Coupling phytoremediation efficiency and detoxification to assess the role of P in the Cu tolerant Ricinus communis L

Applicability of the sigmoid model to estimate heavy metal uptake in maize and sorghum as affected by organic acids

Although assisted phytoremediation using chemical treatments is a suitable technique for the removal of heavy metals (HMs), the estimation of this process using simple models is also crucial. For this purpose, a greenhouse trial was designed to evaluate the effectiveness of citric, oxalic, and tartaric acid on Cd, Pb, Ni, and Zn phytoremediation by maize and sorghum and to estimate this process using sigmoid HMs uptake model. Results showed that mean values of root and shoot dry weight and metals uptake, translocation factor (TF) of Pb and Zn, and uptake efficiency (UE) of Cd in maize were higher than sorghum but the TF of Cd and the phytoextraction efficiency (PEE) and UE of Pb in sorghum were higher than maize. Citric, oxalic, and tartaric acid significantly increased the UE of Pb by 17.7%, 22.5%, and 32.5%, respectively. Tartaric acid significantly increased the mean values of shoot dry weight, shoot Cd, Pb, and Ni uptake, and PEE of Pb and Ni, but decreased TF of Zn. The R2, NRMSE, and KM values indicated the ability of sigmoid HM uptake model in estimating HMs uptake in maize and sorghum treated with organic acids. Thus, tartaric acid was more effective than citric and oxalic acids to enhance phytoremediation potential. Sigmoid HM uptake model is suitable to estimate the HMs uptake in plants treated with organic acids at different growth stages.

Analysis of the Physiological Response and Reactive Oxygen Species in Castor Oil Plant (Ricinus Communis) in the Phytoremediation Processes with Plant Growth Promoter Bacteria (PGPB)

In the phytoremediation processes of mine tailings with Ricinus communis inoculated with PGPB, it was found that the Serratia K120 bacterium favors the translocation of Al, As, Cu, Pb, Cr, Cd, and Mn to the aerial part of the plant, with a significant difference (p < 0.05) concerning for the control. The bioaccumulation factor (BF) was > 1 in Al with all the bacteria, Pb, Serratia K120, Fe, Pantoea 113, Cu, Pb, Cd, Mn in Serratia MC119 and Serratia K120, Fe and As in Serratia K120 and Pantoea 134, indicating that Ricinus communis inoculated with PGPB functions as a hyper accumulating plant. The PGPB help to reduce the stress in the plants generated by the heavy metals, decreasing the H2O2, and increasing the activity of the enzymes SOD, CAT, APX, POX, and GR, for which the bacteria Serratia K120 and Pantoea 113 can be used as bioinoculants to favor phytoremediation processes.

Arbuscular mycorrhizal fungi influence the uptake of cadmium in industrial hemp (Cannabis sativa L.)

Phytoremediation is currently a more environmentally friendly and economical measure for the remediation of cadmium (Cd) contaminated soil. Heavy metal-resistant plant species, Cannabis sativa L. was inoculated with Rhizophagus irregularis to investigate the mechanisms of mycorrhizal in improving the Cd remediation ability of C. sativa. The results showed that after inoculation with R. irregularis, C. sativa root Cd contents increased significantly, and leaf Cd enrichment decreased significantly. At the transcriptional level, R. irregularis down-regulated the expression of the ABC transporter family but up-regulated differentially expressed genes regulating low molecular weight organic acids. The levels of malic acid, citric acid, and lactic acid were significantly increased in the rhizosphere soil, and they were significantly and strongly related to oxidizable Cd concentrations. Then citric acid levels were considerably and positively connected to exchangeable Cd concentrations. Our findings revealed that through regulating the movement of root molecules, arbuscular mycorrhizal fungus enhanced the heavy metal tolerance of C. sativa even more, meanwhile, they changed the Cd chemical forms by altering the composition of low molecular weight organic acids, which in turn affected soil Cd bioavailability.

