Chromium in agricultural soils and crops: A review
Andrea Ertani1, Anna Mietto1, Maurizio Borin1 and Serenella Nardi11 Dipartimento di Agronomia, Animali, Alimenti, Risorse Naturali e Ambiente (DAFNAE), Università di Padova, Viale
dell’Università 16, 35020 Legnaro (Padova), Italy.
Abstract
The mobility and distribution of metals in the environment is related not only to their concentration but also to their availability in the environment. Most chromium (Cr) exists in oxidation states ranging from 0 to VI in soils but the most stable and common forms are Cr(0), Cr(III) and Cr(VI) species. Chromium can have positive and negative effects on health, according to the dose, exposure time and its oxidation state. The last is highly soluble, mobile and toxic to humans, animals and plants. On the contrary, Cr(III) has relatively low toxicity and mobility, and it is one of the micronutrients needed by humans. In addition, Cr(III) can be absorbed on the surface of clay minerals in precipitates or complexes. Thus, the approaches converting Cr(VI) to Cr(III) in soils and waters have received considerable attention. The Cr(III) compounds are sparingly soluble in water and may be found in water bodies as soluble Cr(III) complexes, while the Cr(VI) compounds are readily soluble in water. Chromium is absorbed by plants through carriers of essential ions such as sulphate. Chromium uptake, accumulation and translocation, depend on its speciation. Chromium shortage can cause cardiac problems, metabolic dysfunctions and diabetes. Symptoms of Cr toxicity in plants comprise decrease of germination, reduction of growth, inhibition of enzymatic activities, impairment of photosynthesis and oxidative imbalances. This review provides an overview of the chemical characteristics of Cr, its behavior in the environment, the relationships with plants and aspects of the use of fertilizers.
Keywords: Chromium; Speciation; Agricultural soil, Anthropic pollution
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In the past decades the increased use of chromium (Cr) in several anthropic activities (Gueral et al. 2012; Weng et al. 2013; Qiu et al. 2014) caused soil and water contamination. Differently from other heavy metals like lead, cadmium and copper, chromium primarily presents in both the low-toxic form of Cr(III) and high-toxic Cr(VI).
1 Chromium sources of environmental contamination
Chromium is regarded as an important pollutant released into the environment by industries (Nriagu 1988). Contamination of soils and groundwater caused by the use of this metal in various anthropic activities is a worldwide problem that has been studied by the scientific community for decades, and is still a current issue.
Among heavy metals, Cr is widely utilized in multiple industrial activities. It is used mainly in industrial leather processing and finishing, in the production of refractory steel, drilling muds, electroplating cleaning agents, catalytic manufacture and in the production of chromic acid and specialty chemicals. Chromium compounds are also used in the metalworking industries, cooling tower water, and, until a short time ago, to preserve wood (Shanker et al. 2005). All these activities have contributed to a significant increase in Cr concentration in the environment. The leather and tannery industries in particular, are the most responsible for the stream of this metal into the biosphere (Barnhart, 1997), despite many efforts have been made during the last years to limit the risks of contamination and improve the quality of effluents deriving from these activities. Chromium basic sulfate (Cr2(OH(SO4)3) is mainly used to tan leather; it oxidizes the organic materials of the hide, reducing and precipitating inside the tissue as Cr2O3 or Cr(OH)3 compounds, which soften the leather and work as mordant for the subsequent coloring process. In India, a quantity that varies from 2,000 – 32,000 tons of elementary Cr per year enters the environment through the uncontrolled spillage of contaminated sludge coming from the leather working industries (Zayed and Terry 2003).
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The report of UNIC (2012) shows that Italy is a world leader for production of sole leather with an annual production of 30,000 tons. An average removal of Cr(III) equal to 99.1% was calculated from the abatement efficiency performance of the industrial plants (given by the ratio between the concentration of pollutant before and after entering the treatment plant). A relevant datum is the incidence of environmental costs sustained by the leather industries: they reached 4.27% of turnover and 4.44% of the total operation costs in 2012. These values are significantly higher than those of the past years, and are proof of the important role of environmental protection in this field (UNIC 2012).
