Nickel Hyperaccumulator Research Articles

Community ionomics reveals a diversity of mineral nutrition in a species‐rich shrubland on infertile soil

AbstractQuestionsHigh species richness is observed in certain shrublands on infertile substrates. Mineral nutrients are likely to be the primary limiting resources in these ecosystems, and below‐ground plant interactions may be crucial to understanding their diversity. Using ionomics, we investigated whether there were nutritional variations between plant species that coexist in a shrubland located in an edaphically extreme environment.LocationNew Caledonia.MethodsWe set up a 20 m × 20 m plot in a diverse shrubland (“maquis”) on ultramafic (infertile) substrate, in which we sampled all 475 plants taller than 1 m and characterized their ionome (22 elements).ResultsIn our study, 37 species were identified in the plot, representing all major forms of mycorrhizal symbioses, as well as nitrogen‐fixing plants, cluster rooted and parasitic plants. Notably, both nickel hyperaccumulating and manganese hyperaccumulating species were present. Hypervolume approaches were used to assess ionome overlap among the nine most abundant species, with the results revealing limited overlap. Moreover, it was observed that the rarest species in the plot also had the most functionally distinct features.ConclusionsDifferent nutritional strategies were present in the plot, as demonstrated by the variety of root symbioses and leaf ionomes. Our findings indicate coexistence of multiple species within this infertile shrubland may be achieved by species partitioning into different highly specialized biogeochemical niches.

Hyperaccumulation of nickel but not selenium drives floral microbiome differentiation: A study with six species of Brassicaceae.

Intraspecific variation in flower microbiome composition can mediate pollination and reproduction, and so understanding the community assembly processes driving this variation is critical. Yet the relative importance of trait-based host filtering and dispersal in shaping among-species variation in floral microbiomes remains unknown. Within two clades of Brassicaceae, we compared diversity and composition of floral microbiomes in natural populations of focal nickel and selenium hyperaccumulator species and two of their non-accumulating relatives. We assessed the relative strengths of floral elemental composition, plant phylogenetic distance (host filtering), and geography (dispersal) in driving floral microbiome composition. Species in the nickel hyperaccumulator clade had strongly divergent floral microbiomes, the most of that variation driven by floral elemental composition, followed by geographic distance between plant populations and, lastly, phylogenetic distance. Conversely, within the selenium hyperaccumulator clade, floral microbiome divergence was much lower among the species and elemental composition, geography, and plant phylogeny were far weaker determinants of microbiome variation. Our results show that the strength of elemental hyperaccumulation's effect on floral microbiomes differs substantially among plant clades, possibly due to variation in elements as selective filters or in long-distance dispersal probability in different habitats.

Enhanced phytoaccumulation dynamics of chromium and nickel from spent engine oil-contaminated soil amended with biomass-derived bulking agent

Phytoremediation is a promising sustainable approach for the remediation of soil contaminated with inorganic and organic pollutants. In the lands affected by petroleum-derived compounds, their massive toxicity towards the plants, however, hinders the efficacy of the phytoremediation process. Hence, adopting a green approach to enhance phytoremediation is highly recommended and desirable. Here, sugarcane bagasse (SCB) was used as a bulking agent enhancing the hyperaccumulation of heavy metals chromium (Cr) and nickel (Ni) by sunflower plants from 0.5%, 2.5%, and 5% oil-contaminated soil. Accumulation of Cr and Ni was observed higher in roots than shoots. The bioconcentration factor (BCF) of Cr and Ni was higher in soil amended with SCB than that without amended. The highest rate of Ni and Cr uptake was also noticed in all treatments amended with the bulking agent except in the case of 0.5% oil-contaminated soil without SCB. Overall, the phytoremediation of oil-contaminated soil is most effective when it is amended with SCB as a bulking agent which provides water and air well to plants, improving plant growth and then increasing phytoaccumulation of heavy metals.

