Effects of phosphate and thiosulphate on arsenic accumulation in the species Brassica juncea

Assessment of health risks associated with prediction of vegetable inorganic arsenic concentrations given different soil properties

Inorganic arsenic (iAs) is a carcinogen. Vegetables such as water spinach (Ipomoea aquatica Forssk.) and amaranth (Amaranthus mangostanus L.) are recognized as high-risk sources of iAs exposure because they can accumulate significant amounts of iAs and are widely consumed. To ensure safe cultivation conditions, this study aimed to establish prediction models for iAs concentration in the edible parts of water spinach and amaranth based on soil properties. Subsequently, health risk assessments associated with iAs exposure through the consumption of these vegetables were conducted using prediction models. Soil samples were collected from agricultural fields in Taiwan and used in the pot experiments. Pearson correlation and partial correlation analyses were used to explore the relationship between soil properties, including total As, clay, organic matter, iron oxides and available phosphates, and iAs concentration in edible parts of water spinach and amaranth. Prediction models based on soil properties were developed by stepwise multiple linear regression. Health risk assessments were conducted using the Monte Carlo algorithm. The results indicate that total As and organic matter contents in soil were major predictors of iAs concentration in water spinach, whereas those in amaranth were total As and clay contents. Therefore, higher health risks for consuming water spinach and amaranth are associated with higher levels of organic matter and clay contents in soil, respectively, and these are crucial factors to consider to ensure food safety. This study suggested that As-elevated soils enriched with organic matter and clay contents should be avoided when growing water spinach and amaranth, respectively.

The role of sulfur nutrition in plant response to metal(loid) stress: Facilitating biofortification and phytoremediation

Metal(loid)s contamination poses a serious threat to ecosystem biosafety and human health. Phytoremediation is a cost-effective and eco-friendly technology with good public acceptance, although the process does require a significant amount of time for success. To enhance the phytoremediation efficiency, numerous approaches have been explored, including soil amendments application with chelators to facilitate remediation. Sulfur (S), a macronutrient for plant growth, plays vital roles in several metabolic pathways that can actively affect metal(loid)s phytoextraction, as well as attenuate metal(loid) toxicity. In this review, different forms of S-amendments (fertilizers) on uptake and translocation in plants upon exposure to various metal(loid) are evaluated. Possible mechanisms for S application alleviating metal(loid) toxicity are documented at the physiological, biochemical and molecular levels. Furthermore, this review highlights the crosstalk between S-assimilation and other biomolecules, such as phytohormones, polyamines and nitric oxide, which are also important for metal(loid) stress tolerance. Given the effectiveness and potential of S amendments on phytoremediation, future studies should focus on optimizing phytoremediation efficiency in long-term field studies and on investigating the appropriate S dose to maximize the food safety and ecosystem health.

Field experiments for evaluating the effects of water management and phosphate application on inorganic arsenic accumulation in water spinach (Ipomoea aquatica Forssk.)

Water spinach (Ipomoea aquatica Forssk.) is a commonly planted vegetable in the Southeast Asia; it is a semi-aquatic leafy vegetable with high inorganic arsenic (As) accumulation capability and can be planted under both upland and flooding cultivation conditions. To date, a limited number of field studies have investigated the effect of soil management on As phytotoxicity and accumulation of water spinach. Therefore, in this study, a field experiment was conducted to investigate the effects of water management and phosphate (P) application on the As phytotoxicity and accumulation of water spinach grown in As-contaminated fields (121 mg As kg^-1). Water spinach was planted in the study field with two water management (flooding and upland cultivation) and two P application rates (90 and 180 kg P₂O₅ ha^-1), and continuously harvested three times. Results reveal that the concentration and estimated daily intake (EDI) of inorganic As in the edible parts of water spinach under flooding cultivation were approximately twofold higher than those under upland cultivation. It was also found that the accumulation of As in the shoot of water spinach was strongly related to the As concentrations, rather than P/As molar ratio in pore water due to that P application rates were lower than the maximum capacity for P retention of the tested soil. Moreover, the As phytotoxicity and accumulation of water spinach were reduced at the third harvest relative to the first two harvests because of the increase in iron plaque formation on the root surface and the decrease in the growing temperature during the experimental period. Our results suggest that upland cultivation is the better practice than flooding cultivation for reducing inorganic As accumulation in the edible parts of water spinach grown in As-contaminated soils. Further, ratooning may be a feasible cultivation approach to reducing inorganic As accumulation in water spinach.

