Arsenic Speciation Governs Arsenic Uptake and Transport in Terrestrial Plants

Wheat PHT1;9 acts as one candidate arsenate absorption transporter for phytoremediation

Arsenate (AsV) is one of the most common forms of arsenic (As) in environment and plant high-affinity phosphate transporters (PHT1s) are the primary plant AsV transporters. However, few PHT1s involved in AsV absorption have been identified in crops. In our previous study, TaPHT1;3, TaPHT1;6 and TaPHT1;9 were identified to function in phosphate absorption. Here, their AsV absorption capacities were evaluated using several experiments. Ectopic expression in yeast mutants indicated that TaPHT1;9 had the highest AsV absorption rates, followed by TaPHT1;6, while not for TaPHT1;3. Under AsV stress, further, BSMV-VIGS-mediated TaPHT1;9-silencing wheat plants exhibited higher AsV tolerance and lower As concentrations than TaPHT1;6-silenced plants, whereas TaPHT1;3-silencing plants had similar phenotype and AsV concentrations to control. These suggested that TaPHT1;9 and TaPHT1;6 possessed AsV absorption capacity with the former showing higher activities. Under hydroponic condition, furthermore, CRISPR-edited TaPHT1;9 wheat mutants showed the enhanced tolerance to AsV with decreased As distributions and concentrations, whereas TaPHT1;9 ectopic expression transgenic rice plants had the opposite results. Also, under AsV-contaminated soil condition, TaPHT1;9 transgenic rice plants exhibited depressed AsV tolerance with increased As concentrations in roots, straws and grains. Moreover, Pi addition alleviated the AsV toxicity. These suggested that TaPHT1;9 should be a candidate target gene for AsV phytoremediation.

Negative Impacts of Arsenic on Plants and Mitigation Strategies

Arsenic (As) is a metalloid prevalent mainly in soil and water. The presence of As above permissible levels becomes toxic and detrimental to living organisms, therefore, making it a significant global concern. Humans can absorb As through drinking polluted water and consuming As-contaminated food material grown in soil having As problems. Since human beings are mobile organisms, they can use clean uncontaminated water and food found through various channels or switch from an As-contaminated area to a clean area; but plants are sessile and obtain As along with essential minerals and water through roots that make them more susceptible to arsenic poisoning and consequent stress. Arsenic and phosphorus have many similarities in terms of their physical and chemical characteristics, and they commonly compete to cause physiological anomalies in biological systems that contribute to further stress. Initial indicators of arsenic's propensity to induce toxicity in plants are a decrease in yield and a loss in plant biomass. This is accompanied by considerable physiological alterations; including instant oxidative surge; followed by essential biomolecule oxidation. These variables ultimately result in cell permeability and an electrolyte imbalance. In addition, arsenic disturbs the nucleic acids, the transcription process, and the essential enzymes engaged with the plant system's primary metabolic pathways. To lessen As absorption by plants, a variety of mitigation strategies have been proposed which include agronomic practices, plant breeding, genetic manipulation, computer-aided modeling, biochemical techniques, and the altering of human approaches regarding consumption and pollution, and in these ways, increased awareness may be generated. These mitigation strategies will further help in ensuring good health, food security, and environmental sustainability. This article summarises the nature of the impact of arsenic on plants, the physio-biochemical mechanisms evolved to cope with As stress, and the mitigation measures that can be employed to eliminate the negative effects of As.

Germination and early seedling development of Helichrysum microphyllum Cambess. subsp. tyrrhenicum Bacch., Brullo & Giusso in the presence of arsenates and arsenites

Arsenate, As(V), and arsenite, As(III), are the most available arsenicals present in the soil solutions, in particular in mine polluted substrates, and cause several symptoms of toxicity in plants (like inhibition of seed germination and reduction of seedling development). For these reasons, seeds germination studies are essential for the design of phytoremediation activities of mine sites. Seed germination and seedling development of Helichrysum microphyllum subsp. tyrrhenicum, were evaluated at 15 °C using various concentrations of As(V) and As(III) (0–500 mg/L and 0–200 mg/L, respectively). Seeds were harvested (I) into a mine dump contaminated in As, (II) nearby this site, and (III) faraway the As contaminated area and without mine activities. Seed germination, cotyledons emergence, and seedling mortality were evaluated for 90 days. As(V) and As(III) acted differently, showing a much higher toxicity when arsenite was added than arsenate. The taxon was able to germinate, develop cotyledons, and survive under all arsenate concentrations, whereas arsenite acted on these steps already at 2.5 mg/L. Moreover, a linear decrease in cotyledons emergence was assessed with the increase of both arsenicals’ concentrations, as well as a linear decrease of seedling survival under arsenite. The taxon showed great adaptability to As pollution, giving an important contribution in phytoremediation of mining sites.

Molecular insight into arsenic uptake, transport, phytotoxicity, and defense responses in plants: a critical review

A critical investigation into arsenic uptake and transportation, its phytotoxic effects, and defense strategies including complex signaling cascades and regulatory networks in plants. The metalloid arsenic (As) is a leading pollutant of soil and water. It easily finds its way into the food chain through plants, more precisely crops, a common diet source for humans resulting in serious health risks. Prolonged As exposure causes detrimental effects in plants and is diaphanously observed through numerous physiological, biochemical, and molecular attributes. Different inorganic and organic As species enter into the plant system via a variety of transporters e.g., phosphate transporters, aquaporins, etc. Therefore, plants tend to accumulate elevated levels of As which leads to severe phytotoxic damages including anomalies in biomolecules like protein, lipid, and DNA. To combat this, plants employ quite a few mitigation strategies such as efficient As efflux from the cell, iron plaque formation, regulation of As transporters, and intracellular chelation with an array of thiol-rich molecules such as phytochelatin, glutathione, and metallothionein followed by vacuolar compartmentalization of As through various vacuolar transporters. Moreover, the antioxidant machinery is also implicated to nullify the perilous outcomes of the metalloid. The stress ascribed by the metalloid also marks the commencement of multiple signaling cascades. This whole complicated system is indeed controlled by several transcription factors and microRNAs. This review aims to understand, in general, the plant-soil-arsenic interaction, effects of As in plants, As uptake mechanisms and its dynamics, and multifarious As detoxification mechanisms in plants. A major portion of this article is also devoted to understanding and deciphering the nexus between As stress-responsive mechanisms and its underlying complex interconnected regulatory networks.

