Cadmium minimization in wheat: A critical review

Cadmium Concentrations in New Zealand Wheat: Effect of Cultivar Type, Soil Properties, and Crop Management

Wheat ( L.) grain is a contributing source of dietary Cd in New Zealand, but despite this, there is a dearth of information on Cd concentrations in wheat and the factors that affect uptake. We measured Cd concentrations in 12 wheat cultivars grown in field sites across New Zealand and also assessed the soil, plant, and crop factors that have been reported to affect Cd uptake. We found there was a wide range in grain Cd concentrations (0.004 to 0.205 mg kg fresh weight [FW]). The overall mean concentration (0.066 mg kg FW) was below the maximum limit (ML) of 0.1 mg kg FW. Only 7% of grain samples across seven sites exceeded the ML. There were significant ( < 0.05) differences (2.5-fold) in Cd concentrations in wheat grain between cultivars. No strong significant relationships were found between soil properties and Cd concentrations in grain. Further, management factors affecting grain Cd concentrations were inconsistent. Given the lack of relationships between soils and management practices, the most effective way of limiting Cd uptake in wheat grain at sites where Cd exceeds MLs may be the use of low-Cd-accumulating cultivars. Further studies are required to explore specifically the influence of irrigation and effects of type and rate of fertilizer on Cd uptake in wheat, as well as to better understand the mechanisms of Cd uptake in wheat.

Alleviation of cadmium accumulation in maize (Zea mays L.) by foliar spray of zinc oxide nanoparticles and biochar to contaminated soil

Due to the increase in area of cadmium (Cd)-contaminated soils worldwide, effective measures are necessary to minimize the Cd accumulation in cereals including maize (Zea mays L.) plant. A study was therefore performed to explore the effectiveness of foliar spray of zinc oxide (ZnO) nanoparticle (NPs) alone (0, 50, 75, 100 mg/L) or combined with soil application of biochar (1.0% w/w) on biomass, antioxidant enzyme activity and Cd concentrations in maize plants grown on a Cd-contaminated soil. The results depicted that ZnO NPs alone or in combination with biochar improved the height of maize plants, number of leaves, shoot and roots dry biomass, chlorophyll concentrations and gas exchange attributes. All the amendments reduced the electrolyte leakage, malondialdehyde, and hydrogen peroxide contents while improved the activities of antioxidant enzymes in leaf and roots of maize over the control. The application of 50, 75 and 100 mg/L ZnO NPs reduced the Cd contents in shoots by about 12%, 23, and 61%, and in roots by 18%, 33%, and 53%, respectively, over the control. The Cd concentrations in shoot decreased by 15%, 28%, and 68% and in roots by 14%, 35, and 55% after biochar combined with foliar spray of 50, 75 and 100 mg/L ZnO NPs, respectively. All the amendments improved the Zn concentrations in maize shoots and roots whereas reduced the soil bioavailable Cd. Overall, biochar combined with foliar spray of ZnO NPs could be recommended for safely growing the crops on Cd-contaminated soils.

Protective effect of different forms of nitrogen application on cadmium-induced toxicity in wheat seedlings

Therefore, applying NH₄⁺-N tp PHW-SA caused greater tolerance to Cd toxicity by higher biomass production, photosynthetic capacity, Ca and Cu accumulation, better root development and lower translocation factor of Cd as well as Cd concetration in organelle fraction. The Cd stress inhibited the growth performance of wheat seedlings, the mineral nutrient accumulation, and nitrogen uptake and distribution, and different forms of nitrogen have different protective effects on wheat. In PHW-SA, ANCd treatment caused lower reduction in biomass accumulation, photosynthesis, isotope stable N content, Ca and Cu accumulation, root development inhibition, tissue Cd concentration, and transfer factor, which even led to lower concentrations of Cd in F_co than those in F_cw and F_s in comparison with NNCd treatment. On the other hand, the converse growth performance was recorded in J-11 under ANCd treatment. Meanwhile, the nitrogen absorption preference in PHW-SA was altered along with the enhanced absorption efficiency of nitrogen. Therefore, applying NH₄⁺-N to PHW-SA caused greater tolerance to Cd toxicity by higher biomass production, photosynthetic capacity, Ca and Cu accumulation, better root development and lower translocation factor of Cd as well as Cd concetration in organelle fraction..

Effects of organic-inorganic amendments on the cadmium fraction in soil and its accumulation in rice (Oryza sativa L.)

Cadmium (Cd) stress is a serious concern in agricultural soils worldwide, and increasing accumulation and subsequent transfer to humans via the food chain can have potentially harmful effects. In this study, field experiments were conducted to examine the uptake and translocation of Cd in rice, changes in the soil Cd speciation, and the subsequent effect on Cd accumulation in rice under combined organic (farmyard manure and crop straw) and inorganic (sepiolite, lime, and calcium-magnesium phosphate) soil amendments. The results showed that farmyard manure combined with sepiolite or lime and straw combined with lime or calcium-magnesium phosphate reduced the Cd translocation from the rice roots to the straw and the grains, significantly decreasing the Cd accumulation in brown rice. In addition, straw combined with sepiolite, lime, or calcium-magnesium phosphate reduced the Cd accumulation in brown rice but increased the Cd translocation from the roots to the straw by 37.8-279.3% compared with the control. Organic-inorganic amendments also decreased the soil exchangeable Cd and increased the organic-bound Cd by more than 40%. Fe-Mn oxide-bound Cd also increased but varied with growth. Cd accumulation in brown rice showed a significant positive relationship with soil exchangeable Cd at 90 days after transplantation, while at 30 days, the increase in Fe-Mn oxide- and organic-bound Cd was found to be the key factor in reducing the Cd accumulation in rice. These findings suggest that straw (under rice-rape rotation) and farmyard manure (under rice-wheat rotation) combined with sepiolite or lime are widely applicable as agronomic control techniques aimed at lowering Cd pollution.

Effect of salinity on cadmium tolerance, ionic homeostasis and oxidative stress responses in conocarpus exposed to cadmium stress: Implications for phytoremediation

Contamination of soil with salinity and Cd negatively affects growth and productivity of plants. The proposed study has been planned to explore the effects of salinity on Cd uptake, tolerance and phytoremediation potential of conocarpus (Conocarpus erectus L.). One-month-old uniform plants of conocarpus were exposed to 0, 8.9, 44.5, 89 and 178 µM Cd alone or in combination with 0, 100 and 200 mM NaCl in Hoagland's nutrient solution. Results revealed that shoot and root biomasses, leaf water content and pigment content decreased more in response to combination of Cd and salinity compared to Cd alone. The Na⁺ and Cl^- concentrations in shoot and root were not affected by Cd alone, but increased in Cd + salinity treatments. The K⁺ concentration decreased by Cd alone as well as Cd combination with salinity. Plant Cd uptake increased in the presence of salinity but its translocation from root to shoot remained unaffected. Exposure of plants to Cd alone and Cd + salinity caused oxidative stress via overproduction of H₂O₂ and inducing lipid peroxidation. The activities of antioxidant enzymes such as SOD, CAT, POD and APX increased to mitigate this oxidative stress. It is concluded that the tolerance of conocarpus against Cd stress is decreased in the presence of salinity due to increased uptake of toxic ions and intensification of oxidative stress. Moreover, the Cd uptake behavior of this tree indicates its suitability for phytostabilization of Cd contaminated saline and non-saline soils.

Effect of elemental sulfur and gypsum application on the bioavailability and redistribution of cadmium during rice growth

Recently, concerns over heavy metal contamination of soil have grown. The application of sulfur has been recommended to enhance crop productivity and increase soil cadmium (Cd) immobilization. In this study, a pool experiment was conducted to investigate the effects of two sulfur sources and multiple treatment levels on rice growth and Cd accumulation. The two sulfur forms were elemental sulfur (S⁰) and gypsum, both of which were applied at 0, 0.15, and 0.30 g S kg^-1 soil, for a total of five treatments. The results showed that both S⁰ and gypsum significantly increased rice biomass compared to the control (CK), and rice yield was increased 2.8-4.8 folds. The effect size was greater for gypsum than S⁰. The application of S⁰ reduced the rice grain Cd concentration from 0.61 mg kg^-1 (CK) to 0.41-0.46 mg kg^-1, while gypsum reduced the Cd concentration to 0.24-0.43 mg kg^-1. The lower gypsum application level achieved the greatest reduction in rice grain Cd accumulation. This study further demonstrated that the application of S⁰ and gypsum led to a decrease in the labile Cd percentage and an increase in the stable Cd percentage. In bulk soil, iron and manganese oxide-bound Cd increased by 6.4-7.3% and 0.7-2.0% for the S⁰ and gypsum treatments, respectively. In the rhizosphere, residual Cd increased by >0.6%. Furthermore, this study found that sulfur application reduced Cd transfer from root to shoot, and significantly decreased rice grain Cd accumulation. These findings indicate that sulfur application to paddy soils can promote rice productivity and effectively remediate soil Cd contamination, with a greater effect by gypsum than S⁰.

