Cadmium and lead interactive effects on oxidative stress and antioxidative responses in rice seedlings

Citric acid enhanced phytoextraction of nickel (Ni) and alleviate Mentha piperita (L.) from Ni-induced physiological and biochemical damages

Phytoremediation is considered one of the well-established and sustainable techniques for the removal of heavy metals and metalloids from contaminated sites. The metal extraction ability of the plants can be enhanced by using suitable organic materials in combination with metal-tolerant plants. This experiment was carried out to investigate the phytoextraction potential of Mentha piperita L. for nickel (Ni) with and without citric acid (CA) amendment in hydroponic experiment. The experiment was performed in controlled glass containers with continuous aeration in complete randomized design (CRD). Juvenile M. piperita plants were treated with different concentrations of Ni (100, 250, and 500 μM) alone and/or combined with CA (5 mM). After harvesting the plants, the morpho-physiological and biochemical attributes as well as Ni concentrations in different tissues of M. piperita plants were measured. Results revealed that Ni stress significantly decreased the plant agronomic traits, photosynthesis in comparison to control. Nickel stress enhanced the antioxidant enzymes activities and caused the production of reactive oxygen species (ROS) in M. piperita. The CA treatment under Ni stress significantly improved the plant morpho-physiological and biochemical characteristics when compared with Ni treatments alone. The results demonstrated that CA enhanced the Ni concentrations in roots, stems, and leaves up to 138.2%, 54.2%, and 38%, respectively, compared to Ni-only-treated plants. The improvement in plant growth with CA under Ni stress indicated that CA is beneficial for Ni phytoextraction by using tolerant plant species. Graphical abstract.

Cadmium, Smoking, and Human Blood DNA Methylation Profiles in Adults from the Strong Heart Study

BACKGROUND

The epigenetic effects of individual environmental toxicants in tobacco remain largely unexplored. Cadmium (Cd) has been associated with smoking-related health effects, and its concentration in tobacco smoke is higher in comparison with other metals.

OBJECTIVES

We studied the association of Cd and smoking exposures with human blood DNA methylation (DNAm) profiles. We also evaluated the implication of findings to relevant methylation pathways and the potential contribution of Cd exposure from smoking to explain the association between smoking and site-specific DNAm.

METHODS

We conducted an epigenome-wide association study of urine Cd and self-reported smoking (current and former vs. never, and cumulative smoking dose) with blood DNAm in 790,026 CpGs (methylation sites) measured with the Illumina Infinium Human MethylationEPIC (Illumina Inc.) platform in 2,325 adults 45-74 years of age who participated in the Strong Heart Study in 1989-1991. In a mediation analysis, we estimated the amount of change in DNAm associated with smoking that can be independently attributed to increases in urine Cd concentrations from smoking. We also conducted enrichment analyses and in silico protein-protein interaction networks to explore the biological relevance of the findings.

RESULTS

At a false discovery rate (FDR)-corrected level of 0.05, we found 6 differentially methylated positions (DMPs) for Cd; 288 and 17, respectively, for current and former smoking status; and 77 for cigarette pack-years. Enrichment analyses of these DMPs displayed enrichment of 58 and 6 Gene Ontology and Kyoto Encyclopedia of Genes and Genomes gene sets, respectively, including biological pathways for cancer and cardiovascular disease. In in silico protein-to-protein networks, we observed key proteins in DNAm pathways directly and indirectly connected to Cd- and smoking-DMPs. Among DMPs that were significant for both Cd and current smoking (annotated to PRSS23, AHRR, F2RL3, RARA, and 2q37.1), we found statistically significant contributions of Cd to smoking-related DNAm.

CONCLUSIONS

Beyond replicating well-known smoking epigenetic signatures, we found novel DMPs related to smoking. Moreover, increases in smoking-related Cd exposure were associated with differential DNAm. Our integrative analysis supports a biological link for Cd and smoking-associated health effects, including the possibility that Cd is partly responsible for smoking toxicity through epigenetic changes. https://doi.org/10.1289/EHP6345.

Goethite-modified biochar ameliorates the growth of rice (Oryza sativa L.) plants by suppressing Cd and As-induced oxidative stress in Cd and As co-contaminated paddy soil

Co-contamination of soils with cadmium (Cd) and arsenic (As) in rice growing areas is a serious threat to environment and human health. Increase in soil Cd and As levels curtail the growth and development of rice plants by causing oxidative stress and reduction in photosynthetic activity. Therefore, it is necessary to formulate and evaluate different strategies for minimizing the Cd and As uptake in rice plant. We modified biochar (BC) with goethite and assessed the effects of goethite-modified biochar (GB) application on mitigating Cd and As stress in rice plant. Although BC supply to rice plants enhanced their performance in contaminated soil but application of different GB levels i.e.1.5% GB to the soil resulted in prominent improvements in physiological and biochemical attributes of rice plants grown in Cd and As co-contaminated paddy soil. It was observed that soil amendment with GB increased the plant growth, biomass, photosynthetic pigments, gas exchange attribute of rice plant and suppressed the oxidative stress in rice leaves and roots by increased antioxidant enzymes activities. Supplementing the soil with 1.5% GB incremented the iron plaque (Fe-plaque) formation and enhanced the Cd and As sequestration by Fe-plaque. Application of GB (1.5%) significantly improved the Fe content of Fe-plaque by 68.7%. Maximum Cd (1.57 mg kg^-1) and As (0.85 mg kg^-1) sequestration by Fe-plaque was observed with 1.5% GB treatment. Compared to the control, 1.5% GB treatment application prominently reduced the Cd content in the rice roots and shoots by 42.9%, and 56.7%, respectively and As content in the rice roots and shoots declined by 32.2%, 46.6%, respectively, compared to the control. These findings demonstrate that amending the soil with 1.5% GB can be a potential remediation strategy for checking Cd and As accumulation, reducing oxidative stress and increasing the growth of rice plant.

Application of co-composted farm manure and biochar increased the wheat growth and decreased cadmium accumulation in plants under different water regimes

Toxic trace element pollution in the agricultural soils may negatively affect the plant growth. This study mainly focused on investigating the impact of co-composted biochar and farmyard manure (FYM) on wheat growth and cadmium (Cd) accumulation by plants. The different ratios of FYM and biochar were composted for two and half months and mixed in Cd-contaminated soil at a rate of 2% w/w of each treatment. After this, wheat seeds were sown in the soil at normal soil moisture (70% of soil water holding capacity (WHC)) level. After 50-day of sowing, both normal and drought stress (35% WHC) levels were applied and plants were harvested at 122 days after seed sowing. The results depicted that Cd and drought alone depressed the wheat growth, elevated the oxidative stress and Cd contents in wheat tissues. However, application of co-composted treatments increased the growth, yield, chlorophyll contents and minimized the oxidative stress in the leaves along with the reduction of Cd concentrations in wheat tissues mainly in grains. The amendments enhanced the post-harvest soil pH and minimized the soil bioavailable Cd. The increasing ratios of biochar in the compost were most effective in improving the growth and alleviating Cd toxicity and its concentration in grains. Overall, co-composted biochar and FYM might be suitable for reducing Cd in grains, but the field studies in different soils and plants are required to further explore the effects of these amendments before final recommendations.

Interactive effects of multiple heavy metal(loid)s on their bioavailability in cocontaminated paddy soils in a large region

Heavy metal(loid) pollution in large regions is often highly complex due to the coexistence and interactions of metal(loid)s and complex soil environments. However, the interaction effects of heavy metal(loid)s on their bioavailability in large regions remain obscure. Here, we identified the interaction effects of Pb, Cd and As on their bioavailability in cocontaminated paddy soils of Shaoguan City, China. The results showed that pH (12.40%) was the most important impact factor among the key soil properties of the pH, CEC, and SOM for the bioconcentration factor (BCF) of Cd. As the soil Pb content decreased and the soil Cd content increased, the interaction of soil Pb and Cd increased the BCF of Pb. As the soil Pb content increased and the soil Cd content decreased, the interaction of soil Pb and Cd increased the BCF of Cd. The synergistic interaction of Pb and Cd in soil promoted the accumulation of both Pb and Cd in rice under certain soil conditions. Specifically, when the average pH and CEC were below 6.19 ± 0.82 and above 7.23 ± 2.55 cmol (+)/kg, respectively, the average BCF of Cd varied from 0.70 ± 0.80 to 1.47 ± 0.62 due to the interactive effect of soil Cd and Pb on the BCF of Cd. When the average pH was above 6.19 ± 0.82 and the average SOM was below and above 27.12 ± 20.34 mg/kg, respectively, the corresponding average BCF of Cd varied from 0.70 ± 0.63 to 0.10 ± 0.26. As the average soil Cd and As contents increased, the interactive effect of soil As and Cd on the average BCF of As decreased. The negative interaction of As and Cd in soil mitigated As accumulation in rice at certain soil Cd and As contents. Our results suggest that to achieve the large-scale control of heavy metal(loid) pollution, the interactions among multiple metal(loid)s and soil properties should be considered.