2 Chemistry of chromium
Chromium is the seventh most abundant element in the Earth and the twenty-first on the Earth’s crust (0.1-0.3 mg kg-1) (Katz and Salem 1994). The most stable forms of this metal in the environment and in biological systems are the trivalent Cr(III) and hexavalent Cr(VI) (Becquer et al. 2003). Other intermediate valence states of Cr exist in nature, but they are generally unstable. The trivalent form, which is the most stable in absolute, occurs in the environment under the form of chromite (FeOCr2O3), while Cr(VI) is usually present in association with oxygen to form chromate (CrO42-) or dichromate (Cr2O72-) oxyanions, compounds that are extremely toxic and carcinogenic to all living organisms (Becquer et al. 2003).
Chromium displays different degrees of toxicity depending on its chemical form, and gives rise to a wide spectrum of reactions in the environment (Hrudayanath et al. 2014). For instance, Cr(III) is scarcely mobile and bioavailable, and is naturally present in insoluble inorganic compounds at pH<4; as the pH increases, Cr(III) is more present in hydrolyzed forms, which are anyway not very soluble and tend to bind organic substances (Adriano 1984). On the contrary, Cr(VI) is very mobile because it forms soluble inorganic compounds like chromates, which are highly bioavailable and toxic (Salmani et al. 2016). 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69
The two main forms of Cr can interchange and co-exist in a dynamic balance regulated by three different types of reaction: 1) oxidation/reduction: Cr(VI) is a strong oxidizing agent and in the presence of organic substances it is reduced to Cr(III) under acidic conditions; Cr(III), however, can also be oxidized to Cr(VI) in the presence of oxygen and manganese oxides. It has been reported that manganese oxides mediate the transfer of electrons between Cr (III) and the oxygen that is in air, and that the amount of Cr(III) oxidized to Cr(VI) is proportional to the reduction rate of manganese (Bartlett and James 1988; Bartlett 1997). Oxidation of Cr(III) by manganese oxides is controlled by the surface characteristics of the oxides and Cr(III) concentration. 2) Precipitation/dissolution: the trivalent form is very stable in soil, while the hexavalent form is easily mobilized in both acid and alkaline soils (Adriano 1986). 3) Adsorption/desorption: Cr(III) is strongly adsorbed on the soil particle surfaces, therefore it cannot be easily exchanged with other cations present in soil. It is partially mobile only in acid soils. The solubility of Cr(III) increases when it is linked to certain ligands, including hydroxyl groups, if they are present as free molecules and ions. In the case where ligands are found in macromolecular structures, such as humic acids, Cr(III) solubility is reduced.
The adsorption of Cr(VI) is favored by the positive charges present on the mineral coating of iron oxides (Stollenwerk and Grove 1985) and is accompanied by the concurrent release of OH- groups (Bartlett 1991).
In soil, heavy metals like Cr are distributed in: 1) water solution; 2) exchangeable fraction; 3) organic substance-bound phase; 4) oxides and clay-like minerals-bound phase and crystal lattice core of primary minerals. The distribution of Cr between these phases is controlled by the three main processes mentioned above, that influences the fate of Cr in the environment (Table 1).
Chromium (III) concentrations in soils and sediments are a consequence of silicate alteration in minerals relative to chromite. In ultramafic rocks and serpentinites of ophiolite complexes, chromium concentration is <200 mg kg-1. Chromium(III) liberated during weathering are absorbed on clay
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minerals and precipitates as homogeneous solids or with Al(III)/ Fe(III)-hydroxides (Oze et al., 2007). Chromite [FeCr(III)2O4] is the primary source of Cr in ultramafic and serpentinite rocks, but exhibit low solubility, and there are few naturally occurring oxidants of Cr(III). The only natural oxidants of aqueous Cr(III) at pH < 9 are Mn(IV/III)- oxides and hydrogen peroxide (H2O2) (Oze et al., 2007).
The balance between Cr(III) and Cr(VI) in the soil phases, as well as in water bodies, can be expressed by the following reaction:
Cr2O72- + 6e +14H+ ↔ 2Cr3+ 7H2O
The direction of the reaction is towards the right when pH values (Fig 1) are low and in the presence of electron donator compounds. (Bartlett and Kimble 1976; Cary et al. 1977). The reduction kinetics of Cr(VI) to Cr(III) is simple and direct, but requires more time than oxidation, and it can take many years in natural soils (Bartlett 1991). The Cr that remains in an oxidized form in the soil, and cannot be extracted from a 10 mM solution of (KH)2PO4 at pH 7.2, is considered immobilized, because is precipitated or strongly adsorbed.