Physiological responses of the nickel hyperaccumulator Bornmuellera emarginata under varying nickel dose levels and pH in hydroponics

Abstract Background and aims The nickel hyperaccumulator Bornmuellera emarginata (Brassicaceae) is a species adapted to thrive on naturally nickel-enriched ultramafic soils in the Balkans and a promising candidate for use in nickel agromining. The main aim of this study was to provide insight into the physiological mechanisms of nickel hyperaccumulation in B. emarginata. Methods Bornmuellera emarginata was cultivated under various nickel exposure concentrations (control, 1, 10, and 100 µM nickel in solution), and different pH levels of the hydroponic solution for four weeks. During this period, the plants underwent assessment for various physiological parameters, including photosynthetic pigments, leaf relative water content, tolerance index, and metal accumulation in plant tissues. Results The results show that the translocation factors and bioconcentration factors were > 1 even at 1 µM nickel in solution. This confirms the ability of B. emarginata to hyperaccumulate nickel (up to 6600 mg kg−1) over a wide range of nickel concentrations in hydroponics. Nickel at 100 µM (a concentration that is an order of magnitude higher than the highest soil solution nickel concentration found in ultramafic soils) induced only mild physiological stress symptoms (e.g. a minor proline response). Alterations in the solution pH did not cause any significant effect on nickel accumulation in the plants. Conclusions Bornmuellera emarginata is a highly adapted nickel-tolerant and nickel hyperaccumulating species that shows very little stress responses even to extreme nickel exposure concentrations in hydroponics. This species shows interesting trade-off responses between nickel and other metals, including non-competitive uptake of zinc. The potential for this species to accumulate zinc should therefore be further explored.

Grassland flora of ultramafic areas in Northern Pindus (Greece)

AbstractIn Europe, the largest and most floristically diverse ultramafic (serpentine) outcrops are located on the Balkan Peninsula. In Greece, the Northern Pindus mountain range is characterized as a floristic diversity hotspot, due to the presence of large ultramafic outcrops that harbor a large number of nickel (Ni) hyperaccumulators with potential use in agromining. The work presented here is part of a more in‐depth study of floristic composition and ecology of plant communities in ultramafic grasslands of the Northern Pindus mountain range. In 2021–2022, 120 relevés in three subareas (sA1, sA2, and sA3) were sampled according to the Braun‐Blanquet method. We identified 493 plant taxa belonging to 61 families. The most abundant families were Asteraceae, Poaceae, Fabaceae, Lamiaceae, and Caryophyllaceae. A total of 22 plant taxa are endemic to Greece. Ninety‐five taxa are of Balkan origin while 158 taxa are distributed to the broader Mediterranean region. The highest number of plant life‐forms are hemicryptophytes (56.39%) and therophytes (23.33%). Eight Ni hyperaccumulators including Centaurea thracica, Bornmuellera baldaccii, Bornmuellera emarginata, Bornmuellera tymphaea, Noccaea boeotica, Noccaea tymphaea, Odontarrhena chalcidica, and Odontarrhena smolikana were recorded.

Synchrotron μXRF imaging reveals elemental distribution in the nickel hyperaccumulator Odontarrhena muralis (Brassicaceae) from Serbia

AbstractOdontarrhena muralis (Brassicaceae) is a nickel hyperaccumulator species from the Balkans used as a “metal crop” in nickel phytomining. This study aimed to determine the elemental distribution, focusing on nickel, in fresh‐hydrated plant tissue (stems, leaves and inflorescences), to clarify where nickel is localized at the tissue and cellular scale‐level and to infer the physiological response to its hypertolerance and hyperaccumulation. For the analysis, intact plant organs of O. muralis were subjected to elemental imaging using synchrotron‐based micro‐X‐ray fluorescence (μXRF). The predominant distribution of nickel occurs in the epidermal tissue and at the base of the trichomes, which are also the main sinks for calcium deposition. The obtained results represent a further contribution to the knowledge of the physiological characteristics of this hyperaccumulating “metal crop” species and, consequently, to its application in sustainable metal extraction using phytomining.

