Nickel: an overview of uptake, essentiality and toxicity in plants

Valorisation possibilities of exhausted biosorbents loaded with metal ions - A review

Biosorption is considered one of the most promising methods for removal of metal ions from aqueous effluents, due to its low-cost and eco-friendly characteristics. However, the exhausted biosorbents loaded with metal ions, obtained at the end of biosorption processes, are still a problem which should be solved to increase the applicability of biosorption on an industrial scale. In this study are examined three possibilities for the valorisation of exhausted biosorbents loaded with metal ions, namely: (i) regeneration and reuse of biosorbents in multiple biosorption cycles, (ii) the use of exhausted biosorbents as fertilizers for soils poor in essential microelements, and (iii) the pyrolysis of exhausted biosorbents, under well defined conditions. The main advantages and disadvantages of each valorisation possibility are reviewed in order to find the best way to use these cheap materials in accordance with the principles of the circular economy and thereby contributing to the development of sustainable biosorption technology.

High-potential accumulation and tolerance in the submerged hydrophyte Hydrilla verticillata (L.f.) Royle for nickel-contaminated water

Water contamination by nickel (Ni) has become an increasing concern in recent decades. Hydrilla verticillata (L.f.) Royle has been recognized as a promising accumulator of several potentially toxic elements (PTEs) in phytoremediation, but its Ni-accumulation characteristics and its mechanisms of tolerance to Ni remain largely unknown. This research investigated the biochemical responses of leaves and stems of H. verticillata to various concentrations of Ni (5, 10, 15, 20, and 40 μM) over periods of 7, 14, or 21 days. Plants accumulated considerable Ni to a maximum amount of 1080 mg kg^-1 dry weight (DW) with a maximum bioconcentration factor of 1100; thus, high Ni accumulation was detected in H. verticillata. Low concentrations (5-15 μM) or short durations (less than 14 days) of Ni exposure might promote plant growth without adversely affecting normal metabolism. After peaking at day 14, a decline in bioaccumulation was unexpectedly observed as a long-term effect of Ni toxicity. Malondialdehyde content and the activities of defense-related enzymes changed in a similar pattern after treatment with Ni, increasing with both Ni concentration and exposure time to a peak (often at 5-15 μM on day 14), followed by a decline. Through a comprehensive analysis of all the test parameters, the tolerance thresholds were determined to be > 40.0 μM, 24.0 μM, and 15.8 μM at days 7, 14, and 21, respectively. Hydrilla verticillata could be a "high-potential accumulator" capable of decontaminating aquatic bodies polluted by Ni within the threshold range.

Ethylenediamine disuccinic acid enhanced phytoextraction of nickel from contaminated soils using Coronopus didymus (L.) Sm

In a screenhouse, the applicability of biodegradable chelant ethylenediamine disuccinic acid (EDDS) to enhance Ni-phytoextraction by Coronopus didymus was tested for the first time. This study assayed the hypothesis based upon the role of EDDS on physiological and biochemical alterations and ameliorating phytoextraction capacity of C. didymus under nickel (Ni) stress. Pot experiments were conducted for 6 weeks and C. didymus plants were cultivated in soil artificially contaminated with 30, 50, and 70 mg kg^-1 Ni treatments. Soil was amended with EDDS (2 mmol kg^-1). Plants were harvested, 1 week after EDDS application. At 70 mg kg^-1 Ni level, EDDS application dramatically enhanced the root and shoot Ni concentration from 665 and 644 to 1339 and 1338 mg kg^-1, respectively. Combination of Ni + EDDS induced alterations in biochemical parameters of plants. EDDS addition posed pessimistic effects on growth, biomass, photosynthetic activity and protein content of the plants. Besides, application of EDDS stimulated the generation of superoxide anion, H₂O₂ content and MDA level. However, EDDS assisted mount in antioxidant activities (superoxide dismutase, catalase and glutathione peroxidase) considerably neutralised the toxicity induced by reactive oxygen species in plant tissues. The results revealed EDDS efficacy to ameliorate the performance of antioxidant enzymes and improved Ni translocation in plant tissues, thus strongly marked its affinity to be used together with C. didymus for Ni-phytoextraction.

Epigenetic modifications preserve the hyperaccumulator Noccaea caerulescens from Ni geno-toxicity

The Ni hyperaccumulator Noccaea caerulescens has adapted to live in a naturally stressed environment, evolving a complex pattern of traits to cope with adverse conditions. Evidence is accumulating regarding the important role of epigenetic modifications in regulating plant responses to stress. In this study, we present data from the natural "open-field" adaptation of the Ni hyperaccumulator N. caerulescens to serpentine soil and provide the first evidence of the involvement of epigenetic changes in response to the high Ni content present in plant leaves. The alkaline comet assay revealed the integrity of the nuclei of leaf cells of N. caerulescens grown in a Ni-rich environment, while in the non-tolerant Arabidopsis thaliana exposed to Ni, the nuclei were severely damaged. DNA of N. caerulescens plants grown in situ were considerably hyper-methylated compared to A. thaliana plants exposed to Ni. In addition, qRT-PCR revealed that N. caerulescens MET1, DRM2, and HDA8 genes involved in epigenetic DNA and histone modification were up-regulated in the presence of high Ni content in leaves. Such epigenetic modifications may constitute a defense strategy that prevents genome instability and direct damage to the DNA structure by Ni ion, enabling plants to survive in an extreme environment. Further studies will be necessary to analyze in detail the involvement of DNA methylation and other epigenetic mechanisms in the complex process of metal hyperaccumulation and plants' adaptive response. Environ. Mol. Mutagen. 59:464-475, 2018. © 2018 Wiley Periodicals, Inc.

Multisurface modeling of Ni bioavailability to wheat (Triticum aestivum L.) in various soils

Continual efforts have been made to determine a simple and universal method of estimating heavy metal phytoavailability in terrestrial systems. In the present study, a mechanism-based multi-surface model (MSM) was developed to predict the partition of Ni(II) in soil-solution phases and its bioaccumulation in wheat (Triticum aestivum L.) in 19 Chinese soils with a wide range of soil properties. MSM successfully predicted the Ni(II) dissolution in 0.01 M CaCl₂ extracting solution (R² = 0.875). The two-site model for clay fraction improved the prediction, particularly for alkaline soils, because of the additional consideration of edge sites. More crucially, the calculated dissolved Ni(II) was highly correlated with the metal accumulation in wheat (R² = 0.820 for roots and 0.817 for shoots). The correlation coefficients for the MSM and various chemical extraction methods have the following order: soil pore water > MSM ≈ diffuse gradient technique (DGT) > soil total Ni > 0.43 M HNO₃ > 0.01 M CaCl₂. The results suggested that the dissolved Ni(II) calculated using MSM can serve as an effective indicator of the bioavailability of Ni(II) in various soils; hence, MSM can be used as an supplement for metal risk prediction and assessment besides chemical extraction techniques.