Goethite-modified biochar restricts the mobility and transfer of cadmium in soil-rice system

Cadmium (Cd) contamination of paddy soils has raised serious concerns for food safety and security. Remediation and management of Cd contaminated soil with biochar (BC) and modified biochar is a cost-effective method and has gained due attention in recent years. Goethite-modified biochar (GB) can combine the beneficial effects of BC and iron (Fe) for remediation of Cd contaminated soil. We probed the impact of different BC and GB amendments on Cd mobility and transfer in the soil-rice system. Both BC and GB effectively reduced Cd mobility and availability in the rhizosphere and improved the key growth attributes of rice. Although BC supply to rice plants enhanced their performance in contaminated soil but application of 1.5% GB to the soil resulted in prominent improvements in physiological and biochemical attributes of rice plants grown in Cd contaminated soil. Sequential extraction results depicted that BC and GB differentially enhanced the conversion of exchangeable Cd fractions to non-exchangeable Cd fractions thus restricted the Cd mobility and transfer in soil. Furthermore, supplementing the soil with 1.5% GB incremented the formation of iron plaque (Fe plaque) and boosted the Cd sequestration by Fe plaque. Increase in shoot and root biomass of rice plants after GB treatments positively correlates with incremented chlorophyll contents and gas exchange attributes. Additionally, the oxidative stress damage in rice plants was comparatively reduced under GB application. These findings demonstrate that amending the soil with 1.5% GB can be a potential remediation method to minimize Cd accumulation in paddy rice and thereby can protect human beings from Cd exposure.

Efficacy of Zea mays L. for the management of marble effluent contaminated soil under citric acid amendment; morpho-physiological and biochemical response

The adverse industrial activities discharged contaminated wastewater directly into the water bodies that contain toxic substances such as heavy metals. The contours use of marble industrial effluents may affect the fertility of soil and crop growth. The present study was conducted to investigate the toxic effects of marble industrial effluents (M.E) on Zea mays L under the exogenous application of citric acid (CA) with different combinations such as marble industrial effluent (0, 30%, 60%, 100%) diluted with distilled water and CA (10 mM). The results showed significant decrease in the growth of Zea mays with increasing concentration of marble industrial effluent. The maximum reduction in plant height, root length, number of leaves, leaf area and fresh and dry biomass was observed at the application of 100% M.E as compared to control. Similar to growth conditions the photosynthetic machinery and the activities of antioxidant enzymes (Superoxide dismutase (SOD), Peroxidases (POD), Catalases (CAT), Ascorbate peroxidase (APX)) was also decreased with increasing concentration of M.E. The application of CA significantly alleviated the M.E induced toxic effect on Zea mays and ameliorated the growth, biomass, photosynthesis and antioxidant enzymes activities by reducing the production of reactive oxygen species. The C.A application also enhanced the heavy metal content such as chromium (Cr), cadmium (Cd), Zinc (Zn) in different parts of Zea mays. The results concluded that the Zea mays tolerant varieties can be a potential candidate for the M.E irrigated soil and might be suitable for the phyto-extraction of Cr, Cd and Zn.

Phytotoxicity of silver nanoparticles on Vicia faba: Evaluation of particle size effects on photosynthetic performance and leaf gas exchange

Nanotechnology is an emerging field in science and engineering, which presents significant impacts on the economy, society and the environment. The nanomaterials' (NMs) production, use, and disposal is inevitably leading to their release into the environment where there are uncertainties about its fate, behaviour, and toxicity. Recent works have demonstrated that NMs can penetrate, translocate, and accumulate in plants. However, studies about the effects of the NMs on plants are still limited because most investigations are carried out in the initial stage of plant development. The present study aimed to evaluate and characterize the photochemical efficiency of photosystem II (PSII) of broad bean (Vicia faba) leaves when subjected to silver nanoparticles (AgNPs) with diameters of 20, 51, and 73 nm as well as to micrometer-size Ag particles (AgBulk). The AgNPs were characterized by transmission electron microscopy and dynamic light scattering. The analyses were performed by injecting the leaves with 100 mg L^-1 aqueous solution of Ag and measuring the chlorophyll fluorescence imaging, gas exchange, thermal imaging, and reactive oxygen species (ROS) production. In addition, silver ion (Ag⁺) release from Ag particles was determined by dialysis. The results revealed that AgNPs induce a decrease in the photochemical efficiency of photosystem II (PSII) and an increase in the non-photochemical quenching. The data also revealed that AgNPs affected the stomatal conductance (g_s) and CO₂ assimilation. Further, AgNPs induced an overproduction of ROS in Vicia faba leaves. Finally, all observed effects were particle diameter-dependent, increasing with the reduction of AgNPs diameter and revealing that AgBulk caused only a small or no changes on plants. In summary, the results point out that AgNPs may negatively affect the photosynthesis process when accumulated in the leaves, and that the NPs themselves were mainly responsible since negligible Ag⁺ release was detected.

Physiological and molecular mechanisms of cobalt and copper interaction in causing phyto-toxicity to two barley genotypes differing in Co tolerance

The combined effects of cobalt (Co) and copper (Cu) in their toxicity to plants is poorly studied although these two metals co-exist commonly in soil. In this study, a hydroponic experiment was carried out to investigate the effect of longer exposure of two barley genotypes differing in Co tolerance to the combined Co and Cu stress. The results confirmed the previous findings that Co accumulation in plant tissues was reduced by Cu presence, while Cu accumulation was stimulated by Co presence. Moreover, both single and combined treatments of Co and Cu reduced the mineral (Mn, Zn and K) uptake. Co and Cu applied alone or in combination at rate of 50 μM resulted in the significant reduction of plant growth and increase of oxidative stress (ROS and MDA), and meanwhile the capacity of scavenging active oxygen species (AOS) was enhanced, reflected by increased phytochelatin (PC) and glutathione (GSH and GSSG) content, as well as expression of the related genes (HvPCS1 and HvGR1). Yan66, a Co tolerant genotype was less affected in oxidative stress, and had higher AOS scavenging capacity in comparison with Ea52, a Co sensitive one. Among three HvSOD isoforms, only HvFeSOD expression was up-regulated in the combined treatment relative to control as well as the treatment of Co or Cu alone, while HvCuZnSOD and HvMnSOD were down-regulated and unaffected, respectively. In addition, the expressions of metal transporter genes (HvHMA2, HvHMA3 and HvHMA5) varied with genotype and metal treatments, with the extent being greater in Yan66 on the whole. The results suggest that upon longer exposure to the combined stress of Co and Cu, the greater phyto-toxicity than each element alone is associated with more Cu accumulation stimulated by Co and that, the higher regulation of transporter genes observed in Yan66 could in part explain for its higher metal tolerance ability.

Ascorbate-Mediated Modulation of Cadmium Stress Responses: Reactive Oxygen Species and Redox Status in Brassica napus

The role of ascorbate (AsA) in antioxidant defense system-associated resistance to cadmium (Cd) in oilseed rape plants has not yet been clearly demonstrated. The present study investigated the critical role of exogenous AsA on the physiological and biochemical responses of reactive oxygen species (ROS) and antioxidant scavenging defense systems in oilseed rape (Brassica napus L. cv. Tammi) seedlings exposed to Cd. Cd (10 μM) treatment led to significant reductions in plant growth; increases in the levels of superoxide anion radical, hydrogen peroxide, and malondialdehyde; and increases in Cd uptake and accumulation by the roots and shoots in hydroponically grown 10-day-old seedlings. Moreover, it reduced AsA content and AsA redox ratios, which have been correlated with reductions in glutathione (GSH) and/or nicotinamide adenine dinucleotide phosphate (NADPH) redox status. However, exogenously applying AsA to Cd-exposed seedlings decreased Cd-induced ROS, improved antioxidant defense systems by increasing AsA, GSH, and NADPH contents, and increased Cd uptake and accumulation in both roots and shoots of the plants. These results provided evidence that the enhancement in AsA redox status can be linked to an increase in the GSH and/or NADPH redox ratios through the induction of the AsA-GSH-NADPH cycle. Thus, these results suggest that exogenous AsA application to oilseed rape seedlings under Cd stress might alleviate the overall Cd toxicity by regulating the homeostasis of the AsA-GSH-NADPH cycle, which reestablishes the steady-state cellular redox status.

Oxidative stress induced by cadmium in lettuce (Lactuca sativa Linn.): Oxidative stress indicators and prediction of their genes

Environmental contamination with heavy metals is of concern as plants have the ability to absorb chemical toxicants facilitating the entry of toxic metals into the food chain. Lettuce (Lactuca sativa Linn.) was cultured in four nutrient solutions containing different concentrations of cadmium (0, 3, 6, and 9 mmol). The impact of heavy metal on the morphological features, antioxidant properties and antioxidant enzymes activity were investigated with primary focus on superoxide dismutase, ascorbate peroxidase, peroxidase and catalase enzymes. In silico methods were utilized in the study of the genes of these enzymes. Significant changes were observed in the morphological features of the plant with plants appearing stunted, more spherical and yellow in colour. A decrease in the dry mass of the plant was also detected. The Translocation factor (TF) for cadmium was significantly high in lettuce. Enhanced antioxidant enzymatic activity suggests that these enzymes are integrally involved in the defense mechanism of the plant to heavy metal stress. Also observed was an increase in total soluble protein, and total phenolic content. Total flavonoid content was not significantly affected. Fourteen genes encoding for ascorbate peroxidase and nineteen genes for superoxide dismutase were identified in lettuce. These enzymes varied from each other with regards to the number of exons and amino acids present, as well as their location within the cell. Plants exhibit various response mechanisms to combat heavy metal contamination.

