Heterogeneity of epidermal cells in relation to nickel accumulation in hyperaccumulator plants belonging to the genus <Emphasis Type="Italic">Alyssum</Emphasis> L.

Epidermal cells of some plants are able to accumulate high levels of heavy metals (Zn, Ni, Cd). We studied this ability in plants in the genus Alyssum L. distinguished by tolerance to nickel (Ni). It was established that the predominant Ni accumulation occurred in epidermis, whereas in other tissues lower concentrations of the metal were revealed. It was also found that epidermal cells were characterized by heterogeneity in relation to Ni accumulation. The highest metal amount was accumulated in ordinary epidermal cells and in trichomes. Species-specific features of Ni distribution in leaf tissues in Alyssum spp. were shown. The reasons for the heterogeneity of epidermal cells in relation to Ni accumulation were discussed. We have attempted to resolve the contradictions encountered in the literature concerning the distribution and accumulation of Ni in the leaf tissues of plants belonging to the genus Alyssum L.     

Cellular compartmentation of nickel in the hyperaccumulators Alyssum lesbiacum, Alyssum bertolonii and Thlaspi goesingense.

Nickel uptake and cellular compartmentation were investigated in three Ni hyperaccumulators: Alyssum bertolonii (Desv), Alyssum lesbiacum (Candargy) and Thlaspi goesingense (Hálácsy). The three species showed similar hyperaccumulation of Ni, but T. goesingense was less tolerant to Ni than the two Alyssum species. An addition of 500 mg Ni kg(-1) to a nutrient-rich growth medium significantly increased shoot biomass of all three species, suggesting that the Ni hyperaccumulators have a higher requirement for Ni than normal plants. Energy-dispersive X-ray microanalysis (EDXA) was performed on frozen-hydrated tissues of leaves (all species) and stems (Alyssum only). In all species analysed, Ni was distributed preferentially in the epidermal cells, most likely in the vacuoles, of the leaves and stems. In stems, there was a second peak of Ni in the boundary cells between the cortical parenchyma and the vascular cylinder. The non-glandular trichomes on the leaf surfaces of the two Alyssum species were highly enriched with Ca, but contained little Ni except in the base. In the leaves of T. goesingense, the large elongated epidermal cells contained more Ni than the cells of the stomatal complexes. The role of cellular compartmentation in Ni hyperaccumulation is discussed.     

Nickel distribution in the hyperaccumulator Alyssum serpyllifolium Desf. spp. from the Iberian Peninsula

Abstract

The relative concentration and distribution of nickel (Ni) in vegetative tissues (leaves, stems and trichomes) and reproductive organs (seeds) was studied using energy-dispersive X-ray microanalysis (EDXS) and scanning electron microscopy (SEM) in two previously studied Ni-hyperaccumulator subspecies of Alyssum serpyllifolium Desf. growing naturally in ultramafic soils of the Iberian Peninsula: A. serpyllifolium ssp. lusitanicum Dudley & P. Silva and A. serpyllifolium ssp. malacitanum Rivas Goday ex G. López. Both taxa showed that Ni accumulates preferentially in the leaves, exhibiting a homogeneous distribution on both epidermis surfaces. The highest Ni concentrations were found inside the epidermal cells and at the base of trichome stalks. Ni accumulation in seeds was lower than in the vegetative organs. The location of Ni in these plants allows us to postulate that its accumulation is a protection mechanism against external stress.     

Hyperaccumulator Alyssum murale relies on a different metal storage mechanism for cobalt than for nickel

The nickel (Ni) hyperaccumulator Alyssum murale has been developed as a commercial crop for phytoremediation/phytomining Ni from metal-enriched soils. Here, metal co-tolerance, accumulation and localization were investigated for A. murale exposed to metal co-contaminants. A. murale was irrigated with Ni-enriched nutrient solutions containing basal or elevated concentrations of cobalt (Co) or zinc (Zn). Metal localization and elemental associations were investigated in situ with synchrotron X-ray microfluorescence (SXRF) and computed-microtomography (CMT). A. murale hyperaccumulated Ni and Co (> 1000 microg g(-1) dry weight) from mixed-metal systems. Zinc was not hyperaccumulated. Elevated Co or Zn concentrations did not alter Ni accumulation or localization. SXRF images showed uniform Ni distribution in leaves and preferential localization of Co near leaf tips/margins. CMT images revealed that leaf epidermal tissue was enriched with Ni but devoid of Co, that Co was localized in the apoplasm of leaf ground tissue and that Co was sequestered on leaf surfaces near the tips/margins. Cobalt-rich mineral precipitate(s) form on leaves of Co-treated A. murale. Specialized biochemical processes linked with Ni (hyper)tolerance in A. murale do not confer (hyper)tolerance to Co. A. murale relies on a different metal storage mechanism for Co (exocellular sequestration) than for Ni (vacuolar sequestration).     