A critical review on the effects of zinc at toxic levels of cadmium in plants

Increasing cadmium (Cd) pollution in agricultural soils has raised serious concerns worldwide. Several exogenous substances can be used to mitigate the toxic effects of Cd in plants. Zinc (Zn) is one of the essential plant micronutrients and is involved in several physiological functions in plants. Zn may alleviate Cd toxicity in plants owing to the chemical similarity of Zn with Cd. Published reports demonstrated that Zn can alleviate toxic effects of Cd in plants by increasing plant growth, regulating Cd uptake, increasing photosynthesis, and reducing oxidative stress. Literature demonstrated that the role of Zn on Cd accumulation by plants is very controversial and depends upon several factors including concentrations of Cd and Zn in the medium, exposure duration, plant species and genotypes, and growth conditions. This review highlights the role of Zn in reducing Cd toxicity in plants and provides new insight that proper level of Zn in plants may enhance plant resistance to excess Cd.

Seed priming with silicon nanoparticles improved the biomass and yield while reduced the oxidative stress and cadmium concentration in wheat grains

Cadmium (Cd) is among the non-essential elements for the growth of crops while silicon (Si) is a beneficial element for plant growth. There is little evidence regarding the use of silicon nanoparticles (Si NPs) on the reduction of Cd accumulation in crops especially wheat. The present study determined the impact of seed priming with Si NPs on Cd-induced responses in wheat in terms of growth, yield, photosynthesis, oxidative stress, and Si and Cd accumulation in wheat. Seed priming was done by different levels of Si NPs (0, 300, 600, 900, 1200 mg/L) for 24 h by providing continuous aeration. Afterwards, seeds were sown in soil contaminated with Cd. The results depicted that Si NPs positively affected the wheat growth and chlorophyll contents over the control. The Si NPs diminished the oxidative stress and positively affected the antioxidant enzyme activity. The Si NPs decreased the Cd concentrations in wheat, especially in grains, and increased the Si concentrations in plants. The Si NPs reduced the Cd contents by 10-52% in shoot, by 11-60% in roots, and by 12-75% in grains as compared with respective controls. The study suggested that the use of Si NPs may be a tool for reducing the Cd toxicity in wheat and declining its concentration in grains. Thus, Si NPs application by seed priming method might be helpful in increasing plants biomass and yield while reducing the oxidative stress and Cd uptake in wheat grains.

Silicon decreases cadmium concentrations by modulating root endodermal suberin development in wheat plants

Silicon (Si) can alleviate cadmium (Cd) toxicity in many plants, but mechanisms underlying this beneficial effect are still lacking. In this study, the roles of Si in time-dependent apoplastic and symplastic Cd absorption by roots of wheat plants were investigated. Results showed that, during short-term Cd exposure, the symplastic pathway of Cd in roots was not significantly affected by Si. Cell wall properties and cell wall-bound Cd regarding the apoplastic pathway were unaffected by Si either. Nevertheless, Cd concentrations in the apoplastic fluid of roots were decreased by Si. The reason could be that Si delayed endodermal suberization of roots resulting in promoted apoplastic Cd translocation to shoots, thus decreasing Cd in the apoplastic fluid of roots after short-term Cd stress. By contrast, after long-term Cd stress, cell wall properties and the expression of genes related to Cd influx and transport were unaffected. Intriguingly, Si up-regulated the expression of the Cd efflux-related gene TaTM20 and repressed apoplastic Cd translocation to shoots, which might contribute to decrease of Cd after long-term Cd exposure. Taken together, these results indicate that Si-dependent decrease in root Cd concentrations during short-term Cd exposure helps plants to mitigate Cd toxicity in the long-term.

Composting of municipal solid waste by different methods improved the growth of vegetables and reduced the health risks of cadmium and lead

Reutilization of putrescible municipal solid wastes (MSW) in agriculture can provide valuable plant nutrients. However, it may pose serious noncarcinogenic health risks for a human when contaminants, especially the heavy metals in MSW, end up in plants through the waste-soil-plant continuum. This study examined the effects of composting methods viz. aerobically (AC), anaerobically (ANC), and aerobic-anaerobically (AANC) composted MSW material on (i) fertilizer value: vegetable yield, nitrogen (N) mineralization, and apparent N recovery (ANR); and (ii) associated health risks: selected heavy metal concentration, daily intake of metals (DIM), health risk index (HRI), hazard index (HI), and target hazard quotient (THQ) when applied to a loamy soil. All the aforementioned compost materials were incorporated into the sandy loam soil filled in pots and carrot and spinach were cultivated for 85 and 90 days, respectively. After soil application, between 51 and 56% of the applied organic N was mineralized from ANC material, while the values in case of AC and AANC were 26-31% and 34-40%, respectively. Consequently, dry matter yield and vegetable N uptake from composts were in the order ANC > AANC > AC (P < 0.05). Further, vegetable ANR was the highest from ANC (56 and 56%) than AANC (42 and 45%), and AC (30 and 33%) for spinach and carrot, respectively (P < 0.05). Interestingly, plant uptake of lead and cadmium was lowest from ANC as compared to AC or AANC (P < 0.05), irrespective of the vegetable type. Consequently, DIM, HRI, and THQ for these metals were substantially lower in the former as compared to the latter compost materials. Further, HI from ANC material was 50% lower over the unfertilized control indicating the absence of noncarcinogenic human health risks via vegetable intake. This all indicates that from viewpoint of sustainable waste recycling in agriculture, anaerobic composting is superior to the other composting methods.

Morpho-physiological traits, antioxidant capacity and phytoextraction of copper by ramie (Boehmeria nivea L.) grown as fodder in copper-contaminated soil

Ramie (Boehmeria nivea L.), the oldest fiber crop in China, can also be grown as fodder crop because of its huge biomass production. Moreover, it has the potential to colonize heavy metal-contaminated soils which showed the possibilities of phytoremediation using B. nivea. Therefore, the present study was conducted to investigate the potential of B. nivea for phytoextraction of copper (Cu)-contaminated soil. Moreover, the impact of different concentrations of Cu on growth and antioxidant enzymatic activity by B. nivea were also studied. For this purpose, a pot experiment was conducted to examine the growth, antioxidative response, and localization (distribution) of Cu in B. nivea plant under different Cu concentrations (0, 50, 100, 200, 300, and 400 mg kg^-1 soil). Results revealed that B. nivea tolerated up to 100 mg kg^-1 Cu concentration without a significant decrease in biomass, but further increase in Cu concentration from 200 to 400 mg kg^-1 exhibited a significant reduction in chlorophyll content, fresh and dry biomass, plant height, and number of leaves. It was further observed that B. nivea accumulated more Cu in roots (26 to 53 mg kg^-1), followed by the leaves (23 to 28 mg kg^-1) and stems (14 to 21 mg kg^-1), while the values for both bioaccumulation factor (BAF) and translocation factor (TF) at all treatments were less than 1. Moreover, activities of antioxidative enzymes (superoxide dismutase and peroxidase) were initially increased with the exposure of 50, 100, and 200 mg kg^-1 Cu, but decreased by further increasing the Cu concentration to 300 and 400 mg kg^-1 indicating the oxidative stress which is manifested by high malondialdehyde (MDA) and proline contents also. Thus, based on results, it can be concluded that B. nivea accumulated relatively low Cu contents in aboveground parts and could be grown as fodder crop for phytoremediation of Cu-contaminated sites.

