Correlations between cadmium and mineral nutrients in absorption and accumulation in various genotypes of rice under cadmium stress

Mechanism for Fe(III) to decrease cadmium uptake of wheat plant: Rhizosphere passivation, competitive absorption and physiological regulation

The presence of dissolved Fe(III) and Fe(III)-containing minerals has been found to alleviate cadmium (Cd) accumulation in wheat plants grown in Cd-contaminated soils, but the specific mechanism remains elusive. In this work, hydroponic experiments were conducted to dissect the mechanism for dissolved Fe(III) (0-2000 μmol L-1) to decrease Cd uptake of wheat plants and study the influence of Fe(III) concentration and Cd(II) pollution level (0-20 μmol L-1) on the Cd uptake process. The results indicated that dissolved Fe(III) significantly decreased Cd uptake through rhizosphere passivation, competitive absorption, and physiological regulation. The formation of poorly crystalline Fe(III) oxides facilitated the adsorption and immobilization of Cd(II) on the rhizoplane (over 80.4 %). In wheat rhizosphere, the content of CaCl2-extractable Cd decreased by 52.7 % when Fe(III) concentration was controlled at 2000 μmol L-1, and the presence of Fe(III) may reduce the formation of Cd(II)-organic acid complexes (including malic acid and succinic acid secreted by wheat roots), which could be attributed to competitive reactions. Down-regulation of Cd uptake genes (TaNramp5-a and TaNramp5-b) and transport genes (TaHMA3-a, TaHMA3-b and TaHMA2), along with up-regulation of the Cd efflux gene TaPDR8-4A7A, contributed much to the reduction of Cd accumulation in wheat plants in the presence of Fe(III). The inhibitory effect of Fe(III) on Cd uptake and transport in wheat plants declined with increasing Cd(II) concentration, particularly at 20 μmol L-1. This work provides important implications for remediating Cd-contaminated farmland soil and ensuring the safe production of wheat by using dissolved Fe(III) and Fe(III)-containing minerals.

Regulation of mineral elements in Hordeum brevisubulatum by Epichloë bromicola under Cd stress

In this study, wild barley (Hordeum brevisubulatum) infected (E+) and uninfected (E-) by Epichloë bromicola were used for hydroponic experiments during the seedling stage. Various attributes, such as the effect of fungal endophyte on the growth and development of wild barley, the absorption of cadmium (Cd) and mineral elements (Ca, Mg, Fe, Mn, Cu, Zn), subcellular distribution, and chemical forms were investigated under CdCl2 stress. The results showed that the fungal endophy significantly reduced the Ca content and percentage of plant roots under Cd stress. The Fe and Mn content of roots, the mineral element content of soluble fractions, and the stems in the pectin acid or protein-chelated state increased significantly in response to fungal endophy. Epichloë endophyte helped Cd2+ to enter into plants; and reduced the positive correlation of Ca-Fe and Ca-Mn in roots. In addition, it also decreased the correlation of soluble components Cd-Cu, Cd-Ca, Cd-Mg in roots, and the negative correlation between pectin acid or protein-chelated Cd in stems and mineral elements, to increase the absorbance of host for mineral elements. In conclusion, fungal endophy regulated the concentration and distribution of mineral elements, while storing more Cd2+ to resist the damage caused by Cd stress. The study could provide a ground for revealing the Cd tolerance mechanism of endophytic fungal symbionts.

Effect of brackish water irrigation on cadmium migration in a soil-maize system

Phytoremediation is an effective way to reduce heavy metal content in agricultural soil. The effects of brackish water irrigation on phytoremediation efficiency of plants have not yet been completely understood. In this study, the effects of brackish water irrigation on cadmium (Cd) uptake by maize as the phytoremediator were investigated. In a pot experiment, maize seedlings were grown in soil with exogenously added Cd (0, 5, 10, or 15 mg kg-1) and irrigated with deionized water (T1), natural brackish water (T2), or water with NaCl with salinity equal to that of natural brackish water (T3). Salt stress and cation antagonism caused by brackish water affected maize plant growth and Cd uptake. Under 5, 10, and 15 mg kg-1 Cd, Cd accumulation in maize shoots was 5.55, 7.08, and 5.71 μg plant-1; 4.08, 3.04, and 5.38 μg plant-1; and 2.48, 3.44, and 5.33 μg plant-1 under the T1, T2, and T3 treatments, respectively. Cd accumulation in the shoots was significantly lower under the T2 and T3 treatments than under the T1 treatment at 5 and 10 mg kg-1 Cd; however, no significant differences were observed among all treatments at 15 mg kg-1 Cd. These findings indicated that phytoremediation efficiency decreased in response to both salt stress and cation antagonism caused by brackish water under low soil-Cd concentrations; however, this effect was negligible under high soil-Cd concentration. Therefore, brackish water irrigation can be considered for the phytoremediation of soils contaminated with high Cd levels to save freshwater resources.

Molecular-Assisted Breeding of Cadmium Pollution-Safe Cultivars

Cadmium (Cd) contamination in edible agricultural products, especially in crops intended for consumption, has raised worldwide concerns regarding food safety. Breeding of Cd pollution-safe cultivars (Cd-PSCs) is an effective solution to preventing the entry of Cd into the food chain from contaminated agricultural soil. Molecular-assisted breeding methods, based on molecular mechanisms for cultivar-dependent Cd accumulation and bioinformatic tools, have been developed to accelerate and facilitate the breeding of Cd-PSCs. This review summarizes the recent progress in the research of the low Cd accumulation traits of Cd-PSCs in different crops. Furthermore, the application of molecular-assisted breeding methods, including transgenic approaches, genome editing, marker-assisted selection, whole genome-wide association analysis, and transcriptome, has been highlighted to outline the breeding of Cd-PSCs by identifying critical genes and molecular biomarkers. This review provides a comprehensive overview of the development of Cd-PSCs and the potential future for breeding Cd-PSC using modern molecular technologies.

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.

Phenotypic and genetic dissection of the contents of important metallic elements in hybrid rice grown in cadmium-contaminated paddy fields

Rice (Oryza sativa L.) is a staple food that feeds over half of the world's population, and the contents of metallic elements in rice grain play important roles in human nutrition. In this study, the contents of important metallic elements were determined by ICP-OES, and included cadmium (Cd), zinc (Zn), manganese (Mn), copper (Cu), iron (Fe), nickel (Ni), calcium (Ca), and magnesium (Mg) in brown rice, in the first node from the top (Node 1), in the second node from the top (Node 2), and in roots of 55 hybrids and their parental lines. The heritability of metallic element contents (MECs), the general combining ability (GCA) for MEC, and the correlation between MECs in different organs/tissues of hybrids were also analyzed. The results indicated that: (1) there was a positive correlation between the contents of Cd and Zn in nodes and roots, but a negative correlation between the contents of Cd and Zn in brown rice of the hybrids(2) the GCA for MECs can be used to evaluate the ability of the parental lines to improve the metal contents in brown rice of the hybrids(3) the contents of Cd, Zn, Ca, and Mg in brown rice were mainly affected by additive genetic effects(4) the restorer lines R2292 and R2265 can be used to cultivate hybrids with high Zn and low Cd contents in the brown rice.

Transcriptomic Analysis of Differentially Expressed Genes in Arabidopsis thaliana Overexpressing BnMYB2 from Boehmeria nivea under Cadmium Stress

Boehmeria nivea (ramie) is an important fiber crop with strong tolerance to cadmium (Cd). In our previous study, a novel MYB transcription factor gene from ramie, BnMYB2, was found to positively regulate Cd tolerance and accumulation in the transgenic Arabidopsis thaliana lines. Herein, transcriptome sequencing was performed to identify the differentially expressed genes involved in cadmium response between the wild-type (WT) and BnMYB2 overexpressed lines; 1598 differentially expressed genes (DEGs) were detected in the shoot. GO and KEGG analysis indicated that the majority of DEGs belonged to the categories of transcription factors, plant hormone signal transduction and nitrogen metabolism. The expression level of the Ib subgroup bHLH genes (AtbHLH38, AtbHLH39, AtbHLH100 and AtbHLH101) and nitrogen assimilation-related genes (AtNIA1, AtNIA2, AtNIR1 and AtASN2) were significantly higher than that of WT, accompanied with the positive changes in iron (Fe) and total nitrogen content in the shoot of BnMYB2 overexpression lines. Several DEGs belonging to the bZIP transcription factor family or SAUR family were also found up-regulated in the transgenic plants. These results provide important clues for elucidating how the molecular mechanisms of BnMYB2 regulate plant response to Cd stress.

