Hyperaccumulation Threshold Research Articles

A new species of Odontarrhena (Brassicaceae) endemic to Greek ultramafics: From taxonomy to metal accumulation behavior

AbstractA new species of Odontarrhena (Brassicaceae) is described from Mount Vourinos in western Macedonia, Greece. The species is restricted to the ridge and the northern slopes of Mt. Vourinos above 1500 m a.s.l. and grows on stony slopes and dry rocky grassland on ultramafic soil with elevated trace metal concentrations. Based on morphological, karyological, and nrDNA sequences, the species is tetraploid with supernumerary B‐chromosomes (2n = 4× = 32 + 2B) and related to the Balkan endemic Odontarrhena decipiens. It differs from the latter mainly by the habit of subshrub with woody base and a denser indumentum of stellate trichomes, resulting in a white‐silvery color of the leaves and shoots. The presence of four degenerate positions in the ITS1 region was detected only in the new species from Mt. Vourinos. Shoot Ni concentrations determined by x‐ray fluorescence analysis and atomic absorption spectroscopy were largely variable (6200–18,700 μg g−1 dw), but always significantly higher than in roots. Hydroponic trials confirmed the typical features of Ni hyperaccumulating plants, such as growth stimulating effect at low metal doses and shoot metal concentration above the hyperaccumulation threshold. The new species of Odontarrhena is a Ni hyperaccumulator with substantial aboveground biomass production, making it a promising candidate for phytomining applications.

“Mining” the herbarium for hyperaccumulators: Discoveries of nickel and zinc (hyper)accumulation in the genus Noccaea (Brassicaceae) through X‐ray fluorescence herbarium scanning

AbstractX‐ray fluorescence spectroscopy (XRF) is a relatively new method for non‐destructive elemental analysis of herbarium material that meets the scientific interest in being capable of discovering new hyperaccumulator plant species. Since the genus Noccaea (Brassicaceae family) is known to be one of the most numerous in term of the hyperaccumulator plant species it contains, especially those that hyperaccumulate Ni, the herbarium material available worldwide represents a great resource for expanding our knowledge of their elemental profiles. In this first systematic XRF scanning of herbarium specimens of the genus Noccaea, a total of 794 specimens from the collection of the National Museum of Natural History in Paris (MNHN) were analyzed, and the raw values obtained were corrected using regression formulas against inductively coupled plasma atomic emission spectroscopy data. Hyperaccumulation of Ni was detected in 90 specimens covering 21 taxa, with Ni concentrations reaching up to 48,700 mg kg−1 in Noccaea cappadocica, an ultramaficophyte from Syria. Zinc concentrations above the hyperaccumulation threshold were found in 210 specimens covering 23 taxa, most of which belonged to different subspecies of Noccaea caerulescens, with the highest concentration reaching up to 56,200 mg kg−1 in N. caerulescens subsp. caerulescens. Although the accumulation of Ni and Zn is contrasting in most of the specimens studied, in 10 specimens, predominantly belonging to N. caerulescens, simultaneous hyperaccumulation of Ni and Zn was found. This study also revealed previously unknown hyperaccumulation of Ni in several Noccaea taxa, as well as a simultaneous hyperaccumulation of Ni and Zn that needs to be confirmed by further experimental and field studies.

Dianthus sylvestris subsp. sylvestris as a promising candidate for phytostabilization of copper-contaminated post-mining sites in Alpine ecosystems.

The lack of remediation of inactive mine sites is a serious global concern, as they pose risks to the environment, human health, and safety. The potential of Dianthus sylvestris subsp. sylvestris to remediate post-mining sites contaminated with copper (Cu) at high altitudes, which is a challenging task for most management strategies, was explored in this study. More than 1300 mg Cu kg-1 in shoots were found in plants collected at the Monte Avanza legacy mine site (Alps). However, it is unclear whether this is due to hyperaccumulation or foliar contamination. To address this gap, field samples were washed with two different protocols, and a controlled Cu-tolerance test was conducted. While very high Cu concentrations, exceeding the Cu hyperaccumulation threshold of 300 mg kg-1, were found in samples washed with water, results for the plants cleaned with a more rigorous approach with EDTA suggested Cu exclusion strategy. Under controlled conditions, the plant showed Cu hypertolerance but did not hyperaccumulate Cu. Thus D. sylvestris presents a Cu exclusion strategy rather than hyperaccumulation potential, making it a suitable candidate for Cu phytostabilization at high altitude legacy mine sites. The study emphasizes the need for experiments under controlled conditions when assessing the phytoremediation potential.

