Mycorrhizal fungi enhance accumulation and tolerance of chromium in sunflower (Helianthus annuus)

Chromium (Cr) is a heavy metal risk to human health, and a contaminant found in agricultural soils and industrial sites. Phytoremediation, which relies on phytoextraction of Cr with biological organisms, is an important alternative to costly physical and chemical methods of treating contaminated sites. The ability of the arbuscular mycorrhizal fungus (AM),Glomus intraradices, to enhance Cr uptake and plant tolerance was tested on the growth and gas exchange of sunflower (Helianthus annuus L.). Mycorrhizal-colonized (AM) and non-inoculated (Non-AM) sunflower plants were subjected to two Cr species [trivalent cation (Cr³⁺) Cr(III) , and divalent dichromate anion (Cr₂O₇⁻) Cr(VI) ]. Both Cr species depressed plant growth, decreased net photosynthesis (A) and increased the vapor pressure difference; however, Cr(VI) was more toxic. Chromium accumulation was greatest in roots, intermediate in stems and leaves, and lowest in flowers. Greater Cr accumulation occurred with Cr(VI) than Cr(III). AM enhanced the ability of sunflower plants to tolerate and hyperaccumulate Cr. At higher Cr levels greater mycorrhizal dependency occurred, as indicated by proportionally greater growth, higherA and reduced visual symptoms of stress, compared to Non-AM plants. AM plants had greater Cr-accumulating ability than Non-AM plants at the highest concentrations of Cr(III) and Cr(VI), as indicated by the greater Cr phytoextraction coefficient. Mycorrhizal colonization (arbuscule, vesicle, and hyphae formation) was more adversely affected by Cr(VI) than Cr(III), however high levels of colonization still occurred at even the most toxic levels. Arbuscules, which play an important role in mineral ion exchange in root cortical cells, had the greatest sensitivity to Cr toxicity. Higher levels of both Cr species reduced leaf tissue phosphorus (P). While tissue P was higher in AM plants at the highest Cr(III) level, tissue P did not account for mycorrhizal benefits observed with Cr(VI) plants.     

Effect of Chelate-Assisted Hexavalent Chromium on Physiological Changes,Biochemical Alterations,and Chromium Bioavailability in Crop Plants—An In Vitro Phytoremediation Approach

This study revealed heavy metal–induced physiological and biochemical alterations in crop seedlings by supplementing chelating agents in the nutrient solution. Hexavalent chromium (Cr⁺⁶) induces several toxic effects in hydroponically grown rice, wheat, and green gram seedlings. A noticeable decrease was observed in root length, shoot length, biomass content, and chlorophyll biosynthesis of the seedlings grown in the nutrient solutions supplemented with Cr⁺⁶ at 100 μM. The seedling growth was stimulated with supplement of chelating agents such as EDTA, DTPA, and EDDHA. An increase in proline content was noticed with the application of Cr⁺⁶ (100 μM) in nutrient solutions. Stimulated activities of antioxidant enzymes such as catalase and peroxidase were noticed with increasing concentrations of chromium. Cr bioaccumulation was significantly high in roots of seedlings treated with Cr⁺⁶ at 100 μM in nutrient solution. Shoot translocation of Cr as depicted by transportation index (Ti) values for different crops were enhanced with the application of chelating agents. The total accumulation rate (TAR) for Cr was enhanced with the supplementation of DTPA in rice and wheat, whereas the application of EDDHA was found effective for increasing the accumulation rate of Cr in green gram seedlings. This study demonstates the role of chelating agents in lessening the toxic effects of Cr⁺⁶. The chelating agents supplemented with Cr⁺⁶ in the culture medium enhanced the Cr bioavailability in plants.     

SPECTROSCOPIC DETERMINATION OF THE TOXICITY,ABSORPTION, REDUCTION,AND TRANSLOCATION OF Cr(VI) IN TWO MAGNOLIOPSIDA SPECIES

Hexavalent chromium is a contaminant highly mobile in the environment that is toxic for plants at low concentrations. In this work, the physiological response of Convolvulus arvensis and Medicago truncatula plants to Cr(VI) treatments was compared. C. arvensis is a potential Cr hyperaccumulator well adapted to semiarid conditions that biotransform Cr(VI) to the less toxic Cr(III). M. truncatula is a model plant well adapted to semiarid conditions with a well studied genetic response to heavy metal stress. The results demonstrated that C. arvensis is more tolerant to Cr toxicity and has a higher Cr translocation to the leaves. The inductively coupled plasma optical emission spectroscopy results showed that C. arvensis plants treated with 10 mg Cr(VI) L^–1 accumulated 1512, 210, and 131 mg Cr kg^–1 in roots, stems, and leaves, respectively. While M. truncatula plants treated with the same Cr(VI) concentration accumulated 1081, 331, and 44 (mg Cr kg^–1) in roots, stems, and leaves, respectively. Enzymatic assays demonstrated that Cr(VI) decreased ascorbate peroxidase activity and increased catalase activity in M. truncatula, while an opposite response was found in C. arvensis. The x-ray absorption spectroscopy studies showed that both plant species reduced Cr(VI) to the less toxic Cr(III).     

