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Journal of soil science and plant nutrition

versão On-line ISSN 0718-9516

J. Soil Sci. Plant Nutr. v.10 n.4 Temuco  2010 

J. Soil Sci. Plant Nutr. 10 (4): 428 - 442 (2010)




M. Pigna1*, V. Cozzolino1, A. Giandonato Caporale1, M.L. Mora2, V. Di Meo1, A.A. Jara2 and A.Violante1

1Dipartimento di Scienze del Suolo, della Pianta e dell'Ambiente, Universitá di Napoli Federico II, Napoli, Italy,Corresponding author:

2Departamento de Ciencias Químicas y Recursos Naturales, and Scientific and Technological Bioresource Nucleus (BIOREN-UFRO), Universidad de La Frontera, Temuco, Chile.


In this study we have examinated the results of two experiments on the uptake and distribution of arsenic (As) in roots, shoots, and grain of wheat grown in As-polluted soils and in an unpolluted soil irrigated with As-contaminated water in absence or presence of phosphorus (P) fertilization. Arsenic concentrations in wheat samples of the two experiments are higher than those in the plants grown on uncontaminated soil. In the experiments showed in this work, it is highlighted the role of P fertilization in preventing As uptake and translocation in wheat plants. These findings could have important implications to reduce the potential risk posed to human health by As entering the food-chain.

Keywords: phosphorus fertilization, arsenic,wheat, polluted soils.


There is increasing concern worldwide regarding the contamination of soil with arsenic (As), and the potential risk to human and environmental health arising from such contamination (Smith et al., 1998). Arsenic contamination of soil can occur as a result of both natural sources and anthropic activities, including the use of arsenical pesticides and herbicides, atmospheric deposition, mining activity, waste disposal, and other sources (Mandal and Suzuky, 2002). Furthermore, in spite of the low geochemical abundance of As (average earth crust 1.8 mg kg-1) some degree of arsenic dissolution might cause critical value is only 10 μg L-1. High concentrations of As in groundwater has been reported from several countries icincluding Argentina,Bangladesh, Chile, China, Hungary, Mexico, West Bengal (India), and Vietnam (Smith et al., 1998). New cases of high concentrations in drinking water have been reported when As analyses was included in routine water analysis. These cases included some areas of Italy where elevated as concentrations in groundwater and in soils were recently found (Costagliola et al., 2004).

Typical uncontaminated agricultural soils contain 1-20 mg kg-1 (Wauchope, 1983), but contaminated soils in mining or industrial areas contain levels as high as 45-2600 mg kg-1 (Brandsetter et al., 2002). In many cases, extensive use of As-rich groundwater has led to elevated As concentrations in agricultural soils, which may reduce soil productivity, be toxic to plants, and enter into the crops (Heikens, 2006. Martin et al., 2007). In fact, As is not an essential element for plants, but interferes with plant metabolism, inhibiting plant growth and crop yield (Barrachina et al., 1995; Abedin et al., 2002; Rahman et al., 2004). Therefore, minimizing soil-plant transfer of As is essential for agriculture on As-affected farmland.

In contrast, phosphorus (P), that is a chemical analogue of As (Adriano, 2001) and competes with As in plant uptake (Meharg and Macnair, 1992), is one of 17 essential elements required for plant growth (Raghothama, 1999), and often the overriding nutritional limitation in modern cereal farming (Runge-Metzger, 1995). It is estimated that crop yields on around 30-40 % of the world's arable land are limited by P availability (Runge-Metzger, 1995). The effect of P on the sorption/desorption of As in soil environments has received great attention, especially when P is used as a crop fertilizer (Peryea, 1998). The bioavailable fraction of As in soils to crop plants depends on different physical and chemical properties of soils. In fact soils rich in variable charge minerals (Al or Fe oxides) do not release As easily. Only large additions of P to high anion-fixing soils or alkaline pH or Fe and Al oxide dissolution may affect As solubility (Smith et al, 1998; Violante and Pigna, 2002). However, As sensitivity is intimately linked to P nutrition in plants. In rice seedlings, even cultivars found to be susceptible to arsenate can become more resistant by raising level of intracellular P (Geng et al., 2006; Wang and Duan, 2009). Gultz et al. (2005) reported that P availability and P demand, which are plant specific, have to be taken in account to predict uptake of As by crop plants. Therefore, As toxicity in crops can be more prevalent in situation where As contamination is found coexisting with low available P.