Cutting propagation of Bornmuellera tymphaea, a nickel hyperaccumulator for use in agromining: Effects of rooting media and auxins on stem cuttings

AbstractThe nickel hyperaccumulator Bornmuellera tymphaea has been evaluated for use in agromining in Greece with very promising results. This study examines propagation of this species based on cuttings, which provides rapid multiplication and clonal propagation of genotypes with useful genetic traits and offers obvious advantages over propagation by seed. Apical cuttings, 10 cm long, obtained from plants in the field, were propagated on mist benches under greenhouse conditions. Four treatments based on the rooting medium were evaluated: (a) perlite, (b) perlite + peat 1:1 (v/v), (c) ultramafic soil, and (d) ultramafic soil + perlite 1:1 (v/v). Completely randomized blocks were used in a 4 × 2 factorial design, using 4 different propagation rooting media and 2 levels of auxins (with or without the use of indole‐3‐butyric acid [IBA]), with 3 replicates of 28 cuttings. The response to perlite medium was best in terms of rooting: 75% without IBA and 71.33% with IBA, and the dry weight of roots was 0.18 g and of shoots 0.34 g. Perlite + peat 1:1 (v/v) medium also resulted in rooting in more than 55% of cuttings whereas cuttings grown in ultramafic soil showed very low rooting (<27%). Based on the findings, B. tymphaea has an aptitude for vegetative propagation by cuttings with survival values and rooting greater than 70%. Perlite combined with the application of auxin (ΙΒΑ) is recommended.

Effects of different shading levels on the growth of Bornmuellera emarginata, a nickel hyperaccumulator for use in agromining

AbstractThe Greek endemic Bornmuellera emarginata, known for its Ni hyperaccumulation capacity, is a promising candidate species for use in agromining. At the same time, the installation of photovoltaic systems on agricultural land has recently been increasing. Field observations that populations of this species usually occupy shaded places under shrubs or trees led us to study the effect of different shading levels on the growth of B. emarginata, with a view to combine the cultivation of the species with photovoltaic panels in the same area. The experiment was conducted under greenhouse conditions and lasted 210 days after sowing (DAS). Seeds were sown directly into 2‐L pots filled with ultramafic soil collected from the field. Three plant samplings (81, 140, and 210 DAS) were conducted during the experiment to determine a set of morphological and physiological parameters of the species (fresh and dry plant biomass, plant height, number of lateral shoots, total number of leaves, proline and chlorophyll concentrations) alongside with its nickel yield at different levels of shading (0%, 20%, 35%, 60%, and 90%). Growth of B. emarginata was only significantly affected at the highest shading level. Nickel yield of the species increased over time and was maximized at the end of the experiment. The highest nickel yield per plant was observed at the intermediate shading level (35%). Bornmuellera emarginata can be considered as a candidate nickel hyperaccumulator for cultivation in ultramafic or nickel‐polluted areas under photovoltaic panels with corresponding shading, providing important environmental benefits.

Divergent roles of IREG/Ferroportin transporters from the nickel hyperaccumulator Leucocroton havanensis.

