Dynamic study of the lead (Pb) tolerance and accumulation characteristics of new dwarf bamboo in Pb-contaminated soil

Metal(loid) uptake and physiological response of Coix lacryma-jobi L. to soil potentially toxic elements in a polluted metal-mining area

Coix lacryma-jobi L. is a traditional medicinal plant in east Asia and is an important crop in Guizhou province, southwest China, where there are elevated levels of soil mercury and arsenic (As). Exposure to multiple potentially toxic elements (PTEs) may affect plant accumulation of metal(loid)s and food safety in regions with high geological metal concentrations. Field experiments were conducted to study the effects of PTEs on metal(loid) accumulation and physiological response of C. lacryma in different plant parts at three pollution levels. Total root length, number of root tips, number of branches, and number of root crosses increased with increasing pollution level, with increases in highly polluted areas of 44.2, 57.0, 79.6, and 97.2%, respectively, compared to lightly polluted areas. Under multi-element stress the activity of C. lacryma antioxidant oxidase showed an increase at low and medium PTE concentrations and inhibition at high concentrations. The As contents were all below the maximum limit of cereal food contaminants in China (GB 2762-2022, As < 0.5 mg kg-1). The stems had high Tl bioconcentration factors but the translocation factors from stem to grain were very low, indicating that the stems may be a key plant part restricting Tl transport to the grains. C. lacryma increased root retention and reduced the transport effect, thus reducing metal accumulation in the grains. C. lacryma adapted to PTE stress through root remodeling and enhanced antioxidant enzyme activities.

Lead Tolerance and Remediation Potential of Four Indocalamus Species in Lead-Contaminated Soil

Indocalamus plants are low-growing shrubby bamboos with growth advantages, such as high biomass and strong resistance, and they are rich in germplasm resources in southern China. This study conducted soil lead (Pb) stress experiments on Indocalamus latifolius (Keng) McClure (LA), Indocalamus hunanensis B.M. Yang (HU), Indocalamus chishuiensis Y.L. Yang and Hsueh (CH) and Indocalamus lacunosus Wen (LC). Five Pb treatments (0, 500, 1000, 1500 mg·kg-1 Pb, and 1000 mg·kg-1 Pb + 1000 mg·kg-1 ethylenediamine tetraacetic acid (EDTA)) were established. EDTA was applied to explore the tolerance mechanism of different Indocalamus species after absorbing large amounts of heavy metals. The results were as follows: (1) under Pb treatment, the total relative biomass of LA, HU and LC was <100%, whereas the total relative biomass of CH was >100%; (2) after applying EDTA, the bioconcentration coefficient, translocation factor, and free proline content of the four Indocalamus species increased; and (3) the Pb mobility and distribution rates of the underground parts of the four Indocalamus species were consistently greater than those of the aboveground parts. The Pb mobility and distribution rates in the stems increased after applying EDTA, while those in the leaves decreased, as the plants tended to transfer Pb to their stems, which have lower physiological activity than their leaves.

On the electrokinetic remediation of Pb-contaminated soil: A coupled electro-transport-reaction modelling study based on chemical reaction kinetics

The accumulation of lead (Pb) in soil resulted from industrialization and urbanization poses a threat to human health and the ecosystem. This study proposes a mathematical model for Pb migration and transformation in soil porous media, aiming to guide the design of electrokinetic remediation schemes for Pb-contaminated soils. To improve the validity of the model, the chemical reactions considered in the model are all based on chemical reaction kinetics, which were usually overlooked for model simplification. The model quantitatively describes various physical and chemical processes of Pb at the soil-pore fluid interface and in the pore fluid, including diffusion, electromigration, electroosmosis, electrolytic water reaction, precipitation, adsorption/desorption, protonation/deprotonation reaction, and water self-ionization reaction. The numerical results show that the pH value is a key factor affecting the distribution of Pb in the soil and determining the removal efficiency of Pb. The effects of different enhancement methods on Pb concentration distribution and removal efficiency were evaluated with this model. It was found that placing a cation exchange membrane at the cathode boundary while using 0.01 M nitric acid as anode electrolyte can effectively improve Pb removal efficiency from 3.9% to 93.6%. The developed model can be used to guide the design of the enhanced electrokinetic remediation schemes.