Zinc (hyper)accumulation in Cardamine waldsteinii: from discovery in the herbarium to validation in the field

X-ray Fluorescence Spectroscopy (XRF) is a non-destructive analytical technique that has recently been developed as a tool for high throughput screening for the discovery of trace element hyperaccumulator species in herbarium collections. Herbarium XRF analysis was undertaken of representatives of the Brassicaceae family in the two herbaria in Belgrade (Serbia). This was coupled to field collection and chemical analysis of new samples of Cardamine waldsteinii. The data revealed a potentially new zinc hyperaccumulator species, C. waldsteinii, with 3050 mg kg−1 Zn in its leaves. This compares to 2210 mg kg−1 in the leaves of this species collected from the field. These values are below the notional hyperaccumulation threshold for zinc but are nevertheless unusual. The remarkably high accumulation factor for zinc are suggestive of the exceptional potential of C. waldsteinii for zinc uptake, even in non-metalliferous, zinc-deficient soils. We recommend following up this study with experimental investigations on C. waldsteinii to elucidate its ecophysiology and its zinc tolerance and accumulation characteristics.

Salinity and Heavy Metal Tolerance, and Phytoextraction Potential of Ranunculus sceleratus Plants from a Sandy Coastal Beach.

The aim of the present study was to evaluate tolerance to salinity and different heavy metals as well as the phytoextraction potential of Ranunculus sceleratus plants from a brackish coastal sandy beach habitat. Four separate experiments were performed with R. sceleratus plants in controlled conditions: (1) the effect of NaCl gradient on growth and ion accumulation, (2) the effect of different Na+ and K+ salts on growth and ion accumulation, (3) heavy metal tolerance and metal accumulation potential, (4) the effect of different forms of Pb salts (nitrate and acetate) on plant growth and Pb accumulation. A negative effect of NaCl on plant biomass was evident at 0.5 g L-1 Na+ and growth was inhibited by 44% at 10 g L-1 Na+, and this was associated with changes in biomass allocation. The maximum Na+ accumulation (90.8 g kg-1) was found in the stems of plants treated with 10 g kg-1 Na+. The type of anion determined the salinity tolerance of R. sceleratus plants, as Na+ and K+ salts with an identical anion component had a comparable effect on plant growth: nitrates strongly stimulated plant growth, and chloride treatment resulted in slight but significant growth reduction, but plants treated with nitrites and carbonates died within 4 and 5 weeks after the full treatment, respectively. The shoot growth of R. sceleratus plants was relatively insensitive to treatment with Mn, Cd and Zn in the form of sulphate salts, but Pb nitrate increased it. Hyperaccumulation threshold concentration values in the leaves of R. sceleratus were reached for Cd, Pb and Zn. R. sceleratus can be characterized as a shoot accumulator of heavy metals and a hyperaccumulator of Na+. A relatively short life cycle together with a high biomass accumulation rate makes R. sceleratus useful for dynamic constructed wetland systems aiming for the purification of concentrated wastewaters.

Hyperaccumulator plant discoveries in the Balkans: Accumulation, distribution, and practical applications

Hyperaccumulator plants are able to tolerate extremely high concentrations of metals/metalloids in the soil in which they grow and to accumulate high concentrations in their shoots. To date, a total of 31 hyperaccumulator plant species have been identified in the Balkans, the centre of diversity and speciation in the European flora which is particularly rich in ultramafic areas. A further 8 species have yet to be confirmed through additional studies. Most of the 31 hyperaccumulator taxa (13 taxa or 41.9%) are species of the genus Odontarrhena, all hyperaccumulating Ni, but concentrations of this element above the hyperaccumulation threshold were also found in the genera Bornmuellera and Noccaea (all Brassicaceae), Orobanche (Orobanchaceae), Centaurea (Asteraceae) and Viola (Violaceae). The existence of hyperaccumulators of Tl and Zn is of particular interest because very few species worldwide hyperaccumulate these elements. Multiple metal hyperaccumulation was found in Noccaea kovatsii, as the hyperaccumulation of Zn was found in this species in addition to Ni, the primary accumulated element. Metal hyperaccumulation is discussed in terms of phylogenetic relationships and species distributions, with special attention to their systematics, the detection and recognition of new hyperaccumulating species and the possibilities for their future practical applications in phytotechnologies.