Remediation of uranium-contaminated alkaline soil by rational application of phosphorus fertilizers: Effect and mechanism

In alkaline soil, abundant carbonates will mobilize uranium (U) and increase its ecotoxicity, which is a serious threat to crop growth. However, the knowledge of U remediation in alkaline soils remains very limited. In this study, U-contaminated alkaline soil (tillage layer) was collected from the Ili mining area of Xinjiang, the soil remediation was carried out by using phosphorus (P) fertilizers of different solubility (including KH₂PO₄, Ca(H₂PO₄)₂, CaHPO₄, and Ca₃(PO₄)₂), and the pathways and mechanisms of U passivation in the alkaline soil were revealed. The results showed that water-soluble P fertilizers, KH₂PO₄ and Ca(H₂PO₄)₂, were highly effective at immobilizing U, and significantly reduced the bioavailability of soil U. The exchangeable U was reduced by 70.5 ± 0.1% (KH₂PO₄) and 68.2 ± 1.9% (Ca(H₂PO₄)₂), which was converted into the Fe-Mn oxide-bound and residual phases. Pot experiments showed that soil remediation by KH₂PO₄ significantly promoted crop growth, especially for roots, and reduced U uptake in crops by 94.5 ± 1.0%. The immobilization of U by KH₂PO₄ could be attributed to the release of phosphate anions, which react with the uranyl ion (UO₂²⁺) forming a stable mineral of meta-ankoleite and enhancing the binding of UO₂²⁺ to the soil Fe-Mn oxides. In addition, KH₂PO₄ dissolution produces acidity and P fertilizer, which can reduce soil alkalinity and improve crop growth. The findings in this work demonstrate that a rational application of P fertilizer can effectively, conveniently, and cheaply remediate U contamination and improve crop yield and safety on alkaline farmland.

Phytoaccumulation of zinc and its associated impact on the growth performance and tolerance index of six non-food crop plants grown in Zn-contaminated soil

Crop plant remediation and detoxification of Zn-contaminated soils may pose a significant threat to food safety and, thus, human health. Therefore, the current study was carried out to assess the ability of six non-food crop plants (NFCP); Zea mays L. cultivar 360 (T360), Z. mays cultivar 123 (T123), Helianthus annuus L., Brassica juncea (L.) Czern., Ricinus communis L., and Simmondsia chinensis (Link) C.K. Schneid to remediate and restore Zn-contaminated soils. The investigated plants tolerate 150 mg/kg of Zn content of the soil, where they had tolerance index (TI) > 1 for all growth criteria, except the root dry weight (DW) of S. chinensis. Z. mays T123 and R. communis were the most susceptible plants, while B. juncea and S. chinensis were moderately tolerant, while H. annuus was the most tolerant to high Zn concentrations in a growing medium. Increasing the soil Zn content led to a significant increase (p < 0.05) in Zn concentration in the various tissues of the six NFCPs. The studied NFCP did not translocate Zn to their grains/seeds; consequently, they can be used safely for Zn-contaminated soils. The Zn content in root and shoot was negatively correlated with the TI of their length and weight, while the translocation factor (TF) of Zn from root to shoot was positively correlated to the TI of the root length and weight. The six studied NFCPs were arranged based on their phytoremediation efficiency as follows: B. juncea (31.86%) > Z. mays T123 (31.14%) > Z. mays T360 (27.59%) > H. annuus (20.85%) > S. chinensis (20.29%) > R. communis (15.3%). All tested NFCPs accumulated significant concentrations of Zn in their roots and shoots, a high Zn uptake potential, and biomass at 150-450 mg/kg of Zn treatments, indicating that these plants are good candidates for the implementation of a new strategy of cultivating NFCP for phytoremediation of Zn-contaminated soils.

Effects of microbial agents on cadmium uptake in Solanum nigrum L. and rhizosphere microbial communities in cadmium-contaminated soil

Solanum nigrum L. (S. nigrum) and microbial agents are often used for the remediation of cadmium (Cd)-contaminated soil; however, no studies to date have examined the efficacy of using various microbial agents for enhancing the remediation efficiency of Cd-contaminated soil by S. nigrum. Here, we conducted greenhouse pot experiments to evaluate the efficacy of applying Bacillus megaterium (BM) along with citric acid (BM + CA), Glomus mosseae (BM + GM), and Piriformospora indica (BM + PI) on the ability of S. nigrum to remediate Cd-contaminated soil. The results showed that BM + GM significantly increased the Cd accumulation of each pot of S. nigrum by 104% compared with the control. Application of microbial agents changed the soil microbial communities. Redundancy analysis showed that the activities of Catalase (CAT) and urease (UE), soil organic matter, available N and total Cd were the main influencing factors. By constructing the microbial co-occurrence networks, the soil microbe was divided into four main Modules. BM + GM and BM + PI significantly increased the relative abundance of Module#1 and Module#3, respectively, when compared with the control. Additionally, Module#1 showed a significant positive correlation with translocation factor (TF), which could be regarded as the key microbial taxa. Further research found that Ascomycota, Glomeromycota, Proteobacteria, and Actinobacteria within Module#1 were also significantly correlated with TF, and these key species enriched in BM + GM. Overall, our findings indicate that the BM + GM treatment was the most effective for the remediation of Cd pollution. This treatment method may further affect the rhizosphere microbial community by affecting soil indicators, which might drive the formation of Module#1, thus greatly enhancing the Cd remediation capacity of S. nigrum.