Much of the research on As in grain crops have focused on rice (Oryza sativa L.) (Abedin et al., 2002; Williams et al., 2005; Rahman et al., 2007). The work that has been conducted on wheat is less extensive (Tao et al., 2006; Zhao et al., 2007), with limited information regarding soil-root, root-shoot and shoot-grain assimilation and translocation (Williams et al., 2007). In Tuscany, Italy there is extensive soil contamination with As due to base mining and processing activities (Costagliola et al., 2004). Elevated As concentrations have been reported for Scarlino, close to Grosseto, in southwest Tuscany (ARPAT-UNIFI, 2003). Wheat (T. durum) is the main crop cultivated in this area. Grain is largely used for human consumption and as poultry feed. Also, the straw is used as fodder for cattle. The aim of this work was to evaluate the role of P fertilization on growth, As uptake and partitioning between different plant part (grain, shoot, root) in wheat (T. durum). In one experiment wheat was grown in two contaminated soils (Vetricella and La Botte) and in an uncontaminated soil (Padula) collected from Scarlino. The second experiment examined the effect of P fertilization on wheat grown in an uncontaminated soil, collected from a natural grass in Presenzano, Caserta, Italy, and irrigated with solutions containing As at three different concentrations.



Some physical and chemical properties of the soils utilized in these experiments are presented in Table 1. The analyses were carried out as reported in previous works (Cozzolino et al., 2010; Pigna et al., 2009). A five-step sequential extraction procedure for As (Wenzel et al., 2001) was performed on contaminated soils (Table 1). The first extraction step (Step 1), performed with 0.05 M (NH4)2S04, represents the most labile As of the procedure. The second extraction (Step 2), with 0.05 M NH4H2P04, was used to assess the As fraction that can be specifically replaced by phosphate. Even if not as easily released as the first fraction, this one can also be considered as labile As. The following steps included extractions with 0.2 M ammonium oxalate at pH 3, in the dark (Step 3); with ammonium oxalate and ascorbic acid, in the light (Step 4); and an acid digestion with HN03 65% and HF 50% (Step 5). These three final fractions, representing As bonded to amorphous (Step 3) and crystalline (Step 4) hydrous oxides of Fe and Al, and residual As (Step 5), can be considered as nonlabile As. The sequential extraction was carried out in triplicate on 1 g of soil in 50-ml polypropylene centrifuge tubes to facilitate washing of the soil after each extraction thus minimizing any loss of the solids. The supernatants were separated by centrifugation (1700g) and filtered through 0.45-μm filters. The total soil arsenic concentration was determined by digesting the soils with concentrated HNO3 and HF at 5:1 ratio. Total As concentrations of soil extracts and digests were determined by hydride generation-inductively coupled plasma spectrometry (HG-ICP - AES, Varían, Liberty 150).

Experiment 1. Vegetative responses of wheat grown on contaminated soils and uncontaminated soil with and without additional P

Experiments were conducted for seven months in an unheated greenhouse. Series of twelve pots were filled with 7 kg of each of the three soils with different arsenic content. Wheat (T. durum cv Creso) plants were sown directly in pots at a density of 10 seeds per pot. Fourteen days after sowing the seedlings were thinned to 3 plants per pot. All the pots were fertilized every 2 weeks with 80 mL of nutrient solution containing 29.1 mM N as NH4NO3 and 25 mM K as K2S04. One third of the pots did not receive any phosphorus in the nutrient solution (P0 treatment); another third of the pots received 2.8 mM P as K2HP04, corresponding to a fertilizer distribution of 75 kg ha-1 of P (PI treatment); the last third of the pots received 5.6 mM P (150 kg ha-1) included in the nutrient solution (P2 treatment). Thus, each of the three thesis with soils at different As content received three P treatments, and each treatment was replicated 4 times to give a total of 36 pots. The design was completely randomized and re-arranged every day.