Uptake of yttrium, lanthanum and neodymium in Melastoma malabathricum and Dicranopteris linearis from Malaysia

Plants that naturally accumulate aluminium (Al) may also inadvertently accumulate rare earth elements (REEs) due to the similar chemical properties of Al and REE trivalent ions, and vice versa. In this study, an Al hyperaccumulator plant species, Melastoma malabathricum, and a species known to have a propensity to hyperaccumulate REEs (in addition to Al), Dicranopteris linearis, were evaluated for potential REE accumulation in a one-year pot dosing trial in Sabah, Malaysia. To test whether the Malaysian accessions of D. linearis and M. malabathricum hyperaccumulate REEs (and Al), both species were grown in pots containing soil treated with solutions containing yttrium (Y), lanthanum (La), neodymium (Nd), and a mixture of these three REEs. The results showed that both M. malabathricum and D. linearis accumulated > 1000 µg g−1 Al in their leaves as expected. The shoots of M. malabathricum contained lower REEs than the roots (50 µg g−1 compared to 905 µg g−1). In D. linearis, the mean foliar REE concentrations ranged from 145 to 315 µg g−1, which is below the hyperaccumulation threshold set for REEs (> 1000 µg g−1 REEs). This study revealed that the Malaysian accessions of both M. malabathricum and D. linearis are Al hyperaccumulators, but their REE hyperaccumulation status requires further testing.

Diversity of Ni growth response and accumulation in Central-Eastern Mediterranean Odontarrhena (Brassicaceae) populations on and off serpentine sites

Nickel response and accumulation were analyzed in 12 populations from seven Central-Eastern Mediterranean Odontarrhena taxa on and off serpentine sites to test the presence of metal-induced growth stimulation and its relationship with tolerance and plant Ni concentration. Seedlings were cultivated in hydroponics with increasing NiSO4 concentrations to obtain dose-response curves and to evaluate Ni levels in roots and shoots. In all the accessions, a metal stimulatory effect on growth was present in the low-dose zone and significantly fitted the Brain-Cousens hormetic model. Accessions showed broad variation in tolerance, with the most tolerant plants requiring the highest Ni concentrations in the culture medium for optimal growth. Significant differences were also detected in plant Ni concentrations and a positive relationship was found between tolerance and accumulation. The serpentine and non-serpentine populations of O. chalcidica were similarly capable of hyperaccumulation, suggesting this ability to a be a species-wide trait. In the case of O. muralis, a species that preferentially avoids serpentine soils, the two populations from non-serpentine sites showed the same Ni-enhanced growth of the serpentine accessions of the other species investigated here, but only at the lowest concentration, and reached shoot Ni amounts approaching the Ni hyperaccumulation threshold. Therefore, in Odontarrhena the capacity to tolerate and accumulate Ni could be considered a specieswide trait in all the serpentine taxa, both endemic and facultative. Regarding the metal concentration showed by the accessions in nature, neither Ni levels in the field-collected plants nor those in their natural soils of origin were related to Ni tolerance. On the other hand, Ni accumulation capacity appeared as the main driving factor for the metal concentration in the plants in their native habitats.

Plantago subulata as indicator of potentially toxic elements in the substrate.

Plantago subulata is a facultative serpentinophyte, with predominantly ultramafic distribution in Serbia and Montenegro. The plant samples were collected from ultramafic and non-ultramafic substrate, including two abandoned mining sites, with the aim to assess the accumulation potential of this species. The samples were collected from 10 sites in Serbia and Montenegro and element concentrations in plants and soils were determined. Particularly high concentrations of Fe, Cu, Mn, Zn, and Cd were found in the soil and plant samples from an abandoned iron mining site, Mt. Kopaonik, Suvo Rudište. The concentrations of Ni, Cr, Fe, Co, and Cd were statistically different between soil and plant samples from ultramafic and non-ultramafic bedrock. Considering the fact that concentrations of Zn, Ni, and Cr in roots and shoots were positively correlated with their respective contents in the soil, i.e., the chemical composition of the plant and soil samples reflected the characteristics of the substrate, and for most of the elements analyzed, P. subulata acted as indicator species. For Cd and Pb, only the root concentrations were positively correlated with soil content, indicating exclusion and root sequestration as the main tolerance strategies for these elements. Although below the hyperaccumulation threshold, a strong accumulation capacity of P. subulata was found for Ni and Cu.