Effect of phosphorus fertilizer on phytoextraction using Ricinus communis L. in Cu and Cd co-contaminated soil

Mining activities have led to Cu and Cd contaminated of surrounding agricultural soil. To decrease the Cu and Cd accumulation in crops, the Ricinus communis L. (castor) has been used for phytoremediation. A pot experiment was served to investigate the effect of phosphate fertilizer (Ca(H₂PO₄)₂) on the growth and Cu/Cd uptake of castor in contaminated soil. The results showed that the application of P fertilizer improved the leaf cell morphology, decreased the malonaldehyde (MDA) content of castor leaves, and increased the plant biomass (28.2-34.2%). Besides, phosphate fertilizer still facilitated accumulation Cu and Cd by castor. The addition of phosphate fertilizer increased the contents of Cu in the root of castor, improved the bioconcentration factor (BCF) of Cu, and observably enhanced the accumulation of Cu (up to 201 μg/plant) in castor. Applying phosphorus increased the percentage of residual Cd, diminished the percentage of acid extractable Cd in soil, and the accumulation of Cd in castor was not significantly increased. These results suggest that phosphorus alleviated the stress of heavy metals on castor leaves and enhanced the accumulation of Cu and Cd in castor by promoting the growth of castor.

The mechanism of arbuscular mycorrhizal enhancing cadmium uptake in Phragmites australis depends on the phosphorus concentration

Arbuscular mycorrhizal fungi (AMF) is a vital strategy to enhance the phytoremediation of cadmium (Cd) pollution. However, the function of AMF was influenced by phosphorus (P) concentration. To reveal the effect of AMF on the Cd accumulation of host plants under different P concentrations and how the AMF and P interact, this study comparatively analyzed the regulatory effects of AMF on the Cd response, extraction, and transportation processes of Phragmites australis (P. australis) under different P levels, and explored its physiological, biochemical and molecular biological mechanisms. The study showed that AMF could induce different growth allocation strategies in response to Cd stress. Moreover, AMF promoted plant Cd tolerance and detoxification by enhancing P uptake, Cd passivation, Cd retention in the cell wall, and functional group modulation. Under P starvation treatments, AMF promoted Cd uptake by inducing Cd to enter the iron pathway, increased the transport coefficient by 493.39%, and retained Cd in stems. However, these effects disappeared following the addition of P. Additionally, AMF up-regulated the expression of ZIP, ZIP, and NRAMP genes to promote cadmium uptake at low, medium, and high phosphorus levels, respectively. Thus, the Cd response mechanism of the AMF-P. australis symbiotic system was P dose-dependent.

Plant growth and heavy meal accumulation characteristics of Spathiphyllum kochii cultured in three soil extractions with and without silicate supplementation

A hydroponic method was conducted to test whether Spathiphyllum kochii is tolerant to multiple HMs as well as to evaluate whether sodium silicate promotes plant growth and alleviates HM stress mainly by assessing biomass, HM accumulation characteristics and antioxidant enzyme activities (AEAs). Three soil extractions from an uncontaminated soil, a comparable lightly HM-contaminated soil (EnSE), and a comparable heavily HM-contaminated soil (ExSE) with or without 1 mM sodium silicate supplementation were used. S. kochii showed no obvious symptoms when cultured in EnSE and ExSE, indicating that it was a multi-HM-tolerant species. The biomass and photosynthesis followed the order: UnSE > EnSE > ExSE, but the opposite order was found for HM concentration, AEAs, and malondialdehyde content. Silicate had no effects on the growth and HM bioaccumulation characteristics of S. kochii cultured in UnSE but exhibited a novel role in decreasing HM uptake by 13.61-41.51% in EnSE and ExSE, respectively, corresponding upregulated AEAs, and reduced malondialdehyde contents, resulting in increased biomass and alleviating HM stress. The activities of peroxidase and superoxide dismutase were upregulated by an increase in soil extraction HM concentration and further upregulated by silicate supplementation, indicating that they were important mechanisms alleviating HM stress in S. kochii.