Zinc and iron oxide nanoparticles improved the plant growth and reduced the oxidative stress and cadmium concentration in wheat

The effects of seed priming with zinc oxide (ZnO) and iron (Fe) nanoparticles (NPs) on the growth and cadmium (Cd) accumulation by wheat (Triticum aestivum) were investigated. Seeds of wheat were primed with different concentrations of either ZnO NPs (0, 25, 50, 75, and 100 mg L^-1) or Fe NPs (0, 5, 10, 15, and 20 mg L^-1) for 24 h by continuous aeration and then the seeds were sown in a soil which was contaminated with Cd due to long-term application of sewage water. Plants were grown till maturity under natural conditions with 60-70% moisture contents of total soil water holding capacity throughout the experiment. Plant height, spike length, and dry weights of shoots, roots, spikes, and grains were increased with NPs, in particular with the higher rates of NPs. The results depicted that NPs positively affected the photosynthesis of wheat as compared to the control. The NPs reduced the electrolyte leakage and superoxide dismutase and peroxidase activities in leaves of Cd-stressed wheat. The concentrations of Cd in roots, shoots, and grains were significantly decreased with NPs application. The Cd content in the grains was below the threshold level of Cd (0.2 mg kg^-1) for cereals when the seeds were treated with higher NPs treatments. The application of ZnO NPs increased the Zn concentrations and Fe NPs increased the Fe concentrations in roots, shoots, and grains. Overall, the NPs play a major role in the increase in biomass, nutrients and decrease in Cd toxicity in wheat.

A novel method for predicting cadmium concentration in rice grain using genetic algorithm and back-propagation neural network based on soil properties

Heavy metal pollution is a global ecological safety issue, especially in crops, where it directly threatens regional ecological security and human health. In this study, the back-propagation (BP) neural network optimized by the genetic algorithm (GA) was used to predict the concentration of cadmium (Cd) in rice grain based on influencing factors. As an intelligent information processing system, the GA-BP neural network could learn the laws of Cd movement in the soil-crop system through its own training and use the soil properties to predict the concentration of Cd in grain with high accuracy. The total soil Cd concentration, clay content, Ni concentration, cation exchange capacity (CEC), organic matter (OM), and pH have important impacts and interactions on Cd concentration in rice grain were selected as input factors of the prediction model based on Pearson's correlation analysis and GeoDetector. By using GA to optimize the initial weight, the prediction accuracy of the GA-BP neural network model was optimal compared with the BP neural network model and multiple regression analysis. Based on the Cd concentration predicted in grain by the model, human exposure and health risk can be assessed quickly, enabling measures to be taken in time to reduce the transfer of Cd from soil to the food chain.

Influence of metal-contamination on distribution in subcellular fractions of the earthworm (Metaphire californica) from Hunan Province, China

Earthworms have the ability to accumulate of heavy metals, however, there was few studies that addressed the metals in earthworm at subcellular levels in fields. The distributions of metals (Cd, Cu, Zn, and Pb) in subcellular fractions (cytosol, debris, and granules) of earthworm Metaphire californica were investigated. The relationship between soil metals and earthworms were analyzed to explain its high plasticity to inhabit in situ contaminated soil of Hunan Province, south China. The concentration of Cd in subcellular compartments showed the same pattern as Cu in the order of cytosol > debris > granules. The distribution of Zn and Pb in earthworms indicated a similar propensity for different subcellular fractions that ranked as granules > debris > cytosol for Zn, and granules > cytosol > debris for Pb. The internal metal concentrations in earthworms increased with the soil metals (p<0.05). Significant positive correlations were found between soil Cd and Cd concentrations in cytosol and debris (p<0.01). Moreover, the soil Pb concentration significantly influenced the Pb concentrations in cytosol and debris (p<0.01), similar to that of Cd. The soil Cu concentrations was only associated with the Cu in granules (p<0.05). Soil Zn concentrations correlated with the Zn concentrations in each subcellular fraction (p<0.05). Our results provide insights into the variations of metals partitioning in earthworms at subcellular levels and the relationships of soil metals, which could be one of the detoxification strategies to adapt the long-term contaminated environment.

Transcriptome analysis of Cd-treated switchgrass root revealed novel transcripts and the importance of HSF/HSP network in switchgrass Cd tolerance

Transcriptome analysis of Cd-treated switchgrass roots not only revealed novel switchgrass transcripts and gene structures but also highlighted the indispensable role of HSF/HSP network in switchgrass Cd tolerance. Switchgrass (Panicum virgatum L.), a C4 perennial tall grass, can be used for revegetation of Cd-contaminated soil. In the present study, a comparative transcriptome analysis of Cd-treated switchgrass roots was conducted. The result revealed a total of 462 novel transcripts and refined gene structures of 2337 transcripts. KEGG pathway and Gene Ontology analyses of the differentially expressed genes (DEGs) suggested that activation of redox homeostasis and oxidation-related metabolic processes were the primary response to Cd stress in switchgrass roots. In particular, 21 out of 23 differentially expressed shock transcription factor genes (HSFs), and 22 out of 23 differentially expressed heat shock protein genes (HSPs) had increased expression levels after Cd treatment. Furthermore, over-expressing one HSP-encoding gene in Arabidopsis significantly improved plant Cd tolerance. The result highlighted the activation of the redox homeostasis and the involvement of the HSF/HSP network in re-establishing normal protein conformation and thus cellular homeostasis in switchgrass upon Cd stress. These DEGs, especially those of the HSF/HSP network, could be used as candidate genes for further functional studies toward improved plant Cd tolerance in switchgrass and related species.

Zinc oxide nanoparticles alter the wheat physiological response and reduce the cadmium uptake by plants

An experiment was performed to explore the interactive impacts of zinc oxide nanoparticles (ZnO NPs) and cadmium (Cd) on growth, yield, antioxidant enzymes, Cd and zinc (Zn) concentrations in wheat (Triticum aestivum). The ZnO NPs were applied both in Cd-contaminated soil and foliar spray (in separate studies) on wheat at different intervals and plants were harvested after physiological maturity. Results depicted that ZnO NPs enhanced the growth, photosynthesis, and grain yield, whereas Cd and Zn concentrations decreased and increased respectively in wheat shoots, roots and grains. The Cd concentrations in the grains were decreased by 30-77%, and 16-78% with foliar and soil application of NPs as compared to the control, respectively. The ZnO NPs reduced the electrolyte leakage while increased SOD and POD activities in leaves of wheat. It can be concluded that ZnO NPs (levels used in the study) could effectively reduce the toxicity and concentration of Cd in wheat whereas increase the Zn concentration in wheat. Thus, ZnO NPs might be helpful in decreasing Cd and increasing Zn biofortification in cereals which might be effective to reduce the hidden hunger in humans owing the deficiency of Zn in cereals.

AsA-GSH Cycle and Antioxidant Enzymes Play Important Roles in Cd Tolerance of Wheat

Wheat (Triticum aestivum L.) has relatively high tolerance to cadmium (Cd), but the underlying mechanisms are poorly understood. Growth and physiological parameters of wheat exposed to different Cd concentrations (0, 0.5, 5 and 50 µM) were characterized. The fresh weight, leaf chlorophyll and carotenoid concentrations and photosynthesis parameters did not differ among Cd treatments, suggesting relatively high Cd tolerance in wheat. However, the soluble sugar concentrations increased with the increasing Cd concentration and the soluble protein concentrations decreased in both shoots and roots, suggesting that the Cd application promoted nitrogen metabolism over carbon metabolism. In addition, the higher concentrations of MDA, GSH and AsA and activities of antioxidant enzymes (SOD, POD, and CAT) were observed in leaves and roots in the Cd50 treatment. Our results reveal that wheat can tolerate Cd by enhancing the antioxidant enzymes activities and increasing the concentration of ascorbate and glutathione.

Cow manure-loaded biochar changes Cd fractionation and phytotoxicity potential for wheat in a natural acidic contaminated soil

The current study aims to investigate the implications of amending a soil contaminated with Cd with peanut residues biochar (BP) solely or in combination with cow manure (CMPB) at different rates on phytotoxicity of Cd for wheat plants and its distribution in a mine contaminated soil. Soil pH and EC increased progressively in soils amended with either PB or CMPB. Exchangeable Cd was decreased while its non-exchangeable fractions were increased. Dry weights of wheat straw, roots and grains increased when soils amended with either PB or CMPB, especially at the higher application rate. Such increases were correlated significantly with the extractable soil-Cd. Concentrations of Cd in roots were higher than those in straw; whereas, the concentrations in grains seemed to be the lowest. Generally, values of bio-concentration and translocation factors did not exceed "1" and decreased with application of either PB or CMPB. In conclusion, enriching biochar with cow manure is a recommended strategy to reduce Cd uptake and translocation to straw and seeds. Moreover, Concentrations of Cd did not exceed the permissible levels in grains when soils amended with the highest rate of CMPB.

