Assessment of Sunflower (Helianthus annuus L.) for Phytoremediation of Heavy Metal Polluted Mine Tailings—A Case Study of Nampundwe Mine Tailings Dam, Zambia

Mining activities have led to a generation of large quantities of heavy metals laden wastes which are released into the environment in an unsustainable way causing the contamination of the ecosystems and posing a risk to human health. Most mining companies have not employed any rehabilitation or remediation program of the heavy metal laden waste. The aim of this study was to assess the potential of sunflower for phytoremediation of heavy metal polluted mine tailings. Phytoremediation is an emerging technology in the remediation of mine tailings that uses tolerant plant species to clean up contaminated sites. It uses plants with high biomass and sunflower has been identified as such. These plants can extract, transfer, sequester and stabilize a variety of metals through mechanisms such as phytoextraction, phytostabilization, phytoaccumulation and phytovolatilization. Pot experiments were conducted by growing sunflower (Helianthus annuus L.) in pyrite mine tailings and in agricultural soil as a control. The study showed that the concentration of Cu reduced from 40.76 mg/kg to 36.59 mg/kg, Zn reduced from 3.58 mg/kg to 3.49 mg/kg and Fe reduced from 23.70 mg/kg to 10 mg/kg respectively in the mine tailings after 6 weeks. Analysis of harvested sunflower (roots, stems, leaves) showed that sunflower could remove heavy metals from the tailings and the highest removal efficiency was 53.7% and the highest translocation factor was 0.25. It was concluded that sunflower has the potential to remediate contaminated mine tailings and that phytoremediation is a viable and efficient technology to treat soils contaminated with heavy metals.

Subcellular Cd accumulation characteristic in root cell wall of rice cultivars with different sensitivities to Cd stress in soil

The variations of grain cadmiun（Cd） concentrations, translocation factors（TFs） of Cd from roots to shoots/grains of six rice cultivars, characterized with different Cd-sensitivities in polluted soil were studied, the selected rice cultivars were Xiangzao 17（R1）, Jiayu 211（R2）, Xiangzao 42（R3）, Zhuliangyou 312（R4）, Zhuliangyou 611（R5）, and Jinyou 463（R6）, respectively. The Cd subcellular distribution and Cd binding characteristics on subcellular fractions of rice root cell wall（CW） were further investigated. The results showed that the rice grain Cd contents varied significantly, with a maximum variation of 47.0% among the cultivars, the largest grain Cd content was observed with cultivar R1（Cd-sensitivity cultivar） and the smallest with R5（Cd-tolerance cultivar）. The translocation factors of Cd from roots to shoots（TF_（shoot）） and roots to grains（TF_（grain）） varied greatly among the cultivars. In general, the TFgrain of the cultivars followed the order of R1〉R2〉R3〉R4〉 R6-R5. The Cd concentration（mg kg^（–1） FW） in the fraction of root CW, the fraction of cell wall removing pectin（CW-P） and the fraction of cell wall removing pectin and hemicellulose（CW-P-HC） of the cultivars generally followed the order of CW-P〉CW〉CWP-HC; the ratios of Cd concentration（mg kg–1 FW） in the fraction of CW-P to that of CW were mostly more than 1.10, while the ratios of Cd concentration in the fraction of CW-P-HC to that of CW were mostly less than 0.60, indicating that Cd was mainly stored in the hemicellulose of the root CW. The ratios of Cd of CW-P-HC to CW generally followed the descending order of R1~R2〉R3〉R4〉R5~R6 for the cultivars, which implied that hemicellulose is probably the main subcellular pool for transferring Cd into rice grain, and it restrains the translocation of Cd from shoot to the grain, especially for the Cd-tolerance cultivars（R5 and R6）, the compartmentation of more Cd in hemicellulose in root CW is probably one of the main mechanisms for Cd tolerance of rice cultivars.

