Phytoextraction of metals and metalloids from contaminated soils

Effect of microbial inoculation and EDTA on the uptake and translocation of heavy metal by corn and sunflower

A greenhouse experiment was carried out to study the effect of microbial inoculation and EDTA as synthetic chelator on heavy metals (Zn, Cu, Pb and Cd) uptake into and translocation within corn (Zea mays) and sunflower (Helianthus annuus) grown on contaminated soil. Four treatments were included: the control, EDTA as synthetic chelator, inoculation with arbuscular mycorrhizal fungi (AMF), and the inoculation with yeast. Microbial inoculation increased biomass of both species of agricultural crop plants, but EDTA addition decreased only biomass of sunflower. The results also showed that EDTA was more effective than microbial inoculation at increasing the concentrations of all metals in plants. Generally, there were significant differences among the treatments in the most metals taken up, especially by corn plants, with an order: AM fungi>yeast>EDTA>control except for Pb. It was found that the efficiency of EDTA in increasing Pb uptake was significantly higher than that of microbial inoculation treatments. The most effective treatments to stimulate the translocation of the studied metals from roots to shoots were EDTA and/or AM fungi. Overall, the maximum metal uptake and phytoextraction efficiencies were pronounced for corn either with AM fungi treatment for Zn, Cu and Cd or after EDTA addition for Pb. However, heavy metals uptake was not high enough to achieve extraction rates which would be necessary for practical use.

Biofortification and phytoremediation

Producing nutritious and safe foods sufficiently and sustainably is the ultimate goal of modern agriculture. Past efforts have focused on increasing crop yields, but enhancing the concentrations of mineral micronutrients has become an urgent task because about half of the world population suffers from the malnutrition of iron, zinc, and selenium. Biofortification of these trace elements can be achieved through fertilization, crop breeding or biotechnology. On the other hand, soils contaminated with metals or metalloids may be cleaned up by phytoextraction that combines hyperaccumulation with high biomass production. Progress has been made in identifying inter-species and intra-species variation in trace element accumulation, and mechanistic understanding of some aspects of trace element transport and homeostasis in plants, but much remains to be elucidated.

Isolation and characterization of Ni mobilizing PGPB from serpentine soils and their potential in promoting plant growth and Ni accumulation by Brassica spp

The study was undertaken to assess the effects of Ni mobilizing bacteria on the plant growth and the uptake of Ni by Brassica juncea and Brassica oxyrrhina. Among a collection of Ni resistant bacterial strains isolated from the non-rhizosphere and rhizosphere soils of Alyssum serpyllifolium and Astragalus incanus at a serpentine site in Bragança, north-east of Portugal, nine strains were selected based on their ability to solubilize Ni in soil. Further assessment on plant growth-promoting parameters revealed the intrinsic ability of the Ni mobilizing strains to produce indole-3-acetic acid (IAA), siderophores, utilize 1-aminocyclopropane-1-carboxylic acid (ACC) as the sole N source and solubilize insoluble phosphate. All of the strains tested positive for IAA production and phosphate solubilization. In addition, all the strains, except SRS5 exhibited significant levels of siderophore production. Besides, five isolates showed positive for ACC deaminase activity. In pot experiments, inoculation of plants with Ni mobilizing strains increased the biomass of both B. juncea and B. oxyrrhina. Among the strains, Pseudomonas sp. SRI2, Psychrobacter sp. SRS8 and Bacillus sp. SN9 showed maximum increase in the biomass of the test plants. In addition, the strain SN9 significantly increased the Ni concentration in the root and shoot tissues of B. juncea and B. oxyrrhina. Further, a significantly positive correlation was observed between the bacterial Ni mobilization in soil and the total Ni uptake in both plant species. The findings, therefore, revealed that inoculation of Ni mobilizing plant growth-promoting bacterial strain SN9 increases the efficiency of phytoextraction directly by enhancing Ni accumulation in plant tissues and indirectly by promoting the shoot and root biomass of B. juncea and B. oxyrrhina.

Cadmium tolerance and accumulation in eight potential energy crops

The production of energy crops that can be used for biodiesel production is a sustainable approach for the removal of metal pollutants by phytoremediation. This study investigated the cadmium (Cd) accumulation and tolerance of eight potential energy crops. After growth for 28 days in substrates containing 0, 50, 100 or 200 mg Cd x kg(-1), seedlings were evaluated for growth parameters, chlorophyll content, chlorophyll fluorescence parameters and Cd accumulation. All eight crops were moderately tolerant to Cd toxicity, with four [i.e., hemp (Cannabis sativa), flax (Linum usitatissimum), castor (Ricinus communis) and peanut (Arachis hypogaea)] being more tolerant than the others. Three of these crops (hemp, flax and peanut) had higher Cd accumulation capacities. The roots of peanut and hemp had high bioconcentration factors (BCF>1000), while flax shoots accumulated a higher concentration of Cd (>100 mg/kg). These results demonstrate that it is possible to grow energy crops on Cd-contaminated soil. Hemp, flax and peanut are excellent candidates for phytoremediation.

Cadmium uptake and xylem loading are active processes in the hyperaccumulator Sedum alfredii

Sedum alfredii is a well known cadmium (Cd) hyperaccumulator native to China; however, the mechanism behind its hyperaccumulation of Cd is not fully understood. Through several hydroponic experiments, characteristics of Cd uptake and translocation were investigated in the hyperaccumulating ecotype (HE) of S. alfredii in comparison with its non-hyperaccumulating ecotype (NHE). The results showed that at Cd level of 10 microM measured Cd uptake in HE was 3-4 times higher than the implied Cd uptake calculated from transpiration rate. Furthermore, inhibition of transpiration rate in the HE has no essential effect on Cd accumulation in shoots of the plants. Low temperature treatment (4 degrees C) significantly inhibited Cd uptake and reduced upward translocation of Cd to shoots for 9 times in HE plants, whereas no such effect was observed in NHE. Cadmium concentration was 3-4-fold higher in xylem sap of HE, as compared with that in external uptake solution, whereas opposite results were obtained for NHE. Cadmium concentration in xylem sap of HE was significantly reduced by the addition of metabolic inhibitors, carbonyl cyanide m-chlorophenylhydrazone (CCCP) and 2,4-dinitrophenol (DNP), in the uptake solutions, whereas no such effect was noted in NHE. These results suggest that Cd uptake and translocation is an active process in plants of HE S. alfredii, symplastic pathway rather than apoplastic bypass contributes greatly to root uptake, xylem loading and translocation of Cd to the shoots of HE, in comparison with the NHE plants.