Phosphorus application enhances alkane hydroxylase gene abundance in the rhizosphere of wild plants grown in petroleum-hydrocarbon-contaminated soil

This study assessed the ability of phosphorus (P) fertilizer to remediate the rhizosphere of three wild plant species (Banksia seminuda, a tree; Chloris truncata, a grass; and Hakea prostrata, a shrub) growing in a soil contaminated with total (aliphatic) petroleum hydrocarbon (TPH). Plant growth, photosynthesis (via chlorophyll fluorescence), soil microbial activity, alkane hydroxylase AlkB (aliphatic hydrocarbon-degrading) gene abundance, and TPH removal were evaluated 120 days after planting. Overall, although TPH served as an additional carbon source for soil microorganisms, the presence of TPH in soil resulted in decreased plant growth and photosynthesis. However, growth, photosynthesis, microbial activities, and AlkB gene abundance were enhanced by the application of P fertilizer, thereby increasing TPH removal rates, although the extent and optimum P dosage varied among the plant species. The highest TPH removal (64.66%) was observed in soil planted with the Poaceae species, C. truncata, and amended with 100 mg P kg^-1 soil, while H. prostrata showed higher TPH removal compared to the plant belonging to the same Proteaceae family, B. seminuda. The presence of plants resulted in higher AlkB gene abundance and TPH removal relative to the unplanted control. The removal of TPH was associated directly with AlkB gene abundance (R² > 0.9, p < 0.001), which was affected by plant identity and P levels. The results indicated that an integrated approach involving wild plant species and optimum P amendment, which was determined through experimentation using different plant species, was an efficient way to remediate soil contaminated with TPH.

Rhizobium Inoculation Enhances the Resistance of Alfalfa and Microbial Characteristics in Copper-Contaminated Soil

Some studies have reported the importance of rhizobium in mitigating heavy metal toxicity, however, the regulatory mechanism of the alfalfa-rhizobium symbiosis to resist copper (Cu) stress in the plant-soil system through biochemical reactions is still unclear. This study assessed the effects of rhizobium (Sinorhizobium meliloti CCNWSX0020) inoculation on the growth of alfalfa and soil microbial characteristics under Cu-stress. Further, we determined the regulatory mechanism of rhizobium inoculation to alleviate Cu-stress in alfalfa through plant-soil system. The results showed that rhizobium inoculation markedly alleviated Cu-induced growth inhibition in alfalfa by increasing the chlorophyll content, height, and biomass, in addition to nitrogen and phosphorus contents. Furthermore, rhizobium application alleviated Cu-induced phytotoxicity by increasing the antioxidant enzyme activities and soluble protein content in tissues, and inhibiting the lipid peroxidation levels (i.e., malondialdehyde content). In addition, rhizobium inoculation improved soil nutrient cycling, which increased soil enzyme activities (i.e., β-glucosidase activity and alkaline phosphatase) and microbial biomass nitrogen. Both Pearson correlation coefficient analysis and partial least squares path modeling (PLS-PM) identified that the interactions between soil nutrient content, enzyme activity, microbial biomass, plant antioxidant enzymes, and oxidative damage could jointly regulate plant growth. This study provides comprehensive insights into the mechanism of action of the legume-rhizobium symbiotic system to mitigate Cu stress and provide an efficient strategy for phytoremediation of Cu-contaminated soils.

