Effectiveness of simultaneous applications of lime and zinc/iron foliar sprays to minimize cadmium accumulation in rice

Mitigation effects of foliar supply of different sulfur forms on uptake, translocation and grain accumulation of Cd and As by paddy rice on basis of liming

Co-contamination of Cd and As in strongly acidic paddy soil has posed great challenges for remediation practice due to their distinct properties. Liming is a necessary but inadequate measure for normal growth of paddy rice and for Cd and As remediation in strongly acidic paddy soils rich in iron minerals. A greenhouse rice pot cultivation experiment was conducted to explore the efficiency and mechanisms of how foliar supply of different sulfur forms (K2S, K2SO4) could further mediate the uptake, translocation and grain accumulation of Cd and As by paddy rice on basis of liming. Results showed that compared to liming alone (CK), co-application of liming and foliar supply of K2S (L + FK2S) significantly reduced contents of Cd and As in brown rice by 44.4 % and 24.7 %, respectively. Contrastingly, co-application of liming and foliar supply of K2SO4 (L + FK2SO4) decreased Cd content of brown rice by 55.5 %, but had no effect on As content. Foliar supply of K2S and K2SO4 dramatically facilitated Cd upward transfer from roots to shoots by enhancing root Cd transfer from cell wall into trophoplast. On the other hand, both sulfur forms remarkably elevated sulfur contents in leaves and significantly inhibited Cd translocation from leaves to grain by enhancing vacuolar sequestration of Cd in leaves. Compared to CK and L + FK2SO4 treatment, it was by enhancing glutathione synthesis, cell wall deposition in roots and vacuolar sequestration of As in leaves that L + FK2S showed greater inhibiting effects on transfer of As from roots, stems and leaves to grain. Foliar supply of either sulfate or sulfide could efficiently decrease grain Cd of paddy rice, but only foliar supply of sulfide is effective in reducing grain As.

Prediction of cadmium contents in rice grains from Quaternary sediment-distributed farmland using field investigations and machine learning

The Quaternary sediment-distributed regions of South China are suitable for rice cultivation, which is crucial for ensuring food security. Spatial correlations between soil cadmium (Cd) and rice Cd contents are generally poor, making the evaluation of rice quality and associated health risks challenging. In this study, we developed machine learning algorithms for predicting rice Cd contents using 654 data pairs of soil-rice samples collected in Guangxi province, China. After a comprehensive comparison, our results showed that the random forest (RF) had the better performance than artificial neural network (ANN) based on all the data (RMSEtesting 0.066 vs. 0.099 and R2testing 0.860 vs. 0.688). The feature importance analysis showed that soil CaO, Cd, elevation, and rainfall were the four most important features affecting the rice Cd contents in the study area. Using the established RF-predicated model, the rice Cd contents were predicted at the provincial level with an additional dataset of 1176 paddy soil samples. The prediction result revealed about 23 % of farmland cultivated rice with Cd content over 0.2 mg kg-1 in the study area. Therefore, it is recommended to implement strict measures by local agricultural departments to reduce rice Cd contents and ensure food safety in these areas. Our study provides valuable insights into the prediction of rice Cd contents, thus contributing to ensuring food safety and preventing Cd exposure-associated health risks.

Exploring the mechanism of Cd uptake and translocation in rice: Future perspectives of rice safety

Cadmium (Cd) contamination in rice fields has been recognized as a severe global agro-environmental issue. To reach the goal of controlling Cd risk, we must pay more attention and obtain an in-depth understanding of the environmental behavior, uptake and translocation of Cd in soil-rice systems. However, to date, these aspects still lack sufficient exploration and summary. Here, we critically reviewed (i) the processes and transfer proteins of Cd uptake/transport in the soil-rice system, (ii) a series of soil and other environmental factors affecting the bioavailability of Cd in paddies, and (iii) the latest advances in regard to remediation strategies while producing rice. We propose that the correlation between the bioavailability of Cd and environmental factors must be further explored to develop low Cd accumulation and efficient remediation strategies in the future. Second, the mechanism of Cd uptake in rice mediated by elevated CO2 also needs to be given more attention. Meanwhile, more scientific planting methods (direct seeding and intercropping) and suitable rice with low Cd accumulation are important measures to ensure the safety of rice consumption. In addition, the relevant Cd efflux transporters in rice have yet to be revealed, which will promote molecular breeding techniques to address the current Cd-contaminated soil-rice system. The potential for efficient, durable, and low-cost soil remediation technologies and foliar amendments to limit Cd uptake by rice needs to be examined in the future. Conventional breeding procedures combined with molecular marker techniques for screening rice varieties with low Cd accumulation could be a more practical approach to select for desirable agronomic traits with low risk.

Using Fe biofortification strategies to reduce both Ni concentration and oral bioavailability for rice with high Ni

Due to naturally high Ni or soil Ni contamination, high Ni concentrations are reported in rice, raising a need to reduce rice Ni exposure risk. Here, reduction in rice Ni concentration and Ni oral bioavailability with rice Fe biofortification and dietary Fe supplementation was assessed using rice cultivation and mouse bioassays. Results showed that for rice grown in a high geogenic Ni soil, increases in rice Fe concentration from ∼10.0 to ∼30.0 μg g^-1 with foliar EDTA-FeNa application led to decreases in Ni concentration from ∼4.0 to ∼1.0 μg g^-1 due to inhibited Ni transport from shoot to grains via down-regulated Fe transporters. When fed to mice, Fe-biofortified rice was significantly (p < 0.01) lower in Ni oral bioavailability (59.9 ± 11.9% vs. 77.8 ± 15.1%; 42.4 ± 9.81% vs. 70.4 ± 6.81%). Dietary amendment of exogenous Fe supplements to two Ni-contaminated rice samples at 10-40 μg Fe g^-1 also significantly (p < 0.05) reduced Ni RBA from 91.7% to 61.0-69.5% and from 77.4% to 29.2-55.2% due to down-regulation of duodenal Fe transporter expression. Results suggest that the Fe-based strategies not only reduced rice Ni concentration but also lowered rice Ni oral bioavailability, playing dual roles in reducing rice-Ni exposure.