Chelate assisted phytoextraction for effective rehabilitation of heavy metal(loid)s contaminated lands

The contamination of lands and water by heavy toxic metal(loid)s is an environmental issue that needs serious attention as it poses a major threat to public health. The persistence of heavy metals/metalloids in the environment as well as their potentially dangerous effects on organisms underpins the need to restore the areas contaminated by heavy toxic metal(loid)s. Soil restoration can be achieved through a variety of different methods. Being more cost-effective and environmentally sustainable, phytoremediation has recently replaced traditional processes like soil washing and burning. Many plants have been intensively explored to eliminate various heavy metals from polluted soils through phytoextraction, which is a commonly used phytoremediation approach. The ability of chelants to enhance phytoextraction potential has also received wide attention owing to their ability to elevate the efficiency of plants in removing heavy metal(loid)s. Chelants have been found to improve plant growth and the activity of the defense system. Several chelants, either non-biodegradable or biodegradable, have been reported to augment the phytoextraction efficiencies of various plants. The problem of the leaching of heavy metal(loid)s and secondary pollution caused by non-biodegradable chelants can be overcome by the use of biodegradable chelants to an extent. This review is a brief report focusing on recent articles on chelate-assisted phytoextraction of heavy metal (loids) As, Cd, Cu, Cr, Hg, Ni, Pb, U, and Zn.

Use of Biostimulants as a New Approach for the Improvement of Phytoremediation Performance—A Review

Environmental pollution is one of the most pressing global issues, and it requires priority attention. Environmental remediation techniques have been developed over the years and can be applied to polluted sites, but they can have limited effectiveness and high energy consumption and costs. Bioremediation techniques, on the other hand, represent a promising alternative. Among them, phytoremediation is attracting particular attention, a green methodology that relies on the use of plant species to remediate contaminated sites or prevent the dispersion of xenobiotics into the environment. In this review, after a brief introduction focused on pollution and phytoremediation, the use of plant biostimulants (PBs) in the improvement of the remediation effectiveness is proposed. PBs are substances widely used in agriculture to raise crop production and resistance to various types of stress. Recent studies have also documented their ability to counteract the deleterious effects of pollutants on plants, thus increasing the phytoremediation efficiency of some species. The works published to date, reviewed and discussed in the present work, reveal promising prospects in the remediation of polluted environments, especially for heavy metals, when PBs derived from humic substances, protein and amino acid hydrolysate, inorganic salts, microbes, seaweed, plant extracts, and fungi are employed.

Cannabis sativa L. and Brassica juncea L. grown on arsenic-contaminated industrial soil: potentiality and limitation for phytoremediation

Phytoremediation represents a natural method to remove contaminants from soil. The goal of this study was to investigate the potential of phosphate-assisted phytoremediation by two energy crops, Cannabis sativa L. and Brassica juncea L., for the sustainable remediation of heavily arsenic-contaminated industrial soil. The two species were investigated for uptake, translocation, and physiological effects of arsenic and phosphate in a microcosm test. Although C. sativa and B. juncea were symptomless when grown in arsenic-contaminated soil, an important reduction of biomass (50 and 25%, respectively) was observed as a stress marker. Phytotoxicity and cytotoxicity effects promoted by contaminated soils were investigated in both the species and a model plant for ecotoxicity studies, Vicia faba L., which is the most developed model to test genotoxicity effects in terms of chromosomal aberration and micronuclei presence. The higher amount of arsenic was found in C. sativa and B. juncea roots (on average 1473 and 778 mg kg-1, respectively), but both species were able to uptake and translocate arsenic in leaves and stems, up to 47.0 and 189 mg kg-1, respectively. Phosphate treatment had no effect on arsenic uptake in none of the crop, but significantly improved the plant performance. Biomass production resulted similar to that of B. juncea control plants. Antioxidant enzymatic activities and photosynthetic performance responded differently in the two crops. The present investigation provides new insight for a proficient selection of the most suitable crop species for sustainable phytomanagement of a highly polluted As-contaminated site by coupled phytoremediation-bioenergy approach.