SiO2 nanomaterial as a tool to improve Hordeum vulgare L. tolerance to nano-NiO stress

This work was designed to assess the potential role of silicon dioxide nanomaterial (nano-SiO₂) in enhancing barley's tolerance to nickel oxide nanomaterial (nano-NiO). For this purpose, plants were grown for 14days under nano-NiO (120mgkg^-1) single and co-exposure with nano-SiO₂ (3mgkg^-1). The exposure of barley to nano-NiO caused a significant decrease in growth-related parameters and induced a negative response on the photosynthetic apparatus. However, upon nano-SiO₂ co-exposure, the inhibitory effects of nano-NiO were partially reduced, with lower reductions in fresh and dry biomass, and with the recovery of the photosynthesis-related parameters. Plants growing under nano-NiO stress showed an overproduction of superoxide anion (O₂^.-), which favored the occurrence of oxidative stress and the enhancement of lipid peroxidation (LP), but the co-treatment with nano-SiO₂ reverted this tendency, generally lowering or maintaining the levels of LP and stimulating the redox pathway of thiols. The evaluation of the antioxidant (AOX) system revealed that nano-NiO induced the accumulation of proline, along with a decrease in ascorbate in leaves. Furthermore, superoxide dismutase (SOD) activity was significantly enhanced and catalase (CAT) and ascorbate peroxidase (APX) seemed to have a pivotal role in H₂O₂ detoxification in leaves and roots, respectively. The response of the AOX system was even more prominent upon nano-SiO₂ co-exposure, reinforcing the ameliorating functions of this nanomaterial. Overall, the present study highlighted the protective role of nano-SiO₂ in barley plants under nano-NiO stress, possibly due to the Si-mediated protection against oxidative stress, by a more proactive performance of the plant AOX system.

Involvement of organic acids and amino acids in ameliorating Ni(II) toxicity induced cell cycle dysregulation in Caulobacter crescentus: a metabolomics analysis

Nickel (Ni(II)) toxicity is addressed by many different bacteria, but bacterial responses to nickel stress are still unclear. Therefore, we studied the effect of Ni(II) toxicity on cell proliferation of α-proteobacterium Caulobacter crescentus. Next, we showed the mechanism that allows C. crescentus to survive in Ni(II) stress condition. Our results revealed that the growth of C. crescentus is severely affected when the bacterium was exposed to different Ni(II) concentrations, 0.003 mM slightly affected the growth, 0.008 mM reduced the growth by 50%, and growth was completely inhibited at 0.015 mM. It was further shown that Ni(II) toxicity induced mislocalization of major regulatory proteins such as MipZ, FtsZ, ParB, and MreB, resulting in dysregulation of the cell cycle. GC-MS metabolomics analysis of Ni(II) stressed C. crescentus showed an increased level of nine important metabolites including TCA cycle intermediates and amino acids. This indicates that changes in central carbon metabolism and nitrogen metabolism are linked with the disruption of cell division process. Addition of malic acid, citric acid, alanine, proline, and glutamine to 0.015 mM Ni(II)-treated C. crescentus restored its growth. Thus, the present work shows a protective effect of these organic acids and amino acids on Ni(II) toxicity. Metabolic stimulation through the PutA/GlnA pathway, accelerated degradation of CtrA, and Ni-chelation by organic acids or amino acids are some of the possible mechanisms suggested to be involved in enhancing C. crescentus's tolerance. Our results shed light on the mechanism of increased Ni(II) tolerance in C. crescentus which may be useful in bioremediation strategies and synthetic biology applications such as the development of whole cell biosensor.

Trace metal metabolism in plants

Many trace metals are essential micronutrients, but also potent toxins. Due to natural and anthropogenic causes, vastly different trace metal concentrations occur in various habitats, ranging from deficient to toxic levels. Therefore, one focus of plant research is on the response to trace metals in terms of uptake, transport, sequestration, speciation, physiological use, deficiency, toxicity, and detoxification. In this review, we cover most of these aspects for the essential micronutrients copper, iron, manganese, molybdenum, nickel, and zinc to provide a broader overview than found in other recent reviews, to cross-link aspects of knowledge in this very active research field that are often seen in a separated way. For example, individual processes of metal usage, deficiency, or toxicity often were not mechanistically interconnected. Therefore, this review also aims to stimulate the communication of researchers following different approaches, such as gene expression analysis, biochemistry, or biophysics of metalloproteins. Furthermore, we highlight recent insights, emphasizing data obtained under physiologically and environmentally relevant conditions.

Soil ecological criteria for nickel as a function of soil properties

Scientific information for the chemistry and ecotoxicology of metals in soils has been obtained in the research conducted in recent years. However, the latest "science" obtained from this research has not yet been translated into "regulations" in China. In the present study, the predicted no effect concentrations (PNECs) for Ni which denoted as soil ecological criteria for Ni were derived based on the effects of soil properties on bioavailability/ecotoxicity of Ni, incorporating the differences in species sensitivity as well as in laboratory and field conditions. First, collected all ecotoxicity data of Ni from bioassays in Chinese soils and filtered with given criteria to obtain reliable data. Second, corrected the compiled data with either aging factor or leaching and aging factors simultaneously to eliminate the discrepancy caused by difference between laboratory and realistic field conditions. Prior to being put into a species sensitivity distribution (SSD) method to fit SSD curves, the corrected Ni ecotoxicity data were normalized with Ni ecotoxicity predictive models to modify the variation in Ni ecotoxicity caused by different soil properties. Then, the hazardous concentration for x% of the species (HCx) was figured out from the Ni SSD curve and the ecological criterion of Ni (PNEC) was set equal to HCx. Finally, predictive models for HCx were developed by quantifying the relationship between the Ni HCx and soil properties. Soil pH was the major factor controlling the values of HCx for Ni, with HC5 varying from 6.5 mg/kg in an acidic soil (pH 5.0) to 218.8 mg/kg in an alkaline soil (pH 8.5). The predictive models with parameters of soil pH and organic carbon (%) could calculate HCx with determination coefficients (R²) of 0.90-0.95. When soil cation exchange capacity was taken into account the third parameter, the predictive models could predict HCx more accurate with R² values of 0.94-0.99.

Nitric oxide induces rice tolerance to excessive nickel by regulating nickel uptake, reactive oxygen species detoxification and defense-related gene expression

Soil contamination with nickel (Ni) is a persistent threat to crop production worldwide. The present study examined the putative roles of nitric oxide (NO) in improving Ni-tolerance in rice. Our findings showed that application of exogenous sodium nitroprusside (SNP), a NO donor, significantly improved the growth performance of rice seedlings when grown under excessive Ni. The enhanced Ni-tolerance of rice prompted by SNP could be ascribed to its ability to regulate Ni uptake, decrease Ni-induced oxidative stress as evidenced by reduced levels of hydrogen peroxide, malondialdehyde, and electrolyte leakage in Ni-stressed plants. The positive roles of NO against Ni-toxicity also reflected through its protective effects on photosynthetic pigments, soluble proteins and proline. SNP also boosted antioxidant capacity in Ni-stressed plants by maintaining increased levels of ascorbate, enhanced activities of ROS-detoxifying enzymes, particularly peroxidase (POD) and catalase (CAT) in both roots and shoots compared with Ni-stressed alone plants. Moreover, SNP treatment also upregulated the transcript levels of CAT, POD, ascorbate peroxidase, glutathione reductase and superoxide dismutase genes in shoots under Ni-stress. Using different sulfide compounds and NO scavenger cPTIO, we also provided evidence that NO, rather than other byproducts of SNP, contributed to the improved performance of rice seedlings under Ni-stress. Collectively, our results conclude that exogenous SNP-mediated modulation of endogenous NO enhanced rice tolerance to Ni-stress by restricting Ni accumulation, maintaining photosynthetic performance and reducing oxidative damage through improved antioxidant system, thereby suggesting NO as an effective stress regulator in mitigating Ni-toxicity in economically important rice, and perhaps in other crop plants.

