Two birch clones originating from metal-contaminated sites were exposed for 3 months to soils (sand-peat ratio 1:1 or 4:1) spiked with a mixture of polyaromatic hydrocarbons (PAHs; anthracene, fluoranthene, phenanthrene, pyrene). PAH degradation differed between the two birch clones and also by the soil type. The statistically most significant elimination (p <= 0.01), i.e. 88% of total PAHs, was observed in the more sandy soil planted with birch, the clearest positive effect being found with Betula pubescens clone on phenanthrene. PAHs and soil composition had rather small effects on birch protein complement. Three proteins with clonal differences were identified: ferritin-like protein, auxin-induced protein and peroxidase. Differences in planted and non-planted soils were detected in bacterial communities by 16S rRNA T-RFLP, and the overall bacterial community structures were diverse. Even though both represent complex systems, trees and rhizoidal microbes in combination can provide interesting possibilities for bioremediation of PAH-polluted soils. Birch can enhance degradation of PAH compounds in the rhizosphere.

The correlations among arsenic (As) accumulation in grains and straw, rates of radial oxygen loss (ROL), and porosity of roots using 25 rice cultivars were investigated based on two pot experiments: (1) soil with addition of 100 mg As kg⁻¹ for analysis of As in grains and straw, and (2) deoxygenated solution for analyzing rates of ROL and porosity of roots. The results showed that there were great differences in grain As (0.71-1.72 mg kg⁻¹) and straw As (15.6-31.7 mg kg⁻¹), rates of ROL (7.40-13.24 mmol O2 kg⁻¹ root d.w. h⁻¹), and porosity (20.91-33.08%) among the cultivars. There were significant negative correlations between As in grains or straw and ROL and porosity, and significant positive correlations between rates of ROL and porosities, respectively. Rice cultivars with high porosities tended to possess higher rates of ROL, and had higher capacities for limiting the transfer of As to aboveground tissues. Rice with high radial oxygen loss and porosity of root accumulates low As in grains.

In Brazil's littoral, many iron ore industries are located in areas of restinga, a type of coastal ecosystem; such industries represent stationary sources of iron and acid particulates. The industrial sector is under expansion, threatening the already fragile ecosystem. In the present study, the simulated impact of these emissions was studied on the early establishment stages of two native plant species, Sophora tomentosa and Schinus terebinthifolius. The results indicate that S. tomentosa display deficient germination and low root tolerance when exposed to iron dust and acidity. Toxic iron concentrations were found to accumulate only in the roots of S. tomentosa plants exposed to increasing doses of iron dust. In these plants, initial growth was reduced, leading to the conclusion that S. tomentosa was unable to regulate, and, thus, to overcome, the effects of the iron dust in the substrate. In contrast, the germination, root tolerance index, and initial growth of S. terebinthifolius were not affected by iron dust and acidity, revealing the species' resistance to these pollutants. We propose that under similar field conditions, S. tomentosa would be adversely affected, whereas S. terebinthifolius would thrive. On a long-term basis, this imbalance is likely to interfere in the vegetational composition and dynamics of the affected ecosystem.

Uptake of organic contaminants by plant roots consists of two consecutive steps: sorption to plant roots and entrance into root xylem tissues through epidermal and endodermic membranes. Most research pertaining to phytoremediation assumed that sorption to plant roots is linear and non-competitive. A growing body of evidence, however, is suggesting that sorption to plant roots is nonlinear and competitive. The objective of this study was to examine the concentration effects of chemical constituents on the competitive sorption of trichloroethylene (TCE) and tetrachloroethylene (PCE) to the roots of Typha latifolia. Competitive sorption was clearly demonstrated by the reduced sorption of TCE and PCE in bi-solute systems than in single-solute systems. Concentration is an important factor affecting the extent of competition. In bi-solute systems, the PCE/TCE ratio on root surface approximately reflected the contaminant footprints in solution. The result was attributed to limited high energetically favorable sorption sites on the root surface and similar sorption mechanisms of TCE and PCE. The results hold significant importance for the application of phytomonitoring of organic contaminant mixtures.