An overview of the methods for analyzing the chemical forms of metals in plants

Currently, the occurrence of toxic levels of metals in soils is a serious environmental issue worldwide. Phytoremediation is getting much attention to control metals soil pollution because it is economic and environmentally friendly. However, the methods used to detect metals in plants are not uniform and have depicted poor comparability of the research investigations. Therefore, the present overview is designed to discuss the possible chemical forms of metals in various environmental matrixes and the detection methods employed to identify the chemical forms of metals in plants. Moreover, the in situ and indirect methods to detect metals in plants have also been discussed herein. In addition, the pros and cons of the available techniques have also been critically analyzed and discussed. Finally, key points/challenges and future perspectives of these methods have been highlighted for the scientific community.Novelty statementIn the current review, the possible chemical forms of metals in various environmental matrixes are discussed in detail. Various extraction agents and their efficiency for extracting metals from plants have been clearly illustrated. Further, all the available methods for analyzing the chemical forms of metals in plants have been compared.

Arsenic Toxicity-Induced Physiological and Metabolic Changes in the Shoots of Pteris cretica and Spinacia oleracea

Arsenic is a ubiquitous toxic element that can be accumulated into plant parts. The present study investigated the response of Pteris cretica and Spinacia oleracea to As treatment through the analysis of selected physiological and metabolic parameters. Plants were grown in pots in As(V) spiked soil (20 and 100 mg/kg). Plants’ physiological condition was estimated through the determination of elements, gas-exchange parameters, chlorophyll fluorescence, water potential, photosynthetic pigments, and free amino acid content. The results confirmed differing As accumulation in plants, as well as in shoots and roots, which indicated that P. cretica is an As-hyperaccumulator and that S. oleracea is an As-root excluder. Variations in physiological and metabolic parameters were observed among As treatments. Overall, the results revealed a significant effect of 100 mg/kg As treatment on the analysed parameters. In both plants, this treatment affected growth, N, Mg, S, Mn, and Zn content, as well as net photosynthetic rate, chlorophyll fluorescence, and total free amino acid content. In conclusion, the results reflect the similarity between P. cretica and S. oleracea in some aspects of plants’ response to As treatment, while physiological and metabolic parameter changes related to As treatments indicate the higher sensitivity of S. oleracea.

Unraveling the role of plant growth-promoting rhizobacteria in the alleviation of arsenic phytotoxicity: A review

The toxic metalloid arsenic (As), is a major pollutant of soil and water, imposing severe health concerns on human lives. It enters the food chain mainly through As-contaminated crops. The uptake, translocation and accumulation of As in plant tissue are often controlled by certain soil-inhabiting microbial communities. Among them, indigenous, free-living As-resistant plant growth-promoting rhizobacteria (PGPR) plays a pivotal role in As-immobilization. Besides, the plant's inability to withstand As after a threshold level is actively managed by these PGPR increasing As-tolerance in host plants by a synergistic plant-microbe interaction. The dual functionality of As-resistant PGPR i.e., phytostimulation and minimization of As-induced phytotoxic damages are one of the main focal points of this review article. It is known that such PGPR having the functional arsenic-resistant genes (in ars operon) including As-transporters, As-transforming genes contributed to the As accumulation and detoxification/transformation respectively. Apart from assisting in nutrient acquisition and modulating phytohormone levels, As-resistant PGPR also influences the antioxidative defense system in plants by maneuvering multiple enzymatic and non-enzymatic antioxidants. Furthermore, they are effective in reducing membrane damage and electrolyte leakage in plant cells. As-induced photosynthetic damage is also found to be salvaged by As-resistant PGPR. Briefly, the eco-physiological, biochemical and molecular mechanisms of As-resistant PGPR are thus elaborated here with regard to the As-exposed crops.

The fate of arsenic in rice plants (Oryza sativa L.): Influence of different forms of selenium

As contamination of rice plants has aroused worldwide concern because of the threats posed to human health through its accumulation in the food chain. However, no data are currently available on the effect of Se nanoparticles (SeNPs) on the fate of As in higher plants, and previously reported relationships between As and Se are inconsistent. Therefore, in this study, the possible mediating roles of SeNPs or selenite on the uptake, translocation, subcellular distribution, and transformation of arsenite and arsenate in rice seedlings (Oryza sativa L.) were investigated through hydroponic experiments. The results showed that, when supplied as arsenite and arsenate, selenite significantly increased root As uptake by 71.7% and 45.9% but decreased shoot As content by 48.9% and 52.4%, respectively. In comparison, the reducing effect of SeNPs on shoot As content (37.1%) was only significant in arsenite-treated rice plants. Furthermore, selenite significantly reduced and increased the As content of different shoot and root subcellular fractions, respectively; and SeNPs also led to a dramatic decrease in the As content of the different shoot subcellular fractions of arsenite-treated rice plants. Moreover, As(III) and As(V) content was reduced in rice shoots while enhanced in rice roots by selenite. Generally, neither As(III) nor As(V) content in rice tissues was dramatically changed by SeNPs. Our results indicate that both SeNPs and selenite are effective in mitigating As toxicity in rice plants, although selenite showed a stronger inhibiting effect on As translocation.

Ectopic expression of wheat aquaglyceroporin TaNIP2;1 alters arsenic accumulation and tolerance in Arabidopsis thaliana

Arsenic (As) is one of the most toxic contaminants to food crops, and as such, decreasing crops uptake and accumulation of As cannot be overemphasized. Here, we characterized a functional wheat NIP2;1 homolog of the As transporter, TaNIP2;1. TaNIP2;1 expression was suppressed by arsenite (As(III)) in wheat. Ectopic expression of TaNIP2;1 in the Δfps1 yeast mutant enhanced yeast sensitivity towards As(III). Conversely, the elevated expression of TaNIP2;1 in Δacr3 mutants decreased yeast sensitivity to arsenate (As(V)), demonstrating that TaNIP2;1 showed both influx and efflux transport activities for As(III) in yeasts. This is further supported by increased As concentration in the yeast cells that overproduce TaNIP2;1 in Δfps1, while As concentration decreased in Δacr3. Furthermore, ectopic expression of TaNIP2;1 in Arabidopsis confirmed that TaNIP2;1 can transport As into plants, as supported by increased sensitivity to and uptake of As(III). No change in plant sensitivity was found to Cu(II), Cd(II), Zn(II) or Ni(II), indicating that transport activity of TaNIP2;1 is specific for As(III). Taken together, our data show that TaNIP2;1 may be involved in As(III) transportation in plants. This finding reveals a functional gene that can be manipulated to reduce As content in wheat.

