Breeding Low-Cadmium Wheat: Progress and Perspectives

Genome-wide identification, expression and function analysis of the MTP gene family in tulip (Tulipa gesneriana)

Currently, soil heavy metal contamination is a severe issue, particularly with Cd pollution. The metal tolerance protein (MTP) proteins, as plant divalent cation transporters, play a crucial role in the transport and tolerance of heavy metals in plants. This study conducted comprehensive identification and characterization of the MTP gene family in the tulip. A total of 11 TgMTP genes were identified and phylogenetically classified into three subfamilies. Conserved motif and gene structure analyses unveiled commonalities and variations among subfamily members. Expression profiling demonstrated several TgMTPs were markedly upregulated under Cd exposure, including the TgMTP7.1. Heterologous expression in yeast validated that TgMTP7.1 could ameliorate Cd sensitivity and enhance its tolerance. These results provide primary insights into the MTP gene family in tulip. Phylogenetic relationships and functional analyses establish a framework for elucidating the transporters and molecular mechanisms governing Cd accumulation and distribution in tulip. Key TgMTPs identified, exemplified by TgMTP7.1, may illuminate molecular breeding efforts aimed at developing Cd-tolerant cultivars for the remediation of soil Cd contamination.

Variations in Cadmium and Lead Bioaccessibility in Wheat Cultivars and Their Correlations with Nutrient Components

To reduce the health risks of exposure to Cd and Pb in wheat, a field experiment was conducted to investigate the differences in Cd and Pb bioaccessibility among the grains of 11 wheat cultivars and their relationships with the nutrient compositions of grains. The grain concentrations (Cd: 0.14-0.56 mg kg-1, Pb: 0.08-0.39 mg kg-1) and bioaccessibility (5.28-57.43% and 0.72-7.72% for Cd and Pb in the intestinal phase, respectively) of Cd and Pb differed significantly among the 11 cultivars. A safe wheat cultivar (Shannong16) with a relatively low health risk and the lowest grain Cd and Pb concentrations was selected. Ca, Mg, phytate, and methionine played key roles in affecting Cd and Pb bioaccessibility in wheat, with Ca and phytate significantly negatively correlated with Cd and Pb bioaccessibility. These findings can be used to optimize the selection strategy for safe wheat cultivars for healthy grain production in Cd-polluted farmland.

Keystone taxa of phoD-harboring bacteria mediate alkaline phosphatase activity during biochar remediation of Cd-contaminated soil

Phosphorus (P)-modified biochar can efficiently remediate cadmium (Cd)-contaminated soil. However, the mechanisms of responses of alkaline phosphatase (ALP) and phoD-harboring microorganisms, which are notably sensitive to Cd and P, are not clear during the remediation process. In this study, apple (Malus domestica) tree branches were co-pyrolyzed with tripotassium phosphate (K3PO4) to prepare P-modified biochar, which was used to remediate Cd-soil contaminated soil collected near a mine site. The effect of P-modified biochar on the composition of the phoD-harboring microbial community and its mechanism of interacting with ALP were analyzed. The results showed that the application of P-modified biochar to Cd-contaminated soil promoted the co-precipitation of Cd and phosphate and reduced the content of bioavailable Cd by 69.77 %. P-modified biochar improved the complexity and stability of the soil phoD-harboring microbial community. Furthermore, this study clarified that ALP activity was not completely regulated by the abundance of phoD, but Priestia and Massilia that contain phoD genes dominated the activity of ALP in rhizosphere and bulk soils, respectively. It is notable that bioavailable Cd significantly stimulated Priestia, Massilia, and ALP activity. These findings provide a theoretical basis for the application of P-modified biochar to the remediation of soil contaminated with Cd with respect to P functional microorganisms.