Hidden Nickel Deficiency? Nickel Fertilization via Soil Improves Nitrogen Metabolism and Grain Yield in Soybean Genotypes

Nickel (Ni)-a component of urease and hydrogenase-was the latest nutrient to be recognized as an essential element for plants. However, to date there are no records of Ni deficiency for annual species cultivated under field conditions, possibly because of the non-appearance of obvious and distinctive symptoms, i.e., a hidden (or latent) deficiency. Soybean, a crop cultivated on soils poor in extractable Ni, has a high dependence on biological nitrogen fixation (BNF), in which Ni plays a key role. Thus, we hypothesized that Ni fertilization in soybean genotypes results in a better nitrogen physiological function and in higher grain production due to the hidden deficiency of this micronutrient. To verify this hypothesis, two simultaneous experiments were carried out, under greenhouse and field conditions, with Ni supply of 0.0 or 0.5 mg of Ni kg^-1 of soil. For this, we used 15 soybean genotypes and two soybean isogenic lines (urease positive, Eu3; urease activity-null, eu3-a, formerly eu3-e1). Plants were evaluated for yield, Ni and N concentration, photosynthesis, and N metabolism. Nickel fertilization resulted in greater grain yield in some genotypes, indicating the hidden deficiency of Ni in both conditions. Yield gains of up to 2.9 g per plant in greenhouse and up to 1,502 kg ha^-1 in field conditions were associated with a promoted N metabolism, namely, leaf N concentration, ammonia, ureides, urea, and urease activity, which separated the genotypes into groups of Ni responsiveness. Nickel supply also positively affected photosynthesis in the genotypes, never causing detrimental effects, except for the eu3-a mutant, which due to the absence of ureolytic activity accumulated excess urea in leaves and had reduced yield. In summary, the effect of Ni on the plants was positive and the extent of this effect was controlled by genotype-environment interaction. The application of 0.5 mg kg^-1 of Ni resulted in safe levels of this element in grains for human health consumption. Including Ni applications in fertilization programs may provide significant yield benefits in soybean production on low Ni soil. This might also be the case for other annual crops, especially legumes.

Nickel accumulation and its effect on growth, physiological and biochemical parameters in millets and oats

With the boom in industrialization, there is an increase in the level of heavy metals in the soil which drastically affect the growth and development of plants. Nickel is an essential micronutrient for plant growth and development, but elevated level of Ni causes stunted growth, chlorosis, nutrient imbalance, and alterations in the defense mechanism of plants in terms of accumulation of osmolytes or change in enzyme activities like guiacol peroxidase (POD), catalase (CAT), and superoxide dismutase (SOD). Ni-induced toxic response was studied in seedlings of finger millet, pearl millet, and oats in terms of seedling growth, lipid peroxidation, total chlorophyll, proline content, and enzymatic activities. On the basis of germination and growth parameters of the seedling, finger millet was found to be the most tolerant. Nickel accumulation was markedly lower in the shoots as compared to the roots, which was the highest in finger millet and the lowest in shoots of oats. Plants treated with a high concentration of Ni showed significant reduction in chlorophyll and increase in proline content. Considerable difference in level of malondialdehyde (MDA) content and activity of antioxidative enzymes indicates generation of redox imbalance in plants due to Ni-induced stress. Elevated activities of POD and SOD were observed with high concentrations of Ni while CAT activity was found to be reduced. It was observed that finger millet has higher capability to maintain homeostasis by keeping the balance between accumulation and ROS scavenging system than pearl millet and oats. The data provide insight into the physiological and biochemical changes in plants adapted to survive in Ni-rich environment. This study will help in selecting the more suitable crop species to be grown on Ni-rich soils.

Application of Bacillus megaterium MCR-8 improved phytoextraction and stress alleviation of nickel in Vinca rosea

The current research was performed to evaluate the effect of Bacillus megaterium MCR-8 on mitigation of nickel (Ni) stress in Vinca rosea grown on Ni-contaminated soil (50, 100, and 200 mg Ni kg^-1 soil). The treated plants exhibited reduced growth, biomass, gas exchange capacity, and chlorophyll (Chl) content under Ni stress. The inoculated plants growing in Ni-contaminated media exhibited relatively higher growth, total soluble protein, and proline contents. Similarly, bacterial inoculation improved the activity of antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX) under Ni stress. The Ni stress alleviation in inoculated plants was attributed to the reduced level of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂), enhanced synthesis of protein, proline, phenols, and flavonides in conjunction with improved activity of antioxidant enzymes. The growth-promoting characteristics of microbe such as 1-aminocyclopropane-1-carboxylate deaminase (ACCD) and phosphate solubilization activity, siderophore, and auxin production capability also improved the growth and stress mitigation in inoculated plants. Furthermore, the inoculated plants exhibited higher value for bioconcentration factor (BCF), translocation factor (TF), and resulted in higher loss of Ni content from soil. The current results exhibited the beneficial role of B. megaterium MCR-8 regarding stress alleviation and Ni phytoextraction by V. rosea.

Nickel stressed responses of rice in Ni subcellular distribution, antioxidant production, and osmolyte accumulation

Nickel has been found a key pollutant in farmlands of central and south China, and understanding of Ni toxicity in rice is of great significance in safety production of rice and remediation of Ni polluted paddy soils. The present study aimed to investigate the uptake and subcellular distribution of Ni, antioxidant production, and osmolyte accumulation of rice (Oryza sativa L., cv. yangliangyou 6) plants exposed to excessive Ni concentrations to gain an insight into Ni-induced phytotoxicity. Results revealed that exposure of rice seedlings to high Ni concentrations resulted a decline in root and shoot lengths and fresh weight (FW) and dry weight (DW) of rice plants, which are in connection with the depletion of the contents of photosynthetic pigments. Measurement of Ni concentrations in the roots and shoots showed that Ni was mainly accumulated in roots followed by shoots. Moreover, Ni was mainly deposited in soluble fraction and cell wall, than cell organelle, which suggests that both compartments act as crucial defensive barriers against Ni toxicity in rice plants. Ni also induced its toxicity by damaging oxidative metabolism, as indicated by increased level of hydrogen peroxide and malondialdehyde content. Furthermore, Ni stress also showed a desynchronized antioxidant system by increasing the activities of catalase, peroxidase, and the contents of ascorbic acid and glutathione, whereas decreasing the activity of superoxide dismutase in the roots and shoots of rice plants. Ni stress also triggered the rate of proline accumulation and decreasing the contents of soluble protein and soluble sugar. In crux, our results suggests that excessive Ni inhibited rice growth and induced oxidative stress through inducing ROS formation, while stimulated enzymatic and non-enzymatic antioxidants system appeared as adaptive mechanisms of rice plants against Ni-induced oxidative stress. Furthermore, majority of Ni was located in soluble fraction and modulation in osmolyte accumulation under Ni stress seemed to provide additional defense against oxidative stress.

Microwave irradiation and citric acid assisted seed germination and phytoextraction of nickel (Ni) by Brassica napus L.: morpho-physiological and biochemical alterations under Ni stress

The complex bio-geochemistry of soil allows pollutant to persist for a longer period of time which further decreased the fertility and natural composition of land. Nickel, an inorganic pollutant, coming from a wide range of industrial and manufacturing units possesses serious threat to soil degradation and crop productivity around the world. The present study was carried to evaluate the combined role of microwave irradiation (MR) and citric acid (CA) on the phytoextraction potential of Brassica napus L. under Ni stress. An initial seed germination test was conducted to select effective time scale of MR exposure. Highest seed germination was observed at exposure of 2.45 GHz frequency for 30 s. Healthy seeds of B. napus L. genotype Faisal Canola (RBN-03060) treated with MR at 2.45 GHz for 30 s were sown in plastic pots filled with 5 kg of soil. Nickel and CA applied exogenously in solution form with different combinations to both MR-treated and untreated B. napus plants. The MR-treated plants showed higher growth, biomass, photosynthetic pigments (Chl a, b, total, and carotenoids) and activities of antioxidant enzymes (SOD, POD, APX, CAT) as compared to untreated plants who showed higher reactive oxygen species (MDA, H₂O₂) and electrolyte leakage. Increasing Ni concentration significantly decreased the physiological and biochemical attributes of B. napus both in MR-treated and untreated plants. The addition of CA alleviated Ni-induced toxic effects in both MR-treated and untreated plants by improving antioxidant defense system. The degree of Ni stress mitigation was higher in MR-treated plants. The Ni concentration was higher in root, stem, and leaves of MR-treated plants under CA application as compared to untreated plants. The present study concluded that seeds treated with MR before sowing showed higher accumulation and concentration of Ni from soil, and this phenomenon boosted with the application of CA.