In response to our ever-increasing demand for metals, phytotechnologies are being developed to limit the environmental impact of conventional metal mining. However, the development of these technologies, which rely on plant species able to tolerate and accumulate metals, is partly limited by our lack of knowledge of the underlying molecular mechanisms. In this work, we aimed to better understand the role of metal transporters of the IRON REGULATED 1/FERROPORTIN (IREG/FPN) family from the nickel hyperaccumulator Leucocroton havanensis from the Euphorbiaceae family. Using transcriptomic data, we identified two homologous genes, LhavIREG1 and LhavIREG2, encoding divalent metal transporters of the IREG/FPN family. Both genes are expressed at similar levels in shoots, but LhavIREG1 shows higher expression in roots. The heterologous expression of these transporters in A. thaliana revealed that LhavIREG1 is localized to the plasma membrane, whereas LhavIREG2 is located on the vacuole. In addition, the expression of each gene induced a significant increase in nickel tolerance. Taken together, our data suggest that LhavIREG2 is involved in nickel sequestration in vacuoles of leaf cells, whereas LhavIREG1 is mainly involved in nickel translocation from roots to shoots, but could also be involved in metal sequestration in cell walls. Our results suggest that paralogous IREG/FPN transporters may play complementary roles in nickel hyperaccumulation in plants.

Native Metallophytes on Ultramafic Wooded Grassland in Sta Cruz, Mindoro Occidental, Philippines: Insights Into Phytostabilization and Forest Restoration

<p>The native metallophytes species are the optimum choice to restore degraded areas on ultramafic soil. However, a limited restorative floristic survey on the wooded grassland of Mindoro Occidental had been reported. Four 20 m x 20 m plots were established to rapidly assess the plant diversity of a wooded grassland on ultramafic soil in Sta. Cruz, Mindoro Occidental, Philippines. Diversity index (H`), relative density, relative dominance, and importance value (IV) were computed. Physicochemical characteristics and heavy metal contents of the soil in the site were analytically determined. We identified 43 morpho-species of plants belonging to 25 families. Thirty-six of the morpho-species identified are Philippine natives and typically grow on ultramafic forests. Nine species out of the top 10 trees with the highest IV are native ones, with <em>Buchanania arborescens</em> Blume as the most dominant. The estimate of Relative Cover (%) also showed native species. Poles and sapling dominated the area, suggesting that active regeneration is taking place. Further, 72.94% of the ground cover were represented by tree flora recruits (e.g., <em>B. arborescens</em>, <em>Alstonia macrophylla</em>). The sites have overall moderate diversity (H` index of 2.7). The soil contains a high amount of Nickel, Chromium, Iron, and Manganese. There were no Nickel hyperaccumulators but 22 native species showed Aluminum and Silicon hyperaccumulation. Therefore, the study revealed that the surveyed area is home to important metallophytes that have the potential for phytostabilization and reforestation.</p>

Rearing the Cabbage White Butterfly (Pieris rapae) in Controlled Conditions: A Case Study with Heavy Metal Tolerance.

The cabbage white butterfly (Pieris rapae) is an important system for applied pest control research and basic research in behavioral and nutritional ecology. Cabbage whites can be easily reared in controlled conditions on an artificial diet, making them a model organism of the butterfly world. In this paper, a manipulation of heavy metal exposure is used to illustrate basic methods for rearing this species. The general protocol illustrates how butterflies can be caught in the field, induced to lay eggs in greenhouse cages, and transferred as larvae to artificial diets. The methods show how butterflies can be marked, measured, and studied for a variety of research questions. The representative results give an idea of how artificial diets that vary in components can be used to assess butterfly performance relative to a control diet. More specifically, butterflies were most tolerant to nickel and least tolerant to copper, with a tolerance of zinc somewhere in the middle. Possible explanations for these results are discussed, including nickel hyper-accumulation in some mustard host plants and recent evidence in insects that copper may be more toxic than previously appreciated. Finally, the discussion first reviews variations to the protocol and directions for troubleshooting these methods, before considering how future research might further optimize the artificial diet used in this study. Overall, by providing a detailed video overview of the rearing and measurement of cabbage whites on artificial diets, this protocol provides a resource for using this system across a wide range of studies.

Rhizosphere processes by the nickel hyperaccumulator Odontarrhena chalcidica suggest Ni mobilization.

