Ni Hyperaccumulator Research Articles

Nickel increases productivity, Ca accumulation and reduces blossom-end rot in tomato

ABSTRACT Blossom-end rot (BER) is a physiological disorder related to calcium (Ca) deficiency in tomato fruits. However, adequate Ca supply is not sufficient to inhibit the BER incidence. The Ni-Ca interaction has increased the Ca content in Ni hyperaccumulators plants. However, studies with commercial plants are still incipient. The objective of this work was to evaluate the effect of Ni-Ca interaction in tomato plants, focusing on yield and incidence of BER. We carried out two trials. The first trial two Ca doses was used: sufficient Ca 0.70 g kg−1 (Ca S) and deficient Ca 0,35 g kg−1 (Ca D) and; four Ni doses applied via substrate (0.0; 0.5; 1.0 and; 2.0 mg kg−1). In the second trial, the same Ca doses of the first trial and four doses of Ni (0.0; 0.08; 0.42 and; 0.84 mM) applied, via foliar, at the beginning of flowering were used. We verified that the Ni-Ca interaction increased the tomato fruits yield, the shoot calcium accumulation reduced the BER incidence and increased the time of fruit color change (green to red). Even under stressed conditions such as a low Ca availability. Both Ni application ways showed positive results for fruit yield and decreased BER incidence.

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.

The effect of endophytic fungi on growth and nickel accumulation in Noccaea hyperaccumulators

The role of endophytic fungi isolated from different populations of European Ni hyperaccumulators was investigated in regard to the microorganisms' ability to enhance the hyperaccumulation of Ni in Noccaea caerulescens. Effects of particular species of endophytic fungi on adaptation of N. caerulescens to excess Ni were tested by co-cultivation with single strains of the fungi. Seven of these had a positive effect on plant biomass production, whereas two of the tested species inhibited plant growth; biomass production of inoculated plants was significantly different compared to non-inoculated control. Inoculation with six fungal strains: Embellisia thlaspis, Pyrenochaeta cava, Phomopsis columnaris, Plectosphaerella cucumerina, Cladosporium cladosporioides and Alternaria sp. stimulated the plant to uptake and accumulate more Ni in both roots and shoots, compared to non-inoculated control. P. columnaris was isolated from all plant species sampled. Strains isolated from Noccaea caerulescens and Noccaea goesingensis increased Ni root and shoot accumulation of their native hosts (compared to non-inoculated control). Inoculation of different populations of Noccaea with P. columnaris of foreign origin did not cause its host to accumulate more Ni, with the exception of the Ni-unadapted ecotype of N. goesingensis. Inoculation with P. columnaris from N. caerulescens significantly improved Ni uptake, but the effect of the fungus was not as prominent as in the case of N. caerulescens. By comparing the transcriptomes of N. caerulescens and N. goesingensis from Flatz inoculated with P. columnaris, we showed that enhanced uptake and accumulation of Ni in the plants is accompanied by an upregulation of several genes mainly involved in plant stress protection and metal uptake and compartmentation.

Soil chemistry, elemental profiles and elemental distribution in nickel hyperaccumulator species from New Caledonia

This study aimed to establish elemental profiles and to spatially resolve the elemental distribution in five New Caledonian woody Ni hyperaccumulator plant species (Geissois pruinosa var. pruinosa, Homalium francii, Hybanthus austrocaledonicus, Psychotria gabriellae, and Pycnandra acuminata) originating from the Cunoniaceae, Salicaceae, Violaceae, Rubiaceae, and Sapotaceae families respectively. Using synchrotron-based micro-X-ray Fluorescence (μXRF) imaging of different plant tissues, from the roots to the shoots and reproductive organs, this study aimed to clarify how distribution patterns of nickel, and other physiologically relevant elements, differ between these species. The results show that the tissue-level and cellular-level distribution of nickel in P. gabriellae, H. austrocaledonicus, G. pruinosa var. pruinosa, and H. francii conform with the majority of studied Ni hyperaccumulator plant species globally, including (temperate) herbaceous species, with localization mainly in epidermal cells and phloem bundles. However, P. acuminata has nickel-rich laticifers, which constitute an independent network of cells that is parallel to the vascular bundles and are the main sink for nickel. Synchrotron-based micro-X-ray Fluorescence (μXRF) is a powerful method for investigating how metal hyperaccumulation influences acquisition and spatial distribution of a wide range of elements. This non-invasive method enables investigation into the in vivo distribution of multiple elements and the structure and organisation of cells (e.g. laticifers).