Redox chemistry of nickel in soils and sediments: A review

Knowledge on the redox geochemistry of Ni is behind in comparison to other heavy metals. Hence, this article reviews the direct and indirect impact of redox potential (E_H) on mobilization and release dynamics of Ni in soils and sediments across the world. Nickel can show a different behavior in response to E_H. Mobilization of Ni increased at low E_H in various soils; however, oxic conditions can lead to an increased mobilization of Ni in other soils. Those differences occur because the mobilization of Ni is often indirectly affected by E_H, e.g. through E_H-dependent pH changes, co-precipitation with iron (Fe) and manganese (Mn) (hydr)oxides, complexation with soil organic carbon, similar position of Ni and magnesium (Mg) in the soil solid phase, and/or precipitation as sulphides. Dissolved concentrations of Ni showed a similar pattern like Fe and increased at low E_H in many soils, which might be explained by the reductive dissolution of Fe (hydr)oxides and the release of the co-precipitated/sorbed Ni. Few other studies indicated that Ni might be associated with Mn oxides rather than with Fe oxides. Additionally, the formation of soluble complexes with dissolved organic carbon may contribute to a mobilization of Ni at low E_H. Nickel and Mg are similarly affected by redox changes especially in serpentine soils. This review summarizes the recent knowledge about the redox chemistry of Ni and contributes thus to a better understanding of the potential mobilization, hazard, and eco-toxicity of Ni in frequently flooded soils and sediments as agricultural ecosystems.

Endophytic Paecilomyces formosus LHL10 Augments Glycine max L. Adaptation to Ni-Contamination through Affecting Endogenous Phytohormones and Oxidative Stress

This study investigated the Ni-removal efficiency of phytohormone-producing endophytic fungi Penicillium janthinellum, Paecilomyces formosus, Exophiala sp., and Preussia sp. Among four different endophytes, P. formosus LHL10 was able to tolerate up to 1 mM Ni in contaminated media as compared to copper and cadmium. P. formosus LHL10 was further assessed for its potential to enhance the phytoremediation of Glycine max (soybean) in response to dose-dependent increases in soil Ni (0.5, 1.0, and 5.0 mM). Inoculation with P. formosus LHL10 significantly increased plant biomass and growth attributes as compared to non-inoculated control plants with or without Ni contamination. LHL10 enhanced the translocation of Ni from the root to the shoot as compared to the control. In addition, P. formosus LHL10 modulated the physio-chemical apparatus of soybean plants during Ni-contamination by reducing lipid peroxidation and the accumulation of linolenic acid, glutathione, peroxidase, polyphenol oxidase, catalase, and superoxide dismutase. Stress-responsive phytohormones such as abscisic acid and jasmonic acid were significantly down-regulated in fungal-inoculated soybean plants under Ni stress. LHL10 Ni-remediation potential can be attributed to its phytohormonal synthesis related genetic makeup. RT-PCR analysis showed the expression of indole-3-acetamide hydrolase, aldehyde dehydrogenase for indole-acetic acid and geranylgeranyl-diphosphate synthase, ent-kaurene oxidase (P450-4), C13-oxidase (P450-3) for gibberellins synthesis. In conclusion, the inoculation of P. formosus can significantly improve plant growth in Ni-polluted soils, and assist in improving the phytoremediation abilities of economically important crops.

The mechanisms of nickel toxicity in aquatic environments: An adverse outcome pathway analysis

Current ecological risk assessment and water quality regulations for nickel (Ni) use mechanistically based, predictive tools such as biotic ligand models (BLMs). However, despite many detailed studies, the precise mechanism(s) of Ni toxicity to aquatic organisms remains elusive. This uncertainty in the mechanism(s) of action for Ni has led to concern over the use of tools like the BLM in some regulatory settings. To address this knowledge gap, the authors used an adverse outcome pathway (AOP) analysis, the first AOP for a metal, to identify multiple potential mechanisms of Ni toxicity and their interactions with freshwater aquatic organisms. The analysis considered potential mechanisms of action based on data from a wide range of organisms in aquatic and terrestrial environments on the premise that molecular initiating events for an essential metal would potentially be conserved across taxa. Through this analysis the authors identified 5 potential molecular initiating events by which Ni may exert toxicity on aquatic organisms: disruption of Ca²⁺ homeostasis, disruption of Mg²⁺ homeostasis, disruption of Fe^2+/3+ homeostasis, reactive oxygen species-induced oxidative damage, and an allergic-type response of respiratory epithelia. At the organ level of biological organization, these 5 potential molecular initiating events collapse into 3 potential pathways: reduced Ca²⁺ availability to support formation of exoskeleton, shell, and bone for growth; impaired respiration; and cytotoxicity and tumor formation. At the level of the whole organism, the organ-level responses contribute to potential reductions in growth and reproduction and/or alterations in energy metabolism, with several potential feedback loops between each of the pathways. Overall, the present AOP analysis provides a robust framework for future directed studies on the mechanisms of Ni toxicity and for developing AOPs for other metals. Environ Toxicol Chem 2017;36:1128-1137. © 2016 SETAC.

Bioassessment of heavy metals in the surface soil layer of an opencast mine aimed for its rehabilitation

The contemporary reclamation method in an opencast coal mine closure comprises the use of the preserved surface soil layer (SSL) before mining, and can be directly returned to the areas being rehabilitated. The present study emphasizes a risk in the use of such a SSL in mine rehabilitation due to the possible excessive amount of heavy metals which usually derives from a metal-rich sediment or fluvial character of overburden material. This indication was approved by the bioassessment of cadmium (Cd), chromium (Cr), cobalt (Co), copper (Cu), lead (Pb), nickel (Ni), and zinc (Zn) in root and aerial parts of maize (Zea mays), alfalfa (Medicago sativa), sweet clover (Melilotus officinalis), wheat (Triticum aestivum), barley (Hordeum sativum), white clover (Trifolium repens), pasture (Poales sp.), tomato (Solanum lycopersicum), potato (Solanum tuberosum) and carrot (Daucus carota) grown on SSL in the opencast mine area. The fluvial layers of the investigated mine SSL revealed the excessive existence of Ni and Cr, probably of geogenic origin, according to the X-ray diffraction (XRD) which detected Ni- and Cr-bearing minerals in soil fractions. In addition, the highest residual fraction of these two heavy metals, obtained by sequential extraction analyses, together with all other tested soil parameters, supported this assumption. Nevertheless, the accumulations of Cr in tomato fruit (2.93 mg kg^-1), potato tuber (5.89 mg kg^-1) and carrot root (7.35 mg kg^-1) grown on the investigated SSL were found to exceed a critical level of this element for human nutrition. However, despite the evident excess of Ni in the investigated SSL, a similar trend was not found in edible part of plants. The transfer and mobility of the investigated metals was evaluated using the accumulation factor (AF < 1.0) where the root were the preferential organ for the storage of heavy metals. This investigation could bring an important input for its acceptability of use in soil restoration after mining for food/fodder production, or it could indicate the potential risks of the presence of heavy metals regarding its possible use in improving the human surrounding.