Co-application of combined amendment (limestone and sepiolite) and Si fertilizer reduces rice Cd uptake and transport through Cd immobilization and Si-Cd antagonism

Limestone and sepiolite combined amendment (LS) and silicon (Si) fertilizers are commonly applied for the remediation of Cd-polluted paddy soil. However, it is difficult to further decrease cadmium (Cd) accumulation in rice grains by the individual application of LS or Si fertilizer to heavily Cd-polluted paddy fields. Two seasons of continuous field experiments were conducted in heavily Cd-polluted soil to study how the co-application of LS and Si fertilizer (namely soil-applied Si and foliar-sprayed Si) influences Cd and Si bioavailability in soil and Cd uptake and transport in rice. The results indicated that LS co-applied with soil-applied Si fertilizer treatments can enhance pH, cation exchange capacity (CEC), and available Si content in soil by 0.56-1.26 units, 19.3%-57.2%, and 14.7%-58.9% (p < 0.05), respectively, and reduce the toxicity characteristic leaching procedure (TCLP) extractable Cd content in soil by 26.5%-49.8% (p < 0.05) relative to the control. Furthermore, the co-application of LS and soil and foliar-sprayed Si fertilizer treatments reduced the Cd content in brown rice by 18.8%-70.6% (p < 0.05) compared with the control. Particularly, the brown rice Cd content under the co-application treatment (4500 kg/ha of soil applied LS, 90 kg/ha of Si fertilizer, and 0.4 g/L of foliar-sprayed Si fertilizer) was below 0.20 mg/kg in both seasons. Meanwhile, the Si content in rice was considerably enhanced by LS co-applied with Si fertilizer and negatively (p < 0.05) correlated with the rice Cd content. Therefore, the reduction of Cd bioavailability in soil and the antagonistic effect between Cd and Si in rice might be the key factors regulating Cd accumulation in rice via the co-application of LS and Si fertilizer.

Simultaneous mitigation of arsenic and cadmium accumulation in rice grains by foliar inhibitor with ZIF-8@Ge-132

Simultaneous mitigation of Arsenic (As) and Cadmium (Cd) in rice grains is hardly achieved with conventional soil treatments due to their opposite chemical behaviors in paddy soils. This study evaluates the effectiveness of a novel foliar inhibitor with germanium (Ge) -modified zeolitic imidazolate framework (ZIF-8@Ge-132) in cooperative mitigation of As and Cd in rice grains in a As and Cd co-contaminated paddy field, and the effecting mechanisms are elucidated by a series of advanced techniques. The results showed that the grains inorganic As and Cd was remarkably decreased by 45 % and 66 % by the foliar spay of ZIF-8@Ge-132, respectively. ZIF-8@Ge-132 also reduced the As and Cd contents in rice tissues, except for Cd in leaves, where Cd content increased by 148 %. The image-based measurement of plant phenotypic traits and the elements of image analysis using Laser Ablation-ICP-MS (LA-ICP-MS) and Laser Scanning Confocal Microscopy (LSCM) revealed that the possible mechanisms for the reduction of As and Cd in rice grains were as follows: (i) the thickening of the xylem in roots significantly retarded As and Cd absorption by rice plants. (ii) co-accumulation of Ge and Cd in the leaf vascular system likely contributed to the high Cd retention in rice leaves. (iii) antagonistic effects of Zn suppressed the uptake and transport of As in roots/leaves, resulting a lower As accumulation in rice grains.

Reduction of Cd Uptake in Rice (Oryza sativa) Grain Using Different Field Management Practices in Alkaline Soils

Cadmium contamination and toxicity on plants and human health is a major problem in China. Safe rice production in Cd-contaminated alkaline soils, with acceptably low Cd levels and high yields, remains an important research challenge. To achieve this, a small-scale field experiment with seven different soil amendment materials was conducted to test their effects performance. Two best-performing materials were selected for the large-scale field experiment. Combinations of humic acid, foliar, and/or soil silicon fertilization and deep or shallow plowing were designed. It was found that the combination, including humic acid, soil and foliar silicate fertilization, and shallow plowing (5-10 cm), produced the most desirable results (the lowest soil bioavailable Cd, the lowest grain Cd concentrations, and the highest grain yield). Rice farmers are therefore recommended to implement this combination to attain high grain yield with low Cd concentrations in alkaline soils.

