Reid and Ian Waghorn of RHS Wisley for undertaking much of the field
work associated with the project, and also to Chris Prior of RHS Wisley for advice at many stages of the project, Geoffrey Warren of The University of Reading Department of Soil Science for analysis of soils and plants, and finally Eleanor Allan of Statistical Services Centre (The University of Reading) for statistical advice.
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