3 Chromium in the environment
Chromium is present in all environmental compartments, including water, air and soil, at different concentrations. Cr content in freshwater varies from 0.1 to 117 µg L-1, while in sea water from 0.5 to 50 µg L-1. Chromium may enter the natural waters by weathering of Cr-containing rocks, direct discharge from leaching of soils,. In the aquatic environment Cr may undergo reduction, oxidation, precipitation, sorption and desorption (Kimbrough et al. 1999). The solubility of Cr(III) depends on the pH of the water. Under neutral to basic pH, Cr(III) will precipitate and conversely under acidic pH it will tend to solubilize. The forms of Cr(VI) chromate and dichromate are very soluble under all pH conditions, but they can precipitate with divalent cations (Kimbrough et al. 1999).
Concerning the interaction of Cr(III) and clay minerals, clay adsorption is one of the most promising techniques to remove chromium(III and VI) . For example, bentonite, due to its physical and
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chemical properties, has been considered one of the most promising candidates in decontamination and disposal of high-level heavy metal wastes (Atia et al., 2008; Chen et al., 2011).
In air samples collected in remote areas, such as the Antarctic and Greenland, Cr levels fluctuate between 5.0x10-6 and 1.2x10-3 µg m-3, while in urban areas they can increase to 0.015-0.03 µg m-3 (Nriagu 1988). Under natural conditions, the concentration of Cr in soil ranges between 10 and 50 mg kg-1, but it can reach much higher concentrations depending on the nature of the substrate. For instance, Cr is abundant in mafic and ultramafic igneous rocks because during the first stages of fractioned crystallization it becomes part of such minerals as spinel and pyroxene (De Vivo et al. 2004), while much lower concentrations of the metal are present in acid igneous and sedimentary rocks (Alloway 1995). Concerning the soil properties, poorly orderd alumino-silicate like allophane showed high affinity with chromium.Allophanes and ferryhydrite, in virtue of their high surface area and reactivity, are likely the main clay minerals which bind the metals (Zampella et al.,2010).
Chromium concentration in agricultural soils varies up to values as high as 350 mg kg-1. It has been reported that most chromium in soil is in the trivalent form, and is complexed with mineral structures in the form of mixed Cr3+ and Fe3+ oxides. Indeed, Cr(VI) tends to be reduced to Cr(III) in soil with a high organic matter content. The redox reaction is particularly active in the rhizosphere, where different bacterial reductase enzymes are present, and/or released by plant roots (Branca et al. 1990).
A study by Lapo et al. (2008) showed a map (Fig. 2) of Cr levels distribution in Europe. The data acquired were elaborated by the FOREGS web site of the Association of Geological Surveys of the European Union, and then related to the concentrations of Cr found in the topsoil determined via ICP-AES. From the map it is evident that the areas with higher abundance of Cr are the Greek peninsula, central & northern Italy (mainly in the Po valley), the Balkans, south eastern Czech Republic, southern
England and northern Finland.
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In Italy, the highest levels of Cr usually occur in soil generated and developed on the geological substratum in which chromite is prevalent. In the chapter on the geosphere in the Environmental Data Yearbook (2005), drafted by various Environmental Agencies (ARPA, APPA, ISPRA) in collaboration with National Statistical System (SISTAN), the various environmental indicators of the soil matrix in the Italian territory were analyzed, with reference to the Soil Regions of the Ecopedologic Map of Italy. The content of Cr is generally greater in the lower than in the upper layers (APAT 2005). Veneto, reagion is into Soil Regions 1 and 3, presents an average Cr content between 60 and 70 mg kg-1, despite some variability being highlighted throughout the region. The total Cr content in agricultural soils around some Italian towns can partly be traced back to the natural degradation processes of the geological substrate from which the soil originates (APAT 2003), partly ascribed to the use of substances containing heavy metals during agricultural practices (pesticides and fertilizers).
4 Factors that influence Cr phytoavailability
The availability of Cr in soil depends on various factors, among which the most important are: pH, redox potential, type of minerals present in the matrix, organic content, microbial community structure, nature of the absorbing phase and origin of the contamination by chromium.
In low redox potential environments, Cr(III) is the most stable form; in the pH interval between 4 and 8, Cr(III) is present in the hydrolyzed forms: (CrOH2+, Cr(OH)2+, Cr(OH)3). Under oxidative conditions (high redox potential), usually Cr(VI) is thermodynamically the most stable species in the pH interval from 2 to 14. However, it must be highlighted that in the presence of reducing agents, hexavalent chromium is quickly reduced to Cr(III), which in turn forms stable soluble complexes in acidic conditions. Cr(III) forms complexes with organic ligands, as well as with fluoride, ammonium, cyanide, thiocyanate, oxalate and sulfate. Cr(III) can also bind to organic and inorganic compounds, and then be converted to Cr(VI) through oxidation or dissolution reactions, as mentioned previously.