Effects of combined soil amendments on Cd accumulation, translocation and food safety in rice: a field study in southern China

Excessive Cd content and high Cd/Zn ratio in rice grains threaten human health. To study the reduction effects of combined soil amendments on Cd content and Cd/Zn ratio in rice planting in soils with different Cd contamination levels, we conducted field trials in three regions of Hunan province, China. Six field treatments were designed in each study area, including control (CK), lime alone (L), lime combined with sepiolite (LS), phosphate fertilizer (LP), organic fertilizer (LO) and phosphate fertilizer + organic fertilizer (LPO). The application of the combined amendments reduced the Cd content in rice grains to less than the Food Health Standard of China (0.2 mg/kg) and the Cd/Zn ratio to less than the safety threshold of 0.015. The average reduction rates of grain Cd content under the combined treatments among the three regions increased with the increase in Cd content in the soil. Meanwhile, the amendments also decreased the soil available Cd and Zn concentration significantly. The LO had the highest efficiency on decreasing Cd content in rice grains among these amendments, which is ranged from 44.6% to 52.8% in the three regions compared with CK. Similarly, high reduction rates of Cd/Zn ratio were found in the LO treatment, with an average value of 57.3% among the three regions. The grain Cd contents and Cd/Zn ratios were significantly correlated with the soil available Cd concentrations, plant uptake factor and the straw to rice grain translocation factor (TF_gs) (P < 0.05). The results indicated that the combined soil amendments, especially lime combined with organic fertilizer, would be an effective way to control Cd content in rice.

Combined exogenous selenium and biochemical fulvic acid reduce Cd accumulation in rice

Paddy soil Cd contamination and the related accumulation risk in rice grains have attracted global attention. The application of selenium and humic substances is considered to be a cost-effective Cd mitigation measure. However, the effect of a combined application of the two materials remains unclear. Therefore, a 2-season pot experiment was conducted, wherein sodium selenite (Se) and biochemical fulvic acid (BFA) were applied alone and together. Paddy soils with two levels of Cd contamination were used. The results indicate that Se application alone considerably decreased the rice grain Cd content by 36.1-48.7% compared to the control rice grain Cd concentration, which was above the food safety limit (0.2 mg kg^-1). Although the application of BFA alone decreased the soil pH, it also increased the soil CaCl₂ extractable Cd content by 0.2 to 19.3% and had a limited effect on Cd in the rice grains. The combined application of Se and BFA did not affect the soil pH or the CaCl₂ extractable Cd, and more effectively reduced the Cd contents of the rice grains by 50.2 to 57.1%, except for the control rice grain Cd content, which was below the limit. The combined application of Se and BFA also inhibited Se accumulation in rice grains, maintaining the Se content at a safe level (0.33-0.58 mg kg^-1) compared to Se application alone. The effects of reducing the Cd content of rice grains while safely increasing their Se contents could persist for at least two seasons. Therefore, the combined application of Se and BFA should be recommended to mitigate Cd contamination risks in Cd-contaminated paddy soil.

Metabolomic and antioxidant enzyme activity changes in response to cadmium stress under boron application of wheat (Triticum aestivum)

Boron (B) has previously been shown to inhibit cadmium (Cd) uptake in wheat. Here, we investigated the physiological response of external B application (C for no B added, B for B added, B+Cd for B and Cd added, B/Cd for B 24 h pretreatment before Cd added, B and Cd were 46.2 μM and 5 μM, respectively) on wheat growth under Cd stress. The results showed that the wheat growth was significantly weaker under Cd treatment, while B application did not significantly improve the wheat growth under Cd stress. However, B application decreased Cd concentrations and malondialdehyde (MDA) concentrations of shoot and root. The key enzyme activities including superoxide dismutase (SOD) and peroxidase (POD) significantly increased under Cd treatments while decreased under B treatments. Further, a total of 198, 680 and 204 of the differential metabolites were isolated between B and C treatment, Cd and C treatment and B+Cd and Cd treatment, respectively. The metabolites with up-accumulation in B application (B+Cd) roots were mainly galactaric acid, citric acid, N6-galacturonyl-L-lysine, D-glucose, while the metabolites with down-accumulation were mainly threoninyl-tryptophan and C16 sphinganine. The differential metabolic pathways were mainly concentrated in linoleic acid metabolism, galactose metabolism, sphingolipid metabolism, glycolysis/gluconeogenesis, propanoate metabolism in diabetic complications between B+Cd treatment and B treatment. The results indicate that B alleviates Cd toxicity in winter wheat by inhibiting Cd uptake, increasing antioxidant enzyme activity and changing metabolites.

Foliar application of several reagents reduces Cd concentration in wheat grains

Cadmium (Cd) in agricultural soils can be absorbed by wheat and transferred into the grains, risking human health. In order to find the optimal foliar treatment method to reduce Cd accumulation in wheat grain, nineteen single-factor foliar treatments and multi-factor combination treatments were used to study the effects of different foliar sprays on Cd accumulation of wheat grain. The results showed that the foliar application of ethylenediaminetetraacetate (EDTA), selenium (Se), and sodium nitroprusside (SNP) can significantly reduce Cd concentration in wheat grains by 49.2%, 29.6%, and 28.8%, respectively, in the field. Foliar application of EDTA, Se, zinc (Zn), ascorbic acid (ASC), silicon (Si), and molybdenum (Mo) can significantly reduce Cd concentration of wheat grains by 32.3%, 32.0%, 27.7%, 27.7%, 26.3%, and 25.9%, respectively, in pot experiment. Foliar application of 2 mM EDTA and 2 mM Se exerted excellent effects on controlling the Cd accumulation of wheat grains both in pot and field experiment. Foliar application with 0.1 mM Se or 2 mM EDTA significantly reduced Cd concentrations in grains both in grain filling stage and heading + grain filling stage. Spraying at the filling stage has a better effect on reducing Cd concentration in grains than spraying at the heading stage. In addition, the relationship between Cd concentration in grains and husks was significantly positive, while the Cd concentration in grains and flag leaves was significantly negative. Our research proves that foliar spraying of Se and EDTA is feasible to reduce the Cd concentration in wheat grains, which provides technical guidance for the safe production of wheat in low-Cd-contaminated soils.