Heavy metals distribution in soils surrounding an abandoned mine in NW Madrid (Spain) and their transference to wild flora

The present work concerns the distribution and mobility of heavy metals (Fe, Mn, Cu, Zn and Cd) in the surrounding soils of a mine site and their transfer to wild flora. Thus, soils and plants were sampled from a mining valley in NW Madrid (Spain), and total and extractable heavy metals were analysed. Soils affected by mining activities presented total Cd, Cu and Zn concentrations above toxic thresholds. The percentage of extractable element was highest for Cd and lowest for Cu. A highly significant correlation was observed between the total and extractable concentrations of metals in soils, indicating that, among the factors studied, total metals concentration is the most relevant for heavy metals extractability in these soils. (NH(4))(2)SO(4)-extractable metal concentrations in soils are correlated better with metal concentrations in several plant species than total metals in soils, and thus can be used as a suitable and robust method for the estimation of the phytoavailable fraction present in soils. Twenty-five vascular plant species (3 ferns and 22 flowering plants) were analysed, in order to identify exceptional characteristics that would be interesting for soil phytoremediation and/or reclamation. High Cd and Zn concentrations have been found in the aerial parts of Hypericum perforatum (Cd), Salix atrocinerea (Cd, Zn) and Digitalis thapsi (Cd, Zn). The present paper is, to the best of our knowledge, the first report of the metal accumulation ability of the two latter plant species. The phytoremediation ability of S. atrocinerea for Cd and Zn was estimated, obtaining intervals of time that could be considered suitable for the phytoextraction of polluted soils.

Phytoremediation trials on metal- and arsenic-contaminated pyrite wastes (Torviscosa, Italy)

At a site in Udine, Italy, a 0.7m layer of As, Co, Cu, Pb and Zn contaminated wastes derived from mineral roasting for sulphur extraction had been covered with an unpolluted 0.15m layer of gravelly soil. This study investigates whether woody biomass phytoremediation is a realistic management option. Comparing ploughing and subsoiling (0.35m depth), the growth of Populus and Salix and trace element uptake were investigated in both pot and field trials. Species differences were marginal and species selection was not critical. Impaired above-ground productivity and low translocation of trace elements showed that bioavailable contaminant stripping was not feasible. The most significant finding was of coarse and fine roots proliferation in surface layers that provided a significant sink for trace elements. We conclude that phytostabilisation and effective immobilisation of metals and As could be achieved at the site by soil amelioration combined with woody species establishment. Confidence to achieve a long-term and sustainable remediation requires a more complete quantification of root dynamics and a better understanding of rhizosphere processes.

Phytotoxicity of mercury in Indian mustard (Brassica juncea L.)

This study investigated the phytotoxicity of mercury to Indian mustard (Brassica juncea L.). Two common cultivars (Florida Broad Leaf and Long-standing) were grown hydroponically in a mercury-spiked solution. Mercury exhibited a significant phytotoxicity in these two cultivars of Indian mustard at elevated concentrations (>or=2 mg L(-1)). Mercury uptake induced a significant reduction in both biomass and leaf relative water content. Microscopy studies indicated that elevated mercury concentrations in plants significantly changed leaf cellular structure: thickly stained areas surrounding the vascular bundles; decreases in the number of palisade and spongy parenchyma cells; and reduced cell size and clotted depositions. The palisade chloroplasts exhibited decreases in their amounts and starch grains as well as a loss of spindle shape. However, due to high accumulation of mercury in plants, especially in the roots, Indian mustard might be a potential candidate plant for phytofiltration of contaminated water and phytostabilization of mercury-contaminated soils.

PHYTOREMEDIATION OF INORGANICS: REALISM AND SYNERGIES

There are very few practical demonstrations of the phytoextraction of metals and metalloids from soils and sediments beyond small-scale and short-term trials. The two approaches used have been based on using 1) hyperaccumulator species, such as Thlaspi caerulescens (Pb, Zn, Cd, Ni), Alyssum spp. (Ni, Co), and Pteris vittata (As) or 2) fast-growing plants, such as Salix and Populus spp. that accumulate above-average concentrations of only a smaller number of the more mobile trace elements (Cd, Zn, B). Until we have advanced much more along the pathway of genetic isolation and transfer of hyperaccumulator traits into productive plants, there is a high risk in marketing either approach as a technology or stand-alone solution to clean up contaminated land. There are particular uncertainties over the longer-term effectiveness of phytoextraction and associated environmental issues. Marginally contaminated agricultural soils provide the most likely land use where phytoextraction can be used as a polishing technology. An alternative and more useful practical approach in many situations currently would be to give more attention to crops selected for phytoexclusion: selecting crops that do not translocate high concentrations of metals to edible parts. Soils of brownfield, urban, and industrial areas provide a large-scale opportunity to use phytoremediation, but the focus here should be on the more realistic possibilities of risk-managed phytostabilization and monitored natural attenuation. We argue that the wider practical applications of phytoremediation are too often overlooked. There is huge scope for cross-cutting other environmental agenda, with synergies that involve the recovery and provision of services from degraded landscapes and contaminated soils. An additional focus on biomass energy, improved biodiversity, watershed management, soil protection, carbon sequestration, and improved soil health is required for the justification and advancement of phytotechnologies.

Cadmium tolerance and accumulation characteristics of Bidens pilosa L. as a potential Cd-hyperaccumulator

Recently, researchers are becoming interested in using hyperaccumulators for decontamination of heavy metal polluted soils, whereas few species that hyperaccumulate cadmium (Cd) has been identified in the plant kingdom. In this study, the physiological mechanisms at the seedling stage and growth responses and Cd uptake and accumulation at flowering and mature stages of Bidens pilosa L. under Cd treatments were investigated. At the seedling stage, when soil Cd was lower than 16mgkg(-1), the plant did not show obvious symptom of phytoxicity, and the alterations of chlorophyll (CHL), superoxide dismutase (SOD), peroxidase (POD), malondialdehyde (MDA), and soluble protein (SP) did not have significant differences when compared with the control. At the flowering and mature stages, under low Cd treatments (</=16mgkg(-1)), the application of Cd could facilitate plant growth, resulting in 3.9-11.0% and 5.9-13.8%, respectively, increase in shoots dry biomass compared with the control. The Cd concentrations in stems, leaves and shoots exceeded 100mgkg(-1) when soil Cd was at 8mgkg(-1), and they were positively correlated with Cd concentration in soils, the bioaccumulation factor (BF) and translocation factor (TF) values were all greater than 1.0. Thus, it is clear that B. pilosa has the basic characteristics of a Cd-hyperaccumulator. All the results elementarily indicated that B. pilosa is a potential Cd-hyperaccumulating plant.

Arsenic uptake and speciation in the rootless duckweed Wolffia globosa

Duckweeds are a common macrophyte in paddy and aquatic environments. Here, we investigated arsenic (As) accumulation, speciation and tolerance of the rootless duckweed Wolffia globosa and its potential for As phytofiltration. When grown with 1 microm arsenate, W. globosa accumulated two to 10 times more As than four other duckweed or Azolla species tested. W. globosa was able to accumulate > 1000 mg As kg(-1) in frond dry weight (DW), and tolerate up to 400 mg As kg(-1) DW. At the low concentration range, uptake rate was similar for arsenate and arsenite, but at the high concentration range, arsenite was taken up at a faster rate. Arsenite was the predominant As species (c. 90% of the total extractable As) in both arsenate- and arsenite-exposed duckweed. W. globosa was more resistant to external arsenate than arsenite, but showed a similar degree of tolerance internally. W. globosa decreased arsenate in solution rapidly, but also effluxed arsenite. Wolffia globosa is a strong As accumulator and an interesting model plant to study As uptake and metabolism because of the lack of a root-to-frond translocation barrier.