Urban soil phytomanagement for Zn and Cd in situ removal, greening, and Zn-rich biomass production taking care of snail exposure.

The phytoextraction potential of Arabidopsis halleri (L.) O'Kane & Al Shehbaz and Salix viminalis L. to partially remove Zn and Cd in soil was investigated. In an urban field site, a very short rotation coppice of willows was implemented, and growth parameters were monitored for 3 years. A. halleri was cultivated in the same site with or without fertilizer to improve biomass yield and/or Zn and Cd aerial part concentrations. Effects of harvest and co-cultivation on these two parameters were measured. To determine if willows and A. halleri leaves were risky in case of consumption by a herbivorous invertebrate like the landsnail Cantareus aspersus, metal concentrations of snails fed with Zn- and Cd-enriched and low enriched leaves were compared. Willows and A. halleri grew well on the metal-contaminated soil (1.7 and 616 mg kg-1 Cd and Zn, respectively). The A. halleri Zn foliar concentration reached the Zn hyperaccumulation threshold (> 10,000 mg kg-1 DW) in the presence of NPK fertilizer and although the soil was alkaline (pH > 8.2). Cd concentration increased with harvest and fertilizer. Cd and Zn foliar concentrations of willows were far above baseline values. Laboratory snails exposure revealed that willow leaves ingestion caused a moderate increase of Cd, Pb, and Zn bioaccumulation in snails compared to the one caused by A. halleri ingestion. The soil and plant metal concentrations were reflected by field snail biomonitoring. This study confirmed the interest of selecting A. halleri and willows to partially remove Zn and Cd in the soil and emphasized their potential usefulness in greening urban contaminated area and producing raw materials for green chemistry while paying attention to the environmental pollutant transfer.

Accumulation of trace elements in Tussilago farfara colonizing post-flotation tailing sites in Serbia.

The purpose of this study was to examine the accumulation potential of spontaneously developed Tussilago farfara populations colonizing sites with different levels of anthropogenic pollution. Physical characteristics of the soil are presented, together with the concentrations of macroelements and microelements (Ca, Mg, Fe, S, Al, Pb, Zn, Cu, Cd, Mn, As, Sb, Ag, Ti, and Sr) in both soil and plants. The biological concentration, accumulation, and translocation factors were used to assess the potential for heavy metal accumulation. Considerable differences were found among assessions from unevenly contaminated habitats, particularly in comparison with an unpolluted site. In line with the ore's characteristics, substrate samples from polluted sites were heavily contaminated with Pb, Zn, As, and Sb. Increased levels of microelements were also detected in plant samples from flotation tailings. Despite active absorption of Zn, Cu, Cd, Mn, and Sr by the plants from mining sites, the detected quantities of these elements in all samples were below the hyperaccumulation threshold. However, the obtained results indicate that the use of T. farfara from such sites in traditional medicine could pose a risk to human health due to accumulation of several toxic elements in the plant's aboveground tissues. Additionally, as a successful primary colonizer and stabilizer of technogenic substrates, T. farfara has an important role in the initial phases of revegetation of highly contaminated sites.

Elemental defense of nickel hyperaccumulator seeds against a generalist insect granivore

AbstractHyperaccumulation has been proposed as an elemental defense against herbivores; however, few studies have examined seed defense. This study included two annual Streptanthus species (Brassicaceae) from California serpentine soils: a non‐hyperaccumulator (S. insignis) and three populations (representing Y, P, and U morphs) of a Ni hyperaccumulator (S. polygaloides). Adults of the generalist seed herbivore Tribolium confusum (Coleoptera: Tenebrionidae) were fed either whole or cut seeds, survival was recorded for 7 weeks, and Ni concentrations of both beetles and seeds were determined using ICP‐OES. Survival analysis showed significantly more rapid mortality for beetles consuming S. polygaloides seeds compared to those consuming S. insignis. Mortality of beetles fed whole S. polygaloides seeds was more rapid than those fed cut S. polygaloides seeds. Seeds of the S. polygaloides populations contained approximately 300 µg Ni g−1 whereas S. insignis contained approximately 5 µg Ni g−1. Beetles fed whole S. polygaloides seeds contained more than 2.5‐fold greater Ni concentrations than those fed cut seeds (approximately 60 and 25 µg Ni g−1, respectively), whereas beetles fed either cut or whole S. insignis seeds contained < 0.3 µg Ni g−1. An artificial diet study, using Ni‐amended cornmeal, confirmed that diet Ni concentrations greater than 240 µg Ni g−1 were toxic to T. confusum. We conclude that Ni in S. polygaloides seeds can act as an elemental defense against seed herbivores even at 300 µg Ni g−1, a level below the 1000 µg Ni g−1 hyperaccumulation threshold concentration.

