Interaction of bioaccumulation of heavy metal chromium with water relation, mineral nutrition and photosynthesis in developed leaves of Lolium perenne L

Selenium alleviates chromium stress and promotes chromium uptake in As-hyperaccumulator Pteris vittata: Cr reduction and cellar distribution

Arsenic-hyperaccumulator Pteris vittata exhibits remarkable absorption ability for chromium (Cr) while beneficial element selenium (Se) helps to reduce Cr-induced stress in plants. However, the effects of Se on the Cr uptake and the associated mechanisms in P. vittata are unclear, which were investigated in this study. P. vittata plants were grown for 14 days in 0.2-strength Hoagland solution containing 10 (Cr10) or 100 μM (Cr100) chromate (CrVI) and 1 μM selenate (Se1). The plant biomass, malondialdehyde contents, total Cr and Se contents, Cr speciation, expression of genes associated with Cr uptake, and Cr subcellular distribution in P. vittata were determined. P. vittata effectively accumulated Cr by concentrating 96-99% in the roots under Cr100 treatment. Further, Se substantially increased its Cr contents by 98% to 11,596 mg kg-1 in the roots, which may result from Se's role in reducing its oxidative stress as supported by 27-62% reduction in the malondialdehyde contents. Though supplied with CrVI, up to 98% of the Cr in the roots was reduced to insoluble chromite (CrIII), with 83-89% being distributed on root cell walls. Neither Cr nor Se upregulated the expression of sulfate transporters PvSultr1;1-1;2 or phosphate transporter PvPht1;4, indicating their limited role in Cr uptake. P. vittata effectively accumulates Cr in the roots mainly as CrIII on cell walls and Se effectively enhances its Cr uptake by reducing its oxidative stress. Our study suggests that Se can be used to enhance P. vittata Cr uptake and reduce its oxidative stress, which may have application in phytostabilization of Cr-contaminated soils.

Evaluation of arsenic phytoremediation potential in Azolla filiculoides Lam. plants under low pH stress conditions

The Azolla filiculoides plants were challenged with different arsenic (As) concentration under low pH stress conditions. The growth rate and doubling time of the plants were severely affected by higher As treatments at pH 5.00 when compared with stress pH 4.75 treatments. Hence, pH 5.00 was considered for further studies. In 10-30 μM As treated cultures, after 6 days, the relative growth rate (RGR) of Azolla plants was significantly reduced and in higher concentration of As, the RGR was negatively regulated. The root trait parameters were also significantly affected by increasing concentrations of As. Further, photosynthetic performance indicators also show significant decline with increasing As stress. Overall, the plants treated with 40 and 50 μM of As displayed stress phenotypes like negative RGR, reduced doubling time and root growth, browning of leaves and root withering. The total proline, H2O2, POD and Catalase activities were significantly affected by As treatments. Meantime, 30 μM of As treated cultures displayed 15 μg/g/Fw As accumulation and moderate growth rate. Thus, the Azolla plants are suitable for the phytoremediation of As (up to 30 μM concentration) in the aquatic environment under low pH conditions (5.00). Furthermore, the transcriptome studies on revealed that the importance of positively regulated transporters like ACR3, AceTr family, ABC transporter super family in As (10 μM) stress tolerance, uptake and accumulation. The transporters like CPA1, sugar transporters, PiT were highly down-regulated. Further, expression analysis showed that the MATE1, CIP31, HAC1 and ACR3 were highly altered during the As stress conditions.

Chromiomics: Chromium detoxification and approaches for engineering tolerance in plants

Chromium (Cr) is a heavy metal that poses a grave threat to the ecosystem including plants. Chromium is very harmful to plants due to its effects on many physiological and metabolic pathways culminating in a negative impact on plant's growth, development, and ability to take up nutrients. Plants have developed physiological, biochemical, and molecular ways of defense against Cr, such as by augmenting antioxidant potential to reduce reactive oxygen species (ROS). A number of genes have been discovered to play a significant role in the defense mechanisms of plants against Cr, for example, genes associated with the activation of phytochelatins, metallothioneins, and those of enzymes like glutathione-S-transferases. Along with this, a few miRNAs have been found to be associated in alleviating Cr stress and, to augment plant tolerance by controlling transcription factors, HSPs, and the expression of a few proteins and hormones. Defense pathway genes and miRNAs have been used for the generation of transgenic phytoremediator plants. Not only do the transgenic plants have a higher tolerance to Cr, but they also act as hyperaccumulators for Cr and have the potential to remediate other heavy metals. This article describes about environmental Cr contamination, Cr effects on plants, different genes and miRNAs involved in Cr stress mitigation and use of candidate genes, microRNAs for creating transgenic plant systems for phytoremediation, and the applications of CRISPR technology. It is expected that the integration of omics approach and advanced genomics will offer scope for more effective phytoremediation of Chromium in the coming years.

Phytohormones-mediated strategies for mitigation of heavy metals toxicity in plants focused on sustainable production

KEY MESSAGE

In this manuscript, authors reviewed and explore the information on beneficial role of phytohormones to mitigate adverse effects of heavy metals toxicity in plants. Global farming systems are seriously threatened by heavy metals (HMs) toxicity, which can result in decreased crop yields, impaired food safety, and negative environmental effects. A rise in curiosity has been shown recently in creating sustainable methods to reduce HMs toxicity in plants and improve agricultural productivity. To accomplish this, phytohormones, which play a crucial role in controlling plant development and adaptations to stress, have emerged as intriguing possibilities. With a particular focus on environmentally friendly farming methods, the current review provides an overview of phytohormone-mediated strategies for reducing HMs toxicity in plants. Several physiological and biochemical activities, including metal uptake, translocation, detoxification, and stress tolerance, are mediated by phytohormones, such as melatonin, auxin, gibberellin, cytokinin, ethylene, abscisic acid, salicylic acid, and jasmonates. The current review offers thorough explanations of the ways in which phytohormones respond to HMs to help plants detoxify and strengthen their resilience to metal stress. It is crucial to explore the potential uses of phytohormones as long-term solutions for reducing the harmful effects of HMs in plants. These include accelerating phytoextraction, decreasing metal redistribution to edible plant portions, increasing plant tolerance to HMs by hormonal manipulation, and boosting metal sequestration in roots. These methods seek to increase plant resistance to HMs stress while supporting environmentally friendly agricultural output. In conclusion, phytohormones present potential ways to reduce the toxicity of HMs in plants, thus promoting sustainable agriculture.

Toxic effects of lead on plants: integrating multi-omics with bioinformatics to develop Pb-tolerant crops

MAIN CONCLUSION

Lead disrupts plant metabolic homeostasis and key structural elements. Utilizing modern biotechnology tools, it's feasible to develop Pb-tolerant varieties by discovering biological players regulating plant metabolic pathways under stress. Lead (Pb) has been used for a variety of purposes since antiquity despite its toxic nature. After arsenic, lead is the most hazardous heavy metal without any known beneficial role in the biological system. It is a crucial inorganic pollutant that affects plant biochemical and morpho-physiological attributes. Lead toxicity harms plants throughout their life cycle and the extent of damage depends on the concentration and duration of exposure. Higher levels of lead exposure disrupt numerous key metabolic activities of plants including oxygen-evolving complex, organelles integrity, photosystem II connectivity, and electron transport chain. This review summarizes the detrimental effects of lead toxicity on seed germination, crop growth, and yield, oxidative and ultra-structural alterations, as well as nutrient absorption, transport, and assimilation. Further, it discusses the Pb-induced toxic modulation of stomatal conductance, photosynthesis, respiration, metabolic-enzymatic activity, osmolytes accumulation, and antioxidant activity. It is a comprehensive review that reports on omics-based studies along with morpho-physiological and biochemical modifications caused by lead stress. With advances in DNA sequencing technologies, genomics and transcriptomics are gradually becoming popular for studying Pb stress effects in plants. Proteomics and metabolomics are still underrated and there is a scarcity of published data, and this review highlights both their technical and research gaps. Besides, there is also a discussion on how the integration of omics with bioinformatics and the use of the latest biotechnological tools can aid in developing Pb-tolerant crops. The review concludes with core challenges and research directions that need to be addressed soon.