Flooding regimes alleviate lead toxicity and enhance phytostabilization of salix: Evidence from physiological responses and iron-plaque formation

Aggravated metal pollution in wetland and riparian zones has become a global environmental issue, necessitating the identification of sustainable remediation approaches. Salix exhibits great potential as a viable candidate for metal(loid) remediation. However, the underlying mechanisms for its effectiveness in different flooding regimes with Pb pollution have not been extensively studied. In this study, fast-growing Salix×jiangsuensis 'J172' was selected and planted in different Pb polluted soils (control, 400 and 800 mg ∙ kg-1) under non-flooded and flooded (CF: continuous flooding and IF: intermittent flooding) conditions for 60 days. This study aimed to explore the effects of flooding on Salix growth performance, physiological traits, and the relationship between Pb uptake/translocation and root Fe plaques. Salix×jiangsuensis 'J172' exhibited excellent tolerance and adaptation to Pb pollution with a tolerance index (TI) exceeding 0.6, even at the highest Pb levels. Moreover, the TIs under flooded conditions were higher than that under non-flooded conditions, suggesting that flooding could alleviate Pb toxicity under co-exposure to Pb and flooding. Leaf malondialdehyde (MDA) exhibited a dose-dependent response to Pb exposure; however, CF or IF mitigated the oxidative damage induced by Pb toxicity with decreased MDA content (2.2-11.9%). The superoxide dismutase and peroxidase activities were generally enhanced by flooding, but combined stress (flooding and Pb) significantly decreased catalase activity. Pb was predominantly accumulated in Salix roots, and flooding markedly increased root Pb accumulation by 19.2-173.0% compared to non-flooded condition. Additionally, a significant positive correlation was observed between the iron (Fe) content of the root plaque and root Pb accumulation, indicating that the formation of Fe plaque on the root surface could enhance the phytostabilization of Pb in Salix. The current findings highlight that fast-growing woody plants are suitable for phyto-management of metal-polluted wetlands and can potentially minimize the risk of metal mobility in soils.

Sasa argenteostriata - A potential plant for phytostabilization remediation of lead-zinc tailing-contaminated soil

Phytoremediation is an effective way to remediate metal-contaminated soils. During phytoremediation, plants immobilize heavy metals through the roots to reduce the mobility, toxicity and dispersal of the metals, and the changes in the activity of the roots are often accompanied by changes in the rhizosphere ecosystems, in which rhizobacteria are essential components and interact with roots to maintain the stability of the rhizosphere ecosystem and improve soil health. In this study, the phytoremediation potential of Sasa argenteostriata (Regel) E.G. Camu and the response of rhizobacteria were revealed with different levels of lead-zinc tailing contamination (Pb, Zn, and Cd concentrations of 1197.53, 3243.40, and 185.44 mg/kg for M1 and 2301.71, 6087.95, and 364.00 mg/kg for M2, respectively). The BCF of Sasa argenteostriata increased with increasing soil pollution, and the BCFPb, BCFZn, and BCFCd were 0.19, 0.27, and 0.08, respectively, under the M2 treatment; in contrast, the TF decreased with increasing soil pollution, and the TFPb, TFZn, and TFCd were 0.39, 0.85, and 0.07, respectively, under the M1 treatment. The mobility of Pb in the rhizosphere was higher than that of Zn and Cd, and the percentage of residual (Res) Zn and Cd in the rhizosphere increased, while the acid-soluble (Aci) Pb was significantly higher, leading to obvious uptake of Pb by the roots. Correlation analysis showed that Sasa argenteostriata affected the rhizobacterial community by changing the rhizosphere soil pH, the contents of organic matter and NRFM, and bacteria such as Proteobacteria and MND1, which are highly resistant to heavy metals (HMs), became the dominant species in the community. Further PICRUSt2 analysis showed that reducing metal transport across the membranes and increasing the efficiency of cellular reproduction were the main metabolic mechanisms of bacterial tolerance to HMs. Overall, the roots of Sasa argenteostriata were able to immobilize more heavy metals in PbZn tailing-contaminated soil, reducing the toxicity of HMs in the soil, and then influencing the rhizobacteria to change the community structure and metabolism mechanism to adapt to the HM-contaminated environment, and the soil fertility was increased, which together promoted the health and stability of the soil. This study is the first to illustrate the phytoremediation potential and response of the rhizobacterial community of Sasa argenteostriata under multimetal contamination of PbZn tailings. The results of the study provide some guidance for the practice of lead-zinc tailing-phytoremediation and soil health.