Risk assessment of trace-metal contamination in soils requires predictive models that take into account the interaction of metal ions with other cations (e.g., H⁺ and Ca²⁺) that can change the speciation of trace metals in solution and compete for binding sites on plant roots thus affecting metal uptake and toxicity. Acid-base titrations were used to estimate the types and quantity of cation-binding sites on fresh pea (Pisum sativum L. cv. Lincoln) roots and their binding strength with protons. The roots were found to have three types of cation-binding sites with site densities of 190, 382, and 347 μmolc g⁻¹ (dry weight), respectively. The binding strength with H⁺ was indicated by the equilibrium formation constants (K HLj ). The logK HLj values under different ionic strengths were determined. At zero ionic strength, the logK HLj values are estimated to be 2.5, 5.5, and 8.3, respectively. Complementary experiments were used to validate the titration results. These included an ion exchange experiment, an experiment with HCl extractions, and a KOH neutralization method. Estimates from all four methods were consistent under the experimental conditions. The quantification of the binding capacity and the characteristics of these binding sites will assist in the development of more appropriate solution speciation models that incorporate biotic ligands. The derived parameters will provide the basis on which further development of a biotic ligand model is dependent.

Silicon presents a close relationship with the amelioration of heavy metals phytotoxicity. However, mechanisms of Si-mediated alleviation of metal stress remains poorly understood. This work aimed at studying the relationship between the accumulation of Si, Cd, and Zn and the tolerance and structural alterations displayed by maize plants grown on a Cd and Zn enriched soil treated with doses of Si (0, 50, 100, 150, and 200mg kg⁻¹) as calcium silicate (CaSiO₃). The results showed that the maize plants treated with Si presented not only biomass increasing but also higher metal accumulation. Significant structural alterations on xylem diameter, mesophyll and epidermis thickness, and transversal area occupied by collenchyma and midvein were also observed as a result of Si application. The deposition of silica in the endodermis and pericycle of roots seems to play an important role on the maize tolerance to Cd and Zn stress.

Background, aim, and scope The reclamation of nonferrous metal-polluted soil by phytoremediation requires an overall and permanent plant cover. To select the most suitable plant species, it is necessary to study metal effects on plants over the time, thereby checking that metals remain stored in root systems and not transferred to aerial parts. In this purpose, the seasonal and annual variations of metal bioaccumulation, transfer, and phytotoxicity in Trifolium repens and Lolium perenne grown in a Cd-, Pb-, and Zn-contaminated soil were also studied. Materials and methods The experimental site was located near a closed smelter. In spring 2004, two areas were sown with T. repens and L. perenne, respectively. Thereafter, the samplings of plant roots and shoots and surrounding soils were realized in autumn 2004 and spring and autumn 2005. The soil agronomic characteristics, the Cd, Pb, and Zn concentrations in the surrounded soils and plant organs, as well as the oxidative alterations (superoxide dismutase [SOD], malondialdehyde [MDA], and 8-hydroxy-2'-deoxyguanosine [8-OHdG]) in plant organs were carried out. Results Whatever the sampling period, metal concentrations in soils and plants were higher than background values. Contrary to the soils, the fluctuations of metal concentrations were observed in plant organs over the time. Bioaccumulation and transfer factors confirmed that metals were preferentially accumulated in the roots as follows: Cd>Zn>Pb, and their transfer to shoots was limited. Foliar metal deposition was also observed. The results showed that there were seasonal and annual variations of metal accumulation in the two studied plant species. These variations differed according to the organs and followed nearly the same pattern for the two species. Oxidative alterations were observed in plant organs with regard to SOD antioxidant activities, MDA, and 8-OHdG concentrations. These alterations vary according to the temporal variations of metal concentrations. Discussion Metal concentrations in surrounded soils and plant organs showed the effective contamination by industrial dust emissions. Metals absorbed by plants were mainly stored in the roots. With regard to this storage, the plants seemed to limit the metal transfer to their aerial parts over the time, thereby indicating their availability for metal phytostabilization. Aerial deposition was another source of plant exposure to nonferrous metals. Despite the occurrence of metal-induced oxidative alterations in plant organs, both plant species seemed to tolerate a high metal concentration in soils. Conclusions Taken together, these results indicated that T. repens and L. perenne were able to form a plant cover on highly Cd-, Pb-, and Zn-polluted soils, to limit the metal transfer to their aerial parts and were relatively metal-tolerant. All these characteristics made them suitable for phytostabilization on metal-contaminated soils. These findings also highlighted the necessity to take into account seasonal and annual variations for a future phytomanagement. Recommendations and perspectives In this work, the behavior of plant species grown in metal-polluted soil has been studied during 2 years. Obviously, this time is too short to ensure that metals remain accumulated in the root system and few are transferred in aerial parts over the time. It is why regular monitoring should be achieved during more than a decade after the settlement of the plant cover. This work will be completed by the study of the T. repens and L. perenne effects on mobility of metals in order to evaluate the quantities of pollutants which could be absorbed by the biota and transferred to groundwater. Bioaccessibility tests could be also realized on polluted soils in order to evaluate the phytostabilization impacts on the exposition risks for humans.