Influence of sulfur amendments on heavy metals phytoextraction from agricultural contaminated soils: A meta-analysis

Heavy metal pollution is becoming recurrent and threatens biota biosafety in many agricultural fields. Diverse solutions explore the application of amendments to enable remediation. Sulfur represents a nonmetallic chemical element that actively affects heavy metals phytoextraction, and promotes and alternatively mitigates soil functions. In this study, we conduct a meta-analysis to synthesize the current knowledge on the influence of sulfur amendments on plants heavy metals uptake from contaminated soil media. Random-effects model was used to summarize effect sizes from 524 data points extracted from 30 peer reviewed studies. The phytoextraction of cadmium, chromium and nickel were 1.6-, 3.3-, and 12.6-fold, respectively, higher when sulfur amendment was applied; while copper uptake was 0.3-fold lower. Irrespective of the sulfur type, heavy metal extraction increased with the raising sulfur stress. Individual organs showed significant differences of heavy metal uptake between sulfur applied and non-sulfur treatments, and combined organs did not. The heavy metals uptake in leaves and roots were higher in sulfur applied than non-sulfur applied treatments, while those in grain, husk, and stalks were lower. The heavy metals phytoextraction (response ratio) followed the order roots > leaves > stalk > grain > husk. Moreover, heavy metals uptake was 2-fold higher in the sulfur applied than the non-sulfur treatments under ideal (5.5-8) and alkaline conditions (8-14), and 0.2-fold lower under acidic pH (1-5.5). Cadmium, manganese and nickel, and chromium were the most extracted under sulfur application by Vicia sp., Sorghum sp. and Brassica sp., respectively; while chromium, manganese, and iron were the most uptake without sulfur amendments by Oryza sp., Zea sp. and Sorghum sp., respectively. Our study highlights that the influence of sulfur on heavy metal phytoextraction depends on the single or combined effects of sulfur stress intensity, sulfur compounds, plant organ, plant type, and soil pH condition.

Application of sulphate and cytokinin in assisted arsenic phytoextraction by industrial Cannabis sativa L

Phytoextraction is currently investigated to effectively remediate soil contaminated by metals and provide highly competitive biomass for energy production. This research aimed to increase arsenic (As) removal from contaminated soil using industrial Cannabis sativa L., a suitable energy crop for biofuel production. Assisted phytoextraction experiments were conducted on a microcosm scale to explore the ability of two friendly treatments, sodium sulphate (SO₄) and exogenous cytokinin (CK), in increasing As phytoextraction efficiency. The results showed that the treatments significantly increased As phytoextraction. Cytokinin was the most effective agent for effectively increasing translocation and the amount of As in aerial parts of C. sativa. In fact, the concentration of As in the shoots of CK-treated plants increased by 172% and 44% compared to untreated and SO₄-treated plants, respectively. However, the increased As amount accumulated in C. sativa tissues due to the two treatments negatively affected plant growth. Arsenic toxicity caused a significant decrease in aerial C. sativa biomass treated with CK and SO₄ of about 32.7% and 29.8% compared to untreated plants, respectively. However, for our research purposes, biomass reduction has been counterbalanced by an increase in As phytoextraction, such as to consider C. sativa and CK an effective combination for the remediation of As-contaminated soils. Considering that C. sativa has the suitable characteristics to provide valuable resources for bioenergy production, our work can help improve the implementation of a sustainable management model for As contaminated areas, such as phytoremediation coupled with bioenergy generation.

Assaying the use of sodium thiosulphate as a biostimulant and its effect on cadmium accumulation and tolerance in Brassica oleracea plants

An optimal uptake of mineral elements is crucial to ensure both crop yield and quality. The use of biostimulants is taking relevance to improve the nutrition of crops. Sulphur (S) is one of the elements with great potential within biostimulants. Furthermore, soil contamination by heavy metals such as cadmium (Cd) has become a serious environmental problem. Different studies have suggested the use of thiosulphate (TS) as a biostimulant and to increase the phytoremediation capacity of plants. Therefore, in the present study, we use a crop plant with high S requirements such as Brassica oleracea, to test whether TS serves as a biostimulant and whether affects Cd accumulation and tolerance. B. oleracea plants were grown with two different TS doses (2 mM and 4 mM), under Cd toxicity, and with the combination of Cd toxicity and both TS doses. Parameters of biomass, mineral elements accumulation, and stress tolerance were analyzed. The results showed that TS reduced biomass of B. oleracea plants. The application of 2 mM TS increased Cd accumulation whereas the 4 mM dose reduced it. On the other hand, TS incremented micronutrient accumulation on plants subjected to Cd toxicity and increased Zn contents. Besides, the application of 2 mM to Cd-stressed plants enhanced photosynthesis performance and reduced oxidative stress. Finally, TS increased the antioxidant capacity of B. oleracea plants. Briefly, although TS can not be used as a biostimulant it could be used for Cd phytoremediation purposes and to enhance Zn accumulation in B. oleracea plants.