Foliar application of nutrients on medicinal and aromatic plants, the sustainable approaches for higher and better production

Background

The most important advantages of foliar fertilization are to improve plant growth and crop quality, appropriately manage the nutritional status of plants, enhance disease resistance and regulate nutrient deficiencies.

Main body

The aim of this manuscript is to outline and emphasize the importance of foliar application of nutrients in order to increase both quality and yield of medicinal and aromatic plants. The searches focused on publications from 1980 to July 2021 using PubMed, Google Scholar, Science Direct and Scopus databases. The current manuscript presented many examples of potential of foliar application for medicinal and aromatic plants production systems. Foliar application of Fe and Zn on Anise; Se on Atractylodes; Zn sulfate on Basil, Costmary, Mint and Fenugreek; Se and Fe on Stevia; S and P on castor bean; Zn and Fe on Chamomile; Cu, Mg and ZnSO4 on Damask rose; N and P on Fennel; Se on water spinach and tea; K+ and Ca2+ on Thyme; Zn and K on Spearmint; Zn on Saffron, Ni on Pot marigold; Fe on peppermint, N and P on Mustard had positive and significant impacts.

Conclusion

Observed impacts of foliar fertilization consisted of significant increase of yield, enhanced resistance to insects, pests and diseases, improved drought tolerance and escalated crop quality.

Effect of humic and calcareous substance amendments on the availability of cadmium in paddy soil and its accumulation in rice

Humic substances (HS) are widely known as important components in soil and significantly affect the mobility of metals due to their large surface area and abundant organic functional groups. Calcareous substances (CSs) are also commonly used as robust and cost-effective amendments for increasing the pH of acidic soils and decreasing the mobility of metals in soils. In this study, we developed a new remediation scheme for cadmium (Cd)-contaminated soil remediation by coupling HS and CS. The results showed that regardless of the addition of fulvic acid (FA), all the CS-containing treatments significantly increased the soil pH by 0.32-0.60, and the concentration of bioavailable Cd decreased in the moderately (field experiment soil, maximum 62%) and highly (pot experiment soil, maximum 57%) Cd-contaminated soils. The Cd content in rice (Oryza sativa L.) tissues significantly decreased after all the treatments. The bioaccumulation factors (BAFs) decreased by over 50% in the roots, stems, leaves and husks in all treatments, while the translocation factors (TFs) only significantly decreased in the highly contaminated soil. Among all treatments, the two HS+CS treatments (FA+CaCO₃ and FA+CaO) had the greatest effect on decreasing the concentration of bioavailable Cd in soil and Cd in brown rice grains. The suggested mechanism for the effectiveness of coupled HS and CS was that CS first mitigated the pH and precipitated Cd, followed by a complexation effect between HS and Cd. Although the Cd in rice grains in both cases was higher than the standard limit, HS+CS remediation can be advocated as a robust, simple and cost-effective scheme for Cd remediation if the additive dose is slightly increased, as this approach can simultaneously improve the pH of acidic soil and adsorb Cd in soil.

Screening of Foliar Barrier Agents and Reduces the Absorption and Transport of Cd in Wheat

Different foliar barrier agents (FBA) were used by foliar spraying in first season field and pot experiments to compare their effects on Cadmium (Cd) reduction in wheat grains. The best two FBA (50 µM SNP and 2 mM Na₂EDTA) can significantly reduce Cd concentration in wheat grains, and the filling period was the most effective period for FBA application. Compared with the control (H₂O), foliar spraying 50 µM SNP or 2 mM Na₂EDTA inhibited the moving of Cd from the lower tissue to upper tissue in stem and also significantly reduced the Cd accumulation in grains. Furthermore, compared with normal wheat variety (AK58), foliar spraying 50 µmol SNP or 2 mM Na₂EDTA as the best two FBA significantly reduced Cd concentration in shoots of Cd low accumulation varieties (HZB and HJBY), which can be used for the safe production of wheat in Cd-contaminated farmlands.

Synergistic inhibitory effect of selenium, iron, and humic acid on cadmium uptake in rice (Oryza sativa L.) seedlings in hydroponic culture