Aided phytostabilization of a trace element-contaminated technosol developed on steel mill wastes

Aided phytostabilization of a barren, alkaline metal(loid)-contaminated technosol developed on steel mill wastes, with high soluble Cr and Mo concentrations, was assessed in a pot experiment using (1) Ni/Cd-tolerant populations of Festuca pratensis Huds., Holcus lanatus L., and Plantago lanceolata L. sowed in mixed stand and (2) six soil treatments: untreated soil (UNT), ramial chipped wood (RCW, 500m³ha^-1), composted sewage sludge (CSS, 120t DW ha^-1), UNT soil amended with compost (5% w/w) and either vermiculite (5%, VOM) or iron grit (1%, OMZ), and an uncontaminated soil (CTRL). In the CSS soil, pH and soluble Cr decreased whereas soluble Cu, K, Fe, Mn, Mg, Ni and P increased. The RCW treatment enhanced soluble Fe, Mn, and Mg concentrations. After 15 weeks, shoot DW yield and shoot Cd, Cu, Fe, Mn, Mo, Zn, and Mg removals peaked for F. pratensis grown on the CSS soil, with lowest shoot Cr, Ni and Mo concentrations. Holcus lanatus only grew on the CTRL, UNT, and CSS soils and P. lanceolata on the CTRL soil. Best treatment, F. pratensis grown on the CSS soil, led to a dense grass cover but its shoot Mo concentration exceeded the maximum permitted concentration in forage.

Ecotoxicological relevance of nano-NiO and acetaminophen to Hordeum vulgare L.: Combining standardized procedures and physiological endpoints

The present work aimed to assess the ecotoxicological relevance of acetaminophen (AC) and nickel oxide nanomaterial (nano-NiO) to barley plants. Combining standard procedures and several biochemical determinations, a global approach regarding the biological effects of these two contaminants was performed. After 14 days of growth, the exposure of barley to increased concentrations (0, 87.8, 131.3, 197.5, 296.5, 444.4, 666.6, and 1000 mg kg^-1) of each contaminant resulted in a marked decrease in biomass production and biometric parameters. Photosynthetic pigments and markers of oxidative stress were analyzed to assess if any of the treatments interfered with the physiological performance and with the cellular redox state. Our observations revealed that only nano-NiO induced a negative response in total chlorophylls and carotenoids, confirming the macroscopic phytotoxicity symptoms (chlorosis). However, both contaminants led to a significant increase in lipid peroxidation (LP), superoxide anion (O₂^.-), and cell death for all the tested concentrations, suggesting that AC and nano-NiO cause oxidative stress in barley, even at the lowest applied dose (87.8 mg kg^-1). Comparing the two studied approaches (parameters included in standard protocols and several biochemical determinations), it is concluded that the inclusion of several biochemical endpoints, especially those related to oxidative stress, resulted in a more sensitive analysis and thus, a more sensitive risk evaluation of these two contaminants for barley plants.

The Effect of Cd, Zn and Fe on Seed Germination and Early Seedling Growth of Wheat and Bean

This study was conducted to evaluate the effect of metals on wheat and bean species. The method uses seed germination and early seedling growth of these plants in the presence of various levels (10, 50, 100, 250, 500, 750 and 1000 mg/L) of Cadmium (Cd), Iron (Fe) and Zinc (Zn). The inhibition caused by these metals was depending on the concentration used, the metal itself and the plant species. The species had reduced seed germination, root and shoot lengths, tolerance index and percentphyto-toxicity with increasing concentrations of metals. Cadmium was determined to be the most inhibitory metal on these parameters. This metal affected significantly the germination, root and shoot length of the species tested, as well as the tolerance index and percentphytotoxicity starting from 50 mg Cd/l. Under the Iron stress, in general, the inhibition of germination and root length of wheat was reduced from 500 mg Fe/l. The results showed also that the inhibitory effect of increase of Zn levels was seen in root, shoot and tolerance indices. The findings also revealed that the metal toxicity was as follow: Cd > Fe > Zn. Regarding species, the results showed that bean seemed to be more tolerant to the increase of the three metals than wheat.

Essential and Beneficial Trace Elements in Plants, and Their Transport in Roots: a Review

The essentiality of 14 mineral elements so far have been reported in plant nutrition. Eight of these elements were known as micronutrients due to their lower concentrations in plants (usually ≤100 mg/kg/dw). However, it is still challenging to mention an exact number of plant micronutrients since some elements have not been strictly proposed yet either as essential or beneficial. Micronutrients participate in very diverse metabolic processes, including from the primary and secondary metabolism to the cell defense, and from the signal transduction to the gene regulation, energy metabolism, and hormone perception. Thus, the attempt to understand the molecular mechanism(s) behind their transport has great importance in terms of basic and applied plant sciences. Moreover, their deficiency or toxicity also caused serious disease symptoms in plants, even plant destruction if not treated, and many people around the world suffer from the plant-based dietary deficiencies or metal toxicities. In this sense, shedding some light on this issue, the 13 mineral elements (Fe, B, Cu, Mn, Mo, Si, Zn, Ni, Cl, Se, Na, Al, and Co), required by plants at trace amounts, has been reviewed with the primary focus on the transport proteins (transporters/channels) in plant roots. So, providing the compiled but extensive information about the structural and functional roles of micronutrient transport genes/proteins in plant roots.

Ni exposure impacts the pool of free Fe and modifies DNA supercoiling via metal-induced oxidative stress in Escherichia coli K-12

The biology of nickel has been widely studied in mammals because of its carcinogenic properties, whereas few studies have been performed in microorganisms. In the present work, changes accompanying stress caused by nickel were evaluated at the cellular level using RNA-Seq in Escherichia coli K-12. Interestingly, a very large number of genes were found to be deregulated by Ni stress. Iron and oxidative stress homeostasis maintenance were among the most highly enriched functional categories, and genes involved in periplasmic copper efflux were among the most highly upregulated. These results suggest that the deregulation of Fe and Cu homeostatic genes is caused by a release of free Cu and Fe ions in the cell which in turn activate the Cu and Fe homeostatic systems. The content of Cu was not significantly affected upon the addition of Ni to the growth medium, nor were the Cus and CopA Cu-efflux systems important for the survival of bacteria under Ni stress In contrast the addition of Ni slightly decreased the amount of cellular Fe and activated the transcription of Fur regulated genes in a Fur-dependent manner. Cu or Fe imbalance together with oxidative stress might affect the structure of DNA. Further experiments revealed that Ni alters the state of DNA folding by causing a relaxed conformation, a phenomenon that is reversible by addition of the antioxidant Tiron or the Fe chelator Dip. The Tiron-reversible DNA relaxation was also observed for Fe and to a lesser extent with Cu but not with Co. DNA supercoiling is well recognized as an integral aspect of gene regulation. Moreover our results show that Ni modifies the expression of several nucleoid-associated proteins (NAPs), important agents of DNA topology and global gene regulation. This is the first report describing the impact of metal-induced oxidative on global regulatory networks.

The impact of Ni on the physiology of a Mediterranean Ni-hyperaccumulating plant

High nickel (Ni) levels exert toxic effects on plant growth and plant water content, thus affecting photosynthesis. In a pot experiment, we investigated the effect of the Ni concentration on the physiological characteristics of the Ni hyperaccumulator Alyssoides utriculata when grown on a vermiculite substrate in the presence of different external Ni concentrations (0-500 mg Ni L(-1)). The results showed that the Ni concentration was higher in leaves than in roots, as evidenced by a translocation factor = 3 and a bioconcentration factor = 10. At the highest concentration tested (500 mg Ni L(-1)), A. utriculata accumulated 1100 mg Ni per kilogram in its leaves, without an effects on its biomass. Plant water content increased significantly with Ni accumulation. Ni treatment did not, or only slightly, affected chlorophyll fluorescence parameters. The photosynthetic efficiency (FV/FM) of A. utriculata was stable between Ni treatments (always ≥ 0.8) and the photosynthetic performance of the plant under Ni stress remained high (performance index = 1.5). These findings support that A. utriculata has several mechanisms to avoid severe damage to its photosynthetic apparatus, confirming the tolerance of this species to Ni under hyperaccumulation.

