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Lead in soils and tolerant herbs on roadsides in search of the metal accumulation patterns in diverse species.

Introduction

Pb is one of the toxic heavy metal pollutants, common to all urban systems, challenging the sustainability of humans and many other species. Pb harmfully affects neurovascular systems of humans and is also toxic to all other life forms including marine life, and has negative influence on crop yields (Finster et al., 2004; Li et al., 2005; Mashi et al., 2005). Major reports of Pb contamination in natural environment appear in the 1960s (Cannon and Bowles, 1962; Warren and Delavault, 1962). Over 300 million metric tons of Pb produced to date remains in the environment (Lewis, 1985). Although Pb addition to paint or gasoline is currently not common, the natural content of Pb as a pollutant in poor quality fossil fuels continuously pollute roadside soils and industrial environments all over the world. Apart from contaminated gasoline, wheel balance weight also currently contributes to traffic related Pb pollution (Root, 2000; Suzuki et al., 2009). The quantity of Pb in natural environment depends on traffic volume and the distance from roads (Ward et al., 1977; Fergusson et al., 1980; Hol et al., 1997; Lu, 2003; Bretzel and Calderisi, 2006). Pb from pavement wear or road maintenance significantly exceeds that from automobile exhausts in urban environments (Haal and Harri, 2008). There are many reports of Pb contamination in top soil, air, vegetation and food crops in several urban areas, which also highlights the increase in the levels of Pb in the body of inhabitants along roads of high traffic in different parts of the world (Daines et al., 1970; Page et al., 1971; Davies and Holmes, 1972; Haney et al., 1974; Flanagan et al., 1980; Agrawal et al., 1981; Collins, 1984; Fakayode and Onianwa, 2002; Fakayode and Olu-Owolabi, 2003).

Roadside emissions are considered as chronic line sources of Pb contamination in urban environments (Ona et al., 2006). In general, Pb is believed to be a heavy metal of low solubility (Harrison et al., 1981) and usually finds concentrated on the uppermost part of the soil profile within the first 5 to 10 cm (Gratani et al., 1992). The quantity of Pb in soils and plants near roads is substantially higher than that at 50 meter distance from roads (Okonkwo and Maribe, 2004) and the concentration of Pb in roadside plants decreases with increasing distance from the road edge (Nabulo et al., 2006). Moreover, the uptake of Pb by plants growing near highways is especially high (Madany et al., 1990). Studies carried out on higher plants in polluted areas have revealed that they can be used as monitors of the degree of contamination from many different toxic heavy metal elements (Brown et al., 1995). Plants known to have the ability to phytoextract Pb efficiently from environment are rare (Zhuang et al., 2007). Pb contaminated roadsides are one of the best places to search them out, because greater presence indicates that the hyper tolerant plants on contaminated soils have the genetic potential to hyper accumulate toxic metals (Shu et al., 2002). Geobotanical surveys and plant screenings are necessary for the identification of newer hypermetal-accumulator species (Diez et al., 2006).

In populous developing countries, gasoline with Pb additive concentrations up to 0.45 g [l.sup.-1] is still being used (Sutherland et al., 2003). The people in at least 100 countries including India are still exposed to air polluted with Pb (Singh et al., 2005). Automobile exhausts and other roadside emissions contribute most of environment Pb in the Indian sub continent (Singh et al., 1995; Joshi and Shrivastava, 2003; Akbar, 2006). Moreover, Indian roads are congested with high proportion of old vehicles (Samanta et al., 1995) using leaded gasoline (Kamavisdar et al., 2005). Study of Pb contamination on roadsides and its accumulation in roadside plants is highly relevant in India because of high urban development associated with an exponential rise in the number of vehicles on the roads having no effective pollution control standards. Learning of the degree of such contaminations in the biodiversity-rich tropics has high relevance. In general, assessments of the impact of heavy metal pollution, especially of Pb in roadside vegetations, and the amount of accumulation of the metal in diverse species of roadside plants in various parts of the world are significant to the understanding of Pb tolerance or accumulation or hyper accumulation tendencies of many plants. Knowledge of such plants contributes significantly to experiments for the development of better cost effective technologies of phytoremediations for the management of Pb contaminations. Investigations of its absorption into pollution resistant roadside herbaceous flora would be highly important in the understanding of its flow into food chains as well.

