Accumulation and Soil-to-Plant Transfer Factor of Lead and Manganese in some Plant Species in Semnan Province, Central Iran

Background: Heavy metals have detrimental effects on the health of human being. The values of manganese (Mn) and lead (Pb) in some plant species and soil samples in an arid area of Iran were evaluated in this study. Methods: The values of Pb and Mn in 94 plant samples from 8 plant species and the related soil samples in 2010 were considered in Shahroud and Damghan, central Iran. Moreover, the soil-to-plant transfer factors of these two elements were investigated. Results: Except for barley, the values of Pb in all of the considered plants were higher than the standard level of 0.3 mg/kg. The amounts of Mn in all of the plant species other than pistachio were higher than the standard level of 25 mg/kg. As a whole, the bio-concentration factor of Mn was higher than that of Pb in the study area. Conclusion: None of the soil-to-plant transfer factors exceeded one. Grape recorded the highest amounts of Pb and Mn compared with that of other investigated plant species. However, since its respective bio concentration factor (BCF) was lower than one, it cannot be considered as a hyper accumulator of lead and manganese.


INTRODUCTION
If the levels of metals in soil exceed standard values, the toxicity for cultivated plants may occur, which pose a threat to the health of human if they accumulate in food crops [1].
According to WHO, the selective study of individual foodstuffs is an important step in the estimation of dietary intake of metals [2]. The dominant factor for the accumulation of heavy metals in plant species seems to be related to genotype of different biotic and non-biotic factors [3].In general, every plant species can act as a potential bio-indicator of environmental condition. In this field, some plants can accumulate high amounts of heavy metals in their living tissues especially their leaves [4].
Among different elements, manganese (Mn) and lead (Pb) have been known as heavy and trace elements in the environment. There are few environmental pollutants having detrimental effects on humans, animals and ecosystems like that of Pb. It has been recently phased out from the petrol used in most of the developing countries to reduce these detrimental effects.
However, because of its persistence the longterm effects of this pollutant are still available in the environment [5]. In unpolluted natural soils, it stems mainly from parent materials and the overall mean value of this element for different soil is about 25 mg/kg [6].
On the contrary, despite the fact that Mn is an essential element for plants especially for the proper function of photosynthesis process through an enzyme antioxidant-cofactor, an excess of this micronutrient is toxic for plants [7]. The excess of this element in plant species can alter various processes, such as enzyme activity, absorption, translocation and utilization of other mineral elements (e.g.Ca, Mg, Fe and P), causing oxidative stress [8]. Several oxidation states are available for manganese in environment (II, III, IV, VI and VII). However, the predominant forms in biological systems are II, III and IV. In the soil system, on the other hand, divalent Mn (Mn II) is the most soluble form, while the solubility of other forms is very low [9].Considering the above-mentioned introductory comments, the main objectives of Vol 10, No 3, May-June 2016; http://www.ijt.ir the present study were: (i) to consider the values of lead and manganese in some plant species in Shahroud and Damghan, central Iran and (ii) to consider the soil-to-plant transfer factor of these elements. As some of the investigated plants have been using for edible purposes by local people in the area, consideration of heavy metals is of great importance regarding the health risk that these metals may pose for the consumers.

MATERIALS AND METHODS
Ninety-four sampling locations were selected in 2010 in Shahroud and Deamghan, Semnan Province central Iran, in which plant and the associated soil samples were taken randomly from the same location that the plant samples were prepared to estimate the soil-toplant transfer factors of Pband Mn. The plant species, number of samples for each plant type and the respective sampling part have been given in Table1. For soil samples, the surface soil (0-5cm) was taken from each sampling station. In laboratory, the collected soil samples were airdried and sieved through a 2-mm stainless steel mesh to remove stones and plant roots. Following digestion of soil samples with nitric acid (HNO 3 ) and hydrochloric acid (HCl) in a ratio of 3:1 (HNO 3 :HCl), the total metal concentrations of Pb and Mn were analyzed by inductively coupled plasma (ICP) optical emission spectroscopy(ICP-OES). Plant species were threshed manually, with glumes and dust was removed whereas no washing of grains was performed. Dried samples were ground using a stainless steel grinder (<0.25 mm) and the total content of the above-mentioned heavy metal and trace elements were detected by ICP-OES.

RESULTS
The results of detection of Pb and Mn in the associated plant species in the study area have been illustrated in Figure1 and2.Considering the levels of Pb, the highest amounts were recorded in grape with the average value of 2.03 mg/kg. Whereas, the lowest amounts were found in barley, which has, mean value of 0.228 mg/kg. The order of lead levels in the plant species was as follows: Grape>chives>mulberry>apricot>spindle tree>wheat>pistachio>barley In this respect, except for barley, the values of lead in all of the considered plants were higher than the standard level of 0.3 mg/kg set by WHO for Pb in plant species. On the contrary, the highest values of Mn were also observed in grape with the average level of 115.75 mg/kg and the lowest amount was recorded in pistachio with the mean value of 23.89 mg/kg. In this field, the order of Mn in the respective plant species was as follows: Grape> spindle tree> mulberry> wheat> chives> apricot> barley> pistachio In this field, the values of Mn in all of the plant species other than pistachio were higher than the standard level of 25 mg/kg. On the other hand, a method for expressing the accumulation of metals from soil to above-ground tissues of plants is through bio-concentration factor (BCF), generally obtained by dividing the concentration of the element in plant tissues(grains, shoots, straw, etc) to the total concentration of the same element in the rooted soil. The bio-concentration factor of Pb and Mn for different plant species in the regionis given in Table2.Accordingly, the BCF of Mn is higher than that of Pb. The highest BCF of manganese was recorded for grape ranging from 0.116 to 0.319. While, the lowest observed value was for pistachio fluctuating from 0.019 to 0.048. On the contrary, Pb has most actively accumulated in chives as its bioconcentration factors fluctuate between 0.051 and 0.152.Whereas, the lowest amounts were for barley in which they varied from 0.006 to 0.026. Table2. Bioconcentration factor of manganese and lead in the considered plant species.