Experiment 2. Vegetative responses of wheat grown on an uncontaminated soil and irrigated with water containing As at three different concentrations, with and without additional P

The wheat plants were grown for seven months in an unheated greenhouse. Wheat (Triticum durum cv. Creso) plants were grown in pots filled with 7 kg of the soil planted at a density of 10 seeds per pot, sown directly in the pots, and irrigated during the first 2 weeks with water. After this period the seedlings were thinned to 3 per pot, and were irrigated with water containing arsenate (Na2HAs04) at four different concentrations: 0 (control treatment), 0.5, 1, and 2 mg L-1 of As, until the wheat grain was ripe. The range of arsenate concentrations was chosen to encompass the concentrations occurring in underground waters of the As-affected areas of world. Contaminated water was added as required to maintain moisture content at 60% of water holding capacity. All the pots were fertilized each 2 weeks with 80 mL of nutrient solution containing 29.1 mM N as NH4N03 and 25 mM K as K2S04. Furthermore, in half of the pots 5.6 mM P as K2HP04 was included in the nutrient solution in order to evaluate the influence of Phosphorus on As uptake by plants. Thus, there were 2 treatments without supplemental P (P-) and with supplemental P (P+). The design was completely randomized and rearranged every day, and each treatment was replicated 4 times to give a total of 32 pots. As irrigation was stopped 1 week before harvest.


Contemporaneously, a germination assay was carried out. Thirty surface-sterilized seeds of wheat were placed on a filter paper, collocated on a Petri plate, moistened with aqueous solution of arsenate at four concentrations (0, 0.5, 1.0, 2.0 mg As L-1) Each treatment was replicated four times. The seeds were incubated in dark at 24°C. Germinated seeds were counted 5 days after initiation.

Plant harvest and analysis

Seven months after sowing, wheat plants were harvested by cutting the stem 3-4 cm above the soil. Wheat spikes were collected and dried at 70°C to constant weight. The spikes were then dehusked by hand and the weight of the grain was recorded for each pot. Shoots and roots were washed with tap water and then rinsed twice with deionized water. The dry weights of the roots and shoots were determined after oven drying at 70°C for 48 h. Roots, shoots, and grain were analyzed for total concentration of As. All samples were ground using a PM 200 ball mill (Retsch) and were digested in a microwave (Milestone, Digestor/Dring Ethos 900). A sample of about 0.5 g was accurately weighted into a PTFE pressure vessel and 7 mL of HN03 (65%), 0.5 mL of HF (50%), and 2 mL of H202 were added. All glassware and plasticware were previously acid-washed with 3M HC1, and rinsed with deionized water. Arsenic concentrations in roots, shoots and grain were determined by flow-injection hydride generation atomic absorption spectrometer using a Perkin-Elmer AAnalist 700 interfaced with a FIAS 100 hydride generator. Hydride generation was used for roots, shoots and grain samples due to the lower detection limits of this technique (0.5 |ag L-1). All analyses were carried out in triplicate. In each analytical batch at least, one reagent blank and one internationally certified reference material (CRM), oriental tobacco leaves CTA-OTL-1, was included to assess precision and accuracy of the chemical analysis.

Statistical analysis

Data analyses were performed with Kaleidagraph 3.6. Treatment effects were determined by analysis of variance. Differences were considered as statistically significant at p < 0.05 (Tukey's test).



Experiment 1. Vegetative responses of wheat grown on two contaminated soils and on an uncontaminated soil with and without additional P

Plant growth and As toxicity

The soils investigated in this study originated from sites where As contamination was due to high As content in the geological materials and to mining and industrial activities (Donati et al, 2005). The total As concentrations in the two polluted soils were clearly higher than Italian regulatory limits of As contamination in agricultural soils (20 mg kg-1) (DM 471/99). The highest As concentrations was observed in La Botte soil (304 mg kg-1) (Table 1).

It was observed that plant biomass decreased markedly with increasing As concentration in soils (Table 3). For plants grown without P addition (P0 treatment) there was a decrease in biomass production of 11%, 58%, and 60% compared to the plants that received PI fertilization and of 16%, 65%, and 69% with respect to the plants with P2 fertilization (Table 3), in Padula, Vetricella and La Botte soils, respectively. These findings are comparable to results reported for rice (Abedin et al, 2002) and wheat (Liu et al, 2005). Root biomass decreased significantly (p < 0.001) with increasing concentration of As in soils. In P0 plant series, it decreased by 72%, and 83.6%, respectively in Vetricella and La Botte, compared to plants grown in unpolluted soil. Phosphorus application significantly increased root biomass in all soils and treatments, ameliorating the toxic effects of As in these soils. The interaction between As and P on the growth response was significant in shoots. The shoot biomass (shoot plus leaf biomass) decreased with increasing concentration of As in soils, especially in the P0 plants (Table 2), but P addition, both in PI and P2 treatments, ameliorated the effects of As.