Among the Indian States, Kerala represents one of the most urbanized regions of the country with very high increase in traffic volume on roads. Moreover, the state is situated on the Southern Western Ghats, well known as one of the biodiversity hotspots of the world. Floristic survey of busy roadsides of Kerala has showed 85 species, some of which are identified as hyper resilient species (the highly tolerant with high relative abundance), ecologically significant in many ways (Ray and Jojo, 2009). However, practically no information is available on the degree of Pb contamination of roadside soils in this region. Therefore, investigation of Pb contamination in Kerala roadsides in relation to the metal accumulation in most tolerant herbaceous flora to search out Pb accumulation patterns in different roadside species is highly relevant. Pb in roadside surface soils (0-5 cm) close to the tar-edge of two most busy roadsides of Kerala over 110 kilometers, along with Pb accumulation in the 19 most common species occupying these roadside areas in two different seasons were examined for two years and the metal accumulation patterns are explained.

Materials and Methods

Sampling Area and Collection Procedure

The roadsides investigated are about 55 km each of two roads - the 'Main Central Road' (MC road) and the 'Kottayam-Kumily Road' (KK road) of average traffic densities of 15300 vehicles per day - in Kottayam District (total area 2208 [km.sup.2]; total population 1952901; population density of 884 per [km.sup.2]; latitude 9 [degrees] 15 [sup.l] to 10 [degrees] 21 [sup.l] and longitude 76 [degrees] 22 [sup.l] to 77 [degrees] 25 [sup.l]) of Kerala State, South India (Government of India, 2007; Kerala Government report, 2008). The region has a tropical wet climate with total annual average rainfall of about 3130.33 mm received mainly in two monsoons--southwest monsoon (May to August) and northeast monsoon (September to December), separated by a break of summer (January to April). During the summer also, there are random summer showers in the zone.

Specific urban and rural sampling sites of 1 m distance from tar-edge and 1 km length of both the sides were identified on both the roads. Urban sites were those with high degree of traffic densities and other anthropogenic disturbances (trampling by people and crushing by vehicles) whereas rural sites were those with comparatively lower degree of disturbances of both the types. Samples representing monsoons (south-west monsoon and northeast monsoons; May to December) and summer (January to April) seasons were collected from these sites using quadrat methods (Trivedy and Goel, 1986). Quadrats of 40 cm X 40 cm size, of approximately 0.1 m (Uzbeck, 1981) were used; at each site, 100 to 110 quadrats from both sides of the roads at random (of both the seasons) were observed. Altogether there are eight urban sites (4 from KK road and 4 from MC road) and five different rural sites (3 from KK road and 2 from MC road) (Fig. 1). Total 1350 quadrats were taken from all the sites in two seasons.

[FIGURE 1 OMITTED]

Plants and soil samples of roadsides were collected separately (at random) in pre-washed polythene bags. Phytosociological measures were done in the field and recorded in the field book. Plant species were dug out carefully using a polythene shovel. From all the different sites specimens of each plant species available was collected from different quadrats at random during different months of the monsoon and the summer seasons. Different specimens of each species from an area were kept in a common bag; different species were kept in separate bags. During each sampling time, surface soil samples (about 500 g of top soil 0-5cm) of each area were taken from quadrats at random and the samples from different quadrat of a site were put in separate labeled bags.

Measurement of Pb in Soil Samples

Altogether there were 30-40 packets of monthly soil samples representing both the seasons from a specific roadside site, kept undisturbed for air drying in dust-free racks in the laboratory. Prior to chemical analysis soil samples representing a season (15-20) of each site was divided into three equal groups and each group was mixed thoroughly. The three different composite soil samples from each site were treated as three distinct samples of each season. 500 g of each air dried composite sample was ground separately to pass through a 2 mm sieve. About 5g of the homogenized sample from each group was ground into fine powder using agate mortar and pestle and further dried in hot air oven at 70[degrees]C for 72 hrs to constant weights. Exactly 1 g from each of these finely ground soil samples were weighed out using an electronic balance into properly cleaned 250 ml glass beakers.