Plant Ni uptake in aboveground biomass exceeding concentrations of 1000μgg-1 in dry weight is defined as Ni hyperaccumulation. Whether hyperaccumulators are capable of mobilizing larger Ni pools than non-accumulators is still debated and rhizosphere processes are still largely unknown. The aim of this study was to investigate rhizosphere processes and possible Ni mobilization by the Ni hyperaccumulator Odontarrhena chalcidica and to test Ni uptake in relation to a soil Ni gradient. The Ni hyperaccumulator O. chalcidica was grown in a pot experiment on six soils showing a pseudo-total Ni and labile (DTPA-extractable) Ni gradient and on an additional soil showing high pseudo-total but low labile Ni. Soil pore water was sampled to monitor changes in soil solution ionome, pH, and dissolved organic carbon (DOC) along the experiment. Results showed that Ni and Fe concentrations, pH as well as DOC concentrations in pore water were significantly increased by O. chalcidica compared to unplanted soils. A positive correlation between Ni in shoots and pseudo-total concentrations and pH in soil was observed, although plant Ni concentrations did not clearly show the same linear pattern with soil available Ni. This study shows a clear root-induced Ni and Fe mobilization in the rhizosphere of O. chalcidica and suggests a rhizosphere mechanism based on soil alkalinization and exudation of organic ligands. Furthermore, it was demonstrated that soil pH and pseudo-total Ni are better predictors of Ni plant uptake in O. chalcidica than labile soil Ni. The online version contains supplementary material available at 10.1007/s11104-023-06161-w.

Reflectance spectroscopy as a promising tool for ‘sensing’ metals in hyperaccumulator plants

Main conclusionThe VNIR reflectance spectra of nickel hyperaccumulator plant leaves have spectral variations due to high nickel concentrations and this property could potentially be used for discovery of these plants. Hyperaccumulator plants accumulate high concentrations of certain metals, including manganese, cobalt, or nickel. Of these metals, the divalent ions of nickel have three absorption bands in the visible to near-infrared region which may cause variations in the spectral reflectance of nickel hyperaccumulator plant leaves, but this has not been investigated previously. In this shortproof-of-concept study, the spectral reflectance of eight different nickel hyperaccumulator plant species leaves were subjected to visible and near-infrared and shortwave infrared (VNIR-SWIR) reflectance spectrum measurements in dehydrated state, and for one species, it was also assessed in hydrated state. Nickel concentrations in the plant leaves were determined with other methods and then correlated to the spectral reflectance data. Spectral variations centred at 1000 ± 150 nm were observed and had R-values varying from 0.46 to 0.96 with nickel concentrations. The extremely high nickel concentrations in nickel hyperaccumulator leaves reshape their spectral reflectance features, and the electronic transition of nickel-ions directly contributes to absorption at ~ 1000 nm. Given that spectral variations are correlated with nickel concentrations it make VNIR-SWIR reflectance spectrometry a potential promising technique for discovery of hyperaccumulator plants, not only in the laboratory or herbarium, but also in the field using drone-based platforms. This is a preliminary study which we hope will instigate further detailed research on this topic to validate the findings and to explore possible applications.

Heavy metals tolerance on seed germination and growth of serpentine plant species Alyssum murale L.

Alyssum murale L. is a nickel hyperaccumulator herbaceous perennial species in a natural serpentine soil and adapted to arid, infertile soils. The objective of this study was to investigate how the seed germination, root, and hypocotyl growth of this plant species respond to exposure to different concentrations of heavy metals such as lead (Pb), cadmium (Cd), nickel (Ni), chromium (Cr) and manganese (Mn). The highest germination, even higher than in the control group, was observed at lower concentrations of Ni, Cd, Pb and Mn, while almost all higher concentrations of the metals decreased germination. Germination time was significantly slower (from about 6 to 10 days) for seeds treated with higher metal concentrations, especially Mn, Cd and Ni, and faster (from about 3 to 5 days) for seeds treated with lower concentrations. The strongest inhibitory effect on root and hypocotyl length occurred in treatments with different concentrations of Pb, Cd, and Mn, and seeds treated with lower concentrations of Ni showed a positive effect on root and hypocotyl growth. Seeds treated with Ni showed relatively high tolerance to this metal, presenting the potential for practical use in various fields of phytoremediation technology.