Cellular localization of cadmium and structural changes in maize plants grown on a cadmium contaminated soil with and without liming

The effects of different concentrations of soil cadmium (0, 1, 3, 5, 10, and 20mgkg(-1)) on growth, structural changes and cadmium cellular localization in leaves of maize plants (Zea mays L.) were investigated in a pot experiment. The results showed that the structural changes observed in maize leaves were not only a response to the Cd-induced stress but also a cellular mechanism to reduce the free Cd(+2) in the cytoplasm. However, this mechanism seems to be efficient only up to a Cd concentration in leaves between 27 and 35mgkg(-1) for soils without and with liming, respectively. The cellular response varied with both the Cd concentration in soil and liming. For limed soil, Cd was preferentially accumulated in the apoplast while for unlimed soils Cd was more evenly distributed into the cells. The ability of Cd accumulation depended on the leaf tissue considered. The apoplast collenchyma presented the highest Cd concentration followed by the endodermis, perycicle, xylem, and epidermis. On the other hand, symplast Cd accumulated mainly in the endodermis, bundle sheath cells, parenchyma, and phloem. Based on the structural changes and growth reduction, the critical toxic concentration of soil Cd to maize plants is between 5 and 10mgkg(-1).

Enhanced root-to-shoot translocation of cadmium in the hyperaccumulating ecotype of Sedum alfredii

Sedum alfredii (Crasulaceae) is the only known Cd-hyperaccumulating species that are not in the Brassica family; the mechanism of Cd hyperaccumulation in this plant is, however, little understood. Here, a combination of radioactive techniques, metabolic inhibitors, and fluorescence imaging was used to contrast Cd uptake and translocation between a hyperaccumulating ecotype (HE) and a non-hyperaccumulating ecotype (NHE) of S. alfredii. The K(m) of (109)Cd influx into roots was similar in both ecotypes, while the V(max) was 2-fold higher in the HE. Significant inhibition of Cd uptake by low temperature or metabolic inhibitors was observed in the HE, whereas the effect was less pronounced in the NHE. (109)Cd influx into roots was also significantly decreased by high Ca in both ecotypes. The rate of root-to-shoot translocation of (109)Cd in the HE was >10 times higher when compared with the NHE, and shoots of the HE accumulated dramatically higher (109)Cd concentrations those of the NHE. The addition of the metabolic inhibitor carbonyl cyanide m-chlorophenylhydrazone (CCCP) resulted in a significant reduction in Cd contents in the shoots of the HE, and in the roots of the NHE. Cd was distributed preferentially to the root cylinder of the HE but not the NHE, and there was a 3-5 times higher Cd concentration in xylem sap of the HE in contrast to the NHE. These results illustrate that a greatly enhanced rate of root-to-shoot translocation, possibly as a result of enhanced xylem loading, rather than differences in the rate of root uptake, was the pivotal process expressed in the Cd hyperaccumulator HE S. alfredii.

Manganese uptake and interactions with cadmium in the hyperaccumulator--Phytolacca Americana L

In the present study, the accumulation of Mn and other metals by Phytolacca Americana L. from contaminated soils in Hunan Province, South China, was investigated. Results showed that the average concentrations of Mn in the leaves and roots reached 2198 and 80.4 mg kg(-1) (dry weight), respectively, with a maximum 13,400 mg kg(-1) in the leaves. A significant correlation was found between Mn concentrations in the plant leaves and those in the corresponding soils. Hydroponic experiments were also conducted to study the Cd uptake ability and interactions between Mn and Cd in the plant. It was found that P. americana hyperaccumulated not only Mn, but also Cd in the leaves. In the presence of Cd, adding Mn to the solution significantly improved the plant growth and reduced the concentrations of Cd in all organs of the plant.

Accumulation of zinc, cadmium, and lead in four populations of Sedum alfredii growing on lead/zinc mine spoils

Sedum alfredii Hance is a newly reported zinc (Zn) and cadmium (Cd) hyperaccumulator native to China. In this study, four populations of S. alfredii were collected from Yejiwei (YJW), Jinchuantang (JCT) and Qiaokou (QK) lead (Pb)/Zn mines located in Hunan Province as well as Quzhou (QZ) Pb/Zn mine located in Zhejiang Province for exploring the intraspecies difference of this plant in metal accumulation. Although they grew in the Pb/Zn spoils with relatively similar levels of Zn, Cd and Pb, remarkable differences among the four populations in tissue heavy metal concentrations were observed. The shoot Zn concentration of QZ population (11,116 mg/kg) was highest and nearly five times higher than that of the JCT population (1930 mg/kg). Furthermore, the shoot Cd concentration observed in the QZ population (1090 mg/kg) was also highest and 144 times higher than that found in the JCT population (7.5 mg/kg). As for Pb concentrations in the shoot of different populations, a fourfold difference between the highest and the lowest was also found. Such difference on metal accumulation was population-specific and may be significantly explained by differences in the soil properties such as pH, organic matter (OM), and electrical conductivity (EC). Taking biomass and metal concentration in plants into consideration, the QZ, YJW and QK populations may have high potential for Zn phytoremediation, the QZ population may have the highest potential in Cd phytoremediation, and the QK population may be the most useful in Pb phytoremediation.

Effects of inoculation of plant-growth promoting bacteria on Ni uptake by Indian mustard

In this study, among a collection of Ni resistant bacterial strains isolated from serpentine soil, two plant growth promoting bacteria (PGPB), Ps29C and Bm4C were selected based on their ability to utilize ACC as the sole N source and promote seedling growth in roll towel assay. The Ni resistant PGPB, Ps29C and Bm4C were characterized as Pseudomonas sp. and Bacillus megaterium, respectively, on the basis of their 16s rDNA sequences. Assessment of the parameters of plant growth promotion revealed the intrinsic ability of the strains for the production of IAA, siderophore and solubilization of insoluble phosphate. Further, the plant growth promoting activity of Ps29C and Bm4C on the Indian mustard (Brassica juncea) were assessed with different concentrations of Ni in soil. Inoculation of Ps29C or Bm4C promoted plant growth and protected the plant from Ni toxicity. However, the maximum growth was observed in the plants inoculated with strain Bm4C. Inoculation with Ps29C or Bm4C had little influence on the accumulation of Ni in root and shoot system, but produced a much larger aboveground biomass. The present observations showed that the strains Ps29C and Bm4C protect the plants against the inhibitory effects of nickel, probably due to the production of IAA, siderophore and solubilization of phosphate. The above results provided a new insight into the phytoremediation of Ni contaminated soil.