Chromium distribution in shoots of macrophyte Callitriche cophocarpa Sendtn.

The aim of the study was the analysis of Cr distribution in shoots of the macrophyte Callitriche cophocarpa by means of two X-ray-based techniques: micro X-ray fluorescence (μXRF) and electron probe X-ray microanalysis (EPXMA). Plants were treated with 100 μM (5.2 mg l^?1) chromium solutions for 7 days. Cr was introduced independently at two speciations as Cr(III) and Cr(VI), known for their diverse physicochemical properties and different influence on living organisms. A comparative analysis of Cr(III)-treated plants by EPXMA and μXRF demonstrated high deposition of Cr in epidermal glands/hairs localized on leaves and stems of the plant shoots. Cr in Cr(III)-treated plants was recorded solely in glands/hairs, and the element was not present in any other structures. On the other hand, Cr in Cr(VI)-treated group of plants was rather found in vascular bundles. Moreover, the concentration of Cr in Cr(VI)-treated plants was significantly lower than in plants incubated in Cr(III) solution. The results obtained in this work suggest differences in chromium uptake, transport and accumulation dependent on the oxidative state of the element.     

Effect of different oxidation states of chromium in causing chromosome alterations in cultured CHO cells

Four chromium salts with different oxidation states were tested for their influence in causing chromosome aberrations and sister-chromatid exchange in Chinese hamster ovary cellsin vitro. Cell cultures were treated with CrO₃, K₂Cr₂O₇, CrCl₂ and Cr(NO₃)₃.9H₂O at concentrations of 10^–7, 10^–6, 10^–5 and 10^–4 M for the aberration assay, and 10^–8, 10^–7, 10^–6 and 10^–5 M for the sister-chromatid exchange assay. It was noticed that Cr (VI) compounds-CrO₃ and K₂Cr₂O₇-considerably enhanced the frequencies of aberrations and sister-chromatid exchanges compared to the control cultures. CrCl₂ and Cr(NO₃)₃.9H₂O–Cr (II) and Cr (III) respectively-caused a slight increase in sister-chromatid exchange rates, but the frequencies of aberrations were almost unchanged compared to the controls. These investigations indicate a definite link between the metals and changes produced in the mammalian chromosomes, reaffirming the evidence of carcinogenic potential of Cr (VI) observed by other investigators.Abbreviations BrdU 5-bromo-2-deoxyuridine - CHO Chinese hamster ovary - SCE sister-chromatid exchange     

Investigation of electron-transfer kinetics for bis(benzene) chromium(1+/0) redox couple in acetonitrile/dichloromethane binary mixtures at 298.15 K

The oxidation of bis(benzene) chromium(0) (Bz₂Cr) to bis(benzene) chromium(1+) (Bz₂Cr⁺) in acetonitrile (ACN), dichloromethane (DCM), and acetonitrile (ACN)/dichloromethane (DCM) binary mixtures with n-tetrabutylammonium hexafluorophosphate (TBAPF₆) as background electrolyte has been studied using the method of cyclic voltammetry at a temperature of 298.15 K. The diffusion coefficients (D) have been calculated using the Randles-Sevcik equation. The heterogeneous electron transfer rate constants (k_s) have been evaluated employing the electrochemical rate equation proposed by Nicholson. The one-electron oxidation of Bz₂Cr to produce Bz₂Cr⁺ was found to be either reversible or quasi-reversible and diffusion controlled in the investigated solvent media. The effect of the physical and chemical properties of the solvent medium on the electrochemical behavior of the Bz₂Cr⁺/Bz₂Cr couple has been examined.     

Ecophysiological responses of water hyacinth exposed to Cr3+ and Cr6+

Due to its wide industrial use, chromium (Cr) is considered a serious environmental pollutant of aquatic bodies. In order to investigate the ecophysiological responses of water hyacinth [Eichhornia crassipes (Mart.) Solms] to Cr treatment, plants were exposed to 1 and 10 mM Cr₂O₃ (Cr³⁺) and K₂Cr₂O₇ (Cr⁶⁺) concentrations for two or 4 days in a hydroponic system. Plants exposed to the higher concentration of Cr⁶⁺ for 4 days did not survive, whereas a 2 days treatment with 1 mM Cr³⁺ apparently stimulated growth. Analysis of Cr uptake indicated that most of the Cr accumulated in the roots, but some was also translocated and accumulated in the leaves. However, in plants exposed to Cr⁶⁺ (1 mM), a higher translocation of Cr from roots to shoots was observed. It is possible that the conversion from Cr⁶⁺ to Cr³⁺, which immobilizes Cr in roots, was not total due to the presence of Cr⁶⁺, causing deleterious effects on gas exchange, chlorophyll a fluorescence and photosynthetic pigment contents. Chlorophyll a was more sensitive to Cr than chlorophyll b. Cr³⁺ was shown to be less toxic than Cr⁶⁺ and, in some cases even increased photosynthesis and chlorophyll content. This result indicated that the F_v/F₀ ratio was more effective than the F_v/F_m ratio in monitoring the development of stress by Cr⁶⁺. There was a linear relationship between qP and F_v/F_m. No statistical differences were observed in NPQ and chlorophyll a/b ratio, but there was a tendency to decrease these values with Cr exposure. This suggests that there were alterations in thylakoid stacking, which might explain the data obtained for gas exchanges and other chlorophyll a fluorescence parameters.     