Genotypic variation in grain cadmium concentration in wheat: Insights into soil pollution, agronomic characteristics, and rhizosphere microbial communities

Soil cadmium (Cd) pollution poses a serious threat to both the productivity and quality of wheat. This study aimed to investigate the genotypic variation in grain Cd concentration in wheat through field and pot experiments. Among 273 wheat genotypes, a significant genotypic difference was found in grain Cd concentration, ranging from 0.01 to 0.14 mg kg-1. Two contrasting genotypes, X321 (a low grain Cd accumulator) and X128 (a high grain Cd accumulator), were selected for pot experiments. X321 exhibited a 17.9% greater reduction in yield and a 10.2% lower shoot-to-grain Cd translocation rate than X128 under Cd treatment. Grain Cd content showed a positive correlation with soil available Cd content and a negative correlation with Cu content. Soil catalase activity significantly decreased in X128 under Cd stress, whereas no difference was found in X321. The grains of X321 exhibited a more compact spatial distribution of starch grains and protein matrix than those of X128. Moreover, the size of A-type starch in X128 was larger than in X321. Meanwhile, X128 contained much B-type starch, with some surface pits observed on A-type granules under Cd stress. Cd treatment increased the abundance of rhizosphere microorganism communities, with Ellin6067 and Ramlibacter being enriched in X128 under Cd treatment, which might facilitate Cd uptake. The accumulation of Cd in grains demonstrated a strong positive correlation with the rhizosphere bacterial diversity (correlation coefficient = 0.78). These findings provide new insights into the basis of grain Cd accumulation in wheat and have potential implications for developing new verities with low Cd accumulation to ensure food safety and minimize human exposure.

Molecular-Assisted Breeding of Cadmium Pollution-Safe Cultivars

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

Mercapto-palygorskite efficiently immobilizes cadmium in alkaline soil and reduces its accumulation in wheat plants: A field study

Cadmium (Cd) contamination in wheat fields has become a major environmental issue in many regions of the world. Mercapto-palygorskite (MPAL) is a high-performance amendment that can effectively immobilize Cd in alkaline wheat soil. However, MAPL as an in-situ Cd immobilization strategy for alkaline wheat soil remains to be evaluated on a field-scale and the underlying mechanisms requires further evaluation. Here, MPAL were used as soil amendment to evaluate their immobilization efficiency on Cd-contaminated alkaline soil in the field experiments. The field experiments showed that MPAL application significantly reduced wheat grain Cd concentration from 0.183 mg/kg to 0.056 mg/kg, with Cd concentration in wheat grain treated with MPAL all falling below the limit value of 0.1 mg/kg as defined in China's food safety standard (GB 2762-2022). The maximal immobilization efficiency of MPAL on soil Cd figured out by diethylenetriaminepentaacetic acid (DTPA) extraction was 61.5%. The mechanisms involved in Cd immobilization by MPAL were mainly related to the enhanced sorption of Cd onto Fe oxides, and the removal of amorphous or free Fe oxides from soil had a substantial impact on Cd immobilization efficiency by MPAL. Furthermore, the antagonistic effect between Mn and Cd uptake may also contribute to the reduction of wheat Cd accumulation after MPAL application. The current research can provide theoretical and technical support for the large-scale application of MPAL in Cd-contaminated wheat fields.

Novel mechanisms of cadmium tolerance and Cd-induced fungal stress in wheat: Transcriptomic and metagenomic insights

Although several studies on the effects of cadmium (Cd) on wheat have been reported, the gene expression profiles of different wheat tissues in response to gradient concentrations of Cd, and whether soil microorganisms are involved in the damage to wheat remain to be discovered. To gain further insight into the molecular mechanisms of Cd-resistance in wheat, we sowed bread wheat (Triticum aestivum) in artificially Cd-contaminated soil and investigated the transcriptomic response of the wheat roots, stems, and leaves to gradient concentrations of Cd, as well as the alteration of the soil microbiome. Results indicated that the root bioaccumulation factors increased with Cd when concentrations were < 10 mg/kg, but at even higher concentrations, the bioaccumulation factors decreased, which is consistent with the overexpression of metal transporters and other genes related to Cd tolerance. In the Cd-contaminated soil, the abundance of fungal pathogens increased, and the antimicrobial response in wheat root was observed. Most of the differentially expressed genes (DEGs) of wheat changed significantly when the Cd concentration increased above 10 mg/kg, and the transcriptional response is much greater in roots than in stems and leaves. The DEGs are mainly involved in Cd transport and chelation, antioxidative stress, antimicrobial responses, and growth regulation. COPT3 and ZnT1 were identified for the first time as the major transporters responding to Cd in wheat. Overexpression of the nicotianamine synthase and pectinesterase genes suggested that nicotianamine and pectin are the key chelators in Cd detoxification. endochitinase, chitinase, and snakin2 were involved in the anti-fungal stress caused by Cd-induced cell damage. Several phytohormone-related DEGs are involved in the root's growth and repair. Overall, this study presents the novel Cd tolerance mechanisms in wheat and the changes in soil fungal pathogens that increase plant damage.