Selenium (Se), iron (Fe), and humic acid (HA) are beneficial fertilizers that inhibit cadmium (Cd) uptake in crops and are crucial for agricultural yields as well as human health. However, the joined effect of Se, Fe, and HA on Cd uptake in rice are still poorly understood. Therefore, a hydroponic culture experiment was established to evaluate the combined effect of Se (Se⁴⁺ or Se⁶⁺), Fe, and HA on the biomass, Cd uptake, and Cd translocation of/in rice seedlings. Compared to Se⁶⁺ application, Se⁴⁺ application in most treatments resulted in lower Cd translocations from roots to shoots, leading to a significant decrease in shoot Cd concentrations. Compared to the treatments with Se⁴⁺ or Fe²⁺ application, joined application of Se⁴⁺ and Fe²⁺ inhibited Cd uptake in shoots by decreasing Cd adsorption onto (iron plaque) and uptake by roots, and alleviating Cd translocation from root to shoot. Compared to the treatments with Se⁶⁺ or Fe²⁺ application, joined application of Se⁶⁺ and Fe²⁺ inhibited Cd uptake in shoots by sequestering (retaining) Cd onto root surface (iron plaque). HA inhibited Cd uptake in all treatments by decreasing the bioavailability of Cd in the nutrient solution through complexation. The simultaneous application of Se, Fe, and HA decreased the shoot Cd concentrations the most, followed by the combined application of two fertilizers and their individual application; the mean shoot Cd concentration in the Fe-SeIV-HA2 treatment was the lowest among all the treatments, at only 11.39 % of those in the control treatments. The 3-way ANOVA results indicated that the Cd concentrations in shoots were significantly affected by Se, Fe, HA, and certain of their interactions (Fe×Se and Se×HA) (p< 0.05). The above findings suggest that the joined application of Se, Fe, and HA ameliorated Cd uptake mainly by inhibiting Cd adsorption onto (iron plaque) and uptake by roots and the translocation from roots to shoots (Fe×Se⁴⁺), retaining (sequestering) Cd in iron plaque (Fe×Se⁶⁺), and decreasing Cd availability in nutrient solution (HA).

Effect of a Passivator Synthesized by Wastes of Iron Tailings and Biomass on the Leachability of Cd/Pb and Safety of Pak Choi (Brassica chinensis L.) in Contaminated Soil

Cadmium (Cd) and lead (Pb) carry a high heavy-metal-toxic risk for both animals and plants in soil. In this study, iron-based biochar (T-BC) was prepared by co-pyrolysis using wastes of iron tailings and biomass with urea as the functioning agents. Field-emission scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and toxicity-characteristic leaching procedure (TCLP) methods were employed to analyze the physicochemical characteristics of T-BC. Additionally, a pot trial was conducted to examine the effects of T-BC on the physiological characteristics of pak choi (Brassica campestris L.), the availability of heavy metals, and enzyme activities in the soils. The results show that toxic metals have been volatilized by the roasting process and immobilized within T-BC via the formation of stable metal-compounds during the co-pyrolysis process, which satisfies the requirements of a soil passivator. Incubation experiments showed that the DTPA-extractable Cd and Pb in contaminated soils decreased with an increasing amendment rate. Moreover, in the pot experiments, by adding 1% (w/w) T-BC into soils, the soils benefited from its large adsorption, complex precipitation, and immobilization capacity. Approximately 36% Cd and 29% Pb concentrations of edible parts in pak choi were reduced. The amendment proved promising for the stabilization of Cd and Pb in contaminated soils, while providing a strategy for solving the residual waste of tailings and biomass.

Foliar application of Zn reduces Cd accumulation in grains of late rice by regulating the antioxidant system, enhancing Cd chelation onto cell wall of leaves, and inhibiting Cd translocation in rice

Paddy soil contaminated by cadmium (Cd) has attracted worldwide attention, while foliar spraying of zinc (Zn) could be considered a cost-effective and practical agronomic measure for reducing Cd accumulation in rice grain. However, the effects due to foliar spraying of Zn on different cultivars, as well as the mechanism of subsequent processes taking place are not fully understood up to now. To go a step ahead, a field experiment was conducted with the aim of studying the capability of foliar application of Zn (0.4% ZnSO₄) to reduce Cd concentration in grain in five late rice cultivars (here named JLYHZ, FYY272, JY284, CLY7 and LXY130), and the antioxidant activities and subcellular distribution of Cd in the leaves. The results indicate that foliar Zn application significantly decreased grain yield in JY284, CLY7 and JLYHZ, compared to controls. In addition, foliar application of Zn significantly decreased Cd concentration in grain of the five rice cultivars, while increased Zn concentration. The effect of foliar application of Zn on transport coefficients of Cd varied greatly for the different rice cultivars. Foliar application of Zn significantly decreased the malondialdehyde (MDA) concentration in rice leaves, and increased peroxidase (POD) activity. Also, it changed the distribution of Cd in the soluble fraction in leaves (expressed as proportion), which was significantly decreased, and the proportion of Cd in the cell wall increased. The structural equation model (SEM) revealed the positive effects of flag leaf Cd, first node Cd, old leaf Cd, and root Cd concentration on grain Cd concentration. Flag leaf Cd had the highest standardized total effects on grain Cd concentration, followed by old leaf Cd. These results indicated that foliar application of Zn was effective in reducing grain Cd concentration of late rice by enhancing antioxidant activities and Cd chelation onto cell wall of leaves, and reducing Cd concentrations in leaves.

Effects of arbuscular mycorrhizal fungi on the growth and toxic element uptake of Phragmites australis (Cav.) Trin. ex Steud under zinc/cadmium stress