Progress in phytoremediation of chromium from the environment

As chromium (Cr) in ecosystems affects human health through food chain exposure, phytoremediation is an environmentally friendly and efficient way to reduce chromium pollution in the environment. Here, we review the mechanism of absorption, translocation, storage, detoxification, and regulation of Cr in plants. The Cr(VI) form is more soluble, mobile, and toxic than Cr(III), reflecting how various valence states of Cr affect environmental risk characteristics, physicochemical properties, toxicity, and plant uptake. Plant root's response to Cr exposure leads to reactive oxygen species (ROS) generation and apoptosis. Cell wall immobilization, vacuole compartmentation, interaction of defense proteins and organic ligand with Cr, and removal of reactive oxygen species by antioxidants continue plant life. In addition, the combined application of microorganisms, genetic engineering, and the addition of organic acids, nanoparticles, fertilization, soil amendments, and other metals could accelerate the phytoremediation process. This review provides efficient methods to investigate and understand the complex changes of Cr metabolism in plants. Preferably, fast-growing, abundantly available biomass species should be modified to mitigate Cr pollution in the environment as these green and efficient remediation technologies are necessary for the protection of soil and water ecology.

Sources, impacts, factors affecting Cr uptake in plants, and mechanisms behind phytoremediation of Cr-contaminated soils

Chromium (Cr) is released into the environment through anthropogenic activities and has gained significant attention in the recent decade as environmental pollution. Its contamination has adverse effects on human health and the environment e.g. decreases soil fertility, alters microbial activity, and reduces plant growth. It can occur in different oxidation states, with Cr(VI) being the most toxic form. Cr contamination is a significant environmental and health issue, and phytoremediation offers a promising technology for remediating Cr-contaminated soils. Globally, over 400 hyperaccumulator plant species from 45 families have been identified which have the potential to remediate Cr-contaminated soils through phytoremediation. Phytoremediation can be achieved through various mechanisms, such as phytoextraction, phytovolatilization, phytodegradation, phytostabilization, phytostimulation, and rhizofiltration. Understanding the sources and impacts of Cr contamination, as well as the factors affecting Cr uptake in plants and remediation techniques such as phytoremediation and mechanisms behind it, is crucial for the development of effective phytoremediation strategies. Overall, phytoremediation offers a cost-effective and sustainable solution to the problem of Cr pollution. Further research is needed to identify plant species that are more efficient at accumulating Cr and to optimize phytoremediation methods for specific environmental conditions. With continued research and development, phytoremediation has the potential to become a widely adopted technique for the remediation of heavy metal-contaminated soils.

Effect of stabilization time and soil chromium concentration on Sesbania virgata growth and metal tolerance

Sesbania virgata is a pioneer shrub from the Fabaceae family, native to riparian environments in northeast of Argentina, southern of Brazil and Uruguay. In peri-urban riparian soils, metal contamination is a frequent problem, being its bioavailability partly determined by the stabilization time and frequency of contamination events. The effect of time elapsed between chromium (Cr) soil enrichment and plant seeding and Cr doses on S. virgata tolerance and metal absorption were evaluated. Treatments were developed by adding Cr (80-400 ppm) to the soil and allowing two days or fifteen months to elapse before sowing, and a control treatment without Cr addition. After 150 days from seeding, bioaccumulation and translocation factors, growth parameters (dry biomass and its aerial/radical allocation pattern, stem length and its elongation rate), morphological parameters (root volume and leaf area), and physiological parameters (chlorophyll content) of the specimens were determined. The emergence of S. virgata was inhibited since 150 ppm when Cr was added to the soil two days before seeding, with Cr accumulation in roots starting at 80 ppm (17.4 ± 2.5 mg kg-1). Under 15 months of metal stabilization, S. virgata plants survived across the entire range of Cr doses tested, with accumulation in roots since 100 ppm (35.5 ± 0.2 mg kg-1) and metal translocation to aerial tissues only under 400 ppm. The results obtained showed that S. virgata did not have high BCF and TF values, suggesting that it cannot be classified as bioaccumulator of Cr under the tested conditions. However, its presence in environments contaminated with Cr can be beneficial, as it helps to stabilize the metal in the soil.

Effects of resource availability on the growth, Cd accumulation, and photosynthetic efficiency of three hyperaccumulator plant species

Plants that hyperaccumulate heavy metals such as cadmium (Cd) are important agents of phytoremediation. Availability of resources such as light, nutrients, and water can affect heavy metal accumulation by plants, but the responses of hyperaccumulators to different levels of resource availability remain little studied. To test such responses, three Cd hyperaccumulators, Solanum nigrum, Bidens pilosa, and Taraxacum mongolicum, were grown in Cd-contaminated soil; subjected to three levels of light, nutrient, or water availability; and measured for growth, Cd accumulation, and photosynthetic efficiency. All three species accumulated more total biomass and grew taller if given high than low water or light (each P < 0.001). Species accumulated four to eight times more Cd (190-309 versus 24-68 μg Cd g-1 mass) under high than low light. High water availability increased Cd accumulation by 89% in B. pilosa but decreased it by 31% and 40% in S. nigrum and B. pilosa, respectively. Effects of nutrients on both growth and accumulation varied between species; Cd accumulation by S. nigrum and T. mongolicum was respectively 14% and 54% lower at high than low nutrients, while it was 130% higher in B. pilosa. Light but not water or nutrient availability affected effective and maximum quantum yields and electron transport rate. Findings from this study suggest that low levels of light may constrain phytoremediation of Cd in soil and that testing species of Cd hyperaccumulators individually for responses to levels of water and nutrients will inform selection of species for phytoremediation.

Soil water stress alters differentially relative metabolic pathways affecting growth performance and metal uptake efficiency in a cadmium hyperaccumulator ecotype of Sedum alfredii

Modeling plants for biomass production and metal uptake from surrounding environment is strongly dependent on the moisture content of soil. Therefore, experiments were conducted to find out how soil moisture affects the phenotypic traits, photosynthetic efficiency, metabolic profile, and metal accumulation in the hyperaccumulating ecotype of Sedum alfredii (S. alfredii). A total of six water potential gradients were set: 0 ~ -15 kPa (T1), -15 ~ -30 kPa (T2), -30 ~ -45 kPa (T3), -45 ~ -60 kPa (T4), -60 ~ -75 kPa (T5), and -75 ~ -90 kPa (T6). Different water potential treatments had a significant effect on plant growth and metal uptake efficiency. Compared to T3, T2 was more effective in promoting plant growth and development, with an increase in biomass of 23% and 17% in both fresh weight (FW) and dry weight (DW), respectively. T2 and T3 had the highest cadmium (Cd) content in the shoot (280.2 mg/kg) and (283.3 mg/kg), respectively, whereas T1 had the lowest values (204.7 mg/kg). Cd availability for plants in the soil was affected by moving soil moisture cycles. Changes in soil moisture that were either too high or too low compared to the ideal soil water content for S. alfredii growth resulted in a significant reduction in Cd accumulation in shoots. Tryptophan, phenylalanine, and other amino acids were accumulated in T5, whereas only tryptophan and phenylalanine slightly increased in T1. Sugars and alcohols such as sucrose, trehalose, mannitol, galactinol, and mannobiose increased in T5, while they decreased significantly in T1. Interestingly, in contrast to T1, the two impaired metabolic pathways in T5 (galactose and starch metabolism) were identified to be glucose metabolic pathways. These findings provide scientific information (based on experiments) to improve biomass production and metal uptake efficiency in hyperaccumulating ecotype of S. alfredii for phytoremediation-contaminated agricultural fields.

Comparative Study of Chromium Phytoremediation by Two Aquatic Macrophytes

Chromium (Cr) occurs in several oxidation states from trivalent to hexavalent. However, hexavalent forms are more toxic and mainly produced by anthropogenic activities. A hydroponic experiment was conducted to analyse the comparative remediation of Cr by Marsilea minuta and Pistia stratiotes. Plants were exposed to four concentrations of Cr (0.5, 1.0, 1.5, and 2.0 mM) for 3 days. The highest accumulation of Cr was seen at the 1.5 mM concentration after 3 days in Marsilea (11.96 mg/g) and Pistia (18.78 mg/g). Dry weights decreased and malondialdehyde (MDA) levels increased in response to increasing Cr concentrations. Results indicate that both macrophytes are suitable candidates for Cr phytoremediation. Antioxidant-enzyme activity as a function of metal tolerance is imperative for a coherent understanding of plant physiology under metal stress.