The role of organic acids in the uptake and storage of nickel in hyperaccumulator plant, Brackenridgea palustris ssp. foxworthyi (Elm.) P.O. Karis

The role of low molecular weight organic acids (LMWOAs) in the Philippine nickel (Ni) hyperaccumulating plant, Brackenridgea palustris ssp. foxworthyi (Elm.) P.O. Karis is not yet fully understood. Using High Performance Liquid Chromatography (HPLC), the presence of organic acids such as oxalic, citric and malic acids were determined. Average nickel concentration in the plant tissues followed the ascending order: roots>stem>leaves with values of 7,294.73 µg/g, 7,412.30 µg/g and 9,866.46 µg/g, respectively. Among the organic acids analyzed, only oxalic acid was detected in all the plant tissues at considerable concentration. Linear correlation between oxalic acid and Ni concentrations in 0.025 M HCl plant extracts generated a positive r- value of 0.0437 indicating that as Ni content increases, oxalic acid also increases. This paper suggests that oxalic acid can be synthesized by B.palustris ssp. foxworthyi , therefore, it may acts as a ligand that chelates Ni and other metals to the aboveground tissues were it gets compartmentalized. To our knowledge, this will be the first report on the presence of organic acids in the Philippine endemic Ni hyperaccumulator plant, B. palustris ssp. foxworthyi whose potential was discovered more than thirty (30) years ago.

Fertilization regimes affecting nickel phytomining efficiency on a serpentine soil in the temperate climate zone

Phytomining of nickel (Ni) refers to cropping of selected Ni hyperaccumulator plants on Ni-rich serpentine soils. In this study, the effect of different fertilization regimes on the Ni yield of Odontarrhena chalcidica (syn. Alyssum murale) was evaluated within a field experiment on an Austrian serpentine site. Odontarrhena chalcidica was planted in six treatments: control, fertilized by mineral fertilizer, cow manure, pig manure, compost, and planted at higher plant density. A positive fertilization effect was observed: plants treated with NPK and pig manure produced significantly higher biomass (1.9 t ha−1 for both treatments). Nickel yields showed a clear trend for enhancement upon fertilization (cow manure: 22.7 kg Ni ha−1, pig manure: 21.3 kg Ni ha−1, NPK: 20.6 kg Ni ha−1), but were not significantly different from the control. As a result of Ni accumulation in plants, DTPA-extractable Ni pools were significantly lower after harvesting (average 37.3 mg kg Ni-DTPA−1) compared to the time of planting (average 45.6 mg kg Ni-DTPA−1) in organic fertilization treatments and plots of higher plant density. The application of organic fertilizers contributed also to improved soil quality. We conclude that fertilization can increase the phytomining potential of field-grown Ni hyperaccumulator plants in a soil-friendly manner.

Novel Insights Into the Hyperaccumulation Syndrome in Pycnandra (Sapotaceae).

The discovery of nickel hyperaccumulation, in Pycnandra acuminata, was the start of a global quest in this fascinating phenomenon. Despite recent advances in the physiology and molecular genetics of hyperaccumulation, the mechanisms and tolerance of Ni accumulation in the most extreme example reported to date, P. acuminata, remains enigmatic. We conducted a hydroponic experiment to establish Ni tolerance levels and translocation patterns in roots and shoots of P. acuminata, and analyzed elemental partitioning to gain insights into Ni regulation. We combined a phylogeny and foliar Ni concentrations to assess the incidence of hyperaccumulation within the genus Pycnandra. Hydroponic dosing experiments revealed that P. acuminata can resist extreme Ni concentrations in solution (up to 3,000 µM), and dosing at 100 µM Ni was beneficial to growth. All plant parts were highly enriched in Ni, but the latex had extreme Ni concentrations (124,000 µg g−1). Hyperaccumulation evolved independently in only two subgenera and five species of the genus Pycnandra. The extremely high level of Ni tolerance is posited to derive from the unique properties of laticifers. The evolutionary and ecological significance of Ni hyperaccumulation in Pycnandra is discussed in light of these findings. We suggest that Ni-rich laticifers might be more widespread in the plant kingdom and that more investigation is warranted.

