Environmental pollution with toxic metals has increased dramatically due to
rapid industrialization and surge in population (Al-Kateeb
and Leilah, 2005). The main source of pollution are fossil fuels, fumes
from motor vehicles, industrial production processes, the use of industrial
products and fires, in some areas, natural sources by air blown dusts, vegetation
and sea salt sprays, wild fires, volcanoes, landfill leachates, fertilizers,
sewage, discharges from the power and treatment plants (Bargagli,
1998; Yoon et al., 2006; Yusuf
and Oluwole, 2009). Trace metals deposition in plants from anthropogenic
sources has increased the attention on inorganic pollution and established plants
as passive bio-monitors (Wittig, 1993; Monaci
et al., 2000). A variety of plants species are used as biological
monitors since, they have a tendency to assimilate metals from the surrounding
environment (Djingova et al., 1993; Bargagli,
1998; Kadukova et al., 2008). With the increase
in global metals contamination, plants process metals, might provide efficient
and ecologically sound approaches to sequestration and removal through leaves,
stem and roots. Their concentrations can be correlated with their surrounding
soil (Keane et al., 2001). In the desert, many
plants were found to be biological indicators with broad specificity to inorganic
pollutants (Simon et al., 1996). Metals sequestered
by these plants may be transferred through trophic links from detritus communities
to secondary and tertiary consumers and thus lead to the total contamination
of the arid ecosystem (Lyngby and Brix, 1989;
Sinegani and Ebrahimi, 2007). Metal toxicity was found to have significant
relationship with factors controlling metal tolerance, including available uptake
sites, chemical interaction and ionic speciation. Many plants that accumulate
>1000 or >10000 mg kg-1 of trace metals are categorized as
metals hyper accumulators (Baker and Brooks, 1989).
Ma et al. (2001), Roselli
et al. (2003), Yoon et al. (2006)
and Sinegani and Ebrahimi (2007) observed significant
trace metals mobilization between the plant parts above and below the surface
of the soil with translocation factor (TF)>1. Most plants translocate inorganic
and nutrient constituents from roots to leaves (Roselli
et al., 2003).
Based on the above studies, the objective of the present study was to determine (1) the trace metal concentrations in the leaves, stem and roots of six desert plants in relation to its surrounding soil, (2) Bio-accumulation factor (BAF) and Translocation Factor (TF) and its interrelationship in six plants and (3) in six Kuwaiti Governorate areas.
MATERIALS AND METHODS
Six Kuwait Governorate areas were chosen for our study are namely, (a) G-I
(Jahra): Situated at the North of Kuwait that comprises of residential, industrial,
vegetation and desert areas and significant with thermal, power, desalination
and water treatment plants, (b) G-II (Capital/Kuwait City): The Central Kuwait
zone with industrial and residential areas significant for its business centers,
domestic wastewater outfalls, (c) G-III (Hawalli): Known for its business activities
and residential areas (d) G-IV (Farwaniya): With densely populated residential
areas (e) G-V (Mubarek Al-Kabeer): With moderately populated residential and
recreational activities and (f) G-VI (Ahmedi): The Southern Region of Kuwait
with oil fields, allied industries, scanty residential and green house vegetation
sites (Fig. 1).
||Sampling sites of desert plants in the Kuwait Governorates
GI-GVI: Jahra, Kuwait City, Hawalli, Farwaniya, Mubarek Al-Kabeer and Ahmedi
Desert plants (replicates: 10 Nos.) such as Malva parviflora,
Suaeda aegyptiaca, Chrozophora tinctoria, Fagonia bruguieri,
Gynandriris sisyrinchium and Ducrosia anethifolia were collected
from six Kuwait Governorate areas (Fig. 1) from October 2006
to December 2008. These plants were collected based on their orientation towards
the wind and leeward direction following the methodology of Sinegani
and Ebrahimi (2007). Samples were thoroughly rinsed in deionized distilled
water to remove the dust and soil. They were collected in sterile polyethylene
labeled (Fischer brand, US) zipper bags (34x30 cm x 0.3 mm), stored in Thermo
Cole box and transported to the lab. They were stored at -4°C before analysis.
The thawed plant parts such as leaves, stem and roots were cut into small pieces
(5 cm) placed in a sterile Petri-dish (9 cm).
Soil samples (100 g) below 5 cm from the surface, adjoining each plant species
in the Kuwait Governorate areas were scooped and collected in sterile polyethylene
containers and transported to the laboratory (Keane et
Replicate plant and soil samples (5 and 100 g) from each area were dried until
constant weight at 50°C in an oven (GallenKamp II), respectively. Dried
plants and soil were powdered in the Agate mortar (Reutch), homogenized and
sieved in 1.0 mm sieve mesh and stored in sterile vials (Djingova
et al., 1993; Keane et al., 2001).
Samples (0.2 g) were used for trace metal analysis.