Silicon-based additive on heavy metal remediation in soils: Toxicological effects, remediation techniques, and perspectives

Chemical fertilizer is gaining increasing attention and has been the center of much research which indicating complex beneficial and harmful effects. Chemical fertilizer might cause some environmental hazards to the biosphere, especially in the agricultural ecosystem. The application of silicon (Si) fertilizer in agriculture has been proved to be able to create good economic and environmental benefits. Si is the second most abundant earth crust element. Si fertilizer improves soil quality and alleviates biotic and abiotic crop stress. It is of great significance to understand the function of Si fertilizer in agricultural utilization and environmental remediation. This paper reviews the Si-based fertilizer in farmland use and summarizes prior research relevant with characterization, soil quality improvement, and pollution remediation effects. Its use in agriculture enhances plant silicon uptake, mediates plant salt and drought stress and remediates heavy metals such as Al, As, Cd, Cu, Zn and Cr. This article also summarizes the detoxification mechanism of silicon and its effects on plant physiological activity such as photosynthesis and transpiration. Fertilizer materials and crop fertilizer management were also considered. Foliar spraying is an effective method to improve crop growth and yield and reduce biotic or abiotic stress. Silicon nanoparticle material provides potential with great potential and prospects. More investigation and research are prospected to better understand how silicon impacts the environment and whether it is a beneficial additive.

Heavy Metal Contamination in Oryza sativa L. at the Eastern Region of Malaysia and Its Risk Assessment

Paddy plants tend to accumulate heavy metals from both natural and anthropogenic sources, and this poses adverse risks to human health. The objective of this study was to investigate heavy metal contamination in paddy plants in Kelantan, Malaysia, and its health risk assessment. The bioaccumulation of heavy metals was studied by means of enrichment (EF) and translocation factors (TF). The health risk assessment was performed based on USEPA guidelines. The EF for heavy metals in the studied areas was in the descending order of Cu > As > Cr > Cd > Pb. Meanwhile, Cr and Pb exhibited higher TF values from stem to grain compared with the others. The combined hazard index (HI) resulting from five heavy metals exceeded the acceptable limit (HI >1). The lifetime cancer risk, in both adult and children, was beyond the acceptable limit (10-4) and mainly resulted from exposure. The total cancer risk (CRt) due to simultaneous exposures to multiple carcinogenic elements also exceeded 10-4. In conclusion, intake of heavy metal through rice ingestion is likely to cause both non-carcinogenic and carcinogenic health risks. Further research is required to investigate the extent of heavy metal contamination in agricultural soils and, moreover, to establish human exposure as a result of rice consumption.

Strategies of cadmium and copper uptake and translocation in different plant species growing near an E-waste dismantling site at Wenling, China

This study aimed to explore the interactions between cadmium (Cd) and copper (Cu) during uptake and translocation in plants growing in soil polluted with heavy metals derived from electronic waste (E-waste). We collected the roots, stems, leaves, and root-surrounding soils of ten dominant plant species growing in farmland near an E-waste dismantling site, and analyzed their Cd and Cu concentrations. Among the ten plant species, Echinochloa crus-galli (L.) P. Beauv., Cucurbita moschata (Duch. ex Lam.) Duch. ex Poiret, Phragmites australis (Cav.) Trin. ex Steud., and Benincasa hispida (Thunb.) Cogn. accumulated Cd (2.40-4.56 mg kg^-1) and Cu (19.60-35.21 mg kg^-1) in the roots. In Polygonum hydropiper L. and Sesbania cannabina (Retz.) Poir., the Cd (0.50-0.81 mg kg^-1) and Cu (11.04-15.55 mg kg^-1) concentrations were similar among the three organs. Glycine max (L.) Merr. accumulated more Cu in the roots (16.42 mg kg^-1) than in the stems (5.61 mg kg^-1) and leaves (7.75 mg kg^-1), and accumulated Cd at similar levels in the three organs (0.65-0.99 mg kg^-1). Sesamum indicum L., Bidens pilosa L., and Solidago decurrens Lour. accumulated Cd at similar levels among the three organs (0.16-3.34 mg kg^-1) and accumulated less Cu in the stems (6.89-8.28 mg kg^-1) than in the roots (12.61-21.63 mg kg^-1) and leaves (12.93-22.38 mg kg^-1). S. indicum had a stronger capacity to accumulate and translocate Cd and Cu according to transfer coefficient and translocation factor. The concentrations of Cd and Cu in soils were significantly positively correlated with those in the roots (p<0.01) but not those in the stems and leaves. We detected significantly positive correlations between Cd and Cu concentrations in the roots and leaves (p<0.01) but not in the stems. These results suggest that there is a synergetic strategy of Cd and Cu transport from soils to the roots and from the roots to the leaves, while the stems may not be the key organ controlling Cd and Cu transport in plants. These findings have important implications for the phytoremediation of soils contaminated with Cd and Cu, the mechanisms of plant Cd and Cu transport, and the food safety of agricultural products.

A meta-analysis about the accumulation of heavy metals uptake by Sedum alfredii and Sedum plumbizincicola in contaminated soil

Sedum alfredii and Sedum plumbizincicola typically have high heavy metal (such as Zn and Cd) accumulation capacities with fast growth rates and relatively high Pb tolerance in contaminated soils. We compared the accumulation characteristics of heavy metals in Sedum species through meta-analysis. Furthermore, we analyzed the effects of soil organic matter (SOM) and soil pH on Cd, Pb and Zn accumulation by S. alfredii and S. plumbizincicola and the correlation between various metals. Results showed that the accumulations of Cd and Zn in shoots were higher than that of roots, but Pb accumulated in roots more than shoots. Moreover, there is a significant positive correlation between the accumulation of Zn and Cd in shoots. We found that the heavy metal accumulation rate in shoots was higher with lower soil pH. Sedum species had the highest Cd adsorption capacity in 20-30 g/kg SOM and the highest Zn adsorption capacity in SOM less than 20 g/kg. The accumulation rate of Cd in shoots of S. plumbizincicola was increased with exposure time, while the accumulation rate of Zn was slightly decreased.

Targeting Cd coping mechanisms for stress tolerance in Brassica napus under spiked-substrate system: from physiology to remediation perspective

Cadmium (Cd) is a prevalent, non-essential, carcinogenic, and hazardous heavy metal that reduces plant productivity and capacity of arable land area around the globe. In the present substrate-based pot study, seedlings of Brassica napus 180015 were grown equidistantly in the spiked-substrate medium for 60 days under increasing concentrations of Cd (0, 10, 20, 30, 40, 50 mg kg^-1). Following harvest, the morpho-physio-biochemical, antioxidative, and Cd-induced tolerance responses were evaluated in B. napus under an increasing Cd stress regime. Additionally, these parameters were also investigated to select the plant's threshold tolerance limit for Cd under the spiked-substrate system. B. napus showed dynamic behavior regarding morpho-physio-biochemical attributes, including agronomic features, biomass, photosynthetic pigments, relative water content under increased Cd toxicity. Cd stress-induced hydrogen peroxide (H₂O₂) production with high MDA contents and passive EL, followed by the orchestration of both enzymatic (SOD, POD, APX, CAT, and GR) and non-enzymatic antioxidants (flavonoids, TPC, TPA, proline, and total soluble protein) up to a certain limit. In addition, Cd-induced stress upregulated transcriptional levels of antioxidative enzyme SOD, POD, APX, GR, and MT encoded genes in B. napus. The increasing trend of Cd accumulation in different tissues at the highest Cd concentration was as follows: root > leaf > stem. In spiked substrate system, B. napus demonstrated improved metal extractability performance and a high potential for phyto-management of low to moderate Cd contamination, implying that this study could be used for integrative breeding programs and decontaminating heavy metals in real contaminated scenarios.Novelty statementThis study provides an insight into Cd-coping mechanisms of oilseed rape involved in alleviating toxicity and simultaneous phyto-management of increasing Cd concentration under spiked substrate system. The current study is the first scientific evidence of using a Cd-spiked soilless substrate medium. The present study will further strengthen our understanding of Cd-instigated positive responses in B. napus. Furthermore, it will provide a useful basis for integrative breeding programs and decontaminating heavy metals in real contaminated scenarios.