Coupling nickel chemical speciation and isotope ratios to decipher nickel dynamics in the Rinorea cf. bengalensis-soil system in Malaysian Borneo

Aims. Rinorea cf. bengalensis is a Ni hyperaccumulator which occurs in Sabah (Malaysia), on Borneo Island, that is able to accumulate considerable amounts of Ni and influences the Ni cycle in surface soil layers, both in terms of Ni concentration and Ni isotopic composition. In this study, the biogeochemical processes underpinning Ni isotopic fractionation in the soil-plant system and the mechanisms regulating Ni homeostasis in R. cf. bengalensis plants were elucidated. Two specimens of R. cf. bengalensis of different ages and associated surface soils were collected from ultramafic soils in Sabah. Soil mineralogy, Ni concentrations, speciation and isotopic signatures were subsequently determined in plant and soil samples. Nickel in R. cf. bengalensis leaves is mainly complexed with citrate. Soil Ni available fractions have different δ60Ni values depending on the Ni bearing phases. Rinorea cf. bengalensis specimens take up lighter Ni isotopes and a pronounced isotopic fractionation within the plant is observed, especially in the young specimen. The results suggest that the observed fractionation in the young plant can be attributable to kinetic effects (lighter isotopes move faster), which become less evident in the older specimen, as Ni is redistributed and homogenized through phloem loading and unloading processes.

Evaluation of the performance of nickel hyperaccumulator plants as combustion fuel

Agromining is an emerging technology aiming at recovering metals from soils using hyperaccumulator (HA) plants. Plant cultivation and pyro- and hydrometallurgical processes have been designed for nickel (Ni) recovery. In this chain of processes, dry plants are burnt to obtain Ni-rich ashes that are subsequently treated. To ensure the sustainability and the profitability of the process, energy recovered at the combustion stage would add some value. The combustion of three Ni HA plants (Odontarrhena chalcidica, Leptoplax emarginata and Berkheya coddii) was investigated. Namely the higher and lower heating values (HHV and LHV) were measured with a calorimeter bomb and calculated with correlations, thermo-gravimetric analysis was performed, dry plants and ashes were characterized, and gas emissions were monitored during combustion at two temperatures. In a complementary approach, the self-heating behaviour of plants and ashes were compared, and the activation energy of such phenomenon was determined. These results allowed the identification of the step limiting the self-inflammation of the plants, which gives relevant information for their safe storage.

Transcriptome Response of Metallicolous and a Non-Metallicolous Ecotypes of Noccaea goesingensis to Nickel Excess.

Root transcriptomic profile was comparatively studied in a serpentine (TM) and a non-metallicolous (NTM) population of Noccaea goesingensis in order to investigate possible features of Ni hyperaccumulation. Both populations were characterised by contrasting Ni tolerance and accumulation capacity. The growth of the TM population was unaffected by metal excess, while the shoot biomass production in the NTM population was significantly lower in the presence of Ni in the culture medium. Nickel concentration was nearly six- and two-fold higher in the shoots than in the roots of the TM and NTM population, respectively. The comparison of root transcriptomes using the RNA-seq method indicated distinct responses to Ni treatment between tested ecotypes. Among differentially expressed genes, the expression of IRT1 and IRT2, encoding metal transporters, was upregulated in the TM population and downregulated/unchanged in the NTM ecotype. Furthermore, differences were observed among ethylene metabolism and response related genes. In the TM population, the expression of genes including ACS7, ACO5, ERF104 and ERF105 was upregulated, while in the NTM population, expression of these genes remained unchanged, thus suggesting a possible regulatory role of this hormone in Ni hyperaccumulation. The present results could serve as a starting point for further studies concerning the plant mechanisms responsible for Ni tolerance and accumulation.