Chromium accumulation and toxicity in aquatic vascular plants

Chromium poisoning among leather tanners has long been known. The workers have been found to suffer from ulcers, allergic dermatitis, lung cancer, and liver necrosis due to prolonged contact with chromium salts. One of the highly catastrophic incidences of lung cancer as a result of inhaling dust containing Cr (VI) was reported in 1960 from the Kiryama factory of the Nippon-Denko concern on the island of Hokkaido, Japan. Pollution of water resources, both surface and underground, by indiscriminate discharge of spent wastes of chromium-based industries has become a serious global concern, for it has created an acute scarcity of safe drinking water in many countries. In August 1975 it was observed that underground drinking water in Tokyo near the chromium (VI))-containing spoil heaps contained more than 2000 times the permissible limit of chromium. In Ludhiana and Chennai, India, chromium levels in underground water have been recorded at more than 12 mg/L and 550–1500 ppm/L, respectively. Chromium is widely distributed in nature, occupying 21st position in the index of most commonly occurring elements in the earth’s crust. Chromium occurs in nature in the form of a compound (chromium + oxygen + iron) known as “chromite.” The geographical distribution of chromite mines is uneven. Over 95% of economically viable chromite ores are situated in the southern part of Africa. Its annual global production is ca. 9 million tons, mainly mined in the former Soviet Union, Albania, and Africa. In India, over 90% of chromite deposits are located in Sukinda Valley of Orissa. Chromium occurs in several oxidation states, ranging from Cr²⁺ to Cr⁶⁺, with trivalent and hexavalent states being the most stable and common in the terrestrial environment. Chromium (III) is used for leather tanning because it forms stable complexes with amino groups in organic material. In the presence of excessive oxygen, chromium (III) oxidizes into Cr (VI), which is highly toxic and more soluble in water than are other forms. Chromium (VI) can easily cross the cell membrane, whereas the phosphate-sulphate carrier also transports the chromite anions. On the other hand, Cr (III) does not utilize any specific membrane carrier and hence enters into the cell through simple diffusion. The diffusion is possible only after the formation of appropriate lipophilic ligands. Use of chromium as industrial material was discovered only 100 years ago. It was used for the first time in the production of corrosion-resistant steel (stainless steel) and coatings. Subsequently, chromium was widely deployed in various industries; namely, electroplating, dyes and pigments, textiles, photography, and wood processing. The tanning industry is one of the major users of chromium (III) salts. During leather processing the conversion of putrefactive proteinaceous matter, skin, into non-putricible is carried out by the treatment of chromium sulphate solution. According to an estimate, ca. 32 tons of chromium sulphate salts are used annually in Indian tanneries. As a result of unplanned disposal of spent tannery wastes, ca. 2000–3200 tons of chromium as element escapes into the environment. This has raised severe ecological concern and reduced the forest cover considerably. Aquatic vascular plants play an important role in the uptake, storage, and recycling of metals. The uptake of metals depends on the chemical form present in the system and on the life form of the macrophytes (floating, free floating, well rooted, or rootless). The free-floating species (Eichhornia, Lemna, Pistia) absorb elements through the roots/leaves, whereas the rootless speciesCeratophyllum demersum absorbs mainly through the finally divided leaves. Submerged species showed higher chromium accumulation than do floating and emergent ones. The order is:Elodea canadensis > Lagarosiphon major > Potamogeton crispes > Trapa natans > Phragmitis communis. Roots of water hyacinth (Eichhornia crassipes) showed an accumulation of 18.92 μmol (g dry tissue wt^-1) Cr. AlthoughCeratophyllum demersum andHydrodictyon reticulatum showed lower levels of chromium accumulation, their bioconcentration factor values were very high. Floating-species duckweeds (Lemna, Spirodela) are potential accumulators of heavy metals. They have bioconcentrated Fe and Cu, as high as 78 times their concentration in wastewater. Duckweeds have also shown the ability to accumulate chromium substantially. Although duckweeds attain higher concentrations of chromium in their tissues than do other macrophytes, their bioconcentration factor (BCF) values were much lower than those reported in other aquatic species. A moderate accumulation of chromium has been found in emergent species. Plants ofScirpus validatus andCyperus esculentus accumulated 0.55 kg and 0.73 kg^-1 Cr, respectively. InBacopa monnieri andScirpus lacustris accumulations of 1600 and 739 μg g^-1 dw Cr, respectively, have been reported when exposed to 5 mg L^-1 Cr for 168 hours in solution culture. The accumulation of Cr was greater in the root than the shoot. Higher accumulations of chromium in roots and least in shoots of emergent species have also been recorded. Phytotoxicity of chromium in aquatic environment has not been studied in detail. The mechanism of injury in terms of ultrastructural organization, biochemical changes, and metabolic regulations has not been elucidated. It has been pointed out that while considering the toxicity of heavy metals, a distinction should be made between elements essential to plants and metals that have no proven beneficial biochemical effects. For example, an increased level of chromium may actually stimulate growth without being essential for any metabolic process. In aquatic species—namely,Myriophyllum spicatum— the maximum increase in shoot length was found at 50 μgl^-1 Cr. Higher concentrations up to 1000 μ gl^-1 caused an almost linear reduction both in shoot weight and length. Duckweeds showed relatively greater tolerance to chromium. However, an inhibition of growth inSpirodela andLemna was found at 0.02 mM and 0.00002 mM Cr concentrations, respectively. Mortality ofL. aequinoctialis was found at 0.005 mM Cr and higher concentrations. The effective chromium concentrations (EC-50) for some aquatic species have been reported as follows:Lemna minor, 5.0 mg L^-1, 14 days EC;L. Paucicostata, 1.0 mg L-¹, 20 days EC;Myriophyllum spicatum, 1.9 mg L^-1, 32 days EC; andSpirodela polyrrhiza, 50 mg L^-1, 14 days EC. Chromium toxicity on biochemical parameters showed a reduction in photosynthetic rate at 50 μgl^-1 Cr inMyriophyllum spicatum. Decrease in chlorophyll and protein contents were also recorded inNajas indica, Vallisneria spiralis, andAlternanthera sessilis with an increase in chromium concentration. InLimnanthemum cristatum, a slight reduction in chlorophyll and almost no change in control were found due to chromium toxicity. Submerged species (Ceratophyllum demersum, Vallisneria spiralis) and an emergent one (Alternanthera sessilis) showed decreases in chlorophyll as well as in protein contents when treated with chromium. Chromium-induced morphological and ultrastructural changes have been reported in several aquatic vascular plants: InLemna minor andCeratophyllum demersum, chromium-induced changes in chloroplast fine structure disorganized thylakoids with loss of grain and caused formation of many vesicles in the chloroplast. Chromium (VI) has caused stunting and browning of roots produced from the chromium-treated excised leaves ofLimnanthemum cristatum. At 226 μg/g Cr dry wt leaf tissue concentration, development of brown coloration in the hydathodes of juvenile leaves ofLimnanthemum cristatum is a characteristic chromiuminduced alteration. Aquatic vascular plants and algae may serve as effective bioindicators in respect to metals in aquatic environments. Chromium-induced morphological and ultrastructural changes inLimnanthemum cristatum have significant indicator values and could be used for assessing the level of chromium in ambient water.Wolffia globosa, a rootless duckweed, showed substantial chromium accumulation and high concentration factor (BCF) value at very low ambient chromium concentrations, suggesting its feasibility in detecting chromium pollution in water resources. Methylene blue-stained cells ofScenedesmus acutus become uniformly dark blue during chromium (VI) treatment. This may serve as an indicator of chromium pollution.     