Feasibility of nanoscale zero-valent iron to enhance the removal efficiencies of heavy metals from polluted soils by organic acids

Soil washing with natural chelators to remediate metal-contaminated soils has been gained attention by researchers. However, the abilities of the chelators to remediate the multiple metal polluted soils are less effective. This study employed zero-valent iron nanoparticle (nZVI) to enhance the removal efficiencies of citric (CA), tartaric (TA) and oxalic acids (OA), and evaluate their feasibility. Results showed that metal removal efficiencies increased with the increasing concentration of nZVI and soil-liquid ratio, decreased with the increasing solution pH. The kinetic simulation indicated that pseudo-first-order and pseudo-second-order models could be used for describing the washing processes. Additionally, metal removals were significantly improved by addition of nZVI (p < 0.05). The highest enhancements of soil Cd, Pb and Zn removals under solution pH of 4.0, soil-liquid ratio of 1:20 and washing time of 120 min reached 12.83% (OA- nZVI), 24.92% (CA-nZVI) and 11.64% (OA- nZVI) for mine soil, and 19.24% (TA- nZVI), 18.16% (CA-nZVI) and 8.93% (OA- nZVI) for farmland soil, respectively. After soil washing, the exchangeable forms and the environmental risks of residual metals were markedly diminished in soils. Therefore, the combinations of the organic acids and nZVI are the feasible practices to repair the soils contaminated by heavy metals.

Alleviation of cadmium (Cd) toxicity and minimizing its uptake in wheat (Triticum aestivum) by using organic carbon sources in Cd-spiked soil

Cadmium (Cd)-contamination of agricultural soils has been receiving attention worldwide due to its entry into food crops such as wheat (Triticum aestivum L.). Little is known regarding the use of organic carbon (OC) sources in alleviating Cd toxicity in cereals. The current experiment was aimed to study the effects of different OC sources on the Cd accumulation by wheat. A pot study was conducted to determine the effects of rice husk biochar (RHB), farmyard manure (FYM), and lignite (LT) either alone or in combination on crop growth, Cd bioavailability and health risk assessment. The results proved that the application of OC sources like RHB, FYM, and LT either alone or in combination were highly effective in enhancing the wheat growth and yield as well as in minimizing the phyto-available fraction of Cd and its transfer to edible tissue of wheat. The RHB was the most efficient source in enhancing the plant growth and reducing the Cd concentration in wheat tissues. RHB increased grain yield by 91% and decreased Cd concentration in shoot, roots, grains, and bioavailable fraction of Cd by 67, 69, 62.5, and 74% than control, respectively. The RHB reduced the daily Cd uptake and health risk index in adults in comparison to control. Overall, where un-amended soil resulted in diminished plant productivity, the application of other OC sources also significantly proved their potential to enhance the dry weight and grain yield, suggesting that these OC sources may be used aiming to minimize the Cd concentration in crops. However, there is still a need to explore the potential of different OC sources in combination with other frequently available amendments for their large scale implementation in metal-contaminated soils.

Metal(loid)-resistant bacteria reduce wheat Cd and As uptake in metal(loid)-contaminated soil

This study characterized the effect of the metal(loid)-resistant bacteria Ralstonia eutropha Q2-8 and Exiguobacterium aurantiacum Q3-11 on Cd and As accumulation in wheat grown in Cd- and As-polluted soils (1 mg kg^-1 of Cd + 40 mg kg^-1 of As and 2 mg kg^-1 of Cd + 60 mg kg^-1 of As). The influence of strains Q2-8 and Q3-11 on water-soluble Cd and As and NH₄⁺concentration and pH in the soil filtrate were also analyzed. Inoculation with these strains significantly reduced wheat plant Cd (12-32%) and As (9-29%) uptake and available Cd (15-28%) and As (22-38%) contents in rhizosphere soils compared to the controls. Furthermore, these strains significantly increased the relative abundances of the arsM bacterial As metabolism gene and of Fe- and Mn-oxidizing Leptothrix species in rhizosphere soils. Notably, these strains significantly reduced water-soluble Cd and As concentrations and increased pH and NH₄⁺ concentration in the soil filtrate. These results suggest that these strains increased soil pH and the abundance of genes possibly involved in metal(loid) unavailability, resulting in reduced wheat Cd and As accumulation and highlight the possibility of using bacteria for in situ remediation and safe production of wheat or other food crops in metal(loid)-polluted soils.

Effect of biochar on alleviation of cadmium toxicity in wheat (Triticum aestivum L.) grown on Cd-contaminated saline soil

Soil degradation by salinity and accumulation of trace elements such as cadmium (Cd) in the soils are expected to become one of the most critical issues hindering sustainable production and feeding the increasing population. Biochar (BC) has been known to protect the plants against soil salinity and heavy metal stress. A soil culture study was performed to evaluate the effect of BC on wheat (Triticum aestivum L.) growth, biomass, and reducing Cd and sodium (Na) uptake grown in Cd-contaminated saline soil under ambient conditions. Soil salinity decreased the plant growth, biomass, grain yield, chlorophyll contents, and gas exchange parameters and caused oxidative stress in plants compared with Cd stress alone. Salt stress increased Cd and Na uptake and reduced the potassium (K) and zinc (Zn) uptake by plants. AB-DTPA-extractable Cd and soil electrical conductivity (ECe) increased under salt stress compared to the soil without NaCl stress. Biochar application improved the plant growth and reduced the Cd and Na uptake except in plants treated with higher BC and salt stress (5.0% BC + 50 mM NaCl). Biochar application reduced the oxidative stress in plants and modified the antioxidant enzyme activities, and reduced the bioavailable Cd under salt stress. The positive effects of BC under lower salt stress while the negative effects of BC under higher BC and salt levels indicated that BC doses should be used with great care in higher soil salinity levels simultaneously contaminated with Cd to avoid the negative effects of BC on growth and metal uptake.

Cadmium phytoremediation potential of Brassica crop species: A review

Cadmium (Cd) is a highly toxic metal released into the environment through anthropogenic activities. Phytoremediation is a green technology used for the stabilization or remediation of Cd-contaminated soils. Brassica crop species can produce high biomass under a range of climatic and growing conditions, allowing for considerable uptake and accumulation of Cd, depending on species. These crop species can tolerate Cd stress via different mechanisms, including the stimulation of the antioxidant defense system, chelation, compartmentation of Cd into metabolically inactive parts, and accumulation of total amino-acids and osmoprotectants. A higher Cd-stress level, however, overcomes the defense system and may cause oxidative stress in Brassica species due to overproduction of reactive oxygen species and lipid peroxidation. Therefore, numerous approaches have been followed to decrease Cd toxicity in Brassica species, including selection of Cd-tolerant cultivars, the use of inorganic and organic amendments, exogenous application of soil organisms, and employment of plant-growth regulators. Furthermore, the coupling of genetic engineering with cropping may also help to alleviate Cd toxicity in Brassica species. However, several field studies demonstrated contrasting results. This review suggests that the combination of Cd-tolerant Brassica cultivars and the application of soil amendments, along with proper agricultural practices, may be the most efficient means of the soil Cd phytoattenuation. Breeding and selection of Cd-tolerant species, as well as species with higher biomass production, might be needed in the future when aiming to use Brassica species for phytoremediation.

Interactive effect of 24-epibrassinolide and silicon alleviates cadmium stress via the modulation of antioxidant defense and glyoxalase systems and macronutrient content in Pisum sativum L. seedlings

BACKGROUND

This study assessed the effects of 24-epibrassinolide (EBL, 10-7M) and silicon (2 mM) on the alleviation of cadmium (Cd, 150 mg L-1) toxicity in Pisum sativum L. seedlings via the modulation of growth, antioxidant defense, glyoxalase system, and nutrient uptake.

RESULTS

Shoot and root lengths declined by 46.43% and 52.78%, respectively, following Cd stress. Shoot and root dry weights also declined with Cd toxicity. Biochemical and physiological aspects exhibit significant decline including total chlorophyll (33.09%), carotenoid (51.51%), photosynthetic efficiency (32.60%), photochemical quenching (19.04%), leaf relative water content (40.18%), and gas exchange parameters (80.65%). However, EBL or Si supplementation alone or in combination modulates the previously mentioned parameters. Cadmium stress increased proline and glycine betaine (GB) contents by 4.37 and 2.41-fold, respectively. Exposure of plants to Cd stress increased the accumulation of H2O2, malondialdehyde content, electrolyte leakage, and methylglyoxal, which declined significantly with EBL and Si supplementation, both individually and in combination. Similarly, Cd stress adversely affected enzymatic and non-enzymatic antioxidants, but EBL and/or Si supplementation maintained antioxidant levels. Glyoxalase I (GlyI) accumulated after Cd stress and increased further with the application of EBL and Si. However, GlyII content declined after Cd stress but increased with supplementation of EBL and Si. Cadmium accumulation occurred in the following order: roots > shoots>leaves. Supplementation with EBL and Si, individually and in combination reduced Cd accumulation and enhanced the uptake of macronutrients and micronutrients in shoots and roots, which declined with Cd toxicity.