Background, aim, and scope The success of phytoextraction depends upon the identification of suitable plant species that hyperaccumulate heavy metals and produce large amounts of biomass using established agricultural techniques. In this study, the Mediterranean saltbush Atriplex halimus L., which is a C4 perennial native shrub of Mediterranean basin with an excellent tolerance to drought and salinity, is investigated with the main aim to assess its phytoremediation potential for Pb and Cd removal from contaminated soils. In particular, the influence of soil salinity in metal accumulation has been studied as there is notable evidence that salinity changes the bioavailability of metals in soil and is a key factor in the translocation of metals from roots to the aerial parts of the plant. Materials and methods Three pot experiments were conducted under greenhouse conditions for a 10-week period with A. halimus grown in soil artificially polluted with 20 ppm of Cd and/or 800 ppm of Pb and irrigated with three different salt solutions (0.0%, 0.5%, and 3.0% NaCl). Soil measurements for soil characterization were performed with the expiration of the first week of plant exposure to metals and NaCl, and at the end of the experimental period, chlorophyll content, leaf protein content, leaf specific activity of guaiacol peroxidase (EC 1.11.1.7), shoot water content, biomass, and Cd and Pb content in the plant tissues were determined. Additionally, any symptoms of metal or salt toxicity exhibited by the plants were visually noted during the whole experimental period. Results The experimental data suggest that increasing salinity increases cadmium uptake by A. halimus L. while in the case of lead there was not a clear effect of the presence of salt on lead accumulation in plant tissues. A. halimus developed no visible signs of metal toxicity; only salt toxicity symptoms were observed in plants irrigated with 3% NaCl solutions. Chlorophyll content, leaf protein content, shoot water content, and biomass were not negatively affected by the metals; instead, there was even an increase in the amount of photosynthetic pigments in plants treated with both metals and salinity. The specific activity of guaiacol peroxidase seems to have a general tendency for increase in plants treated with the metals in comparison with the respective controls but a statistically significant difference exists only in plants treated with the metal mixture and saline conditions. Discussion The data revealed that lead and cadmium accumulation in plant tissues was kept generally at low levels. Salinity was found to have a positive effect on cadmium uptake by the plant and this may be related to a higher bioavailability of the metal in soil due to decreased Cd sorption on soil particles. On the other hand, salinity did not influence in a clear way the uptake of Pb by the plant probably because of lead's limited mobility in soils and plant tissues. Cd and Pd usually decrease the chlorophyll content and biomass and change water relations in plants; however, A. halimus was found not to be affected indicating that it is a Cd- and Pb-tolerant plant. Guaiacol peroxidase activity as one of the parameters expressing oxidative damage and extent of stress in plants was not generally found to be significantly affected under the presence of metals in most plants suggesting that the extent of stress in plants was minimal, while only for plants treated with the metal mixture and low salinity the enzyme activity was elevated confirming that this enzyme serves as an antioxidative tool against the reactive oxygen species produced by the metals. Conclusions Atriplex halimus L. is a Pb- and Cd-tolerant plant but metal concentrations achieved in plant tissues were kept generally at low levels; however, metal accumulation in shoots, especially for Cd, considered together with its high biomass production, rapid growth, and deep root system able to cope with poor structure and xeric characteristics of several polluted soils suggest that this plant deserves further investigation. Recommendations and perspectives Phytoextraction by halophytes is a promising alternative for the remediation of heavy metal contaminated sites affected by salinity since saline depressions often indicate sites of industrial effluents accumulation, contaminated by heavy metals, including Pb and Cd. Halophytes are also promising candidates for the removal of heavy metals from non-saline soils. Furthermore, the use of such plants can be potentially viewed as an alternative method for soil desalination where salt is removed from the soil instead of being washed downwards by water or other solutions.

Background, aim, and scope The use of plants and associated microorganisms to remove, contain, inactivate, or degrade harmful environmental contaminants (generally termed phytoremediation) and to revitalize contaminated sites is gaining more and more attention. In this review, prerequisites for a successful remediation will be discussed. The performance of phytoremediation as an environmental remediation technology indeed depends on several factors including the extent of soil contamination, the availability and accessibility of contaminants for rhizosphere microorganisms and uptake into roots (bioavailability), and the ability of the plant and its associated microorganisms to intercept, absorb, accumulate, and/or degrade the contaminants. The main aim is to provide an overview of existing field experience in Europe concerning the use of plants and their associated microorganisms whether or not combined with amendments for the revitalization or remediation of contaminated soils and undeep groundwater. Contaminations with trace elements (except radionuclides) and organics will be considered. Because remediation with transgenic organisms is largely untested in the field, this topic is not covered in this review. Brief attention will be paid to the economical aspects, use, and processing of the biomass. Conclusions and perspectives It is clear that in spite of a growing public and commercial interest and the success of several pilot studies and field scale applications more fundamental research still is needed to better exploit the metabolic diversity of the plants themselves, but also to better understand the complex interactions between contaminants, soil, plant roots, and microorganisms (bacteria and mycorrhiza) in the rhizosphere. Further, more data are still needed to quantify the underlying economics, as a support for public acceptance and last but not least to convince policy makers and stakeholders (who are not very familiar with such techniques).