Effects of different cellular and subcellular characteristics on the atmospheric Pb uptake, distribution and morphology in Tillandsia usneoides leaves

Lead (Pb) is a widespread highly toxic and persistent environmental pollutant. Plant leaves play a key role in accumulating atmospheric Pb, but its distribution in different cells and subcellular structures and the factors affecting it have been little studied. Here, Tillandsia usneoides, an indicator plant for atmospheric heavy metals, was treated with an aerosol generation device to analyze Pb contents in different cells (three types of cells in leaf surface scales, epidermal cells, mesophyll cells, vascular bundle cells), subcellular structures (cell wall, cell membrane, vacuoles, and organelles) and cell wall components (pectin, hemicellulose 1 and 2, and cellulose). Results show the different cells of T. usneoides leaves play distinct roles in the process of Pb retention. The outermost wing cells are structures that capture external pollutants, while mesophyll cells, as the aggregation site after material transport, ring cells, disc cells, epidermal cells, and vascular cells are material transporters. Pb was only detected in the cell wall and pectin, indicating the cell wall was the dominant subcellular structure for Pb retention, while pectin was the main component affecting Pb retention. FTIR analysis of cell wall components indicated the esterified carboxyl (CO) functional group in pectin may function in absorbing Pb. Pb entered leaf cells mainly in the form of low toxicity and activity to enhance its resistance.

Physiological tolerance of black locust (Robinia pseudoacacia L.) and changes of rhizospheric bacterial communities in response to Cd and Pb in the contaminated soil

Woody plants possess great potential for phytoremediation of heavy metal-contaminated soil. A pot trial was conducted to study growth, physiological response, and Cd and Pb uptake and distribution in black locust (Robinia pseudoacacia L.), as well as the rhizosphere bacterial communities in Cd and Pb co-contaminated soil. The results showed that R. pseudoacacia L. had strong physiological regulation ability in response to Cd and Pb stress in contaminated soil. The total chlorophyll, malondialdehyde (MDA), soluble protein, and sulfhydryl contents, as well as antioxidant enzymes (superoxide dismutase, peroxidase, catalase) activities in R. pseudoacacia L. leaves under the 40 mg·kg-1 Cd and 1000 mg·kg-1 Pb co-contaminated soil were slightly altered. Cd uptake in R. pseudoacacia L. roots and stems increased, while the Pb content in the shoots of R. pseudoacacia L. under the combined Cd and Pb treatments decreased in relative to that in the single Pb treatments. The bacterial α-diversity indices (e.g., Sobs, Shannon, Simpson, Ace, and Chao) of R. pseudoacacia L. rhizosphere soil under Cd and Pb stress were changed slightly relative to the CK treatment. However, Cd and Pb stress could significantly (p < 0.05) alter the rhizosphere soil microbial communities. According to heat map and LEfSe (Linear discriminant analysis Effect Size) analysis, Bacillus, Sphingomonas, Terrabacter, Roseiflexaceae, Paenibacillus, and Myxococcaceae at the genus level were notably (p < 0.05) accumulated in the Cd- and/or Pb-contaminated soil. Furthermore, the MDA content was notably (p < 0.05) negatively correlated with the relative abundances of Isosphaeraceae, Gaiellales, and Gemmatimonas. The total biomass of R. pseudoacacia L. was positively (p < 0.05) correlated with the relative abundances of Xanthobacteraceae and Vicinamibacreraceae. Network analysis showed that Cd and Pb combined stress might enhance the modularization of bacterial networks in the R. pseudoacacia L. rhizosphere soil. Thus, the assembly of the soil bacterial communities in R. pseudoacacia L. rhizosphere may improve the tolerance of plants in response to Cd and/or Pb stress.