CONCLUSION

The application of 24-EBL and Si, individually and in combination, alleviated the adverse effects of Cd by improving growth, biochemical parameters, nutrient uptake, osmolyte accumulation, and the anti-oxidative defense and glyoxalase systems in Pisum sativum seedlings.

A multi-surface model to predict Cd phytoavailability to wheat (Triticum aestivum L.)

The prediction of metal bioavailability in soils is critical to metal risk assessments and the amount of dissolved metal in soils is a key factor determining bioavailability. Because the recently developed geochemical multi-surface models (MSMs) offer a promising tool for the determination of metal partitioning in soils, in this study, a MSM based on generic parameters was used to assess the bioavailability of Cd in wheat (Triticum aestivum L.) growing in 12 soils with a wide range of properties. The amount of MSM-calculated dissolved Cd correlated strongly with the amount of Cd uptake by wheat (R²=0.873 for roots and R²=0.837 for shoots), and the model's performance was better than that of chemical extraction methods (0.01M CaCl₂, 0.43M HNO₃ and soil total Cd). The reactive fraction of soil organic matter, the soil/solution ratio, and the inclusion/exclusion of background cations influenced the calculation results. The best calculation condition was optimized. The application of the MSM was also examined in 84 wheat-soil samples from the field. The amount of Cd in wheat seeds had a stronger correlation with the amount of MSM-predicted Cd than with the amount of Cd obtained using chemical extraction methods. Our results suggested that MSM-calculated Cd is an effective indicator of the bioavailability of Cd in soils and demonstrated the utility of the method as a tool to assess the risk of Cd contamination in soils.

Effect of foliar-applied iron complexed with lysine on growth and cadmium (Cd) uptake in rice under Cd stress

Contamination of soils with cadmium (Cd) is a serious problem worldwide. Rice (Oryza sativa L.) is reported to accumulate relatively higher Cd contents in consumable parts and is considered a main source of Cd toxicity to humans from rice-derived products. The aim of this pot trial was to investigate the effect of foliar-applied iron (Fe) complexed with lysine on growth, photosynthesis, Cd concentration in plants, oxidative stress, and activities of antioxidants of rice in soil contaminated with Cd. Rice seedlings (30-day-old) were transferred to the soil, and after 2 weeks, different concentrations of Fe-lysine (0, 1.5, 3.0, 4.5, 6.0, and 7.5 mg L^-1) were applied as a foliar spray once in a week for 4 weeks and plant samples were taken after 10 weeks of growth in the soil under ambient conditions. Foliar supply of Fe-lysine complex significantly enhanced the plant height, dry weights of plants, concentration of chlorophyll, and gas exchange attributes in Cd-stressed rice. Fe-lysine decreased the Cd concentrations in plants while increasing the Fe concentrations in rice seedlings being maximum with Fe-lysine of 6.0 mg L^-1. Electrolyte leakage decreased while activities of key antioxidant enzymes increased with Fe-lysine compared to the control. According to the present results, Fe-lysine complex can effectively be used to reduce Cd concentrations in rice and probably in other crop species.

Deciphering adverse effects of heavy metals on diverse wheat germplasm on irrigation with urban wastewater of mixed municipal-industrial origin

The current study provides one of the first attempts to identify tolerant, moderately sensitive, and highly sensitive wheat genotypes on the basis of heavy metal accumulation, biochemical attributes, and human health risk assessments on urban wastewater (UW) irrigation. Mean heavy metals (Fe, Co, Ni, Cu, Zn, Pb, Cd, Cr, Mn) and macro-nutrients (Na, K, Ca, Mg) levels increased in the roots, stem, and grains of studied genotypes. Except K (stem > root > grain), all metals were accumulated in highest concentrations in roots followed by stem and grains. Principal component analyses (PCA) identified three groups of UW-irrigated genotypes which were confirmed by hierarchical agglomerative cluster analyses (HACA). Wheat genotypes with the lowest metal accumulation were regarded as tolerant, whereas those with maximum accumulation were considered highly sensitive. Tolerant genotypes showed the lowest hazard quotient for heavy metals, i.e., Co, Mn, Cd, Cu, Fe, Pb, and Cr, and hazard index (HI) values (adults, 2.04; children, 2.27) than moderately and highly sensitive genotypes. Higher health risks (HI) associated with moderate (adults 2.26; children 2.53) and highly sensitive (adults 2.52; children 2.82) genotypes revealed maximum uptake of heavy metals. The heatmap showed higher mean biochemical levels of chlorophyll, carotenoids, membrane stability index (MSI%), sugars, proteins, proline, superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) in tolerant genotypes than remaining genotypes. With the lowest metal accumulation and advanced biochemical mechanisms to cope with the adverse effects of heavy metals in their plant bodies, tolerant genotypes present a better option for cultivation in areas receiving UW or similar type of wastewater.

Effect of humic and fulvic acid transformation on cadmium availability to wheat cultivars in sewage sludge amended soil

The high nutrients and organic matter (OM) content of sewage sludge make it an excellent fertilizer to enhance soil fertility and crop production. However, the presence of adsorbed and precipitated forms of heavy metals, especially cadmium (Cd), can be a major problem for such a utilization of sludge. This pot study aims at producing safe food with minimal Cd concentrations from sewage sludge amended soils. Two wheat cultivars (NARC-11 and Shafaq-06) were sown in soil amended with sewage sludge with rates 0, 15 and 30 g kg^-1 soil. Application of sewage sludge resulted in enhancement of wheat grain yield while Cd concentrations in wheat grains of both cultivars remained within permissible limits (24.1 to 58.6 μg kg^-1 dry weight). Fourier transform infrared (FTIR) spectroscopic analysis revealed more spectral changes in fulvic acids than in humic acids, which showed a higher humification degree, making them chemically and biologically more stable for Cd retention. Sequential extraction data of Cd after NARC-11 harvest exhibited a significant decrease in mobile fractions (exchangeable and reducible fractions were reduced by 3.6 and 5.2%, respectively) and increase in immobile fraction (the oxidizable and residual fractions increased by 7 and 1.8%, respectively). It is concluded that sewage sludge application could be useful for the improvement of wheat production due to formation of stable humate complexes and decrease in Cd availability.

Farmyard manure alone and combined with immobilizing amendments reduced cadmium accumulation in wheat and rice grains grown in field irrigated with raw effluents

Cadmium (Cd) stress is a serious concern in agricultural soils worldwide whereas little is known about the impact of farmyard manure (FYM) alone or combined with limestone, lignite and biochar on Cd concentrations in plants. Wheat was grown in Cd-contaminated field amended with control (T₁), FYM @ 0.1% (T₂), FYM + limestone @ 0.05% each (T₃), FYM + lignite @ 0.05% each (T₄), FYM + biochar @ 0.05% each (T₅) and subsequent rice was grown without additional use of amendments. Soil application of amendments increased straw and grain yield and thousand grain weight being maximum in FYM + limestone treatment. Wheat and rice straw yield increased by 19% and 10% in T₃ than control respectively. Photosynthetic pigments increased with the supply of amendments than control. Amendments decreased Cd concentration, total Cd uptake in straw and grains and Cd harvest index of both crops and the maximum reduction in these parameters was recorded with where FYM + limestone (T₃). Cd concentration in wheat and rice straw decreased by 78.5% and 65% in T₃ than control, respectively. The highest benefit to cost ratio was obtained in FYM + limestone (T₃). Ammonium bicarbonate - diethylenetriamine penta acetic acid (AB-DTPA) extractable Cd of the post-harvest soil reduced whereas Cd immobilization index and soil pH increased with the supply of all treatments than control being maximum in T_3. The present study revealed that field management with FYM + limestone increased plant yield and reduced Cd concentrations in grains.