The Ameliorative Role of 5-Aminolevulinic Acid (ALA) Under Cr Stress in Two Maize Cultivars Showing Differential Sensitivity to Cr Stress Tolerance

Heavy metal (HM) contamination of the environment is a serious threat to sustainable crop production. Among the HMs, chromium (Cr) is one of the most toxic HMs that is known to negatively affect growth and metabolic activities of diverse crop plants. The present study was designed to investigate the ameliorative role of 5-aminolevulinic acid (ALA) under Cr stress in two maize (Zea mays L.) cultivars showing differential sensitivity to Cr tolerance. ALA is a biosynthesis precursor and it has a dominant regulatory effect related to physiological, respiratory, and photosynthesis processes in various plant species. Three concentrations of Cr (0, 5, and 10 mg kg⁻¹) were tested under the graded levels of ALA application (0, 12.5, and 25 mg L⁻¹). The results indicated that Cr stress differentially reduced plant growth attributes, gas exchange characteristics, photosynthetic pigments, and biomass in both the cultivars. Oxidative stress increased as evidenced in the form of electrolyte leakage, malondialdehyde, and hydrogen peroxide (H₂O₂) accumulation in plants. The anti-oxidative enzyme activities, that is, catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD) both in the leaves and roots of maize cultivars decreased due to Cr stress. The concentration of Cr increased in roots and shoots of maize under Cr levels without ALA. Under Cr stress, ALA exogenous application markedly enhanced plant growth, photosynthetic pigments, gas exchange capacity, and biomass. Furthermore, ALA application decreased the Cr-induced oxidative stress in maize cultivars by improving the activities of CAT, POD, and SOD in plants. After ALA application, the Cr concentrations and total Cr uptake by plants differently decreased in both cultivars. The 6103 cultivar of maize was found to be a tolerant cultivar against Cr stress due to its strong defensive system with a higher rate of antioxidant enzyme activities. On the other hand, the other maize cultivar (9108) was found to be a sensitive cultivar against Cr stress due to its weak defense system with higher contents of reactive oxygen species. These findings suggest that ALA can play a regulatory role in maintaining optimum plant growth and efficient photosynthetic processes under Cr-challenged habitats in maize. Thus, ALA application may be used as a sustainable remedial strategy to alleviate Cr-induced stress in maize cultivars.