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Inorganic complexes have low solubility, consequently their transport in groundwater is reduced. On the contrary, organic complexes are much more soluble and mobile in soil.
Naqvi and Ritzi (2000) reported that Cr is accumulated in clay and humus following the trend humus>clay, and that accumulation is proportional to the exposure time. The same authors showed that in soils rich in organic matter a significant portion of Cr is linked to the organic fraction and the iron oxides. The content of water-soluble and exchangeable Cr is very low with a resulting low mobility and bioavailability of the element in these soils.
Bartlett (1997) evidenced that in soils containing high Cr concentrations both the mobility and bioavailability of this metal increase, because of the significant presence of Cr in the soluble and exchangeable fraction. In addition, if the pH conditions are favorable and MnO2 is present, Cr(III) can be oxidized to the toxic hexavalent form, even if this transformation is quite rare.
5 Toxicity and health effects of chromium
Chromium is listed among the top 129 environmental pollutants and, in the hexavalent form, it is considered as one of the 14 most harmful substances for the health of living organisms (USEPA 2000). The human body can be exposed to Cr or some of its compounds through inhalation (breathing), ingestion (eating/drinking) and dermal contact (skin penetration).
Chromium can have positive and negative effects on human and animal health, according to the dose, exposure time and its oxidation state. Cr(III) is an essential nutrient for humans and mammals, and the ideal daily intake for normal glucose, protein and fatty acid metabolism is between 50 and 200 µg per day-1 (WHO 2000). Its lack in the diet can cause serious cardiac problems, metabolic dysfunctions and diabetes, but its excess presence in the body has dangerous health effects. Hexavalent chromium compounds are considered to be 10-100 times more toxic than trivalent chromium compounds because they tend to act as strong oxidants (Katz and Salem 1994; Zayed et al. 1998). If present among the components of tannery products, Cr(VI) can cause allergic reactions and skin
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eruptions. Inhalation causes ulceration and perforation of the mucous of the nasal septum, irritation of the larynx and pharynx, bronchitis, bronchospasm and edema, coughing, asthma and itching of the nose. Other pathologies identified in people exposed to Cr(VI) include gastric disturbances and ulcers, weakness of the immune system, kidney and hepatic damage, genetic mutations, lung cancer (Kotaś and Stasicka 2000).
6 Chromium uptake, translocation and accumulation by plants
Plant roots produce organic acids, including citric and malic acid, which act as ligands for the metals present in insoluble forms in soil, thus influencing their redox behavior. Studies on the role of organic acids in Cr toxicity in tomato plants (Lycopersicon esculentum) showed that in the presence of organic acids Cr accumulation was enhanced (Srivastava et al. 1998).
Organic acids such as citric acid, aspartic acid and oxalic acid can convert inorganic chromium into organic, making it more soluble for a long period of time, therefore available for plant uptake (Srivastava et al., 1998).
Chromium is not an essential element for plants, so its absorption does not occur through specific mechanisms. The toxicity of Cr relies on its speciation, which determines metal uptake, transport and accumulation. In turn, the translocation and accumulation of Cr inside the plant depend on the plant species, the oxidation state of the metallic ions, and also its concentration in the growth medium.
Independent uptake mechanisms for Cr(VI) and Cr(III) were reported for the first time in bean plants (Skeffington et al. 1976). The application of metabolic inhibitors reduced the absorption capacity of Cr(VI) in plants, while it did not interfere with Cr(III) uptake. These experiments provided evidence that Cr(VI) transport through the plasma membranes requires metabolic energy, while Cr (III) transport is mediated by a passive mechanism. The influx of Cr(VI) compounds was also shown to be inhibited by sulfate, leading to the hypothesis that Cr(VI) is absorbed by the root cells in an active process involving sulfate transporters (Cervantes et al. 2001; Skeffington et al. 1976). Higher Cr accumulation
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is generally observed in roots than in leaves (Gupta and Sinha 2006; Calliari et al. 1993). However the immobilization of the Cr(III) by cation binding capacity of the cell wall is the major process influencing the root accumulation of the metal (Dheeba et al., 2014; Skeffington et al. 1976). The reason for the high Cr accumulation in roots is mainly because most Cr is stored in the vacuoles of the root cells as a protective mechanism (Mangabeira et al. 2006; Shanker et al. 2004). Indeed, in this way plants can tolerate Cr to some extent as it is less toxic when sequestered (Shanker et al. 2004).