Arsenic accumulation by red fescue (Festuca rubra) growing in mine affected soils - Findings from the field and greenhouse studies

Soils strongly enriched in arsenic in historical mining sites pose the environmental risk. Phytostabilization is a reasonable method for their remediation. A suitability of red fescue (Festuca rubra L.) for this purpose was examined. Plant and soil material was collected from four study objects: mine dumps in Złoty Stok and Czarnów and two areas formerly flooded by tailings. Total As in soils ranged 72-48900 mg/kg, while the shoots and roots of red fescue contained 1.5-65.5 and 2.3-824 mg/kg As, respectively. Bioaccumulation BAF and translocation TF factors were typical for excluders, however, in most cases, As in shoots exceeded 4 mg/kg, an EU threshold for As in fodder. A greenhouse experiment, that involved treatment with mineral fertilizers, manure, and forest litter, was performed to closer examine the factors governing As uptake by red fescue. A stress-resistant cultivar Leo-Pol was used as a test plant. Grass shoots were harvested after 6 and 12 weeks. Manure treatment increased strongly As extractability but did not increase As uptake by plants. Though, As concentrations in plants were in the pot experiment by manifold higher than those in the field. Particularly high (66.5-1580 mg/kg) was As in the second shoot harvest. Differences between the field and greenhouse data indicate that the populations of red fescue, that develop in As rich sites, are specifically As-tolerant. Possible mechanisms of tolerance are discussed. The conclusion is that the commercial cultivar, despite declared stress-resistance, cannot be used for phytostabilization of barren As-rich soils.

Multi-Component Antioxidative System and Robust Carbohydrate Status, the Essence of Plant Arsenic Tolerance

Arsenic (As) contaminates the food chain and decreases agricultural production through impairing plants, particularly due to oxidative stress. To better understand the As tolerance mechanisms, two contrasting tobacco genotypes: As-sensitive Nicotiana sylvestris and As-tolerant N.tabacum, cv. 'Wisconsin' were analyzed. The most meaningful differences were found in the carbohydrate status, neglected so far in the As context. In the tolerant genotype, contrary to the sensitive one, net photosynthesis rates and saccharide levels were unaffected by As exposure. Importantly, the total antioxidant capacity was far stronger in the As-tolerant genotype, based on higher antioxidants levels (e.g., phenolics, ascorbate, glutathione) and activities and/or appropriate localizations of antioxidative enzymes, manifested as reverse root/shoot activities in the selected genotypes. Accordingly, malondialdehyde levels, a lipid peroxidation marker, increased only in sensitive tobacco, indicating efficient membrane protection in As-tolerant species. We bring new evidence of the orchestrated action of a broad spectrum of both antioxidant enzymes and molecules essential for As stress coping. For the first time, we propose robust carbohydrate metabolism based on undisturbed photosynthesis to be crucial not only for subsidizing C and energy for defense but also for participating in direct reactive oxygen species (ROS) quenching. The collected data and suggestions can serve as a basis for the selection of plant As phytoremediators or for targeted breeding of tolerant crops.

Optimized extraction of inorganic arsenic species from a foliose lichen biomonitor

To assess the two most toxicologically relevant species of As, namely arsenite (As(III)) and arsenate (As(V)), chromatographic separations often require two separate chromatographic columns to address the co-elution of arsenobetaine (AsB) with As(III). This issue is typically observed using conventional isocratic methods on anion exchange columns, increasing cost and analysis time. Here, we optimize the extraction of inorganic As from a lichen air biomonitor and develop an isocratic method for the chromatographic separation of five common As species on a PRP X-100 anion exchange column, resulting in the complete baseline separation of all species under study. This method was then applied to lichen biomonitors from an urban and rural site to demonstrate its use. In order of abundance, the various arsenic species in lichens from the urban site in South Africa were As(V) > As(III) > AsB > dimethylarsinic acid (DMA) > monomethylarsonic acid (MMA), and As(V) > AsB > As(III) > DMA > MMA for the rural site, where MMA was present in extremely low, non-quantifiable concentrations in lichens from both sites. Total concentrations of As were higher in samples from the urban site (6.43 ± 0.25 μg/g) than in those from the rural site (1.87 ± 0.05 μg/g), with an overall extraction efficiency of 19% and 40%, respectively. The optimized method utilized relatively inexpensive solvents and is therefore low-cost and eco-friendly in comparison with conventional chromatographic techniques. This is the first study which addresses the optimized extraction and characterization of As species in a South African lichen biomonitor of air pollution. Graphical abstract .

Low arsenate influx rate and high phosphorus concentration in wheat (Triticum aestivum L.): A mechanism for arsenate tolerance in wheat plants

Two wheat (Triticum aestivum L.) cultivars differing in arsenic (As)-tolerance were used to investigate the effects of phosphorus (P) concentration and nutrient solution pH on As(V) toxicity and As(V) uptake kinetics, and to illustrate the mechanism of As(V) tolerance in wheat seedlings. Low pH and low phosphate concentration enhanced wheat uptake of As, resulting in high As toxicity. The As(V)-tolerant cultivar MM45 exhibited higher relative root elongation than non-tolerant cultivar HM29 in all treatments, except that no genotypic difference was recorded for the solution P at 100 μmol L^-1 or greater. Wheat seedling As(V) tolerance was positively correlated with P concentration in roots and shoots. In short-term (30 min) As(V)-uptake kinetics experiments, the maximum influx rate (V_max) of As(V) increased with decreasing solution pH (from 7.0 to 6.0). Compared with HM29, although MM45 had lower V_max, its Michaelis-Menten constant (K_m) did not exceed that of HM29 in all treatments. The V_max values of both cultivars were not significantly affected by phosphate treatments, except for HM29 which had significantly higher V_max value in the presence of phosphate at pH 7.0. The K_m values of the two cultivars increased by 9- to 20-fold when phosphate was present as opposed to absent from the uptake solution. This study showed that the V_max values are mainly increased by high pH and As(V) uptake K_m is mainly increased by phosphate presence. Decreased As(V) influx rates during early stages and increased P concentration in plant tissues are associated with increased As tolerance in wheat seedlings.