Wheat Selenium-binding protein TaSBP-A enhances cadmium tolerance by decreasing free Cd2+ and alleviating the oxidative damage and photosynthesis impairment

Cadmium, one of the toxic heavy metals, robustly impact crop growth and development and food safety. In this study, the mechanisms of wheat (Triticum aestivum L.) selenium-binding protein-A (TaSBP-A) involved in response to Cd stress was fully investigated by overexpression in Arabidopsis and wheat. As a cytoplasm protein, TaSBP-A showed a high expression in plant roots and its expression levels were highly induced by Cd treatment. The overexpression of TaSBP-A enhanced Cd-toleration in yeast, Arabidopsis and wheat. Meanwhile, transgenic Arabidopsis under Cd stress showed a lower H2O2 and malondialdehyde content and a higher photochemical efficiency in the leaf and a reduction of free Cd2+ in the root. Transgenic wheat seedlings of TaSBP exhibited an increment of Cd content in the root, and a reduction Cd content in the leaf under Cd2+ stress. Cd2+ binding assay combined with a thermodynamics survey and secondary structure analysis indicated that the unique CXXC motif in TaSBP was a major Cd-binding site participating in the Cd detoxification. These results suggested that TaSBP-A can enhance the sequestration of free Cd2+ in root and inhibit the Cd transfer from root to leaf, ultimately conferring plant Cd-tolerance via alleviating the oxidative stress and photosynthesis impairment triggered by Cd stress.

Temporal Comparative Transcriptome Analysis on Wheat Response to Acute Cd Toxicity at the Seedling Stage

Cadmium (Cd) is a non-essential and toxic metal that accumulates in plant's tissues and diminishes plant growth and productivity. In the present study, differential root transcriptomic analysis was carried out to identify Cd stress-responsive gene networks and functional annotation under Cd stress in wheat seedlings. For this purpose, the Yannong 0428 wheat cultivar was incubated with 40 µm/L of CdCl₂·2.5H₂O for 6 h at three different seedling growth days. After the quality screening, using the Illumina Hiseq 2000 platform, more than 2482 million clean reads were retrieved. Following this, 84.8% to 89.3% of the clean reads at three time points under normal conditions and 86.5% to 89.1% of the reads from the Cd stress condition were mapped onto the wheat reference genome. In contrast, at three separate seedling growth days, the data analysis revealed a total of 6221 differentially expressed genes (DEGs), including 1543 (24.8%) up-regulated genes and 4678 (75.8%) down-regulated genes. In total, 120 DEGs were co-expressed throughout all the growth days, whereas 1096, 1088, and 2265 DEGs were found to be selectively up-/down-regulated at 7d, 14d, and 30d, respectively. However, the clustering of DEGs, through utilizing the Kyoto Encyclopedia of Genes and Genomes (KEGG), revealed that the DEGs in the metabolic category were frequently annotated for phenylpropanoid biosynthesis. In comparison, a considerable number of DEGs were linked to protein processing in the endoplasmic reticulum under the process of genetic information processing. Similarly, in categories in organismal systems and cellular processes, DEGs were found in plant hormone signal transduction pathways, and DEGs were identified in the plant-pathogen interaction pathway, respectively. However, DEGs in "endocytosis pathways" were enriched in environmental information processing. In addition, in-depth annotations of roughly specific heavy metal stress-response genes and pathways were also mined, and the expression patterns of eight DEGs were studied using quantitative real-time PCR. The results were congruent with the findings of RNA sequencing regarding transcript abundance in the studied wheat cultivar.