Species adaptation in serpentine soils in Lesbos Island (Greece): metal hyperaccumulation and tolerance

Serpentine (ultramafic) soils, containing relatively high nickel and other metal concentrations, present a stressful environment for plant growth but also a preferred substrate for some plants which accumulate nickel in their tissues. In the present study we focused on: (1) the relationships between serpentine soils of Lesbos Island (Greece) and serpentinophilic species in order to test their adaptation to the ‘serpentine syndrome’, and (2) the Ni-hyperaccumulation capacity of Alyssum lesbiacum, a serpentine endemic, Ni-hyperaccumulating species, recorded over all its distribution for the first time. We sampled soil and the most abundant plant species from the four serpentine localities of Lesbos Island. Soil and leaf elemental concentrations were measured across all the sites. Our results confirmed our hypothesis that serpentinophilic species are adapted to elevated heavy metal soil concentrations but restricting heavy metal concentration in their leaves. We demonstrated that different A. lesbiacum populations from Lesbos Island present differences in Ni hyperaccumulation according to soil Ni availability. Our results highlighted the understanding of serpentine ecosystems through an extensive field study in an unexplored area. Alyssum lesbiacum and Thlaspi ochroleucum emerge as two strong Ni hyperaccumulators with the former having a high potential for phytoextraction purposes.     

Cellular and subcellular compartmentation of Ni in the Eurasian serpentine plants Alyssum bracteatum, Alyssum murale (Brassicaceae) and Cleome heratensis (Capparaceae)

This study investigated the cellular and subcellular compartmentation of Ni in the Eurasian serpentine species Alyssum murale, Alyssum bracteatum and Cleome heratensis and a non-serpentine population of A. murale (as a control) grown in hydroponic culture. Plant growth responses and Ni uptake clearly revealed the higher Ni tolerance of serpentine plants than the non-serpentine plants. Serpentine A. murale and A. bracteatum grew better at elevated (0.01 mM) Ni in the nutrient solution, supporting the view that the Ni hyperaccumulators have a higher requirement for Ni than normal plants. Low shoot Ni content of C. heratensis in response to the high Ni treatments indicated that this species employs an avoidance strategy for Ni tolerance. Energy-dispersive X-ray microanalysis showed that Ni was highly concentrated in the cell walls and cell lumen, most likely the vacuoles, of leaf epidermis of A. murale and A. bracteatum rather than in the mesophyll cells. EDX spectra from leaves of the non-serpentine A. murale suggested that Ni accumulated in both epidermal and mesophyll cells but not in the epidermal cell walls. Growth reduction and Ni toxicity in plants of the non-serpentine A. murale could be due to accumulation of Ni in the lumen of leaf mesophyll cells. Our data suggest that cellular and subcellular compartmentation are both possible mechanisms for Ni tolerance employed by the serpentine A. murale and A. bracteatum.     

Nickel and zinc effects,accumulation and distribution in ruderal plants Lepidium ruderale and Capsella bursa-pastoris

Ruderal plants can grow in polluted areas, but little is known about heavy metal accumulation and distribution in them. Here Ni and Zn accumulation, distribution and effects were investigated in Lepidium ruderale and Capsella bursa-pastoris grown at 5–30 µM Ni(NO₃)₂ or 10–80 µM Zn(NO₃)₂. Metal contents were measured by flame atomic absorption spectrophotometry and tissue distribution of metals was studied histochemically. Ni was more toxic than Zn for both plants. When metal-induced growth-inhibiting effects were compared at various metal concentrations in solution, L. ruderale was more tolerant to Ni, whereas C. bursa-pastoris to Zn. However, when compared at similar Zn or Ni contents in roots, root growth of C. bursa-pastoris was more tolerant than that of L. ruderale. On the contrary, at similar Zn or Ni contents in shoots, shoot growth of L. ruderale was more tolerant. Both plants are excluders maintaining low metal levels in shoots. In roots, Ni located in protoplasts while Zn was also detected in cell walls. Metal accumulation in root apices resulted in growth inhibition. Ni accumulation in root cortex constrained metal translocation into central cylinder and then to shoots, where it located only in conductive tissues and epidermis, particularly in leaf trichomes of C. bursa-pastoris. Zn was translocated to shoots more actively and distributed in all shoot tissues, being accumulated in leaf vascular bundles and epidermis. To conclude, these patterns of Ni and Zn distribution are aimed at metal sequestration in roots and leaf epidermis, thus keeping mesophyll from metal penetration and pigment degradation.     