Alleviation of chromium stress in plants using metal and metal oxide nanoparticles

Chromium (Cr), one of the hazardous pollutants, exists predominantly as Cr(VI) and Cr(III) in the environment. Cr(VI) is more toxic than Cr(III) due to its high mobility and solubility. Elevated levels of Cr in agricultural soil due to various anthropogenic activities cause Cr accumulation in plants, resulting in a significant reduction in plant yield and quality due to Cr-induced physiological, biochemical and molecular alterations. It can infiltrate the food chain through crop plants and cause harmful effects in humans via biomagnification. Cr(VI) is linked to cancer in humans. Therefore, mitigation strategies are required to remediate Cr-polluted soils and limit its accumulation in plants for safe food production. Recent research on metal and metal oxide nanoparticles (NPs) has shown that they can effectively reduce Cr accumulation and phytotoxicity. The effects of these NPs are influenced by their type and dose, exposure method, plant species and experimental settings. In this review, we present an up-to-date compilation and comprehensive analysis of the existing literature regarding the process of uptake and distribution of Cr and impact and potential mechanisms of metal and metal oxide nanoparticles led mitigation of Cr-induced stress in plants. We have also discussed recent developments, existing research gaps and future research directions in the field of Cr stress mitigation by NPs in plants. Overall, this review can provide valuable insights in reducing Cr accumulation and toxicity using metal and metal oxide nanoparticles, thereby promoting safe and sustainable cultivation of food and phytostabilization of Cr-polluted soil.

Hexavalent chromium-reducing plant growth-promoting rhizobacteria are utilized to bio-fortify trivalent chromium in fenugreek by promoting plant development and decreasing the toxicity of hexavalent chromium in the soil

BACKGROUND

Fenugreek is known to have good anti-diabetes properties. Moreover, several studies accounted that the trivalent form of chromium [Cr(III)] also have anti-diabetic properties. However, its hexavalent form i.e., Cr(VI) is known to be highly toxic and carcinogenic to living beings and retarded plant growth even if it is present in low concentration in soil. Many plant growth-promoting rhizobacteria (PGPR) are reported to have the potential to reduce the Cr(VI) into Cr(III) in soil. In view of the above, the present objective was designed to effectively utilize Cr(VI) reducing PGPRs for the growth and development of fenugreek plant in Cr(VI) amended soil, apart from reducing Cr(VI) in soil and fortification of Cr(III) in the aerial part of plants.

METHODS

The experiment was carried out to evaluate the effect of Cr(VI)-reducing PGPRs viz. Bacillus cereus (SUCR44); Microbacterium sp. (SUCR140); Bacillus thuringiensis (SUCR186) and B. subtilis (SUCR188) on growth, uptake and translocation of Cr as well as other physiological parameters in fenugreek grown under artificially Cr(VI) amended soil (100 mg kg^-1 of Cr(VI) in soil).

RESULTS

The aforementioned concentration of Cr(VI) in soil cause severe reduction in root length (41 %), plant height (43 %), dry root (38 %) and herb biomass (48 %), when compared with control negative (CN; uninoculated plant not grown in Cr(VI) contaminated soil). However, the presence of Microbacterium sp.-SURC140 (MB) mitigates the Cr toxicity resulting in improved root length (92 %), plant height (86 %), dry root (74 %) and herb biomass (99 %) as compared with control positive (CP; uninoculated plants grown in Cr(VI) contaminated soil). The maximum reduction in bioavailability (82 %) of Cr(VI) in soil and its uptake (50 %) by the plant were also observed in MB-treated plants. However, All Cr(VI)-reducing PGPRs failed to decrease the translocation of Cr to the aerial parts. Moreover, the plant treated with MB observed diminution in relative water content (13 %), electrolyte leakage (16%) and lipid peroxidation (38 %) as well as higher chlorophyll (37 %) carotenoids (17 %) contents and antioxidants (18%) potential.

CONCLUSION

This study demonstrates that MB can lower the Cr(VI) toxicity to the plant by reducing the bioavailable Cr(VI), consequently reducing the Cr(VI) toxicity level in soil and helping in improving the growth and yield of fenugreek. Additionally, Cr(III) uptakes and translocation may improve the effectiveness of fenugreek in treating diabetes.

Toxicity and tolerance of nickel in sunflower (Helianthus annuus L.)

This study aimed at exploration of nickel (Ni) application (0, 10, 20, 30, and 40 mg L^-1) on physiological and biochemical attributes of sunflower cultivars (Hysun-33 and SF-187) grown in sand culture. Results revealed a significant decrease in vegetative parameters in both sunflower cultivars by increasing Ni concentration, although low levels of Ni (10 mg L^-1) improved growth attributes to some extent. Among photosynthetic attributes, 30 and 40 mg L^-1 Ni application severely reduced the photosynthetic rate (A), stomatal conductance (g_s), water use efficiency (WUE), and Ci/Ca ratio but improved the transpiration rate (E) in both sunflower cultivars. The same level of Ni application also reduced leaf water potential, osmotic potentials, and relative water contents but increased leaf turgor potential and membrane permeability. At low level (10 and 20 mg L^-1), Ni improved the soluble proteins, while high Ni concentration decreased it. The opposite was true for total free amino acids and soluble sugars. To conclude, the high Ni concentration in various plant organs had a strong impact with the changes in vegetative growth, physiological and biochemical attributes. A positive correlation of growth, physiological, water relations, and gas exchange parameters at low levels of Ni and negative correlation at higher Ni level confirmed that the supplementation of low Ni levels greatly modulated studied attributes. Based on observed attributes, Hysun-33 showed high tolerance to Ni stress as compared to SF-187.

Chromium toxicity, speciation, and remediation strategies in soil-plant interface: A critical review

In recent decades, environmental pollution with chromium (Cr) has gained significant attention. Although chromium (Cr) can exist in a variety of different oxidation states and is a polyvalent element, only trivalent chromium [Cr(III)] and hexavalent chromium [Cr(VI)] are found frequently in the natural environment. In the current review, we summarize the biogeochemical procedures that regulate Cr(VI) mobilization, accumulation, bioavailability, toxicity in soils, and probable risks to ecosystem are also highlighted. Plants growing in Cr(VI)-contaminated soils show reduced growth and development with lower agricultural production and quality. Furthermore, Cr(VI) exposure causes oxidative stress due to the production of free radicals which modifies plant morpho-physiological and biochemical processes at tissue and cellular levels. However, plants may develop extensive cellular and physiological defensive mechanisms in response to Cr(VI) toxicity to ensure their survival. To cope with Cr(VI) toxicity, plants either avoid absorbing Cr(VI) from the soil or turn on the detoxifying mechanism, which involves producing antioxidants (both enzymatic and non-enzymatic) for scavenging of reactive oxygen species (ROS). Moreover, this review also highlights recent knowledge of remediation approaches i.e., bioremediation/phytoremediation, or remediation by using microbes exogenous use of organic amendments (biochar, manure, and compost), and nano-remediation supplements, which significantly remediate Cr(VI)-contaminated soil/water and lessen possible health and environmental challenges. Future research needs and knowledge gaps are also covered. The review's observations should aid in the development of creative and useful methods for limiting Cr(VI) bioavailability, toxicity and sustainably managing Cr(VI)-polluted soils/water, by clear understanding of mechanistic basis of Cr(VI) toxicity, signaling pathways, and tolerance mechanisms; hence reducing its hazards to the environment.

Selenium and molybdenum synergistically alleviate chromium toxicity by modulating Cr uptake and subcellular distribution in Nicotiana tabacum L

Chromium (Cr) is a harmful heavy metal that poses a serious threat to plants and animals. Selenium (Se) and molybdenum (Mo) are two beneficial elements for plant growth and resistance. However, their interactive effects on Cr uptake and distribution are poorly understood. Therefore, a hydroponics experiment was conducted to explore the effects of the use of Se and Mo alone and simultaneously on mitigating Cr toxicity. In this study, Nicotiana tabacum L. seedlings were exposed to control, 50 µM Cr, 50 μM Cr + 2 μM Se, 50 μM Cr + 1 μM Mo, or 50 μM Cr + 2 μM Se + 1 μM Mo in Hoagland solution. After 2 weeks, the plant biomass, Cr, Se and Mo contents, photosynthesis, leaf ultrastructure, antioxidant system, subcellular distribution and associated gene expression in Nicotiana tabacum L. were determined. The results showed that simultaneous use of Se and Mo promoted tobacco growth under Cr stress, as evidenced by reducing reactive oxygen species (ROS) content and reducing Cr translocation factor (TF) and inducing a 51.3% reduction in Cr content in shoots. Additionally, Se-Mo interactions increased the levels of glutathione (GSH) and phytochelatin (PC) and the distribution of Cr in the cell walls and organelles. Furthermore, the relative expression of PCS1 was upregulated, while those of NtST1 and MSN1 were downregulated. The results concluded that the simultaneous use of Se and Mo effectively alleviated Cr toxicity in Nicotiana tabacum L., which not only offers an efficient way for crops to resist Cr toxicity but also provides evidence for the benefit of Se combined with Mo.