The grain yield of wheat was greatly affected by the application of P, ranging from 24.5 to 12.4 g pot-1 for the P2 treatment; from 23.6 to 8.6 g pot-1, PI treatment; 21.20 to 2.10 g pot-1, P0 treatment (Table 2). For the non contaminated soil (Padula), the difference between P2 and P0 treatment was lower (14%) than for the two polluted soil, especially La Botte soil, where the increases in grain yield with respect with P0 treatment were 76% for PI treatment and 84% for P2 treatment. According to our results, root biomass production responded more strongly to increasing soil As concentration compared to the shoot, especially in P0 plants. In the experiments reported here, in the presence of P supply, high As concentrations in the soils determined a moderate inhibition in wheat growth, including grain yield, especially at the highest P supply, compared to wheat growth in P0 treatment. Phosphorus has a protective effect with respect to As toxicity in wheat plant, particularly in La Botte soil, characterized by low P availability (Table 1). Studies on As toxicity have shown that root biomass decreased significantly (p < 0.001) with increasing concentration of As in soils. In P0 plant series, it decreased by 72%, and 83.6%, respectively in Vetricella and La Botte, compared to plants grown in unpolluted soil. Pt application significantly increased root biomass in all soils and treatments, ameliorating the toxic effects of As in these soils. The interaction between As and P on the growth response was significant in shoots. The shoot biomass (shoot plusleaf biomass) decreased with plant species not resistant to As suffer considerable stress upon exposure, with symptoms ranging from inhibition of root growth to plant death (Meharg and Macnair, 1991; Paliouris and Hutchinson, 1991; Barrachina et al, 1995). Arsenate acts as a phosphate (Pi) analogue and is transported across the plasma membrane via a Pi cotransport systems (Ullrich-Erebius et al., 1989). Once inside the cytoplasm arsenate competes with Pi, for example replacing Pi in ATP to form unstable ADP-As, leading to the disruption of energy flows in cells (Meharg, 1994). The effects of P nutrition on the mitigation of arsenate toxicity symptoms seem to be: 1) high plant P status leads to a down-regulation of the arsenate/Pi plasma-lemma transporters; 2) high cellular Pi levels results in greater competition with arsenate for biochemical processes where arsenate substitutes for Pi (Meharg, 2005).

As concentration in wheat roots, shoots and grain increased significantly with the increase of total As concentrations in polluted soils and As concentrations of Fl and F2 fractions (labile fractions) (Figure 1). In fact, the total As and the sum of the Fl and F2 fractions were highly correlated with As root concentrations (r = 0.976 and r = 0.897, respectively) as well as with shoot (r = 0.958 and r = 0.745) and grain concentration (r = 0.870 and r = 0.674) in P0 treatment. High correlation coefficients were found also for As bonded to amorphous (F3) and crystalline (F4) hydrous oxides of Fe and Al (r = 0.855 and r = 0.998, respectively), but not for residual fractions. The results were almost the same in PI and P2 treatments, although the correlation coefficients were lower. Arsenic concentration in the roots increased particularly in the P0 treatment. By increasing As concentration in the soils As root concentrations ranged from 0.28 to 3.50 in P2 plants and from 0.30 to 6.2 mg As kg-1 in P0 plants (Figure 1). Similar results were found in other studies investigating the influence of P on As accumulation by wheat (Tao et al, 2006).

Shoot As concentrations also rose with increasing As concentration in soil, more so for the P0 treatment. The highest shoot As concentration (2.45 mg kg -1) was measured in the plants grown in La Botte soil in the absence of P fertilization. Our results are in agreement with those reported in previous studies (Barrachina et al, 1995; Quaghebeur and Rengel, 2003). The ability of As to accumulate in wheat has already been shown. Greenhouse pot experiments on wheat grown in soil spiked with 50 mg As kg-1 resulted in shoot levels of ~ 3 mg kg-1 d.wt (Williams et al, 2007). In wheat grown in As-polluted soil (range 53.8-709 mg kg-1), contaminated by the Aznalcollar mine spill, levels of ~ 20 mg As kg-1 were detected in the shoots (Soriano and Fereres, 2003).