Cadmium-zinc accumulation and photosystem II responses of Noccaea caerulescens to Cd and Zn exposure.

A population of the metallophyte Noccaea (Thlaspi) caerulescens originating from a Zn-enriched area at Røros Copper Mine (Norway) was studied. N. caerulescens tolerance to accumulate Cd and Zn was evaluated in hydroponic experiments by chlorophyll fluorescence imaging analysis. In the field-collected N. caerulescens mother plants, Zn shoot concentrations were above Zn hyperaccumulation threshold while, in hydroponic experiments under 40-μM Cd exposure, shoot Cd concentrations were clearly above Cd hyperaccumulation threshold. Cadmium ions and, to a less extent, Zn were mainly retained in the roots. Exposure to Cd enhanced Zn translocation to the shoot, while decreased significant total Ca2+ uptake, suggesting that Cd uptake occurs through Ca2+ transporters. Nevertheless, it increased Ca2+ translocation to the leaf, possibly for photoprotection of photosystem II (PSII). Exposure to 800μM Zn or 40μM Cd resulted in increased Fe3+ uptake suggesting that in N. caerulescens, Cd uptake does not take place through the pathway of Fe3+ uptake and that conditions that lead to Cd and Zn accumulation in plants may also favor Fe accumulation. Despite the significant high toxicity levels of Zn and Cd in leaves, under Zn and Cd exposure, respectively, the allocation of absorbed light energy at PSII did not differ compared to controls. The results showed that N. caerulescens keep Cd and Zn concentrations in the mesophyll cells in non-toxic forms for PSII and that the increased Ca and Fe accumulation in leaves alleviates the toxicity effects. Chlorophyll fluorescence imaging revealed that PSII of N. caerulescens resisted better the phytotoxic effects of 20 times higher Zn than Cd exposure concentration. Overall, it is concluded that the use of chlorophyll fluorescence imaging constitutes a promising basis for investigating heavy metal tolerance of plants.

How metal hyperaccumulating plants can advance Zn biofortification

Zn hyperaccumulating plants reach Zn concentrations in their leaves that are several hundredfold higher than the target values set for Zn-biofortified crops. This extreme trait is relevant for Zn biofortification in various ways that are being discussed based on what is known about Zn hyperaccumulating plants. First, Zn hyperaccumulation is the result of changes in metal homeostasis networks shared by all higher plants. It is not strictly dependent on high Zn levels in the soil. Thus, mechanistic insights gained from the study of Zn hyperaccumulators can support the breeding and engineering of Zn-biofortified crops. Second, certain plant families, the Brassicaceae in particular, appear to be genetically predispositioned to evolve enhanced Zn accumulation, suggesting the existence of intermediate species with elevated Zn concentrations in their leaves below the hyperaccumulation threshold or the potential to breed for such plants. This calls for extended screening especially among potential vegetable crops, ideally guided by knowledge about the at least partially convergent evolution of Zn hyperaccumulation. Third, the introduction of leafy vegetables with high Zn concentrations or even of Zn hyperaccumulator leaves into diets may represent a valid complementary biofortification approach.