Arbuscular mycorrhizas contribute to phytostabilization of uranium in uranium mining tailings

Uranium (U) tailings pose environmental risks and call for proper remediation. In this paper medic and ryegrass plants were used as host plants to examine whether inoculation with an AM fungus, Glomus intraradices, would help phytostabilization of U tailings. The need of amending with uncontaminated soil for supporting plant survival was also examined by mixing soil with U tailing at different mixing ratios. Soil amendment increased plant growth and P uptake. Ryegrass produced a more extensive root system and a greater biomass than medic plants at all mixing ratios. Medic roots were extensively colonized by G. intraradices whereas ryegrass were more sparsely colonized. Plant growth was not improved by mycorrhizas, which, however, improved P nutrition of medic plants. Medic plants contained higher U concentrations and showed higher specific U uptake efficiency compared to ryegrass. In the presence of U tailing, most U had been retained in plant roots, and this distribution pattern was further enhanced by mycorrhizal colonization. The results suggest a role for AM fungi in phytostabilization of U tailings.

The effects of exogenous plant growth regulators in the phytoextraction of heavy metals

The term "assisted phytoextraction" usually refers to the process of applying a chemical additive to contaminated soil in order to increase the metal uptake by crop plants. In this study three commercially available plant growth regulators (PGRs) based on cytokinins (CKs) were used to boost the assisted phytoextraction of Pb and Zn in contaminated soil collected from a former manufactured gas-plant site. The effects of EDTA treatment in soil and PGR treatment in leaves of Helianthus annuus were investigated in terms of dry weight biomass, Pb and Zn accumulation in the upper parts of the plants, Pb and Zn phytoextraction efficiency and transpiration rate. Metal solubility in soil and its subsequent accumulation in shoots were markedly enhanced by EDTA. The combined effects of EDTA and cytokine resulted in an increase in the Pb and Zn phytoextraction efficiency (up to 890% and 330%, respectively, compared to untreated plants) and up to a 50% increase in foliar transpiration. Our results indicate that exogenous PGRs based on CKs can positively assist the phytoextraction increasing the biomass production, the metal accumulation in shoots and the plant transpiration. The observed increase in biomass could be related to its action in stimulation of cell division and shoot initiation. On the other hand, the increase in metal accumulation in upper parts of plant could be related to both the role of PGRs in the enhancement of plant resistance to stress (as toxic metals) and the increase in transpiration rate, i.e. flux of water-soluble soil components and contaminants by the regulation of stomatal opening.

Variation in root-to-shoot translocation of cadmium and zinc among different accessions of the hyperaccumulators Thlaspi caerulescens and Thlaspi praecox

Efficient root-to-shoot translocation is a key trait of the zinc/cadmium hyperaccumulators Thlaspi caerulescens and Thlaspi praecox, but the extent of variation among different accessions and the underlying mechanisms remain unclear. Root-to-shoot translocation of Cd and Zn and apoplastic bypass flow were determined in 10 accessions of T. caerulescens and one of T. praecox, using radiolabels (109)Cd and (65)Zn. Two contrasting accessions (Pr and Ga) of T. caerulescens were further characterized for TcHMA4 expression and metal compartmentation in roots. Root-to-shoot translocation of (109)Cd and (65)Zn after 1 d exposure varied 4.4 to 5-fold among the 11 accessions, with a significant correlation between the two metals, but no significant correlation with uptake or the apoplastic bypass flow. The F(2) progeny from a cross between accessions from Prayon, Belgium (Pr) and Ganges, France (Ga) showed a continuous phenotype pattern and transgression. There was no significant difference in the TcHMA4 expression in roots between Pr and Ga. Compartmentation analysis showed a higher percentage of (109)Cd sequestered in the root vacuoles of Ga than Pr, the former being less efficient in translocation than the latter. Substantial natural variation exists in the root-to-shoot translocation of Cd and Zn, and root vacuolar sequestration may be an important factor related to this variation.

Highly efficient xylem transport of arsenite in the arsenic hyperaccumulator Pteris vittata

The hyperaccumulator Pteris vittata translocates arsenic (As) from roots to fronds efficiently, but the form of As translocated in xylem and the main location of arsenate reduction have not been resolved. Here, P. vittata was exposed to 5 microM arsenate or arsenite for 1-24 h, with or without 100 microM phosphate. Arsenic speciation was determined in xylem sap, roots, fronds and nutrient solutions by high-performance liquid chromatography (HPLC) linked to inductively coupled plasma mass spectrometry (ICP-MS). The xylem sap As concentration was 18-73 times that in the nutrient solution. In both arsenate- and arsenite-treated plants, arsenite was the predominant species in the xylem sap, accounting for 93-98% of the total As. A portion of arsenate taken up by roots (30-40% of root As) was reduced to arsenite rapidly. The majority (c. 80%) of As in fronds was arsenite. Phosphate inhibited arsenate uptake, but not As translocation. More As was translocated to fronds in the arsenite-treated than in the arsenate-treated plants. There was little arsenite efflux from roots to the external solution. Roots are the main location of arsenate reduction in P. vittata. Arsenite is highly mobile in xylem transport, possibly because of efficient xylem loading, little complexation with thiols in roots, and little efflux to the external medium.

Response of sugarcane to increasing concentrations of copper and cadmium and expression of metallothionein genes

Sugarcane (Saccharum spp.) offers the potential to be a phytoremediator species due to its outstanding biomass production, but its prospective metal accumulation and tolerance have not been fully characterized. Sugarcane plantlets were able to tolerate up to 100microM of copper in nutrient solution for 33 days, with no significant reduction in fresh weight, while accumulating 45mgCukg(-1) shoot dry weight. Higher levels of copper in solution (250 and 500microM) were lethal. Sugarcane displayed tolerance to 500microM Cd without symptoms of toxicity, accumulating 451mgCdkg(-1) shoot dry weight after 33 days, indicating its potential as Cd phytoremediator. DNA gel blot analyses detected 8 fragments using a metallothionein (MT) Type I probe, while 10 were revealed for the MT Type II and 8 for MT Type III. The number of genes for each type of MT in sugarcane might be similar to the ones identified in rice considering the interspecific origin of sugarcane cultivars. MT Type I gene appeared to present the highest level of constitutive expression, mainly in roots, followed by MT Type II, corroborating the expression pattern described based on large-scale expressed sequence tags sequencing. MT Type II and III genes were more expressed in shoots, where MT I was also importantly expressed. Increasing Cu concentration had little or no effect in modulating MT genes expression, while an apparent minor modulation of some of the MT genes could be detected in Cd treatments. However, the level of response was too small to explain the tolerance and/or accumulation of Cd in sugarcane tissues. Thus, cadmium tolerance and accumulation in sugarcane might derive from other mechanisms, although MT may be involved in oxidative responses to high levels of Cd. Sugarcane can be considered a potential candidate to be tested in Cd phytoremediation.