Synergistic effect of Bacillus and Rhizobium on cytological and photosynthetic performance of Macrotyloma uniflorum (Lam.) Verdc. Grown in Cr (VI) contaminated soil

Macrotyloma uniflorum (horse gram) is considered an under-utilized legume crop despite its nutritional and medicinal values. In India, it has wide acceptance among farming communities. This investigation emphasized on the possible application of two endosymbionts (Bacillus sp. AS03 and Rhizobium sp. AS05) of horse gram cultivated on Cr (VI)-contaminated soil. The photosynthetic performance (PIφ) of Cr treated plants co-inoculated with AS03 and AS05 was significantly improved compared with non-inoculated Cr treated plants based on photosynthetic yield, which was evidenced from the rise in the fluorescence at I-P transient and rate of photosynthesis (pN), indicating synergistic action between plant and bacteria (AS03 and AS05). The smooth electron transport from PS II to PS I was achieved in the Cr stressed plants inoculated with both the bacterial strains. The detrimental effects of Cr toxicity on the root tips were also minimized with bioinoculation as revealed from mitotic index. Plants with dual inoculation of AS03 and AS05 had significantly lesser chromosomal aberration in the roots. Dual inoculation biochar or seed inoculation have beneficial impact on the plant photosynthetic performance along with improved growth of roots in plants treated with Cr (VI). The results of the current work suggest the possitive effect of dual inoculation of Cr tolerant endosymbionts, Bacillus sp. (AS03) and nodulating Rhizobium sp. (AS05), in reducing cytological as well as physiological stress of plants in Cr (VI) contaminated soil.

Immobilization of hexavalent chromium in soil-plant environment using calcium silicate hydrate synthesized from coal gangue

The presence of excessive hexavalent chromium (Cr(VI)) in the contaminated soils and plants has become a global environmental issue due to its toxicity and carcinogenicity. This work investigated the feasibility of immobilizing Cr(VI) in the soil-plant environment using calcium silicate hydrate (C-S-H) synthesized from coal gangue. The results revealed that the C-S-H amendment increased soil pH and organic matter (OM), which further promoted Cr(VI) immobilization. Results also revealed that exchangeable and carbonate bound fractions of Cr were either converted into Fe/Mn oxide and OM bound fractions of Cr or hardly released residual fraction of Cr due to C-S-H treatment. The C-S-H accelerated conversion of Cr(VI) into Cr(III) promoting plant growth and alleviating the toxic effect of Cr(VI). Cr(VI) was mainly immobilized and accumulated in the plant roots which resulted in comparatively lower Cr(VI) content in the edible part of plants. The exchangeable fraction of Cr in soil could be used as a bioavailability evaluation index of Cr(VI) in plants. In short, C-S-H was proved to be a practical and environmentally friendly amendment for in-situ immobilization of Cr(VI) contaminated soil.

Management of chromium(VI)-contaminated soils through synergistic application of vermicompost, chromate reducing rhizobacteria and Arbuscular mycorrhizal fungi (AMF) reduced plant toxicity and improved yield attributes in Ocimum basilicum L

An integrated approach involving vermicompost, chromate reducing bacteria and AMF was tested to manage the toxic impacts of Cr(VI) on Ocimum basilicum as a model plant. Pot experiments were conducted on O. basilicum plants in an artificially Cr(VI)-contaminated soil in two phases of experiment as bioinoculants experiment and vermicompost experiment. In the first phase of the bioinoculants experiment the series of gradient concentrations of Cr(VI) (0, 25, 50 and 100 mg kg^–1 in soil) were evaluated with previously isolated four efficient Cr(VI)-reducing rhizo-bacterial strains (Bacillus Cereus strain SUCR 44, BC; Microbacterium sp. strain SUCR 140, MB; Bacillus thuringiensis strain SUCR186, BT; and Bacillus subtilis strain SUCR188; BS) along with Arbuscular Mycorrhizal Fungus—Glomus fasciculatum (GF) in alone and in co-inoculation form. In the second experiment (vermicompost) the best performing strain (MB) was tested alone or in combination with GF along with different doses of vermicompost. It was observed that vermicompost by itself could be useful in decreasing the bioavailable Cr(VI), uptake of Cr besides improving the nutritional status of plants. The vermicompost also played an important and indirect role and improved herb yield by supporting the multiplication of MB (Microbacterium sp.), an efficient chromate reducing rhizobacteria, that further decreased the bioavailable and toxic form of Cr and improved population and colonization of GF too. The translocation of Cr(VI) was averted through improved colonization of GF, also prevented higher accumulation of Cr in aerial parts (leafy herb) of O. basilicum.

Phytoextraction by Moso Bamboo under high level chromium stress in mediterranean conditions

In this study a bamboo species, Moso Bamboo (MB) - Phyllostachys pubescens - has been selected for its heavy metal accumulation capacity and translocation potential to restore Cr-contaminated soil. Experiments have been conducted so to evaluate the capability of MB to remove Cr from soil, growing under Mediterranean conditions, irrigated with water containing 180 mgCr/L, at flow rate of 600 mm/year. The soil has been contaminated by the irrigation water. When the concentration of Cr in soil reached 300 mgCr/kg, Cr phytoextraction by MB from soil at the same irrigation rate of 600 mm/year with uncontaminated water has been evaluated. Cr removal from soil was approx. 42% after 6 weeks and 60.7% after 12 weeks, starting from a Cr content in soil of approximately 300 mg/kg. MB growing in Cr contaminated soil has shown Cr concentration per gram of dry biomass in aerial parts greater than the underground parts of the plants. After 12 weeks of cultivation, the quantity of Cr in roots and rhizome was measured as 1.79 mg/g, while in stems and leaves as 2.49 mg/g. Results shown a bioconcentration factor of 0.77, 0.65, 0.18, 0.08, after 6 weeks and 0.64, 0.98, 0.53, 0.26 after 12 weeks for roots, rhizomes, stems and leaves, respectively and a translocation factor equal to 0.23 and 0.11 after 6 weeks and 0.83 and 0.40 after 12 weeks, for stems and leaves, respectively.

Chromium phytoextraction using Phyllostachys pubescens (Moso Bamboo)

In this study, a bamboo species, the Phyllostachys pubescens - Moso Bamboo (MB) -, was selected for its heavy metals accumulation and translocation potential to restore Chromium (Cr)-contaminated soil. In order to evaluate the MB Chromium growth, tolerance and the potential for phytoremediation using MB to restore Cr-contaminated soil, pot experiments were carried out in simulated Mediterranean conditions in a laboratory, in a controlled environment, at a temperature of 20 °C. The results showed that MB growth rate was 4.28 cm/week on average, with an irrigation flow of 1.644 mm/d. MB tolerance was tested over a 12-week irrigation period with the addition of Cr-contaminated water. Cr removal from soil was 43% starting from a Cr content of approx. 200 mg/kg dry weight (dw) and the quantity of Cr per gram of root and rhizome was equal to 1.31 mg/g dw, while the quantity of Cr per gram of stem and leaves was equal to 0.86 mg/g dw, after 12 weeks. Pot experiments confirm that phytoremediation using plants such as MB provides an alternative approach for handling Cr-contaminated soil.