Chelator-buffered nutrient solution is ineffective in extracting Ni from seeds of Alyssum

Hyperaccumulator species of the genera Alyssum can accumulate 100 times more Ni than normal crops and are therefore used for phytomining and phytoextraction of nickel contaminated soils. Basic studies on the physiology and metal uptake mechanisms of these plants are needed to increase efficiency and uptake capacity of Nickel (Ni) by hyperaccumulators. Recent attempts to disclose if those hyperaccumulator species require higher Ni level than normal plants failed because of the high Ni content in the seeds (7000-9000 microg g(-1)). In this study, we attempted to use chelator buffered nutrient solution to deplete Ni from the seed/seed coat and to obtain low Ni seedlings of Alyssum cultivars to be used in physiology studies. HEDTA-buffered nutrient solution did not deplete Ni from the seeds, perhaps because Ni was mainly localized within the seedling embryonic tissues with greatest Ni enrichment in the cotyledons and hypocotyls. We could not observe any positive correlation between seed fitness and germination capacity with seed Ni content. Investigation of nickel localization in Alyssum seeds using synchrotron X-ray microfluorescence (micro-SXRF) showed that nickel is localized in the embryonic tissues with greatest Ni enrichment observed in the cotyledons and hypocotyl.     

Localisation of nickel and mineral nutrients Ca,K, Fe,Mg by Scanning Electron Microscopy microanalysis in tissues of the nickel-hyperaccumulator Alyssum bertolonii Desv. and the non-accumulator Alyssum montanum L.

Plants of the nickel-hyperaccumulator Alyssum bertolonii Desv. and of the non-accumulator A. montanum L. growing on a serpentine site in Tuscany, Italy, and plants of A. montanum from a nearby non-serpentine site were analysed for metal concentration and localisation. The leaves of A. bertolonii contained 160 times more nickel than those of A. montanum from the same site, thus demonstrating its hyperaccumulation capacity towards this metal. On the other hand, both species showed an inversion of the Ca/Mg ratio in their organs relative to the soil. Nickel localisation in plant tissues was examined by Scanning Electron Microanalysis (SEM/EDX). In A. bertolonii, a specific pattern of nickel distribution was detected, with the highest concentrations present in parenchyma and sclerenchyma cells for the roots; in the shoots, the highest amounts of nickel were found in the stem epidermis, the leaf epidermal surface, and the leaf trichome base. This particular nickel tissue distribution pattern was not found in the non-accumulator A. montanum growing on serpentine soil. Other mineral nutrients, namely Mg, Ca, K, Fe, instead, had a similar distribution in the two species. The A. montanum plants from the non-serpentine site had very low nickel levels in their tissues, and these were of the same magnitude as those found in A. bertolonii plants grown in a greenhouse on commercial horticultural soil with low nickel concentration. In A. bertolonii plants, the tissue-specific allocation patterns appeared to depend on the degree of nickel hyperaccumulation, which is, in turn, directly linked to the soil characteristics.     

Comparison of essential and non-essential element distribution in leaves of the Cd/Zn hyperaccumulator Thlaspi praecox as revealed by micro-PIXE

A detailed localization of elements in leaf tissues of the field-collected Cd/Zn hyperaccumulator Thlaspi praecox (Brassicaceae) growing at a highly metal-polluted site was determined by micro-proton-induced X-ray emission (micro-PIXE) in order to reveal and compare nutrient and non-essential element accumulation patterns in the case of multiple metal accumulation within particular leaf tissues, including the detailed distribution between apoplast and symplast regions. On the larger scans, the highest concentrations of metals were observed in the epidermis, S and Ca in the palisade mesophyll, Cl in the spongy mesophyll and vascular bundles, and P and K in the vascular bundles. On the more detailed scans, the highest Cd, Pb, Cl and K concentrations were observed in vascular bundle collenchyma. The relative element distribution (%) was calculated based on concentrations of elements in particular leaf tissues and their relative weight portions, indicating that most of the accumulated Zn was located in epidermises, while the majority of Cd and Pb was distributed within the mesophyll. Detailed scans of epidermal/mesophyll tissues revealed that Zn was mainly accumulated and detoxified in the symplast of large vacuolated epidermal cells, Cd in the mesophyll symplast, and Pb in the mesophyll symplast and apoplast.     