Lead Toxicity in Cereals: Mechanistic Insight Into Toxicity, Mode of Action, and Management

Cereals are the major contributors to global food supply, accounting for more than half of the total human calorie requirements. Sustainable availability of quality cereal grains is an important step to address the high-priority issue of food security. High concentrations of heavy metals specifically lead (Pb) in the soil negatively affect biochemical and physiological processes regulating grain quality in cereals. The dietary intake of Pb more than desirable quantity via food chain is a major concern for humans, as it can predispose individuals to chronic health issues. In plant systems, high Pb concentrations can disrupt several key metabolic processes such as electron transport chain, cellular organelles integrity, membrane stability index, PSII connectivity, mineral metabolism, oxygen-evolving complex, and enzymatic activity. Plant growth-promoting rhizobacteria (PGPR) has been recommended as an inexpensive strategy for remediating Pb-contaminated soils. A diverse group of Ascomycetes fungi, i.e., dark septate endophytes is successfully used for this purpose. A symbiotic relationship between endophytes and host cereal induces Pb tolerance by immobilizing Pb ions. Molecular and cellular modifications in plants under Pb-stressed environments are explained by transcription factor families such as bZIP, ERF, and GARP as a regulator. The role of metal tolerance protein (MTP), natural resistance-associated macrophage protein (NRAMP), and heavy metal ATPase in decreasing Pb toxicity is well known. In the present review, we provided the contemporary synthesis of existing data regarding the effects of Pb toxicity on morpho-physiological and biochemical responses of major cereal crops. We also highlighted the mechanism/s of Pb uptake and translocation in plants, critically discussed the possible management strategies and way forward to overcome the menace of Pb toxicity in cereals.

Interactions between decabromodiphenyl ether and lead in soil-plant system

Pot experiments were conducted under abiotic conditions to investigate the interactive influence of decabromodiphenyl ether (BDE-209) and lead (Pb) on the seed germination, germ length, root exudation and physiological characteristics of tall fescue (Festuca arundinaceae), and the uptake, accumulation of Pb and BDE-209 in the plant tissues. Results show that seed germination and germ length were impacted by Pb but less influenced by BDE-209. BDE-209 spiking (10 and 50 mg/L) could alleviate the toxicity of high Pb concentration on seed germination and growth. The chlorophyll content was significantly increased at 500 mg/kg Pb but declined at 2000 mg/kg Pb. Low-level Pb contamination (500 mg/kg) activated antioxidase activity; however, 2000 mg/kg Pb significantly reduced the antioxidase activity. Plant biomass slightly decreased at 500 mg/kg Pb but significantly declined at 2000 mg/kg Pb. The addition of a moderate dosage of BDE-209 (10-50 mg/kg) lessened Pb phytotoxicity, leading to improved plant growth relative to the case of Pb spiking alone. The exudate secretion was significantly enhanced by Pb addition, but BDE-209 spiking only caused slightly increased secretion. Pb could interfere with BDE-209 adsorption and translocation of tall fescue by affecting physiological behavior of the plant, but BDE-209 exhibited little influence on the Pb fate in the plant. Overall, BDE-209 had slight interference on the impact of Pb towards tall fescue. The results demonstrate the complex interactive effects of organic pollutants and heavy metals in the soil-plant system.

The effect of lead pollution on nutrient solution pH and concomitant changes in plant physiology of two contrasting Solanum melongena L. cultivars

Lead (Pb) is highly toxic to plants because it severely affects physiological processes by altering nutrient solution pH. The current study elucidated Pb-induced changes in nutrient solution pH and its effect on physiology of two Solanum melongena L. cultivars (cv. Chuttu and cv. VRIB-13). Plants were grown in black plastic containers having 0, 15, 20, and 25 mg L^-1 PbCl₂ in nutrient solutions with starting pH of 6.0. pH changes by roots of S. melongena were continuously monitored for 8 days, and harvested plants were analyzed for physiological and biochemical attributes. Time scale studies revealed that cv. Chuttu and cv. VRIB-13 responded to Pb stress by causing acidification and alkalinization of growth medium during the first 48 h, respectively. Both cultivars increased nutrient solution pH, and maximum pH rise of 1.21 units was culminated by cv. VRIB-13 at 15 mg L^-1 Pb and 0.8 units by cv. Chuttu at 25 mg L^-1 Pb treatment during the 8-day period. Plant biomass, photosynthetic pigments, ascorbic acid, total amino acid, and total protein contents were significantly reduced by Pb stress predominantly in cv. Chuttu than cv. VRIB-13. Interestingly, chlorophyll contents of cv. VRIB-13 increased with increasing Pb levels. Pb contents of roots and shoots of both cultivars increased with applied Pb levels while nutrient (Ca, Mg, K, and Fe) contents decreased predominately in cv. Chuttu. Negative correlations were identified among Pb contents of eggplant roots and shoots and plant biomasses, leaf area, and free anthocyanin. Taken together, growth medium alkalinization, lower root to shoot Pb translocation, and optimum balance of nutrients (Mg and Fe) conferred growth enhancement, ultimately making cv. VRIB-13 auspicious for tolerating Pb toxicity as compared with cv. Chuttu. The research outcomes are important for devising metallicolous plant-associated strategies based on plant pH modulation response and associated metal uptake to remediate Pb-polluted soil.

Copper alleviates cobalt toxicity in barley by antagonistic interaction of the two metals

Cobalt (Co) commonly co-exists with copper (Cu) in natural soils, but the information about their combined effects on plants is poorly available. In this study, we hydroponically investigated the combined effects of Co and Cu on two barley genotypes differing in Co toxicity tolerance to reveal the interaction pattern of these two metals. The results showed that single treatment of Co or Cu at the dose of 100 μM led to a significant decrease of growth and photosynthetic rate, and a significant increase of lipid peroxidation, ROS radicals as well as anti-oxidative enzyme (SOD, CAT and GR) activities and glutathione content, with the extent of effect being less in Yan66 than Ea52. The combined treatment of Co and Cu alleviated the toxicity of both metals in comparison with each metal treatment alone, as reflected by improved growth and photosynthesis, and much slight oxidative stress. The alleviation of metal toxicity upon combined treatment is mainly attributed to a drastic reduction of Co uptake and its translocation from roots to shoots. It may be suggested that interaction of Co and Cu on their uptake and movement in plants is antagonistic.

Reactive oxygen species, auxin and nitric oxide in metal-stressed roots: toxicity or defence

The presented review is a summary on the current knowledge about metal induced stress response in plants, focusing on the roles of reactive oxygen species, auxin and nitric oxide in roots. The article focuses mainly on the difference between defence and toxicity symptoms of roots during metal-induced stress. Nowadays, pollution of soils by heavy metals is a rapidly growing issue, which affects agriculture and human health. In order to deal with these problems, we must first understand the basic mechanisms and responses to environmental conditions in plants growing under such conditions. Studies so far show somewhat conflicting data, interpreting the same stress responses as both symptoms of defence and toxicity. Therefore, the aim of this review is to give a report about current knowledge of heavy metal-induced stress research, and also to differentiate between toxicity and defence, and outline the challenges of research, focusing on reactive oxygen and nitrogen species, auxin, and the interplay among them. There are still remaining questions on how reactive oxygen and nitrogen species, as well as auxin, can activate either symptoms of toxicity or defence, and adaptation responses.

Calcium amendment improved the performance of fragrant rice and reduced metal uptake under cadmium toxicity

Cadmium (Cd) toxicity has detrimental effects on plant metabolism and yield formation. This study examined the effects of Cd stress in rice and the possible role of calcium (Ca) in mitigating oxidative damage caused by Cd in two fragrant rice cultivars, i.e., Guixiangzhan and Meixiangzhan 2. The experimental treatments were composed of various Ca and Cd levels as individual, i.e., Ca at 2.5 and 5.0 mg/kg soil (Ca1 and Ca2, respectively), Cd at 50 and 100 mg/kg soil (Cd50 and Cd100, respectively), and combined, i.e., Ca1+Cd50, Ca1+Cd100, Ca2+Cd50, and Ca2+Cd100. Plants without Ca and Cd application were taken as control (CK). Results showed that Cd stress led to a substantial decline in the photosynthetic pigments, i.e., Chl a, Chl b, and carotenoids, while enhanced oxidative damage in terms of increased levels of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) and electrolyte leakage (EL) in both rice cultivars. Moreover, Cd stress hampered the activities of enzymatic antioxidants, i.e., superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), with lowest antioxidant activities were recorded at Cd100. The overall trend (lowest to highest) for antioxidant activities across treatments was recorded as Cd100 < Ca2+Cd100 < Cd50 < Ca1+Cd100 < CK < Ca1 < Ca1+Cd50 < Ca2+Cd50 < Ca2. Similarly, Ca amendment improved the proline, soluble protein, and soluble sugar contents in both rice cultivars under Cd stress condition. Comparing Ca2 with CK, the yield and related components, i.e., number of panicles, spikelets per panicle, seed setting rate, 1000 grain weight, and grain yield, were found to increase by 13.08, 2.39, 4.03, 5.86, and 27.53% for Guixiangzhan and 16.48, 5.19, 6.87, 15.44, and 51.16% for Meixiangzhan, respectively. Furthermore, Cd contents in roots, stems, leaves, and grains increased with increased Cd concentration applied and reduced with Ca amendment. The Cd contents in grains for all Ca+Cd levels are statistically at par with each other and significantly lower (P < 0.05) than those for individual Cd application. Hence, Ca amendment can be an appropriate approach to ameliorate the toxic effects of Cd in crops grown under Cd-contaminated soils.