Digestion was performed by adding 12 ml of aquaregia (3:1, v/v, concentrated HCl to concentrated HNO3) into the beaker covered with watch glasses on a hotplate for 3 h at 110[degrees]C. After evaporation to near dryness carefully, the sample was diluted with 20 ml of 2% (v/v with water) nitric acid and transferred into a 100 ml volumetric flask after filtering through Whatman no. 42 filter paper and diluted to 100ml with double distilled water (Chen and Ma, 2001; Hseu et al., 2002) and used for chemical analysis. Average of the three samples of each season was taken as the average amount of Pb of the site in that specific season. Thus the average Pb content of all the thirteen sites for both the seasons was found out.

Measurement of Pb in Plant Samples

Plant specimens were uprooted along with the entire roots from the field using a polythene shovel. From each site, mature plants (plant in the flowering stage) from different quadrats (where the species were available) of a site were brought to the laboratory, each species in separately labeled packet. In the laboratory, after each collection, all specimens of each species from different quadrats of a site were washed together thoroughly many times with tap water followed by distilled water to remove completely the adhered dust and dirt, and air-dried. After the air-drying all specimens of each species from a site were separated into roots and shoots and kept in separate paper bags labeled as root or shoot of a species with a site code; specimens were then dried in hot air oven at 70[degrees]C for 72 hrs to constant weights and kept undisturbed for chemical analyses.

For each species, for each rural or urban site, there were many mature plants (plant in the flowering stage) collected at random from different quadrats of both the seasons. However, all species were not available at all sites. The number of sites for a species varied from 7 to 13. Altogether 30-40 specimens were collected for each available species from a site belonging to the two seasons. At the time of chemical analysis (carried out after field studies) the roots and shoots of all plant specimens of each species from a site were powdered separately using agate mortar and pestle and thoroughly mixed. About 25 g each of the root and shoot sample of a species from a site was then further ground to fine powder. Shoot and root sample preparations for chemical analysis of all species were carried out similarly. Shoot samples of all the species were analyzed, whereas the analysis of root samples was carried out for only four species of grasses, which had significantly high root mass.

The digestion procedure was carried out on a hot plate as per standard methods (Zarcinas, 1987; Ho and Tai, 1988; APHA, 1998; Fakayode and Onianwa, 2002). 500 mg each of powdered sample of root and shoot of different species from each site were placed separately into 250 ml beakers (in triplicate). 10 ml of concentrated nitric acid was added and the mixture was heated for 45 min at 90[degrees]C, swirled if frothing occurred and occasionally washed down the sides of the beaker with double distilled water. Then the temperature was increased to 120[degrees]C and the digestion was continued at this temperature until about 1 ml of acid remained. The heating was continued and concentrated HN[O.sub.3] was added in necessary volume until digestion became complete as was shown by a light colored, clear solution. The samples were not permitted to dry during digestion. After the digestion, the extract was cooled and diluted to 20 ml with 1% v/v nitric acid. The extract was filtered through Whatman no. 42 filter paper into a 100 ml volumetric flask and made up to 100 ml by adding double distilled water, and used for chemical analysis. Reagent blanks for both the plant and soil analysis were also prepared in all cases for calibrations. Average of Pb in a species from all the sites was taken as the average Pb of the species on roadsides.

All the chemicals used were analytical grade compounds of Merck Company. Reagent bottles, beakers, and volumetric flasks were cleaned by soaking overnight in 2 N hydrochloric acid, rinsed with water and oven dried at 60[degrees]C. Chemical analyses for Pb of both soil and plant samples were carried out in a Flame Atomic Absorption Spectrophotometer (Perkin Elmer model 3110) at the Chemical Oceanography Lab of the Department of Marine Science, Cochin University of Science and Technology, Kochi, Kerala, India. Concentrations of the metal in both the soil and plant samples were computed as mg metal per kg dry sample (mg [kg.sup.-1]). In order to compare the Pb content of soils with that of plant samples, the average of the metal in sites from where plant specimens of each species were collected was used. Statistical analyses such as ANOVA and correlations were carried out using SPSS package and MATLAB.