In vivo localization of manganese in the hyperaccumulator Gossia bidwillii (Benth.) N. Snow & Guymer (Myrtaceae) by cryo-SEM/EDAX

Gossia bidwillii (Myrtaceae) is a manganese (Mn)-hyperaccumulating tree native to subtropical eastern Australia. It typically contains foliar Mn levels in excess of 1% dry weight. However, in G. bidwillii and other Mn-hyperaccumulating species, the cellular and subcellular localization of Mn has not been measured. Quantitative in vivo cryo-scanning electron microscopy (SEM)/energy dispersive X-ray analysis (EDAX) was used to localize Mn and other elements in tissue collected from mature trees growing in a natural population. Cryo-SEM showed that the leaf mesophyll is differentiated as a double-layer palisade mesophyll above spongy mesophyll. Transmission electron microscopy (TEM) revealed that the palisade and epidermal cells are highly vacuolated. EDAX data were used to estimate in situ vacuolar Mn concentrations of all cell types in fresh cryo-fixed leaf tissues. The highest average vacuolar Mn concentration of over 500 mM was found in the upper-layer palisade mesophyll, while the lowest concentration of around 100 mM was found in the spongy mesophyll. Qualitative in vivo cryo-SEM/EDAX was employed to further investigate the spatial distribution of Mn in fresh leaf tissues and young bark tissue, which was also found to have a high Mn concentration. It is concluded that Mn distribution in G. bidwillii is quantitatively different to metal distribution in other hyperaccumulating species where the highest localized concentrations of these elements occur in non-photosynthmetic tissues such as epidermal cells and associated dermal structures including trichomes and leaf hairs.     

Effects of Ni Deficiency on Some Nitrogen Metabolites in Cowpeas (Vigna unguiculata L. Walp)

Cowpeas grown in nutrient solutions, from which Ni had been removed by a ligand exchange technique, accumulated urea in most tissues. Urea levels were highest (up to 3.1 percent dry weight) in necrotic leaf tips. Urea accumulation in Ni-deficient cowpea tissues amounted to about 1 percent of the total N. The accumulation of urea was presumably associated with the catabolism of N compounds in older tissues and the redistribution of N catabolites within the plant during the reproductive growth. The exclusion of N salts from the nutrient media at a late stage of growth, either with or without added Ni, led to a general amelioration of urea accumulation and a lower level of the related amino acid, arginine, in root and stem tissue. Plant leaves that contained toxic levels of urea and displayed necrotic symptoms had tissue Ni levels ranging from less than 0.01 to 0.15 μg Ni per gram dry weight. Nickel concentrations in tissue from plants not treated with Ni, were initially very low, but increased as the cowpeas matured. Apparently, there was a source of Ni contamination in the Ni-deficient growth media which provided a source of Ni for uptake by the plants during growth. Ureide levels were low and unaffected by Ni deprivation. No evidence for free purines or uric acid accumulation in plant tissues could be found. It is hypothesized that Ni (and urease) participates in the normal N metabolism of these plants during the reproductive phase of growth.     

玉米叶表皮短细胞发育过程的形态特征及栓质细胞作用研究

采用大田试验,直接撕表皮或对叶片进行固定处理,结合单染、复染、荧光染色等多种细胞学显色方法,利用光学显微镜、荧光显微镜和扫描电子显微镜系统观察玉米叶表皮短细胞的发生时期、发育过程、分布规律以及形态结构特征,研究K⁺和H₂O₂在栓质细胞中的分布变化与表皮其它细胞中K⁺和H₂O₂的分布及气孔器开关的关系,为进一步挖掘短细胞的新功能提供细胞学依据。结果表明：（1）短细胞是同步发生在玉米多叶位新表皮组织形成过程中,所有植株从第7新生叶,大部分第6叶,极少数第5叶的基部同时开始发生短细胞,之后新生的高位叶也均发生短细胞,并随着叶位的升高叶片各部位短细胞密度均增大,所有植株的1~4叶（因不再生长）均无短细胞出现。（2）初期发育的叶表皮细胞进行不对称分裂,生成相互交替的长、短细胞,有的短表皮细胞横（垂直叶脉）分裂,形成栓质细胞和硅质细胞对;栓质细胞基部与叶肉细胞相邻,硅质细胞嵌在栓质细胞和表皮细胞间偏上。（3）有短细胞发生的叶片,宏观背面发亮且覆有蜡质层,微观表皮细胞的着色特性发生了变化;栓质细胞为面包形柱状细胞,硅质细胞为哑铃形扁细胞。（4）气孔器张开时,栓质细胞中没有K⁺和H₂O₂的积累;气孔器关闭时,栓质细胞中积累了大量的K⁺和H₂O₂,且栓质细胞中K⁺和H₂O₂的积累始终与副卫细胞中K⁺和H₂O₂的积累变化一致,而硅质细胞和长细胞没有K⁺和H₂O₂的积累。该研究确定了玉米叶表皮短细胞发生的时期;展示了其发育过程的形态学变化特征;发现栓质细胞中K⁺和H₂O₂的积累随气孔器开关呈周期性变化,且与副卫细胞中K⁺和H₂O₂的积累变化保持一致。     