The seed germination properties of two hyperaccumulator plant species with the potential for Ni agromining

The aim of this study is to investigate the effect of different nickel concentrations and light in combination with storage conditions and storage time on the seed germination ability of two serpentine-endemic nickel hyperaccumulating species (Bornmuellera emarginata and B. tymphaea). The seeds of both species were collected from natural populations in the Pindus Mountain range, Greece in early July and stored in a refrigerator (4?C) and in laboratory conditions (22?C). The seeds were exposed to a range of nickel concentrations typical of non-ultramafic-ultramafic gradient in two light environments (12 h photoperiod and continuous darkness). The nickel concentration only had a significant effect on B. emarginata, decreasing its seed germination rate with increasing Ni concentrations. The storage temperature significantly affected the germination percentage of both species and it was higher at 4?C compared to 22?C. A higher germination rate (> 60%) was observed for 5-8-month-old seeds, but both species generally showed significantly higher germination rates in the tests conducted seven months after seed ripening in the field. A higher germination rate was observed in a 12-hour photoperiod than in continuous darkness only for B. tymphaea. This study provides guidelines on the germination capacity of two obligate nickel hyperaccumulators with a potential for use in agromining systems.

Nickel Hyperaccumulator Biochar Sorbs Ni(II) from Water and Wastewater to Create an Enhanced Bio-ore.

Nickel (Ni) hyperaccumulators make up the largest proportion of hyperaccumulator plant species; however, very few biochar studies with hyperaccumulator feedstock have examined them. This research addresses two major hypotheses: (1) Biochar synthesized from the Ni hyperaccumulator Odontarrhena chalcidica grown on natural, metal-rich soil is an effective Ni sorbent due to the plant's ability to bioaccumulate soluble and exchangeable cations; and (2) such biochar can sorb high concentrations of Ni from complex solutions. We found that O. chalcidica grew on sandy, nutrient-poor soil from a Minnesota mining district but did not hyperaccumulate Ni. Biochar prepared from O. chalcidica biomass at a pyrolysis temperature of 900 °C sorbed up to 154 mg g-1 of Ni from solution, which is competitive with the highest-performing Ni sorbents in recent literature and the highest of any unmodified, plant-based biochar material reported in the literature. Precipitation, cation exchange, and adsorption mechanisms contributed to removal. Ni was effectively removed from acidic solutions with initial pH > 2 within 30 min. O. chalcidica biochar also removed Ni(II) from a simulated Ni electroplating rinsewater solution. Together, these results provide evidence for O. chalcidica biochar as an attractive material for simultaneously treating high-Ni wastewater and forming an enhanced Ni bio-ore.

Relict lineages with extreme ecology and physiology: metal hyperaccumulation on ultramafic substrates in New Caledonian Alseuosmineae (Asterales)

ABSTRACT Background Relict lineages are an important component of biodiversity, but it is unclear under what circumstances these groups persist. A potential example of such a group is the Alseuosmineae (Asterales) of Oceania. This clade contains the three small families – Alseuosmiaceae, Argophyllaceae and Phellinaceae. The clade has highest diversity in New Caledonia, where there are extensive ultramafic substrates, creating an extreme edaphic environment. Aims Using several lines of evidence we aimed to show that Alseuosmineae qualify as a relict lineage, with specialised adaptations for ultramafic substrates. Methods Molecular phylogenetic and dating analyses were carried out on representatives from all Alseuosmineae genera. Metal concentration in 33 out of 44 Alseuosmineae species was measured in herbarium specimens with X-ray fluorescence spectrometry. Results Dating analyses indicated that Alseuosmineae began diversifying about 75–80 million years ago, and had much slower net diversification rates than other Asterales groups. One-third of the species occur on ultramafic substrates in New Caledonia. Six are categorised as nickel hyperaccumulators, and two as manganese hyperaccumulators. Conclusions Extinction was probably paramount in the history of Alseuosmineae, especially for continental species. We postulate that their adaptation to ultramafic substrates and metal accumulation may have contributed to their survival until today.