The evaluation of heavy metal accumulation and application of a comprehensive bio-concentration index for woody species on contaminated sites in Hunan, China

Fast-growing metal-accumulating woody plants are considered potential candidates for phytoextraction of metals. Shuikoushan mining, one of the biggest Pb and Zn production bases in China, presents an important source of the pollution of environment during the last 100 years. Over 150 km(2) of fertile soil have been contaminated by the dust, slag, and tailings from this mining. The goal of the present work has been to determine the content of Pb, Zn, Cd, and Cu in wild woody plants (18 species) naturally growing in this area. Two hundred five plant and soil samples from 11 contaminated sites were collected and analyzed. In addition, to assess the ability of multi-metal accumulation of these trees, we proposed a predictive comprehensive bio-concentration index (CBCI) based on fuzzy synthetic assessment. Our data suggest some adult trees could also accumulate a large amount of metals. Pb concentrations in leaves of Paulownia fortunei (Seem.) Hemsl. (1,179 mg/kg) exceeded the hyperaccumulation threshold (1,000 mg/kg). Elevated Pb concentrations (973.38 mg/kg) were also found in the leaves of Broussonetia papyrifera (L.) Vent., with a Pb bio-concentration factor of up to 0.701. Endemic species, Zenia insignis Chun exhibited huge potential for Zn and Cd phytoextraction, with the highest concentrations of Zn (1,968 mg/kg) and Cd (44.40 mg/kg), characteristic root nodules, and fast growth rates in poor soils. As for multi-metal accumulation ability, native species B. papyrifera was calculated to have the most exceptional ability to accumulate various metals simultaneously (CBCI 2.93), followed by Amorpha fruticosa L. (CBCI 2.72) and Lagerstroemia indica L. (CBCI 2.53). A trend of increasing metal from trunks to leaves (trunks < branches < leaves) and towards fine roots has been shown by metal partitioning between tissues. The proposed CBCI would allow for the selection of suitable trees for phytoremediation in the future.

Transformation of Tobacco with ScMTII Gene‐Enhanced Cadmium and Zinc Accumulation

AbstractGenetic transformation is gaining importance for developing plant types suitable to metal accumulate and/or hyperaccumulate. In this study, the transgenic tobacco plant which transferred the ScMTII gene from Saccharomyces cerevisiae to wild type tobacco cultivar Petite Havana (SR1) was grown on soils with low and high cadmium (Cd) and zinc (Zn) concentrations in a growth chamber for 6 weeks and compared to wild type tobacco for Cd and Zn accumulation. Cadmium and Zn accumulations in the transgenic and wild type tobacco plants were increased with the increasing Cd and Zn concentrations. Unlike Zn, the transgenic plant accumulated significantly higher amount of Cd compared to the wild type control plants. Shoot Cd concentrations of transgenic tobacco in higher Cd dosages reached the above the hyperaccumulation threshold value of 100 mg Cd kg−1 in the dry weight (DW). Transgenic tobacco accumulated 354, 400, 372, and 457 mg Cd kg−1 DW, for 10, 20, 40, and 80 mg Cd kg−1 soil treatments, respectively. These values are 3.5–4.5‐fold higher than that of Cd hyperaccumulation threshold value. With 10 mg kg−1 Cd treatment, the bioconcentration factor (BCF) of transgenic tobacco plants for Cd reached up to 35 in which the threshold value for BCF should be at least 10. Our results showed that the transgenic tobacco may be used as a good Cd hyperaccumulator plant and for phytoextraction of Cd contaminated soils, but not for Zn.

Gentle remediation at the former “Pertusola Sud” zinc smelter: Evaluation of native species for phytoremediation purposes

The master plan for a soil clean-up of the former zinc smelter “Pertusola Sud” (Crotone, Italy) considered gentle remediation options for a specific area where both by-products and industrial wastes had been disposed in the past. Although the soil is severely contaminated by metals (Cd, Cu, Ge, Hg, In, Pb, Tl and Zn) and metalloids (As and Sb), several plant species grow spontaneously in this area. Plants and soil samples were collected and analysed for trace element concentrations.In the shoots of Dittrichia viscosa the Cd concentration (112mgkg−1) exceeded the hyperaccumulation threshold. Phragmites australis and Silene bellidifolia concentrated about 30 and 40mgkg−1 of Tl in their shoots, respectively. Sb accumulation in leaves of Acacia saligna, Eucalyptus camaldulensis and P. australis was more than 20 times higher than herbs. The highest Zn concentration in shoots was recorded in D. viscosa (1172mgkg−1). The phytoremediation potential of plants was evaluated considering the concentration of metals and metalloids in the plant tissues and also the bioconcentration factor (BF) and translocation factor (TF). The plant requirements for Sb phytoextraction were verified for P. australis (1.66 BF, TF 9.02), and for E. camaldulensis (BF 1.11, TF 1.71) and Galactites elegans (BF 2.30, TF 1.37) for Tl. Scirpoides holoschoenus could be considered for phytostabilization and recommended to restore a green cover on bare soils at the Pertusola Sud site.