Quantification and feed to food transfer of total and inorganic arsenic from a commercial seaweed feed

Seaweed has a long-associated history of use as a supplemented livestock feed, providing nutrients and vitamins essential to maintaining animal health. Some species of seaweed, particularly the fucoids, are well-known accumulators of the metalloid arsenic (As). Arsenic toxicity to humans is well established even at low exposure levels and is considered a class 1 human carcinogen. As mankind's appetite for livestock produce continues to grow unabated, there is a concern that consumption of livestock produce reared on a diet supplemented with seaweed animal feed (SAF) may pose a threat to the human population due to potentially high levels of As present in seaweed. To address this concern and provide end users, including industry, consumers, policymakers and regulators with information on the exposure associated with As in commercial seaweed animal feed, the estimated daily intake (EDI) of As was calculated to evaluate potential human exposure levels. Using As data from a commercially available seaweed meal over a five-year period (2012-2017) a population exposure assessment was carried out. A Monte Carlo simulation model was developed to characterise the feed to food transfer of As from animal feed to animal produce such as beef, milk, chicken, and eggs. The model examined initial levels in seaweed, inclusion rate in animal feed, animal feeding rates and potential transfer to food produced from a supplemented diet of SAF. The analysis of seaweed animal feed showed that inorganic As was a small fraction of the total As found in seaweed meal (80:1). Statistical analysis found significant differences in the concentration of As in seaweed animal feed depending on the grain size (p < 0.001), with higher As concentrations in smaller sized grain fractions. Due to several detoxification steps and subsequent rapid excretion from the bodies of livestock, a very low carryover rate of As compounds from seaweed animal feed into livestock produce was observed. The EDI calculated in this study for the livestock produce evaluated at the 95th confidence interval was <0.01% of suggested safe levels of inorganic As intake. The threat to the general population as a result of consumption of livestock products reared on a diet consisting of SAF is found to be negligible.

Two facets of world arsenic problem solution: crop poisoning restriction and enforcement of phytoremediation

This review provides insights into As toxicity in plants with focus on photosynthesis and sugar metabolism as important arsenic targets and simultaneously defence tools against accompanying oxidative stress. Heavy metal contamination is a great problem all over the world. Arsenic, a metalloid occurring naturally in the Earth's crust, also massively spreads out in the environment by human activities. Its accumulation in crops poses a severe health risk to humans and animals. Besides the restriction of human-caused contamination, there are two basic ways how to cope with the problem: first, to limit arsenic accumulation in harvestable parts of the crops; second, to make use of some arsenic hyperaccumulating plants for phytoremediation of contaminated soils and waters. Progress in the use of both strategies depends strongly on the level of our knowledge on the physiological and morphological processes resulting from arsenic exposure. Arsenic uptake is mediated preferentially by P and Si transporters and its accumulation substantially impairs plant metabolism at numerous levels including damages through oxidative stress. Rice is a predominantly studied crop where substantial progress has been made in understanding of the mechanisms of arsenic uptake, distribution, and detoxification, though many questions still remain. Full exploitation of plant potential for soil and water phytoremediations also requires deep understanding of the plant response to this toxic metalloid. The aim of this review is to summarize data regarding the effect of arsenic on plant physiology with a focus on mechanisms providing increased arsenic tolerance and/or hyperaccumulation. The emphasis is placed on the topic unjustifiably neglected in the previous reviews - i.e., carbohydrate metabolism, tightly connected to photosynthesis, and beside others involved in plant ability to cope with arsenic-induced oxidative and nitrosative stresses.

Arsenic-phosphorus interactions in the soil-plant-microbe system: Dynamics of uptake, suppression and toxicity to plants

High arsenic (As) concentrations in the soil, water and plant systems can pose a direct health risk to humans and ecosystems. Phosphate (Pi) ions strongly influence As availability in soil, its uptake and toxicity to plants. Better understanding of As(V)-Pi interactions in soils and plants will facilitate a potential remediation strategy for As contaminated soils, reducing As uptake by crop plants and toxicity to human populations via manipulation of soil Pi content. However, the As(V)-Pi interactions in soil-plant systems are complex, leading to contradictory findings among different studies. Therefore, this review investigates the role of soil type, soil properties, minerals, Pi levels in soil and plant, Pi transporters, mycorrhizal association and microbial activities on As-Pi interactions in soils and hydroponics, and uptake by plants, elucidate the key mechanisms, identify key knowledge gaps and recommend new research directions. Although Pi suppresses As uptake by plants in hydroponic systems, in soils it could either increase or decrease As availability and toxicity to plants depending on the soil types, properties and charge characteristics. In soil, As(V) availability is typically increased by the addition of Pi. At the root surface, the Pi transport system has high affinity for Pi over As(V). However, Pi concentration in plant influences the As transport from roots to shoots. Mycorrhizal association may reduce As uptake via a physiological shift to the mycorrhizal uptake pathway, which has a greater affinity for Pi over As(V) than the root epidermal uptake pathway.

Relative influence of sediment variables on mangrove community assembly in Leizhou Peninsula, China

Effective conservation of mangroves requires a complete understanding of vegetation structure and identification of the variables most important to their assembly. Using canonical correspondence analysis (CCA) combined with variation partition, we determined the independent and joint effects of sediment variables, including physicochemical characteristics and heavy metals, on mangrove community assemblies in the overstory and understory in Leizhou Peninsula, China. The results indicated that the contributions of sediment physicochemical variables to community assembly were greater than were those of heavy metals, particularly in overstory vegetation. However, the independent contributions of heavy metals were higher in understory mangrove vegetation than in the overstory. The TOC, TP, and salinity of the sediment, distance from the coastline, and concentration of As were limiting factors for mangrove assembly in overstory vegetation, while understory vegetation may be affected to a greater degree by the distance from the coastline, electrical conductivity, and concentration of As and Pb in the sediment.

Micro-distribution of arsenic species in tissues of hyperaccumulator Pteris vittata L

Arsenic (As) contamination and its harmful consequences have gained increasing attention in research. Phytoextraction, which uses the As hyperaccumulator Pteris vittata L., is a well-established technology adopted in many countries. However, the hyperaccumulation mechanisms of this plant remain controversial. This study investigated the species and the micro-distribution of As species in three P. vittata L. ecotypes after exposure to arsenite (AsIII) and arsenate (AsV) for 7d. Arsenic-accumulating abilities and preferences to As species varied among different ecotypes. The reduction of AsV into AsIII, oxidation of AsIII into AsV, and chelation of AsIII with thiols were all observed in P. vittata. The reduction of As mainly occurred in the rhizoid, whereas oxidation and chelation mainly occurred in the aboveground parts. Correlation analyses showed that the As concentration in pinna was significantly correlated with the AsV percentage in paraxial and abaxial epidermis (positive), AsIII-GSH percentage in paraxial epidermis (positive), and AsIII percentage in paraxial and abaxial epidermis (negative). Results indicated that oxidation and chelation reactions contributed to the accumulation of As in P. vittata.

Mechanisms of metal toxicity in plants

Metal toxicity in plants is still a global problem for the environment, agriculture and ultimately human health.