Nickel hyperaccumulation, elemental profiles and agromining potential of three species of Odontarrhena from the ultramafics of Western Iran

The profiles of trace and major elements in three Odontarrhena species from the ultramafics of Western Iran (O. callichroa, O. penjwinensis and O. inflata) were evaluated to provide detailed information on their soil-plant relationships and potentials for agromining. The mean concentrations of Ni in leaf dry matter of these three species were 877, 3,270 and 2,720 mg kg−1, respectively. The mean concentrations of total soil Ni at sites Mazi Ban, Kamyaran and Ghala Ga were 1,470, 2,480, 1,030 mg kg−1, respectively. The Bioconcentration Factor (BCF) for Ni was >1 in O. penjwinensis and O. inflata, but not in O. callichroa. A positive relationship between shoot Ni and soil pH was found for all three species. They display Ni hyperaccumulation in the leaves, but with pronounced variation in the Ni concentrations attained. Odontarrhena penjwinensis emerged as the most promising potential candidate for future Ni agromining. The progress made in this study will enable further consideration of the three Odontarrhena species, especially O. penjwinensis, for any future commercial Ni agromining of the serpentinic ultramafic soils in Western Iran.

The parameters determining hyperaccumulator rhizobacteria diversity depend on the study scale

Soils harbor some of the most diverse microbiomes on Earth and are essential for both nutrient cycling and carbon storage. Numerous parameters, intrinsic to plant physiology, life history and the soil itself, can influence the structure of rhizomicrobial communities. While our knowledge of rhizosphere microbial diversity is increasing, opinion is divided as to whether the factors that most impact this diversity are abiotic, climatic or plant selection. Here we focused on the rhizosphere bacterial diversity of nickel hyperaccumulator plants (28 species from Mediterranean or tropical climates). We showed, by leveraging 16S Illumina sequencing of 153 ultramafic rhizosphere soils, that bacterial genetic diversity was highest in Mediterranean habitats where plant diversity was the lowest. Concerning those parameters driving this diversity, we demonstrated that climate drives bacterial diversity, in particular with the annual temperature variation. Focusing on each region, we underlined the substantial role of soil physicochemical parameters. Our results highlight the importance of considering spatial scale when explaining bacterial community diversity.

Interpopulation variation in nickel hyperaccumulation and potential for phytomining by Odontarrhena penjwinensis from Western Iran

The Ni hyperaccumulator Odontarrhena penjwinensis (Brassicaceae) is a species endemic to the ultramafic soils of Western Iran. Seven ultramafic outcrops and the populations of O. penjwinensis they support were investigated for Ni and other elemental concentrations to assess their potential for future Ni agromining operations. Nickel hyperaccumulation was confirmed in at least one individual plant sampled from all seven populations. The highest and lowest concentrations of Ni in leaves were recorded for two sites (3270 and 660 mg kg−1, respectively). The highest concentrations of Mn, Cr, and Co were 3110, 79, and 193 mg kg−1, respectively. A positive relationship was found between the concentrations of Ni and Fe, and Mg and Ca in roots and shoots. Correlation analysis of edaphic factors also revealed positive correlations between Ni concentration in the shoots, water-holding capacity, and soil pH, whereas it was negatively correlated for soil organic matter. The highest Ni yield (c. 0.4 g per plant), biomass (115 g per plant) and maximum plant height (43 cm) were recorded for one population where the total soil Ni concentration was 2550 mg kg−1. This O. penjwinensis population is likely to be a promising candidate for the development of Ni agromining in the region.