Phytoextraction of arsenic forms in selected tree species growing in As-polluted mining sludge

The aim of this study was to determine the phytoextraction of inorganic (As(III), As(V)) and organic arsenic (As_org) forms in six tree species: Acer platanoides, Acer pseudoplatanus, Betula pendula, Quercus robur, Tilia cordata and Ulmus laevis. Plants were grown in a pot experiment using As-polluted mining sludge for 90 days. Arsenic (As_total) was accumulated mainly in the roots of all six tree species, which were generally thinner, shorter and/or black after the experiment. The highest concentration of As(III) and As(V) was determined in the roots of A. pseudoplatanus and A. platanoides (174 and 420 mg kg^-1, respectively). High concentrations of As(III) were also recorded in the shoots of B. pendula (11.9 mg kg^-1) and As(V) in the aerial parts of U. laevis and A. pseudoplatanus (77.4 and 70.1 mg kg^-1). With some exceptions, the dominant form in the tree organs was As_org, present in mining sludge in low concentration. This form has a decisive influence on As phytoextraction by young tree seedlings even though its BCF value was the only one lower than 1. The obtained results highlight the important role of speciation studies in assessing the response of plants growing in heavily polluted mining sludge.

Arsenic uptake by arugula (Eruca vesicaria, L.) cultivars as affected by phosphate availability

To assess the importance of variation among arugula (Eruca vesicaria subsp. sativa) cultivars in the ability to accumulate arsenic (As) in above-ground tissues, uptake of As by 16 cultivars was measured in the field and in hydroponic culture. In the field trial on soil contaminated by past pesticide use, As soil-plant uptake coefficients varied by a factor of 2.7 among different cultivars, approaching a value of one for the strongest accumulators. Compared to the field assay, hydroponically grown arugula accumulated much lower concentrations of As when nutrient solutions contained standard (high) concentrations of phosphate along with 1.0 mg L^-1 As in the form of soluble arsenate. However, As accumulation was much greater in hydroponic culture using low-P nutrient solutions, an indication that phosphate strongly competed with arsenate for root uptake. Analysis of arugula roots after exposure to arsenate at 1.0 mg As L^-1 and low phosphate revealed from 24 to 400 times greater As concentration in roots than tops, with S concentrations significantly greater in As-exposed than control roots. This indicated greater sulfate uptake by roots exposed to arsenate, and suggested that thiol-mediated As immobilization occurred in the roots which strongly restricted translocation to the tops.

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.

Phosphate alleviates arsenate toxicity by altering expression of phosphate transporters in the tolerant barley genotypes

The contribution of the phosphate transporters (PHTs) in uptake of arsenate (As⁵⁺) and phosphate (P) is a widely recognized mechanism. Here we investigated how P regulates the uptake of As⁵⁺ and the subsequent effects on growth and relative expression of PHTs. The study was conducted on 3 barley genotypes differing in As tolerance (ZDB160, As-tolerant; ZDB115, moderately tolerant; ZDB475, As-sensitive) using a hydroponic experiment. There were 3 As⁵⁺ (0, 10 and 100µM) and 3P (0, 50 and 500µM) levels. The results showed that the negative effect of As stress on plant growth, photosynthesis and cell ultra-structure is As dose and barley genotype dependent, confirming the distinctly genotypic difference in As tolerance. As uptake and accumulation in plant tissues are closely associated with inhibited extent of growth and photosynthesis, with the tolerant genotype ZDB160 having lower As content than other two genotypes. The toxic effect caused by As stress could be alleviated by P addition, mainly due to reduced As uptake. Moreover, the tolerant genotype showed relatively lower expression PHTs than sensitive ones upon exposure to both As stress and P addition, suggesting regulation of PHTs expression is a major mechanism for relative uptake of As and P, in subsequence affecting As tolerance. Moreover, among 6 PHTs examined in this study, the expressions of PHT1.3, PHT1.4 and PHT1.6 showed the marked difference among the three barley genotypes in responses to As stress and P addition, indicating further research on the contribution of phosphate transporters to As and P uptake should be focused on these PHTs.