Arbuscular mycorrhizal symbiosis influences arsenic accumulation and speciation in Medicago truncatula L. in arsenic-contaminated soil

In two pot experiments, wild type and a non-mycorrhizal mutant (TR25:3-1) of Medicago truncatula were grown in arsenic (As)-contaminated soil to investigate the influences of arbuscular mycorrhizal fungi (AMF) on As accumulation and speciation in host plants. The results indicated that the plant biomass of M. truncatula was dramatically increased by AM symbiosis. Mycorrhizal colonization significantly increased phosphorus concentrations and decreased As concentrations in plants. Moreover, mycorrhizal colonization generally increased the percentage of arsenite in total As both in shoots and roots, while dimethylarsenic acid (DMA) was only detected in shoots of mycorrhizal plants. The results suggested that AMF are most likely to get involved in the methylating of inorganic As into less toxic organic DMA and also in the reduction of arsenate to arsenite. The study allowed a deeper insight into the As detoxification mechanisms in AM associations. By using the mutant M. truncatula, we demonstrated the importance of AMF in plant As tolerance under natural conditions.

Food safety and bioavailability evaluations of four vegetables grown in the highly arsenic-contaminated soils on the Guandu Plain of northern Taiwan

Arsenic contamination in a large area of agricultural fields on the Guandu Plain of northern Taiwan was confirmed in a survey conducted in 2006, but research concerning the relationship between bioavailable As concentrations in contaminated soils and crop production in Taiwan is not available. Pot experiments were conducted to examine the growth and accumulation of As in four vegetable crops grown in As-contaminated soils and to assess As intake through consumption. The phytotoxic effects of As in soils were not shown in the pot experiments in which vegetable crops were grown in soils contaminated with different As levels in situ collected from Guandu Plain (120–460 mg/kg) or artificially spiked As-contaminated soils (50–170 mg/kg). Experimental results showed that the bioavailable As extracted with 5 M NaHCO₃ from soils can be used to estimate As concentrations in vegetables. The As concentrations in the vegetables were compared with data shown in the literature and As limits calculated from drinking water standards and the provisional tolerance weekly intake (PTWI) of inorganic As established by the Food and Agriculture Organization of the United Nations/World Health Organization (FAO/WHO). Although the As levels in the vegetables were not high and the bioavailability of As in the soils was quite low, long-term consumption may result in higher As intake in the human body.

Levels of toxic arsenic species in native terrestrial plants from soils polluted by former mining activities

Ten native terrestrial plants from soils polluted by former mining activities (Mónica mine, NW Madrid, Spain), with high total arsenic concentration levels (up to 3500 μg g(-1)), have been studied to determine the fraction of arsenic present as toxic forms (inorganic and methylated species), which present a higher mobility and therefore the potential risk associated with their reintegration into the environment is high. Roots and aboveground parts were analyzed separately to assess possible transformations from translocation processes. Extractions were carried out with deionized water by microwave-assisted extraction at a temperature of 90 °C and three extraction steps of 7.5 min each. Total extracted arsenic concentrations were determined by inductively coupled plasma atomic emission spectrometry, showing extraction percentages from 9 to 39% (calculated as the ratio between total extracted arsenic (Asext) and total arsenic (AsT) concentrations in plants). Speciation studies, performed by high performance liquid chromatography-photo-oxidation-hydride generation-atomic fluorescence spectrometry, showed the main presence of arsenate (As(v)) (up to 350 μg g(-1)), followed by arsenite (As(iii)), in both plant parts. Monomethylarsonic acid (MMA) and trimethylarsine oxide (TMAO) were also found only in some plants. On the other hand, the use of 0.5 mol L(-1) acetic acid as an extractant led to higher extraction percentages (33-87%), but lower column recoveries, probably due to the extraction of arsenic compounds different to the toxic free ions studied, which may come from biotransformation mechanisms carried out by plants to reduce arsenic toxicity. However, As(v) concentrations increased up to 800 μg g(-1) in acid medium, indicating the probable release of As(v) from organoarsenic compounds and therefore a higher potential risk for the environment.

Phytoremediation of lead (Pb) and arsenic (As) by Melastoma malabathricum L. from contaminated soil in separate exposure

This study was conducted to investigate the uptake of lead (Pb) and arsenic (As) from contaminated soil using Melastoma malabathricum L. species. The cultivated plants were exposed to As and Pb in separate soils for an observation period of 70 days. From the results of the analysis, M. malabathricum accumulated relatively high range of As concentration in its roots, up to a maximum of 2800 mg/kg. The highest accumulation of As in stems and leaves was 570 mg/kg of plant. For Pb treatment, the highest concentration (13,800 mg/kg) was accumulated in the roots of plants. The maximum accumulation in stems was 880 mg/kg while maximum accumulation in leaves was 2,200 mg/kg. Only small amounts of Pb were translocated from roots to above ground plant parts (TF < 1). However, a wider range of TF values (0.01-23) for As treated plants proved that the translocation of As from root to above ground parts was greater. However, the high capacity of roots to take up Pb and As (BF > 1) is indicative this plants is a good bioaccumulator for these metals. Therefore, phytostabilisation is the mechanism at work in M. malabathricum's uptake of Pb, while phytoextraction is the dominant mechanism with As.

Arsenic toxicity on Ludwigia octovalvis in spiked sand

Wilting, especially of the leaves, was observed as an initial symptom of arsenate [As(V)] to Ludwigia octovalvis (Jacq.) P. H. Raven. The plants tolerated As(V) levels of 39 mg kg⁻¹ for as long as 35 days of exposure. After 91 days, the maximum concentration of As uptake in the plant occurred at As(V) concentration of 65 mg kg⁻¹ while As concentration in the stems, roots and leaves were 6139.9 ± 829.5, 1284.5 ± 242.9 and 1126.1 ± 117.2 mg kg⁻¹, respectively. In conclusion, As(V) could cause toxic effects in L. octovalvis and the plants could uptake and accumulate As in plant tissues.

Influence of soil properties and phosphate addition on arsenic uptake from polluted soils by velvetgrass (Holcus lanatus)

Four kinds of soil material were used in a pot experiment with velvetgrass (Holcus lanatus). Two unpolluted soils: sand (S) and loam (L) were spiked with sodium arsenite (As II) and arsenate (As V), to obtain total arsenic (As) concentrations of 500 mg As kg(-1). Two other soils (ZS I, ZS III), containing 3320 and 5350 mg As kg(-1), were collected from Zloty Stok where gold and arsenic ores were mined and processed for several centuries. The effects of phosphate addition on plants growth and As uptake were investigated. Phosphate was applied to soils in the form of NH4H2PO4 at the rate 0.2 g P/kg. Average concentrations of arsenic in the shoots of velvetgrass grown in spiked soils S and L without P amendment were in the range 18-210 mg As kg(-1) d.wt., whereas those in plants grown on ZS I and ZS II soils were considerably lower, and varied in the range 11-52 mg As kg(-1) d.wt. The addition of phosphate caused a significant increase in plant biomass and therefore the total amounts of As taken up by plants, however, the differences in As concentrations in the shoots of velvetgrass amended and non-amended with phosphate were not statistically significant.