The Rice Cation/H+ Exchanger Family Involved in Cd Tolerance and Transport

Cadmium (Cd), a heavy metal toxic to humans, easily accumulates in rice grains. Rice with unacceptable Cd content has become a serious food safety problem in many rice production regions due to contaminations by industrialization and inappropriate waste management. The development of rice varieties with low grain Cd content is seen as an economic and long-term solution of this problem. The cation/H⁺ exchanger (CAX) family has been shown to play important roles in Cd uptake, transport and accumulation in plants. Here, we report the characterization of the rice CAX family. The six rice CAX genes all have homologous genes in Arabidopsis thaliana. Phylogenetic analysis identified two subfamilies with three rice and three Arabidopsis thaliana genes in both of them. All rice CAX genes have trans-member structures. OsCAX1a and OsCAX1c were localized in the vacuolar while OsCAX4 were localized in the plasma membrane in rice cell. The consequences of qRT-PCR analysis showed that all the six genes strongly expressed in the leaves under the different Cd treatments. Their expression in roots increased in a Cd dose-dependent manner. GUS staining assay showed that all the six rice CAX genes strongly expressed in roots, whereas OsCAX1c and OsCAX4 also strongly expressed in rice leaves. The yeast (Saccharomyces cerevisiae) cells expressing OsCAX1a, OsCAX1c and OsCAX4 grew better than those expressing the vector control on SD-Gal medium containing CdCl₂. OsCAX1a and OsCAX1c enhanced while OsCAX4 reduced Cd accumulation in yeast. No auto-inhibition was found for all the rice CAX genes. Therefore, OsCAX1a, OsCAX1c and OsCAX4 are likely to involve in Cd uptake and translocation in rice, which need to be further validated.

Foliar Zinc-Selenium and Nitrogen Fertilization Affects Content of Zn, Fe, Se, P, and Cd in Wheat Grain

The grain yield and concentrations of Fe, Zn, Se, Cd, and P in two winter wheat genotypes and in vitro bioaccessibility of Fe and Zn under the effect of different nitrogen fertilization and Zn-Se foliar application were evaluated. The total grain Fe, Zn, and Se concentrations, as well as Fe and Zn concentrations, after in vitro digestion were under the strongest effect of foliar Zn-Se application. On the other hand, Fe and Zn bioaccessibility (%) were under the most substantial effect of genotype. Regarding the need to increase concentrations of essential micronutrients in wheat grain, foliar Zn-Se application is a reliable and accepted agricultural practice, but to improve mineral bioaccessibility in human nutrition, foliar Zn-Se application should be combined with the most responsive genotypes. For this reason, further research on the genotype specificity of wheat regarding micronutrient bioaccessibility should be carried out.

Iron and copper micronutrients influences cadmium accumulation in rice grains by altering its transport and allocation

Cadmium (Cd) contamination in rice paddy fields constitutes a serious health issue in some parts of China. Here we study the potential for remediation of Cd contaminated alkaline paddy soil with low iron (Fe) and high copper (Cu) background by altering the concentrations of Fe and Cu in the growing media, which has been only seldom considered. We assessed how these two micronutrients (Cu and Fe) affect the absorption and transport of Cd in rice. Adding Cu significantly increased rice biomass and grain yield by reducing root Cd influx and Cd upward transport which, consequently, lowered Cd concentrations in roots, culms and leaves. However, excessive Cu also promoted a relatively higher Cd allocation in grains, especially under Fe deficiency, likely because Cu significantly increased the proportion of bioavailable Cd in leaves. Contrastingly, Fe did not alleviate the toxic effects of Cd on rice growth and yield, but it significantly reduced Cd transfer towards grains, which might be attributed to a sharp decrease in the proportion of bioavailable Cd in leaves. Our results demonstrated that Cd remediation may be achieved through altering Fe and Cu inputs, such that Cd accumulation in rice grains is reduced.

Understanding Potential Heavy Metal Contamination, Absorption, Translocation and Accumulation in Rice and Human Health Risks

Rice is a worldwide staple food and heavy metal contamination is often reported in rice production. Heavy metal can originate from natural sources or be present through anthropogenic contamination. Therefore, this review summarizes the current status of heavy metal contamination in paddy soil and plants, highlighting the mechanism of uptake, bioaccumulation, and health risk assessment. A scoping search employing Google Scholar, Science Direct, Research Gate, Scopus, and Wiley Online was carried out to build up the review using the following keywords: heavy metals, absorption, translocation, accumulation, uptake, biotransformation, rice, and human risk with no restrictions being placed on the year of study. Cadmium (Cd), arsenic (As), and lead (Pb) have been identified as the most prevalent metals in rice cultivation. Mining and irrigation activities are primary sources, but chemical fertilizer and pesticide usage also contribute to heavy metal contamination of paddy soil worldwide. Further to their adverse effect on the paddy ecosystem by reducing the soil fertility and grain yield, heavy metal contamination represents a risk to human health. An in-depth discussion is further offered on health risk assessments by quantitative measurement to identify potential risk towards heavy metal exposure via rice consumption, which consisted of in vitro digestion models through a vital ingestion portion of rice.

Effects of Soil pH and Mineral Nutrients on Cadmium Uptake by Rice Grain in the Pearl River Delta, China

Alluvial soils are rich in mineral nutrients, and contain high heavy metals, especially Cd. The interactions of mineral nutrients with Cd in soil-rice grain systems on natural condition of alluvial plain are highlighted in this study. 110 pairs of rice grain and soil (0-20 cm) samples from the Pearl River Delta were investigated and measured. The results indicated that pH, organic matter, cation exchange capacity, clay, Ca, Cd, Fe, Mn and Zn are the most important soil characteristics controlling Cd uptake by rice grain. There are synergetic interactions between Cd and mineral elements in the soils, and antagonistic interactions between them in the rice grains. It could provide useful information for the risk assessment of heavy metals in the soils of alluvial plain.

Evaluation of Heavy Metal Contamination in Paddy Plants at the Northern Region of Malaysia Using ICPMS and Its Risk Assessment

Heavy metals from natural and anthropogenic sources accumulate in soil and plants and as a consequence represent important environmental contamination problems. Nevertheless, food safety issues and adverse health risks make this one of the most serious environmental issues. The aim of the present study was to assess heavy metal contamination in the paddy plants from the northern area of Malaysia using Inductively Coupled Plasma Mass Spectrometry (ICPMS) and its risk assessment. In total, the heavy metals (As, Cd, Cu, Cr, and Pb) of the samples of paddy plants harvested from Kedah areas were extracted using an acid digestion method, while the heavy metals for soil samples using ammonium acetate. The heavy metal concentrations were then analysed using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The enrichment (EF) and translocation factors (TF) of heavy metals were calculated, and health risk assessment (HRA) was performed. The EF values for heavy metals from the soil to roots, roots to stems, stems to leaves, and stems to grains followed the order Cu > As > Cr > Cd > Pb, whereas Cr and Pb were characterized by greater TF values from stem to grain than the other elements. The average daily dose (ADD) for both children and adults is below the safe value intake for each of the studied elements. The combined hazard index (HI) of five elements was beyond the acceptable value (HI >1). The carcinogenic risk, as exemplified by lifetime cancer risk (LCR), indicated that single exposure to As or Cr, in both adults and children, was greater than 10⁻⁴. The total cancer risk (CRt) resulting from multiple exposure to carcinogenic elements exceeded the acceptable value (CRt >1 ×10⁻⁴) in both adults and children. Overall, exposure to heavy metals through rice consumption poses potential non-carcinogenic and carcinogenic health risks to the local residents in the northern area; thus, regular monitoring of pollution in the area is crucial.