     

Long-Term Exposure to [Cr3O(O2CCH2CH3)6(H2O)3]+ in Wistar Rats Fed Normal or High-Fat Diets Does Not Alter Glucose Metabolism

The essentiality of chromium(III) has been the subject of much debate, particularly in healthy subjects. Chromium(III)-containing supplements are widely used for body mass loss, building of lean muscle mass, and improving glucose and lipid metabolism. [Cr₃O(O₂CCH₂CH₃)₆(H₂O)₃]⁺, Cr3, is one of the most-studied chromium nutritional supplements. The current study evaluates the effects of long-term (15 months) supplementation with Cr3 on body mass and glucose metabolism in Wistar rats on traditional and cafeteria-style (high fat, high carbohydrate) diets. Male Wistar rats were randomly assigned to one of four treatment groups: (1) control diet (milled Harlan Teklad LM-485 rodent diet), (2) control diet?+?1 mg Cr3/kg body mass/day, (3) a cafeteria-style (CAF) diet (high fat, high carbohydrate), or (4) CAF diet?+?1 mg Cr3/kg/day. Cr3 supplementation had no effect on fasting blood glucose levels or blood glucose levels in response to glucose and insulin challenges. Rats consuming the CAF?+?Cr3 diet tended to have a significantly higher body mass than rats consuming the CAF diet, but necropsy results showed no difference in visceral fat or body wall thickness between groups. These data suggest that long-term Cr3 supplementation does not significantly affect body mass in rats consuming a normal diet or glucose levels or metabolism in rats consuming either diet.     

Impact of picolinic acid on the chromium accumulation in fodder radish and komatsuna

Effects of picolinic acid (2-pyridinecarboxylic acid) and chromium(III) picolinate was studied on the chromium (Cr) accumulation of fodder radish (Raphanus sativus L. convar. oleiformis Pers., cv. Leveles olajretek) and komatsuna (Brassica campestris L. subsp. napus f. et Thoms. var. komatsuna Makino, cv. Kuromaru ) grown in a pot experiment. Control cultures, grown in an uncontaminated soil (UCS; humous sand with pH_KCl 7.48, sand texture with 12.4% clay+silt content, organic carbon 0.56%, CaCO₃ 2.2%, CEC 6.2 cmol_c kg^–1, Cr 10.6 mg kg^–1), accumulated low amounts of chromium (less than 5.4 g g^–1) in their roots or shoots. When this UCS was artificially contaminated with 100 mg kg^–1 Cr (CrCl₃) later picolinic acid treatment promoted the translocation of chromium into the shoots of both species. In fodder radish shoots Cr concentration reached 30.4 g g^–1 and in komatsuna shoots 44.5 g g^–1. Application of ethylene diamine tetra-acetic acid (EDTA) to this Cr contaminated soil had similar effect to picolinic acid. When the UCS was amended with leather factory sewage sediment (which resulted in 853 mg kg^–1 Cr in soil), Cr mobilization was observed only after repeated soil picolinic acid applications. From a galvanic mud contaminated soil (brown forest soil with pH_KCl 6.77, loamy sand texture with 26.6% clay+silt content, organic carbon 1.23%, CaCO₃ 0.7%, CEC 24.5 cmol_c kg^–1, Cd 5.0 mg kg^–1, Cr 135 mg kg^–1, and Zn 360 mg kg^–1) the rate of Cr mobilization was negligible, only a slight increase was observed in Cr concentration of fodder radish shoots after repeated picolinic acid treatments of soil. Presumably picolinic acid forms a water soluble complex (chromium(III) picolinate) with Cr in the soil, which promotes translocation of this element (and also Cu) into the shoots of plants. The rate of complex formation may be related to the binding forms and/or concentration of Cr in soil and also to soil characteristics (i.e. pH, CEC), since the rate of Cr translocation was the following: artificially contaminated soil > leather factory sewage sediment amended soil > galvanic mud contaminated soil. Four times repeated 10 mg kg^–1 chromium(III) picolinate application to UCS multiplied the transport of chromium to shoots, as compared to single 10 mg kg^–1 CrCl₃ treatment. This also suggests that chromium(III) picolinate is forming in the picolinic acid treated Cr-contaminated soils, and plants more readily accumulates and translocates organically bound Cr than ionic Cr. Picolinic acid promotes Cr translocation in soil-plant system. This could be useful in phytoextraction (phytoremediation) of Cr contaminated soils or in the production of Cr enriched foodstuffs.     