The Metabolomics Response of Solanum melongena L. Leaves to Various Forms of Pb

Due to activities like mining and smelting, lead (Pb) enters the atmosphere in various forms in coarse and fine particles. It enters plants mainly through leaves, and goes up the food chain. In this study, PbXn (nano-PbS, mic-PbO and PbCl2) was applied to eggplant (Solanum melongena L.) leaves, and 379 differential metabolites were identified and analyzed in eggplant leaves using liquid chromatography-mass spectrometry. Multivariate statistical analysis revealed that all three Pb treatments significantly altered the metabolite profile. Compared with nano-PbS, mic-PbO and PbCl2 induced more identical metabolite changes. However, the alterations in metabolites related to the TCA cycle and pyrimidine metabolism, such as succinic acid, citric acid and cytidine, were specific to PbCl2. The number of differential metabolites induced by mic-PbO and PbCl2 was three times that of nano-PbS, even though the amount of nano-PbS absorbed by leaves was ten times that of PbO and seven times that of PbCl2. This suggests that the metabolic response of eggplant leaves to Pb is influenced by both concentration and form. This study enhances the current understanding of plants' metabolic response to Pb, and demonstrates that the metabolomics map provides a more comprehensive view of a plant's response to specific metals.

Effect of cornstalk biochar on phytoremediation of Pb-contaminated soil by females and males of Populus deltoides (Salicaceae)

Soil pollution with lead (Pb) has become a serious global concern, adversely affecting the forest ecosystem. This study was conducted to investigate the effects of corn straw on the remediation efficiency of Pb-contaminated soil using Populus deltoides. Female and male P. deltoides cuttings were subjected to soil spiked with 900 mg kg-1 Pb and amended with 5% (v/v) corn straw biochar for 90 days. Under Pb stress, the addition of biochar significantly increased the total biomass accumulation by 29% in females and 26% in males. However, without the addition of biochar, the biomass accumulation was significantly reduced by 11% in females and 3% in males under Pb stress. Females showed a higher uptake and accumulation of Pb in roots and leaves, while males accumulated more Pb in roots and stems and exhibited an increased anti-oxidative capacity. Biochar addition alleviated Pb toxicity in both male and female P. deltoides by immobilizing Pb ion in the soil, reducing Pb uptake and translocation, promoting nutrient uptake, and improving the diversity and stability of the soil bacteria community. Under Pb stress, the relative abundances of metal-resistance bacteria significantly increased, such as the abundance of Bacteroidetes in females and the abundances of Actinobacteria, Firmicutes, and Planctomycetes in males. In brief, the males under biochar addition exhibited promising potential as candidates for phytoremediation of Pb-contaminated soil. This study provides new insights into mechanisms underlying sexually differential responses to Pb stress in the presence of biochar amendment.