Elucidating the Potential of Vertical Flow-Constructed Wetlands Vegetated with Different Wetland Plant Species for the Remediation of Chromium-Contaminated Water

Water scarcity is one of the key global challenges affecting food safety, food security, and human health. Constructed wetlands (CWs) provide a sustainable tool to remediate wastewater. Here we explored the potential of vertical flow-CWs (VF-CWs) vegetated with ten indigenous wetland plant species to treat chromium (Cr)-contaminated water. The wetland plants were vegetated to develop VF-CWs to treat Cr-contaminated water in a batch mode. Results revealed that the Cr removal potential of VF-CWs vegetated with different wetland plants ranged from 47% to 92% at low (15 mg L−1) Cr levels and 36% to 92% at high (30 mg L−1) Cr levels, with the maximum (92%) Cr removal exhibited by VF-CWs vegetated with Leptochloa fusca. Hexavalent Cr (Cr(VI)) was reduced to trivalent Cr (Cr(III)) in treated water (96–99 %) of all VF-CWs. All the wetland plants accumulated Cr in the shoot (1.9–34 mg kg−1 dry weight (DW)), although Cr content was higher in the roots (74–698 mg kg−1 DW) than in the shoots. Brachiaria mutica showed the highest Cr accumulation in the roots and shoots (698 and 45 mg kg−1 DW, respectively), followed by Leptochloa fusca. The high Cr level significantly (p < 0.05) decreased the stress tolerance index (STI) percentage of the plant species. Our data provide strong evidence to support the application of VF-CWs vegetated with different indigenous wetland plants as a sustainable Cr-contaminated water treatment technology such as tannery wastewater.

A combined approach utilizing UAV 3D imaging methods, in-situ measurements, and laboratory experiments to assess water evaporation and trace element uptake by tree species growing in a red gypsum landfill

The extractive industry is increasingly faced with problems of managing contaminated sites. The red gypsum landfill at the Ochsenfeld site is representative of the typology byproduct storage of the Ti-extraction activity. The management of the elemental content and the water body are the issues at this site. The aim of this study was to evaluate the canopy conductance (g_cmax) of various tree species and the content of elements in the leaves, utilizing the opportunity of a demonstration plantation setup in 2014 with sixteen tree species, combined with a growth chamber experiment. We combined the gas exchange measurements with the data from two multispectral cameras with RGB and NIR bands embarked on an unmanned aerial vehicle (UAV). In the field, Ostrya carpinifolia, Maclura pomifera, and Rhus copallina had the highest g_cmax of all planted tree species, and the high transpiration rate in O. carpinifolia was confirmed in a pot-based controlled experiment. Except R. copallina, the species with a high Mn content (O. carpinifolia, Betula pendula, and Salix aquatica grandis) had high stomatal conductance. O. carpinifolia could therefore be a species to exploit in the management of landfill leachates, especially in the context of climate change since this species is well adapted to dry environments.

Chromium biogeochemical behaviour in soil-plant systems and remediation strategies: A critical review

Chromium (Cr) is a toxic heavy metal that is heavily discharged into the soil environment due to its widespread use and mining. High Cr levels may pose toxic hazards to plants, animals and humans, and thus have attracted global attention. Recently, much progress has been made in elucidating the mechanisms of Cr uptake, transport and accumulation in soil-plant systems, aiming to reduce the toxicity and ecological risk of Cr in soil; however, these topics have not been critically reviewed and summarised to date. Accordingly, based on available data-especially from the last five years (2017-2021)-this review traces a plausible link among Cr sources, levels, chemical forms, and phytoavailability in soil; Cr accumulation and translocation in plants; and Cr phytotoxicity and detoxification in plants. Additionally, given the toxicity and hazard posed by Cr(VI) in soils and the application of reductant materials to reduce Cr(VI) to Cr(III) for the remediation of Cr(VI)-contaminated soils, the reduction and immobilisation mechanisms by organic and inorganic reductants are summarised. Finally, some priority research challenges concerning the biogeochemical behaviour of Cr in soil-plant systems are highlighted, as well as the environmental impacts resulting from the application of reductive materials and potential research prospects.

Individual and combinatorial effects of SNP and NaHS on morpho-physio-biochemical attributes and phytoextraction of chromium through Cr-stressed spinach (Spinacia oleracea L.)

Chromium (Cr) is a toxic heavy metal that contaminates soil and water resources after its discharge from different industries. A pot experiment was conducted to determine the effects of single and/or combined application of sodium nitroprusside (SNP) (250 μM) and sodium hydrogen sulfide (NaHS) (1 mM) on growth, photosynthetic pigments, gas exchange characteristics, oxidative stress biomarkers, antioxidant machinery (enzymatic and non-enzymatic antioxidants), ion uptake, organic acid exudation, and Cr uptake of spinach (Spinacia oleracea L.) exposed to severe Cr stress [Cr: 0 (no Cr), 150, and 300 μM]. Our results depicted that Cr addition to the soil significantly (P < 0.05) decreased plant growth and biomass, gas exchange attributes, and mineral uptake by S. oleracea when compared to the plants grown without the addition of Cr. However, Cr toxicity boosted the production of reactive oxygen species (ROS) by increasing the content of malondialdehyde (MDA), which is the indication of oxidative stress in S. oleracea, and was also manifested by hydrogen peroxide (H₂O₂) content and electrolyte leakage to the membrane-bound organelles. The results showed that the activities of various antioxidative enzymes, such as superoxidase dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX), and the content of non-enzymatic antioxidants, such as phenolic, flavonoid, ascorbic acid, and anthocyanin, initially increased with an increase in the Cr concentration in the soil. The results also revealed that the levels of soluble sugar, reducing sugar, and non-reducing sugar were decreased in plants grown under elevating Cr levels, but the accumulation of the metal in the roots and shoots of S. oleracea, was found to be increased, and the values of bioaccumulation factor were <1 in all the Cr treatments. The negative impacts of Cr injury were reduced by the application of SNP and NaHS (individually or combined), which increased plant growth and biomass, improved photosynthetic apparatus, antioxidant enzymes, and mineral uptake, as well as diminished the exudation of organic acids and oxidative stress indicators in roots of S. oleracea by decreasing Cr toxicity. Here, we conclude that the application of SNP and NaHS under the exposure to Cr stress significantly improved plant growth and biomass, photosynthetic pigments, and gas exchange characteristics; regulated antioxidant defense system and essential nutrient uptake; and balanced organic acid exudation pattern in S. oleracea.

Phytoextraction of Cr(VI)-Contaminated Soil by Phyllostachys pubescens: A Case Study

This work presents the results of experimental tests to evaluate the effects of prolonged contamination by Cr on Moso Bamboo (MB) (Phyllostachys pubescens) and the adaptability of the MB to the Mediterranean climate. A preliminary test on the MB was developed in the laboratory, simulating irrigation under Mediterranean conditions (600 mm per year) and tropical conditions (1800 mm per year), to evaluate the rate of growth and the MB’s capability for Cr phytoextraction from contaminated soil. The tolerance of MB to Cr was also performed showing a good response of the plant to 100 mg Cr/L solution, utilized for irrigation of the pots. The results show that the rate of MB’s removal of Cr from soil ranged from 49.2% to 61.7% as a function of the soil degree of contamination, which varied from approx. 100 mg/kg to 300 mg/kg. The distribution of Cr in the various sections of the bamboo revealed that the greater percentage was present in rhizomes: 42%, equal to 114 mg Cr for 600 mm per year, and 50%, equal to 412 mg Cr for 1800 mm per year. A noteworthy diffusion of the metal towards the outermost parts of the plant was shown. The values of Cr retained in the stems and leaves of MB tissues were quite high and varied from 1100 mg/kg to 1700 mg/kg dry weight.

An actinomycete strain of Nocardiopsis lucentensis reduces arsenic toxicity in barley and maize

Accumulation of arsenic in plant tissues poses a substantial threat to global crop yields. The use of plant growth-promoting bacterial strains to mitigate heavy metal toxicity has been illustrated before. However, its potential to reduce plant arsenic uptake and toxicity has not been investigated to date. Here, we describe the identification and characterization of a Nocardiopsis lucentensis strain isolated from heavy metal contaminated soil. Inoculation with this bioactive actinomycete strain decreased arsenic root and shoot bioaccumulation in both C3 and C4 crop species namely barley and maize. Upon arsenate treatment, N. lucentensis S5 stimulated root citric acid production and the plant's innate detoxification capacity in a species-specific manner. In addition, this specific strain promoted biomass gain, despite substantial tissue arsenic levels. Detoxification (metallothionein, phytochelatin, glutathione-S-transferase levels) was upregulated in arsenate-exposed shoot and roots, and this response was further enhanced upon S5 supplementation, particularly in barley and maize roots. Compared to barley, maize plants were more tolerant to arsenate-induced oxidative stress (less H₂O₂ and lipid peroxidation levels). However, barley plants invested more in antioxidative capacity induction (ascorbate-glutathione turnover) to mitigate arsenic oxidative stress, which was strongly enhanced by S5. We quantify and mechanistically discuss the physiological and biochemical basis of N. lucentensis-mediated plant biomass recovery on arsenate polluted soils. Our findings substantiate the potential applicability of a bactoremediation strategy to mitigate arsenic-induced yield loss in crops.