Phosphorus acquisition strategies of wheat are related to biochar types added in cadmium-contaminated soil: Evidence from soil zymography and root morphology

Biochar application for the remediation of cadmium (Cd)-contaminated soils may result in a relative deficiency of phosphorus (P) due to the disruption of soil nutrient balance. However, the P acquisition strategies of plants in such situation are still unclear. In this study, analyses on soil zymography and root morphology were combined for the first time to investigate the effects of pristine and P-modified biochars from apple tree branches on the P acquisition strategies of wheat under Cd stress. The results show that the application of pristine biochar exacerbated the soil's relative P deficiency. Wheat was forced to improve foraging for P by forming longer and thinner roots (average diameter 0.284 mm) as well as releasing more phosphatase to promote P mobilization in the soil. Moreover, bioavailable Cd affected the P acquisition strategies of wheat through stimulating the release of phosphatase from roots. The P-modified biochar maintained high levels of Olsen-P (>100 mg kg^-1) in the soil over time by slow release, avoiding the creation of relative P deficiency in the soil; and increased the average root diameter (0.338 mm) and growth performance index, which promoted shoot growth (length and biomass). Furthermore, the P-modified biochar reduced DTPA-extracted Cd concentration in soils by 79.8 % (pristine biochar by 26.9 %), and decreased the Cd translocation factor from root to shoot as well as Cd concentration in the shoots. Therefore, P-modified biochar has a great potential to regulate the soil element balance (carbon, nitrogen, and P), promote wheat growth, and remediate the Cd-contaminated soil.

Effects of application of rice husk biochar and limestone on cadmium accumulation in wheat under glasshouse and field conditions

Cadmium (Cd) has seriously threatened the safe production of food crops. Passivator amendments are commonly used to control the soil Cd availability. Yet, few studies are tested to explore the effect of the combination of various amendments. Here, we investigated the effects of different amendments (2% rice husk biochar, 2% limestone, and 1% rice husk biochar + 1% limestone) on the growth and Cd accumulation of wheat in pot and field experiments. The results showed that under the low soil Cd condition, the maximum increase of soil pH (1.83) was found in the limestone treatment compared to CK in pot experiment. Compared with the CK, the treatment of rice husk biochar decreased soil Cd availability and grain Cd content by about 25% and 31.2%, respectively. In contrast, under high soil Cd condition, the highest soil pH was observed in limestone, while the lowest soil Cd availability and grain Cd concentrations were found in rice husk biochar treatment. In the field experiment, the treatment of 1% rice husk biochar + 1% limestone caused a significant increase of soil pH by about 28.2%, whereas the treatment of 2% rice husk biochar reduced soil Cd availability and grain Cd content by about 38.9% and 38.5% compared to the CK. Therefore, rice husk biochar showed great potential to reduce Cd availability and ensure safe food production.

The Effect of "Production during Remediation" of Plants in Cd-Contaminated Soil

In order to find suitable plants for "production during remediation" in wheat fields moderately contaminated by cadmium (Cd), five plants-green amaranth, oil sunflower, broomcorn, maize, and waxy maize-were planted in pots to study their enrichment characteristics and remediation effects in Cd-contaminated soil. The results showed that the highest bioaccumulation and translocation factors were greater than 0.5 for oil sunflower, which had the strongest Cd-enrichment ability in Cd-contaminated soil, but its biomass was small, and the Cd content of the grain exceeded the standard (GB2762-2017). The Cd content in the grains of broomcorn, maize, and waxy maize was less than 0.1 mg∙kg^-1, which is lower than the national food safety standard on limiting pollutants in food (GB2762-2017). Broomcorn accumulated 0.429 mg∙pot^-1 for Cd, with a Cd-extraction efficiency of 1.73%, which were higher than other plants. Taking the risk-screening values in GB15618-2018 "Soil Environmental Quality Standard" as the target, it will take 80 years to remediate using broomcorn, which has the highest extraction efficiency, based on cultivating remediation plants once per year. However, in view of the scarcity of arable land resources in China and the objective of safe production during remediation, the use of broomcorn can be considered for production during remediation for the given degree of Cd contamination of the soil.