Arbuscular mycorrhizal fungi improve growth and tolerance of Platycladus orientalis under lead stress

Platycladus orientalis is a significant woody plant for phytoremediation in heavy metals contaminated soils. The growth and tolerance of host plants under the lead (Pb) stress were enhanced by arbuscular mycorrhizal fungi (AMF). To evaluate the adjustment by AMF on growth and activity of antioxidant system of P. orientalis under Pb stress. The two-factor pot experiment was conducted with three AM fungal treatments (noninoculated, Rhizophagus irregularis, and Funneliformis mosseae) and four Pb levels (0, 500, 1000, and 2000 mg kg-1). AMF increased dry weight, phosphorus uptake, root vitality, and total chlorophyll content of P. orientalis in spite of Pb stress. Compared with nonmycorrhizal treatments, mycorrhizal P. orientalis had lower H2O2 and malondialdehyde (MDA) contents under Pb stress. AMF increased Pb uptake in roots and decreased the Pb translating to the shoots yet under Pb stress. Total glutathione and ascorbate in roots of P. orientalis were decreased by AMF inoculation. Mycorrhizal P. orientalis had higher superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and glutathione S-transferase (GST) activities in shoots and roots than nonmycorrhizal counterparts. Mycorrhizal P. orientalis under Pb stress showed higher expression of PoGST1 and PoGST2 in roots than that in CK treatments. Future studies will explore the function of induced tolerance genes by AMF of P. orientalis under Pb stress.

Physiological and metabolomics responses of Hydrangea macrophylla (Thunb.) Ser. and Hydrangea strigosa Rehd. to lead exposure

Hydrangea is a potential remediation plant for lead (Pb) pollution. Plant roots communicate with soil through the release of root exudates. It is crucial to study rhizoremediation mechanisms to understand the response of root exudates to contamination stress. Here, we investigated the physiological responses and metabolomic profiling of two Hydrangea species, a horticultural cultivar (Hydrangea macrophylla (Thunb.) Ser.) and a wild type (Hydrangea strigosa Rehd.), under Pb-free and Pb-stressed conditions for 50 days. The results showed that Pb treatment adversely affected the biomass and root growth of the two species. H. strigosa was a Pb-tolerant species with higher superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activities and more ascorbic acid (AsA) content in roots. Metabolomic profiling showed that 181 and 169 compounds were identified in H. macrophylla and H. strigosa root exudates, respectively, among which 18 showed significant differences between H. macrophylla and H. strigosa under Pb exposure. H. strigosa showed significantly (P < 0.05) higher secretion of sucrose, glycolic acid, and nonanoic acid than H. macrophylla after Pb treatment. Pb stress promoted fatty acid metabolism in H. strigosa, suppressed amino acid metabolism in H. macrophylla, and promoted a higher carbohydrate metabolism in H. strigosa compared with H. macrophylla. This study provides a possible mechanism for the high Pb absorption potential of Hydrangea.

Tolerance and Heavy Metal Accumulation Characteristics of Sasa argenteostriata (Regel) E.G. Camus under Zinc Single Stress and Combined Lead–Zinc Stress

Sasa argenteostriata (Regel) E.G. Camus is a gramineous plant with the potential for phytoremediation. In this study, we aimed to determine its tolerance to zinc stress and combined lead–zinc stress and the effect of zinc on its absorption and accumulation characteristics of lead. The results showed that S. argenteostriata had good tolerance to zinc stress, and S. argenteostriata was not significantly damaged when the zinc stress concentration was 600 mg/L. Under both zinc stress and combined lead–zinc stress, the root was the main organ that accumulated heavy metals in S. argenteostriata. The presence of zinc promoted the absorption of lead by the root of S. argenteostriata, and the lead content in the root under PZ1, PZ2, PZ3 and PZ4 treatments was 2.15, 4.31, 4.47 and 6.01 times that of PZ0 on the 20 days. In the combined lead–zinc stress treatments, the toxicity of heavy metals to S. argenteostriata was mainly caused by lead. Under high concentrations of combined lead–zinc stress (PZ4), the proportion of zinc in the leaf of S. argenteostriata on the 20 days increased, which was used as a tolerance strategy to alleviate the toxicity of lead.