DISCUSSION
Both soil and atmosphere contribute to the levels of Pb in plant species among them the lowest contents are found in cereals and grains (<1 mg/kg) while higher amounts are found in roots and tubers (>1 mg/kg). Elevated levels of lead have also been found in leafy vegetables (>2 mg/kg) [5].
Next to industrial activities and vehicles, mining activities are among the main contributing factors to the release of lead to the environment [5]. In mining areas, Pb may be dispersed due to the erosion and chemical weathering of tailings [10]. Mining activity is one of the main sources of heavy metal release and its subsequent soil pollution in the study area. Semnan is the largest province of Iran with known gold and base metal [11]. For instance, it hosts five gold and base metal deposits, i.e., Gandy (Au-Ag-Pb-Zn), Abolhassani (Pb-Zn-Ag-Au), Cheshmeh Hafez (Pb-Zn), Chalu (Cu), Chahmosa (Cu), pousideh (Cu), Baghu and Arghash (Au-Sb) deposit. The discharge of heavy metals due to the widespread mining area in the region has been reported [12,13]. In addition, deposition of Pbin the air on the plant's leave and its following uptake by the plant during photosynthesis is another path of entry for Pb to plant species [14].
The highest amounts of metals (Pb and Mn) were observed in grape. One of the possible reasons for this might be because this species can absorb metals from both soil and aerosols in the atmosphere simultaneously. For instance, the heavy metal levels (including Pband Mn) decreased about 3 times following water washing of leaves implying that the pollution mainly takes place via aerosols [15]. In this field, the amounts found in this study were lower than that recorded by Angelova et al. in the leaves of grape in Bulgaria [15]. The other plant which showed a high level of lead was chives which is a sought-after vegetable consumed by local people. The values of heavy metals including Pb were higher than normal in areas in which the vegetables are irrigated with waste water [16]. The findings of this study confirm resultsof Bigdeli and Seilsepour [17] on the investigation of heavy metals in vegetables irrigated with wastewater in Shahre-Rey, Iran. The amounts of Pb found in the current research are far lower than reported earluer [18] on some medicinal herbal products in Iranian market.
In general, Pbis mainly accumulated near the soil surface as it is absorbed by the soil's organic matter. However, some soil parameters such as soil's acidity may facilitate its mobility in the soil [5]. The mean concentration of Pbin wheat grain in 16 different studies conducted in various countries was mostly high ranging from 0.015 to 22.6mg/kg [19]. The amounts of Pb detected in wheat grains by Eriksson [20] ranged from 0.005 to 0.011mg/kg (mean 0.007), while, the levels for barley grains fluctuated between 0.007 and 0.028 mg/kg(mean 0.013) [20] in which both of them were lower than that of the current research. In Iran, the health risk of heavy metals via consumption of cereals and vegetables was studied in Isfahan, Iran, so total non-carcinogenic hazard of As and Pb was greater than 1 indicating that there was a high risk for the health of consumer through consumption of these food products [21].
The results of this study are comparable with those reported earlier in which the mean value of Mn in wheat grain samples was equal to 49.8 mg/kg. However, the concentration value of Vol 10, No 3, May-June 2016; http://www.ijt.ir Mn found in wheat grain [19] in Pakistan was 4.9mg/kg, which is significantly less than that of the current research.
In the previous studies, elevated levels of Mn has been found in the soils of the study area attributed to the presence of ultramafic rocks and weathering and leaching of elements from them [13]. In area with elevated levels of Mn active absorption of Mn is happening across the soilroot interface resulting in its accumulation in plants [22]. It might be one of the contributing factors for the higher than normal values of Mn in plant species in this study [22]. Mn is mainly accumulated in the root of wheat and its translocation in the mature plant is restricted due to the importance of xylem in the transport of this element [23]. In other words, Mn transport from roots to shoots was visualized in the xylem and was essentially immobile in the phloem [24].
Regarding Table2, all of the calculated bio-concentration factors (BCFs) found in this study was lower than one indicating the low amount of accumulation of heavy metals in the considered plants. Considering Table2, the BCFs of Mn were higher than that of Pb. One of the possible reasons for this higher value may be because Mn is an essential element for plant species so; it is more actively absorbed from the soil by the root system of the respective species compared. The bio-concentration factor of Mn [25] ranged between 0.005 and 0.167 in the consumed parts of different plants in Gabon, which waslower than that of the current study.
Among the 36plants [26], G. pennelliana showed the highest BCF for Pb (BCF=11). The BCF of Pb in this study was lower than that found by Kim et al. [27] in P. thunbergii (BCF=5-58), and in the same range of those reported by Stoltz and Greger [28] (BCF=0.004-0.45).

CONCLUSION
The values of manganese and lead in eight plant species were considered in this study. Grape recorded the highest amounts of Pb and Mn compared with that of other plant species in the area of study. However, since it's respective BCF was lower than one so, it cannot be considered as a hyper accumulator of Pb and Mn. The amount of these two elements was higher than standard values in most of the plant species indicating the high health risk for the consumers. However, their respective bioconcentration factor was low implying that this high accumulation is due to elevated levels of these two elements in the soil rather than their high accumulation capability. Cavalieri Correlating manganese X-rayabsorption near-