Soil amendments affect the potential of Gomphrena claussenii for phytoremediation of a Zn- and Cd-contaminated soil

This study assessed the impact of inorganic and organic amendments upon zinc (Zn) and cadmium (Cd) availabilities in leachates collected from a Cd- and Zn-contaminated soil, while also evaluating the beneficial use of the tested amendments for decreasing metal availability, hence improving the phytoremediation potential of Gomphrena claussenii Moq. Plants were grown for 60 days in a Zn-smelting-affected soil containing 45,000 and 621 mg kg^-1 of Zn and Cd, respectively (pseudo-total concentrations), after application of the following amendments: limestone, calcium silicate, sewage sludge, triple superphosphate, and red mud. Zinc and Cd availabilities in the soil decreased following the addition of limestone, calcium silicate, and red mud. These amendments were effective in reducing metal mobility and availability, positively affecting plant growth. Plants grown in the soil amended with limestone and calcium silicate accumulated Zn mainly in the roots, while Cd was translocated to plant shoots, with smaller amounts being detected in the roots. Reductions of Zn and Cd concentrations in the leachate were found by adding red mud, with this decrease for Zn being less pronounced compared to what was verified after the application of limestone and calcium silicate. Moreover, the use of red mud resulted in a higher Zn:Cd ratio in the leachate, which favored a greater absorption and transport of Zn from root to shoot. In conclusion, the tested soil amendments reduced the availability of excessive concentrations of Cd and Zn in naturally contaminated soil, which resulted in improved growth and survival of Zn- and Cd-tolerant G. claussenii plants, with the application of limestone, calcium silicate, and red mud - i.e., alkaline amendments - standing out as the best combinations with G. Claussenii when designing a strategy to achieve optimal phytoremediation.

Heavy metals uptake and its impact on the growth dynamics of the riparian shrub Ricinus communis L. along Egyptian heterogenic habitats

Heavy metals are well known for their toxicity and become significant environmental pollution with a continually rising technology and public outcry to ensure the safest and healthiest environment. The present study aims to investigate the uptake capability of heavy metals and its impact on the growth dynamics of Ricinus communis L. (castor bean), along various habitats in Qalyubia Province, Egypt. Three composite plants and soil samples were collected from four different habitats: urban (residential area), canal banks, field edges, and drain banks. The samples were analyzed for nutrients and heavy metals. At the same time, forty quadrats (5 × 5 m) were selected to represent the micro-variations of castor bean in the selected habitats to determine its growth criteria and normalized vegetation index (NDVI). The lowest size index, volume, and number of leaves of castor bean were recorded along canal banks and they were characterized by high soil heavy metal concentration, especially Zn, Cu, and Ni, while the highest values were recorded along field edges with lower heavy metal concentration. Moreover, the NDVI indicated that castor bean from most studied habitats, except field edges, was healthy population. This study revealed that the leaves collected from all habitats were considered to be toxic with Cu. The bioconcentration factor (BF) of the investigated heavy metals was greater than 1. The BF order for heavy metals uptake by castor bean leaves was Fe > Ni > Mn > Cu > Zn. Consequently, the species selected in the present study can be used as a biomonitor of these heavy metal polluted soils. Moreover, it could be used as a phytoremediator, taken into consideration its use in all medicinal purposes.

Use and treatment of chicken feathers as a natural adsorbent for the removal of copper in aqueous solution

PURPOSE

Copper is a heavy metal that causes considerable deterioration to human health and ecosystems, so their elimination in water bodies is of great interest. Present investigation shows the efficiency of chicken feather as a natural adsorbent and its subsequent degradation in order to have an integral treatment and avoid the unconscious disposition.

METHODS

Optimal conditions of adsorption process were determined using the Response Surface Methodology (RSM)-Box-Behnken design (BBD) with three variables (pH, temperature and adsorbent dose). After that, the optimal conditions were used to analize the adsorption isotherms by Langmuir, Freundlich and Temkin models; also thermodynamics parameters Gibbs free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) were obtained. Finally, the biodegradation of the residue denominated "adsorbent-adsorbate" complex was evaluated through monitoring the soluble protein production, keratinolytic activity, ninhydrin positive products, sulfhydryl groups, and gravimetrically analysis.

RESULTS

The optimum conditions for the adsorption were 30°C and pH 3, the Langmuir model was better described the adsorption process at 30°C, while at 40°C was Temkin model. The chicken feather turned out a natural adsorbent competitive with respect to others used in the removal of copper in liquid systems; obtaining qmax of 7.84 and 11.48 mg/g at 30 and 40°C, respectively; it was also a favorable and spontaneous process. Finally the adsorbent used was degraded by a keratinolytic consortium.

CONCLUSIONS

In this study, chicken feather was used as a low cost adsorbent for copper efficiently and with the feasibility that the adsorbent can be biodegraded and release the metal.