Aquatic plants for phytostabilization of cadmium and zinc in hydroponic experiments

Cadmium (Cd) may be toxic to aquatic plants even at modest concentrations, and excessive quantities of zinc (Zn) decrease plant performance. The Cd and Zn phytoremediation potential of several aquatic plant species (Thalia geniculate, Cyperus alternifolius, Canna indica, Eichhornia crassipes, Pistia stratiotes) and one grass species (Vetiveria zizanioides) was evaluated in hydroponic experiments. Vetiveria zizanioides, E. crassipes, and P. stratiotes experienced reduced growth performance in the presence of Cd as determined from biomass production, survival rate, and crown root number (CN); however, they accumulated high quantities of metals in their tissues, particularly in roots. Root accumulation is considered a key characteristic of so-called excluder species. In this study, only E. crassipes and P. stratiotes had bioconcentration factors and translocation factors (> 1000 and < 1, respectively) suitable for high phytostabilization of Cd. Furthermore, V. zizanioides and P. stratiotes showed the highest percent metal uptake from solution and removal capacity for Zn (~70% and ~2 mg d^-1 g^-1, respectively). Emergent aquatic species (particularly C. alternifolius and T. geniculate) adapted and lived well in Cd- and Zn-contaminated solution and took up high quantities of Cd and Zn in roots, and are therefore considered strong excluders. Beneficial uses of such species in contaminated wetlands include stabilizing toxic metals and limiting erosion. Plant tissue can be applied to other uses, including as a biomass fuel. In field situations, the candidate species may work best when grown together, since each plant genotype possesses a different potential to control Cd and Zn.

Signaling molecule methylglyoxal ameliorates cadmium injury in wheat (Triticum aestivum L) by a coordinated induction of glutathione pool and glyoxalase system

Methylglyoxal (MG) now is found to be an emerging signaling molecule. It can relieve the toxicity of cadmium (Cd), however its alleviating mechanism still remains unknown. In this study, compared with the Cd-stressed seedlings without MG treatment, MG treatment could stimulate the activities of glutathione reductase (GR) and gamma-glutamylcysteine synthetase (γ-ECS) in Cd-stressed wheat seedlings, which in turn induced an increase of reduced glutathione (GSH). Adversely, the activated enzymes related to GSH biosynthesis and increased GSH were weakened by N-acetyl-L-cysteine (NAC, MG scavenger), 2,4-dihydroxy-benzylamine (DHBA) and 1,3-bischloroethyl-nitrosourea (BCNU, both are specific inhibitors of GR), buthionine sulfoximine (BSO, a specific inhibitors of GSH biosynthesis), and N-ethylmaleimide (NEM, GSH scavenger), respectively. In addition, MG increased the activities of glyoxalase I (Gly I) and glyoxalase II (Gly II) in Cd-treated seedlings, followed by declining an increase in endogenous MG as comparision to Cd-stressed seedlings alone. On the contrary, the increased glyoxalase activity and decreased endogenous MG level were reversed by NAC and specific inhibitors of Gly I (isoascorbate, IAS; squaric acid, SA). Furthermore, MG alleviated an increase in hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) in Cd-treated wheat seedlings. These results indicated that MG could alleviate Cd toxicity and improve the growth of Cd-stressed wheat seedlings by a coordinated induction of glutathione pool and glyoxalase system.

Biochar effects on uptake of cadmium and lead by wheat in relation to annual precipitation: a 3-year field study

Biochar has been widely studied for its ability to reduce plant uptake of heavy metals by lowering metal bioavailabilities through adsorption and pH-driven fixation reactions. However, the long-term effect of biochar on heavy metal bioavailabilities in alkaline soils under natural redox condition is rarely studied. Here, we report a study examining the effects of biochar on bioavailability and partitioning of cadmium (Cd) and lead (Pb) among different soil fractions over 3 years in a field study with wheat (Triticum aestivum L.). Plots were established on two similar soils having low and high levels of contamination, both of which were amended in the first year with wheat straw biochar at 0, 20, and 40 t ha^-1. Precipitation patterns varied greatly over the study period, with 2014 having record drought, which was followed by 2 years having extreme flooding events. Results showed a significant increase in grain yield and reductions in Cd and Pb concentrations in wheat grain in the biochar-amended soils in 2014. In contrast, bioavailable (exchangeable) heavy metal concentrations and plant uptake of Cd and Pb were significantly higher in the subsequent very wet years in 2015 and 2016, where the effects of biochar were much more variable and had an overall lesser effect on reducing heavy metal uptake. The results suggest that fluctuations in soil pH and redox caused by periodic drought and flood cycles strongly drive metal cycling through mobilization and immobilization of metals associated with different mineral phases. Under these conditions, biochar may have reduced efficacy for reducing heavy metal uptake in wheat.

Cadmium accumulation in edible flowering cabbages in the Pearl River Delta, China: Critical soil factors and enrichment models

Although many previous studies have reported the soil pH and organic matter to be the most critical factors that affect the transfer of Cd in soil-crop systems in temperate zones, the behavior of Cd transfer is different in the Pearl River Delta (PRD), which is located in a subtropical zone with different climate and soil conditions. Therefore, we must determine the critical environmental factors that influence the transfer of Cd in the soil-vegetable system in the PRD region. Such knowledge can improve the safety of vegetables. In this study, the soil geochemical properties are investigated to explore the key soil factors that control the uptake of Cd by flowering cabbage, a popular leaf vegetable in China, from soils in the PRD region. The Cd contents in vegetables were most positively correlated to soil oxalate-Cd (p < 0.01), which indicates that amorphous Cd is the most available form for uptake into the cabbages. With the characteristics of rich in Fe oxide and Al oxide in the PRD soils, soil Fe and Al oxides were found to be the most relevant to the transfer factors of Cd from the soils to the cabbages. Soil secondary minerals are the key factor that affects the transfer of Cd, thereby influencing the migration and fate of Cd in soil-cabbage systems, with DCB-Fe significantly decreasing the Cd accumulation in cabbages. Additionally, models were developed to predict the enrichment of Cd in flowering cabbages, in which oxalate-Cd, DCB-Fe, and NaOAc-Al in soils were determined to be the most important factors that affect the Cd enrichment in flowering cabbages. In this study, we determine the important role of soil secondary minerals in affecting the transfer of Cd in soil-cabbage systems in the PRD. These observations are important to evaluate the accumulation of Cd in vegetables in subtropical zones.

Ammonium N influences the uptakes, translocations, subcellular distributions and chemical forms of Cd and Zn to mediate the Cd/Zn interactions in dwarf polish wheat (Triticum polonicum L.) seedlings

Ammonium (NH₄⁺) would influence the uptake and translocation of Cd and Zn to mediate their interactions in wheat. Thus, the effects of NH₄⁺ on Cd and Zn uptake, translocation, subcellular distributions and Cd chemical forms in dwarf polish wheat (DPW, Triticum polonicum L.) under Cd, Zn and Cd + Zn stresses with lack or supply of NH₄⁺ was investigated. The biomasses of root and shoot were reduced by NH₄⁺. NH₄⁺ enhanced Cd and Zn uptakes, but inhibited their translocations. Under lack and supply of NH₄⁺, Zn inhibited Cd uptakes, but promoted Cd translocations. Meanwhile, NH₄⁺ reinforced the inhibition of Cd uptake and the promotion of Cd translocation caused by Zn. Cd inhibited Zn uptake and promoted Zn translocation under lack of NH₄⁺. Meanwhile, Cd slightly reduced the Zn uptake, but did not affect the translocation under supply of NH₄⁺. Therefore, NH₄⁺ alleviated the inhibition of Zn uptake and partly reduced the promotion of Zn translocation stimulated by Cd. NH₄⁺ and Zn changed the subcellular distributions and chemical forms of Cd. NH₄⁺ and Cd also influenced the subcellular distributions of Zn. The changed subcellular distributions and chemical forms were associated with Cd and Zn uptakes and translocations, which physiologically revealed and illustrated NH₄⁺ participates in Cd/Zn interactions.