Heavy metal-induced oxidative stress on seed germination and seedling development: a critical review

Heavy metal contamination in soils can influence plants and animals, often leading to toxicosis. Heavy metals can impact various biochemical processes in plants, including enzyme and antioxidant production, protein mobilization and photosynthesis. Hydrolyzing enzymes play a major role in seed germination. Enzymes such as acid phosphatases, proteases and α-amylases are known to facilitate both seed germination and seedling growth via mobilizing nutrients in the endosperm. In the presence of heavy metals, starch is immobilized and nutrient sources become limited. Moreover, a reduction in proteolytic enzyme activity and an increase in protein and amino acid content can be observed under heavy metal stress. Proline, is an amino acid which is essential for cellular metabolism. Numerous studies have shown an increase in proline content under oxidative stress in higher plants. Furthermore, heat shock protein production has also been observed under heavy metal stress. The chloroplast small heat shock proteins (Hsp) reduce photosynthesis damage, rather than repair or help to recover from heavy metal-induced damage. Heavy metals are destructive substances for photosynthesis. They are involved in destabilizing enzymes, oxidizing photosystem II (PS II) and disrupting the electron transport chain and mineral metabolism. Although the physiological effects of Cd have been investigated thoroughly, other metals such as As, Cr, Hg, Cu and Pb have received relatively little attention. Among agricultural plants, rice has been studied extensively; additional studies are needed to characterize toxicities of different heavy metals on other crops. This review summarizes the current state of our understanding of the effects of heavy metal stress on seed germination and seedling development and highlights informational gaps and areas for future research.

Physiological responses of Elaeocarpus glabripetalus seedlings exposed to simulated acid rain and cadmium

Combined effects of cadmium (Cd) and acid rain on physiological characteristics in Eleocarpus glabripetalus seedlings were investigated under controlled conditions. The single Cd treatment and the combined Cd and acid rain treatment increased growth at low Cd concentrations, while decreased growth and photosynthesis at high Cd²⁺ concentrations. A low Cd²⁺ concentration (50 mg kg^-1) combined with different acid rain treatments increased the seedling biomass. A high Cd²⁺ concentration (100 mg kg^-1) under different acid rain treatments significantly decreased the biomass, the Fe content, chlorophyll fluorescence and photosynthetic parameters. Relative electric conductivity, malondialdehyde (MDA) content and peroxidase (POD) activity were increased while the reduced glutathione (GSH) content and catalase (CAT) activity were significantly lower at high Cd²⁺ concentration under acid rain. The results indicated that the combination of a high concentration of Cd²⁺ and acid rain aggravated the toxic effect of Cd²⁺ or acid rain alone on the growth and physiological parameters of E. glabripetalus due to serious damage to the chloroplast structure. These results provide novel insights into the combined effects of Cd²⁺and acid rain on woody plants and might also serve as a guide to evaluate forest restoration and biological safety in areas with Cd²⁺and acid rain pollution.

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.

Hydrogen gas promotes the adventitious rooting in cucumber under cadmium stress

Hydrogen gas (H2) plays an important role in plant development and stress responses. Here, cucumber (Cucumis sativus L.) explants were used to investigate the roles of H2 in adventitious root development under cadmium (Cd) stress and its physiological mechanism. The results showed that hydrogen-rich water (HRW) promoted adventitious rooting under Cd stress and 50% HRW obtained the maximal biological response. Compared with Cd treatment, HRW + Cd treatment significantly reduced the content of malondialdehyde (MDA), hydrogen peroxide (H2O2), superoxide radical (O2-), thiobarbituric acid reactive substances (TBARS), ascorbic acid (AsA) and reduced glutathione (GSH), as well as relative electrical conductivity (REC), lipoxygenase (LOX) activity, AsA/docosahexaenoic acid (DHA) ratio, and GSH/oxidized glutathione (GSSG) ratio, while increasing DHA and GSSG content. HRW + Cd treatment also significantly increased in the activity and related gene expression of ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDHAR) and glutathione reductase (GR). Additionally, HRW + Cd treatment increased the contents of osmotic adjustment substances, as well as the activities of peroxidase (POD) and polyphenol oxidase (PPO), while significantly decreasing indoleacetic acid oxidase (IAAO) activity. In summary, H2 could induce adventitious rooting under Cd stress by decreasing the oxidative damage, increasing osmotic adjustment substance content and regulating rooting-related enzyme activity.

Combined toxic effects and DNA damage to two plant species exposed to binary metal mixtures (Cd/Pb)

Acute and long-term (3-, 10- and 56-day exposure) laboratory toxicity tests were carried out to assess the individual and combined toxic effects of cadmium (Cd) and lead (Pb) in Brassica oleracea and Trifolium repens. In addition to morphological parameters, this work also used comet assay to address endpoints in relation to genotoxicity. Bioaccumulation was measured to demonstrate the influence of the mixture on the concentrations of each metal in the plant. The statistical method reported by Ince et al. (1999) was used to evaluate the types of interaction between Cd and Pb in each treatment and concerning their combined effect. This study concludes that the combined effects of binary metal combinations of Cd/Pb on morphological parameters are most often additive, sometimes antagonistic and more rarely synergistic, thus extending the findings of previous publications on this subject. DNA damage analysis revealed concentration- and time-dependent interactions. Synergistic effects of mixed metals (more breaks than individually applied metals) are observed in T. repens after a short exposure. Antagonistic effects are statistically significant after 10 days-exposure, suggesting competition between metals. At 56 days, the rate of DNA damage observed in plants exposed to the Cd/Pb mixture was similar to that measured in plants exposed to lead only and was significantly lower than the rate of DNA damage induced by Cd. This supports the idea that there may be competition between metals and also strengthens the hypothesis that long-term reparation mechanisms may be implemented. Cd/Pb co-exposure does not significantly influence the bioaccumulation of each metal. It is nevertheless important to note that a statistically significant 'interaction' is not necessarily biologically relevant and should therefore be considered with caution when assessing heavy metals combined effects.

Cadmium accumulation and its effects on physiological and biochemical characters of summer savory (Satureja hortensis L.)

The objective of this study was to determine the effects of cadmium (Cd) toxicity on accumulation, growth, physiological responses, and biochemical characters in summer savory (Satureja hortensis L.). Plants were subjected to different levels of Cd concentrations including 0 (control), 2.5, 5, and 15 mg L^-1 in the growing medium. Cd exposure led to a significant increase in root and shoot Cd content. Calculation of bioaccumulation factor, translocation factor, and transfer coefficient revealed that Cd mostly accumulated in roots of S. hortensis and root to shoot transport was effectively restricted. Cd toxicity negatively affected plant growth and significantly reduced chlorophyll content. Contrarily, proline, soluble and reducing carbohydrates, anthocyanin content, and the activity of antioxidant enzymes significantly increased as a result of Cd exposure. Cd application led to a significant increase in essential oil content of S. hortensis. GC-MS analysis revealed that percentage main constitute of S. hortensi, carvacrol, which determines the quality of oil increased under the highest Cd treatment. Based on our findings, S. hortensis can be considered an invaluable alternative crop for mildly Cd-contaminated soils. Besides, due to the high potential of Cd accumulation in the root, S. hortensis may offer a feasible tool for phytostabilization purposes.