Role of Ferrous Sulfate (FeSO4) in Resistance to Cadmium Stress in Two Rice (Oryza sativa L.) Genotypes

The impact of heavy metal, i.e., cadmium (Cd), on the growth, photosynthetic pigments, gas exchange characteristics, oxidative stress biomarkers, and antioxidants machinery (enzymatic and non-enzymatic antioxidants), ions uptake, organic acids exudation, and ultra-structure of membranous bounded organelles of two rice (Oryza sativa L.) genotypes (Shan 63 and Lu 9803) were investigated with and without the exogenous application of ferrous sulfate (FeSO4). Two O. sativa genotypes were grown under different levels of CdCl2 [0 (no Cd), 50 and 100 µM] and then treated with exogenously supplemented ferrous sulfate (FeSO4) [0 (no Fe), 50 and 100 µM] for 21 days. The results revealed that Cd stress significantly (p < 0.05) affected plant growth and biomass, photosynthetic pigments, gas exchange characteristics, affected antioxidant machinery, sugar contents, and ions uptake/accumulation, and destroy the ultra-structure of many membranous bounded organelles. The findings also showed that Cd toxicity induces oxidative stress biomarkers, i.e., malondialdehyde (MDA) contents, hydrogen peroxide (H2O2) initiation, and electrolyte leakage (%), which was also manifested by increasing the enzymatic antioxidants, i.e., superoxidase dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) and non-enzymatic antioxidant compounds (phenolics, flavonoids, ascorbic acid, and anthocyanin) and organic acids exudation pattern in both O. sativa genotypes. At the same time, the results also elucidated that the O. sativa genotypes Lu 9803 are more tolerant to Cd stress than Shan 63. Although, results also illustrated that the exogenous application of ferrous sulfate (FeSO4) also decreased Cd toxicity in both O. sativa genotypes by increasing antioxidant capacity and thus improved the plant growth and biomass, photosynthetic pigments, gas exchange characteristics, and decrease oxidative stress in the roots and shoots of O. sativa genotypes. Here, we conclude that the exogenous supplementation of FeSO4 under short-term exposure of Cd stress significantly improved plant growth and biomass, photosynthetic pigments, gas exchange characteristics, regulate antioxidant defense system, and essential nutrients uptake and maintained the ultra-structure of membranous bounded organelles in O. sativa genotypes.

Salicylic acid mediated reduction in grain cadmium accumulation and amelioration of toxicity in Oryza sativa L. cv Bandana

Contamination of agricultural fields with Cadmium (Cd) due to several agricultural practices is increasing worldwide. The rice plants can easily take up Cd and accumulate it into different parts, including the grains, posing a threat to human health even at low concentration exposure. Several phytohormones, including Salicylic acid (SA) have been investigated since long for its alleviating properties under various biotic and abiotic stress conditions. In the present study, 100 μM SA application to ameliorate 25 μM Cd stress was studied for 72 h in hydroponics in Oryza sativa cv. Bandana seedlings. Pot experiments were done with same treatment condition and plants were grown till maturity. SA application to Cd exposed rice seedlings alleviated the stress condition, which was established by several physiological, biochemical, histochemical and gene expression analysis. SA treatment to Cd stressed seedlings showed elevated photosynthetic pigment content, on-protein thiol content and relieved the Cd induced growth inhibition considerably. It lowered the accumulation of ROS like, O2^- and H₂O₂ with a regulated antioxidative enzymatic activity. SA application in Cd exposed rice seedlings had upregulated expression of OsHMA3 and OsPCS1 whereasOsNRAMP2 gene was downregulated. Co-application of SA and Cd led to higher yield and improved agronomic traits in comparison to only Cd exposed plants under pot experimentation. Daily intake of Cd and Carcinogenic risk were also reduced by 99.75% and 99.99% respectively in the SA treated Cd stressed plants. SA positively affected the growth and tolerance of rice seedlings to Cd stress. Hence, SA addition to Cd contaminated soil can ensure rice cultivation without posing health risk to consumers.

Nutrient allocation might affect the cadmium accumulation of Bermuda grass (Cynodon dactylon)

Hydroponic experiments with different cadmium (Cd) concentrations were conducted to explore the impacts of nutrient allocation on the Cd enrichment of Bermuda grass. The results demonstrated with the Cd concentration increase, the accumulation of nitrogen (N), phosphorus (P) and potassium (K) in roots, stems and leaves increased first and then decreased, while the accumulation of Cd had no significant difference. The biomass and N, P and K accumulation of CD1 and CD2 were significantly different from those of CD3 and CD4, but there was no significant difference in Cd accumulation. The root N, P and K distribution ratio of CD4 increased by 47.9%, 114.3% and 64.3% compared with those of CD2 treatment, the values of stem decreased by 29.4%, 22.4% and 17.2%, and the values of leaves increased by 15.8%, 19.8% and 23.6% respectively. The K ratio of root and leaf increased and that of stem decreased. Cd reduced N and K distribution ratio of stem and increased N and K distribution ratio of root and leaf. Pearson correlation analysis showed that the accumulation of N, P and K in stems was positively correlated with the accumulation of Cd in stems, and the accumulation of N, P and K in roots, stems and leaves were positively correlated with the Cd accumulation in leaves. Bermuda grass can schedule the nutrient allocation to adapt to the Cd absorption and enrichment in different organs under different Cd concentrations. In conclusion, nutrient allocation might affect the Cd accumulation of Bermuda grass.

Identification of Durum Wheat Cultivars and Their Tetraploid Relatives with Low Cadmium Content

In this study, 71 durum wheat cultivars (Triticum durum Desf.), 22 emmer wheat (Triticum dicoccum L.) and 11 wild emmer (Triticum dicoccoides L.) genotypes were genetically characterized to determine the alleles associated with high cadmium (Cd) content. After genotypic characterization, 14 cultivars selected among all genotypes with low and high Cd content were phenotyped by a pot experiment to verify the genotypic data. Identification of 32 durum wheat, one emmer wheat and four wild emmer genotypes showed that they have alleles associated with high Cd content, while 68 genotypes of which 39 durum wheat, 21 emmer wheat and 7 wild emmer cultivars had alleles associated with low Cd content, respectively. Moreover, phenotypic data obtained from pot experiment were similar to the molecular data. To sum up, the marker successfully classified durum wheat cultivars into either high or low accumulators and these results can be safely used in breeding programs to improve new durum wheat cultivars with alleles associated with low Cd content. Due to routine use of phosphorus fertilizers in agricultural areas and other anthropogenic factors related to Cd toxicity, new durum wheat cultivars with low Cd content should be urgently developed for safe production of macaroni or other types of wheat products for human and animal consumption.

Effects of biochar and crop straws on the bioavailability of cadmium in contaminated soil

Numerous studies have been investigated the potential of biochar (BC) derived from various materials and crop straw (CS) to decrease the bioavailability of heavy metals in soil contaminated with cadmium (Cd), and thereby reduce their potential risk to human health and the ecological environment. However, little attention has been given to the comparison of heavy metal remediation efficiency using BC and CS such as peanut vine (PV) and rice straw (RS), especially in soil contaminated with Cd. Here, we explore if Cd bioavailability is affected in contaminated soil by BC and CS. Peanuts were grown in plastic pots, which contained BC or CS at 5% (dry weight, w/w) in controlled environment mesocosms. The bioavailability of Cd in contaminated soil was measured by Cd concentration in the plant and the concentrations of various forms of Cd in the soil. At the same plant age, growth with BC (compared with PV and RS) led to 13.56% and 8.28% lower rates of Cd content in the aboveground parts, 40.65% and 35.67% lower rates of Cd content in the seeds, yet 9.08% and 7.09% lower rates of Cd content in the roots, yet 35.80% and 28.48% lower rates of exchangeable Cd content in the soil. Moreover, BC amendment enhanced the biomass of peanut and physiological quality. Thus, BC had a greater impact on immobilizing Cd in the soil. The results imply that BC was more significantly (P < 0.05) remarkable in decreasing the Cd bioavailability and improving the biomass of peanut. BC has greater potential for enhancing soil quality and promoting peanut growth. In conclusion, this research demonstrates an understanding of employing BC as a promising inexpensive and eco-friendly amendment to remediate soil contaminated with Cd.