Anccumulation of Cr(III) and Cr(VI) is species-specific and can be evaluated by the transfer factor (TF), calculated by the ratio between chromium content in the roots and soil Cervantes et al. (2001). This factor gives indications on the mobility and transfer of the elements from the soil to the plant. Plants usually present very low values of TF. In Datura innoxia the TF tends to decrease in response to increases in Cr concentration (Vernay et al. 2008). In a study performed by Sauerbeck (1991), among tested heavy metals (Cd, Zn Cu, Ni) Cr showed the lowest TF values. The Cr concentrations measured in the root tissues of some plant species (spinach, oats, carrots, peas, beans, radish) was very low, despite the presence of Cr in the treated soil (Sauerbeck 1991).
7 Toxic effects of chromium in plants
Once hexavalent chromium enters the plant cells via sulphate transporters and gets reduced to Cr (III) by enzymatic and nonenzymatic processes. Through this process, reactive oxygen species (ROS) are produced that exert harmful effects on cells by interacting with protein as well as nucleic acid (Cheung and Gu 2007). Cr(III) is having much less toxicity and bioavailability (He et al. 2009) as it readily forms insoluble hydroxide/oxides above pH ~ 5.5. In fact, the biological cell membranes are nearly impermeable to Cr (III). As such, detoxification of Cr(VI) by its reduction to Cr (III) is of great environmental importance. In plants, Cr(VI) compounds are very toxic because they negatively influence their growth and interfere with some essential metabolic processes (Panda and Patra 2000; Panda 2007; Shaker et al. 2009). As indicated previously, the biological effects and toxicity of
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chromium depend on its oxidation state: hexavalent chromium is highly toxic for many organisms, while trivalent chromium is relatively innocuous (Zayed and Terry 2003). However, some studies on plants have demonstrated that even Cr(III) produces serious problems in tissues, but at higher concentrations than Cr(VI) (Skeffington et al. 1976; Cervantes et al. 2006).
At a 3.2 mM concentration, Cr(III) inhibits germination in Glycine max, Vigna ratiata, Vigna
angularis and Lablab purpureis (Jun et al. 2009). The reduced seed germination can be attributed to the
depressive effect exerted by Cr on amylase activity and the resulting reduced transport of sugars towards the plant embryo (Zeid 2001).
Chromium can also cause a significant reduction in root growth, generally attributed to inhibition of cell division or extension of the cellular cycle. It was observed that the lengths of Glycine max and
Vigna ratiata roots were limited by 3.2 mM of Cr(III) (Jun et al. 2009). The scarce root growth in
plants treated with Cr limits plant absorption of nutrients and water from the growth medium (Calliari et al. 1993). In this paper, the authors studied the effects of chromium present in compost and its potential toxic effects on the growth and accumulation of barley leaves and roots were assayed. In particular, three types of compost (A, B and C) were tested during the growth of barley seedlings: A and B were from 60% urban (A) and tannery (B) sludge, 30% chicken manure, 10% poplar bark, while compost C was obtained by combining compost A and Cr2(SO4)3, with a chromium content of 300 mg/kg in compost A and 3000 mg/kg in composts B and C. The maximum growth was observed in barley seedlings exposed to the lower Cr concentration (compost A), with maximum values achieved after 24 days, while lowest values of biomass production were observed with compost C, indicating that the concentration of 3000 mg/kg injured plants.
The lower flow of nutritional elements to the above-ground parts results in reduced stem extension and plant height (Shanker et al. 2005). A decrease in the size of Curcumas sativus, Lactuca sativae and
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Panicum miliaceum was reported by Gorsuch et al. (1995). A significant drop in height was also
observed in Helianthus annuus L. plants grown in soil containing 60 mg kg-1 Cr (Fozia et al. 2008). In addition to root growth inhibition, Cr affects the leaf biomass production, causing a reduction in leaf area and number of leaves per plant. Sharma et al. (1995) found a 50% decrease in the number of leaves on wheat plants grown in the presence of 0.5 mM Cr (VI). Other visible effects of Cr on plants include a reduction in the production and number of flowers per plant in Vallisneria spiralis L., an increase in cauliflower seed deformity and lighter oat pods (McGrath 1982; Biacs et al. 1995; Chatterjee and Chatterjee 2000; Vajpayee et al. 2001).