Heavy metal distribution of natural and reclaimed tidal riparian wetlands in south estuary, China

We evaluated the distribution and accumulation of Cd, Cr, Cu, Ni, Pb and Zn in two plant species （Scirpus tripueter Linn. and Cyperus malaccensis Lam.）, in water and soils sampled from the reclaimed tidal riparian wetlands （RTRWs） and the natural riparian wetlands （NRWs） in the Pearl River Estuary （PRE）. The results showed that the concentrations of studied heavy metals in soils exceeded the eco-toxic threshold recommended by US EPA. The concentrations of Cd, Cr and Zn in plants may lead to toxic effiects. The heavy metal concentrations were high in water and low in soils of RTRWs compared with that in the NRWs. The accumulation of heavy metals in the roots of plants was higher in NRWs than those in RTRWs while the opposite result was found for heavy metal accumulation in shoots. Based on the bioaccumulation and translocation factors, the plants in NRWs had a higher capacity to accumulate heavy metals while higher abilities to transport heavy metals from roots to shoots were observed in RTRWs. Heavy metal contaminations in RTRWs were dominated by anthropogenic sources from both side uplands and river water, whereas in NRWs, the metal accumulations were simultaneously affected by anthropogenic and natural factors

A Meta-Analysis on Phenotypic Variation in Cadmium Accumulation of Rice and Wheat:Implications for Food Cadmium Risk Control

In some densely-populated countries, farmland has been widely cadmium (Cd) contaminated, and the utilization of the contaminated farmland for crop production is currently unavoidable. This necessitates the use of low-Cd crops (i.e., pollution-safe cultivars, the crop varieties with the ability to accumulate a low level of Cd in their edible parts when grown on polluted soil) in these areas and highlights the importance of knowledge on phenotypic variation in crop Cd accumulation for food Cd risk control. Studies on phenotypic variation in heavy metal accumulation started decades ago for a wide range of crops, and synthesis of the scattered experimental results in the literature is in need. We built a Low-Cd Crops Database based on literature research, and relevant meta-analysis was performed to quantitatively explore the phenotypic variation in Cd uptake and translocation of rice and wheat. Considerable variability existed among rice (median grain Cd bioconce nt ration factor (BCF) of 0.10) and wheat (median grain Cd BCF of 0.21) phenotypes in grain Cd accumulation, and this variability was labile to soil pH and the level of Cd stress. Wheat statistically had a higher root-to-shoot Cd-translocating ability than rice, highlighting potential food Cd risks and the importance of growing low-Cd wheat in slightly Cd-contaminated regions. Meanwhile, no correlations were detected among soil-to-root, root-to-shoot, and shoot-to-grain translocation factors, implying that Cd uptake and internal translocation in crops were probably controlled by different underlying gene tic mechanisms. Root-to-shoot Cd transport could be a favorable target trait for selecting and breeding low-Cd rice and wheat. In all, this review provides a comprehensive low-Cd crop list for remediation practice and a systematic meta-analysis inferring food Cd risks based on plant capacity for Cd accumulation and desired traits for low-Cd crop breeding.

Enhanced efficiency of cadmium removal by Boehmeria nivea(L.) Gaud. in the presence of exogenous citric and oxalic acids

Boehmeria nivea（L.） Gaud. is a potential candidate for the remediation of Cd contaminated sites. The present investigation aims to explore Cd tolerance threshold and to quickly identify the role of exogenous organic acids in Cd uptake and abiotic metal stress damage.Elevated Cd levels（0–10 mg/L） resulted in an obvious rise in Cd accumulation, ranging from268.0 to 374.4 in root and 25.2 to 41.2 mg/kg dry weight in shoot, respectively. Citric acid at1.5 mmol/L significantly facilitated Cd uptake by 26.7% in root and by 1-fold in shoot,respectively. Cd translocation efficiency from root to shoot was improved by a maximum of66.4% under 3 mmol/L of oxalic acid. Citric acid exhibited more prominent mitigating effect than oxalic acid due to its stronger ligand affinity for chelating with metal and avoiding the toxicity injury of free Cd ions more efficiently. The present work provides a potential strategy for efficient Cd remediation with B. nivea.

Effects of soil amendments, foliar sprayings of silicon and selenium and their combinations on the reduction of cadmium accumulation in rice