The effects of Ni(2+) on electrical signaling of Nitellopsis obtusa cells

The effect of nickel (Ni) on the generation of plant bioelectrical signals was evaluated in Nitellopsis obtusa, a Characean model organism. Conventional glass-microelectrode technique and K(+)-anaesthesia method in current-clamp and voltage-clamp modes were used for the measurement and analysis of electrical parameters. Ni(2+) treatment rapidly influenced the action potential (AP) parameters namely, excitation threshold, AP peak and duration, membrane potential at various voltages and dynamics of ion currents. We conclude that altered electrical signaling pathway in the test organism constituted the early target for Ni toxicity imposition. The observed Ni interference could be ascribed to disturbed [Ca(2+)]cyt content, impaired Cl(-) and K(+) channels activity resulting in decreased excitability and repolarization rate in generated AP.

Implication of citrate, malate and histidine in the accumulation and transport of nickel in Mesembryanthemum crystallinum and Brassica juncea

Citrate, malate and histidine have been involved in many processes including metal tolerance and accumulation in plants. These molecules have been frequently reported to be the potential nickel chelators, which most likely facilitate metal transport through xylem. In this context, we assess here, the relationship between organics acids and histidine content and nickel accumulation in Mesembryanthemum crystallinum and Brassica juncea grown in hydroponic media added with 25, 50 and 100 µM NiCl2. Results showed that M. crystallinum is relatively more tolerant to Ni toxicity than B. juncea. For both species, xylem transport rate of Ni increased with increasing Ni supply. A positive correlation was established between nickel and citrate concentrations in the xylem sap. In the shoot of B. juncea, citric and malic acids concentrations were significantly higher than in the shoot of M. crystallinum. Also, the shoots and roots of B. juncea accumulated much more histidine. In contrast, a higher root citrate concentration was observed in M. crystallinum. These findings suggest a specific involvement of malic and citric acid in Ni translocation and accumulation in M. crystallinum and B. juncea. The high citrate and histidine accumulation especially at 100µM NiCl2, in the roots of M. crystallinum might be among the important factors associated with the tolerance of this halophyte to toxic Ni levels.

Dandelion (Taraxacum officinale) and Agrimony (Agrimonia eupatoria) as Indicators of Geogenic Contamination of Flysch Soils in Eastern Slovakia

Contents of potentially toxic elements Fe, Mn, Cr, Ni, Co, V, Cu, and Mo were determined in common dandelion (Taraxacum officinale) and agrimony (Agrimonia eupatoria) to show their usefulness as bioindicators of geogenic soil pollution. Both plants were collected on geochemically anomalous soils developed on flysch sedimentary rocks (Paleogene) of Eastern Slovakia, which also are composed of weathered detritus of some ultramafic rocks. Generally, contents of the investigated association of potentially toxic elements are highly increased in these "serpentine"-like soils. Elevated concentrations were detected in both shoots and roots of the plants. The highest values, which exceed world average values for plants, were observed for Ni content. They ranged from 1.7 to 16.3 mg kg(-1) in dandelion and from 1.6 to 22.6 mg kg(-1) in agrimony. Essential elements, such as Mo, Cu, and Mn, were the most concentrated in plants, whereas Co, V, and Cr were the least concentrated. Although the bioindication value of the common dandelion for anthropogenic soil pollution is well known, it is not mentioned for agrimony in literature, and no data exist to indicate the geogenic pollution for both plants. Dandelion and agrimony are widely used as herbal drugs; therefore, our intention also was to point out another fact, namely, possible high uptake of potentially toxic elements by herbal plants growing on similar soils.

Macronutrient composition of nickel-treated wheat under different sulfur concentrations in the nutrient solution

The effect of different sulfate(VI) sulfur (2, 6, and 9 mM S) levels and nickel(II) chloride (0, 0.0004, 0.04 and 0.08 mM Ni) in the nutrient solution on productivity and macronutrient (N, P, K, Ca, Mg, S) status and accumulation in spring wheat (Triticum aestivum L.) Zebra cv. was studied. Ni treatment reduced the biomass and disturbed the balance and accumulation of macronutrients in wheat. Intensive S nutrition, especially with 6 mM S, at least partially increased the biomass, improved ionic equilibrium, and enhanced nutrient accumulation in Ni-exposed plants in spite of increased Ni accumulation. Admittedly, the dose 9 mM S reduced Ni accumulation in shoots but increased accumulation thereof in roots. Compared to 6 mM, the dose 9 mM was less effective in improving the mineral status of Ni-treated wheat.

Jasmonic Acid Modulates the Physio-Biochemical Attributes, Antioxidant Enzyme Activity, and Gene Expression in Glycine max under Nickel Toxicity

In present study, we evaluated the effects of Jasmonic acid (JA) on physio-biochemical attributes, antioxidant enzyme activity, and gene expression in soybean (Glycine max L.) plants subjected to nickel (Ni) stress. Ni stress decreases the shoot and root length and chlorophyll content by 37.23, 38.31, and 39.21%, respectively, over the control. However, application of JA was found to improve the chlorophyll content and length of shoot and root of Ni-fed seedlings. Plants supplemented with JA restores the chlorophyll fluorescence, which was disturbed by Ni stress. The present study demonstrated increase in proline, glycinebetaine, total protein, and total soluble sugar (TSS) by 33.09, 51.26, 22.58, and 49.15%, respectively, under Ni toxicity over the control. Addition of JA to Ni stressed plants further enhanced the above parameters. Ni stress increases hydrogen peroxide (H2O2) by 68.49%, lipid peroxidation (MDA) by 50.57% and NADPH oxidase by 50.92% over the control. Supplementation of JA minimizes the accumulation of H2O2, MDA, and NADPH oxidase, which helps in stabilization of biomolecules. The activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) increases by 40.04, 28.22, 48.53, and 56.79%, respectively, over the control in Ni treated seedlings and further enhancement in the antioxidant activity was observed by the application of JA. Ni treated soybean seedlings showed increase in expression of Fe-SOD by 77.62, CAT by 15.25, POD by 58.33, and APX by 80.58% over the control. Nevertheless, application of JA further enhanced the expression of the above genes in the present study. Our results signified that Ni stress caused negative impacts on soybean seedlings, but, co-application of JA facilitate the seedlings to combat the detrimental effects of Ni through enhanced osmolytes, activity of antioxidant enzymes and gene expression.

Arbuscular mycorrhiza affects nickel translocation and expression of ABC transporter and metallothionein genes in Festuca arundinacea

Mycorrhizal fungi are key microorganisms for enhancing phytoremediation of soils contaminated with heavy metals. In this study, the effects of the arbuscular mycorrhizal fungus (AMF) Funneliformis mosseae (=Glomus mosseae) on physiological and molecular mechanisms involved in the nickel (Ni) tolerance of tall fescue (Festuca arundinacea = Schedonorus arundinaceus) were investigated. Nickel addition had a pronounced negative effect on tall fescue growth and photosynthetic pigment contents, as well as on AMF colonization. Phosphorus content increased markedly in mycorrhizal plants (M) compared to non-inoculated (NM) ones. However, no significant difference was observed in root carbohydrate content between AMF-inoculated and non-inoculated plants. For both M and NM plants, Ni concentrations in shoots and roots increased according to the addition of the metal into soil, but inoculation with F. mosseae led to significantly lower Ni translocation from roots to the aboveground parts compared to non-inoculated plants. ABC transporter and metallothionein transcripts accumulated to considerably higher levels in tall fescue plants colonized by F. mosseae than in the corresponding non-mycorrhizal plants. These results highlight the importance of mycorrhizal colonization in alleviating Ni-induced stress by reducing Ni transport from roots to shoots of tall fescue plants.