Nickel hyperaccumulation in New Caledonian Hybanthus (Violaceae) and occurrence of nickel-rich phloem in Hybanthus austrocaledonicus.

Hybanthus austrocaledonicus (Violaceae) is a nickel (Ni) hyperaccumulator endemic to New Caledonia. One of the specimens stored at the local herbarium had a strip of bark with a remarkably green phloem tissue attached to the sheet containing over 4 wt% Ni. This study aimed to collect field samples from the original H. austrocaledonicus locality to confirm the nature of the green 'nickel-rich phloem' in this taxon and to systematically assess the occurrence of Ni hyperaccumulation in H. austrocaledonicus and Hybanthus caledonicus populations. X-ray fluorescence spectroscopy scanning of all collections of the genus Hybanthus (236 specimens) was undertaken at the Herbarium of New Caledonia to reveal incidences of Ni accumulation in populations of H. austrocaledonicus and H. caledonicus. In parallel, micro-analytical investigations were performed via synchrotron X-ray fluorescence microscopy (XFM) and scanning electron microscopy with X-ray microanalysis (SEM-EDS). The extensive scanning demonstrated that Ni hyperaccumulation is not a characteristic common to all populations in the endemic Hybanthus species. Synchrotron XFM revealed that Ni was exclusively concentrated in the epidermal cells of the leaf blade and petiole, conforming with the majority of (tropical) Ni hyperaccumulator plants studied to date. SEM-EDS of freeze-dried and frozen-hydrated samples revealed the presence of dense solid deposits in the phloem bundles that contained >8 wt% nickel. The occurrence of extremely Ni-rich green phloem tissues appears to be a characteristic feature of tropical Ni hyperaccumulator plants.

Intra-specific variation in zinc, cadmium and nickel hypertolerance and hyperaccumulation capacities in Noccaea caerulescens

The study aimed at characterizing the patterns of natural variation in the tolerance and accumulation capacities for zinc (Zn), cadmium (Cd), and nickel (Ni) between and within edaphic ecotypes of the Zn/Cd/Ni hyperaccumulator, Noccaea caerulescens. Tolerance was assessed in a hydroponic ‘sequential exposure’ test, using the lowest concentration that completely arrested root growth as an end point. Accumulation was measured as the foliar metal concentration after six weeks of growth at 5 µM Zn, 2 µM Cd, or 1 µM Ni. Zn and Cd tolerance were positively correlated, and highest in the calamine ecotype. Ni tolerance was without significant ecotypic variation. The ultramafic ecotype was as Zn-tolerant as the non-metallicolous one, but much more sensitive to Cd. The accumulation capacities for Zn, Cd and Ni were all positively correlated and without significant ecotypic variation. Zn hyperaccumulation capacity was species-wide, but Cd and Ni hyperaccumulation capacities were lacking in four populations (all calamine). There is considerable independent variation among populations regarding their Zn, Cd, and Ni accumulation capacities. This variation is most pronounced within the calamine ecotype, because some populations apparently had adopted an exclusion strategy for Zn or Cd hypertolerance, whereas others had not.

Ecophysiology of nickel hyperaccumulating plants from South Africa - from ultramafic soil and mycorrhiza to plants and insects.