Agricultural soil contamination by cadmium (Cd) is becoming one of the most serious environmental issues and public concerns. In this study, factorial arrangements of treatments were designed to explore the effects of two soil amendments, sodium sulfide-biofuel ash (SSBA) and lime (0.1%), and three foliar applications, silicon (Si, 2.5 mmol L-1), selenium (Se, 40 mg L-1), and their combination (SS), on Cd reduction in rice (Oryza sativa L.) in a pot experiment, which were then verified in a field experiment. Compared with the control without amendment but with spraying of deionized water, both SSBA and lime significantly reduced concentrations of CaCl2-extractable Cd in soil by 30%–39% and 31%–40% and Cd in brown rice by 44%–63% and 53%–72% in the pot and field experiments, respectively. Foliar Si, Se, and SS applications significantly reduced Cd accumulation in brown rice by 62%–64%, 72%–83%, and 39%–73%, respectively, increased rice grain yield, and improved antioxidant enzyme activities in rice leaves but with different trends in the pot and field experiments. Combinations of SSBA and lime with Si, Se, and SS had a non-significant synergistic effect on Cd reduction in brown rice compared to only foliar spraying or soil amendment in both pot and field experiments, although SSBA + Se and SSBA + Si reduced Cd concentration in brown rice by 16%–34% and 14%–24% compared to only foliar Si and Se and soil SSBA applications, respectively. Soil lime application and foliar Si spraying were the most cost-effective strategies to reduce Cd accumulation in brown rice in the field and pot experiments, respectively. Although soil amendments and foliar treatments were individually effective, their combinations failed to generate a significant synergistic reduction of Cd concentration in brown rice.

Rice(Oryza sativa L.)seedlings enriched with zinc or manganese:Their impacts on cadmium accumulation and expression of related genes

Cadmium(Cd)contamination in paddy soils means that the rice produced there may be unsafe for human consumption.A hydroponic study was conducted to enrich rice seedlings with zinc(Zn)or manganese(Mn),and the uptake and transport characteristics of Cd in these Zn-and Mn-rich seedlings were subsequently investigated using a greenhouse pot trial.The results showed that hydroponic cultivation in 10-50μmol L^(-1) Zn(ZnSO_(4)·7 H_(2)O)or50-250μmol L^(-1) Mn(MnSO_(4)·H_(2)O)for 30 d had no significant impact on rice growth,while the accumulation of Zn and Mn was 7.31-18.5 and 25.4-47.7 times higher,respectively,than in the control(no Zn or Mn addition).The accumulation of Cd in the Zn-and Mn-rich rice plants was 26.3%-38.6% and 34.4%-44.5% lower than that in the control,respectively,and the translocation factors of Cd from roots to shoots also decreased by 23.3%-41.3% and 25.3%-37.0%,respectively,after transplanting to Cd-contaminated soils.Furthermore,the relative expression levels of OsIRT1(Oryza sativa iron-regulated transporter 1)were downregulated by 40.1%-59.3% and 16.0%-25.9%,respectively,in the Zn-and Mn-rich seedling roots.This downregulation may indicate a possible mechanism contributing to the reductions in Cd absorption.Field experiments confirmed that the Zn-and Mn-rich seedlings produced brown rice(unpolished rice grains)with significantly decreased concentrations of Cd(34.2%-44.4%).This study provides an innovative method for reducing the food safety risks from rice grown on slightly to moderately Cd-contaminated paddy soils.

Phenotypic variations of wheat cultivars from the North China Plain in response to cadmium stress and associated single nucleotide polymorphisms identified by a genome-wide association study

Understanding the genetic mechanisms for cadmium (Cd) uptake and translocation in common wheat (Triticum aestivum) is of significance in food Cd contamination control. In this study, a diverse panel of 132 wheat cultivars was collected from the North China Plain. The cultivars were evaluated in terms of their phenotypic variations in response to Cd stress and subjected to a genome-wide association study (GWAS) to identify single nucleotide polymorphisms (SNPs) associated with the phenotypic variations at the seedling stage. Significant phenotypic variations with high heritability were observed among the wheat cultivars exposed to 40 μmol L-1 Cd for the studied traits, including root length (RL), shoot length (SL), root and shoot dry biomasses (RDW and SDW, respectively), root and shoot Cd concentrations (RCD and SCD, respectively), and Cd translocation factor (TF). Mean RCD, SCD, and TF ranged from 1.0 to 33.8, 0.125 to 2.022, and 0.009 to 0.321 mg g-1, respectively. Cluster analysis showed that wheat cultivars with higher RL, SL, RDW, and SDW under Cd stress were able to accumulate more Cd in root, leading to a lower Cd TF. Mixed linear model-based association analysis detected 17 novel significant marker-trait associations (MTAs), four of which were significant at a genome-wide scale. Most of the significant MTAs controlled Cd TF and explained 17.17%–26.47% of the phenotypic variations. Some of the SNP loci were physically close to a reported Cd-related quantitative trait locus or gene on wheat chromosomes. Results of this study provided a list of wheat cultivars with the potential of low Cd accumulation and enriched our knowledge on the genetic basis of Cd uptake and translocation in wheat. Pyramiding breeding of superior alleles detected in this study may additionally reduce Cd accumulation of improved wheat cultivars with excellent agronomic traits.