Arbuscular mycorrhizal fungi (AMF) play an important role in improving plant tolerance and accumulation of zinc (Zn) and cadmium (Cd). The growth, physiology and absorption of elements and transport in Phragmites australis (P. australis) were investigated under Zn and Cd stress to identify the transport mechanisms of toxic trace elements (TE) under the influence of AMF. Thus, AMF were observed to alleviate the toxic effects of Zn and Cd on P. australis by increasing plant biomass and through different regulatory patterns under different TE concentrations. The activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX) increased under Zn stress, and the activities of SOD, catalase (CAT), peroxidase (POD), and APX significantly increased under high concentrations of Cd. AMF differ in their strategies of regulating the transport of different metals under TE stress. Under Zn stress, the concentration of Zn in P. australis decreased by 10-57%, and the effect on Zn translocation factor (TF_Zn) was concentration-dependent. AMF increased the TF_Zn under low concentration stress, but decreased under high concentration stress. Under Cd stress, the concentration of Cd increased by as much as 17-40%, and the TF_Cd decreased. AMF were also found to change the interaction of Zn×Cd. In the absence of AMF, Cd exposure decreased the Zn concentrations in P. australis at Zn_100 mg/L and Zn_300 mg/L, while it increased the contents of Zn at Zn_700 mg/L. The opposite trend was observed following treatment with AMF. However, regardless of the concentration of Cd, the addition of Zn decreased the concentration of Cd in both treatments in both the presence and absence of AMF. Under different TE stress conditions, the regulation of metal elements by AMF in host plants does not follow a single strategy but a trade-off between different trends of transportations. The findings of our study are important for applying AMF-P. australis systems in the phytoremediation of Zn-Cd co-contaminated ecosystems.

Transfer mechanism and bioaccumulation risk of potentially toxic elements in soil-rice systems comparing different soil parent materials

Although the accumulation of potentially toxic elements in soil and crops has attracted widespread attention, the characteristics of the transfer and accumulation of potentially toxic elements in soil-crop systems with different soil parent materials are still not clear. Soil and crop samples were collected from agricultural regions with different soil parent materials in Guangxi, China. This study analyzed the concentrations of Cd, Zn, and Fe in the roots, straws, and seeds of rice (Oryza sativa L.) and soils with Quaternary sediments and clastic rocks as the parent materials. The concentration of several potentially toxic elements in rice tissue from the two areas followed the order of Croot> Cstraw> Cseed. The transport capability of Cd and Zn from roots to straws is higher than straws to seeds, and Fe showed a strong capability for transport from straws to seeds. In general, the transfer capacity of potentially toxic elements in the soil-rice system in the Quaternary sediments area was stronger than that in the soil-rice system in the clastic rocks area. Soil pH and minerals, which were represented by major elements, were the main factors affecting the transfer of metals from soil to seeds. This approach could help to evaluate the bioaccumulation risk of potentially toxic elements in crops in different areas quantitatively.

Application of different foliar iron fertilizers for enhancing the growth and antioxidant capacity of rice and minimizing cadmium accumulation

Iron (Fe) fertilizer can reduce cadmium (Cd) uptake and toxicity in rice, but the underlying mechanisms of Cd mitigation by different fertilizers are poorly understood. Here, pot experiments in rice were conducted to characterize the effects of four types of foliar-applied Fe fertilizer (chelated ferrous Fe, ferric Fe, ionic ferrous Fe, and ferric Fe) at three doses (20, 50, and 100 mg L^-1) on photosynthetic capacity, antioxidant ability, yield, and Cd accumulation in Cd-contaminated soil. The results showed that foliar Fe application increased the net photosynthesis rate by 19.3%, peroxidase (POD) by 18.2%, superoxide dismutase (SOD) by 26.9%, and catalase (CAT) by 19.6%, and led to a 7.2% increase in grain yield compared with the control. Moreover, foliar Fe application significantly reduced Cd accumulation by 15.9% in brown rice and decreased the translocation of Cd from roots to other plant tissues. Overall, application of moderate doses (50 mg L^-1) of chelated ferrous Fe was the most effective method for reducing Cd uptake (decreasing the Cd concentration in brown rice by 29.0%) and toxicity in rice (decreasing malondialdehyde by 23.2% and increasing POD, SOD, and CAT by 54.4%, 51.6%, and 45.7%, respectively), which may stem from the fact that chelated ferrous Fe was a more stable and bioavailable source of Fe for rice. The Cd concentration in rice had negative relationship with Fe concentration, and the translocation of Cd from root to the other tissues was reduced by the higher Fe nutrition status in leaf, suggesting that a high Fe supply may decrease Cd content by inhibiting the expression of the Fe transport system. These results indicate that foliar application of chelated ferrous Fe provides a promising alternative approach for enhancing growth and controlling Cd accumulation in rice plants. Furthermore, these results advance our understanding of the associations between plant Fe nutrition status and Cd accumulation.

Soil and foliar applications of silicon and selenium effects on cadmium accumulation and plant growth by modulation of antioxidant system and Cd translocation: Comparison of soft vs. durum wheat varieties

Minimization of Cd accumulation in wheat is an effective strategy to prevent Cd hazard to human. This study compared and highlighted the roles of soil and foliar applications of Se and Si effects on Cd accumulation and toxicity in soft and durum wheat. Soil Se (0.5-1.0 mg kg^-1) and Si (3-6 mg kg^-1) applications provided an effective strategy to reduce wheat grain Cd concentrations of both wheat varieties by 59-61 % and 16-30 %, but foliar Se (0.125-0.25 mM) and Si (2.5-5 mM) application reduced grain Cd of soft wheat by 20-36 %. Both soil and foliar Se and Si applications significantly alleviated Cd toxicity by regulation of Cd transport genes, as reflected by increased the grain yield and antioxidant enzymes activities, and reduced MDA in wheat tissues. Selenium applications were more effective than Si on the reduction of Cd-induced toxicity and concentrations in soft wheat, but not in durum wheat due to more tolerant to Cd. Downregulation of influx transporter (TaNramp5) and upregulation of efflux transporter (TaTM20 and TaHMA3) in soft wheat may contribute to the Si/Se-dependent Cd mitigation and enhance the tolerance to toxic Cd. Overall, Se/Si applications, especially soil Se, can be efficiently used for reducing grain Cd uptake from Cd-contaminated soils.