Multiscale and age-dependent leaf nickel in the Ni-hyperaccumulator <Emphasis Type="Italic">Leptoplax emarginata</Emphasis>

Nickel-hyperaccumulator plants are of interest due to their potential use in agromining. We aimed to characterize leaf traits and Ni concentration variabilities occurring between individual plants, leaves of differing age or between various leaf tissues, in a single Greek population of the Ni-hyperaccumulator Leptoplax emarginata (Boiss.) O.E. Schulz. We linked these results to ecophysiological characteristics and other element concentrations at leaf and leaf tissue scales. We measured leaf gas exchanges, stomatal density, and we carried out rapid freezing and freeze-drying processes on leaf sections before microanalysis with scanning electron microscopy and energy-dispersive spectrometry. Leaf or leaf-tissue Ni concentrations were influenced by a combination of individual plant and leaf age factors. The greatest Ni concentrations were found in the highly transpiring young and thin leaves with the greatest stomatal densities. Indeed, Ni was statistically seven times more concentrated in both epidermis layers than in their bulk neighbour leaf counterparts, whatever the leaf age. In both epidermis layers, increases in the Ni–S and Mg–S correlations from the oldest leaves to the youngest ones were observed. The Mg:Ni, Ca:Ni and P:Ni mass ratios decreased from the oldest leaves to the mature leaves. We would recommend time-series characterization of leaf traits belonging to at least three plant replicates in order to take into account the allogamous character of many Ni-hyperaccumulator plants. Long-distance Ni transport via the xylem is predominant in the Ni-hyperaccumulator L. emarginata whereas a complementary redistribution via the phloem should also occur. The perspectives of this study are the validation and refinement of this process over shorter periods using relevant tracers.     

The Accumulation of Zinc and Nickel in Irankoh Indigenous Plant Species on a Contaminated Land

The accumulation of heavy metals by plants determines both the micronutrient content and the toxic metal content of our food. A field survey of higher terrestrial plants growing on a metalliferous site of the Iranian arid mountain in Isfahan was conducted to identify species accumulating exceptionally large concentrations of Zn and Ni in shoots and roots. Plant samples were collected from Irankoh areas near the Bama Pb and Zn mine. Sampling was carried out in Spring 2004 and analyzed for DTPA (Diethylene triamine pentaacetic acid) extractable Zn and Ni by atomic absorption spectrophotometry. Mean total and available Zn in the studied soils were 259.7 μgg^?1 and 5.067 μgg^?1, respectively. Soil total and available Ni were relatively low (58.9 μgg^?1 and 0.143 μgg^?1 respectively). Zinc concentrations were considerably high in shoots of Stachys inflate, Ebenus stellata, and Astragalus glaucanthus (556.88, 508.8, and 449.53 μgg^?1, respectively). Nickel concentrations were markedly high in shoots of Teucrium polium, Alyssum bracteatum, and Ebenus stellata (13.21, 10.98, and 8.84 μgg^?1, respectively). Zinc translocation factor (TF or shoot/root concentration ratio) was higher than Ni TF in most plant species. Zinc and Ni enrichment factors and shoot/root concentration ratios were also significantly high in Stachys inflate, Ebenus stellata, Astragalus glaucanthus Teucrium polium, Stipa barbata, Bromus tectorum, and Alyssum bracteatum. Results suggest that these plants could be good candidates for use in the revegetation and phytoremediation of Zn and Ni contaminated lands in arid regions.     