Chromium can interfere with other physiological processes of plants (Anjum et al., 2016). In
particular, it reduces the photosynthesis efficiency in terms of CO2 fixation, the transport of electrons, photophosphorylation and photosynthetic enzyme activity (Shanker et al. 2009). These effects can be related to alterations of chloroplast ultrastructure caused by Cr(VI), in particular to the scarce development of the lamellar system and disorganization of the mesophyll cells (Schiavon et al. 2009). The levels of transpiration, stomatal conductance and assimilation of CO2 can also be reduced in plants exposed to chromium (Schiavon et al. 2009). The variations in the levels of stomatal conductance could be ascribed to modification of the cellular structure of the spongy parenchyma and a consequent reduction in the size of the stomata on the mesophyll. With regard to the decrease in transpiration levels, it was hypothesized that Cr might affect water movement in the xylem.
Chromium can alter the uptake activity of the plasma membrane in root cells limiting the absorption and accumulation of important elements, such as N, P, K, Fe, Mg, Mn, Mo, Zn, Cu, Ca and B (Shanker et al. 2005). As an example, Cr(III) interferes with the absorption of Fe in dicotyledons, inhibiting the reduction of Fe(III) to Fe(II) or entering in competition with Fe(II) in the absorption sites (Shanker et al. 2004) (Tab.2). 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281
Cr(VI) also interferes with nitrogen assimilation, causing a decrease in the levels of nitrate reductase, nitrite reductase, glutamine synthetase, glutamate dehydrogenase and urease (Shanker et al. 2009). Furthermore, Cr increases the activity of antioxidant enzymes (Fig. 3). Superoxide dismutase (SOD) and catalase (CAT) are enzymes involved in the detoxification of heavy metals in plants. SOD activity increases by 29% in pea plants exposed to 20 µM Cr(VI) (Dixit et al. 2002). In B. juncea activity of the SOD and CAT enzymes was induced by low Cr(VI) concentrations (0.2 and 2 µM), while activity of the glutathione reductase (GR) and glutathione S-transferase (GST) increased significantly in the presence of high levels of the metal (20 µM) (Pandey et al. 2005). The increased activity of antioxidant enzymes can be considered as a direct response to the production of superoxide radicals caused by the block induced by Cr in the electron transport chain in the chloroplasts, with possible damage to the membranes (Shanker et al. 2005). The effects of chromium on plant growth and development are shown in Table 2.
The research conducted by Silva (1977) was aimed at evaluating whether the use of organic fertilizers deriving from tanning residues could determine an appreciable accumulation of Cr in plants, especially in edible tissues. The study was performed on plants growing on either soil that was fertilized with 0, 5, 10 q ha-1 of hydrolyzed leather (2.9% Cr tot) or soil that was not fertilized with it. The study was performed on soil that had been fertilized with 0, 5, 10 q ha-1 of leather (2.9% Cr tot) compared with control plants that had not been fertilized with hydrolyzed leather. The examined plants were: grape, peach, maize, mixed meadow, rice, tomato. In many cases, the concentrations of Cr among species were equal to, and in some cases lower than, the control value. In addition, no Cr(VI) was found in the analyzed percolated water, indicating the stability of Cr(III) at least in common soil conditions (Silva 1977). Conclusions 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305
This paper summarizes the literature about Cr toxicity in the environment, especially in water, soil and plants. Chromium exists in three oxidative states Cr(0), Cr(III), and Cr(VI), which are the most stable forms of Cr. As Cr(0) is the metallic form, Cr(III) and Cr(VI) are the most present in water and soils. Both in water and in soil, Cr undergoes a transformations such as reduction, oxidation, sorption, desorption and precipitation. The solubility of Cr(III) depend on pH: Cr(VI) is extremely soluble under all pH values. In plants Cr is a nonessential element whereas not exist a specific mechanism for its uptake. Cr(III) uptake is a passive process, whereas Cr(VI) uptake is achieved by carriers of others elements such as sulphate. Chromium accumulates principally on plant roots apparatus, being translocated to leaves in small amount, independently of Cr species. Chromium influence several processes in plants, such as germination, growth, yield and several physiological processes. The species of Cr in plants are toxic at different degrees at different stages of plant growth and development. The toxicity of Cr(VI) is due to the formation of Reactive oxygen species (ROS) during the reduction of Cr(VI) to Cr(III) inside the cell: high concentrations of ROS cause oxidative stress which explains most of the Cr toxicity symptoms in plants.
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