High level of zinc triggers phosphorus starvation by inhibiting root-to-shoot translocation and preferential distribution of phosphorus in rice plants

Since the urbanization and industrialization are wildly spread in recent decades, the concentration of Zn in soil has increased in various regions. Although the interactions between P and Zn has long been recognized, the effect of high level of Zn on P uptake, translocation and distribution in rice and its molecular mechanism are not fully understood. In this study, we conducted both hydroponic culture and field trial with different combined applications of P and Zn to analyze the rice growth and yield, the uptake, translocation and distribution of P and Zn, as well as the P- and Zn-related gene expression levels. Our results showed that high level of Zn decreased the rice biomass and yield production, and inhibited the root-to-shoot translocation and distribution of P into new leaves by down-regulating P transporter genes OsPT2 and OsPT8 in shoot, which was controlled by OsPHR2-OsmiR399-OsPHO2 module. High Zn supply triggered P starvation signal in root, thereafter increased the activities of both root-endogenous and -secreted acid phosphatase to release more Pi, and induced the expression OsPT2 and OsPT8 to uptake more P for plant growth. On the other hand, high level of P significantly decreased the Zn concentrations in both root and shoot, and the root uptake ability of Zn through altering the expression levels of OsZIPs, which were further confirmed by the P high-accumulated mutant osnla1-2 and OsPHR2-OE transgenic plant. Taken together, we revealed the physiological and molecular mechanisms of P-Zn interactions, and proposed a working model of the cross-talk between P and Zn in rice plants. Our results also indicated that appropriate application of P fertilizer is an effective strategy to reduce rice uptake of excessive Zn when grown in Zn-contaminated soil.

A novel role of sulfate in promoting Mn phytoextraction efficiency and alleviating Mn stress in Polygonum lapathifolium Linn

A hydroponic method was performed to explore the effects of sulfate supply on the growth, manganese (Mn) accumulation efficiency and Mn stress alleviation mechanisms of Polygonum lapathifolium Linn. Three Mn concentrations (1, 8 and 16 mmol L^-1, representing low (Mn1), medium (Mn8) and high (Mn16) concentrations, respectively) were used. Three sulfate (S) levels (0, 200, and 400 μmol L^-1, abbreviated as S0, S200 and S400, respectively) were applied for each Mn concentration. (1) The average biomass (g plant^-1) of P. lapathifolium was ordered as Mn8 (6.36) > Mn1 (5.25) > Mn16 (4.16). Under Mn16 treatment, S addition increased (P < 0.05) biomass by 29.96% (S200) and 53.07% (S400) compared to that S0. The changes in the net photosynthetic rate and mean daily increase in biomass were generally consistent with the changes in biomass. (2) Mn accumulation efficiency (g plant^-1) was ordered as Mn8 (99.66) > Mn16 (58.33) > Mn1 (27.38); and S addition increased (p < 0.05) plant Mn accumulation and Mn transport, especially under Mn16 treatment. (3) In general, antioxidant enzyme activities (AEAs) and malondialdehyde (MDA) in plant leaves were ordered in Mn16 > Mn8 > Mn1. Sulfate addition decreased (P < 0.05) AEAs and MDA under Mn16 treatment, while the changes were minor under Mn1 and Mn8 treatments. (4) Amino acid concentrations generally increased with increasing Mn concentration and S level. In summary, the medium Mn treatment promoted plant growth and Mn bioaccumulation; sulfate, especially at 400 µmol L^-1 S, can effectively promote plant growth and Mn accumulation efficiency. The most suitable bioremediation strategy was Mn16 with 400 µmol L^-1 S.