Results

Average amount of Pb in soils at all the different sites (urban and rural) during the two different seasons are given in tables 1 and 2 respectively. Statistical analyses revealed that there is no significant difference in the amount of Pb in different urban sites (P = 0.3654) or the different rural sites (P = 0.3297) during both the seasons. Comparison of Pb in different urban and rural sites also showed that there was no significant difference in the amount of Pb in the soils of the urban and rural sites, neither during the Monsoon (P = 0.0970) nor during the Summer (P = 0.1822). Positive correlations were found for Pb in soils over the two seasons at both the urban and rural zones (Fig. 2 & 3); at urban site it was weak (r = 0.071) whereas at the rural site it was strong (r = 0.991).

Relative abundance of the different species of plants on the whole length of roadsides was studied and the mean Pb content in shoots of 19 of them and roots of four of them, in relation to the mean Pb in soils from where they were collected, were calculated (Table 3). Statistical analysis showed that the mean of Pb in soils corresponding to soils from where each species was collected (including urban and rural sites) was significantly different (P = 0.000) and the mean of Pb of both the shoots and roots of different plant species was also significantly different (P = 0.000) for both shoot and roots; the relative abundance of the different species studied were also significantly different. Comparisons between Pb in the shoots and that in different soil samples from where each species was collected (Figures 4 &5) showed that positive correlations between the amount of Pb in soils and plant tissues exist for all species and in certain species the strength of correlations is very strong.

Discussion

Comparison of seasonal differences of Pb in roadside soils over the urban and rural areas of two most busy roadsides of a densely populated region of South India has shown that the average Pb in these soils varied from 17.9 to 74.80 mg [kg.sup.-1]. Pb content in normal soil may be up to 15 mg [kg.sup.-1] .Lau and Wong (1982). But according to Lewis (1985), soils with lead levels above the range of 7 to 20 mg kg-1 are considered contaminated. Anyway, compared to the reports of Holmgren et al. (1993) of Pb contamination in agricultural soils or that of Kabata-Pendias and Pendias (2001) in certain undisturbed soils, the degree of Pb contamination on these roadside soils may be considered moderate. Seasonal variations in the amount of Pb in different sites were not significant, which agrees with the observations of Harrison et al. (1981) and Gratani et al. (1992) that Pb is highly insoluble in water and immobile so that monsoon washout cannot affect the Pb content on roadsides significantly; but does not agree with the observations of Li et al. (2005) that Pb is comparatively more mobile than other heavy metals. These contrasting observations on the mobility of Pb might be due to differences in chemical forms of the element in different soils. However, the search of Pb accumulation pattern of different species on these slightly contaminated roadsides was quite rational because the knowledge of even the less contaminated soils enable finding out plant species for further experiments to find out toxic metal excluders/accumulators/hyper-accumulators, which has many ecological applications.

One of the important applications of metal accumulation pattern in plants is in phytoremediation. Phytoremediation is a newly emerging, promising, environment cleaning technology for removing pollutants such as toxic heavy metals which demand better and newer metal hyper-accumulator plant species. Although about 775 species of plants with phytoremediation potentials for 19 key metallic elements is known, selecting an appropriate plant species with hyper accumulation minimum of 100 [micro]g g dry weight for removing Pb from contaminated environment is a real challenge (McIntyre, 2003). Moreover, research is currently underway to determine whether there are some plant species that can accumulate greater quantities of Pb; otherwise costly physical removal is the only available technology for the time being, because lead is quite immobilized in soils (Rosen 2002). Soils close to busy streets are the most Pb polluted zones in urban areas (Singer and Hanson, 1969; Rolfe et al., 1977) and, therefore, estimation of Pb in the most tolerant species close to the taredge has applications. Metal accumulation patterns of the species reported in these investigations provide the possibility for further experimentations with them to explore their hyper accumulation potentials.