As-Hg Compound Pollution: Rice Growth, Yield, and Environmental Safety Limits

With rapid economic and industrial development, the potentially toxic element (PTE) pollution of farmlands in China has become increasingly serious. Based on the Hakanson potential ecological risk index (RI) and national food pollutant limit standards (GB 2762-2017), the effects of exogenous potentially toxic elements (PTEs), i.e., arsenic (As) and mercury (Hg), on rice growth were studied. Common varieties of hybrid (YY 538, CY 84) and conventional (XS 134, JH 218) rice were grown in pots having representative paddy soil from Zhejiang Province, China. The results showed that As and Hg in soil significantly inhibited growth; however, the plant height of hybrid rice was less affected. Further, there were significant correlations between concentrations of PTEs in soil and rice grains; however, hybrid rice had a stronger ability to absorb and accumulate PTEs. The comprehensive potential ecological RI of soil PTEs was 39.4–89.9, which is below the level of ‘minor ecological hazard’, and the contribution rate of Hg was 79.9%–84.2%; as such, Hg posed the greatest potential ecological risk in the study area. The ecological risk limits of As and Hg in the safe production of different rice varieties decreased in the order XS 134 > JH 218 > YY 538 > CY 84. The ecological risk limits of As and Hg for conventional rice were higher, and the risk limits of As and Hg for hybrid rice were lower, than the limit of the soil environmental quality assessment index in the national “Soil quality control criteria for soil pollution in agricultural land (for trial implementation) (Trial)” (GB15618-2018). The health risk limit of soil As in the typical paddy soil of China decreased in the order XS 134 > JH 218 > CY 84 > YY 538. The health risk limit of Hg decreased in the order XS 134 > JH 218 > YY 538 > CY 84. The health risk limit of As was lower, whereas the risk limit of Hg was higher, than the limit of the soil environmental quality evaluation index in the national “Soil quality control criteria for soil pollution in agricultural land (for trial implementation) (Trial)”(GB15618-2018).

Cd accumulation, biomass and yield of rice are varied with silicon application at different growth phases under high concentration cadmium-contaminated soil

Many reports suggest that exogenous Si addition could reduce Cd translocation to aerial part and make grain safe for rice production. But it remains unclear whether its supplementation during different growth phases can differentially impact Cd uptake in rice. Here, Positive effects of Si applied at different growth phases on biomass and yield of rice were observed. Dry weight of shoot including stem, leaf and grain was enhanced significantly by 15% when Si added at transplanting stage. Grain-yields supplied with Si at transplanting, jointing and heading stages were increased obviously by 14%, 11% and 12%, respectively. Higher percentage of filled spikelet and lower unfilled spikelet number per panicle were found when Si supplied at jointing stage. The increases of Cd accumulation in rice plants by Si application were mainly manifested in roots. Compared to CK, Cd accumulation in root when Si applied at transplanting and tillering stages were elevated by 75% and 64%, respectively. While Cd accumulation in aboveground tissues were all declined by Si addition. Bioconcentration and translocation factors were decreased significantly when Si added at jointing stage. In addition, higher soil pH, lowest available Cd-concentration in soil and Cd²⁺ concentration in xylem sap at 15d after flowering were found when the Si was applied at jointing stage. Overall, Si application at transplanting would be more beneficial to the growth of rice and increased Cd content in root, while Si supplied at jointing would be more favorable for grain filling and reducing Cd accumulation in shoot.

Biopriming with Piriformospora indica ameliorates cadmium stress in rice by lowering oxidative stress and cell death in root cells

Piriformospora indica is known for plant growth promotion and abiotic stress alleviation potential in several agricultural crops. However, a systemic analysis is warranted to explore potential application of this important fungus to augment heavy metal tolerance in rice. The present study explores potential of P. indica in ameliorating the effect of cadmium (Cd) stress in rice cultivars N22 and IR64. Seedlings inoculated with P. indica recorded significantly higher root-shoot length and biomass as compared to non-inoculated plants under control and Cd stress, respectively. Moreover, P. indica inoculated stressed roots accumulated more Cd as compared to non-inoculated stressed roots in both the varieties. Interestingly, cell death and reactive oxygen species (ROS) accumulation were significantly lower in the inoculated plant roots as compare with non-inoculated roots under Cd stress. The results emphasized significantly higher accumulation of Cd in fungal spores could reduce ROS accumulation in root cells resulting in lower cell death.

Bioaccumulation of Cadmium Affects Development, Mating Behavior, and Fecundity in the Asian Corn Borer, Ostrinia furnacalis

Heavy metal pollution is becoming an increasingly serious problem in agricultural ecosystems. Heavy metals such as cadmium (Cd) accumulate in the food chain and may lead to detrimental effects on the physiological functions of living organisms, including herbivorous insects. One such example is the Asian Corn Borer, Ostrinia furnacalis (Lepidoptera: Pyralidae). However, how Cd can affect the development and reproduction of O. furnacalis is largely unknown. In this study, we exposed larvae of O. furnacalis to a diet containing Cd and investigated the effects of Cd on the development, mating behavior, and fecundity of the insect. We showed that Cd accumulates in the larvae and inhibits development by extending larval and pupal duration and decreasing the survival rate. The excretion of Cd through multiple routes during the larval and pupal stages resulted in low levels of residual Cd in the adult insects, which were not fed with Cd. However, the mating behavior and fecundity of these insects were significantly affected, compared to control insects. This suggests that the bioaccumulation of heavy metals such as Cd has long lasting and detrimental effects on O. furnacalis over the entire life cycle, affecting fecundity, even when specimens are only exposed at an early life stage.

Subcellular distribution and tolerance of cadmium in Canna indica L

Canna indica L. is a promising species for heavy metal phytoremediation due to its fast growth rate and large biomass. However, few studies have investigated cadmium (Cd) tolerance mechanisms. In the present study, Canna plants were cultivated under hydroponic conditions with increasing Cd concentrations (0, 5, 10, 15 mg/L). We found that the plants performed well under 5 mg/L Cd²⁺ stress, but damage was observed under higher Cd exposure, such as leaf chlorosis, growth inhibition, a decreased chlorophyll content, and destruction of the ultrastructure of leaf cells. Additionally, Canna alleviated Cd toxicity to a certain extent. After Canna was exposed to 5, 10 and 15 mg/L Cd²⁺ for 45 d, the highest Cd concentration was exhibited in roots, which was almost 17-47 times the Cd concentration in leaves and 8-20 times that in stems. At the subcellular level, cellular debris and heat-stable proteins (HSPs) were the main binding sites for Cd, and the proportion of Cd in the two subcellular fractions accounted for 71.4-94.2% of the total Cd. Furthermore, we found that granules could participate in the detoxification process when Cd stress was enhanced. Our results indicated that Canna indica L. can tolerate Cd toxicity by sequestering heavy metals in root tissues, fencing out by cell wall, and binding with biologically detoxified fractions (granules and HSPs).

Functional Haplotype and eQTL Analyses of Genes Affecting Cadmium Content in Cultivated Rice

BACKGROUND

Rice is a major food resource for Asian countries including Korea. However, most Asian countries are facing food safety problems due to cropland contamination by heavy metals. Thus, this study was conducted to investigate genetic factors affecting the expression of cadmium (Cd) gene, and to confirm differences in Cd translocation among cultivars because the current molecular understanding of Cd uptake-transport mechanisms remains insufficient. Associations between genotypes and gene expression level of Cd-related genes such as NRAMP, MTP, and HMA gene families in the rice core collection were analyzed at the genomic level.

RESULTS

Os01g0956700, Os05g0128400 and Os11g0485200 showed strong associations between expression level and genotype in the rice core collection, the regulatory factors that associated with these genes in cis and trans were founded. The association between the expression level and genotype of the candidate gene (Os01g0611300: metal tolerance protein) predicted to affect Cd content in rice by a previous genome-wide association study (GWAS) was also analyzed. Furthermore, as a result of the phylogeny and haplotype analyses of the candidate gene, high-Cd tolerance cultivars were selected. The correlations between Cd and other inorganic components (Mg, Mn, Fe, Cu and Zn) in the roots, stems, leaves and unpolished grain of selected rice cultivars were analyzed.

CONCLUSION

Therefore, these results may be useful for understanding the uptake-transport mechanisms of Cd and other inorganic components via molecular genetics and may help rice breeders develop new low-Cd cultivars in the near future.