Regression models to stratify the copper toxicity responses and tolerance mechanisms of Glycine max (L.) Merr. plants

MAIN CONCLUSION

Root antioxidant defense, restricted root-to-shoot Cu translocation, altered nutrient partition, and leaf gas exchange adjustments occurred as tolerance mechanisms of soybean plants to increasing soil Cu levels. The intensive application of copper (Cu) fungicides has been related to the accumulation of this metal in agricultural soils. This study aimed to evaluate the effects of increasing soil Cu levels on soybean (Glycine max) plants. Soybean was cultivated under greenhouse conditions in soils containing different Cu concentrations (11.2, 52.3, 79.4, 133.5, 164.0, 205.1, or 243.8 mg kg-1), and biochemical and morphophysiological plant responses were analyzed through linear and nonlinear regression models. Although Cu concentrations around 50 mg kg-1 promoted some positive effects on the initial development of soybean plants (e.g., increased root length and dry weight), these Cu concentrations also induced root oxidative stress and activated defense mechanisms (such as the induction of antioxidant response, N and S accumulation in the roots). At higher concentrations, Cu led to growth inhibition (mainly of the root), nutritional imbalance, and damage to the photosynthetic apparatus of soybean plants, resulting in decreased CO2 assimilation and stomatal conductance. In contrast, low translocation of Cu to the leaves, conservative water use, and increased carboxylation efficiency contributed to the partial mitigation of Cu-induced stress. These responses allowed soybean plants treated with Cu levels in the soil as high as 90 mg kg-1 to maintain growth parameters higher than or similar to those of plants in the non-contaminated soil. These data provide a warning for the potentially deleterious consequences of the increasing use of Cu-based fungicides. However, it is necessary to verify how the responses to Cu contamination are affected by different types of soil and soybean cultivars.

Phosphate: Coupling the functions of fertilization and passivation in phytoremediation of manganese-contaminated soil by Polygonum pubescens blume

Phosphate (P) fertilization is a commonly used agronomic practice. However, research on bioremediation is very limited. This study's principal objective was to evaluate the role of P in the growth and heavy metals (HMs) accumulation of Polygonum pubescens Blume cultured in Mn-contaminated soil. To this end, the effects of sodium dihydrogen phosphate (SDP) and single superphosphate (SSP) on the growth, Mn bioremediation efficiency, organ HMs, and physiological parameters related to antioxidant stress of P. pubescens were examined. The results showed that both SDP and SSP increased soil pH and available P but decreased available HMs. Phosphate significantly (P < 0.05) promoted P. pubescens height and biomass. Average height increased by 36.1% and 32.6% with SDP and SSP, respectively, with corresponding biomass increases of 71.8% and 135%. Phosphate significantly (P < 0.05) reduced Mn concentrations, especially in leaves, where the values decreased by >50.0% for DSP and SSP. Total Mn significantly (P < 0.05) decreased with DSP amendment but significantly (P < 0.05) increased by 38.5% with SSP (200 mg kg^-1) through an increase in biomass. Phosphate significantly (P < 0.05) decreased all organ HM concentrations and translocation, indicating that less HM stress occurred with P amendment. The changes in reactive oxygen species, antioxidants and non-antioxidant materials further supported these results. Pearson correlation analysis revealed negative relationships between soil available P and HMs, indicating a novel role of P in HM passivation. The uncommonly high Ca concentrations in leaves suggested that Ca plays a vital role in promoting growth and alleviating HM stress in P. pubescens, which warrants further study.

Effects of low molecular weight organic acids on Cu accumulation by castor bean and soil enzyme activities

Chelating agents have been considered as an important phytoremediation strategy to enhance heavy metal extraction from contaminated soil. A pot experiment was conducted to explore the effects of low molecular weight organic acids (LMWOAs) on the phytoremediation efficiency of copper (Cu) by castor bean, and soil enzyme activities. Results indicated that the addition of all the three kinds of LMWOAs (citric, tartaric, oxalic acids) did not decrease the biomass of castor bean, despite the fact they reduced the concentration of chlorophyll-a in leaves compared to the control. The Cu concentrations in the roots and shoots significantly increased by 6-106% and 5-148%, respectively, in the LMWOAs treatments so that the total accumulation of Cu by whole plants in all the LMWOAs treatments increased by 21-189% in comparison with the control. The values of the translocation factor (TF) and bio-concentration factor (BCF) of Cu in castor bean also rose following the addition of LMWOAs, indicating that the LMWOAs enhanced the uptake and transportation of Cu. Moreover, the application of LMWOAs did not significantly change the soil pH but significantly increased the activity of soil enzymes (urease, catalase, and alkaline phosphatase). The addition of exogenous LMWOAs increased the available Cu significantly in the soil, thus promoted the phytoextraction efficiency of Cu by castor bean. These results will provide some new insights into the practical use of LMWOAs for the phytoremediation of heavy-metal-contaminated soil employing castor bean.