Relative Abundance (RA) of a species in a natural community always explains its relative importance among the general group, and the value also suggests the degree of competitive environmental relationships of a species among the other species in a general community. Moreover, the value of RA depends on the degree of biodiversity as well. In the highly disturbed roadsides, especially in the 1m margins close to the tar-edges where the disturbance is very high and vegetation is quite sparse, species richness is expected to be quite low. Hernandez and Pastor (2008) reported negative influence of metal contamination on species richness. The three grass species, Axonopus compressus (Sw.) P.Beauv., Eleusine indica (L.) Gaertn., and Cynodon dactylon (L.) Pers., together formed RA of 43.16 in the 1m margin close to the taredge of these roadsides. It may also be noted that among all the species observed on these roadsides, Axonopus compressus (Sw.) P.Beauv. alone showed RA of 21. 19. Therefore, Axonopus compressus (Sw.) P.Beauv. was observed to be the most resilient or hyper tolerant plant among all species growing close to tar-edges on tropical roadsides in this region. RA of all the other plants recorded was comparatively very low, often near to 1 or below. Pb content in shoots of almost all species with RA 1 or above was calculated in the present investigation. For those with RA less than 1, the criterion for analysis of Pb content was their frequency on the roadsides. Altogether the Pb content of the shoots of 19 plants and that of the roots of four grasses with the highest relative abundance were found out and the data compared. On the basis of the RA as the degree of resilience, metal accumulation potentials of different species may be further experimented.

The amounts of Pb in the shoots of all the 19 species and the soils from where they were collected were found to be significantly different, varying from 44. 08 mg kg-1 (where grew the plant Chloris barbata Sw.) to 66.47 mg [kg.sup.-1] (where grew Leucas aspera (Willd.) Spreng). All species showed a positive correlation between Pb in the shoots and the average of that in the soils from where they were collected. However, the increase or decrease in average Pb content of the different species was in no way corresponding to the increase or decrease of Pb content in soils from where the species were collected. On the contrary, Leucas aspera (Willd.) Spreng., the plant with one of the very low amounts of Pb in shoots, was the plant growing in soils with the highest average Pb content. Diverse strengths of correlation of the mean Pb in shoots / roots of plants and that of soils (Fig. 4 & 5) revealed the species specific accumulation of Pb very clearly. These were observations similar to that of Baes and Ragsdale (1981), Cheng (2003) and Sun et al. (2007), who reported species specificity earlier for accumulation of Pb or other metals in plants. Another observation was that among the 19 roadside species studied, 12 were well known medicinal plants, which point to the health hazards of collecting medicinal plants from roadsides.

Comparison of Pb in shoots of the 19 different plant species varied from 20.49 mg kg-1 (Cleome rutidosperma) to 61.00 mg kg-1 (Pilea microphylla). Among the 19 species studied, the species, Pilea microphylla, showed Pb content in shoots above that in the soils where they grew. Therefore, this species as per the criteria of IPCS, (1989) is a hyper Pb accumulator and may be tried as a species with potential for Pb removal from contaminated soils. Experiment with this plant is continuing further to establish the fact. Apart from species specific characteristics, hyper accumulation is controlled by a number of different environmental factors such as soil type, pH, temperature, soil organic composition and many others (Anton and Mathe-Gaspar, 2005). Moreover, roadsides being multiple contaminated sites, accumulation of one metal may be inhibiting the accumulation of others. Therefore, the other species also cannot be ignored as species with no potential uses in phytoremediation. However, considering the tolerant growth in highly disturbed and contaminated soils as one of the criteria for deciding the phytoremediation potential (Shu, et al. 2002), these species also may be subjected to further experimental investigations. This is important, especially because the hyper accumulation potentials of some of these species such as Eleusine indica and Cynodon dactylon for other metals are already reported (Wong and Lau, 1985; Anoliefo et al., 2006), whereas the others were quite new to be identified.