Selection of low-cadmium and high-micronutrient wheat cultivars and exploration of the relationship between agronomic traits and grain cadmium

The cadmium (Cd) and micronutrient contents in grains were used as screening indicators through a pot experiment, and the hierarchical cluster analysis was used to select wheat cultivars with low Cd and high micronutrient contents. The potential human health risks caused by wheat intake and the relationship between the Cd concentration in wheat grains and 12 agronomic traits were also investigated using the risk assessment model and logistic equation fitting, respectively. Yannong-23, Zhongmmai-175, and Luyuan-502, the main wheat cultivars promoted in the Huang-Huai-Hai region of China, were screened for low Cd accumulation and high micronutrient. Health risk assessment results demonstrated that children showed a high noncarcinogenic risk and that adults posed a high carcinogenic risk. The results of the agronomic trait analysis showed that low-Cd accumulation wheat cultivars had high spikelet number and fresh and dry weights of root, stem, and leaf (p < 0.05). Logistic curve fitting results showed that among all agronomic traits, the root dry weight was the most suitable factor with remarkable goodness of fit and showed a significant negative correlation. The Cd concentration in wheat grains could be predicted by the logistic curve equation obtained by fitting this trait. Results provided theoretical support for the safe use of slightly to moderately contaminated farmland, formulation of health risk management policies for different populations, and breeding of high-quality wheat.

Technical solutions for minimizing wheat grain cadmium: A field study in North China

The minimization of Cd pollution in wheat is urgently needed in many parts of the world. Thus, the aims of the present study were to evaluate the feasibility of popular technologies (i.e., soil amendment and low-Cd wheat cultivar) at sites with different Cd risk levels (high and low) and to propose a risk-based strategy for safe grain production. At a high-Cd site, wheat variety JM22 yielded significantly lower grain Cd than SX828, regardless of soil amendment (biochar, sepiolite, and microbial agent YZ1). Neither biochar nor sepiolite amendment reduced grain Cd, DTPA-Cd, or bioconcentration factors, possibly due to low dosage. Metagenomic sequencing and quantitative PCR showed that YZ1 colonization had little effect on rhizospheric fungal community structure and could not be sustained through winter. At a low-Cd site, significantly lower grain Cd was observed in JM22, LX99, and JM262, which could be used as low-Cd cultivars in the study area. Interestingly, the grain Cd of JM22 was linearly correlated with soil Cd (R² = 0.84), which allowed the inference of a soil Cd threshold of 1.55 mg·kg^-1, below which JM22 alone was capable of producing safe grain. Cost-benefit analysis also indicated that the use of low-Cd cultivars is promising for pollution control. This study provides viable technical solutions for minimizing the grain Cd of wheat grown in northern China.

A path toward concurrent biofortification and cadmium mitigation in plant-based foods

Millions of people are anemic due to inadequate consumption of foods rich in iron and zinc. Plant-based foods provide most of our dietary nutrients but may also contain the toxic heavy metal cadmium (Cd). A low level of Cd silently enters the body through the diet. Once ingested, Cd may remain for decades. Hence, prolonged intake of Cd-containing foods endangers human health. Research that leads towards micronutrient enrichment and mitigation of Cd in foods has therefore dual significance for human health. The breeding of Cd-tolerant cultivars may enable them to grow on Cd-polluted soils; however, they may not yield Cd-free foods. Conversely, sequestration of Cd in roots can prevent its accumulation in grains, but this mechanism also retains nutrients, hence counteracting biofortification efforts. A specific restriction of the Cd absorption capacity of crops would prevent Cd entry into the plant system while maintaining micronutrient accumulation and may thus be a solution to the dilemma. After recapitulating existing strategies employed for the development of Cd-tolerant and biofortified cultivars, this Viewpoint elaborates alternative approaches based on directed evolution and genome editing strategies for excluding Cd while enriching micronutrients in plant foods, which will concurrently help to eradicate malnutrition and prevent Cd intoxication.