Trace Metal Analysis
Plant leaves, stem and roots and soil samples were predigested in
HNO3: HCl (Aristar grade v/v ratio of 3:1) in a polystyrene sterile
centrifuge tube and then waited overnight. The soil samples were treated further
with 1% HF for the complete mineralization and digestion (Bu-Olayan
and Thomas 2002). The samples diluted in de-ionized water (50 mL) and digested
in an automatic microwave digester (Spectroprep CEM) was measured in the Analytik
Jena, Zeenit-650 to determine the metals concentration.
Trace metals translocation in these plants from shoot to root was measured
using TF which is given below:
where, Cs and Cr are metal concentrations (μg g-1)
in the shoot and root, respectively.
Wherein, TF>1 indicates that the plant translocate metals effectively from
root to the shoot (Baker and Brooks, 1989). Further,
trace metals BAF in these plants was determined by calculating the ratio of
metal concentration in the aerial parts to that of the soil as given below:
where, Cp and Cso are metal concentrations in aerial
parts of the plant (μg g-1) and in soil (μg g-1),
BAF was categorized further as hyper-accumulators, accumulator and excluder
to those samples which accumulated metals >10 μg g-1, >1
and <1, respectively (Ma et al., 2001).
Quality assurance employing replicates, standard trace metals (ICP grade), blanks and Standard Reference Material: Orchard leaves (SRM 1571) for desert plants and Montana soil (SRM 2711) for soil samples from National Institute Standard Technology (NIST) assessed the precision of the instrument. Recoveries of samples (98±2%) in agreement with certified values were considered as a part of quality control. Pearsons correlation coefficient was used to show correlation significance of trace metals concentrations among studied variables.
Trace Metal Concentrations in Desert Plants and Soil
The total mean trace metal concentrations were in the sequence of C.
tinctoria (8.25 μg g-1), M. parviflora (7.96
μg g-1), S. aegyptiaca (6.12 μg g-1),
G. sisyrinchium (6.06 μg g-1) and D. anethifolia
(5.82 μg g-1), F. bruguieri (5.51 μg g-1)
(Table 1). Governorate-wise, the mean metal concentrations
were observed in the sequence of GII>GI>GVI>GIII>GIV>GV (Table
The mean trace metal concentrations in the desert plant leaves, shoot and roots were 7.36, 6.72 and 5.78 μg g-1, respectively (Table 3). The trace metal concentration in soil collected from the surrounding sampled plants was measured as 6.78 μg g-1. Further, high trace metals concentration was observed in soil collected near C. tinctoria (7.98 μg g-1) followed by M. parviflora (7.91 μg g-1) > S. aegyptiaca (6.67 μg g-1) > G. sisyrinchium (6.59 μg g-1) > D. anethifolia (5.92 μg g-1) > F. bruguieri (5.63 μg g-1). The overall mean trace metal levels in the three parts of these plants with soil samples showed similar sequence as observed in Table 3.
Governorate wise analysis showed the three parts of the sampled plants with
trace metal concentrations in the sequence of GII>GI>GVI>GIII>GIV>GV
Metals-wise analysis showed mean trace metal concentration in Al (14.16 μg
g-1) followed by Cu (10.71 μg g-1) > Ni (4.83
μg g-1) >Fe (4.60 μg g-1)>Pb (2.89 μg
g-1) >V (2.52 μg g-1) (Table 5).
||Total mean metals levels (μg g-1) in the desert
plant species of Kuwait Governorates
||Mean metals levels (μg g-1) in each desert
plants from the six Kuwait Governorates
|GI-GVI: Kuwait Governorates; SP1-SP6: Malva sp.,
Fagonia sp., Suaeda sp., Ducrosia sp., Chrozophora
sp. and Gynandris sp.
||Species-wise mean metal levels (μg g-1) in
the desert plant parts and soil from Kuwait Governorates
|SP1-SP6: Malva sp., Fagonia sp., Suaeda sp.,
Ducrosia sp., Chrozophora sp. and Gynandris sp.
||Governorate-wise mean metal levels (μg g-1)
in the desert plant parts and soil
|GI-GVI: Kuwait Governorates
||Metal-wise levels (μg g-1) in each species
of Kuwait desert plants
|SP1-SP6: Malva sp., Fagonia
sp., Suaeda sp., Ducrosia sp., Chrozophora sp.
and Gynandris sp.
||Mean metal wise levels (μg g-1) in desert
plants from each Kuwait Governorate
|GI-GVI: Kuwait Governorates
||Translocation and bioaccumulation factor for six desert plants
||Trace metals translocation and bioaccumulation factor on Kuwaiti
Species-wise analysis revealed high trace metals concentrations in C. tinctoria
(17.16 μg g-1). The mean concentration of each trace
metal in each Governorate (Table 6) showed similar trace metals
concentrations in sequence to that of the sequence observed in Table
Bioaccumulation Factor (BAF) and Translocation Factor (TF) in the Samples
TF in all the six desert plants were higher than 1. TF and BAF were observed
in the sequence of Gynandriris sisyrinchium>Suaeda aegyptiaca>Chrozophora
tinctoria>Malva parviflora>Ducrosia anethifolia>Fagonia
bruguieri (Table 7, 8). Results showed
the metals concentrations <1000 μg kg-1 in all the investigated
samples. Findings indicated TF < BAF in metals and species wise analysis
(Table 7, 8).