Selenium decreases methylmercury and increases nutritional elements in rice growing in mercury-contaminated farmland

Methylmercury (MeHg) in rice grains grown in Hg-contaminated areas has raised environmental health concerns. Pot experiments found that selenium (Se) could reduce MeHg levels in rice grains. However, relatively high levels of Se (up to 6 mg/kg) were applied in these pot experiments, which may have adverse effects on the soil ecology due to the toxicity of Se. The aims of this work were thus to study 1) the effect of low levels of Se on the accumulation and distribution of Hg, especially MeHg, in rice plants grown in a real Hg-contaminated paddy field and 2) the effect of Se treatment on Se and other nutritional elements (e.g., Cu, Fe, Zn) in grains. A field study amended with different levels of Se was carried out in Hg-contaminated paddy soil in Qingzhen, Guizhou, China. The levels of MeHg and total Hg were studied using cold vapor atomic fluorescence spectrometry (CVAFS) and inductively coupled plasma mass spectrometry (ICP-MS). The distribution and relative quantification of elements in grains were examined by synchrotron radiation X-ray fluorescence analysis (SR-XRF). This field study showed that low levels of Se (0.5 μg/mL, corresponding to 0.15 mg Se/kg soils) could significantly reduce total Hg and MeHg in rice tissues. Se treatment also reduced Hg distribution in the embryo and endosperm and increased the levels of Fe, Cu, Zn and Se in grains and especially embryos. This field study implied that treatment with an appropriate level of Se is an effective approach to not only decrease the level of MeHg but to also increase the levels of nutritional elements such as Fe, Cu, Zn and Se in rice grains, which could bring beneficial effects for rice-dependent residents living in Hg-contaminated areas.

Advances in the Uptake and Transport Mechanisms and QTLs Mapping of Cadmium in Rice

Cadmium (Cd), as a heavy metal, presents substantial biological toxicity and has harmful effects on human health. To lower the ingress levels of human Cd, it is necessary for Cd content in food crops to be reduced, which is of considerable significance for ensuring food safety. This review will summarize the genetic traits of Cd accumulation in rice and examine the mechanism of Cd uptake and translocation in rice. The status of genes related to Cd stress and Cd accumulation in rice in recent years will be summarized, and the genes related to Cd accumulation in rice will be classified according to their functions. In addition, an overview of quantitative trait loci (QTLs) mapping populations in rice will be introduced, aiming to provide a theoretical reference for the breeding of rice varieties with low Cd accumulation. Finally, existing problems and prospects will be put forward.

Screening of candidate gene responses to cadmium stress by RNA sequencing in oilseed rape (Brassica napus L.)

Cadmium (Cd) stress is one of the most serious threats to agriculture in the world. Oilseed rape (Brassica napus L.) is an important oil crop; however, Cd can easily accumulate in rapeseed and thus harm human health through the food chain. In the first experiment, our purpose was to measure the Cd accumulation in mature B. napus plants and its influences on fatty acid composition. The results showed that most Cd was accumulated in the root, and the seed fatty acid content was considerably different at different Cd toxicity levels. In the second experiment, 7-day-old B. napus seedlings stressed by Cd (1 mM) for 0 h (CK-0h), 24 h (T-24h), or 72 h (T-72h) were submitted to physiological and biological analyses, RNA-Seq and qRT-PCR. In total, 5469 and 6769 differentially expressed genes (DEGs) were identified in the comparisons of "CK-0h vs T-24h" and "CK-0h vs T-72h", respectively. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses showed that the photosynthetic and glutathione (GSH) pathways were significantly enriched in response to Cd stress. Key factors in the response to Cd stress included BnPCS1, BnGSTU12, BnGSTU5, and BnHMAs. The transcription factors BnWRKY11 (BnaA03g51590D), BnWRKY28 (BnaA03g43640D), BnWRKY33 (BnaA03g17820D), and BnWRKY75 (BnaA03g04160D) were upregulated after Cd exposure. The present study revealed that upregulation of the genes encoding GST and PCS under Cd stress promoted the formation of low-molecular weight complexes (PC-Cd), and upregulation of heavy metal ATPase genes induced PC-Cd transfer to vacuoles. These findings may provide the basis for the molecular mechanism of the response of B. napus to Cd.

Effect of Cd on growth, physiological response, Cd subcellular distribution and chemical forms of Koelreuteria paniculata

Koelreuteria paniculata were cultivated in nutrient solution with different concentrations of Cd (0, 50, 150, 250 and 500 µM) and sampled after 90 days. The resistance, translocation, accumulation and stress responses in Koelreuteria paniculata were investigated by hydroponic experiments. The results showed that Koelreuteria paniculata is an efficient Cd excluder that can tolerate high concentrations of Cd (up to 150-250 µM of Cd). The concentration of Cd never exceeds 5 ppm in leaves and 10 ppm in roots. The high concentration of Cd (≥ 250 µM) had a toxic effect on K. paniculata and significantly restricted the plant growth. The accumulation ability of Cd by different plant tissues followed the sequence of roots > leaves > stems. The bioconcentration factors and translocation factors both were less than 1. Cd has the highest content in the cell wall and is migrated to soluble fractions and organelles at high concentrations. Undissolved Cd phosphate, pectates and protein-bound Cd were the predominant forms. The low concentration of Cd (≤150 µM) promoted the synthesis of soluble proteins, AsA and GSH, while high concentration of Cd clearly inhibited the physiological and biochemical process, caused membrane lipid peroxidation and severe membrane damages, and increased MDA and H₂O₂ contents. POD, CAT and SOD exhibited positive and effective responses to low concentration Cd stress, but could not remove the toxicity caused by high concentration Cd stress. The content of IAA, GA and ZT decreased and ABA content was significantly increased under high-concentration Cd stress.

Physiological responses to cadmium stress in strawberry treated with pomegranate peel-activated carbon

Many reports have already been published regarding the removal of heavy metals from aqueous solutions onto activated carbon. However, the biosorbents' effect on the plant response still needs further investigation. In this study, activated carbon derived from the pomegranate peel [pomegranate activated carbons (PAC)] was used to see the effects of the addition of PAC on growing strawberry in Cd-contaminated sand. Cd accumulation and toxicity to strawberry was investigated by measuring the concentration of Cd in plant tissues and various biochemical activities of plant. Our results suggested that PAC had a high sorption capacity for Cd. Strawberry plant tried to deal with the Cd-induced oxidative stress by strengthening its antioxidant competences and decreasing Cd absorption. In comparison with the control, PAC applied to the sand decreased the level of lipid peroxidation and enhanced the carotenoid content. The greater tolerance of strawberry toward the level of Cd due to the application of PAC was associated with improving the physical conditions of the soil, increasing the amounts of some essential elements and decreasing the level of Cd absorption. Gaviota strawberry cultivar exposed to 5 or 10 mg kg^-1 Cd was able to adopt a new metabolic equilibrium, allowing the plant to cope with this metal.

Cadmium and lead accumulations and agronomic quality of a newly bred pollution-safe cultivar (PSC) of water spinach

Breeding for pollution-safe cultivars (PSCs) can reduce pollutant accumulation in crops. However, the PSC breeding would face the risk of nutritional quality reduction, which is usually ignored in conventional breeding programs targeting to increase crop yield or nutritional quality. Thus, the doubt whether the risk would exist has to be clarified for supporting the PSC breeding. In the present study, a newly bred Cd/Pb-PSC of water spinach (Ipomoea aquatic Forsk.) and its parents (QLQ with low-Cd/Pb accumulation ability and T308 with high yield) of water spinach were employed to clarify the above-mentioned issue. Yields, and concentrations of Cd, Pb, nitrite, and organic and inorganic nutrients in shoots of the three experimental lines were determined. There were no significant differences in Cd/Pb concentration between the new PSC and QLQ, in nitrite content between the new PSC and its two parents and in yield between the new PSC and T308. It is decisively significant that shoot concentrations of organic and inorganic nutrients in the Cd/Pb-PSC were as high as those in one of its parents. It is affirmed that the breeding operations (crossing and consequently continuous selfing) for lowering Cd/Pb accumulation capacity of water spinach would not lower the nutritional values of the obtained Cd/Pb-PSCs from the breeding, which should be a pillar that supports the feasibility to minimize Cd/Pb pollution in vegetables using PSC-breeding method.