Nickel Availability in Soil as Influenced by Liming and Its Role in Soybean Nitrogen Metabolism

Nickel (Ni) availability in soil varies as a function of pH. Plants require Ni in small quantities for normal development, especially in legumes due its role in nitrogen (N) metabolism. This study investigated the effect of soil base saturation, and Ni amendments on Ni uptake, N accumulation in the leaves and grains, as well as to evaluate organic acids changes in soybean. In addition, two N assimilation enzymes were assayed: nitrate reductase (NR) and Ni-dependent urease. Soybean plants inoculated with Bradyrhizobium japonicum were cultivated in soil-filled pots under two base-cation saturation (BCS) ratios (50 and 70%) and five Ni rates - 0.0; 0.1; 0.5; 1.0; and 10.0 mg dm(-3) Ni. At flowering (R1 developmental stage), plants for each condition were evaluated for organic acids (oxalic, malonic, succinic, malic, tartaric, fumaric, oxaloacetic, citric and lactic) levels as well as the activities of urease and NR. At the end of the growth period (R7 developmental stage - grain maturity), grain N and Ni accumulations were determined. The available soil-Ni in rhizosphere extracted by DTPA increased with Ni rates, notably in BCS50. The highest concentrations of organic acid and N occurred in BCS70 and 0.5 mg dm(-3) of Ni. There were no significant differences for urease activity taken on plants grown at BSC50 for Ni rates, except for the control treatment, while plants cultivated at soil BCS70 increased the urease activity up to 0.5 mg dm(-3) of Ni. In addition, the highest values for urease activities were reached from the 0.5 mg dm(-3) of Ni rate for both BCS treatments. The NR activity was not affected by any treatment indicating good biological nitrogen fixation (BNF) for all plants. The reddish color of the nodules increased with Ni rates in both BCS50 and 70, also confirms the good BNF due to Ni availability. The optimal development of soybean occurs in BCS70, but requires an extra Ni supply for the production of organic acids and for increased N-shoot and grain accumulation.

Effects of Ni stress on the uptake and translocation of Ni and other mineral nutrition elements in mature wheat grown in sierozems from northwest of China

Effects of heavy metal on uptake of mineral nutrition elements in plants have attracted widespread interest and been widely explored. This paper reports the translocation and accumulation behaviors of Ni in the organs of mature wheat plants by means of pot experiment using the sierozem collected from northwestern China as experimental soil. Effect of Ni on accumulation of Cu, Mn, Ca, and Mg is also demonstrated. It was found that influence of Ni on wheat plants differed greatly at different Ni levels. Ni content in the organs of wheat plants increased with the increase in Ni level, and the increasing rate decreased when the Ni level was higher than 400 mg/kg. Ni was mainly accumulated in the roots and less distributed in the shoots, shells, and grains. When the Ni level was lower than 400 mg/kg, the bioconcentration factor (BCF) of the roots was higher than 1, suggesting that Ni was taken in against a concentration gradient. The average translocation factor (TF) of wheat plants was 0.221, indicating the weak ability of wheat plants in translocating Ni toward the aboveground parts. Since Ni is readily accumulated in the grains of wheat plants at lower Ni level, concerns in health risks might be raised. Excess Ni in wheat plants could inhibit the transfer of Cu, Mn, and Mg to grains, leading to the accumulation of Ca, Mg, and Mn in the shoots and shells of wheat plants. The increase in Ni content can disturb the uptake and distribution of mineral nutrition elements in the organs of plants, resulting in the toxic effect of Ni on wheat plants. Results from this study provide a scientific support to prevent or control heavy metal pollution in an arid region.

Can Clethra barbinervis Distinguish Nickel and Cobalt in Uptake and Translocation?

Clethra barbinervis Sieb. et Zucc. accumulates Nickel (Ni) and Cobalt (Co) at high concentrations., We hypothesized that C. barbinervis cannot distinguish between Ni and Co because of the similar chemical properties of these two elements. To confirm this hypothesis and understand the role of these elements in C. barbinervis, we conducted a hydroponic split-root experiment using Ni and Co solutions. We found that the bioconcentration factor (BCF; metal concentration of each tissue/metal concentrations of each treatment solution) of Ni and Co did not significantly differ in the roots, but the BCF for Co was higher than that for Ni in the leaves. The leaves of C. barbinervis accumulated Ni or Co at high concentrations. We also found the simultaneous accumulation of Ni and Co by the multiple heavy metal treatments (Ni and Co) at high concentrations similar to those for the single treatments (Ni or Co). Elevated sulfur concentrations occurred in the roots and leaves of Co-treated seedlings but not in Ni. This result indicates that S was related to Co accumulation in the leaves. These results suggest that C. barbinervis distinguishes between Ni and Co during transport and accumulation in the leaves but not during root uptake.

Comparative study on the sensitivity of turions and active fronds of giant duckweed (Spirodela polyrhiza (L.) Schleiden) to heavy metal treatments

Standard ecotoxicological test procedures use only active forms of aquatic plants. The potential effects of toxicants on vegetative propagules, which play an important role in the survival of several aquatic plant species, is not well understood. Because turion-like resting propagules overwinter on the water bottom in temperate regions, they could be exposed to contaminants for longer periods than active plants. Due to its turion producing capability, giant duckweed (Spirodela polyrhiza) is widely used in studying morphogenesis, dormancy, and activation mechanisms in plants. It is also suitable for ecotoxicological purposes. The present work aims to compare the growth inhibition sensitivity of active (normal frond) and overwintering (turion) forms of S. polyrhiza to concentrations of nickel (Ni), cadmium (Cd) and hexavalent chromium (Cr) ranging from 0 to 100mgL(-1). The results indicated that in general, resting turions have higher heavy metal tolerance than active fronds. Cd proved to be the most toxic heavy metal to S. polyrhiza active frond cultures because it induced rapid turion formation. In contrast, the toxicity of Ni and Cr were found to be similar but lower than the effects of Cd. Cr treatments up to 10mgL(-1) did not result in any future negative effects on turion activation. Turions did not survive heavy metal treatments at higher concentrations of Cr. Cd and Ni treatments affected both the floating-up and germination of turions but did not significantly affect the vigor of sprouts. Higher concentrations (of 100mgL(-1)) Cd completely inhibited germination.

Accumulation of nickel ions in seedlings of Vicia sativa L. and manifestations of oxidative stress

The accumulation of nickel ions in the roots and shoots of vetch seedlings (Vicia sativa L.) at increasing concentrations of nickel chloride in the medium was studied. It was shown that the accumulation of nickel in the shoots was increased when the concentration of nickel chloride in the medium was more than 50 μM. The bioconcentration factor and sustainability index for vetch seedlings were calculated under the experimental conditions. The obtained results were similar to parameters for other plants, grown on a nutrient medium or soil substrate. First, the obtained results allowed estimate the limits of nickel chloride concentrations for four of five zones, which correspond to the theoretical concept of dose-response curves in the studies on the influence of physiologically essential heavy metals on plants (Prasad 2010). Some parameters of oxidative stress caused by the presence of nickel chloride in the medium were shown. It seems that at low nickel concentrations in the medium in vetch seedlings the increase of several biochemical parameters (catalase activity and proline) caused by the high amylase activity in seeds.