As concentration in grain in unpolluted soil (Padula) was only ~ 0.03 mg kg-1 in all P treatments, increasing, in the most polluted soil (La Botte), to 0.62, 0.50, and 0.44 mg kg-1 in P0, PI, and P2 treatments respectively. These values are similar or slightly higher compared to those found by Williams et al, (2007) in wheat grain. They reported levels of ~ 0.21 mg kg-1in Scottish (East Coast) wheat grain, where the maximum soil As levels were of 18 mg kg-1, and of 0.50 mg kg-1 in English (Cornwall and Devon counties) wheat grain, where the maximum As levels in soil was 201 mg kg-1. Greater As accumulation levels (0.74 mg kg-1) were observed in south-eastern Asia wheat (Norra et al, 2005), which was comparable to grain levels of 0.75, 0.71 and 0.69 mg kg-1 found in wheat grown in a greenhouse experiment in 50 mg As kg-1spiked soil (Williams et al, 2007).

In the present study, although As was primarily bound with Fe and Al oxides (Table 1), plant growth was stunted in the two contaminated soils in absence of P addition, especially in the soil with the highest As concentrations in more labile soil fractions and with low P availability (See Table 1; Cozzolino et al., 2010). Phosphorus fertilization seems to reduce the effects of As toxicity, promoting plant growth without increasing As concentrations in above-ground parts of plants and limiting shoot to grain transfer, particularly at highest P level.

These observations corroborate the reports by Meharg (1994) that P restricted the transfer of As from soil to the above-ground plant organs. Also, P nutrition could be involved in the decrease in reactive oxygen species and non-protein thiols production, formed during exposure to As in the shoot that cause tissue damage and lipid peroxidation, affecting shoot-grain transfer. Geng et al., (2006), in two different genotypes of rice seedlings, found higher stress resulting from As exposure in genotype with higher overproduction of enzymatic antioxidants and non-protein thiols.

Experiment 2. Vegetative responses of wheat grown on uncontaminated soil irrigated with water containing As at three different concentrations, with and without additional P

Plant growth and As toxicity

A downward trend was observed in plant growth with increasing As concentration in irrigation water (Table 4). For plants grown without P addition (P-) there was a decrease in biomass production of 15%, 52%, and 67% as As concentration in the irrigation water increased, with respect to control treatment. This reduction was less severe in the P+ conditions, 12%, 16%,and 26% respectively (Table 3). These findings are similar to results reported for rice (Abedin et al., 2002) and wheat (Liu et al., 2005).


Root biomass decreased significantly (p < 0.001) with increasing concentration of As in irrigation water. In P- plants, by 54%, 68%, 79% at 0.5, 1, 2 mg As I/1 respectively, while by 4.2%, 19%, 33% in P+ plants. Pt application increased significantly root biomass in all treatment. Liu et al, (2005) reported a significant decline in root biomass production in wheat seedlings with the increase in arsenite and arsenate concentrations for all six varieties of Triticum aestivum studied. Quaghebeur and Rengel (2003), in the non tolerant species Holcus lanatus, found that at increasing As concentration in nutrient solution there was a decrease in the root and shoot dry weight, accentuated when the plants had no P supplied. Sneller et al. (1999) in hydroponic experiment did not observe any inhibition in root growth with up to concentrations of 0.58 mg of As L-1 in high-P (3.1 mg L-1) treatment (on Silene vulgaris) but found 75% root growth inhibition in low P (0.31 mg L-1) treatment. However, Abedin et al., (2002) observed no significant differences in rice root biomass as a result of P application.

The dry weight of shoots (shoot plus leaf biomass) was significantly influenced by the As X P interaction; the shoot biomass decreased with increasing concentration of As in irrigation water, especially in the P- plants. The grain yield of wheat (mass of grain pot-1) was affected by the application of As in irrigation water. Grain yield was found to range significantly from 8.4 to 2.2 g pot-1 from the control to the highest As level, in P- plants (Table 3), a decrease in yield of 74%. While P+ plants exposed to 0.5 and 2 mg L-1 showed little/or moderate decrease in grain yield (17 to 33% respectively) with weights ranging from 21.25 to 14.30 g pot-1. At lower As application rates there was no effect on root/shoot ratios in P+ plants, while at the lowest As treatment, a clear decrease of ratio was observed in P- plants (Table 3). It was demonstrated that root biomass responded more strongly to As application compared to the shoot, especially in P- plants.