Differential responses of 23 maize cultivar seedlings to an arbuscular mycorrhizal fungus when grown in a metal-polluted soil

Modern breeding efforts have been accelerating crop improvement and yielding numerous cultivars with distinct genetic traits; however, interactions between different cultivars and their root-associated arbuscular mycorrhizal fungi (AMF) are not clear. Herein, we selected the 22 most common commercial maize (Zea mays) varieties in China and an inbred line (B73) to study the differential responses of these 23 cultivars to mycorrhizal inoculation when grown in an arable soil polluted by multiple metals (Pb, Zn, and Cd). We found that the different cultivars exhibited significant variations in plant metal accumulation, ranging from strong metal exclusion (ZYY9) to strong metal accumulation (B73). Mycorrhizal colonization substantially altered metal uptake and repartitioning, while bioaugmenting the inherent characteristics of metal accumulation; for example, the AMF enhanced leaf accumulation of the metal-accumulator B73, and markedly reduced the root uptake of the metal-excluder ZYY9. However, such AMF-induced alterations were also substantially dependent on plant organs (roots and shoots) and metal species. We found that the extent of the AMF-induced leaf alterations was substantially greater than that of the root alterations. Similarly, the number of instances where the AMF significantly altered the Zn and Cd accumulation was far higher than the number of instances where Pb accumulation was significantly altered by AMF. In addition, the presence of AMF appeared to trigger the maize antioxidant systems, which may have alleviated the toxicity of excessive Cd, increased the leaf chlorophyll content, augmented the net photosynthetic rate, and promoted the growth of 17.39% of the maize cultivars. Our results suggest that a future crop breeding challenge is to produce cultivars for safe production or phytoremediation, thereby optimizing the combinations of crop cultivars and their root-associated AMF in slightly to moderately metal-polluted arable soils.

Comparative effects of Tagetes patula L. extraction, mercapto-palygorskite immobilisation, and the combination thereof on Cd accumulation by wheat in Cd-contaminated soil

Phytoextraction and in situ immobilisation are two of the most commonly used remediation techniques for Cd-contaminated farmland. In theory, phytoextraction followed by immobilisation can reduce the total Cd and available Cd contents of the soil, making it suitable for the remediation of heavily Cd-contaminated alkaline soil. However, the real remediation efficiency is uncertain, and it is also unknown whether phytoextraction affects subsequent wheat Cd accumulation. In this study, two seasonal pot experiments were conducted to determine the effects of S,S-ethylenediamine disuccinic acid (EDDS)-assisted Tagetes patula L. (T. patula) extraction, mercapto-palygorskite (MPAL) immobilisation, and the combination thereof on subsequent Cd accumulation in wheat. EDDS application significantly increased the Cd content in the subsequent-soil solution, but the EDDS-activated Cd could not be absorbed by wheat roots. T. patula extraction decreased the subsequent soil pH value by 0.1-0.2 pH units, increased the available Cd content by 0.19 mg/kg, but had no effect on subsequent wheat Cd accumulation. The Cd absorption capacity of wheat roots and the Cd translocation capacity of wheat stems to grains of high-Cd wheat were higher than that of low-Cd wheat cultivar. The application of MPAL had no effect on soil pH value, but significantly decreased soil available Cd and exchangeable Cd contents by 17.78-36.76% and 21.13-52.63%; it also increased the Fe/Mn oxide-bound Cd fraction by 14.02-64.00%. MPAL application decreased the wheat grain Cd concentrations from 0.51 to 0.13 mg/kg (high-Cd wheat) and 0.35 to 0.05 mg/kg (low-Cd wheat), but had no negative effect on Fe, Mn, Cu, and Zn elements. Compared with the single MPAL application treatments, the combination treatments had no inhibition effect on Cd accumulation in wheat. MPAL is an efficient amendment, and considering the remediation efficiency, stability, and time of these methods, the combination of MPAL application with a low-Cd accumulation wheat cultivar represents a suitable proposal to ensure the safe production of wheat in Cd-contaminated alkaline soil.