Biochar application increased the growth and yield and reduced cadmium in drought stressed wheat grown in an aged contaminated soil

Cadmium (Cd) and drought stress in plants is a worldwide problem, whereas little is known about the effect of biochar (BC) under combined Cd and drought stress. The current study was conducted to determine the impact of BC on Cd uptake in wheat sown in Cd-contaminated soil under drought stress. Wheat was grown in a soil after incubating the soil for 15 days with three levels of BC (0%, 3.0% and 5.0% w/w). Three levels of drought stress (well-watered, mild drought and severe drought containing 70%, 50%, and 35% of soil water holding capacity respectively) were applied to 45-d-old wheat plants. Drought stress decreased plant height, spike length, chlorophyll contents, gas exchange parameters, root and shoot dry biomasses and grain yields. Drought stress also caused oxidative stress and decreased the antioxidant enzymes activities whereas increased the Cd concentration in plants. Biochar increased morphological and physiological parameters of wheat under combined drought and Cd stress and reduced the oxidative stress and Cd contents and increased antioxidant enzymes activities. The decrease in Cd concentration with BC application in drought-stressed plant might be attributed to BC-induced increase in crop biomass production and reduction in oxidative stress. These results indicate that BC could be used as an amendment in metal contaminated soil for improving wheat growth and reducing Cd concentrations under semiarid conditions.

Residual effects of biochar on growth, photosynthesis and cadmium uptake in rice (Oryza sativa L.) under Cd stress with different water conditions

Soil cadmium (Cd) contamination and drought stress are among the main issues hindering global food security. Biochar has been used to reduce metal uptake by plants and water stress mitigation, but long-term residual effects of biochar under Cd stress at different moisture levels needs to be investigated. A following rice (Oryza sativa L.) was grown after wheat on Cd-contaminated soil amended with different levels of biochar (0, 3.0, and 5.0%, w/w). Thirty five days old plants were irrigated with three moisture levels including zero drought as a control (1-2 cm water layer on soil), mild drought (MD, 50% of soil water holding capacity, WHC), and severe drought (SD, 35% of soil WHC) for an accompanying 35 days. Plant height, biomass and photosynthesis were reduced whereas oxidative stress increased under MD and SD than control in un-amended soil while opposite trends were observed in plants grown in biochar amended soil. At the same biochar addition, Cd concentrations in seedlings were lower in continuous flooding than MD and SD treatments. The biochar supply reduced the bioavailable Cd in the soil whereas increased the soil EC and pH than the control treatment. In conclusion, continuous flooding plus residual biochar can be strategized in mitigating Cd-contamination in paddy soils and decreased Cd concentrations in rice which may reduce the potential risks to humans.

Differential expression of vacuolar and defective cell wall invertase genes in roots and seeds of metalliferous and non-metalliferous populations of Rumex dentatus under copper stress

Acid invertase activities in roots and young seeds of a metalliferous population (MP) of Rumex dentatus were previously observed to be significantly higher than those of a non-metalliferous population (NMP) under Cu stress. To date, no acid invertase gene has been cloned from R. dentatus. Here, we isolated four full-length cDNAs from the two populations of R. dentatus, presumably encoding cell wall (RdnCIN1 and RdmCIN1 from the NMP and MP, respectively) and vacuolar invertases (RdnVIN1 and RdmVIN1 from the NMP and MP, respectively). Unexpectedly, RdnCIN1 and RdmCIN1 most likely encode special defective invertases with highly attenuated sucrose-hydrolyzing capacity. The transcript levels of RdmCIN1 were significantly higher than those of RdnCIN1 in roots and young seeds under Cu stress, whereas under control conditions, the former was initially lower than the latter. Unexpected high correlations were observed between the transcript levels of RdnCIN1 and RdmCIN1 and the activity of cell wall invertase, even though RdnCIN1 and RdmCIN1 do not encode catalytically active invertases. Similarly, the transcript levels of RdmVIN1 in roots and young seeds were increased under Cu stress, whereas those of RdnVIN1 were decreased. The high correlations between the transcript levels of RdnVIN1 and RdmVIN1 and the activity of vacuolar invertase indicate that RdnVIN1 and RdmVIN1 might control distinct vacuolar invertase activities in the two populations. Moreover, a possible indirect role for acid invertases in Cu tolerance, mediated by generating a range of sugars used as nutrients and signaling molecules, is discussed.

Effect of different forms of N fertilizers on the hyperaccumulatorSolanum nigrumL. and maize in intercropping mode under Cd stress

In the present study, we investigated the effects of different forms of nitrogen fertilizers on the hyperaccumulator Solanum nigrum L. and maize in intercropping mode under cadmium (Cd) stress and explored the physiological response mechanism. This research lays the foundation for the appropriate use of nitrogen (N) fertilizer, reduced costs of ecological restoration, and phytoremediation of environmental pollution by using this intercropping system. The main greenhouse pot experiment was conducted using 1.92 mg kg⁻¹ Cd-contaminated soil. NH₄⁺–N fertilizer and NO₃⁻–N fertilizer treatments were performed along with no nitrogen fertilizer treatment as the control. The results indicate that intercropping could decrease the Cd uptake of maize compared with maize monocropping, but the biomass of maize would decrease under the intercropping mode. The application of N fertilizer to the maize–S. nigrum intercropping system could increase the total biomass of maize and S. nigrum. Compared with the NO₃⁻–N fertilizer treatment, the Cd content of stem, leaf and grain tissues of S. nigrum significantly increased by 9.43%, 22.2%, and 8.33%, respectively, under the NH₄⁺–N fertilizer treatment. The bioconcentration and translocation factors of S. nigrum significantly increased by 11.1% and 15.3%. Moreover, the Cd content of stem, leaf, and grain tissues of maize decreased by 26.5%, 21.2%, and 21.4%, respectively. The bioconcentration and translocation factors of maize significantly decreased by 38.8% and 46.7%. The application of N fertilizers promoted the accumulation of Cd in maize roots, while Cd content decreased in maize shoots. Compared with NO₃⁻–N fertilizer, NH₄⁺–N fertilizer can improve Cd accumulation in various S. nigrum tissues under intercropping, which could reduce Cd accumulation in maize under intercropping. Therefore, the application of NH₄⁺–N fertilizer is recommended for satisfactory bioremediation when using the Cd-hyperaccumulator S. nigrum and for supporting the safe production of maize in Cd-contaminated soil, thus enabling the goal of simultaneous agricultural production and remediation.

In the present study, we investigated the effects of different forms of nitrogen fertilizers on the hyperaccumulator Solanum nigrum L. and maize in intercropping mode under cadmium (Cd) stress and explored the physiological response mechanism.

Effect of zinc-lysine on growth, yield and cadmium uptake in wheat (Triticum aestivum L.) and health risk assessment

Cadmium (Cd) is among the most widespread toxic trace elements found in agricultural soils due to various anthropogenic activities. The role of micronutrient-amino chelates on reducing Cd toxicity in crop plants is recently introduced. The current study was conducted to highlight the role of foliar application of zinc-lysine (Zn-lys) complex on biochemical and growth parameters and Cd uptake in wheat (Triticum aestivum) grown in aged Cd-contaminated soil. Foliar concentration of Zn-lys (0, 10, 20, and 30 mg L^-1) was applied at different time intervals (2nd, 3rd, 5th and 7th week of sowing) and plants were harvested at maturity. Folliar application of Zinc-lys significantly increased the photosynthesis, grain yield, enzyme activities and Zn contents in different plant tissues. Zinc-lys reduced Cd contents in grains, shoot and root as well as reduced the oxidative stress in wheat linearly in a dose-additive manner. Taken together, Zn-lys chelate efficiently improved wheat growth and fortified Zn contents while reduced Cd concentration in plant in a Zn-deficient Cd-contaminated soil. Although, health risk index (HRI) from the soil sampling area seems to be lower than <1 for Cd but may exceed due to long-term consumption of grains produced from such contaminated soil. Foliar applied Zn-lys reduced HRI which may help to reduce health risks associated with Cd.

Contrasting Effects of Organic and Inorganic Amendments on Reducing Lead Toxicity in Wheat

Contamination of agricultural soils with lead (Pb) is a widespread problem which is mainly due to anthropogenic activities. The present study investigated the effect of organic and inorganic amendments on wheat growth and reducing Pb concentration in the plant. A greenhouse experiment was conducted on Pb spiked soil (500 mg kg^-1 of soil) with the application of farmyard manure, poultry manure, gypsum and di-ammonium phosphate (DAP). Plants were harvested after 120 days of growth and analyzed for Pb concentration in different plant parts. Under Pb stress, amendments significantly (p < 0.05) increased grain yield and root dry weights compared to the control. Grain yield and dry weights of shoots and roots were maximum with DAP compared to the control and other amendments. Amendments increased transpiration rate, stomatal conductance, chlorophyll contents and photosynthetic rate. The lowest Pb concentration was found in DAP treated plants which were about 88%, 84%, and 85% lower in root, shoots and grains than control respectively. DAP fertilizer was the most effective in improving these parameters than control and other amendments and can be used to reduce Pb concentration in wheat and probably other crops.