An overview of 30 years of studies related to South African nickel hyperaccumulators is presented. Only five species have so far been identified as Ni hyperaccumulator plants among very rich and diversified South African flora. All of them occur on soils derived from ultramafic (serpentine) rocks and belong to the family Asteraceae: Berkheya coddii Roessler, Berkheya zeyheri subsp. rehmannii var. rogersiana, Berkheya nivea, Senecio coronatus, Senecio anomalochrous. Several techniques and methods were used to investigate ecophysiological aspects of the Ni hyperaccumulation phenomenon, from basic field and laboratory studies, to advanced instrumental methods. Analysis of elemental distribution in plant parts showed that in most cases the hyperaccumulated metal was stored in physiologically inactive tissues such as the foliar epidermis. However, an exception is Berkheya coddii, which has a distinctly different pattern of Ni distribution in leaves, with the highest concentration in the mesophyll. Such a distribution suggests that different physiological mechanisms are involved in the Ni transport, storage location and detoxification, compared to other hyperaccumulator species. Berkheya coddii is a plant with high potential for phytoremediation and phytomining due to its large biomass and potentially high Ni yield, that can reach 7.6% of Ni in dry mass of leaves. Senecio coronatus is the only known hyperaccumulator with two genotypes, hyperaccumulating and non-hyperaccumulating, growing on Ni-enriched/metalliferous soil. Detailed ultrastructural studies were undertaken to characterize specialized groups of cells in the root cortex of Ni-hyperaccumulating genotype, that are not known from any other hyperaccumulator. The occurrence of arbuscular mycorrhiza (AM) in Ni-hyperaccumulating plants was found for the first time in South African hyperaccumulator plants, and this type of symbiosis has been proved obligatory in all of them. There is a significant influence of mycorrhiza on the concentration and distribution of several elements. Three highly specialized herbivore insects feeding only on Ni hyperaccumulator plants were identified: Chrysolina clathrata (formerly Chrysolina pardalina), Epilachna nylanderi and Stenoscepa sp. The Ni-elimination strategies of these specialised insects have been established. Microbiological studies have revealed several genera of fungi and bacteria isolated from B. coddii leaves as well as presence of specialised, Ni-resistant yeasts in the C. clathrata gut. Understanding ecophysiological response to harsh environment broadens our knowledge and can have practical applications in cleaning polluted environments through phytomining/agromining. Finally, conservation aspects are also discussed and lines for future research are proposed.

Use and validation of magnetic properties for differentiating nickel hyperaccumulators and non-nickel hyperaccumulators in ultramafic regions

Nickel hyperaccumulators are plants that can absorb a large amount of Ni in their tissues. Nickel hyperaccumulators have a strong potential to be used as bioindicators in mineral explorations. Previous studies show that analysis of magnetic susceptibility has a potential to accelerate the screening of Ni hyperaccumulators. In this study, we investigated the use magnetic characterizations for screening directly Ni hyperaccumulators and non-Ni hyperaccumulators from ultramafic regions. For comparison, the characterization of Ni hyperaccumulators was also performed on Alyssum murale and Alyssum corsicum, which were cultivated on lateritic soils. The metals concentrations in the tissues were determined via inductively coupled plasma optical emission spectroscopy analysis, atomic absorption spectroscopy analysis, and X-ray fluorescence analysis. The magnetic characterizations were conducted by magnetic susceptibility measurement, acquisition of hysteresis curves, and analysis of the thermomagnetic properties. The results showed that the concentration of Ni in A. murale and A. corsicum were correlated to the magnetic susceptibility values. Concurrently, all samples had negative magnetic susceptibility values were non-Ni hyperaccumulators. Two new species of Ni hyperaccumulators were found in Halmahera and Sulawesi and have high potential for use in phytoremediation, phytomining or mineral exploration.

Citric acid-assisted accumulation of Ni and other metals by Odontarrhena muralis: Implications for phytoextraction and metal foliar distribution assessed by μ-SXRF

Odontarrhena muralis is one of the most promissing plant species for Ni phytomining, and soil amendments can further increase its Ni phytoextraction ability. Here we investigated whether Ni phytomining/phytoremediation using this Ni hyperaccumulator can benefit from applying citric acid to a serpentine soil that is naturally enriched in Ni (>1000 mg kg−1). Synchrotron micro X-ray fluorescence (μ-SXRF) was used to image Ni and other metal distributions in whole fresh leaves of O. muralis. Leaf Ni accumulation in plants grown on citric acid-amended soil increased up to 55% while Co, Cr, Fe, Mn, and Zn concentrations were 4-, 14-, 6-, 7- and 1.3-fold higher than the control treatment. O. muralis presented high bioconcentration factors (leaf to soil concentration ratio) to Ni and Zn whereas Cr was seemingly excluded from uptake. The μ-SXRF images showed a uniform distribution of Ni, preferential localization of Co in the leaf tip, and clear concentration of Mn in the base of trichomes. The citric acid treatments strongly increased the Co fluoerescence intensity in the leaf tip and altered the spatial distribution of Mn across the leaf, but there was no difference in Ni fluorescence counts between the trichome-base region and the bulk leaf. Our data from a serpentine soil suggests that citrate treatment enhances Ni uptake, but Co is excreted from leaves even in low leaf concentrations, which can make Co phytoming using O. muralis unfeasible in natural serpentine soils.