Foliar Application of Zn-EDTA at Early Filling Stage to Increase Grain Zn and Fe, and Reduce Grain Cd, Pb and Grain Yield in Rice (Oryza sativa L.)

The accumulation of Cd and Pb in rice grains poses a potential threat to human health, which is a subject of increasing concern across the globe. We examined the effect that foliar spraying of Zn-ethylenediaminetetraacetate (Zn-EDTA) (0.3% and 0.5% w/v) during the early-grain filling stage has on rice grain yield and Cd, Pb, Zn and Fe contents in rice tissues via a field experiment. The grain yield significantly decreased with the foliar application of 0.5% Zn-EDTA. In rice grain, foliar spraying of 0.5% Zn-EDTA significantly decreased the Cd and Pb contents, but increased the Zn and Fe contents. The main reasons for the decrease in the Cd and Pb content in grain were the inhibition of Cd and Pb by roots and the increased Fe content in grain via Zn-EDTA application. The foliar spraying of Zn-EDTA decreased the grain yield and Cd and Pb contents, while increased the Zn and Fe contents in grains.

Increasing soil moisture faciliates the outcomes of exogenous sulfate rather than element sulfur in reducing cadmium accumulation in rice (Oryza sativa L.)

Cadmium (Cd) contamination in paddy soils and the related pollution risk of rice grain have received increasing attention. Agronomic measures, such as the application of sulfur and changes in water regimes, were reported to mitigate the accumulation of Cd in rice. However, there is limited information on the combined effects of sulfur application and water regimes. Therefore, a pot experiment was conducted to investigate the effects of two sulfur forms, three water regimes and multiple sulfur application rates on Cd accumulation in rice. The sulfur was applied as SO₄^2- (S^VI, replacing the traditional fertilizers by SO₄^2--containing fertilizers), and element S (S⁰) was applied at 0, 10, 20, 30 and 40 mg S kg^-1 soil. The water regimes were continuous flooding (F), flooding-moist alternation (FM), and moist irrigation (M), for a total of 30 treatments. The results indicated that application of S^VI exceeding 30 mg S kg^-1 significantly reduced the Cd concentrations in brown rice by 31.1-56.3%, and the Cd concentrations decreased with increasing amount of irrigation water. Similar reductions in Cd concentrations in rice shoots and rice straw collected at tillering and maturity stages were observed after application of S^VI. However, the effect of S⁰ application on Cd accumulation in grain was not significant under different water regimes. Furthermore, this study found that application of both S^VI and S⁰ inhibited the transfer of Cd from rice roots to shoots in most cases. These findings indicate that replacing traditional fertilizers with SO₄^2--containing fertilizers, especially combined with increased irrigation, could be a potential approach to mitigate Cd accumulation in rice growing in Cd-contaminated acidic paddy soils.

Foliar application of selenium and zinc to alleviate wheat (Triticum aestivum L.) cadmium toxicity and uptake from cadmium-contaminated soil

Due to the large area of agricultural soils contaminated by Cd worldwide, cost-effective and practical method for safety food production are necessary. The roles of micronutrient on reducing Cd accumulation in crops are recently introduced. In the current study, a pot-culture experiment in the greenhouse was conducted to study the foliar spraying of Se (Na₂SeO₄) and Zn (ZnSO₄) on physiological and growth parameters, as well as Cd concentrations in wheat plants grown in Cd-contaminated soil. The foliar was sprayed with four concentration of Se and Zn (0, 10, 20, and 40 mg L^-1) at different growth stage (tillering, elongating and heading) and whole wheat plants were collected after maturity. Both foliar spraying with Se and Zn significantly enhanced the photosynthesis, tissue biomass and antioxidant enzyme activity. Additionally, Se and Zn application can also increase Se and Zn concentrations in different plant tissues. Selenium and Zn decreased malondialdehyde (MDA) and Cd concentrations in wheat grains, hulks, leaves, stalks and root in a dose-additive manner. Overall, Se and Zn both efficiently enhanced the wheat growth and Se and Zn concentrations, and simultaneously decreased the Cd concentration in wheat plant. Compared with Zn, Se more efficiently improved wheat growth and reduced Cd concentration in the wheat in a Cd-contaminated soil. Present results suggest that use of foliar spraying, especially Se, could be a cost-effective strategy and could be recommended for remediation of light-or moderate-polluted soils contaminated by Cd.

Remediation of Cd-contaminated acidic paddy fields with four-year consecutive liming

Liming has been widely used to remediate Cd-contaminated acidic soils, but the effects of consecutive liming are still unclear. Four-year liming experiments were conducted to assess the remediation of Cd-contaminated acidic paddy fields in a double rice cropping system. With four-year consecutive liming (quicklime, 2.25 t ha^-1 per season), the soil pH was increased by an average of 0.57 units (0.10-1.16 units), while the soil DTPA-Cd and available Fe and Mn were reduced by 9%, 19% and 31% (p < 0.05), respectively. The exchangeable plus water-soluble Cd fraction in soil was reduced by 17%, while the soil carbonate-, Fe/Mn oxide- and organic-bound Cd fractions were increased by 23%, 41% and 10% (p < 0.05), respectively. The Cd in rice grain was reduced by 55% for early rice and 63% for late rice (p < 0.05) and in some cases was reduced to below the Chinese allowable limit (0.2 mg kg^-1). High annual fluctuations in rice grain Cd could be caused by variations in the field water regime and in rainfall. With consecutive liming, the soil pH, DTPA-Cd and rice grain Cd varied greatly in the first three seasons and then remained relatively less variable. Meanwhile, soil available nutrients (N, P and K) and rice grain yield were little affected by liming. Soil DTPA-Cd linearly decreased with increasing soil pH, while the reduction of Cd in rice grain logarithmically decreased with increasing soil pH and the reduction in soil DTPA-Cd in the heading stage, indicating potential implications for forecasting rice grain Cd content. Therefore, consecutive liming with quicklime can be recommended for the remediation of Cd-contaminated acidic paddy fields, though supplementary measures are still necessary.