Improved understanding of hyperaccumulation yields commercial phytoextraction and phytomining technologies

This paper reviews progress in phytoextraction of soil elements and illustrates the key role of hyperaccumulator plant species in useful phytoextraction technologies. Much research has focused on elements which are not practically phytoextracted (Pb); on addition of chelating agents which cause unacceptable contaminant leaching and are cost prohibitive; and on plant species which offer no useful phytoextraction capability (e.g., Brassica juncea Czern). Nickel phytoextraction by Alyssum hyperaccumulator species, which have been developed into a commercial phytomining technology, is discussed in more detail. Nickel is ultimately accumulated in vacuoles of leaf epidermal cells which prevents metal toxicity and provides defense against some insect predators and plant diseases. Constitutive up-regulation of trans-membrane element transporters appears to be the key process that allows these plants to achieve hyperaccumulation. Cadmium phytoextraction is needed for rice soils contaminated by mine wastes and smelter emissions with 100-fold more soil Zn than Cd. Although many plant species can accumulate high levels of Cd in the absence of Zn, when Cd/Zn>100, only Thlaspi caerulescens from southern France has demonstrated the ability to phytoextract useful amounts of Cd. Production of element-enriched biomass with value as ore or fertilizer or improved food (Se) or feed supplement may offset costs of phytoextraction crop production. Transgenic phytoextraction plants have been achieved for Hg, but not for other elements. Although several researchers have been attempting to clone all genes required for effective hyperaccumulation of several elements, success appears years away; such demonstrations will be needed to prove we have identified all necessary processes in hyperaccumulation.

Effects of exogenous chelators on phytoavailability and toxicity of Pb in Zinnia elegans Jacq

Chelate-enhanced phytoremediation is considered as an effective method for the extraction of lead (Pb) by plants. However, more detailed studies are needed to evaluate the effect of exogenous chelators on phytoavailability and toxicity of Pb in plants, then to find out the proper applied concentration of chelators to minimize the combined toxicity to the plants and maximize phytoavailable Pb. To clarify these questions, the seed germination test of Zinnia elegans Jacq. exposed to solutions containing Pb and four types of chelators including sodium ethylenediamine tetra-acetic acid (Na2EDTA), oxalic acid, tartaric acid and citric acid was observed. The results showed that the roots and shoots treated with equimolar chelators and Pb were longer than those treated with half and two folds of the molar concentrations of Pb. The growth of seedlings was inhibited by surplus addition of chelators, and the toxicity of complexes was less than that of Pb and chelators. In particular, 2.4 mM EDTA and 1.2 mM oxalic acid significantly (P<0.05) increased Pb uptake when the seeds were treated with 2.4 mM Pb. In the 4.8 mM Pb solution, Pb accumulation in the seedlings was markedly (P<0.05) increased by 4.8 mM EDTA, 2.4 mM tartaric acid, 4.8 mM tartaric acid and 2.4 mM citric acid, and amounted to 6752.4, 6453.8, 6541.4 and 6598.3 microg g(-1), respectively. With the superfluous addition of chelators, Pb accumulation in the seedlings decreased in a concentration-dependent manner. When Pb was used at 2.4 mM, an equimolar concentration of EDTA not only increased Pb uptake but also stimulated the seedling growth. Thus, chelating agents in their appropriately concentrations could counteract Pb toxicity, but superfluous chelators resulted in less Pb uptake and growth inhibition of the seedlings.

The influence of EDDS and EDTA on the uptake of heavy metals of Cd and Cu from soil with tobacco Nicotiana tabacum

Phytoextraction, the use of plants to extract contaminants from soils and groundwater, is a promising approach for cleaning up soils contaminated with heavy metals. In order to enhance phytoextraction the use of chelating agents has been proposed. This study aims to assess whether ethylene diamine disuccinate (EDDS), a biodegradable chelator, can be used for enhanced phytoextraction purposed, as an alternative to ethylene diamine tetraacetate (EDTA). EDDS revealed a higher toxicity to tobacco (Nicotiana tabacum) in comparison to EDTA, but no toxicity to microorganisms. The uptake of Cu was increased by the addition of EDTA and EDDS, while no increase was observed in the uptake of Cd. Both chelating agents showed a very low root to shoot translocation capability and the translocation factor was lower than the one of the control. Heavy metals where significantly more phytoavailable than in the control, even after harvesting, resulting in a high heavy metal leaching possibility, probably owing to a low biodegradation rate of EDDS. New seedlings which were transplanted into the EDDS treated pots 7d after the phytoextraction experiment, showed signs of necrosis and chlorosis, which resulted in a significantly lower biomass in comparison to the control. The seedlings on the EDTA treated pots showed no toxicity signs. Contrary to previous opinions the results of this study revealed the chelating agents EDTA and EDDS as unsuitable for enhanced phytoextraction using tobacco.

Chelate assisted phytoextraction of heavy metals from soil. Effect, mechanism, toxicity, and fate of chelating agents

The low-cost, plant-based phytoextraction technique has often been described as a promising technique to remediate heavy metal contaminated agricultural land. The application of chelating agents has shown positive effects in increasing the solubility of heavy metals in soil and therefore in enhancing phytoextraction. This paper gives an overview of the chelating agents applied in recent studies. Various synthetic aminopolycarboxylic acids, such as ethylene diamine tetraacetic acid, and natural ones such as, ethylene diamine disuccinate and nitrilotriacetic acid, are described. Additionally, results of the application of natural low molecular weight organic acids, such as citric and tartaric acid are given. The effectiveness of these different chelating agents varies according to the plant and the heavy metals used. Furthermore, a focus is laid on the chelating agents fate after application and on its toxicity to plants and soil microorganisms, as well as it degradation. The rate of degradation is of great importance for the future of chelate assisted phytoextraction as it has a direct impact on the leaching probability. An effective prevention of leaching will be crucial for the acceptance and the economic breakthrough of enhanced phytoextraction, but a satisfactory solution to this key issue has so far not been found. Possibly further experiments in the field of enhanced phytoextraction will be able to solve this major problem, but over decades various greenhouse experiments and recently field experiments have resulted in different observations. Therefore, it is questionable if further research in this direction will lead to a promising solution. Phytoextraction has possibly reached a turning point in which it should distance itself from chelate assisted phytoextraction and focus on alternative options.

Light quanta modulated characteristics of Ni uptake by Brassica juncea seedlings: the interdependence of plant metal concentration and biomass

The relationships between the concentration of metal in the growth medium, Cs, the concentration of metal absorbed by the plant, Cp, and the total biomass achieved, M, all of which are factors relevant to the efficiency of metal uptake and tolerance by the plant, have been investigated via the physiological response of Brassica juncea seedlings to Ni stress. The factorial growth experiments treated the Ni concentration in agar medium and the diurnal light quanta as independently variable parameters. Observations included the evidence of light enhancement of Ni toxicity in the root, as well as at the whole-plant level. The shoot mass index possibly is an indicator of the amount of shoot metal sequestration in B. juncea, as are the logarithmic variation of Cp with Cs and the power-law dependence of M on Cp. The sum total of these observations indicates that, for the Ni accumulating plant B. juncea, the overall metabolic allocation to either growth or metal tolerance of the plant is important. Neither a rapid biomass increase nor a high metal absorbed concentration favored the removal of high metal mass from the medium. Rather, the plants with a moderate rate of biomass growth and a moderate absorbed metal concentration demonstrated the ability to remove the maximum mass of metal from the medium. The implication of these results as related to the extant model of phyoextraction efficiency is discussed.