Too much is bad--an appraisal of phytotoxicity of elevated plant-beneficial heavy metal ions

Heavy metal ions such as cobalt (Co), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), and zinc (Zn) are considered essential/beneficial for optimal plant growth, development, and productivity. However, these ions readily impact functions of many enzymes and proteins, halt metabolism, and exhibit phytotoxicity at supra-optimum supply. Nevertheless, the concentrations of these heavy metal ions are increasing in agricultural soils worldwide via both natural and anthropogenic sources that need immediate attention. Considering recent breakthroughs on Co, Cu, Fe, Mn, Mo, Ni, and Zn in soil-plant system, the present paper: (a) overviews the status in soils and their uptake, transport, and significance in plants; (b) critically discusses their elevated level-mediated toxicity to both plant growth/development and cell/genome; (c) briefly cross talks on the significance of potential interactions between previous plant-beneficial heavy metal ions in plants; and (d) highlights so far unexplored aspects in the current context.

Ni-induced Changes in Nitrate Assimilation and Antioxidant Metabolism of Verbascum olympicum Boiss.: Could the Plant be Useful for Phytoremediation or/and Restoration Purposes?

Verbascum olympicum Boiss. (Scrophulariaceae) were studied as a candidate plant for remediating the Ni polluted soils. The metabolic responses, such as nitrate assimilation (nitrate reductase and glutamine synthetase activity) and antioxidant system activity [superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) activity], of this species exposed to nickel in Hoagland's nutrient medium were investigated as remediation performance parameters. The accumulation of nickel and the variations in the content of some elements (B, Cu, Fe, Mg, Mn, Mo and Zn) and some growth parameters, such as the water content, biomass production, and contents of chlorophyll and soluble protein, were also examined. The accumulation of Ni in both the roots and leaves varied depending on the exposure times and doses. Increased oxidative stress was suggested by the increases in the activities of SOD, CAT and APX. Although some element contents were inhibited by Ni treatments, these inhibitory effects was decreased depending on the time, and even these elements are accumulated in roots. These results are the novelties in the use of this species in biotechnology.

Heavy Metal Residues in Soil and Accumulation in Maize at Long-Term Wastewater Irrigation Area in Tongliao, China

Soil and plant samples were collected from Tongliao, China, during the maize growth cycle between May and October 2010. Heavy metals, such as Cr, Pb, Ni, and Zn, were analyzed. The concentrations of Cr, Pb, Ni, and Zn in the wastewater-irrigated area were higher than those in the topsoil from the groundwater-irrigated area. The concentrations of metals in the maize increased as follows: Pb < Ni < Zn < Cr. In addition, Cr, Pb, and Ni mainly accumulated in the maize roots, and Zn mainly accumulated in the maize fruit. The results of translocation factors (TF) and bioconcentration factors (BCF) of maize for heavy metals revealed that maize is an excluder plant and a potential accumulator plant and can serve as an ideal slope remediation plant. In addition, the increasing heavy metal contents in soils that have been polluted by wastewater irrigation must result in the accumulation of Cr, Pb, Ni, and Zn in maize. Thus, the pollution level can be decreased by harvesting and disposing of and recovering the plant material.

Comparative Ni tolerance and accumulation potentials between Mesembryanthemum crystallinum (halophyte) and Brassica juncea: Metal accumulation, nutrient status and photosynthetic activity

Saline soils often constitute sites of accumulation of industrial and urban wastes contaminated by heavy metals. Halophytes, i.e. native salt-tolerant species, could be more suitable for heavy metal phytoextraction from saline areas than glycophytes, most frequently used so far. In the framework of this approach, we assess here the Ni phytoextraction potential in the halophyte Mesembryanthemum crystallinum compared with the model species Brassica juncea. Plants were hydroponically maintained for 21 days at 0, 25, 50, and 100μM NiCl2. Nickel addition significantly restricted the growth activity of both species, and to a higher extent in M. crystallinum, which did not, however, show Ni-related toxicity symptoms on leaves. Interestingly, photosynthesis activity, chlorophyll content and photosystem II integrity assessed by chlorophyll fluorescence were less impacted in Ni-treated M. crystallinum as compared to B. juncea. The plant mineral nutrition was differently affected by NiCl2 exposure depending on the element, the species investigated and even the organ. In both species, roots were the preferential sites of Ni(2+) accumulation, but the fraction translocated to shoots was higher in B. juncea than in M. crystallinum. The relatively good tolerance of M. crystallinum to Ni suggests that this halophyte species could be used in the phytoextraction of moderately polluted saline soils.

Rhizobium tibeticum activated with a mixture of flavonoids alleviates nickel toxicity in symbiosis with fenugreek (Trigonella foenum graecum L.)

The objective of this study is to explore the response of an activated Rhizobium tibeticum inoculum with a mixture of hesperetin (H) and apigenin (A) to improve the growth, nodulation, and nitrogen fixation of fenugreek (Trigonella foenum graecum L.) grown under nickel (Ni) stress. Three different sets of fenugreek seed treatments were conducted, in order to investigate the activated R. tibeticum pre-incubation effects on nodulation, nitrogen fixation and growth of fenugreek under Ni stress. Group (I): uninoculated seeds with R. tibeticum, group (II): inoculated seeds with uninduced R. tibeticum group (III): inoculated seeds with induced R. tibeticum. The present study revealed that Ni induced deleterious effects on rhizobial growth, nod gene expression, nodulation, phenylalanine ammonia-lyase (PAL) and glutamine synthetase activities, total flavonoids content and nitrogen fixation, while the inoculation with an activated R. tibeticum significantly improved these values compared with plants inoculated with uninduced R. tibeticum. PAL activity of roots plants inoculated with induced R. tibeticum and grown hydroponically at 75 and 100 mg L(-1) Ni and was significantly increased compared with plants receiving uninduced R. tibeticum. The total number and fresh mass of nodules, nitrogenase activity of plants inoculated with induced cells grown in soil treated up to 200 mg kg(-1) Ni were significantly increased compared with plants inoculated with uninduced cells. Plants inoculated with induced R. tibeticum dispalyed a significant increase in the dry mass compared with those treated with uninduced R. tibeticum. Activation of R. tibeticum inoculum with a mixture of hesperetin and apigenin has been proven to be practically important in enhancing nodule formation, nitrogen fixation and growth of fenugreek grown in Ni contaminated soils.

Nickel accumulation in leaves, floral organs and rewards varies by serpentine soil affinity

Serpentine soils are edaphically stressful environments that host many endemic plant species. In particular, serpentine soils are high in several heavy metals (e.g. nickel, cobalt and chromium) and these high heavy metal concentrations are thought, in part, to lead to varying levels of plant adaptation and soil affinities (i.e. endemic vs. non-endemic plant species). It is unclear, however, whether serpentine endemics vs. non-endemics differ with respect to heavy metal uptake into either vegetative or reproductive organs. Here, we use nickel as a model to determine whether plant heavy metal uptake varies with the level of endemism in several non-hyperaccumulating species. Under controlled greenhouse conditions, we grew seven plant species from the Brassicaceae family that vary in their degrees of affinity to serpentine soil from low (indifferent) to medium (indicator) and high (endemic) in soil that was nickel supplemented or not. We quantified nickel concentrations in leaves, pistils, anthers, pollen and nectar. While nickel concentrations did not vary across organs or affinities when grown in control soils, under conditions of nickel supplementation endemic species had the lowest tissue concentrations of nickel, particularly when considering leaves and pistils, compared with indifferent/indicator species. Species indifferent to serpentines incorporated higher concentrations of nickel into reproductive organs relative to leaves, but this was not the case for indicator species and endemics where nickel concentration was similar in these organs. Our findings suggest that endemic species possess the ability to limit nickel uptake into above-ground tissues, particularly in reproductive organs where it may interfere with survival and reproduction. Indifferent species accumulated significantly more nickel into reproductive organs compared with leaves, which may limit their reproductive potential relative to endemic species when growing on serpentine soils. Additional work determining the fitness consequences of these differences will further our understanding of edaphic endemism.