Effects of S,S-ethylenediamine disuccinic acid on the phytoextraction efficiency of Solanum nigrum L. and soil quality in Cd-contaminated alkaline wheat soil

Degradable chelating agent-assisted phytoextraction is a promising method for the remediation of Cd-contaminated agricultural soil. However, there are limited studies that have examined the effect of chelating agents on soil solutions and soil quality in alkaline soil. In this study, the effects of S,S-ethylenediamine disuccinic acid (EDDS) on the growth and phytoextraction of Solanum nigrum L. (S. nigrum) were studied using pot experiments. The influence of EDDS on the soil solutions, heavy metal contents, and soil enzyme activities was evaluated. EDDS application increased the height of S. nigrum by 7.25-29.25 cm and increased the biomass of stem and leaf by 4.26-14.95 and 1.14-10.78 g/pot, respectively. The Cd concentrations in the leaves and berries of S. nigrum were 1.21-2.17 and 1.7-9.47 times higher than that of the control, respectively, and the Cd extraction amount in the shoots of S. nigrum increased by 22.78-256.16 μg/pot after EDDS application. The chelation of EDDS on heavy metals reached a peak after 7 days of application, decreased gradually with the degradation of EDDS, and disappeared after 30 days of application. Soil pH, available metals, metal speciation, and soil urease were significantly related to the application time of EDDS. Importantly, EDDS application 45 days before S. nigrum harvest treatments decreased the available metal concentrations and improved soil pH and urease activity. However, when EDDS was applied 15 days before S. nigrum harvest, the available Cd and Pb concentrations significantly increased and caused additional Pb pollution. Considering the chelation and degradation effects, the environmental implication, and the cost of EDDS, the results of this study showed that one application of EDDS was better than two applications, a 45-day application before harvest was preferred to a 15-day application, and application of 1-3 mM EDDS 30-45 days before S. nigrum harvest was the most promising application method for the remediation of Cd-contaminated alkaline soil.

Phytoexclusion of heavy metals using low heavy metal accumulating cultivars: A green technology

Heavy metal (HM) pollution of farmland is a serious problem worldwide and consumption of HM-contaminated food products poses significant public health risks. Phytoexclusion using low HM accumulating cultivars (LACs) is a promising and practical technology to mitigate the risk of HM contamination of agricultural products grown in polluted soils, and does not alter cultivation practices, is easy to apply, and is economical. This review provides an overview of the major scientific advances accomplished in the field of LACs worldwide. The LACs concept and identification criteria are presented, and the known LACs among currently cultivated grain crops and vegetables are re-evaluated. The low HM accumulation by LACs is affected by crop ecophysiological features and soil physicochemical characteristics. Taking low Cd accumulating cultivars as an example, it is known that they can efficiently exclude Cd from entering their edible parts in three ways: 1) decrease in root Cd uptake by reducing organic acids secretion in the rhizosphere and transport protein production; 2) restriction of Cd translocation from roots to shoots via enhanced Cd retention in the cell wall and Cd sequestration in vacuoles; and 3) reduction in Cd translocation from shoots to grains by limiting Cd redirection and remobilization mediated through nodes. We propose an LAC application strategy focused on LACs and optimized to work with other agronomic measures according to the classification of HM risk level for LACs, providing a cost-effective and practical solution for safe utilization of large areas of farmland polluted with low to moderate levels of HMs.

Effects of mercapto-palygorskite on Cd distribution in soil aggregates and Cd accumulation by wheat in Cd contaminated alkaline soil