Phytoremediation of mixed-contaminated soil using the hyperaccumulator plant Alyssum lesbiacum: evidence of histidine as a measure of phytoextractable nickel

In this study we examine the effects of polycyclic aromatic hydrocarbons (PAHs) on the ability of the hyperaccumulator plant Alyssum lesbiacum to phytoextract nickel from co-contaminated soil. Planted and unplanted mesocosms containing the contaminated soils were repeatedly amended with sorbitan trioleate, salicylic acid and histidine in various combinations to enhance the degradation of two PAHs (phenanthrene and chrysene) and increase nickel phytoextraction. Plant growth was negatively affected by PAHs; however, there was no significant effect on the phytoextraction of Ni per unit biomass of shoot. Exogenous histidine did not increase nickel phytoextraction, but the histidine-extractable fraction of soil nickel showed a high correlation with phytoextractable nickel. These results indicate that Alyssum lesbiacum might be effective in phytoextracting nickel from marginally PAH-contaminated soils. In addition, we provide evidence for the broader applicability of histidine for quantifying and predicting Ni phytoavailability in soils.

Role of microbial inoculation and chitosan in phytoextraction of Cu, Zn, Pb and Cd by Elsholtzia splendens--a field case

A field experiment was carried out to study the effect of microbial inoculation on heavy metal phytoextraction by Elsholtzia splendens and whether chitosan could have a synergistic effect with the microbial inocula. The microbial inocula consisted of a consortium of arbuscular mycorrhizal fungi and two Penicillium fungi. Three treatments were included: the control, inoculation with microbial inocula, and the inoculation combined with chitosan. Microbial inoculation increased plant biomass especially shoot dry weight, enhanced shoot Cu, Zn and Pb concentrations but did not affect Cd, leading to higher shoot Cu, Zn, Pb and Cd uptake. Compared with microbial inoculation alone, chitosan application did not affect plant growth but increased shoot Zn, Pb and Cd concentrations except Cu, which led to higher phytoextraction efficiencies and partitioning to shoots of Zn, Pb and Cd. These results indicated synergistic effects between microbial inocula and chitosan on Zn, Pb and Cd phytoextraction.

Zinc adsorption and desorption characteristics in root cell wall involving zinc hyperaccumulation in Sedum alfredii Hance

Radiotracer techniques were employed to characterize (65)Zn adsorption and desorption in root-cell-wall of hyperaccumulating ecotype (HE) and non-hyperaccumulating ecotype (NHE) species of Sedum alfredii Hance. The results indicated that at the end of a 30 min short time radioisotope loading period, comparable amounts of (65)Zn were accumulated in the roots of the two ecotypes Sedum alfredii, whereas 2.1-fold more (65)Zn remains in NHE root after 45-min desorption. At the end of 60 min uptake period, no difference of (65)Zn accumulation was observed in undesorbed root-cell-wall of Sedum alfredii. However, 3.0-fold more (65)Zn accumulated in desorbed root-cell-wall of NHE. Zn(2+) binding in root-cell-wall preparations of NHE was greater than that in HE under high Zn(2+) concentration. All these results suggested that root-cell-wall of the two ecotypes Sedum alfredii had the same ability to adsorb Zn(2+), whereas the desorption characteristics were different, and with most of (65)Zn binding on root of HE being available for loading into the xylem, as a result, more (65)Zn was translocated to the shoot.

Revisiting the plant hyperaccumulation criteria to rare plants and earth abundant elements

The several established criteria to define a hyperaccumulator plant were applied to a rare and endangered species, Plantago almogravensis, and to the 3rd most abundant element in the earth crust, Al. Using the most common criteria, P. almogravensis undoubtedly is an Al hyperaccumulator plant. If the recent proposed requirements were considered, most of them matching those for a plant to be used in phytoextraction, it can only be considered an unusual accumulator of Al. A discussion is made concerning the several criteria of a hyperaccumulator plant in order to include rare and endemic ones and abundant elements. In ecological terms, the enrichment in Al and Fe observed may account for the differences in the vegetation pattern. Due to the rarity and endangered nature of this plant, the contribution of this work is also relevant for the ecological understanding and the development of conservation options of this endemic species.

Chemical mutagenesis--a promising technique to increase metal concentration and extraction in sunflowers

Since most of the metal-hyperaccumulating wild plants only produce very low biomass and many high-yielding crops accumulate only moderate amounts of metals, the current research is mainly focused on overcoming these limitations and the optimization of metal phytoextraction. The main goal of the present study was the improvement of metal concentration and extraction properties of Helianthus annuus L by chemical mutagenesis (the non-GMO approach). Sunflowers--hybrid cultivar Salut and inbred lines-were treated with the chemical mutagen ethyl methanesulfonate (EMS). The effect of chemical mutagenesis on metal concentration in and extraction by new sunflower M1 and M2 mutants was directly assessed on a metal-contaminated field in Raft, Switzerland. Mutants of the M2 generation showed a 2-3 times higher metal shoot concentration than the control plants. The best M2 sunflower "giant mutant" 14/185/04 showed a significantly enhanced metal extraction ability: 7.5 times for Cd, 9.2 times for Zn, and 8.2 times for Pb in aboveground parts, as compared to the control plants. Theoretical calculations for the phytoextraction potential of new sunflower variants note that the best sunflower mutant can produce up to 26 t dry matter per hectare and remove 13.3 kg Zn per hectare and year at the sewage sludge contaminated site of Raft; that is a gain factor of 9 compared to Zn extraction by sunflower controls. Furthermore, the use of sunflower oil and biomass for technical purposes (lubricants, biodiesel, biogas) should produce an additional value and improve the economical balance of phytoextraction.

Growth of Lygeum spartum in acid mine tailings: response of plants developed from seedlings, rhizomes and at field conditions

Lygeum spartum is a native species in semiarid Mediterranean areas that grows spontaneously on acid mine tailings. We aimed to study the suitability of this plant for phytostabilization. L. spartum was grown from both seeds and rhizomes in acid mine tailings with various fertilizer and lime treatments. Untreated soils had a solution pH of 2.9 with high concentrations of dissolved salts (Electrical Conductivity 25 dS m(-1)) and Zn (3100 mg L(-1)). Plants grown on untreated soil had high shoot metal concentrations (>4000 mg kg(-1)Zn). Liming increased the solution pH to 5.5 and reduced the dissolved salts by more than 75%, resulting in lower shoot metal accumulation. Plants grown from rhizomes accumulated less metal than those grown from seeds. Plants collected in the field had metal concentrations an order of magnitude less than plants raised in the growth chamber. These differences may be due to the higher moisture content and homogeneous nature of the soils used in the pot experiment.