A multivariate analysis of physiological and antioxidant responses and health hazards of wheat under cadmium and lead stress

Soil contamination with heavy metals is a global issue confronting the environmental pollution and human/animal health. Much work has been done on physiological and antioxidant responses of wheat in hydroponic experiments and health risks from individual heavy metal contamination to human, but limited information is available on their combined application in soil. Therefore, this pot study delineates the uptake of lead and cadmium, as well as physiological responses of wheat and associated health risks under different levels of alone and combined Cd and Pb treatments. Metal uptake increased with their increasing applied levels. The highest Cd (4.24, 1.38, and 0.92 mg kg^-1) and Pb (763.33, 39.63, and 16.35 mg kg^-1) concentrations in root, shoot, and grain, respectively, were observed at highest applied levels (0.4 mM Cd and 10 mM Pb). Furthermore, all the treatments increased lipid peroxidation and activities of superoxide dismutase, catalase, ascorbate peroxidase, and peroxidase, while decreased total chlorophyll contents and membrane stability index. Under combined application of Cd and Pb, the toxicity and detoxification responses of wheat increased compared to alone treatments. Multivariate analysis further confirmed the toxicity and accumulation pattern of metals under alone and combined treatments. Target hazard quotient values of Cd and Pb were < 1 under alone and combined treatments. The health hazard index values of Pb (97.07 and 87.89%) were higher than those of Cd (2.93 and 12.10%) in combined application for human and buffalo, respectively. This study highlights that the multi-metal contamination (Cd and Pb) is detrimental for wheat growth and human/animal health.

Plant-lead interactions: Transport, toxicity, tolerance, and detoxification mechanisms

Natural and human activities introduced an excess level of toxic lead (Pb) to the environment. Pb has no known biological significance and its interactions with plants lead to the production of reactive oxygen species (ROS). Pb and/or ROS have the potential to cause phytotoxicity by damaging the tissue ultrastructure, cellular components, and biomolecules. These damaging effects may possibly result in the inhibition of normal cellular functioning, physiological reactions, and overall plant performances. ROS play a dual role and act as a signaling molecule in plant defense system. This system encircles enzymatic and non-enzymatic antioxidative mechanisms. Catalase, superoxide dismutase, peroxidase, and enzymes from the ascorbate-glutathione cycle are the major enzymatic antioxidants, while non-enzymatic antioxidants include phenols, flavonoids, ascorbic acid, and glutathione. Pb removal from contaminated sites using plants depend on the plant's Pb accumulation capacity, Pb-induced phytotoxicity, and tolerance and detoxification mechanisms plants adopted to combat against this phytotoxicity. However, the consolidated information discussing Pb-plant interaction including Pb uptake and its translocation within tissues, Pb-mediated phytotoxic symptoms, antioxidative mechanisms, cellular, and protein metabolisms are rather limited. Thus, we aimed to present a consolidated information and critical discussions focusing on the recent studies related to the Pb-induced toxicity and oxidative stress situations in different plants. The important functions of different antioxidants in plants during Pb stress have been reviewed. Additionally, tolerance responses and detoxification mechanisms in the plant through the regulation of gene expression, and glutathione and protein metabolisms to compete against Pb-induced phytotoxicity are also briefly discussed herein.

Single and combined effects of Cd and Pb on the growth, medium pH, membrane potential and metal contents in maize (Zea mays L.) coleoptile segments

The mechanisms of the toxic effects of Cd and Pb on plant cell growth are still poorly understood. In particular, little is known about their interactive effects, which usually occur in the environment. Moreover, the data that do exist in the literature are controversial. This study describes experiments that were performed with maize (Zea mays) coleoptile segments, which is a classical model system for studies of plant cell elongation growth. Cadmium and lead, which were added at 0.1 mM, reduced the endogenous and IAA-induced elongation growth of maize coleoptile cells. When both metals were added together or in sequence, their effect on IAA-induced growth was more toxic. The medium pH changes, which were measured simultaneously with growth, indicated that while Pb stopped IAA-induced proton extrusion, Cd only partially diminished it. Although Cd was generally more accumulated than Pb in the maize coleoptile segments, when IAA was added together with Pb, it significantly increased the accumulation of the metal. The short-term electrophysiological experiments showed that the addition of Cd caused the depolarisation of the membrane potential (E_m), whereas Pb caused membrane hyperpolarisation. In the long-term electrophysiological experiments, it was found that the Cd-induced E_m changes are complex. In conclusion, these results suggest that the effects of Cd and Pb as well as their combination on the elongation growth of maize coleoptile cells and the accumulation of the metals result, among others, from different ionic mechanisms by which each metal change the membrane potential of the cells.

The impact of environmental contamination on the generation of reactive oxygen and nitrogen species - Consequences for plants and humans

Environmental contaminants, such as heavy metals, nanomaterials, and pesticides, induce the formation of reactive oxygen and nitrogen species (RONS). Plants interact closely with the atmosphere, water, and soil, and consequently RONS intensely affect their biochemistry. For the past 30 years researchers have thoroughly examined the role of RONS in plant organisms and oxidative modifications to cellular components. Hydrogen peroxide, superoxide anion, nitrogen(II) oxide, and hydroxyl radicals have been found to take part in many metabolic pathways. In this review the various aspects of the oxidative stress induced by environmental contamination are described based on an analysis of literature. The review reinforces the contention that RONS play a dual role, that is, both a deleterious and a beneficial one, in plants. Environmental contamination affects human health, also, and so we have additionally described the impact of RONS on the coupled human - environment system.

Root iron plaque alleviates cadmium toxicity to rice (Oryza sativa) seedlings

Iron plaque (IP) on root surface can enhance the tolerance of plants to environmental stresses. However, it remains unclear the impact of Fe²⁺ on cadmium (Cd) toxicity to rice (Oryza sativa) seedlings. In this study, the effects of different Fe²⁺ and Cd²⁺ concentration combinations on rice growth were examined hydroponically. Results indicated that Fe²⁺ concentration up to 3.2 mM did not damage rice roots while induced IP formation obviously. Cd²⁺ of 10 μM repressed rice growth significantly, while the addition of 0.2 mM Fe²⁺ to 10 μM Cd²⁺ solution (Cd+Fe) did not damage rice roots, indicating that Fe²⁺ could ameliorate Cd toxicity to rice seedlings. Microstructure analysis showed Cd+Fe treatment induced the formation of IP with dense and intricate network structure, Cd adsorption on the root surface was reduced significantly. Cd concentration of rice roots and shoots and Cd translocation from roots to shoots with Fe+Cd treatment were reduced by 34.1%, 36.0% and 20.1%, respectively, in comparison to a single Cd treatment. Noteworthy, the removal of IP resulted in a larger loss of root biomass under Cd treatment. In addition, Cd+Fe treatment increased the activities of root superoxide dismutase and catalase by 105.5% and 177.4%, and decreased H₂O₂ and O₂·^- accumulation of rice roots by 56.9% and 35.9%, and recovered Cd-triggered electrolyte leakage obviously, when compared with a single Cd treatment. The results from this experiment indicated that the formed dense IP on rice roots decreased Cd absorption and reactive oxygen species accumulation, and Fe²⁺ supply alleviated Cd toxicity to rice seedlings.

Comparative analysis of cadmium-induced stress responses by the aromatic and non-aromatic rice genotypes of West Bengal

Constant exposure of the living ecosystems to heavy metals, like cadmium (Cd), induces a detectable change at the biochemical and genetic level. Repeated application of phosphate fertilizers in paddy fields, leads to increase in Cd content of soil. Cd being highly mobile is transported to shoot and grain, thereby entering into the food chain of animal system. In the present study, treatment of 7-day old rice seedlings with 10 μM cadmium chloride resulted in Cd toxicity across the seven non-aromatic and six aromatic rice cultivars and landraces used for the study. Free proline and malondialdehyde content of treated samples were higher in comparison to the untreated samples, which indicated Cd induced tissue damage in plants. Photosynthetic pigment content of treated samples was also found to be much lower in comparison to the untreated samples, which is probably due to peroxidation of membrane, leading to compromised and lower photosynthetic efficiency of treated plants. At the genetic level, Randomly Amplified Polymorphic DNA assay was found to efficiently detect the genetic polymorphisms (caused by alterations in DNA bases) induced by Cd. Production of unique polymorphic bands in Cd-treated plants helps in assessment of the degree of damage Cd imparts on the plant system. Cluster analysis was performed and the rice genotypes were grouped into five distinct clusters, with IR64 and Tulsibhog in two distinct groups. Based on the variability in responses, the 13 rice genotypes were grouped into sensitive and tolerant ones.