Metal accumulating plants, their importance. Nickel hyperaccumulator Leucocroton havanensis Borhidi

Over 700 of vegetable species in the world are capable of accumulating in their tissues large amounts of metal elements (Ni, Cu, Co, Cr, Mn and Zn, among others); that is why they are called metal hyperaccumulators.This physiological process has been interpreted as an adaptation of plants to the metal polluted soils where they live. Leucocroton havanensis Borhidi is an endemic Ni hyperaccumulator species of xeromorphic thorn bushes on “serpentine soils” in Cuba. Due to the high concentrations of Ni, Cr, Co, Mn, Mg and Fe, together with the low concentrations of Caand macronutrients such as N, P, and K, these metal polluted soils host a high representation of endemic plants with distinct physiological characteristics. The L. havanensis species is classified as endangered in the Red List of Cuban Flora due to the reduced amount of existing population because of human economic activity. Phytoremediation and agro-mining have been implemented in several countries within the context of sustainable development. These new technologies use hyperaccumulating plants as an alternative way to extract metals from the soil. Damage to the environment is thus avoided and the phytogenetic resources of the miningareas are preserved. The ecological function of metal hyperaccumulation may be related to the defense of these species against pathogens and herbivores. The Hyperaccumulated Ni in the leaves of Streptanthus polygaloides protect the plant from the attack of bacteria and fungi and from the Ambigolimax valentianus slugs.

Nickel phytomining from industrial wastes: Growing nickel hyperaccumulator plants on galvanic sludges

Nickel (Ni) is used in numerous industrial processes, with large amounts of Ni-rich industrial wastes produced, which are largely sent to landfill. Nickel recovery from waste materials that would otherwise be disposed is of particular interest. Nickel phytomining represents a new technology in which hyperaccumulator plants are cultivated on Ni-rich substrates for commercial metal recovery. The aim of this study was to investigate the possibility of Ni transfer from industrial waste into plant biomass, to support recovery processes from bio-ores. Different industrial galvanic sludges (containing 85–150 g kg−1 Ni) were converted into artificial substrates (i.e. technosols) and the Ni hyperaccumulator Odontarrhena chalcidica (formerly Alyssum murale) was cultivated on these Ni-rich matrices. A greenhouse pot experiment was conducted for three months including an ultramafic soil control and testing fertilized (NPK) and unfertilized replicates. The results showed that fertilization was effective in improving plant biomass for all the substrates and that O. chalcidica was capable of viably growing on technosols, producing a comparable biomass to O. chalcidica on the control (ultramafic soil). On all technosols, O. chalcidica achieved Ni shoot concentrations of more than >1000 mg Ni kg −1 and maximum Ni uptake was obtained from one of the technosols (26.8 g kg −1 Ni, unfertilized; 20.2 g kg −1 Ni, fertilized). Nickel accumulation from three of the technosols resulted to be comparable with the control ultramafic soil. This study demonstrated the feasibility of transferring Ni from toxic waste into the biomass of Odontarrhena chalcidica and that phytomining from galvanic sludge-derived technosols can provide similar Ni yields as from natural ultramafic soils.