Aluminum and Heavy Metal Accumulation in Tea Leaves: An Interplay of Environmental and Plant Factors and an Assessment of Exposure Risks to Consumers

Environmental and plant factors (soil condition, variety, season, and maturity) and exposure risks of aluminum (Al), manganese (Mn), lead (Pb), cadmium (Cd), and copper (Cu) in tea leaves were investigated. The concentrations of these metals in tea leaves could not be predicted by their total concentrations in the soil. During any one season, there were differences in Al, Mn, and Cd levels between tea varieties. Seasonally, autumn tea and/or summer tea had far higher levels of Al, Mn, Pb, and Cd than did spring tea. Tea leaf maturity positively correlated with the concentrations of Al, Mn, Pb, and Cd, but negatively with Cu. The calculated average daily intake doses (mg/ [kg•d]) for these metal elements were 0.14 (Al), 0.11 (Mn), 2.70 × 10^-3 (Cu), 2.80 × 10^-4 (Pb), and 2.88 × 10^-6 (Cd). The hazard quotient values of each metal were all significantly lower than risk level (=1), suggesting that, for the general population, consumption of tea does not result in the intake of excessive amounts of Al, Mn, Pb, Cd, or Cu. This study identified the factors that can be monitored in the field to decrease consumer exposure to Al and Mn through tea consumption.

PRACTICAL APPLICATION

Environmental and plant factors influence aluminum and heavy metal accumulation in tea leaves. Consumers of tea are not ingesting excessive Al, Mn, Pb, Cd, or Cu. Trackable factors were identified to manage exposure levels.

Selenium mitigates cadmium-induced oxidative stress in tomato (Solanum lycopersicum L.) plants by modulating chlorophyll fluorescence, osmolyte accumulation, and antioxidant system

Pot experiments were conducted to investigate the role of selenium in alleviating cadmium stress in Solanum lycopersicum seedlings. Cadmium (150 mg L^-1) treatment caused a significant reduction in growth in terms of height and biomass accumulation and affected chlorophyll pigments, gas exchange parameters, and chlorophyll fluorescence. Selenium (10 μM) application mitigated the adverse effects of cadmium on growth, chlorophyll and carotenoid contents, leaf relative water content, and other physiological attributes. Lipid peroxidation and electrolyte leakage increased because of cadmium treatment and selenium-treated plants exhibited considerable reduction because of the decreased production of hydrogen peroxide in them. Cadmium-treated plants exhibited enhanced activity of antioxidant enzymes that protected cellular structures by neutralizing reactive free radicals. Supplementation of selenium to cadmium-treated plants (Cd + Se) further enhanced the activity of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR) by 19.69, 31.68, 33.14, and 54.47%, respectively. Osmolytes, including proline and glycine betaine, increased with selenium application, illustrating their role in improving the osmotic stability of S. lycopersicum under cadmium stress. More importantly, selenium application significantly reduced cadmium uptake. From these results, it is clear that application of selenium alleviates the negative effects of cadmium stress in S. lycopersicum through the modifications of osmolytes and antioxidant enzymes.

Adaptive response of arbuscular mycorrhizal symbiosis to accumulation of elements and translocation in Phragmites australis affected by cadmium stress

Arbuscular mycorrhizal (AM) fungi have been reported to play a central role in improving plant tolerance to cadmium (Cd)-contaminated sites. This is achieved by enhancing both the growth of host plants and the nutritive elements in plants. This study assessed potential regulatory effects of AM symbiosis with regard to nutrient uptake and transport, and revealed different response strategies to various Cd concentrations. Phragmites australis was inoculated with Rhizophagus irregularis in the greenhouse cultivation system, where it was treated with 0-20 mg L^-1 of Cd for 21days to investigate growth parameters, as well as Cd and nutritive element distribution in response to AM fungus inoculation. Mycorrhizal plants showed a higher tolerance, particularly under high Cd-level stress in the substrate. Moreover, our results determined the roots as dominant Cd reservoirs in plants. The AM fungus improved Cd accumulation and saturated concentration in the roots, thus inhibiting Cd uptake to shoots. The observed distributions of nutritive elements and the interactions among these indicated the highest microelement contribution to roots, Ca contributed maximally in leaves, and K and P contributed similarly under Cd stress. In addition, AM fungus inoculation effectively impacted Mn and P uptake and accumulation while coping with Cd toxicity. This study also demonstrated translocation factor from metal concentration (TF) could be a good parameter to evaluate different transportation strategies induced by various Cd stresses in contrast to the bioconcentration factor (BCF) and translocation factor from metal accumulation (TF').

Cadmium accumulation characteristics of low-cadmium rice (Oryza sativa L.) line and F1 hybrids grown in cadmium-contaminated soils

Cadmium (Cd) pollution has threatened severely to food safety and human health. A pot experiment and a field experiment were conducted to investigate the difference of Cd accumulation between rice (Oryza sativa L.) lines and F₁ hybrids in Cd-contaminated soils. The adverse effect on biomass of rice lines was greater than that of F₁ hybrids under Cd treatments in the pot experiment. The variations of Cd concentration among rice cultivars in different organs were smaller in stem and leaf, but larger in root and ear. Average proportion of Cd in root of F₁ hybrids was 1.39, 1.39, and 1.16 times higher than those of rice lines at the treatment of 1, 2, and 4 mg Cd kg^-1 soil, respectively. Cd concentrations in ear of F₁ hybrids were significantly lower than rice lines with the reduction from 29.24 to 50.59%. Cd concentrations in brown rice of all F₁ hybrids were less than 0.2 mg kg^-1 at 1 mg Cd kg^-1 soil, in which Lu98A/YaHui2816, 5406A/YaHui2816, and C268A/YaHui2816 could be screened out as cadmium-safe cultivars (CSCs) for being safe even at 2 mg Cd kg^-1 soil. C268A/YaHui2816 showed the lowest Cd concentration in root among F₁ hybrids, while Lu98A/YaHui2816 and 5406A/YaHui2816 showed lower capability of Cd translocation from root to shoot under Cd exposure, which eventually caused the lower Cd accumulation in brown rice. The lower level of Cd translocation contributed to reducing the accumulation of Cd in brown rice had been validated by the field experiment. Thus, Lu98A/YaHui2816, 5406A/YaHui2816, and C268A/YaHui2816 could be considered as potential CSCs to cultivate in Cd-contaminated soils (<2 mg Cd kg^-1 soil).

Role of Rhizophagus irregularis in alleviating cadmium toxicity via improving the growth, micro- and macroelements uptake in Phragmites australis

Arbuscular mycorrhizal (AM) fungi have been used to alleviate heavy metal stress on plant growth and uptake of micro- and macroelements. A greenhouse pot experiment was conducted to verify the effects of AM fungus Rhizophagus irregularis on the growth, physiological characteristics, total Cd, and element uptake of Phragmites australis under different Cd stress (in the range of 0-20 mg L^-1). The results showed that the symbiosis could effectively alleviate Cd toxicity with greater root biomass, higher photosynthesis rate, and lower levels of malonaldehyde (MDA) and proline than non-mycorrhizal plants could. However, reduced transpiration rate (Tr) and stomatal conductance (g _s) indicated R. irregularis protected host plants from Cd stress (≥5 mg L^-1) via the stomatal closure. Although micro- and macroelements displayed differently in the presence of Cd, higher concentrations were still detected in mycorrhizal plants in contrast to non-mycorrhizal plants. Moreover, step multiple regression significantly demonstrated Pn_max, stem diameter (Sd), and g _s were the important factors with regard to total Cd uptake in the symbiosis, but Mn affected to non-mycorrhizal plants. These results suggested R. irregularis could alleviate the competition between Mn and Cd by altering plant physiology. This work clearly demonstrated that R. irregularis can be able to support P. australis growth better even though under high Cd stress (>1 mg L^-1), suggesting its good potential for practical use in high Cd-contaminated areas.