Background, aim, and scope Selenium is a trace metalloid of global environmental concern. The boundary among its essentiality, deficiency, and toxicity is narrow and mainly depends on the chemical forms and concentrations in which this element occurs. Different plant species--including Brassica juncea--have been shown to play a significant role in Se removal from soil as well as water bodies. Furthermore, the interactions between such plants, showing natural capabilities of metal uptake and their rhizospheric microbial communities, might be exploited to increase both Se scavenging and vegetable biomass production in order to improve the whole phytoextraction efficiency. The aim of the present study was to evaluate the capability of selenite removal of B. juncea grown in hydroponic conditions on artificially spiked effluents. To optimize phytoextraction efficiency, interactions between B. juncea and rhizobacteria were designedly elicited. Materials and methods Firstly, B. juncea was grown on water-filtering agriperlite beds in the presence of three different selenite concentrations, namely, 0.2, 1.0, and 2.0 mM. Plant growth was measured after 3 and 6 weeks of incubation in order to establish the selenite concentration at which the best plant biomass production could be obtained. Afterwards, water-filtering agriperlite beds were inoculated either with a selenium-acclimated microbial community deriving from the rhizosphere of B. juncea grown, erstwhile, in a selenite-amended soil or with axenic cultures of two bacterial strains, vicelike Bacillus mycoides SeITE01 and Stenotrophomonas maltophilia SeITE02, previously isolated and described for their high resistance to selenite. These latter were seeded separately or as a dual consortium. Selenite was amended at a final concentration of 1.0 mM. Total Se content in plant tissues (both shoots and roots), plant biomass production, and persistence of bioaugmented microbial inocula during the experimental time were monitored. Moreover, parameters such as bioconcentration factor (BF) and phytoextraction efficiency (PE) were determined at the end of the testing run to evaluate the effects of the different bioaugmentation strategies adopted on selenite phytoextraction efficiency of B. juncea. Results A general but significant increase in capacity to extract and transport selenium to the epigeous plant compartments was recorded in B. juncea grown in beds augmented with microbial inocula, except for the treatment with B. mycoides SeITE01 alone. Nevertheless, a severe decrease in vegetable biomass production was observed after all microbial treatments with the exception of the plants that had received only S. maltophilia SeITE02. Actually, an increase in selenium phytoextraction efficiency up to 65% was observed in B. juncea, when this bacterial strain was inoculated. Discussion Emendation of B. juncea grown in water-filtering beds with a Se(IV)-acclimated microbial community caused a higher Se uptake along with a reduction of plant biomass yield with respect to plants grown without addition of the same bacterial inoculum. The increase of selenium BF in shoots suggests that the Se(IV)-acclimated microbial community not only elicited the plant capacity to absorb selenite, but also did improve the capacity to transport the metalloid to the epigeous compartments. On the other hand, the reduction in plant biomass yield might be related exactly to this improved capability of B. juncea to accumulate selenium at concentrations that are actually toxic for plants. Differently, addition of two selenite-resistant bacterial strains, namely, S. maltophilia SeITE02 and B. mycoides SEITE01, had weaker effects on plant biomass production when compared to those recorded in the presence of the Se(IV)-adapted microbial community. In particular, inoculation of water-filtering beds with the SeITE02 strain alone was the sole strategy resulting in a positive effect on both plant biomass production in stressful conditions and the capacity of shoots to accumulate selenium. In fact, its putative ability of reducing Se(IV) to organo-Se compounds significantly enhanced either selenium absorption by the plants or active metalloid translocation to epigeous parts. Conclusions Bioaugmentation with the bacterial strain S. malthophila SeITE02 is suggested to elicit selenite phytoextraction efficiency in B. juncea. Recommendations Manipulation of synergistic interactions between plants having phytoextraction capabilities and their associated rhizobacteria may enhance already consolidated treatment processes aimed to detoxify selenite laden wastewater.