Nickel localization in tissues of different hyperaccumulator species of Euphorbiaceae from ultramafic areas of Cuba

Pantropical species of the genera Phyllanthus and Euphorbia and the Cuban endemic genus Leucocroton from the Euphorbiaceae family, were selected for nickel localization microanalysis. Scanning Electron Microscopy coupled with Energy Dispersive X-ray Microanalysis (SEM-EDX) was used for qualitative detection of nickel in the selected Ni-hyperaccumulator species: Euphorbia helenae, Leucocroton linearifolius, L. flavicans Phyllanthus orbicularis, P. discolor and P. xpallidus, all collected from Cuban ultramafic soils. The leaves and stems from the Euphorbiaceae species analyzed were the organs with higher nickel accumulation. Elemental mapping of leaves and stem tissues from these species have been compared. The highest Ni concentrations were found in the laticifer tubes of stems and the epidermis tissues of leaves in all the analyzed species, suggesting a general pattern of the Euphorbiaceae family for nickel accumulation. The high nickel concentrations and its rather homogeneous distribution found in leaves of these Ni-hyperaccumulating plants suggest a possible role in protection mechanisms against environmental stress, such as UV irradiation.     

The hyperaccumulator Alyssum murale uses complexation with nitrogen and oxygen donor ligands for Ni transport and storage

The Kotodesh genotype of the nickel (Ni) hyperaccumulator Alyssum murale was examined to determine the compartmentalization and internal speciation of Ni, and other elements, in an effort to ascertain the mechanism used by this plant to tolerate extremely high shoot (stem and leaf) Ni concentrations. Plants were grown either hydroponically or in Ni enriched soils from an area surrounding an historic Ni refinery in Port Colborne, Ontario, Canada. Electron probe micro-analysis (EPMA) and synchrotron based micro X-ray fluorescence (μ-SXRF) spectroscopy were used to determine the metal distribution and co-localization and synchrotron X-ray and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopies were used to determine the Ni speciation in plant parts and extracted sap. Nickel is concentrated in the dermal leaf and stem tissues of A. murale bound primarily to malate along with other low molecular weight organic ligands and possibly counter anions (e.g., sulfate). Ni is present in the plant sap and vasculature bound to histidine, malate and other low molecular weight compounds. The data presented herein supports a model in which Ni is transported from the roots to the shoots complexed with histidine and stored within the plant leaf dermal tissues complexed with malate, and other low molecular weight organic acids or counter-ions.     

Nickel and zinc hyperaccumulation by Alyssum murale and Thlaspi caerulescens (Brassicaceae) do not enhance survival and whole-plant growth under drought stress

Nickel and Zn hyperaccumulation by Alyssum murale and Thlaspi caerulescens bear substantial energetic costs and should confer benefits to the plant. This research determined whether metal hyperaccumulation can increase osmotic adjustment and resistance to water stress (drought). Alyssum murale and Thlaspi caerulescens treated with low or high concentrations of Ni or Zn were exposed to moderate (?0·4 MPa) and severe (?1·0 MPa) water stresses using aqueous polyethylene glycol. In the absence of metals both water deficits inhibited shoot growth. Nickel and Zn hyperaccumulation did not ameliorate growth inhibition by either level of water stress. The water stress did not induce major changes in shoot metal concentrations of these constitutive hyperaccumulators. Moreover, metal hyperaccumulation had minimal effects on the osmolality of leaf‐sap extracts, relative water content of the shoots, or rate of evapotranspiration. It is concluded that Ni or Zn hyperaccumulation does not augment whole‐plant capacity for drought resistance in A. murale and T. caerulescens.     

Improving the growth of Ni-hyperaccumulating plants in serpentine quarry tailings

Phytomining techniques based on metal-hyperaccumulating plants can be implemented in serpentine quarry wastes for Ni recovery. However, strategies must be developed to overcome the unfavourable plant growth conditions that these substrates present and to optimize Ni yields. In this study, the Ni hyperaccumulators Alyssum serpyllifolium, Alyssum inflatum, and Alyssum bracteatum were evaluated for their Ni phytoextraction efficiency from quarry tailings. Effects of two organic amendments, composted municipal sewage sludge and cow manure, on plant growth and physiological status and Ni removal were determined. Organic amendments were incorporated at two addition rates (5% and 20% w/w). The best-performing hyperaccumulators were A. inflatum and A. serpyllifolium. Organic amendments improved plant biomass production, photosynthetic efficiency and nutrition, but reduced shoot Ni concentrations. However, the stimulation in biomass resulted in significantly enhanced Ni yields. The most promising results were found using low addition rates and after manure incorporation.