Mechanisms and uncertainties of Zn supply on regulating rice Cd uptake

Application of Zinc (Zn) is considered an effective measure to reduce Cadmium (Cd) uptake and toxicity in Cd-contaminated soils for many plant species. However, interaction between Zn and Cd in rice plant is complex and uncertain. In this study, four indica rice cultivars were selected to evaluate the effect of Zn exposure in an EGTA-buffered nutrient solution under varying Zn activities and a field level of Cd activity to characterize the interaction between Zn and Cd in rice. Severe depression in shoots' biomass, tiller number, and SPAD (Soil and Plant Analyzer Development) value were found at both Zn deficiency and Zn phytotoxicity levels among four tested rice cultivars. There existed a strong antagonism interaction between Zn and Cd in both shoot and root from Zn deficiency to Zn phytotoxicity. The reduction of Cd accumulation in roots and shoots could be explained by the competition between Zn and Cd as well as the dilution effect of increasing biomass. The conflicting effect of Zn supply on Cd uptake may be attributed to the increasing transfer ratio of Cd from root to shoot with the increasing Zn²⁺ activities and the strong depression of Fe and Mn in shoots with the increasing Zn²⁺ activities as well as the variation of genotypes. Balance between Zn and Cd should be considered in field application.

Selenium spiked in soil promoted zinc accumulation of Chinese cabbage and improved its antioxidant system and lipid peroxidation

Selenium (Se) and zinc (Zn) are necessary mineral nutrients for human body but millions of people have an inadequate intake of them, and eat food enriched with Se and Zn may minimize these problems. Chinese cabbage is an important food in people's daily life. The aim of this study was to evaluate the effects of single Se, Zn and their combination treatment in soil on their accumulation, antioxidant system and lipid peroxidation in roots and leaves of Chinese cabbage using soil pot culture experiment. When 0.5 mg kg^-1 Se +30 mg kg^-1 Zn and 1.0 mg kg^-1 Se +30 mg kg^-1 Zn were spiked in soils, Zn concentrations in roots and leaves of Chinese cabbage were significantly increased (p < 0.05) by 20.2%, 37.8% and 17.9%, 34.1% respectively compared to the treatment of 30 mg kg^-1 Zn added, and the latter was significantly higher (p < 0.05) than that of former, indicating Se significantly promoted Zn accumulation. Almost all physiological indexes including POD, SOD, CAT, APX, GR, Chlorophyll a, Chlorophyll b, Carotenoids, MDA and Free proline in the treatments of Se or Zn spiked were significantly improved (p < 0.05) or basically unaffected compared to the control without Se or Zn added. The biomass change trends were similar with these indexes either. These results showed that the addition in soil of Se and Zn significantly increased their accumulation in Chinese cabbage without affected its formal growth. Particularly, the addition of Se promoted Zn accumulation. The conclusions were more important reference for the production practice of cash crop enriched of Se and Zn either.

Effectiveness of simultaneous foliar application of Zn and Mn or P to reduce Cd concentration in rice grains: a field study

Excess cadmium (Cd) in agricultural soils can be taken up by rice plants and concentrated in the grain, presenting a human health risk. In this study, we field tested the effects of three foliar treatments (zinc (Zn) alone, or combined with manganese (ZnMn) or phosphorus (ZnP)) on the Cd concentration and grain yield of six rice cultivars (C Liangyou 7, Fengyuanyou 272, Xiangwanxian 12, Tianyouhuazhan, Xiangwanxian 13, and Jinyou 284) at the grain filling stage. Our results showed that rice yield and Cd, Zn, Mn, P, and K concentrations were significantly different among the cultivars (p < 0.05); for example, Jinyou 284 recorded lower Cd levels than any other cultivar. Application of Zn, ZnMn, and ZnP had no significant effect on rice yield and Mn, P, and K concentrations for all cultivars. Compared with the control, Cd concentrations after treatment with Zn, ZnMn, and ZnP decreased by 19.03-32.55%, 36.63-55.78% (p < 0.05), and 25.72-49.10%, respectively, while Zn concentrations increased by 11.02-29.38%, 10.63-32.67%, and 11.97-36.82%, respectively. There was a significant negative correlation between Cd and Zn concentrations (p < 0.01). All three treatments increased Zn and reduced Cd concentration in rice grains, though ZnMn was most effective. Therefore, cultivar selection and Zn fertilizer application are effective strategies to minimize Cd concentration in rice grains. However, the lowest result still exceeded the Chinese Cd safety limit (0.2 mg Cd kg^-1) by a factor of 2.6, demonstrating that additional effective measures should be simultaneously used to further reduce the accumulation of Cd in rice grains.