Localization of nickel in the hyperaccumulator plant Breynia cernua (Poir.) Mull.Arg. discovered in the nickeliferous laterites of Zambales, the Philippines

A newly discovered nickel (Ni) hyperaccumulator plant, Breynia cernua (Phyllanthaceae) from the nickeliferous laterites of Acoje, Zambales, has elevated Ni concentrations. Its computed bioaccumulation factor is 1.7 and the translocation factor is 4.6. Leaf tissues which contain the highest Ni concentrations were analyzed using optical microscopy and Scanning Electron Microscopy with Energy-Dispersive X-ray (SEM/EDX). The results indicated that most of the Ni are stored in the epidermal cells of the leaves, followed by the mesophyll cells then the vascular bundles. The accumulation of Ni found in the subcellular locations, probably in the vacuoles, may prevent phytotoxicity that could affect the normal function of other cellular structures. A possible resistance mechanism through strain avoidance towards Ni accumulation makes B. cernua capable of tolerating elevated Ni concentrations in its tissues. The tolerance of B. cernua to Ni phytotoxicity suggests possible applications of this hyperaccumulator plant in phytoextraction technology.

A nickel phytomining field trial using Odontarrhena chalcidica and Noccaea goesingensis on an Austrian serpentine soil

Phytomining of nickel (Ni) is based on the cropping of Ni hyperaccumulators on Ni-rich serpentine soils. The efficiency of this approach is dependent on shoot nickel concentration and harvestable biomass. In a field experiment conducted on an Austrian serpentine site, the phytomining efficiency of the two plant species Odontarrhena chalcidica (syn. Alyssum murale) and Noccaea goesingensis was evaluated. O. chalcidica was planted in three treatments: control, sulphur application (0.46 g S kg−1 soil) and intercropping with the legume Lotus corniculatus. For N. goesingensis the treatments control, high-density planting (110 plants m−2) and intercropping were implemented. Given the experimental set-up, shoot biomass, shoot Ni concentration and thus the total amount of harvested Ni were on average higher for O. chalcidica. The highest Ni yield was achieved with O. chalcidica, reaching 55 kg Ni ha−1 in the sulphur treatment. N. goesingensis showed the maximum yield in the high-density treatment with 36 kg Ni ha−1. However, high-density planting of N. goesingensis and sulphur application to O. chalcidica plots did not significantly increase the Ni yield compared to the control. Intercropping with L. corniculatus tended to decrease the shoot biomass of both species. Planting of the hyperaccumulators led to a decrease of DTPA-extractable Ni and to an increase of soil pH, with the exception of sulphur-amended plots. Likewise, rhizosphere soil pH was higher than bulk soil values. Our data suggest that in particular O. chalcidica is suitable for Ni phytomining on the tested site. Measures to further increase the Ni yield and to optimise crop management will be evaluated in follow-up experiments.

Soil amendments affecting nickel uptake and growth performance of tropical ‘metal crops’ used for agromining

Ultramafic soils are usually marginal in macronutrients (nitrogen (N), phosphorus (P), potassium (K) and calcium (Ca)) for growth of crop plants. Commercial nickel (Ni) agromining is dependent on attaining high yield and high Ni concentration in harvestable biomass of Ni hyperaccumulator species. We previously reported on the biomass responses of two promising tropical ‘metal crops’ (Phyllanthus rufuschaneyi and Rinorea cf. bengalensis) to rates of N, P, and K fertilisers. Calcium, sulphur (S) and organic matter amendments have varied effects on the biomass production and Ni uptake in temperate Ni hyperaccumulator species used in agromining, but the trends in tropical ‘metal crops’ are not reported to-date. We investigated the effects of these amendments on the growth performance and the Ni (and other elements) uptake in P. rufuschaneyi and R. cf. bengalensis The experiments consisted of a large 12-month randomised growth trial in large pots in Sabah (Malaysia) using ultramafic soils under different treatment levels of soluble Ca and S, and organic matter amendments. We found that Ca and S additions had no significant effects on the growth of P. rufuschaneyi and R. cf. bengalensis. Organic matter amendments had strong positive effect on the growth of R. cf. bengalensis (p < 0.05), but we recorded significant negative growth response in P. rufuschaneyi. Whereas Ca and S additions improved the Ni uptake in these species, organic matter amendments significantly reduced the shoot Ni concentrations in both species. Our findings indicate that Ca and S additions are important in the agronomy of tropical ‘metal crops’ to be used in economic Ni agromining, but organic matter amendments may not be useful.