Effect of bacterial inoculation of strains of pseudomonas aeruginosa,alcaligenes feacalis and bacillus subtilis on germination,growth and heavy metal (cd,cr, and ni) uptake of brassica juncea

Bacterial inoculation may influence Brassica juncea growth and heavy metal (Ni, Cr, and Cd) accumulation. Three metal tolerant bacterial isolates (BCr3, BCd33, and BNi11) recovered from mine tailings, identified as Pseudomonas aeruginosa KP717554, Alcaligenes feacalis KP717561, and Bacillus subtilis KP717559 were used. The isolates exhibited multiple plant growth beneficial characteristics including the production of indole-3-acetic acid, hydrogen cyanide, ammonia, insoluble phosphate solubilization together with the potential to protect plants against fungal pathogens. Bacterial inoculation improved seeds germination of B. juncea plant in the presence of 0.1 mM Cr, Cd, and Ni, as compared to the control treatment. Compared with control treatment, soil inoculation with bacterial isolates significantly increased the amount of soluble heavy metals in soil by 51% (Cr), 50% (Cd), and 44% (Ni) respectively. Pot experiment of B. juncea grown in soil spiked with 100 mg kg^?1 of NiCl₂, 100 mg kg^?1 of CdCl₂, and 150 mg kg^?1 of K₂Cr₂O₇, revealed that inoculation with metal tolerant bacteria not only protected plants against the toxic effects of heavy metals, but also increased growth and metal accumulation of plants significantly. These findings suggest that such metal tolerant, plant growth promoting bacteria are valuable tools which could be used to develop bio-inoculants for enhancing the efficiency of phytoextraction.     

Distribution and speciation of chromium accumulated in Gynura pseudochina (L.) DC.

Soil and water contamination with chromium is an issue of recent concern in Thailand due to increases in industrial activity. Gynura pseudochina (L.) DC., a chromium tolerance plant, could be employed to address this problem via phytoremediation. To understand the tolerance mechanism, this study investigated the speciation and distribution of chromium accumulated in G. pseudochina (L.) DC. using AAS, XAFS, μ-XANES, μ-XRF imaging and EPR. The plants were separately treated with K₂Cr₂O₇ and Cr₂(SO₄)₃ in a hydroponic system. μ-XRF imaging clarified the distributions of Cr, Fe, Zn, Ca, Cl, K and S within the samples. In G. pseudochina (L.) DC. treated with Cr(VI) solution, the Cr was mainly distributed in the vascular bundle and periderm of the tuber, the stem xylem, the vein and the epidermis, including the trichome of the leaf tissues. This Cr distribution corresponded to those of Cu, Fe and Zn. In G. pseudochina (L.) DC. treated with Cr(III) solution, the Cr was distributed in the periderm of the tuber, the stem cortex, and the epidermis and parenchyma of the leaf tissues. μ-XANES and XAFS indicated that highly toxic Cr(VI) was reduced to the intermediate Cr(V) and accumulated as less toxic Cr(III), and EXAFS spectra showed that the reduced Cr(III) was bound to oxygen ligands. The coordination number (N) and the interatomic distance (R) to the first shell were approximately 3–4 (N) and 2 Å (R), respectively. EPR spectra of the plant samples treated with Cr(VI) revealed the presence of Cr(V) and Cr(III). Thus, Cr(III) and Cr(VI) were taken up into the vascular system and transported from the roots to the leaves. Cr(III) was distributed via the symplast system to the ground tissue and accumulated mainly in the stem cortex. Cr(VI) was transported to the xylem via the apoplast system, and the adsorption of Cr(VI) and its reduction to Cr(V) and Cr(III) occurred on oxygen ligands in the lignocellulosic structure of the xylem and vein.     

Effect of heavy metal pollution on mineral absorption in sunflower (Helianthus annuus L.) hybrids

The present study was conducted in a potted experiment to examine the effects of chromium pollution on absorption of mineral nutrients and some morpho-physiological attributes of two sunflower (Helianthus annuus L.) hybrids (FH-331 and FH-259) in the presence and absence of ethylene diamine tetra acetic acid (EDTA) used as a chelating agent. Four concentrations of chromium (Cr³⁺) i.e., 0, 20, 30 and 40 mg kg^?1 with and without 0.3 g kg^?1, EDTA as chelating agent were applied to 25-day-old sunflower plants. A gradually decreasing trend in absorption of all minerals and other parameters studied were observed. Different treatments of Cr³⁺ as well as Cr³⁺ and EDTA significantly reduced root and shoot fresh weight; however, root, shoot and achene Cr³⁺ contents of two sunflowers hybrids under higher chromium and EDTA stress varied significantly whereas movement of Cr³⁺ contents to leaves was non-significant. Absorption of Na⁺, K⁺, N₂ and P through roots and shoots significantly reduced with increasing concentration of Cr³⁺ treatments. In fact addition of EDTA to the medium further enhanced the toxicity of chromium.     