In general, certain grasses have higher root mass than shoots, especially in disturbed environments (Ray et al. 1993). Certain grasses have already been tried to assess their Pb accumulation potentials (Antiochia et al. 2007; Paz-Alberto et al. 2007). Since the four grasses, Eleusine indica, Cynodon dactylon, Axonopus compressus, and Cyperus compressus were found to be the most resilient or hyper tolerant species on these disturbed and contaminated environments, Pb accumulation in the roots of these grasses was also estimated. It was interesting to note that in all of these grasses, Pb accumulation was found higher in their roots than in their shoots, varying from 38 to 89 mg kg-1 (Fig 6). In all of them, Pb in roots was found much higher than that in the shoots. In the grass Cynodon dactylon, Pb per unit dry weight in the root was more than that in the soil suggesting it as a plant with hyper accumulation potential as per the criteria of IPCS (1989). The overall impression was that since all of these grasses have very high tolerance towards roadside disturbance and contamination, they may be utilized as phytostabilizers (Mendez and Maier, 2008) in preventing leaching out of lead from contaminated sites including roadsides, and this needs further experimental verifications, which are continuing. Although Pb accumulation in plants is often higher in roots than shoots (Dahmani-Muller et al., 2000; Mbila and Thompson, 2004), root accumulation of the other monocot and dicot species was ignored in this investigation. This was because of the fact that even if the metal accumulation is higher in the roots of such plants, the practical utility of them will be negligible due to negligibly low amount of roots in them.

Comparison of the relative abundance of Pb in the shoots of all plants and roots of grasses revealed no significant correlations between both the values. In general, the species with the highest relative abundance were not the species with the highest amount of Pb in their shoots. Increase in the relative abundance of species in a disturbed environment is a clear sign of its higher degree of tolerance (hyper tolerance). The most hyper tolerant species found on these roadsides were not the plants with the highest amount of metals in their shoots. Thus, it appeared that all the hyper tolerant species are not metal accumulators or hyper accumulators; they include metal evaders as well. However, this observation requires further experimentations in this regard.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Conclusions

Pb is a toxic metal spreading mainly through roadsides globally. Phytoremediation is an emerging technology of cleaning of metal contaminated soils requiring more of hyper accumulator species, especially for specific metals. Hyper accumulator species for Pb are quite rare and are in high demand. Search of patterns of Pb accumulation in 19 very tolerant species growing close to 1 m margins of tar-edges revealed accumulation of Pb in significantly different quantities in their shoots and grasses in their roots as well. Positive correlations were found between Pb in plants and that in the soils where they grew. In some of the species, the correlations were very strong. The two species, Pilea microphylla (L.) Liebm. and Cynodon dactylon (L.) Pers. were found accumulating Pb in amounts higher than that in soils; the former in their shoots and the latter in their roots. These species might have the potentials in phytoextraction of Pb from contaminated sites and hence are subjected to further experiments. All the 19 species can also be experimentally assessed for their Pb hyper accumulation potentials because of their stable growth in the highly disturbed and contaminated sites. Phytosabilization potentials of grasses with high amount of roots and high affinity towards Pb may be experimented further. The overall finding was that heavy metal accumulation patterns in plants of even mildly contaminated places such as tropical roadsides provide information of new species to be subjected to further experimentation for the generation of better plant resources for phytoremediations.

Acknowledgements

Support extended to the second author for carrying out this research work under the Faculty Improvement Programme (FIP) of University Grants Commission (UGC), New Delhi, is gratefully acknowledged.

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J. G. Ray (1) and Jojo George (2)

(1) Environment Science Research Laboratory, Postgraduate Department of Botany, St. Berchmans' College, Changanacherry, Kerala, India - 686101.

(1) Corresponding author: E-mail: methikalamray@yahoo.com;

(2) PresentAddress: Department of Botany, St. Dominic's College, Kanjirapally, Kerala, India--686512. E-mail: jojoaerathu@yahoo.com
Table 1: Pb in soils at urban sites of two roadsides in two seasons.