Seed Priming with Brassinosteroids Alleviates Chromium Stress in Rice Cultivars via Improving ROS Metabolism and Antioxidant Defense Response at Biochemical and Molecular Levels

This research was performed to explore the vital role of seed priming with a 0.01 µM concentration of brassinosteroids (EBL) to alleviate the adverse effects of Cr (100 µM) in two different rice cultivars. Seed priming with EBL significantly enhanced the germination attributes (germination percentage, germination energy, germination index, and vigor index, etc.), photosynthetic rate as well as plant growth (shoot and root length including the fresh and dry weight) under Cr toxicity as compared to the plants primed with water. Cr toxicity induced antioxidant enzyme activities (SOD, POD, CAT, and APX) and ROS level (MDA and H₂O₂ contents) in both rice cultivars; however, a larger increment was observed in YLY-689 (tolerant) than CY-927 (sensitive) cultivar. EBL application stimulatingly increased antioxidant enzyme activities to scavenge ROS production under Cr stress. The gene expression of SOD and POD in EBL-primed rice plants followed a similar increasing trend as observed in the case of enzymatic activities of SOD and POD compared to water-primed rice plants. Simultaneously, Cr uptake was observed to be significantly higher in the water-primed control compared to plants primed with EBL. Moreover, Cr uptake was significant in YLY-689 compared to CY-927. In ultra-structure studies, it was observed that EBL priming relieved the rice plants from sub-cellular damage. Conclusively, our research indicated that seed priming with EBL could be adopted as a promising strategy to enhance rice growth by copping the venomous effect of Cr.

Cr, Ni, and Zn removal from landfill leachate using vertical flow wetlands planted with Typha domingensis and Canna indica

Chromium (Cr), Nickel (Ni), and zinc (Zn) removal from landfill leachate using mesocosm-scale vertical flow wetlands, the effect of recirculation, and the ability of macrophytes to retain metals were evaluated. Wetlands were filled with coarse sand and light expanded clay aggregates and planted with Typha domingensis or Canna indica. Wetlands were operated using intermittent loading, with and without recirculation. Raw leachate was diluted and spiked with metals to reach the following concentrations: 0.2 mg L^-1 Cr , 0.2 mg L^-1 Ni, and0.2 mg L^-1 Zn and 1.0 mg L^-1 Cr, 1.0 mg L^-1 Ni, and 1.0 mg L^-1 Zn. Wetlands planted with T. domingensis presented higher metal removal than those planted with C. indica. Recirculation enhanced metal removal efficiencies significantly, being for T. domingensis/C. indica: 60/54, 49/47, 61/47% for Cr, Ni, and Zn at 0.2 mg L^-1, and 80/71, 76/62, 73/59% for Cr, Ni, and Zn at 1.0 mg L^-1, respectively. Metals were efficiently retained by macrophytes. Plant biomass and metal concentrations in roots were significantly higher than in shoots. Scanning electron microscopy and X-ray microanalysis showed that metals were absorbed by internal root tissues. A hybrid wetland planted with T. domingensis may be implemented to improve not only metal but also chemical oxygen demand and total nitrogen removals.

Alleviating Cr(VI) stress in horse gram (Macrotyloma uniflorum Var. Madhu) by native Cr-tolerant nodule endophytes isolated from contaminated site of Sukinda

Sukinda chromite mine of Odisha is a heavily polluted site, generating huge overburden dumps. The present experiment was designed to evaluate the potential of two native nodule endophytic bacterial strains, viz. Bacillus aryabhattai AS03 (MT645244) and Rhizobium pusense AS05 (MT645243), isolated from contaminated sites to be considered remediation tool to minimize the effect of Cr toxicity on Macrotyloma uniflorum var. Madhu. The two nodule endophytic bacterial strains AS03 and AS05 exhibited tolerance to 1800 and 3000 ppm of Cr(VI) respectively in vitro when cultured alone. AAS analysis confirmed higher accumulation of Cr(VI) in roots and less accumulation in shoots which is dose-specific (bio-inoculant) either treated alone or combined. Complete absence of Cr accumulation approximately 99% in shoots of Macrotyloma was observed owing to synergistic effect of both the strains (biochar-based formulation). This study also suggests increased shoot and root length, nodule nos., and leghemoglobin content of the plant at 60 days indicating the plant growth-promoting effects of both the strains. ROS and antioxidant enzymes of the plant recorded decreasing trend in inoculated plants. However, a significant increment in transpiration rate, total photosynthetic rate, intracellular CO2 conc., and stomatal conductance in leaves was observed owing to dual inoculation. Our findings corroborate the supremacy of synergistic effect of both the strains applied in the form of biochar-based biofertilizer in enhancing growth and tolerance index of M. uniflorum cultivated in Cr(VI)-stressed soil. This investigation depicts the efficiency of the two nodule bacteria as a mixed inoculant to alleviate Cr toxicity and making the seeds safe for consumption.

Azotobacter vinelandii helps to combat chromium stress in rice by maintaining antioxidant machinery

Chromium (Cr) causes toxic effects in plants by generating reactive oxygen species (ROS) which create oxidative environment. Azotobacter vinelandii helps in growth and development of many crops; however, its role in Cr stress tolerance in rice has not been explored. Here, we report the new function of Azotobacter vinelandii strain SRI Az3 (Accession number JQ796077) in providing Cr stress tolerance in Oryza sativa (var. IR64). The efficiency of the strain was checked under different concentrations (50, 100, 150, 200 and 250 µM) of Cr stress and it was observed that it provides stress tolerance to rice plant up to 200 µM concentration. Different agronomic growth parameters were found to be better in this strain of Azotobacter vinelandii-inoculated rice plants as compared to un-inoculated one. The agronomic growth and photosynthetic characteristics such as net photosynthetic rate (PN), stomatal conductance (gs), intercellular CO₂ (Ci) were also found to be significantly increased with increasing concentration of Azotobacter vinelandii inoculation. The activities of antioxidant enzymes were significantly higher (35%) in rice plants inoculated with Azotobacter vinelandii as compared with un-inoculated rice plant. All these positive effects of Azotobacter vinelandii help rice to survive from the toxic effect of Cr.

Advances and Applications of Water Phytoremediation: A Potential Biotechnological Approach for the Treatment of Heavy Metals from Contaminated Water

Potable and good-quality drinking water availability is a serious global concern, since several pollution sources significantly contribute to low water quality. Amongst these pollution sources, several are releasing an array of hazardous agents into various environmental and water matrices. Unfortunately, there are not very many ecologically friendly systems available to treat the contaminated environment exclusively. Consequently, heavy metal water contamination leads to many diseases in humans, such as cardiopulmonary diseases and cytotoxicity, among others. To solve this problem, there are a plethora of emerging technologies that play an important role in defining treatment strategies. Phytoremediation, the usage of plants to remove contaminants, is a technology that has been widely used to remediate pollution in soils, with particular reference to toxic elements. Thus, hydroponic systems coupled with bioremediation for the removal of water contaminants have shown great relevance. In this review, we addressed several studies that support the development of phytoremediation systems in water. We cover the importance of applied science and environmental engineering to generate sustainable strategies to improve water quality. In this context, the phytoremediation capabilities of different plant species and possible obstacles that phytoremediation systems may encounter are discussed with suitable examples by comparing different mechanistic processes. According to the presented data, there are a wide range of plant species with water phytoremediation potential that need to be studied from a multidisciplinary perspective to make water phytoremediation a viable method.