Cadmium Bioavailability, Uptake, Toxicity and Detoxification in Soil-Plant System

This review summarizes the findings of the most recent studies, published from 2000 to 2016, which focus on the biogeochemical behavior of Cd in soil-plant systems and its impact on the ecosystem. For animals and people not subjected to a Cd-contaminated environment, consumption of Cd contaminated food (vegetables, cereals, pulses and legumes) is the main source of Cd exposure. As Cd does not have any known biological function, and can further cause serious deleterious effects both in plants and mammalian consumers, cycling of Cd within the soil-plant system is of high global relevance.The main source of Cd in soil is that which originates as emissions from various industrial processes. Within soil, Cd occurs in various chemical forms which differ greatly with respect to their lability and phytoavailability. Cadmium has a high phytoaccumulation index because of its low adsorption coefficient and high soil-plant mobility and thereby may enter the food chain. Plant uptake of Cd is believed to occur mainly via roots by specific and non-specific transporters of essential nutrients, as no Cd-specific transporter has yet been identified. Within plants, Cd causes phytotoxicity by decreasing nutrient uptake, inhibiting photosynthesis, plant growth and respiration, inducing lipid peroxidation and altering the antioxidant system and functioning of membranes. Plants tackle Cd toxicity via different defense strategies such as decreased Cd uptake or sequestration into vacuoles. In addition, various antioxidants combat Cd-induced overproduction of ROS. Other mechanisms involve the induction of phytochelatins, glutathione and salicylic acid.

Effect of cadmium accumulation on mineral nutrient levels in vegetable crops: potential implications for human health

Consumption of vegetables is often the predominant route whereby humans are exposed to the toxic metal Cd. Health impacts arising from Cd consumption may be influenced by changes in the mineral nutrient content of vegetables, which may occur when plants are exposed to Cd. Here, we subjected model root (carrot) and leaf (lettuce) vegetables to soil Cd concentrations of 0.3, 1.5, 3.3, and 9.6 μg g(-1) for 10 weeks to investigate the effect of Cd exposure on Cd accumulation, growth performance, and mineral nutrient homeostasis. The findings demonstrated that Cd accumulation in lettuce (20.1-71.5 μg g(-1)) was higher than that in carrot (3.2-27.5 μg g(-1)), and accumulation exceeded the maximum permissible Cd concentration in vegetables when soil contained more than 3.3 μg g(-1) of Cd. There was a marked hormetic effect on carrot growth at a soil Cd concentration of 3.3 μg g(-1), but increasing the Cd concentration to 9.6 μg g(-1) caused decreased growth in both crops. Additionally, in most cases, there was a positive correlation between Cd and the mineral nutrient content of vegetables, which was due to physiological changes in the plants causing increased uptake and/or translocation. This may suggest a general mechanism whereby the plant compensated for disrupted mineral nutrient metabolism by increasing nutrient supply to its tissues. Increased nutrient levels could potentially offset some risks posed to humans by increased Cd levels in crops, and we therefore suggest that changes in mineral nutrient levels should be included more widely in the risk assessment of potentially toxic metal contamination. Graphical abstract The Cd concentration (μg g-1 in dry matter) in the root, shoot and translocation factor (TF) of Cd from root to shoot in the carrot and lettuce, and the percentage of root Cd to the gross Cd contents (%) in carrot (C) and lettuce (D) exposed to soil Cd (0 (control), 1, 3, and 9 μg g-1) for 70 days. Values are means ± SD (n = 5).

Increasing CO2 differentially affects essential and non-essential amino acid concentration of rice grains grown in cadmium-contaminated soils

Environmental pollution by both ambient CO2 and heavy metals has been steadily increasing, but we do not know how fluctuating CO2 concentrations influence plant nutrients under high Cd pollution, especially in crops. Here, we studied the effects of elevated CO2 and Cd accumulation on proteins and amino acids in rice under Cd stress. In this pot experiment, we analyzed the amino-acid profile of 20 rice cultivars that accumulate Cd differently; the plants were grown in Cd-containing soils under ambient conditions and elevated CO2 levels. We found that although Cd concentrations appeared to be higher in most cultivars under elevated CO2 than under ambient CO2, the effect was significant only in seven cultivars. Combined exposure to Cd and elevated CO2 strongly decreased rice protein and amino acid profiles, including essential and non-essential amino acids. Under elevated CO2, the ratios of specific amino acids were either higher or lower than the optimal ratios provided by FAO/WHO, suggesting that CO2 may flatten the overall amino-acid profile, leading to an excess in some amino acids and deficiencies in others when the rice is consumed. Thus, Cd-tainted rice limits the concentration of essential amino acids in rice-based diets, and the combination with elevated CO2 further exacerbates the problem.

Cadmium stress in rice: toxic effects, tolerance mechanisms, and management: a critical review

Cadmium (Cd) is one of the main pollutants in paddy fields, and its accumulation in rice (Oryza sativa L.) and subsequent transfer to food chain is a global environmental issue. This paper reviews the toxic effects, tolerance mechanisms, and management of Cd in a rice paddy. Cadmium toxicity decreases seed germination, growth, mineral nutrients, photosynthesis, and grain yield. It also causes oxidative stress and genotoxicity in rice. Plant response to Cd toxicity varies with cultivars, growth condition, and duration of Cd exposure. Under Cd stress, stimulation of antioxidant defense system, osmoregulation, ion homeostasis, and over production of signaling molecules are important tolerance mechanisms in rice. Several strategies have been proposed for the management of Cd-contaminated paddy soils. One such approach is the exogenous application of hormones, osmolytes, and signaling molecules. Moreover, Cd uptake and toxicity in rice can be decreased by proper application of essential nutrients such as nitrogen, zinc, iron, and selenium in Cd-contaminated soils. In addition, several inorganic (liming and silicon) and organic (compost and biochar) amendments have been applied in the soils to reduce Cd stress in rice. Selection of low Cd-accumulating rice cultivars, crop rotation, water management, and exogenous application of microbes could be a reasonable approach to alleviate Cd toxicity in rice. To draw a sound conclusion, long-term field trials are still required, including risks and benefit analysis for various management strategies.

Correlations in metal release profiles following sorption by Lemna minor

Following the rapid uptake of contaminants in the first few hours of exposure, plants typically attempt to cope with the toxic burden by releasing part of the sorbed material back into the environment. The present study investigates the general trends in the release profiles of different metal(loid)s in the aquatic macrophyte Lemna minor and details the correlations that exist between the release of metal(loid) species. Water samples with distinct contamination profiles were taken from Nilüfer River (Bursa, Turkey), Yeniçağa Lake (Bolu, Turkey), and Beyşehir Lake (Konya, Turkey) and used for release studies; 36 samples were tested in total. Accumulation and release profiles were monitored over five days for 11 metals and a metalloid ((208)Pb, (111)Cd, (52)Cr,(53)Cr,(60)Ni,(63)Cu,(65)Cu,(75)As,(55)Mn, (137)Ba, (27)Al, (57)Fe, (66)Zn,(68)Zn) and correlation, cluster and principal component analyses were employed to determine the factors that affect the release of these elements. Release profiles of the tested metal(loid)s were largely observed to be distinct; however, strong correlations have been observed between certain metal pairs (Cr/Ni, Cr/Cu, Zn/Ni) and principal component analysis was able to separate the metal(loid)s into three well-resolved groups based on their release.

Screening of Cd-safe genotypes of Chinese cabbage in field condition and Cd accumulation in relation to organic acids in two typical genotypes under long-term Cd stress

A 65-day field experiment was conducted to select cadmium (Cd)-safe genotypes (CSGs) among 21 Chinese cabbage genotypes in a low Cd-contaminated soil (0.66 mg kg(-1)). Seven CSGs were identified based on their Cd tolerance, shoot Cd concentrations, Cd enrichment factors (EFs), and translocation factors (TFs). Then, Beijingxin3, a typical CSG, together with Qiuxiang, a typical non-CSG for comparison, was selected for a subsequent 80-day field micro-plot experiment under four levels of Cd stress to evaluate the reliability of CSG screening and the role of organic acids in Cd accumulation and tolerance. Beijingxin3 was confirmed to be safe to grow in soil with Cd level up to 3.39 mg kg(-1), with Cd accumulation in its shoots well below the permitted level, and Qiuxiang was still poor in tolerating low Cd stress (1.31 mg kg(-1)). With increasing the Cd stress, Cd accumulation and citrate concentrations increased in shoots and roots of both genotypes, and oxalate concentrations increased significantly in Beijingxin3 roots. Both oxalate and citrate concentrations were significantly positively related to Cd accumulation for Beijingxin3 roots. High accumulation in oxalate and citrate induced by Cd stress in Beijingxin3 roots could benefit its internal tolerance to long-term Cd stress with more Cd accumulation in its roots and less Cd accumulation in its shoots.