 Soil Lead (mg [kg.sup.-1])
Sl Sites
No Road 1 Monsoon Summer
 MC Road

1 Changanacherry 74.80 59.80

2 Chingavanam 27.90 33.4

3 Kottayam 64.80 39.90

4 Ettumanoor 39.80 33.80

 Road 2
 KK Road

5 Mundakayam 34.80 62.80

6 Kanjirapally 54.60 30.90

7 Ponkunnam 40.70 67.60

8 Pampady 49.70 32.3

Mean Value with SD 48.387 [+ or -] 15.786 45.062 [+ or -] 15.55

Table 2: Pb in soils at rural sites of two roadsides in two seasons.

 Soil Lead (mg [kg.sup.]-1)
Sl Sites
No Road 1 Monsoon Summer
 MC Road

1 Thuruthy 38.70 42.10

2 Pattithanam 34.70 39.80

 Road 2
 KK Road

5 Chotty 17.90 26.90

6 Kodungoor 35.80 41.00

7 Kothala 20.90 26.70

Mean Value with SD 29.60 [+ or -] 9.484 33.86 [+ or -] 7.811

Table 3: Pb in plant samples in relation to corresponding soil
samples.

Sl Name of species Pb in soil and plant
No samples (mg [kg.sup.-1])

 Mean of Pb in
 soils from where a
 species is
 collected.

 Mean SD

1 Eleusine indica (L.) 48.71 [+ or -] 15.29
 Gaertn.

2 Cynodon dactylon 51.05 [+ or -] 15.60
 (L.) Pers.

3 Axonopus 46.76 [+ or -] 12.75
 compressus (Sw.)
 P.Beauv.

4 Cyperus compressus 57.58 [+ or -] 18.85
 L.
5 Chloris barbata Sw. 44.08 [+ or -] 14.51

6 Kyllinga nemoralis 47.75 [+ or -] 14.96
 (J.R & G. Fors.)
 Dandyex Hutch. &
 Dalz.

7 Hedyotis corymbosa 49.87 [+ or -] 15.96
 (L) Lam.

8 Scoparia dulcis L. 49.70 [+ or -] 16.89

9 Cleome 49.66 [+ or -] 17.81
 rutidosperma DC.

10 Vernonia cinerea 52.36 [+ or -] 18.53
 (L.)

11 Phyllanthus amarus 54.67 [+ or -] 15.32
 Schum. & Thonn

12 Pilea microphylla 47.77 [+ or -] 12.92
 (L.) Liebm.

13 Portulaca oleracea 50.29 [+ or -] 15.73
 L.

14 Amaranthus viridis 50.68 [+ or -] 18.11
 L.

15 Euphorbia hirta L. 51.30 [+ or -] 14.98

16 Eclipta prostrata 51.83 [+ or -] 18.30
 (L.) L.

17 Leucas aspera 66.47 [+ or -] 7.64
 (Willd.) Spreng.

18 Peperomia pellucida 53.77 [+ or -] 19.95
 (L.) Kunth

19 Aerva lanata (L.) 66.47 [+ or -] 07.64
 Juss. ex Schult.

 Pb in soil and plant
 samples (mg [kg.sup.-1])
Sl Name of species
No Pb in plant shoot