Deciphering the growth, organic acid exudations, and ionic homeostasis of Amaranthus viridis L. and Portulaca oleracea L. under lead chloride stress

Lead (Pb) stress adversely affects in planta nutrient homeostasis and metabolism when present at elevated concentration in the surrounding media. The present study was aimed at investigation of organic acid exudations, elemental contents, growth, and lipid peroxidation in two wild plants (Amaranthus viridis L. and Portulaca oleracea L.), exhibiting differential root to shoot Pb translocation, under Pb stress. Plants were placed in soil spiked with lead chloride (PbCl₂) concentrations of 0, 15, 30, 45, or 60 mg Pb/kg soil, in rhizoboxes supplied with nylon nets around the roots. The plant mucilage taken from root surfaces, mirroring the rhizospheric solution, was analyzed for various organic acids. Lead stress resulted in a release of basified root exudates from both plants. Exudates of P. oleracea roots showed a higher pH. In both plants, the pH rising effect was diminished at the highest Pb treatment level. The exudation of citric acid, glutamic acid (in both plants), and fumaric acid (in P. oleracea only) was significantly increased with applied Pb levels. In both plant species, root and shoot Pb contents increased while nutrients (Ca, Mg, and K) decreased with increasing Pb treatment levels, predominantly in A. viridis. At 60 mg Pb/kg soil, shoot Na content of A. viridis was significantly higher as compared to untreated control. Higher Pb treatment levels decreased plant fresh and dry masses as well as the quantity of photosynthetic pigments due to enhanced levels of plant H₂O₂ and thiobarbituric acid reactive substances in both species. Photosynthetic, growth, and oxidative stress parameters were grouped into three distinct dendrogram sections depending on their similarities under Pb stress. A positive correlation was identified between Pb contents of studied plants and secretion of different organic acids. It is concluded that Pb stress significantly impaired the growth of A. viridis and P. oleracea as a result of nutritional ion imbalance, and the response was cultivar-specific and dependent on exogenous applied Pb levels. Differential lipid oxidation, uptake of nutrients (Ca, Mg, and K) and exudation of citric acid, fumaric acid, and glutamic acid could serve as suitable physiological indicators for adaptations of P. oleracea to Pb enriched environment. The findings may help in devising strategies for Pb stabilization to soil colloids.

Effects of antimony on redox activities and antioxidant defence systems in sunflower (Helianthus annuus L.) plants

The alterations induced by the toxicity of antimony (Sb) in the roots and leaves of sunflower plants were determined. The plants were grown hydroponically with different concentrations of Sb, a heavy metal which reduces biomass production and growth. There was preferential accumulation of Sb in the tissues of the roots, with the concentrations in the leaves being much lower. The accumulation of other mineral elements was also altered, especially that of Fe and Zn. Chlorophyll content declined, as also did the photosynthetic efficiency, but the carotenoid content remained unaltered. The total content of phenolics, flavonoids, and phenylpropanoid glycosides rose, evidence of their participation in the defence response. Increases were observed in the amount of superoxide anion in both roots and leaves, and in lipid peroxidation levels, especially with the highest Sb concentration of 1.0 mM. The induced oxidative stress leads to a strong increase in the SOD, POX and APX antioxidant activities, while the GR activity was only increased in the leaves and at the 1.0 mM Sb concentration. In contrast, the DHAR activity increased considerably in both organs. The GSNOR activity increased only in roots, and the total RSNOs increased. The total amount of AsA + DHA increased in roots and remained unaltered in leaves, whereas that of GSH + GSSG decreased considerably in all cases. As a whole, these results are evidence for the development of a strong oxidative stress induced by Sb, with there being a clear imbalance in the content of the compounds that constitute the AsA/GSH cycle. 0.5 mM Sb enhances GST expression, especially in leaves. This, together with the increase that was observed in the amount of GSH, may play an important part in detoxification. This oxidative stress affects both the phenolic and the ROS/RNS metabolic processes, which seems to implicate their involvement in the plant's defence and response to the stress.

The utility of biochemical, physiological and morphological biomarkers of leaf sulfate levels in establishing Brachylaena discolor leaves as a bioindicator of SO2 pollution

The negative impacts of air pollution have made monitoring of air quality increasingly important, especially in heavily industrialized areas such as the South Durban Basin (SDB), in South Africa. Bioindicators such as trees can complement conventional air quality monitoring and be used to prioritise vulnerable areas. This study assessed the utility of Brachylaena discolor DC. tree leaves as a bioindicator of sulfur dioxide pollution. This involved correlating ground level sulfur dioxide concentrations ([SO₂]) with B. discolor leaf sulfate concentrations ([sulfate]) at three industrial (treatment) sites within the SBD and control site at which [SO₂] was significantly lower for all four seasons. Based on this significant positive correlation between the aforementioned (p = 0.005; ρ = 0.667) leaf sulfate levels were related to various biochemical (intracellular hydrogen peroxide production, total aqueous [TAA] and enzymic antioxidants [superoxide dismutase and catalase], lipid peroxidation [LPO] and electrolyte leakage), physiological (leaf chlorophyll fluorescence [LCF] and relative chlorophyll content) and morphological (leaf area [LA]) biomarkers of stress measured on leaves from trees at the treatment and control sites. Annual [SO₂] and leaf [sulfate] at the treatments were significantly (p < 0.05) higher than the control and high by global standards. TAA, LPO, electrolyte leakage, LCF, and LA were significantly (p < 0.05) correlated with seasonal [sulfate]. Except for superoxide dismutase, catalase and relative chlorophyll content all other biomarkers could differentiate between the treatment sites and the control. However, TAA, electrolyte leakage and LA were the only biomarkers sensitive enough to reflect differences in annual [sulfate] across the treatment sites; these should be used to establish B. discolor leaves as a bioindicator of SO₂ pollution.

Yield and quality responses, plant metabolism and metal distribution pattern in aromatic rice under lead (Pb) toxicity

Present study was planned to assess the yield and quality responses, plant physio-biochemical characters and Pb distribution pattern in two aromatic rice cultivars viz., Guixiangzhan (GXZ) and Nongxiang-18 (NX-18) under four different Pb-levels viz., control (0), low (400), medium (800) and high (1200) mg kg^-1of soil. Results revealed that Pb toxicity increased H₂O₂, lipid peroxidation and electrolyte leakage while inhibited photosynthetic pigments production, but such increment was higher in NX-18 than GXZ. Furthermore, Pb toxicity variably affected protein, proline and soluble sugars and the activities of enzymatic antioxidants viz., superoxide dismutase (SOD), peroxidases (POD), catalases (CAT) and ascorbate peroxidases (APX) and non-enzymatic anti-oxidants viz., reduced glutathione (GSH) and oxidized glutathione (GSSG) in both rice cultivars. The reductions in osmolyte accumulation and antioxidant activities were more severe in NX-18 than GXZ. Pb toxicity severely reduced yield and quality related attributes and plant biomass accumulation; however the reductions were more apparent NX-18 than GXZ. Furthermore, NX-18 accumulated less Pb proportions in roots (∼84%), and transferred more towards shoot, leaves, ears (at panicle heading (PH)) and grains (at maturity (MAT)) than GXZ (∼91% root Pb proportions). Hence, the ability of GXZ to retain higher Pb contents in roots and less towards upper plant parts, higher osmolyte accumulation and antioxidant activities and less yield reduction as compare to NX-18 might be an adaptive response of GXZ under Pb toxicity.

Influence and interaction of iron and cadmium on photosynthesis and antioxidative enzymes in two rice cultivars

In this study, a soil pot experiment was conducted to investigate the changes in photosynthesis and antioxidative enzymes in two rice varieties (Shendao 6 and Shennong 265) supplied with iron (Fe), cadmium (Cd), and Fe and Cd together. The concentrations of Fe and Cd in the soil were 0, 1.0 g Fe·kg^-1 and 0, 2.0 mg Cd·kg^-1, respectively. Photosynthetic indices and antioxidative enzyme activities were recorded at different rice growth stages. At the early stage, Cd showed a transient stimulatory effect on the photosynthetic rate of Shennong 265. For Shendao 6, however, Cd showed a transient stimulatory effect on photosynthetic rate, intercellular CO₂ concentration, stomatal conductance and transpiration efficiency. In addition, the results show that Cd can also enhance the superoxide dismutase (SOD) and peroxidase (POD) activities, but reduce the malondialdehyde (MDA) and soluble protein contents in the two rice cultivars. Subsequently, Cd starts to inhibit photosynthesis and SOD activity until the ripening stage, causing the lowest photosynthetic rate and SOD activity at this stage. In contrast, Fe alleviates the Cd-induced changes at earlier or later growth stage. Notably at the later growth stage, the results show that the interaction between Fe and Cd increases the SOD and catalase (CAT) activities, while decreasing the lipid peroxidation and promoting photosynthesis. As a result, it ultimately increases the biomass. The results from this study suggest that Fe (as Fe fertilizer) is a promising alternative for agricultural use to enhance the plant development and, simultaneously, to reduce Cd toxicity in extensively polluted soils.

Lead (Pb) Toxicity; Physio-Biochemical Mechanisms, Grain Yield, Quality, and Pb Distribution Proportions in Scented Rice

Lead (Pb) caused interruptions with normal plant metabolism, crop yield losses and quality issues are of great concern. This study assessed the physio-biochemical responses, yield and grain quality traits and Pb distribution proportions in three different fragrant rice cultivars i.e., Meixiangzhan-2, Xinagyaxiangzhan and Basmati-385. Plants were exposed to 400, 800, and 1,200 ppm of Pb while pots without Pb were taken as control (0 ppm). Our results showed that Pb toxicity significantly (P < 0.05) reduced photosynthetic pigments (chlorophyll contents and carotenoids) and inducted oxidative stress with increased production of hydrogen peroxide (H2O2), malanodialdehyde (MDA) and leaves leachates; while such effects were more apparent in Xinagyaxiangzhan than other two rice cultivars. Pb stress differentially affected the production protein, proline and soluble sugars; however the production rates were higher at heading stage (HS) than maturity stage (MS). Furthermore, Pb stress altered superoxide dismutase (SOD), peroxidases (POD), catalases (CAT) and ascorbate peroxidases (APX) activities and glutathione (GSH) and oxidized glutathione (GSSG) production in all rice cultivars at both HS and MS. All Pb levels reduced the yield and yield components of all rice cultivars; nonetheless such reductions were observed highest in Xinagyaxiangzhan (69.12%) than Meixiangzhan-2 (58.05%) and Basmati-385 (46.27%) and resulted in grain quality deterioration. Significant and positive correlations among rice yields with productive tillers/pot and grains per panicle while negative with sterility percentage were also observed. In addition, all rice cultivars readily taken up the Pb contents from soil to roots and transported upward in different proportions with maximum in roots followed by stemss, leaves, ears and grains. Higher proportions of Pb contents in above ground plant parts in Xinagyaxiangzhan possibly lead to maximum losses in this cultivar than other two cultivars; while less damage in Basmati-385 might be related to strong anti-oxidative defense system and lower proportions of Pb contents in its aerial parts.