Physiological and transcriptome response to cadmium in cosmos (Cosmos bipinnatus Cav.) seedlings

To date, several species of Asteraceae have been considered as Cd-accumulators. However, little information on the Cd tolerance and associated mechanisms of Asteraceae species Cosmos bipinnatus, is known. Presently, several physiological indexes and transcriptome profiling under Cd stress were investigated. C. bipinnatus exhibited strong Cd tolerance and recommended as a Cd-accumulator, although the biomasses were reduced by Cd. Meanwhile, Cd stresses reduced Zn and Ca uptake, but increased Fe uptake. Subcellular distribution indicated that the vacuole sequestration in root mainly detoxified Cd under lower Cd stress. Whilst, cell wall binding and vacuole sequestration in root co-detoxified Cd under high Cd exposure. Meanwhile, 66,407 unigenes were assembled and 41,674 (62.75%) unigenes were annotated in at least one database. 2,658 DEGs including 1,292 up-regulated unigenes and 1,366 down-regulated unigenes were identified under 40 μmol/L Cd stress. Among of these DEGs, ZIPs, HMAs, NRAMPs and ABC transporters might participate in Cd uptake, translocation and accumulation. Many DEGs participating in several processes such as cell wall biosynthesis, GSH metabolism, TCA cycle and antioxidant system probably play critical roles in cell wall binding, vacuole sequestration and detoxification. These results provided a novel insight into the physiological and transcriptome response to Cd in C. bipinnatus seedlings.

Biochemical and Genetical Responses of Phoenix dactylifera L. to Cadmium Stress

The cadmium (Cd), a heavy metal, causes toxicity, which leads to hampering the growth and development of the plant. The molecular and biochemical approaches were used for the investigation of antioxidant system response and genotoxicity in date palm (Phoenix dactylifera L.) cv. Sagai in pot experiment having Cd. The root length was more affected than the shoot length as more accumulation of Cd occurs in roots. Fresh weights of root and shoot were reduced significantly in treated plants as compared to the control. The proline content was increased at low concentration of Cd (300 µM-CdCl2) than the medium and high concentrations (600 and 900 µM-CdCl2), respectively. The thiobarbituric acid reactive substances (TBARS) content was increased at 600 and 900 µM-CdCl2 compared to the plants treated at 300 µM-CdCl2 and controls. Antioxidant enzymatic assay was performed under Cd stress and compared with control plants. The catalase (CAT) and superoxide dismutase (SOD) activities were found to be high in plants treated with CdCl2 at 300 µM compared to at 600 and 900 µM-CdCl2, respectively. The genotoxicity of Cd was assessed using the inter-simple sequence repeat (ISSR) marker where all treated and control plants were clustered into three main groups based on genetic similarity. P. dactylifera plants were found to be more divergent at high Cd stress as compared to control and plants treated at low concentration of Cd.

Role of Streptomyces pactum in phytoremediation of trace elements by Brassica juncea in mine polluted soils

The industrial expansion, smelting, mining and agricultural practices have increased the release of toxic trace elements (TEs) in the environment and threaten living organisms. The microbe-assisted phytoremediation is environmentally safe and provide an effective approach to remediate TEs contaminated soils. A pot experiment was conducted to test the potential of an Actinomycete, subspecies Streptomyces pactum (Act12) along with medical stone compost (MSC) by growing Brassica juncea in smelter and mines polluted soils of Feng County (FC) and Tongguan (TG, China), respectively. Results showed that Zn (7, 28%), Pb (54, 21%), Cd (16, 17%) and Cu (8, 10%) uptake in shoot and root of Brassica juncea was pronounced in FC soil. Meanwhile, the Zn (40, 14%) and Pb (82, 15%) uptake in the shoot and root were also increased in TG soil. Shoot Cd uptake remained below detection, while Cu decreased by 52% in TG soil. The Cd and Cu root uptake were increased by 17% and 33%, respectively. Results showed that TEs uptake in shoot increased with increasing Act12 dose. Shoot/root dry biomass, chlorophyll and carotenoid content in Brassica juncea were significantly influenced by the application of Act12 in FC and TG soil. The antioxidant enzymatic activities (POD, PAL, PPO and CAT) in Brassica juncea implicated enhancement in the plant defense mechanism against the TEs induced stress in contaminated soils. The extraction potential of Brasssica was further evaluated by TF (translocation factor) and MEA (metal extraction amount). Based on our findings, further investigation of Act12 assisted phytoremediation of TEs in the smelter and mines polluted soil and hyperaccumulator species are suggested for future studies.

A critical review on effects, tolerance mechanisms and management of cadmium in vegetables

Cadmium (Cd) accumulation in vegetables is an important environmental issue that threatens human health globally. Understanding the response of vegetables to Cd stress and applying management strategies may help to reduce the Cd uptake by vegetables. The aim of the present review is to summarize the knowledge concerning the uptake and toxic effects of Cd in vegetables and the different management strategies to combat Cd stress in vegetables. Leafy vegetables grown in Cd contaminated soils potentially accumulate higher concentrations of Cd, posing a threat to food commodities. The Cd toxicity decreases seed germination, growth, biomass and quality of vegetables. This reduces the photosynthesis, stomatal conductance and alteration in mineral nutrition. Toxicity of Cd toxicity also interferes with vegetable biochemistry causing oxidative stress and resulting in decreased antioxidant enzyme activities. Several management options have been employed for the reduction of Cd uptake and toxicity in vegetables. The exogenous application of plant growth regulators, proper mineral nutrition, and the use of organic and inorganic amendments might be useful for reducing Cd toxicity in vegetables. The use of low Cd accumulating vegetable cultivars in conjunction with insolubilizing amendments and proper agricultural practices might be a useful technique for reducing Cd exposure in the food chain.

Foliar application of aspartic acid lowers cadmium uptake and Cd-induced oxidative stress in rice under Cd stress

Cadmium (Cd) contamination of farmland soils is a widespread problem around the globe, and rice (Oryza sativa L.) tends to accumulate more Cd and is considered as one of the major sources of Cd intake in humans, especially consuming rice-derived products. The current study investigated the effects of foliar applied aspartic acid (Asp) on growth parameters, biomass, chlorophyll concentration, gas exchange characteristics, Cd uptake, and antioxidative capacity in the shoots and roots of rice seedlings exposed to Cd stress. For this, 30-day-old rice nursery was transferred in the soil with aged Cd contamination (2.86 mg kg^-1). After 2 weeks of growth, different concentrations (0, 10, 15, and 20 mg L^-1) of Asp were foliar applied four times with a 7-day interval, and the crop was harvested after 10 weeks of transplanting. Foliar applied Asp increased the plant height, shoot and root dry weight, chlorophyll concentration, and gas exchange parameters, while it reduced the Cd concentrations in both shoots and roots as well as shoot to root Cd translocation factor compared to the control. Foliar application of Asp reduced the malondialdehyde content and electrolyte leakage in rice parts compared to the control in a dose-additive manner. The activities of key antioxidant enzymes increased while peroxidase activity decreased by exogenous Asp. The increase in plant weight and photosynthesis might be due to lower Cd concentrations in plants which may reduce the oxidative stress and also help the plants to minimize direct damage caused by Cd to the photosynthetic organs.

Remediation of heavy metal contaminated soils by using Solanum nigrum: A review

Heavy metals are among the major environmental pollutants and the accumulation of these metals in soils is of great concern in agricultural production due to the toxic effects on crop growth and food quality. Phytoremediation is a promising technique which is being considered as an alternative and low-cost technology for the remediation of metal-contaminated soils. Solanum nigrum is widely studied for the remediation of heavy metal-contaminated soils owing to its ability for metal uptake and tolerance. S. nigrum can tolerate excess amount of certain metals through different mechanism including enhancing the activities of antioxidant enzymes and metal deposition in non-active parts of the plant. An overview of heavy metal uptake and tolerance in S. nigrum is given. Both endophytic and soil microorganisms can play a role in enhancing metal tolerance in S. nigrum. Additionally, optimization of soil management practices and exogenous application of amendments can also be used to enhance metal uptake and tolerance in this plant. The main objective of the present review is to highlight and discuss the recent progresses in using S. nigrum for remediation of metal contaminated soils.