 Mean SD

1 Eleusine indica (L.) 28.66 [+ or -] 12.29
 Gaertn.

2 Cynodon dactylon 34.62 [+ or -] 15.79
 (L.) Pers.

3 Axonopus 28.82 [+ or -] 14.67
 compressus (Sw.)
 P.Beauv.

4 Cyperus compressus 30.55 [+ or -] 9.24
 L.

5 Chloris barbata Sw. 23.88 [+ or -] 6.17

6 Kyllinga nemoralis 31.00 [+ or -] 4.98
 (J.R & G. Fors.)
 Dandyex Hutch. &
 Dalz.

7 Hedyotis corymbosa 40.95 [+ or -] 9.42
 (L) Lam.

8 Scoparia dulcis L. 23.09 [+ or -] 11.16

9 Cleome 20.49 [+ or -] 8.18
 rutidosperma DC.

10 Vernonia cinerea 26.00 [+ or -] 12.13
 (L.)

11 Phyllanthus amarus 26.26 [+ or -] 10.16
 Schum. & Thonn

12 Pilea microphylla 61.00 [+ or -] 25.89
 (L.) Liebm.

13 Portulaca oleracea 37.25 [+ or -] 11.59
 L.

14 Amaranthus viridis 24.75 [+ or -] 12.02
 L.

15 Euphorbia hirta L. 34.60 [+ or -] 11.37

16 Eclipta prostrata 20.80 [+ or -] 12.00
 (L.) L.

17 Leucas aspera 22.00 [+ or -] 2.53
 (Willd.) Spreng.

18 Peperomia pellucida 43.33 [+ or -] 29.14
 (L.) Kunth

19 Aerva lanata (L.) 50.60 [+ or -] 20.03
 Juss. ex Schult.

 Pb in soil and plant
 samples (mg [kg.sup.-1])
Sl Name of species
No Pb in plant root

 Mean SD

1 Eleusine indica (L.) 48.50 [+ or -] 18.65
 Gaertn.

2 Cynodon dactylon 89.00 [+ or -] 24.04
 (L.) Pers.

3 Axonopus 46.00 [+ or -] 22.54
 compressus (Sw.)
 P.Beauv.

4 Cyperus compressus 38.00 [+ or -] 28.21
 L.

5 Chloris barbata Sw. - -

6 Kyllinga nemoralis - -
 (J.R & G. Fors.)
 Dandyex Hutch. &
 Dalz.

7 Hedyotis corymbosa - -
 (L) Lam.

8 Scoparia dulcis L. - -

9 Cleome - -
 rutidosperma DC.

10 Vernonia cinerea - -
 (L.)

11 Phyllanthus amarus - -
 Schum. & Thonn

12 Pilea microphylla - -
 (L.) Liebm.

13 Portulaca oleracea - -
 L.

14 Amaranthus viridis - -
 L.

15 Euphorbia hirta L. - -

16 Eclipta prostrata - -
 (L.) L.

17 Leucas aspera - -
 (Willd.) Spreng.

18 Peperomia pellucida - -
 (L.) Kunth

19 Aerva lanata (L.) - -
 Juss. ex Schult.

 Relative
Sl Name of species abundance of
No plants

 Mean SD

1 Eleusine indica (L.) 13.59 [+ or -] 04.63
 Gaertn.

2 Cynodon dactylon 08.38 [+ or -] 08.48
 (L.) Pers.

3 Axonopus 21.19 [+ or -] 19.93
 compressus (Sw.)
 P.Beauv.

4 Cyperus compressus 01.57 [+ or -] 01.53
 L.

5 Chloris barbata Sw. 02.62 [+ or -] 08.83

6 Kyllinga nemoralis 02.08 [+ or -] 01.84
 (J.R & G. Fors.)
 Dandyex Hutch. &
 Dalz.

7 Hedyotis corymbosa 03.41 [+ or -] 02.74
 (L) Lam.

8 Scoparia dulcis L. 02.06 [+ or -] 01.08

9 Cleome 02.37 [+ or -] 01.63
 rutidosperma DC.

10 Vernonia cinerea 02.05 [+ or -] 01.29
 (L.)

11 Phyllanthus amarus 01.26 [+ or -] 01.16
 Schum. & Thonn

12 Pilea microphylla 03.77 [+ or -] 03.43
 (L.) Liebm.

13 Portulaca oleracea 03.11 [+ or -] 03.10
 L.

14 Amaranthus viridis 01.99 [+ or -] 02.07
 L.

15 Euphorbia hirta L. 00.95 [+ or -] 00.85

16 Eclipta prostrata 00.64 [+ or -] 00.64
 (L.) L.

17 Leucas aspera 00.18 [+ or -] 00.38
 (Willd.) Spreng.

18 Peperomia pellucida 00.07 [+ or -] 00.20
 (L.) Kunth

19 Aerva lanata (L.) 00.56 [+ or -] 00.57
 Juss. ex Schult.
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Date:May 1, 2010
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