Solanum nigrum grown in contaminated soil: effect of arbuscular mycorrhizal fungi on zinc accumulation and histolocalisation

Zn tissue accumulation in Solanum nigrum grown in a non-contaminated and a naturally contaminated Zn matrix and the effect of inoculation with different arbuscular mycorrhizal fungi (AMF) on metal uptake were assessed. S. nigrum grown in the contaminated soil always presented higher Zn accumulation in the tissues, accumulating up to 1622 mg Zn kg(-1). The presence of both Glomus claroideum and Glomus intraradices enhanced the uptake and accumulation of Zn by S. nigrum (up to 83 and 49% higher Zn accumulation, respectively). The main deposits of the metal were found in the intercellular spaces and in the cell walls of the root tissues, as revealed by autometallography, with the inoculation with different AMF species causing no differences in the location of Zn accumulation. These findings indicate that S. nigrum inoculated with selected heavy metal tolerant AMF presents extracting and accumulating capacities, constituting a potentially suitable remediation method for Zn polluted soils.

Cadmium-induced inhibition of photosynthesis and long-term acclimation to cadmium stress in the hyperaccumulator Thlaspi caerulescens

Acclimation of hyperaccumulators to heavy metal-induced stress is crucial for phytoremediation and was investigated using the hyperaccumulator Thlaspi caerulescens and the nonaccumulators T. fendleri and T. ochroleucum. Spatially and spectrally resolved kinetics of in vivo absorbance and fluorescence were measured with a novel fluorescence kinetic microscope. At the beginning of growth on cadmium (Cd), all species suffered from toxicity, but T. caerulescens subsequently recovered completely. During stress, a few mesophyll cells in T. caerulescens became more inhibited and accumulated more Cd than the majority; this heterogeneity disappeared during acclimation. Chlorophyll fluorescence parameters related to photochemistry were more strongly affected by Cd stress than nonphotochemical parameters, and only photochemistry showed acclimation. Cd acclimation in T. caerulescens shows that part of its Cd tolerance is inducible and involves transient physiological heterogeneity as an emergency defence mechanism. Differential effects of Cd stress on photochemical vs nonphotochemical parameters indicate that Cd inhibits the photosynthetic light reactions more than the Calvin-Benson cycle. Differential spectral distribution of Cd effects on photochemical vs nonphotochemical quenching shows that Cd inhibits at least two different targets in/around photosystem II (PSII). Spectrally homogeneous maximal PSII efficiency (F(v)/F(m)) suggests that in healthy T. caerulescens all chlorophylls fluorescing at room temperature are PSII-associated.

Uptake and accumulation of arsenic by 11 Pteris taxa from southern China

A field survey was conducted at a deserted arsenic (As) mine in Guangxi Province, China to explore new potential As hyperaccumulators. In addition, young plants of 11 Pteris taxa were grown in glasshouse conditions for 12 weeks on As-amended soils with 0, 50 and 200 mg As kg(-1). Results of the field survey showed that the fern Pteris fauriei accumulated over 1000 mg As kg(-1) in its fronds. Of the 11 Pteris taxa, Pteris aspericaulis, Pteris cretica var. nervosa, P. fauriei, Pteris multifida, P. multifida f. serrulata, and Pteris oshimensis were all found to hyperaccumulate As in addition to P. cretica 'Albo-Lineata' and Pteris vittata (already reported as As hyperaccumulators). However, Pteris ensiformis, Pteris semipinnata and Pteris setuloso-costulata showed no evidence of As hyperaccumulation. Results also revealed a constitutive property of As hyperaccumulation in different populations of P. cretica var. nervosa, P. multifida, P. oshimensis and P. vittata.

Initial studies for the phytostabilization of a mine tailing from the Cartagena-La Union Mining District (SE Spain)

Mine tailings are one of the main environmental problems in post-mining landscapes and their removal is often complicated due to their high heavy metal content and dimensions. In this sense, using plant species for in situ stabilization may be an interesting and low cost option. Moreover, there are some plant species that have adapted to these conditions and are usually present at these contaminated sites. In this study, a mine tailing located in South-East Spain was investigated in order to establish lines for further phytostabilization research. A plot sampling design was carried out in order to characterize the soil properties. In addition, two plant species that have naturally colonized some parts of the tailing, Hyparrhenia hirta and Zygophyllum fabago, were sampled, including the analyses of their respective rhizospheric soils. The results of plot soil samples showed pH values from ultra acid to slightly alkaline. The electrical conductivity values were around 4dSm(-1) in plots with vegetation and 8dS m(-1) in the plot without vegetation. Total metal concentrations were high (4000 mg kg(-1) for Pb, 9000-15000 mg kg(-1) for Zn). DTPA- and water-extractable Zn were 5% and 3% of the total, respectively. H. hirta accumulated around 150 mg kg(-1) Pb in both shoots and roots. Zn concentration was 750 mg kg(-1) in Z. fabago shoots. DTPA-extractable Zn and Cu were positively correlated to plant uptake.

Zinc accumulation in Solanum nigrum is enhanced by different arbuscular mycorrhizal fungi

Solanum nigrum was found to proliferate in sediments with high levels of metal pollution. The effect of Zn on plant growth and tissue metal accumulation was assessed. The response of the plant to the inoculation with four different isolates of arbuscular mycorrhizal fungi (AMF) (Glomus sp. BEG140, Glomus claroideum, Glomus mosseae and Glomus intraradices) was studied. While the isolates of AMF did not have a significant (P<0.05) influence on mycorrhizal colonisation, increasing Zn concentration to high levels (500 and 1000 mgkg(-1)) induced significant (P<0.05) decrease of the AMF colonisation. In general, the presence of AMF did not affect the growth and biomass of S. nigrum individuals. However, the level of metal in the matrix affected S. nigrum growth; plants grown at 100 mgkg(-1) had significantly (P<0.05) lower leaf, stem, root and total biomass than control ones and plants growing at 500 and 1000 mgkg(-1) had the significantly (P<0.05) lowest biomass. Plants inoculated with the AMF G. claroideum and G. intraradices presented significantly (P<0.05) higher Zn accumulation in all plant tissues. In general, the stem tissues had the higher Zn content while the leaves registered the lowest values, which indicate a high translocation of the metal. AMF inoculation had no significant (P<0.05) influence on the metal translocation within the plant. This study suggests that inoculation with the AMF G. claroideum or G. intraradices, can enhance the Zn accumulation in the tissues of S. nigrum, not affecting the plant translocation capacities.