Uptake and translocation of arsenite by Pteris vittata L.: effects of glycerol, antimonite and silver

AsIII uptake in living cells is through aquaglyceroporin transporters, but it is unknown in arsenic-hyperaccumulator Pteris vittata. We investigated the effects of AsIII analogs glycerol and antimonite (SbIII) at 0-100 mM and aquaporin inhibitor AgNO(3) at 0-0.1 mM on the uptake of 0.1 mM AsIII or AsV by P. vittata over 1-2 h. Glycerol or SbIII didn't impact AsIII or AsV uptake by P. vittata (p < 0.05), with As concentrations in the fronds and roots being 4.4-6.3 and 3.9-6.2 mg/kg. However, 0.01 mM AgNO(3) reduced As concentrations in the fronds and roots by 64% and 58%. Hence, AsIII uptake in P. vittata might be via an aquaporin transporter different from glycerol and SbIII transporters. Further as AsIII analogs and aquaporin inhibitor had no impact on AsV uptake, AsIII and AsV were likely taken up by different transporters in P. vittata. Our results imply a different AsIII transporter in P. vittata from other plants.

Arsenic uptake and speciation and the effects of phosphate nutrition in hydroponically grown kikuyu grass (Pennisetum clandestinum Hochst)

BACKGROUND

This work focuses on the accumulation and mobility properties of arsenic (As) and the effects of phosphate (P) on its movement in Pennisetum clandestinum Hochst (kikuyu grass), grown hydroponically under increasing arsenate (As(V)) concentrations. The uptake of both ions and the relative kinetics show that phosphate is an efficient competitive inhibitor of As(V) uptake. The P/As uptake rate ratios in roots indicate that P is taken up preferentially by P/As transporters. An arsenite (As(III)) efflux from roots was also found, but this decreased when the arsenate concentration in the solution exceeded 5 μM.

METHODS

Increases in both arsenite and arsenate concentrations in roots were observed when the arsenate concentration in the solution was increased, and the highest accumulation of As(III) in roots was found when plants were grown at 5 μM As(V). The low ratios of As accumulated in shoots compared to roots suggest limited mobility of the metalloid within Kikuyu plants.

RESULTS

The results indicate that arsenic resistance in kikuyu grass in conditions of moderate exposure is mainly dependent on the following factors: 1) phosphate nutrition: P is an efficient competitive inhibitor of As(V) uptake because of the higher selectivity of membrane transporters with respect to phosphate rather than arsenate; and 2) a detoxification mechanism including a reduction in both arsenate and arsenite root efflux.

CONCLUSIONS

The As tolerance strategy of Kikuyu limits arsenate uptake and As translocation from roots to shoots; therefore, this plant cannot be considered a viable candidate for use in the phytoextraction of arsenic from contaminated soils or water.

Study on arsenate tolerant and sensitive cultivars of Zea mays L.: differential detoxification mechanism and effect on nutrients status

The study identifies sensitive and tolerant cultivars of Zea mays L. (cv. Azad kamal (AK) and Azad uttam (AU)) towards As(V) induced stress, based upon growth biochemical parameters and metal(loid) levels in a sand culture. As(V) (μgg⁻¹ dw) accumulation was lower in cv. AK (31 ± 1 and 107 ± 30) than cv. AU (34.5 ± 3.3 and 132.6) in leaves and roots, respectively, which correlated with lower levels of malondialdehyde and H₂O₂. No definite trend of Mn, Cu, Zn, Fe, Ca, K and Na accumulation signifies that As(V) has little influence on their uptake. Total chlorophyll and protein levels increased in cv. AK and decreased in cv. AU at 7d. Higher levels of SOD and GR in cv. AK and conversely higher levels of APX, GPX and CAT in cv. AU could be a possible differential detoxification mechanism between the cultivars. The results indicate that cv. AK seems to be arsenate tolerant than cv. AU. We assure that the undertaken study does not involve humans or experimental animals and were conducted in accordance with national and institutional guidelines for the protection of human subjects and animal welfare.

The effect of arsenic contamination on amino acids metabolism in Spinacia oleracea L

Changes of amino acid concentrations (proline, glutamate, asparagine, aspartate, alanine) and glutamate kinase activity (GKA) in plants under arsenic chronic stress reported here reveal their role in plant arsenic stress adaptation. Results of the pot experiment confirmed the toxic effect of arsenic at tested levels (As1=25 mg As kg(-1) soil, As2=50 mg As kg(-1) soil, As3=75 mg As kg(-1) soil) for spinach. Growing available arsenic contents in soil were associated with the strong inhibition of above-ground biomass and with the enhancement of As plant content. The changes of glutamate, asparagine, aspartate and proline levels in the plants showed strong linear dependences on arsenic concentration in plants (R2=0.60-0.90). Compared to the untreated control, concentrations of free proline and aspartate of As3 treatment were enhanced up to 381% and 162%, respectively. The significant changes of glutamate were observed on As2 and As3 treatments (increased level up to 188, i.e. 617%). Arsenic in plants was shown to be an inhibitor of glutamase kinase activity (R2=0.91). Inhibition of GKA resulted in an increase in the content of glutamate that is used in synthesis of phytochelatins in plant cells. Concentration of alanine did not have a confirmed linear dependence on arsenic concentration in plant (R2=0.05). The changes of its concentrations could be affected by changes of pH in plant cell or induction of alanine aminotransferase by hypoxia.

Uptake and transport of roxarsone and its metabolites in water spinach as affected by phosphate supply

Roxarsone (ROX) is widely used as a feed additive in intensive animal production. While an animal is fed with ROX, the As compounds in the manure primarily occur as ROX and its metabolites, including arsenate (As[V]), arsenite (As[III]), monomethylarsonic acid (MMA), and dimethylarsinic acid (DMA). Animal manure is commonly land applied with phosphorous fertilizers in China. A pot experiment was conducted to investigate the phytoavailability of ROX, As(V), As(III), MMA, and DMA in water spinach (Ipomoea aquatica), with the soil amended with 0, 0.25, 0.50, 1.0, and 2.0 g PO(4)/kg, respectively, plus 2% (w/w manure/soil) chicken manure (CM) bearing ROX and its metabolites. The results indicate that this species of water spinach cannot accumulate ROX and MMA at detectable levels, but As(V), As(III), and DMA were present in all plant samples. Increased phosphorous decreased the shoot As(V) and As(III) in water spinach but did not affect the root As(V). The shoot DMA and root As(III) and DMA were decreased/increased and then increased/decreased by elevated phosphorous. The total phosphorous content (P) in plant tissue did not correlate with the total As or the three As species in tissues. Arsenate, As(III), and DMA were more easily accumulated in the roots, and phosphate considerably inhibited their upward transport. Dimethylarsinic acid had higher transport efficiency than As(V) and As(III), but As(III) was dominant in tissues. Conclusively, phosphate had multiple effects on the accumulation and transport of ROX metabolites, which depended on their levels. However, proper utilization of phosphate fertilizer can decrease the accumulation of ROX metabolites in water spinach when treated with CM containing ROX and its metabolites.