Recent advances in biochar engineering for soil contaminated with complex chemical mixtures: Remediation strategies and future perspectives

Heavy metal/metalloids (HMs) and polycyclic aromatic hydrocarbons (PAHs) in soil have caused serious environmental problems, compromised agriculture quality, and have detrimental effects on all forms of life including humans. There is a need to develop appropriate and effective remediation methods to resolve combined contaminated problems. Although conventional technologies exist to tackle contaminated soils, application of biochar as an effective renewable adsorbent for enhanced bioremediation is considered by many scientific researchers as a promising strategy to mitigate HM/PAH co-contaminated soils. This review aims to: (i) provide an overview of biochar preparation and its application, and (ii) critically discuss and examine the prospects of (bio)engineered biochar for enhancing HMs/PAHs co-remediation efficacy by reducing their mobility and bioavailability. The adsorption effectiveness of a biochar largely depends on the type of biomass material, carbonisation method and pyrolysis conditions. Biochar induced soil immobilise and remove metal ions via various mechanisms including electrostatic attractions, ion exchange, complexation and precipitation. PAHs remediation mechanisms are achieved via pore filling, hydrophobic effect, electrostatic attraction, hydrogen bond and partitioning. During last decade, biochar engineering (modification) via biological and chemical approaches to enhance contaminant removal efficiency has garnered greater interests. Hence, the development and application of (bio)engineered biochars in risk management, contaminant management associated with HM/PAH co-contaminated soil. In terms of (bio)engineered biochar, we review the prospects of amalgamating biochar with hydrogel, digestate and bioaugmentation to produce biochar composites.

Synergistic and concentration-dependent toxicity of multiple heavy metals compared with single heavy metals in Conocarpus lancifolius

While heavy metals (HMs) naturally occur in soil, anthropogenic activities can increase the level of these toxic elements. Conocarpus lancifolius Engl. (Combretaceae) was investigated as a potential phytoremediator of soils contaminated with HM containing crude oil. This study assessed the potential of C. lancifolius (CL), a locally available plant species in Kuwait, for resolving local issues of the HM-contaminated soils. The absorption, accumulation, and distribution of three toxic HMs (Cd, Ni, and Pb) and essential metals (Fe, Mg, and metalloid Se) were examined, and their role in plant toxicity and tolerance was evaluated. Conocarpus lancifolius plants were exposed to two different concentrations of single and mixed HMs for 30 days. The accumulation of HMs was determined in the roots, leaves, stems, and the soil using ICP/MS. Biomass, soil pH, proline and protein content, and bioaccumulation, extraction, and translocation factors were measured. The bioaccumulation, extraction, and transcription factors were all >1, indicating CC is a hyperaccumulator of HM. The HM accumulation in CL was concentration-dependent and depended on whether the plants were exposed to individual or mixed HMs. The C.C leaves, stems, and roots showed a significant accumulation of antioxidant constituents, such as proline, protein, Fe, Mg, and Se. There was an insignificant increase in the soil pH, and a decrease in plant biomass and a significant increase in protein, and osmoprotective-proline as a result of the interaction of mixed heavy metals that are more toxic than single heavy metals. This study indicates that C. lancifolius is a good candidate for phytoremediation of multiple HM-contaminated soils. Further studies to establish the phyto-physiological effect of multiple heavy metals are warranted.

Chromium Stress in Plants: Toxicity, Tolerance and Phytoremediation

Extensive industrial activities resulted in an increase in chromium (Cr) contamination in the environment. The toxicity of Cr severely affects plant growth and development. Cr is also recognized as a human carcinogen that enters the human body via inhalation or by consuming Cr-contaminated food products. Taking consideration of Cr enrichment in the environment and its toxic effects, US Environmental Protection Agency and Agency for Toxic Substances and Disease Registry listed Cr as a priority pollutant. In nature, Cr exists in various valence states, including Cr(III) and Cr(VI). Cr(VI) is the most toxic and persistent form in soil. Plants uptake Cr through various transporters such as phosphate and sulfate transporters. Cr exerts its effect by generating reactive oxygen species (ROS) and hampering various metabolic and physiological pathways. Studies on genetic and transcriptional regulation of plants have shown the various detoxification genes get up-regulated and confer tolerance in plants under Cr stress. In recent years, the ability of the plant to withstand Cr toxicity by accumulating Cr inside the plant has been recognized as one of the promising bioremediation methods for the Cr contaminated region. This review summarized the Cr occurrence and toxicity in plants, role of detoxification genes in Cr stress response, and various plants utilized for phytoremediation in Cr-contaminated regions.

Comparative growth analysis of okra (Abelmoschus esculentus) in the presence of PGPR and press mud in chromium contaminated soil

Chromium is a toxic heavy metal. Plants, animals and human metabolic processes are disturbed due to higher levels of chromium. PGPR are involved in seed germination, growth improvement, metabolic process and in most of the physiological processes of plants. Press mud in soil provides substrate to the microbes. PGPR can convert the more toxic form of Cr (VI) into less toxic form Cr (III). This study was conducted to find out the reduction potential of pre-isolated rhizobacteria and their role in strengthening of plant growth and physiological attributes. Soil collected from the research area was spiked with 20 mg kg-1 of Cr (VI) by using potassium dichromate (K₂Cr₂O₇) salt before sowing. Results revealed that Cr (VI) significantly suppressed the shoot length, root length and photosynthetic rate of okra up to 19, 37 and 31%, respectively. However, inoculation decreases the uptake of Cr (VI) in root and shoot up to 37 and 31% and by press mud 33 and 20%, respectively. Combined application of inoculation and press mud significantly recovered the negative impact of chromium and plant growth was almost at par compared with contaminated treatment without inoculation.

Absorption and interaction mechanisms of uranium & cadmium in purple sweet potato(Ipomoea batatas L.)

The purpose of this study was to reveal the absorption and interaction mechanisms of uranium (U) & cadmium (Cd) in corps. Purple sweet potato (Ipomoea batatas L.) was selected as the experimental material. The absorption behavior of U and Cd in this crop and the effects on mineral nutrition were analyzed in a pot experiment. The interactions between U and Cd in purple sweet potato were analyzed using UPLC-MS metabolome analysis. The pot experiment confirmed that the root tuber of the purple sweet potato had accumulated U (1.68-5.16 mg kg^-1) and Cd (0.78-2.02 mg kg^-1) and would pose a health risk if consumed. Both U and Cd significantly interfered with the mineral nutrient of the roots. Metabolomics revealed that a total of 4865 metabolites were identified in roots. 643 (419 up; 224 down), 526 (332 up; 194 down) and 634 (428 up; 214 down) different metabolites (DEMs) were identified in the U, Cd, and U + Cd exposure groups. Metabolic pathway analysis showed that U and Cd induced the expression of plant hormones (the first messengers) and cyclic nucleotides (cAMP and cGMP, second messengers) in cells and regulated the primary/secondary metabolism of roots to induce resistance to U and Cd toxicity.

Physiological, ultrastructural, biochemical, and molecular responses of glandless cotton to hexavalent chromium (Cr6+) exposure

Glandless cotton can be grown to obtain cotton seeds free of toxic gossypol for use as both food and feed. However, they are not grown normally due to their lesser productivity and higher susceptibility to biotic stress. Great attention has been paid to biotic stresses rather than abiotic stresses on glandless cotton. Chromium (Cr) is a common pollutant of soil and considered a serious threat to plants due to its adverse effects on different functions. Although numerous studies are available on the toxicity of Cr⁶⁺ in various plants. However, its adverse effects and mechanism of toxicity in glandless cotton can seldom be found in the literature. This study examined the Cr⁶⁺ effect on glandless cotton in comparison to glanded cotton. Four pairs of glanded and glandless cotton near-isogenic lines (NILs) were exposed to different doses (0, 10, 50, and 100 μM/L) of Cr⁶⁺ for seven days, and biochemical, physiological, molecular, and ultrastructure changes were observed, which were significantly affected by Cr⁶⁺ at high concentrations in all NILs. The effect of Cr⁶⁺ on ionic contents shows the same trend in glanded and glandless NILs except for manganese (Mn²⁺) that show inhibition in glandless (ZMS-12w and Coker-312w) and enhance in the glanded NIL (ZMS-17). The gene expression of superoxide dismutase (SOD) and peroxidase (POD) revealed similar trends as enzyme activities in glandless NILs. The principal component analysis (PCA) and Agglomerative hierarchical clustering (AHC) results of all NILs from morpho-physiological traits, cluster ZMS-16, and ZMS-17 into Cr⁶⁺ sensitive group. While the glandless NILs have the potential to cope with the Cr toxicity by increasing the antioxidant enzyme activity and their gene expression. This study also revealed that Cr⁶⁺ tolerance in cotton is genotypic and has an independent mechanism in the root that not related to low gossypol.