Phytochelatins play key roles for the difference in root arsenic accumulation of different Triticum aestivum cultivars in comparison with arsenate uptake kinetics and reduction

In the previous studies, we have found that arsenic (As) accumulation in roots of bread wheat (Triticum aestivum L.) seedlings were significantly different among different wheat cultivars, and As(V) tolerant wheat cultivars have much higher capacities of root As accumulation. However, the reason for the difference remains unclear. Four wheat cultivars with high (MM45 and FM8) or low (QF1 and HM29) levels of arsenic (As) accumulation were selected to investigate the relationship between root As(V) uptake kinetics and root As accumulation. MM45 and HM29 were also used to examine As(V) reduction ability and non-protein thiol (cysteine [Cys], glutathione [GSH], and phytochelatins [PCs]) concentrations in wheat seedlings. MM45 had the lowest Michaelis-Menten constant (K_m) and maximum influx rate (V_max). No difference in the K_m values was found among the three other cultivars. No difference in As(V) reduction capacity was observed between MM45 and HM29. GSH and PC₂ were significantly induced by 10 μM As(V) in roots of wheat seedlings, particularly in MM45. Synthesis of GSH and PCs was completely suppressed in the presence of l-buthionine sulfoximine (BSO), a specific inhibitor of γ-glutamylcysteine synthetase. BSO markedly decreased the As tolerance of wheat seedlings and decreased the accumulation of As in roots, but increased As accumulation in shoots. No significant difference in As concentrations was found between MM45 and HM29 under the BSO treatment. GSH and PCs are the reason why As accumulation and As(V) tolerance differ in roots of different wheat cultivars.

Phosphorus deficiency modifies As translocation in the halophyte plant species Atriplex atacamensis

Most arsenic in surface soil and water exists primarily in its oxidized form, as arsenate (As(V); AsO₄^3-), which is an analog of phosphate (PO₄^3-). Arsenate can be taken up by phosphate transporters. Atriplex atacamensis Phil. is native to northern Chile (Atacama Desert), and this species can cope with high As concentrations and low P availability in its natural environment. To determine the impact of P on As accumulation and tolerance in A. atacamensis, the plants were cultivated in a hydroponic system under four treatments: no As(V) addition with 323µM phosphate (control); 1000µM As(V) addition with 323µM phosphate; no As(V) and no phosphate; 1000µM As(V) addition and no phosphate. Phosphate starvation decreased shoot fresh weight, while As(V) addition reduced stem and root fresh weights. Arsenate addition decreased the P concentrations in both roots and leaves, but to a lesser extent than for P starvation. Phosphorus starvation increased the As concentrations in roots, but decreased it in shoots, which suggests that P deficiency reduced As translocation from roots to shoots. Arsenate addition increased total glutathione, but P deficiency decreased oxidized and reduced glutathione in As(V)-treated plants. Arsenate also induced an increase in S accumulation and nonprotein thiol and ethylene synthesis, and a decrease in K concentrations, effects that were similar for the P-supplied and P-starved plants. In contrast, in As(V)-treated plants, P starvation dramatically decreased total soluble protein content and increased lipid peroxidation, compared to plants supplied with P. Phosphorus nutrition thus appears to be an important component of A. atacamensis response to As toxicity.

The transportation and accumulation of arsenic, cadmium, and phosphorus in 12 wheat cultivars and their relationships with each other

Pot experiments were conducted to investigate the difference in arsenic (As), cadmium (Cd), and phosphorus (P) uptake, accumulation, and translocation among 12 wheat cultivars and their relationships with each other in soil "naturally" contaminated with both As and Cd. As, Cd, and P concentrations in wheat grain, straw, and root differed significantly (p<0.05) among the 12 wheat cultivars. The grain As concentration was not correlated with straw and root As, or the total As content in plants, but was significantly (p<0.05) correlated with As translocation factors (TFs), i.e., TFs(Grain/Root) and TFs(Grain/Straw). The grain Cd concentration was positively correlated with the total Cd content and TFs(Grain/Straw). The grain P concentration was positively correlated with straw and root P. Both As and Cd concentrations in wheat grains were correlated with P in wheat straw and grain. Compared with As, Cd was more easily transported to the wheat grain, and the rachis played a key role in ensuring this difference. A significant positive correlation was observed between root As and Cd, but no significant relationship was detected between grain As and Cd concentrations. The lack of a relationship between grain As and Cd suggests the possibility of selecting cultivars in which little As and Cd accumulation occurs in the wheat grain.