Simultaneous extraction of arsenic and selenium species from rice products by microwave-assisted enzymatic extraction and analysis by ion chromatography-inductively coupled plasma-mass spectrometry

A microwave-assisted enzymatic extraction (MAEE) method was developed for the simultaneous extraction of arsenic (As) and selenium (Se) species in rice products. The total arsenic and selenium content in the enzymatic extracts were determined by inductively coupled plasma mass spectrometry (ICP-MS), while the speciation analysis was performed by ion chromatography coupled to inductively coupled plasma-mass spectrometry (IC-ICP-MS). The main factors affecting the enzymatic extraction process were evaluated in NIST SRM-1568a rice flour. The optimum extraction conditions were 500 mg of sample, 50 mg of protease XIV, and 25 mg of alpha-amylase in aqueous medium during 40 min at 37 degrees C. The extraction recoveries of total As and Se reached 100 +/- 3 and 80 +/- 4%, respectively. The species stability study during the MAEE process did not show transformation of the target species in rice products. The results of As speciation obtained for SRM-1568a were in agreement with previous studies of As speciation performed on the same reference material. The proposed method was applied to the determination of As and Se species in rice and rice-based cereals. Arsenite [As(III)], arsenate [As(V)], dimethylarsinic acid (DMA), and selenomethionine (SeMet) were the predominant species identified in rice products.

Differential response of arsenic stress in two varieties of Brassica juncea L

Present study showed the toxicity caused by Arsenite (As(III)) and its detoxification responses in two varieties (Varuna and Pusa Bold) of Brassica juncea. Comparisons were made in leaves and roots of both the varieties, which showed that the accumulation pattern in both the varieties were dose and duration dependent, being more in roots for two days and in leaves for four days. Increase/decrease of antioxidant enzymes activities (SOD, CAT, GPX) showed not much changes at the given concentrations except that the enzyme activities showed significant increase at the lower concentrations. Semi quantitative RT-PCR analysis of PCS showed more expression of its transcript in P. Bold as compared to Varuna variety due to As(III) stress. The analysis of isoenzyme pattern in leaves of P. Bold showed five and two major bands of SOD and GPX, respectively. As(III) treatment leads to the activation of MAPK activity indicating role of this important cascade in transducing As(III) mediated signals. The data presented indicates the differential responses in both the varieties and also that the increased tolerance in P. Bold may be due to the defensive role of antioxidant enzymes, induction of MAPK and up regulation of PCS transcript which is responsible for the production of metal binding peptides.

Stability of arsenic peptides in plant extracts: off-line versus on-line parallel elemental and molecular mass spectrometric detection for liquid chromatographic separation

The instability of metal and metalloid complexes during analytical processes has always been an issue of an uncertainty regarding their speciation in plant extracts. Two different speciation protocols were compared regarding the analysis of arsenic phytochelatin (As(III)PC) complexes in fresh plant material. As the final step for separation/detection both methods used RP-HPLC simultaneously coupled to ICP-MS and ES-MS. However, one method was the often used off-line approach using two-dimensional separation, i.e. a pre-cleaning step using size-exclusion chromatography with subsequent fraction collection and freeze-drying prior to the analysis using RP-HPLC-ICP-MS and/or ES-MS. This approach revealed that less than 2% of the total arsenic was bound to peptides such as phytochelatins in the root extract of an arsenate exposed Thunbergia alata, whereas the direct on-line method showed that 83% of arsenic was bound to peptides, mainly as As(III)PC(3) and (GS)As(III)PC(2). Key analytical factors were identified which destabilise the As(III)PCs. The low pH of the mobile phase (0.1% formic acid) using RP-HPLC-ICP-MS/ES-MS stabilises the arsenic peptide complexes in the plant extract as well as the free peptide concentration, as shown by the kinetic disintegration study of the model compound As(III)(GS)(3) at pH 2.2 and 3.8. But only short half-lives of only a few hours were determined for the arsenic glutathione complex. Although As(III)PC(3) showed a ten times higher half-life (23 h) in a plant extract, the pre-cleaning step with subsequent fractionation in a mobile phase of pH 5.6 contributes to the destabilisation of the arsenic peptides in the off-line method. Furthermore, it was found that during a freeze-drying process more than 90% of an As(III)PC(3) complex and smaller free peptides such as PC(2) and PC(3) can be lost. Although the two-dimensional off-line method has been used successfully for other metal complexes, it is concluded here that the fractionation and the subsequent freeze-drying were responsible for the loss of arsenic phytochelatin complexes during the analysis. Hence, the on-line HPLC-ICP-MS/ES-MS is the preferred method for such unstable peptide complexes. Since freeze-drying has been found to be undesirable for sample storage other methods for sample handling needed to be investigated. Hence, the storage of the fresh plant at low temperature was tested. We can report for the first time a storage method which successfully conserves the integrity of the labile arsenic phytochelatin complexes: quantitative recovery of As(III)PC(3) in a formic acid extract of a Thunbergia alata exposed for 24 h to 1 mg As(v) L(-1) was found when the fresh plant was stored for 21 days at 193 K.

Difference of toxicity and accumulation of methylated and inorganic arsenic in arsenic-hyperaccumulating and -hypertolerant plants

The arsenic (As) hyperaccumulators, Pteris vittata and Pteris cretica and an As-tolerant plant Boehmeria nivea, were selected to compare the toxicity, uptake, and transportation of inorganic arsenate (As(V)) and its methylated counterpart dimethylarsinic acid (DMA). The XANES method was used to elucidate the effect of As species transformation on As toxicity and accumulation characteristics. Significantly higher toxicity and lower accumulation of DMAthan inorganic As(V) was shown in the As hyperaccumulators and the As-tolerant plant. Reduction of As(V) was commonly found in the plants. Arsenic complexation with thiols, which have less mobility in plants and usually occur in As-tolerant plants, was also found in rhizoids of P. cretica. Plants with greater ability to form As-thiolate have lower ability for upward transport of As. Demethylation of DMA occurred in the three plants. The DMA component decreased from the rhizoids to the fronds in both hyperaccumulators, while this tendency is reverse in B. nivea.