Cadmium (Cd) contamination in alkaline soils is a serious issue in China. As the basic structural units of soil, soil aggregates play an important role in the migration and transformation of heavy metal. However, there are few studies on the effects of adding amendments on Cd distribution in soil aggregates in alkaline soils. In this study, pot experiments were conducted to assess the effects of mercapto-palygorskite (MPAL) on soil aggregates and Cd accumulation in wheat. The results showed that MPAL application had no effect on wheat yield but significantly reduced the Cd uptake by the roots and the Cd transport to the adjacent internode. Application of 0.1% MPAL reduced the Cd concentration in two wheat grains (0.57 and 0.44 mg/kg, control) to 0.10 and 0.09 mg/kg in moderately Cd-contaminated soil, which are below the China national standard limit of 0.1 mg/kg (GB 2762-2017). MPAL application had no effect on soil pH, cation exchange capacity, mass proportion and mean weight diameter of soil aggregates, but increased soil organic matter content. Importantly, MPAL application promoted the migration of Cd from large particles (>0.25 mm) to small particles (<0.048 mm), reduced the unstable Cd fractions in >0.25 mm soil particles of clay soil and in >0.075 mm soil particles of sandy soil, and increased the stable Cd fractions in bulk soils and soil aggregates. The effects of MPAL addition on soil aggregates (grain size fraction metals loading and accumulation factor) of sandy soil were more prominent than on those of clay soil. Under MPAL treatments, wheat grains Cd concentration was significantly positively correlated with the available Cd in >0.075 mm soil particles and the total Cd in >0.25 mm soil particles. These results indicated that MPAL application in alkaline soils promoted the migration of Cd to micro-aggregates and inhibited the uptake and transport of Cd by wheat roots, thus reducing the Cd concentration in wheat grains.

Accentuating the Role of Nitrogen to Phosphorus Ratio on the Growth and Yield of Wheat Crop

Nitrogen (N) and Phosphorus (P) deficiency is a major yield limiting factor across the globe and their proper management plays a vital role in optimizing crop yield. This field experiment was conducted to assess the impact of soil and plant nitrogen N and P ratio on the growth and yield of wheat (Triticum aestivum L.) in alkaline calcareous soil. The study consisted of various levels of nitrogen (0, 40, 80, and 160 kg ha−1 as urea) and phosphorus (0, 30, 60, and 90 kg P2O5 ha−1 as diammonium phosphate), and was carried out in randomized complete block design (RCBD) with factorial arrangement having three replications. The result showed that the addition of 160 kg N ha−1 significantly improved biological yield (10,052 kg ha−1), grain weight (3120 kg ha−1), chlorophyll content at tillering stage soil plant analysis development (SPAD) value (35.38), N uptake in straw (33.42 kg ha−1), and K uptake in straw (192 kg ha−1) compared to other N levels. In case of P, 90 kg P2O5 ha−1 had resulted maximum biological yield (9852 kg ha−1), grain yield (3663 kg ha−1), chlorophyll content at tillering stage (SPAD value 34.36), P (6.68 mg kg−1) and K (171 kg ha−1) uptake in straw. The sole use of N and P have positively influenced the biological and grain yield but their interaction didn’t response to biological yield. The present study reveals that SPAD value (chlorophyll meter) is the better choice for determining plant N and P concentrations to estimate the yield potential.

Screening stably low cadmium and moderately high micronutrients wheat cultivars under three different agricultural environments of China

Heavy metal-contaminated farmland, especially for cadmium (Cd), is being used for agricultural production in large scale due to the increasing food demand. Thus, minimizing the influx of Cd to the human food chain is urgently needed. Screening for stably low Cd and moderately high micronutrient wheat cultivars is one of the most feasible and effective approaches to ensure food safety and quality. Here, the Cd accumulation by 72 wheat cultivars was identified in field 1, and the stability of Cd accumulation in these cultivars was tested in fields 2 and 3. The effects of Cd on micronutrient (zinc (Zn), copper (Cu), iron (Fe), manganese (Mn), and boron (B)) uptake in grains were also investigated. Nine of the 24 low-Cd wheat cultivars identified by screening showed stably low-Cd and moderately high micronutrient concentrations in grain. Nine cultivars exhibited unstable low-Cd accumulation characteristics, and another five cultivars contained significantly lower Zn concentrations in grain in at least two experimental fields. One low-Cd cultivar also had low Zn, Cu and Mn concentrations in grain. The accumulation of Cd in wheat grain had little effects on the uptake of Fe. Grain Cd concentration correlated positively with the dry weight of stem and root, and negatively with the spike length. There was no correlation between grain Cd concentration and wheat yield. The selected cultivars with stably low-Cd accumulation and moderately high micronutrient concentrations in grain are recommended for cultivation in slightly to moderately Cd-contaminated farmland to ensure food safety for the growing human population.