Relationship between heavy metal concentrations of herbaceous plants and soils at four Pb-Zn mining sites in Yunnan,China

This paper studied the relationship between heavy metal concentrations of herbaceous plants and soils at four Pb-Zn mining sites in Yunnan,China.50 herbaceous plant samples of 9 plant species from 4 families and 50 soil samples were collected and then ana1yzed for the tota1 concentrations of Pb,Cd,and Zn.The results showed that the average concentrations of Pb,Cd,and Zn in soil samples were 3772.83,168.81,and 5385.65 mg/kg,respectively.The average concentrations of Pb,Cd,and Zn were 395.68,28.14,and 1664.20 mg/kg in the shoots,and 924.12,57.25,and 1778.75 mg/kg in the roots,respectively.Heterospecific plants at the same site and conspecific plants at various sites had different average levels of Pb,Cd,and Zn,both in the shoots and the roots.Enrichment coefficients of Pb,Cd,and Zn were greater than 1 in 2,3,and 9 herbaceous plant samples,respectively.Translocation factors of Pb,Cd,and Zn were greater than 1 in 10,17,and 25 herbaceous plant samples,respectively.In all 50 samples,the concentrations of Pb,Cd,and Zn between the shoots and the roots,the shoots,and the soils,and the roots and the soils had significant positive relationships.

Nutrient uptake,physiological responses and growth of tobacco(Nicotiana tabacum L.)in soil under composite salt stress

High soil salinity imposes osmotic stress and ion toxicity in plants,leading to substantial crop yield loss worldwide.Understanding of the quantitative and dynamic physiological responses to composite soil salt stress is limited and needs to be expanded.In this study,physiological,nutritional,and biomass yield parameters of tobacco(Nicotiana tabacum L.)grown in soil with five levels of composite soil salinity(CSS),basal CSS level(control,CK)and 3(T_(1)),6(T_(2)),9(T_(3)),and 12(T_(4))times the basal CSS level,under greenhouse were determined at days 30,60,and 90 after transplanting.Leaf dry biomass significantly(P<0.05)increased at the low salinity levels applied(T_(1) and T_(2))at all three time points,whereas it progressively declined as the CSS level further increased.The leaf physiological and photosynthetic responses were more adversely affected by CSS at the early growth stage(day 30).A path coefficient analysis demonstrated that leaf proline content had the largest direct effect(-0.66),and leaf Cu content had the most significant indirect effect(0.49)on leaf dry biomass of plants.The results suggest that lower CSS levels(T_(1) and T_(2))could stimulate tobacco growth(leaf biomass yield,in particular),and higher leaf proline and Cu levels at the early growth stage may potentially increase the ability of tobacco plants to withstand the adverse effects of salinity,which could be considered for future research and development of salinity management strategies.

Uptake and accumulation of di-n-butyl phthalate in six leafy vegetables under hydroponic conditions

The uptake and accumulation of di-n-butyl phthalate(DBP)in six leafy vegetables was investigated under hydroponic conditions.The test vegetables were six varieties of Brassica campestris ssp.,including Kangresijiqing(KRSJQ),Xiadiqing(XDQ),Ziyoucai(ZYC),Aijiaohuang(AJH),Shanghaiqing(SHQ)and Gaogengbai(GGB).The root concentration factor(RCF),translocation factor(TF)and transpiration stream concentration factor(TSCF)were calculated in order to compare the difference of uptake and accumulation behaviours of DBP in vegetable varieties.The results showed that DBP was easily concentrated in vegetable roots,but was poorly translocated from the roots to the shoots.Among the six vegetables,the ability of concentrating DBP from the solution to shoots was the highest in GGB,followed by ZYC,KRSJQ,AJH,SHQ and XDQ.High concentrations of DBP(5.0 mg/L)seem to inhibit normal physiological activity in the vegetables,which resulted in a higher RCF and a lower TF and TSCF than in low-concentration treatment.The results will help to evaluate the safety of agricultural products and to provide evidence for screening DBP pollution-safe vegetable cultivars.