Identification of As accumulation plant species growing on highly contaminated soils

Soils from the alluvial flats of the Turia River, Valencia, Spain, which were highly contaminated by decades of industrial activity, were surveyed for native plant species that could be candidates useful in phytoremediation. Concentrations of heavy metals and arsenic (As) in soils reached 25,000 mg Kg(-1) Pb, 12,000 mg Kg(-1) Zn, 70 mg Kg(-1) Cd, and 13500 mg Kg(-1) As. The predominant vegetation was collected and species identified. Soil samples and the corresponding plant shoots were analyzed to determine the amount of As accumulated by the various plant species. Several were able to tolerate more than 1000 mg Kg(-1) As in the soil. Bassia scoparia (Chenopodiaceae) survive in soil with 8375 mg Kg(-1) As. Arsenic accumulation in shoots of the various plant species investigated ranged from 0.1 to 107 mg Kg(-1) dw. Bassia scoparia (Chenopodiaceae), Inula viscosa (Asteraceae), Solanum nigrum (Solanaceae), and Hirschfeldia incana (Brassicaceae) had the highest values for As accumulation.

Arsenic speciation in plants growing in arsenic-contaminated sites

Concentrations of total arsenic and of arsenic species were determined by ICPMS and HPLC-ICPMS in terrestrial plant samples. The arsenic concentration in plant samples from the contaminated sites ranged from 1.14 to 98.5 mg kg(-1) (dry mass). However, a very high value, exceeding largely this range was found in a moss sample growing in the contaminated area (1750 mg kg(-1)). Plants growing in a non-contaminated area with similar geological characteristics contained 0.06-0.58 mg As kg(-1). Plant samples from different species were selected and extracted with water, water/methanol (9+1, v/v), and water/methanol (1+1, v/v). Water/methanol (9+1, v/v) was selected as extractant for the speciation analysis for all the plant samples. The extraction efficiencies ranged from 3.0% to 41.4%, with good agreement between samples from the same plant species. Arsenite and/or arsenate were found in all the plant samples. Additionally, methylarsonate (MA), dimethylarsinate (DMA), trimethylarsine oxide (TMAO) and tetramethylarsonium ion (TETRA) were also identified in several plants, and in some cases MA and DMA were the main species found. TMAO, which is usually found as a trace constituent in organisms, was also a significant arsenical in one of the studied samples, where it constituted 24% of the extracted arsenic. In the present study, the patterns of arsenic species varied with the plant species and much higher proportion of organoarsenicals was found in plants from the more contaminated sites.

Biologically active compounds of semi-metals

Semi-metals (boron, silicon, arsenic and selenium) form organo-metal compounds, some of which are found in nature and affect the physiology of living organisms. They include, e.g., the boron-containing antibiotics aplasmomycin, borophycin, boromycin, and tartrolon or the silicon compounds present in "silicate" bacteria, relatives of the genus Bacillus, which release silicon from aluminosilicates through the secretion of organic acids. Arsenic is incorporated into arsenosugars and arsenobetaines by marine algae and invertebrates, and fungi and bacteria can produce volatile methylated arsenic compounds. Some prokaryotes can use arsenate as a terminal electron acceptor while others can utilize arsenite as an electron donor to generate energy. Selenium is incorporated into selenocysteine that is found in some proteins. Biomethylation of selenide produces methylselenide and dimethylselenide. Selenium analogues of amino acids, antitumor, antibacterial, antifungal, antiviral, anti-infective drugs are often used as analogues of important pharmacological sulfur compounds. Other metalloids, i.e. the rare and toxic tellurium and the radioactive short-lived astatine, have no biological significance.

Examination of arsenic(III) and (V) uptake by the desert plant species mesquite (Prosopis spp.) using X-ray absorption spectroscopy

This study describes the effects of Arsenic(III) and (V) on the growth and their uptake by the desert plant mesquite (Prosopis spp.). Seedlings were sown in agar-based medium containing a modified Hoagland's nutrient solution. After 1 week, the seedlings were transplanted to arsenic (As) treated agar media that contained 5 mgL(-1) of As either As(III) (As(2)O(3)) or As(V) (As(2)O(5)). The plants were harvested after 14 days of growth and sectioned into roots, stems, and leaves. After digestion, As concentrations in the roots, stems, and leaves were determined using inductively coupled plasma-optical emission spectroscopy (ICP-OES). Our results showed that the As concentrations from As(V) were significantly higher than the As concentrations from As(III) in all portions of the plant. Plants exposed to As(V) concentrated (mg As kg(-1) d wt) about 770+/-191, 326+/-94, and 119+/-18 in roots, stems, and leaves, respectively. X-ray absorption spectroscopy (XAS) showed that As(V) was reduced to As(III) inside the mesquite plant. In addition, greater than 90% of the As(III) found in the mesquite plants was bound to sulfur ligands in the roots, stems and leaves.

Response of pepper plants (Capsicum annum L.) on soil amendment by inorganic and organic compounds of arsenic

The influence of soil contamination by inorganic and organic arsenic compounds on uptake, accumulation, and transformation of arsenic in pepper (Capsicum annum L.) was investigated in greenhouse pot experiments under controlled conditions. Pepper plants were cultivated in substrate amended by aqueous solutions of arsenite, arsenate, methylarsonic acid (MA), and dimethylarsinic acid (DMA) applied individually into cultivation substrate at concentrations of 15 mg As per kg of substrate. The plant availability of the arsenicals increased in the order arsenite = arsenate < MA < DMA. The highest arsenic concentrations were found in roots followed by stems, leaves, and fruits regardless of arsenic compound applied. In the control samples of pepper fruits, As(III), As(V), and DMA were present (25%, 37%, and 39% of the water-extractable arsenic). In control stems + leaves and roots, As(V) was the major compound (63% and 53% in a phosphate buffer extract) followed by As(III) representing 33% and 42%. Additionally, low concentrations (not exceeding 5%) of DMA and MA were detected as well. In all the soils analyzed after the first harvest of pepper fruits, arsenate was the dominating compound followed by arsenite. Methylarsonic acid, methylarsonous acid, and DMA were present at varying concentrations depending on the individual soil treatments. In the treated plants, the arsenic compounds in plant tissues reflected predominantly the extractable portions of arsenic compounds present in soil after amendment, and this pattern was more significant in the first part of vegetation period. The results confirmed the ability of generative parts of plants to accumulate preferably organic arsenic compounds, whereas in the roots and aboveground biomass, mainly inorganic arsenic species are present. Evidently, the source of soil arsenic contamination affects significantly the extractable portions of arsenic compounds in soil and subsequently the distribution of arsenic compounds within the plants.