Evaluation of Lupinus Species to Accumulate Heavy Metals From W aste Waters

Several Lupinus species, for example, Lupinus albus, Lupinus luteus, Lupinus angustifolius, and Lupinus hispanicus were used to accumulate Mn(II), Cd(II), Pb(II), Cr(III), Cr(VI), Hg²⁺, and CH₃Hg⁺ from waste waters. The influence of different species concentrations (50 and 100 mg L^-1) and pH on growing behavior as well as the resulting distribution of metals in the plants were investigated. The results obtained showed that lupins were able to germinate and to grow in the presence of the metals mentioned above, even when they were present at levels as high as 50 mg L^-1. Accumulation of Pb(II), Cr(III), and Cd(II) was higher in roots than in shoots. As far as mercury is concerned, the highest CH₃Hg and Hg²⁺ accumulation was detected in roots, but fast transport toward the leaves was noticed. In contrast to mercury, the uptake of chromium seems to be influenced by the chemical form of the analyte, remaining Cr(VI) in solution. No differences in growing behavior and accumulation were observed for the four Lupinus species studied. Even though plants were exposed only a relatively short time to the metal solutions, metal concentrations of approximately 2 g/kg of dry matter were detected in the young lupins plants. The feasibility of utilizing Lupinus plants for the removal of heavy metals from wastewater was also investigated. Lupins were able to grow under extreme conditions (wastewater, pH lower than 2) and to remove 98% of the initial amount of toxic metals present in the sample.     

Development of a Multi-Species Biotic Ligand Model Predicting the Toxicity of Trivalent Chromium to Barley Root Elongation in Solution Culture

Little knowledge is available about the influence of cation competition and metal speciation on trivalent chromium (Cr(III)) toxicity. In the present study, the effects of pH and selected cations on the toxicity of trivalent chromium (Cr(III)) to barley (Hordeum vulgare) root elongation were investigated to develop an appropriate biotic ligand model (BLM). Results showed that the toxicity of Cr(III) decreased with increasing activity of Ca²⁺ and Mg²⁺ but not with K⁺ and Na⁺. The effect of pH on Cr(III) toxicity to barley root elongation could be explained by H⁺ competition with Cr³⁺ bound to a biotic ligand (BL) as well as by the concomitant toxicity of CrOH²⁺ in solution culture. Stability constants were obtained for the binding of Cr³⁺, CrOH²⁺, Ca²⁺, Mg²⁺ and H⁺ with binding ligand: log K_CrBL 7.34, log K_CrOHBL 5.35, log K_CaBL 2.64, log K_MgBL 2.98, and log K_HBL 4.74. On the basis of those estimated parameters, a BLM was successfully developed to predict Cr(III) toxicity to barley root elongation as a function of solution characteristics.     

Chromium interference in iron nutrition and water relations of cabbage

Cabbage (Brassica oleracea var. capitata cv. Snowball), known to be responsive to potentially toxic elements, was investigated for chromium (Cr³⁺) effect on iron metabolism and water relations. After 6 weeks growth in sand culture, a set of plants was supplied with 500 μM Cr³⁺ (CrCl₃), superimposed over the full nutrient solution (control). Exposure to excess Cr³⁺ led to increased accumulation of Cr, more in roots than in leaves, and to the development of toxicity symptoms. In decreasing chlorophyll concentration and the activities of heme enzymes, catalase and peroxidase, the excess Cr³⁺ effect resembled Fe deficiency. These changes, associated with decrease in Fe accumulation in Cr³⁺ treated plants, indicate that by reducing absorption of Fe, Cr³⁺ impairs the Fe requiring steps of chlorophyll and heme biosynthesis. In spite of lower water saturation deficit, the leaves of Cr³⁺ treated plants showed decrease in leaf water potential, associated with increase in diffusive resistance and lowering of transpiration rate, indicating development of water stress. Enhanced accumulation of proline in Cr³⁺ treated plants also suggested this. Observed changes in water stress parameters in Cr³⁺ stressed plants indicate that plant exposure to excess supply of Cr³⁺ reduces the physiological availability of water.     

Short-term chromium(VI) stress induces different photosynthetic responses in two duckweed species, <Emphasis Type="Italic">Lemna gibba</Emphasis> L. and <Emphasis Type="Italic">Lemna mino</Emphasis>r L.

Physiological responses of two duckweed species, Lemna gibba and Lemna minor, to hexavalent chromium [Cr(VI)] were studied in axenic cultures using short-term (48 h) treatments by K₂Cr₂O₇ (0–200 μM). Chlorophyll (Chl) fluorescence parameters and photosynthetic pigment composition of plants were screened to determine the effects of Cr(VI) exposures. The two duckweed species exhibited different sensitivity in the applied Cr(VI) concentration range. Chl fluorescence parameters of dark-adapted and light-adapted plants and electron transport inducibility were more sensitive to Cr(VI) in L. minor than in L. gibba. We also found fundamental differences in quantum yield of regulated, Y(NPQ), and nonregulated, Y(NO), non-photochemical quenching between the two species. As Cr(VI) concentration increased in the growth medium, L. minor responded with considerable increase of Y(NPQ) with a parallel significant increase of Y(NO). By contrast, in L. gibba only 200 μM Cr(VI) in the growth medium resulted in elevation of Y(NPQ) while Y(NO) remained more or less constant within the regarding Cr(VI) concentration range during 48 h. Photosynthetic pigment content did not change considerably during the short-term Cr(VI) treatment but decrease of Chl a/b and increase of Car/Chl ratios were observed in good accordance with the changes in Chl fluorescence parameters. The data suggest that various duckweed species respond with different sensitivity to the same ambient concentrations of Cr(VI) in the growth medium, and presumably to other environmental stresses too, which may have an influence on their competitive relations when heavy metal pollution occurs in aquatic ecosystem.     