Germination, Physiological Responses and Gene Expression of Tall Fescue (Festuca arundinacea Schreb.) Growing under Pb and Cd

Cadmium (Cd) and lead (Pb) are recognized as the most toxic metal ions due to their detrimental effects not only to plants, but also to humans. The objective of this study was to investigate the effects of Cd and Pb treatments on seed germination, plant growth, and physiological response in tall fescue (Festuca arundinacea Schreb.). We employed six treatments: CK (nutrient solution as control), T1 (1000 mg L-1 Pb), T2 (50 mg L-1 Cd), T3 (150 mg L-1 Cd), T4 (1000 mg L-1 Pb+50 mg L-1 Cd), T5 (1000 mg L-1 Pb+150 mg L-1 Cd). Antagonistic and synergistic actions were observed in tall fescue under Pb and Cd combined treatments. Under low Cd, plants exhibited higher relative germination rate, germ length, VSGR, catalase (CAT) and peroxidase (POD) activities. Additionally, in the shoots, the gene expression level of Cu/Zn SOD, FeSOD, POD, GPX, translocation factors, MDA, EL, and soluble protein contents were reduced under Pb stress. Conversely, under high Cd level, there was a decline in NRT, Pb content in shoots, Pb translocation factors, CAT activity; and an increase in VSGR, Pb content in roots, gene expression level of Cu/ZnSOD and POD in tall fescue exposed to Pb2+ regimes. On the other hand, tall fescue plants treated with low Cd exhibited lower relative germination rate, germination index, germ length, NRT, Cd content in roots. On the other hand there was higher Cd content, Cd translocation factor, CAT and POD activities, and gene expression level of Cu/Zn SOD, FeSOD, POD, GPX under Pb treatment compared with single Cd2+ treatment in the shoots. However, after high Cd exposure, plants displayed lower NRT, Cd content, CAT activity, and exhibited higher Cd contents, Cd translocation factor, MDA content, gene expression level of Cu/ZnSOD and GPX with the presence of Pb2+ relative to single Cd2+ treatment. These findings lead to a conclusion that the presence of low Cd level impacted positively towards tall fescue growth under Pb stress, while high level of Cd impacted negatively. In summary, antioxidant enzymes responded to Cd and Pb interaction at an early stage of exposure, and their gene expression profiles provided more details of the activation of those systems.

Cadmium Uptake and Distribution in Fragrant Rice Genotypes and Related Consequences on Yield and Grain Quality Traits

Cadmium (Cd) toxicity has varying consequences on plants growth and development. This research focused on examining the influence of Cd toxicity on fragrant rice genotypes and its consequences on yield, yield related parameters, and grain quality traits. Randomized complete block design comprising five different fragrant rice cultivars (Meixiangzhan 2, Xiangyaxiangzhan, Guixiangzhan, Basmati, and Nongxiang 18) and four Cd levels (0, 50, 100, and 150 mg/kg soil) was used. The results showed that, with increased levels of soil Cd toxicity, Cd uptake in roots and distribution to other organs increased in dose dependent manner. Uptake and accumulation were higher in roots than in shoots with the highest uptake in both roots and shoots observed in Meixiangzhan 2, followed by Nongxiang 18, Basmati, Xiangyaxiangzhan, and Guixiangzhan cultivars. With increased Cd toxicity, yield and yield parameters were affected in all cultivars. Grain quality was also found affected under Cd stress condition. The results suggested that soil Cd toxicity has negative consequences on rice performance and uptake varies among cultivars. Conclusively, Cd toxicity impaired rice yield formation and grain quality by altering yield components (panicles number, spikelet per panicles, and spikelet setting (%)); however, Guixiangzhan variety performed better, while Meixiangzhan 2 performed less in terms of minimum Cd uptake and distribution to grains, yield, and grain quality reduction under Cd stress condition.

Lead-induced stress, which triggers the production of nitric oxide (NO) and superoxide anion (O2·-) in Arabidopsis peroxisomes, affects catalase activity

Lead (Pb) contamination has a toxic effect on plant metabolisms, leading to a decrease in biomass production. The free radical nitric oxide (NO) is involved in the mechanism of response to a wide range of abiotic stresses. However, little is known about the interplay between Pb-induced stress and NO metabolism. Peroxisomes are sub-cellular compartments involved in multiple cellular metabolic pathways which are characterized by an active nitro-oxidative metabolism. Thus, Arabidopsis thaliana mutants expressing cyan fluorescent protein (CFP) through the addition of peroxisomal targeting signal 1 (PTS1), which enables peroxisomes to be visualized in vivo by confocal laser scanning microscopy (CLSM) combined with fluorescent probes for nitric oxide (NO), superoxide anion (O₂^·-) and peroxynitrite (ONOO^-), were used to evaluate the potential involvement of these organelles in the mechanism of response to 150 μM lead-induced stress. Both NO and O₂^·- radicals, and consequently ONOO^-, were overproduced under Pb-stress. Additionally, biochemical and gene expression analyses of peroxisomal enzymes, including the antioxidant catalase (CAT) and two photorespiration enzymes, such as glycolate oxidase (GOX) and hydroxypyruvate reductase (HPR), show that, under Pb-stress, only the catalase was negatively affected, while the two photorespiration enzymes remained unaffected. These results corroborate the involvement of plant peroxisomal metabolisms in the mechanism of response to lead contamination and highlight the importance of the peroxisomal NO metabolism.

A role for CK2 β subunit 4 in the regulation of plant growth, cadmium accumulation and H2O2 content under cadmium stress in Arabidopsis thaliana

Protein kinase CK2, which consists of two α and two β subunits, plays an essential role in plant development and is implicated in plant responses to abiotic stresses, including salt and heat. However, the function of CK2 in response to heavy metals such as cadmium (Cd) has not yet been established. In this study, the transgenic line CKB4ox, which overexpresses CKB4 encoding the CK2β subunit and has elevated CK2 activity, was used to investigate the potential role of CK2 in response to Cd stress in Arabidopsis thaliana. Under Cd stress, CKB4ox showed reduced root growth and biomass accumulation as well as decreased chlorophyll and proline contents compared with wild type. Furthermore, increased Cd accumulation and a higher H₂O₂ content were found in CKB4ox, possibly contributing to the inhibition of CKB4ox growth under Cd stress. Additionally, altered levels of Cd and H₂O₂ were found to be associated with decreased expression of genes involved in Cd efflux, Cd sequestration and H₂O₂ scavenging. Taken together, these results suggest that elevated expression of CKB4 and increased CK2 activity enhance the sensitivity of plants to Cd stress by affecting Cd and H₂O₂ accumulation, including the modulation of genes involved in Cd transport and H₂O₂ scavenging. This study provides direct evidence for the involvement of plant CK2 in the response to Cd stress.

Elodea nuttallii exposure to mercury exposure under enhanced ultraviolet radiation: Effects on bioaccumulation, transcriptome, pigment content and oxidative stress

The hypothesis that increased UV radiation result in co-tolerance to Hg toxicity in aquatic plants was studied at the physiological and transcriptomic level in Elodea nuttallii. At the transcriptomic level, combined exposure to UV+Hg enhanced the stress response in comparison with single treatments, affecting the expression level of transcripts involved in energy metabolism, lipid metabolism, nutrition, and redox homeostasis. Single and combined UV and Hg treatments dysregulated different genes but with similar functions, suggesting a fine regulation of the plant to stresses triggered by Hg, UV and their combination but lack of co-tolerance. At the physiological level, UV+Hg treatment reduced chlorophyll content and depleted antioxidative compounds such as anthocyanin and GSH/GSSG in E. nuttallii. Nonetheless, combined exposure to UV+Hg resulted in about 30% reduction of Hg accumulation into shoots vs exposure to Hg alone, which was congruent with the level of expression of several transporter genes, as well as the UV effect on Hg bioavailability in water. The findings of the present work underlined the importance of performing experimentation under environmentally realistic conditions and to consider the interplay between contaminants and environmental variables such as light that might have confounding effects to better understand and anticipate the effects of multiple stressors in aquatic environment.