In the experiments reported here, As concentrations of 0.5 - 2 mg L-1, had a moderate inhibition in plant growth, including grain yield, in the presence of P¡. The differences in mean biomass between P- and P+ treatments at the higher As concentrations, also in these experiments, highlighted the role of P in preventing As toxicity and growth inhibition in wheat.

Arsenic concentration and contents in wheat roots, shoots and grain

Arsenic concentration in wheat root, shoot and grain, increased significantly with increaing As in irrigation water (Figure 2). Arsenic concentration in the roots increased particularly when P was added. By increasing As concentration in irrigation water from 0.5 to 2.0 mg As L-1 As root concentrations ranged from 0.63 to 2.06 and from 1.4 to 3.2 mg As kg-1 , respectively in P- and P+ plants. Similar results were found in other studies (Tao et al., 2006), investigating the influence of Pt on As accumulation by wheat.

Shoot As concentrations (mg kg-1) increased in plants irrigated with As contaminated water, but no significant difference were found at 0.5-1.0 mg As L-1 treatments and no difference were observed due to Pt application. The highest shoot As concentration of 0.65 mg kg -1was measured in the highest As treatment. As concentration in grain at the control treatment was only 0.05 mg kg-1 which increased to 0.24 mg kg-1 at concentration of 0.5 mg As L-1, in P-plants, although remained constant for the next two arsenate treatments In P+ plants, grain As concentrations was found to range slightly from 0.05 to 0.14 mg As kg-1 from the control to the highest As level (Figure 2). These results could indicate that at higher levels of As in irrigation water, the toxic element causes severe toxicity to wheat plant resulting in reduced growth rate and lowered translocation of As, as well as other nutrients from soil solution into the wheat grain. Similar results have been reported by Rahman et al. (2007) in rice plants and by Carbonell-Barrachina et al. (1997) in tomato and bean plants.

Figure 3 shows the As partitioning in wheat plant tissues. In P- plants for the highest As concentration in irrigation water As was found to be 72% in root, 21.5% in shoot and 6.5% in grain; whereas in P+ plants, at the same As concentration, 81% was in the root, 15.5% in shoot and 3.5% in grain. Results indicate that regardless P treatment, most of the As accumulated in wheat plant tissues, remains in root and the smallest amount in the grain, although this behaviour was more pronounced in P+ plants. In fact, P fertilization increased total As uptake (3.99 vs. 2.88 mg kg-1, see Pigna et al., 2009), but the increase was restricted to the root. In rice plants, Abedin et al. (2002) also observed that a very large amount of As retained in root compared to its content in straw and grain. Other literature studies (Duxbury et al., 2002; Rahman et al., 2007) also reported similar results. Carbonell-Barrachina et al. (1997) in a similar experiment, found that in tomato plants, 83,2% of all the adsorbed As remained in the root system, 16,8% in the shoots and only 7,3% reached the leaves, while in bean plants only accumulated 13,2% of the total plant As in the roots.

As allocation to the above ground portion of cereal crops is undesirable, as it will cause contamination of the human food-chain, with wheat being a staple in many countries around the globe. The results presented here indicate P supply may result in lower As allocation to the above ground (Figure 3), which has practical application in soil-crop systems. Other authors (Lu et al, 2010) indicated also that in rice, maintaining higher P concentration and P/As molar ratio in rice shoot by variety breeding or soil management is one of the means for decreasing arsenic accumulation in rice grain.



Arsenic concentrations in wheat samples of the two experiments are higher than those in the plants grown on uncontaminated soil, and, although the daily intake of As calculated in these cases was lower than the daily permissible intake of As (0.15 mg, see Cozzolino et al., 2010 and reference therein), very low possible As intake from others foods (vegetables, meat, fish, eggs and milk) would be enough to increase the daily intake of arsenic above the limit value.

In the experiments showed in this work, it is highlighted the role of P fertilization in preventing As uptake in wheat plants. These findings could have important implications for human health and agricultural systems, since they suggest that it may limit the ingestión of As through the consumption of crops grown on contaminated soils and reduce yield losses.



This research was supported by the Italian Research Program of National Interest (PRIN), year 2008. DiSSPAPA Number 229.



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