Accumulation of Pb, Cu, and Zn in native plants growing on a contaminated Florida site

Contamination of heavy metals represents one of the most pressing threats to water and soil resources as well as human health. Phytoremediation can be potentially used to remediate metal-contaminated sites. This study evaluated the potential of 36plants (17species) growing on a contaminated site in North Florida. Plants and the associated soil samples were collected and analyzed for total metal concentrations. While total soil Pb, Cu, and Zn concentrations varied from 90 to 4100, 20 to 990, and 195 to 2200mg kg(-1), those in the plants ranged from 2.0 to 1183, 6.0 to 460, and 17 to 598mg kg(-1), respectively. None of the plants were suitable for phytoextraction because no hyperaccumulator was identified. However, plants with a high bioconcentration factor (BCF, metal concentration ratio of plant roots to soil) and low translocation factor (TF, metal concentration ratio of plant shoots to roots) have the potential for phytostabilization. Among the plants, Phyla nodiflora was the most efficient in accumulating Cu and Zn in its shoots (TF=12 and 6.3) while Gentiana pennelliana was most suitable for phytostabilization of sites contaminated with Pb, Cu and Zn (BCF=11, 22 and 2.6). Plant uptake of the three metals was highly correlated, whereas translocation of Pb was negatively correlated with Cu and Zn though translocation of Cu and Zn were correlated. Our study showed that native plant species growing on contaminated sites may have the potential for phytoremediation.

Physicochemical and biological characterisation of different dredged sediment deposit sites in France

The aim of this work is to determine sediment properties, metal contents and transfers of Cd and Zn from dredged sediments to plants. To this end 10 deposit sites with different contexts were visited in France. The main agronomic characteristics and metal contents for surface soil layers were measured, the plant species present at the sites, such as Brassicaceae and Fabaceae, were listed, and the distribution of their root systems described. Soil characteristics such as available P (Olsen) varied between sites, with values ranging from 0.01 to 0.49 g kg(-1). Total contents and enrichment factors were studied, highlighting metal contamination in most of the sites. Despite carrying out principal component analyses, it was not possible to group deposits by age or geographical localisation. However, deposits could be distinguished as a function of proximity of industrial facilities, sediment grain size and carbonate content. Associations between metals were also highlighted: (1) Cd, Pb and Zn, and (2) Al, Cr, Cu and Fe. Consequently, we propose classifying them as technogenic anthrosols.

Arbuscular mycorrhizae increase the arsenic translocation factor in the As hyperaccumulating fern Pteris vittata L

Phytoremediation techniques are receiving more attention as decontaminating strategies. Phytoextraction makes use of plants to transfer contaminants from soil to the aboveground biomass. This research is devoted to study the effects of arbuscular mycorrhizae (AM) on growth and As hyperaccumulation in the Chinese brake fern Pteris vittata. We grew for 45 days P. vittata sporophytes, infected or not infected with the AM fungi Glomus mosseae or Gigaspora margarita, in a hydroponic system on quartz sand. As-treated plants were weekly fed with 25 ppm As. The As treatment produced a dramatic increase of As concentration in pinnae and a much lower increase in roots of both mycorrhizal and control plants. Mycorrhization increased pinnae dry weight (DW) (G. margarita = G. mosseae) and leaf area (G. margarita > G. mosseae), strongly reduced root As concentration (G. mosseae > G. margarita), and increased the As translocation factor (G. mosseae > G. margarita). The concentration of phosphorus in pinnae and roots was enhanced by both fungi (G. margarita > G. mosseae). The quantitatively different effects of the two AM fungi on plant growth as well as on As and P distribution in the fern suggest that the As hyperaccumulation in P. vittata can be optimized by a careful choice of the symbiont.

Synthesis of low molecular weight thiols in response to Cd exposure in Thlaspi caerulescens

In this study, we investigated the accumulation of phytochelatins (PCs) and other low molecular weight (LMW) thiols in response to Cd exposure in two contrasting ecotypes differing in Cd accumulation. Using a root elongation test, we found that the highly accumulating ecotype Ganges was more tolerant to Cd than the low Cd-accumulation ecotype Prayon. L-buthionine-(S,R)-sulphoximine (BSO), a potent inhibitor of the gamma-glutamylcysteine synthetase gamma-ECS) (an enzyme involved in the PC biosynthetic pathway), increased the Cd sensitivity of Prayon, but had no effect on Ganges. Although PC accumulation increased in response to Cd exposure, no significant differences were observed between the two ecotypes. Cd exposure induced a dose-dependent accumulation of both Cys and a still unidentified LMW thiol in roots of both ecotypes. Root accumulation of Cys and this thiol was higher in Ganges than in Prayon; the ecotypic differences were more pronounced when the plants were treated with BSO. These findings suggest that PCs do not contribute to the Cd hypertolerance displayed by the Ganges ecotype of Thlaspi caerulescens, whereas Cys and other LMW thiols might be involved.

Urban environmental geochemistry of trace metals

As the world's urban population continues to grow, it becomes increasingly imperative to understand the dynamic interactions between human activities and the urban environment. The development of urban environmental geochemistry has yielded a significant volume of scientific information about geochemical phenomena found uniquely in the urban environment, such as the distribution, dispersion, and geochemical characteristics of some toxic and potentially toxic trace metals. The aim of this paper is to provide an overview of the development of urban environmental geochemistry as a field of scientific study and highlight major transitions during the course of its development from its establishment to the major scientific interests in the field today. An extensive literature review is also conducted of trace metal contamination of the urban terrestrial environment, in particular of urban soils, in which the uniqueness of the urban environment and its influences on trace metal contamination are elaborated. Potential areas of future development in urban environmental geochemistry are identified and discussed.

Zinc compartmentation in root, transport into xylem, and absorption into leaf cells in the hyperaccumulating species of Sedum alfredii Hance

Sedum alfredii Hance can accumulate Zn in shoots over 2%. Leaf and stem Zn concentrations of the hyperaccumulating ecotype (HE) were 24- and 28-fold higher, respectively, than those of the nonhyperaccumulating ecotype (NHE), whereas 1.4-fold more Zn was accumulated in the roots of the NHE. Approximately 2.7-fold more Zn was stored in the root vacuoles of the NHE, and thus became unavailable for loading into the xylem and subsequent translocation to shoot. Long-term efflux of absorbed 65Zn indicated that 65Zn activity was 6.8-fold higher in shoots but 3.7-fold lower in roots of the HE. At lower Zn levels (10 and 100 microM), there were no significant differences in 65Zn uptake by leaf sections and intact leaf protoplasts between the two ecotypes except that 1.5-fold more 65Zn was accumulated in leaf sections of the HE than in those of the NHE after exposure to 100 microM for 48 h. At 1,000 microM Zn, however, approximately 2.1-fold more Zn was taken up by the HE leaf sections and 1.5-fold more 65Zn taken up by the HE protoplasts as compared to the NHE at exposure times >16 h and >10 min, respectively. Treatments with carbonyl cyanide m-chlorophenylhydrazone (CCCP) or ruptured protoplasts strongly inhibited 65Zn uptake into leaf protoplasts for both ecotypes. Citric acid and Val concentrations in leaves and stems significantly increased for the HE, but decreased or had minimal changes for the NHE in response to raised Zn levels. These results indicate that altered Zn transport across tonoplast in the root and stimulated Zn uptake in the leaf cells are the major mechanisms involved in the strong Zn hyperaccumulation observed in S. alfredii H.