Investigating the effect of cadmium and aluminium on growth and stress-induced responses in the micropropagated medicinal plant Hypoxis hemerocallidea

Hypoxis hemerocallidea is a highly utilized medicinal plant in South Africa. Its cultivation has received considerable attention in order to meet the high demand. High levels of cadmium (Cd) and aluminum (Al) in H. hemerocallidea plants sold in traditional medicinal markets was previously reported. The present study used an in vitro propagation model to investigate the uptake of Cd and Al by H. hemerocallidea and their effect on plant growth, elemental uptake and some stress-induced responses such as pigment, malondialdehyde (MDA), proline content and ultrastructural changes. Shoot and root growth of plantlets exposed to Cd, Cd:Al and high concentrations of Al was significantly reduced. Highest concentrations of Cd accumulated in the corms of Cd-treated plantlets while highest Al concentrations occurred in the leaves and roots. There was higher accumulation of Cd and Al when applied singularly compared to the Cd:Al combination treatments. Cd and Al also reduced accumulation of trace elements in micropropagted H. hemerocallidea with lowest concentrations in the Cd:Al combination treatments. Exposure to Cd, Al and Cd:Al significantly reduced the level of chlorophyll but increased the levels of carotenoids, MDA and proline. Ultrastructural changes were also observed in H. hemerocallidea exposed to Cd and Al. All these factors contributed to the inhibition of plant growth and could potentially affect the ability of this important medicinal plant to synthesize bioactive compounds. It is thus necessary to understand heavy metal stress-induced responses in this highly valued medicinal plant to ensure a high quality product for the consumer.

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.

Hydrogen Peroxide, Signaling in Disguise during Metal Phytotoxicity

Plants exposed to excess metals are challenged by an increased generation of reactive oxygen species (ROS) such as superoxide ([Formula: see text]), hydrogen peroxide (H2O2) and the hydroxyl radical ((•)OH). The mechanisms underlying this oxidative challenge are often dependent on metal-specific properties and might play a role in stress perception, signaling and acclimation. Although ROS were initially considered as toxic compounds causing damage to various cellular structures, their role as signaling molecules became a topic of intense research over the last decade. Hydrogen peroxide in particular is important in signaling because of its relatively low toxicity, long lifespan and its ability to cross cellular membranes. The delicate balance between its production and scavenging by a plethora of enzymatic and metabolic antioxidants is crucial in the onset of diverse signaling cascades that finally lead to plant acclimation to metal stress. In this review, our current knowledge on the dual role of ROS in metal-exposed plants is presented. Evidence for a relationship between H2O2 and plant metal tolerance is provided. Furthermore, emphasis is put on recent advances in understanding cellular damage and downstream signaling responses as a result of metal-induced H2O2 production. Finally, special attention is paid to the interaction between H2O2 and other signaling components such as transcription factors, mitogen-activated protein kinases, phytohormones and regulating systems (e.g. microRNAs). These responses potentially underlie metal-induced senescence in plants. Elucidating the signaling network activated during metal stress is a pivotal step to make progress in applied technologies like phytoremediation of polluted soils.

Different Growth and Physiological Responses to Cadmium of the Three Miscanthus Species

Miscanthus has been proposed as a promising crop for phytoremediation due to its high biomass yield and remarkable adaptability to different environments. However, little is known about the resistance of Miscanthus spp. to cadmium (Cd). To determine any differences in resistance of Miscanthus to Cd, we examined plant growth, net photosynthetic rate (Pn), activities of anti-oxidant and C₄ photosynthetic enzymes, concentrations of Cd in leaves and roots, and observed the chloroplast structure in three Miscanthus species treated with 0, 10, 50, 100 or 200 μM Cd in solutions. Miscanthus sinensis showed more sensitivity to Cd, including sharp decreases in growth, Pn, PEPC activity and damage to chloroplast structure, and the highest H₂O₂ and Cd concentrations in leaves and roots after Cd treatments. Miscanthus sacchariflorus showed higher resistance to Cd and better growth, had the highest Pn and phosphoenolpyruvate carboxylase (PEPC) activities and integrative chloroplast structure and the lowest hydrogen peroxide (H₂O₂) and leaf and root Cd concentrations. The results could play an important role in understanding the mechanisms of Cd tolerance in plants and in application of phytoremediation.

Water deficit and aluminum interactive effects on generation of reactive oxygen species and responses of antioxidative enzymes in the seedlings of two rice cultivars differing in stress tolerance

Aluminum (Al) is a major constraint to crop productivity in acid soils, whereas water deficit severely limits crop production in arid and semi-arid regions of the world. The objective of the present study was to examine the effects of both stresses, Al excess and water deficit, individually and in combination on the production of the reactive oxygen species (ROS) superoxide anion (O2˙(-)), hydrogen peroxide (H2O2), hydroxyl radical, and lipid peroxidation and the activity of antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), and guaiacol peroxidase (GPX) in the seedlings of two rice (Oryza sativa L.) cvs. Malviya-36 (sensitive to water deficit and Al) and Vandana (tolerant to water deficit and Al). When 15-day grown seedlings were exposed to water deficit (created with 15% polyethylene glycol, PEG-6000) or Al (1 mM AlCl3) treatment or both treatments together for 24-72 h, the lengths and fresh weights of root/shoot declined in the seedlings of the sensitive cultivar, whereas in the tolerant seedlings, either little or insignificant decline in these parameters was observed due to the treatments. Biochemical determinations and histochemical studies revealed that under a similar level of water deficit, Al, or combined treatment, seedlings of sensitive cultivar showed a higher level of production of O2˙(-), H2O2, hydroxyl radical, and lipid peroxides compared to the tolerant seedlings. Seedlings of tolerant cultivars, both in roots and shoots, had constitutively higher activity levels of antioxidative enzymes SOD, CAT, and GPX and showed a greater increase in activity under water deficit or Al treatment alone or in combination compared to the similarly treated seedlings of sensitive cultivar. Our results suggest that a lower constitutive level of ROS and a high antioxidative enzyme capacity are associated with tolerance to both water deficit and Al excess in rice seedlings.

Methyl Jasmonate Alleviates Cadmium-Induced Photosynthetic Damages through Increased S-Assimilation and Glutathione Production in Mustard

The effect of methyl jasmonate (MeJA) in mitigation of 50 μM cadmium (Cd) toxicity on structure and function of photosynthetic apparatus in presence or absence of 1.0 mM [Formula: see text] was investigated in mustard (Brassica juncea L. cv. Ro Agro 4001) at 30 days after sowing. Plants exhibited increased oxidative stress, impaired photosynthetic function when grown with Cd, but MeJA in presence of sulfur (S) more prominently ameliorated Cd effects through increased S-assimilation and production of reduced glutathione (GSH) and promoted photosynthetic functions. The transmission electron microscopy showed that MeJA protected chloroplast structure against Cd-toxicity. The use of GSH biosynthetic inhibitor, buthionine sulfoximine (BSO) substantiated the findings that ameliorating effect of MeJA was through GSH production. MeJA could not alleviate Cd effects when BSO was used due to unavailability of GSH even with the input of S. The study shows that MeJA regulates S-assimilation and GSH production for protection of structure and function of photosynthetic apparatus in mustard plants under Cd stress.

Leaf proteome analysis provides insights into the molecular mechanisms of bentazon detoxification in rice

Bentazon is a widely used herbicide that selectively removes broad-leaf weeds by competing with plastoquinone for the binding site in the D1 protein and interrupting the PET (photosynthetic electron transfer) chain. However, monocotyledonous plants, such as rice, show strong resistance to bentazon due to CYP81A6 induction, which results in herbicide detoxification. Here, we confirmed that rice was sensitive to bentazon treatment during the initial exposure period, in which bentazon rapidly inhibited photosynthesis efficiency and electron transfer, based on results of chlorophyll fluorescence analysis. In order to gain a comprehensive, pathway-oriented, mechanistic understanding of the effects directly induced by bentazon, we employed 2D-DIGE (two-dimensional difference gel electrophoresis) to analyze the leaf proteome after 8h of bentazon treatment coupled with individual protein identification by MALDI-TOF (Matrix assisted laser desorption/ionization-time of flight) MS/MS. Proteomic analyses revealed that bentazon induced the relative upregulation or downregulation of 30 and 71 proteins (by 1.5-fold or more, p<0.05), respectively. The pathways involved include photosynthesis processes, carbohydrate metabolism, antioxidant systems, and DNA stabilization and protein folding. Protein analysis data revealed that bentazon primarily suppressed photosynthesis processes, and showed inhibitory effects on carbohydrate metabolism and ATP synthesis, whereas several stress response proteins were induced in response to bentazon. Importantly, we identified a 519kD protein containing two histidine kinase-like ATPase domains and a C3HC4 RING type zinc finger domain which may function as a transcript factor to drive expression of detoxification genes such as CYP81A6, leading to bentazon tolerance. This study identifies, for the first time, a candidate transcription factor that could up-regulate CYP81A6 expression, and provides a foundation for further research to advance our knowledge of mechanisms of bentazon resistance in rice.