Sorption of Pb(II) onto biochar is enhanced through co-sorption of dissolved organic matter

Biochar plays an important role in controlling migration of pollutants in soils. However, little information is available on the interactions between soil-derived dissolved organic matter (DOM), biochar and soluble metal species. The aim of this work was to present the adsorption process of soil DOM by biochar (corn straw biochar produced at 700 °C) and to determine whether co-sorption of DOM would change the affinity for Pb(II). The adsorption rates of biochar and biochar + DOM for Pb(II) were best fitted with a pseudo-second order kinetic model, and the equilibrium adsorption isotherm data agreed well with both the Langmuir and Freundlich models. Adsorption of DOM to biochar reached equilibrium after 15 h with an uptake of 52% of the supplied DOM. We used fluorescence excitation-emission matrices (EEMs) with parallel factor (PARAFAC) analysis to demonstrate that protein-like, fulvic acid-like and humic acid-like substances were the primary constituents of the DOM, which were quenched over time in the presence of biochar. Synchronous fluorescence spectra indicated that the protein-like structures were the predominant fluorescence substances in DOM. Two-dimensional correlation spectroscopy (2D-COS) showed the binding of DOM to biochar resulted in the quenching of fluorescence in the order: protein-like substances > humic-like substances (280 > 355 nm). Data supports the notion that DOM can increase the adsorption capacity of biochar for metal-ions.

Lead tolerance and accumulation characteristics of Cubana Kordes rose in lead-contaminated soil

This study was conducted to determine lead tolerance and accumulation characteristics of Cubana Kordes rose, which is used in landscaping studies in areas with heavy traffic. In the study, 0%, 3%, and 6% leonardite was added to the sand growing medium, and Pb was applied at different doses (0, 200, 400, 800, and 1600 mg Pb kg^-1). At the end of the experiment, the effect of Pb application on plant physiological properties was not statistically significant. The Pb concentration of flower and stem was between 4.50 and 8.92 mg kg^-1 and 8.47 and 543.25 mg kg^-1, respectively. The Pb concentration in the stem increased with an increase in the dose of Pb. The Pb concentration in the root was between 4.00 and 50.35 mg kg^-1 and increased with an increase in the dose of Pb (p < 0.05). The available Pb concentration in the soil varied between 0.05 and 448.79 mg kg^-1. The transfer factor value varied between 1.84 and 18.73 and the bioaccumulation factor value ranged between 0.00 and 10.46. The amount of Pb removed from the soil by the stem was between 124.7 and 8346.6 µg kg^-1. From the results, we determined that Pb accumulated at a higher rate in the stem than in the root and the flower of Cubana Kordes roses. We found that these roses could tolerate the accumulation of Pb, and hence, they have a great potential to be used in the remediation of soil contaminated by Pb.

A Comparison of the Effect of Lead (Pb) on the Slow Vacuolar (SV) and Fast Vacuolar (FV) Channels in Red Beet (Beta vulgaris L.) Taproot Vacuoles

Little is known about the effect of lead on the activity of the vacuolar K+ channels. Here, the patch-clamp technique was used to compare the impact of lead (PbCl2) on the slow-activating (SV) and fast-activating (FV) vacuolar channels. It was revealed that, under symmetrical 100-mM K+, the macroscopic currents of the SV channels exhibited a typical slow activation and a strong outward rectification of the steady-state currents, while the macroscopic currents of the FV channels displayed instantaneous currents, which, at the positive potentials, were about three-fold greater compared to the one at the negative potentials. When PbCl2 was added to the bath solution at a final concentration of 100 µM, it decreased the macroscopic outward currents of both channels but did not change the inward currents. The single-channel recordings demonstrated that cytosolic lead causes this macroscopic effect by a decrease of the single-channel conductance and decreases the channel open probability. We propose that cytosolic lead reduces the current flowing through the SV and FV channels, which causes a decrease of the K+ fluxes from the cytosol to the vacuole. This finding may, at least in part, explain the mechanism by which cytosolic Pb2+ reduces the growth of plant cells.