Sulfur application alleviates chromium stress in maize and wheat

This study aimed to examine the influence of increasing doses of chromium (Cr) (26, 39, and 52 mg kg−1 soil) and elemental sulfur (S) (60 mg kg−1 soil) on growth, yield, and mineral nutrition in wheat and maize. Macro- and micronutrients and Cr concentrations were determined in the aboveground parts of plants. All examined doses of Cr caused a marked decrease in the fresh and dry weight of maize. Wheat was more tolerant than maize, and lower Cr doses caused a small but statistically significant increase in the total yield. Wheat accumulated more than twofold Cr than maize, and the concentrations increased with higher Cr concentrations in the soil. The application of S significantly improved the total biomass production and lowered the Cr content in both plants. Cr changed the mineral nutrition in both cereals, but the pattern of changes observed was not the same. Applying S alleviated some adverse effects caused by the Cr. Hence, it is concluded that the application of elemental S may be an effective strategy to reduce adverse effects in plants grown on soil contaminated by heavy metals, especially Cr.

Accumulation of Airborne Toxic Elements and Photosynthetic Performance of Lolium multiflorum L. Leaves

In this study, we aimed to investigate the accumulation of airborne trace elements in Lolium multiflorum leaves concerning photosynthetic activity parameters. Five sites for four 28-day series of plant exposition were selected. The concentration of trace elements in leaves was measured after each series, while photosynthetic activity parameters were measured three times during each series. Net photosynthesis rate (PN) and stomatal conductance (gs) were mostly negatively associated with all analyzed trace elements, unlike to CO2 concentrations (Ci). Arsenic was found with opposite trend in two exposure series. The high accumulation of Cd and Pb in plants recorded at two sites was mostly related to lowest PN and gs. Similar tendency for PN was found at sites and series with the highest Cr and Ni content in plants. L. multiflorum revealed a medium-level accumulation of trace elements and a low tolerance of the photosynthetic process to the presence of trace elements in ambient air.

Effect of panchakavya (organic formulation) on phytoremediation of lead and zinc using Zea mays

Chelate-assisted phytoextraction is proposed to be an effective approach for the removal of metals from contaminated soil. Organic chelators can improve this biological technique by increasing metal solubility. The aim of this study was to investigate the possibility of improving the phytoextraction of lead (Pb) and zinc (Zn) by the application of panchakavya, a traditional Indian organic formulation. Panchakavya was prepared by fermentation process in open environment using cow dunk, cow ghee, cow urine, cow milk, cow curd, tender coconut water, crude jaggery, and mashed bananas. Soil metal fraction studies indicate that the panchakavya treatment decreased (73%) water-soluble fraction of Pb. Plant growth analysis indicated the application of panchakavya to increase Zea mays fresh root weight, shoot biomass and superoxide dismutase level in Zn contaminated soil. Similarly, a significant increase in the Zn accumulation (12% in shoots and 9% in roots) was observed in panchakavya treated plants. However, when compared to control plants, panchakavya treatment significantly decreased (32% in shoots and 37% in roots) Pb accumulation in Z. mays. Obtained results point out that panchakavya could potentially increase the phytoremediation of Zn in Z. mays.

Chromium Morpho-Phytotoxicity

Chromium (Cr) is considered as one of the chronic pollutants that cause damage to all living forms, including plants. Various industries release an excessive amount of Cr into the environment. The increasing accumulation of Cr in agricultural land causes a significant decrease in the yield and quality of economically important crops. The Cr-induced biochemical, molecule, cytotoxic, genotoxic, and hormonal impairments cause the inhibition of plant growth and development. In the current study, we reviewed Cr morpho-phytotoxicity related scientific reports published between 2009 to 2019. We mainly focused on the Cr-induced inhibition of seed germination and total biomass production. Furthermore, Cr-mediated reduction in the root, branches, and leave growth and development were separately discussed. The Cr uptake mechanism and interference with the macro and micro-nutrient uptake were also discussed and visualized via a functional model. Moreover, a comprehensive functional model has been presented for the Cr release from the industries, its accumulation in the agricultural land, and ultimate morpho-phytotoxicity. It is concluded that Cr-reduces plant growth and development via its excess accumulation in the plant different parts and/or disruption of nutrient uptake.

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.

Chromium-Induced Reactive Oxygen Species Accumulation by Altering the Enzymatic Antioxidant System and Associated Cytotoxic, Genotoxic, Ultrastructural, and Photosynthetic Changes in Plants

Chromium (Cr) is one of the top seven toxic heavy metals, being ranked 21st among the abundantly found metals in the earth’s crust. A huge amount of Cr releases from various industries and Cr mines, which is accumulating in the agricultural land, is significantly reducing the crop development, growth, and yield. Chromium mediates phytotoxicity either by direct interaction with different plant parts and metabolic pathways or it generates internal stress by inducing the accumulation of reactive oxygen species (ROS). Thus, the role of Cr-induced ROS in the phytotoxicity is very important. In the current study, we reviewed the most recent publications regarding Cr-induced ROS, Cr-induced alteration in the enzymatic antioxidant system, Cr-induced lipid peroxidation and cell membrane damage, Cr-induced DNA damage and genotoxicity, Cr-induced ultrastructural changes in cell and subcellular level, and Cr-induced alterations in photosynthesis and photosynthetic apparatus. Taken together, we conclude that Cr-induced ROS and the suppression of the enzymatic antioxidant system actually mediate Cr-induced cytotoxic, genotoxic, ultrastructural, and photosynthetic changes in plants.

Chromium Bioaccumulation and Its Impacts on Plants: An Overview

Chromium (Cr) is an element naturally occurring in rocky soils and volcanic dust. It has been classified as a carcinogen agent according to the International Agency for Research on Cancer. Therefore, this metal needs an accurate understanding and thorough investigation in soil-plant systems. Due to its high solubility, Cr (VI) is regarded as a hazardous ion, which contaminates groundwater and can be transferred through the food chain. Cr also negatively impacts the growth of plants by impairing their essential metabolic processes. The toxic effects of Cr are correlated with the generation of reactive oxygen species (ROS), which cause oxidative stress in plants. The current review summarizes the understanding of Cr toxicity in plants via discussing the possible mechanisms involved in its uptake, translocation and sub-cellular distribution, along with its interference with the other plant metabolic processes such as chlorophyll biosynthesis, photosynthesis and plant defensive system.

Ecological risk assessment of mercury and chromium in greenhouse soils

Very little information is available about Hg and Cr evolution in greenhouse soils. This paper presents the results of determining Hg and Cr in greenhouse soils in a semi-arid region in the southern Iberian Peninsula (Almería, Spain), and assessing the enrichment level and the Potential Ecological Risk Index (PERI) according to crop age. Hakanson's approach was used to evaluate the PERI. To investigate the behaviour of Hg and Cr in greenhouse soils over time, samples were grouped into values in soils for blocks according to crop age: 0 years, 5-10 years, 10-20 years, more than 20 years. The results showed that 74% of GS exceeded the obtained background level (37.1 μg kg^-1) for Hg, with 43% (48.9 mg kg^-1) for Cr. Temporal patterns indicated that these elements are accumulating in greenhouse soils and this trend was very significant for Hg. After more than 20 intensive crop-farming years, concentrations and the PERI had clearly increased. Although the ecological risk was moderate, our observations suggest that the farming practices performed in the last 35 years have allowed these metals to accumulate. In fact, the 15% of the studied soils presented a considerable potential risk and were the soils that had been used longer.

Phytoremediation potential of Miscanthus sinensis for mercury-polluted sites and its impacts on soil microbial community

Phytoremediation potential of Miscanthus sinensis and its impacts on soil microbial community and nutrients were evaluated by pot experiment at soil mercury concentration from 1.48 to 706 mg kg^-1. The changes in biomass yield in dry mass, chlorophyll content, and SOD activity indicated Miscanthus sinensis was tolerant to higher levels of soil mercury exposure, and could grow even if at soil mercury up to 706 mg kg^-1. Mercury bioconcentration and translocation factors were close to or greater than 1 when exposed to soil mercury up to 183 mg kg^-1, demonstrating Miscanthus sinensis a potential phytoremediator for mercury-polluted soils. Miscanthus sinensis planting could significantly improve the diversity and abundance of soil microbial community, but might cause potential loss of soil nitrogen and phosphorus in the early and middle of its growth. In a word, the study indicated Miscanthus sinensis was a promising energy crop linking biofuel production and phytoremediation of mercury-contaminated sites.