Chromium accumulation, translocation and chemical speciation in vegetable crops

Trivalent chromium (Cr³⁺) is essential for animal and human health, whereas hexavalent Cr (CrO₄ ²⁻) is a potent carcinogen and extremely toxic to animals and humans. Thus, the accumulated Cr in food plants may represent potential health hazards to animals and humans if the element is accumulated in the hexavalent form or in high concentrations. This study was conducted to determine the extent to which various vegetable crops absorb and accumulate Cr³⁺ and CrO₄ ²⁻ into roots and shoots and to ascertain the different chemical forms of Cr in these tissues. Two greenhouse hydroponic experiments were performed using a recirculating-nutrient culture technique that allowed all plants to be equally supplied with Cr at all times. In the first experiment, 1 mg L⁻¹ Cr was supplied to 11 vegetable plant species as Cr³⁺ or CrO₄ ²⁻, and the accumulation of Cr in roots and shoots was compared. The crops tested included cabbage (Brassica oleracea L. var. capitata L.), cauliflower (Brassica oleracea L. var. botrytis L.), celery (Apium graveolens L. var. dulce (Mill.) Pers.), chive (Allium schoenoprasum L.), collard (Brassica oleracea L. var. acephala DC.), garden pea (Pisum sativum L.), kale (Brassica oleracea L. var. acephala DC.), lettuce (Lactuca sativa L.), onion (Allium cepa L.), spinach (Spinacia oleracea L.), and strawberry (Fragaria ×  ananassaDuch.). In the second experiment, X-ray absorption spectroscopy (XAS) analysis on Cr in plant tissues was performed in roots and shoots of various vegetable plants treated with CrO₄ ²⁻ at either 2 mg Cr L⁻¹ for 7 d or 10 mg Cr L⁻¹ for 2, 4 or 7 d. The crops used in this experiment included beet (Beta vulgaris L. var. crassa (Alef.) J. Helm), broccoli (Brassica oleracea L. var. Italica Plenck), cantaloupe (Cucumis melo L. gp. Cantalupensis), cucumber (Cucumis sativus L.), lettuce, radish (Raphanus sativus L.), spinach, tomato (Lycopersicon lycopersicum (L.) Karsten), and turnip (Brassica rapa L. var. rapifera Bailey). The XAS speciation analysis indicates that CrO₄ ²⁻ is converted in the root to Cr³⁺ by all plants tested. Translocation of both Cr forms from roots to shoots was extremely limited and accumulation of Cr by roots was 100-fold higher than that by shoots, regardless of the Cr species supplied. Highest Cr concentrations were detected in members of the Brassicaceae family such as cauliflower, kale, and cabbage. Based on our observations and previous findings by other researchers, a hypothesis for the differential accumulation and identical translocation patterns of the two Cr ions is proposed. Received: 27 February 1998 / Accepted: 2 April 1998     

Biosorption of chromium(III) by two brown algae Macrocystis pyrifera and Undaria pinnatifida: Equilibrium and kinetic study

Two brown algae, Macrocystis pyrifera and Undaria pinnatifida, were employed to remove Cr(III) from aqueous solutions. Both seaweeds were characterized in terms of alginate yields. The alginate contents were 20 and 30% of the dry weight for M. pyrifera and U. pinnatifida, respectively. Kinetics experiments were carried out at different initial pH values. Cr(III) biosorption was affected by the solution pH. The highest metal uptake was found at pH 4 for both biosorbents. Different models were applied to elucidate the rate‐controlling mechanism: pseudo‐first‐order, pseudo‐second‐order, external mass transfer and intra‐particle diffusion. The application of Langmuir, Freundlich and Dubinin–Radushkevich models to the equilibrium data showed a better fitting to the first model. The maximum Cr(III) sorption capacity (q_m) and the affinity coefficient (b) were very similar for both biosorbents: 0.77 mmol/g and 1.20 L/mmol for M. pyrifera and 0.74 mmol/g and 1.06 L/mmol for U. pinnatifida. The free energy of the sorption process was estimated using the Dubinin–Radushkevich isotherm. The values indicate that the processes are chemical sorptions. To evaluate the significance of the ion‐exchange mechanism, the light metals (Ca²⁺, Na⁺, Mg²⁺ and K⁺) and pH were measured during the experiments.