Elevated nicotianamine levels in Arabidopsis halleri roots play a key role in zinc hyperaccumulation.

Zn deficiency is among the leading health risk factors in developing countries. Breeding of Zn-enriched crops is expected to be facilitated by molecular dissection of plant Zn hyperaccumulation (i.e., the ability of certain plants to accumulate Zn to levels >100-fold higher than normal plants). The model hyperaccumulators Arabidopsis halleri and Noccaea caerulescens share elevated nicotianamine synthase (NAS) expression relative to nonaccumulators among a core of alterations in metal homeostasis. Suppression of Ah-NAS2 by RNA interference (RNAi) resulted in strongly reduced root nicotianamine (NA) accumulation and a concomitant decrease in root-to-shoot translocation of Zn. Speciation analysis by size-exclusion chromatography coupled to inductively coupled plasma mass spectrometry showed that the dominating Zn ligands in roots were NA and thiols. In NAS2-RNAi plants, a marked increase in Zn-thiol species was observed. Wild-type A. halleri plants cultivated on their native soil showed elemental profiles very similar to those found in field samples. Leaf Zn concentrations in NAS2-RNAi lines, however, did not reach the Zn hyperaccumulation threshold. Leaf Cd accumulation was also significantly reduced. These results demonstrate a role for NAS2 in Zn hyperaccumulation also under near-natural conditions. We propose that NA forms complexes with Zn(II) in root cells and facilitates symplastic passage of Zn(II) toward the xylem.

Evolution of nickel hyperaccumulation by Stackhousia tryonii (Celastraceae), a serpentinite-endemic plant from Queensland, Australia

To elucidate the evolutionary origin of nickel (Ni) hyperaccumulation by the Australian serpentinite-endemic plant Stackhousia tryonii Bailey, phylogenetic analyses of chloroplast and nuclear DNA for Stackhousia and its close relatives were combined with assays of plant-tissue Ni concentrations. Thirty-five plants from 20 taxa were analysed by sequencing nuclear rDNA (ITS) and the plastid trnL–F region. Phylogenetic analysis of sequence data was conducted under maximum parsimony and Bayesian search criteria. In all, 100 plants from 39 taxa, including all 33 Stackhousia species, were analysed for Ni concentration by radial inductively coupled plasma atomic-emission spectrometry (ICP–AES). In phylogenetic analyses, S. tryonii was monophyletic, nested within a monophyletic Stackhousia. Only S. tryonii contained concentrations of Ni above the hyperaccumulation threshold (0.1%; 1000 ppm), containing between 0.25% (2500 ppm) and 4.1% (41 000 ppm) Ni by dry weight. Nickel-hyperaccumulation ability appears to have been acquired once during diversification of Stackhousia, by S. tryonii.

Revisiting copper and cobalt concentrations in supposed hyperaccumulators from SC Africa: influence of washing and metal concentrations in soil

Metal concentrations have been determined in shoots of 12 species considered as Cu and/or Co hyperaccumulators, collected from five subpopulations in a Cu/Co mine in Katanga. Samples have been subjected to three cleansing protocols (water, alconox, alconox + EDTA). Cu/Co concentrations were significantly higher when demineralised water was used to wash samples compared to more aggressive solutions. Washing effect was largest for species with velvety (Acalypha cupricola) or sticky (Haumaniastrum katangense) indumentum. Element concentrations in shoots varied by two orders of magnitude, i.e. 45–2,891 mg kg−1 Cu (median: 329 mg kg−1) and 21–1,971 mg kg−1 Co (median: 426 mg kg−1) and were generally lower than previously published values. Only 9.3% of data exceeded the hyperaccumulation threshold (1,000 mg kg−1) for Cu and 13% for Co. Shoot concentrations varied by up to two orders of magnitude among subpopulations within species which was partly explained by variation of metal concentrations in the soil. Although the species considered in this paper undisputedly accumulate Co and Cu up to very high concentrations that require specific shoot tolerance mechanisms, the concept of hyperaccumulation, as defined for other metals like Zn and Ni, needs to be critically re-examined for Cu and Co. Our results suggest that many species so far regarded as Co/Cu hyperaccumulators might eventually turn out to be indicators.