Among the six sampled desert plants (Table 1) high trace
metals concentrations in C. tinctoria attributes to their rough surface
leaves that allow deposition of dust particles that contain inorganic pollutants
(Yoon et al., 2006; Sinegani
and Ebrahimi, 2007). These plants are able to limit competition from other
plants by taking up inorganic constituents from deep ground water, accumulating
it in their foliage and from there depositing it in the surface soil where it
builds up concentrations temporarily detrimental to other plants (Al-Kateeb
and Leilah, 2005; Kadukova et al., 2008).
Table 2 showed higher trace metal concentrations in Governorate
(GII) than the other five sampled sites. High trace metals concentrations in
this area attribute to the congested industrial and residential areas and support
the findings of Bu-Olayan and Thomas (2002). Leaves
revealed higher trace metal concentrations than shoot and roots irrespective
of the sampled species (Table 3). This could be attributed
to (1) the larger exposure area of the leaves than the other parts to wind action,
(2) surface adsorption of particulate matter and (3) high rate of assimilation
of trace metals from the environment agreed with the findings of Keane
et al. (2001) and Al-Kateeb and Leilah (2005).
Trace metal concentrations in soil was comparatively higher than in roots and
shoots but lower than that of the leaves in the sampled plants (Table
3). This indicates the mobilization of trace metals from soil through the
roots to the shoot, leaves of these plants, which was also found in the literature
(Baker and Brooks, 1989; Djingova
et al., 1993; Keane et al., 2001;
Sinegani and Ebrahimi, 2007). However, this phenomenon
of metals levels in soil with parts of the plants was in contrary to the findings
of Bu-Olayan and Thomas (2002) in date palms. This could
be attributed to the variations observed in the (a) morphological, (b) absorption
area, (c) phyllotaxis of the leaves and (d) effect of different metals concentrations
analyzed to that of the present study. Table 4 indicated Governorate
wise analysis with similar sequence of trace metals concentrations (Table
2). Low trace metal concentrations in GV attributes to restricted human
interferences, allied industries and efficient air monitoring system and supports
the earlier findings (Wittig, 1993; Bargagli,
1998; Monaci et al., 2000). Species wise
study showed high Al concentrations among the other metals studied (Table
5). This may be attributed to the rapid industrialization, anthropogenic
resources and frequent dust storms in this arid ecosystem. Comparatively, Fe
concentration was lower than the concentration of the other metals due to their
assimilation in plants (Djingova et al., 1993).
Meanwhile, Pb and V showed low concentrations that attributes to their immobility
of effective translocation in the plants (Simon et al.,
1996; Yoon et al., 2006). Species-wise analysis
showed high trace metals concentration in C. tinctoria than the other
species. These species are observed especially in wastewater contaminated outlets
that are enriched in high trace metals concentrations. Thus, this plant could
be used for the assessment of individual biomarker of inorganic pollution.
The mean Governorate-wise analysis showed high trace metals concentration in GII when compared to the metals concentrations from the other Governorate samples (Table 6).
The sampled desert plants showed both TF and BAF >1 (Table
7, 8) and hence, they could be labeled as accumulators
of pollution as described earlier (Lyngby and Brix, 1989;
Baker and Brooks, 1989). However, these samples showed
metal concentrations < 1000 μg kg-1 and thus, none could
be classified as hyper-accumulators (Baker and Brooks, 1989;
Ma et al., 2001). Tolerance limits of these species
to trace metals accumulation estimation through BAF and TF could be used in
phytoremediation process (Roselli et al., 2003;
Sinegani and Ebrahimi, 2007). Pearsons correlation
revealed that high correlation (R2 = 0.89) was found between TF and
BAF for species wise. However, metal wise correlation between TF and BAF was
found less correlated (R2 = 0.58) than species wise TF and BAF. This
suggests that each type of plant follow a specific pattern to metals sequestration
that varies because of environmental factors and metals accumulation (Ma
et al., 2001; Rosselli, 2003; Yoon
et al., 2006).
The present investigation revealed that these plants could be used as potential tool: (1) to study the trace metals pollution levels in each Governorate, (2) to determine the interrelationship between BAF and TF and (3) for site specific studies to inorganic pollutants in the arid environment.
Authors are thankful to the Kuwait Foundation for the Advancement of Sciences, for the financial support of our project (KFAS-2006-1401-02). Authors are also thankful to the the Research Administration and SAF (GS 01/01, GS01/05) Kuwait University, for their support and sample analysis, respectively.