Split application of silicon in cadmium (Cd) spiked alkaline soil plays a vital role in decreasing Cd accumulation in rice (Oryza sativa L.) grains

Combined application of silicon and nitric oxide jointly alleviated cadmium accumulation and toxicity in maize

Cadmium (Cd) contamination is a serious threat to plants and humans. Application of silicon (Si) or nitric oxide (NO) could alleviate Cd accumulation and toxicity in plants, but whether they have joint effects on alleviating of Cd accumulation and toxicity are not known. Therefore, the combined effect of Si and NO application on maize growth, Cd uptake, Cd transports and Cd accumulation were investigated in a pot experiment. Here, we reported that Cd stress decreased growth, caused Cd accumulation in plants. The combined application of Si and NO triggered a significant response in maize, increasing plant growth and reducing Cd uptake, accumulation, translocation and bioaccumulation factors under Cd stress. The grain Cd concentration was decreased by 66 % in the Si and NO combined treatment than Cd treatment. Moreover, the combined application of Si and NO reduced Cd health risk index in maize more effectively than either treatment alone. This study provided new evidence that Si and NO have a strong joint effect on alleviating the adverse effects of Cd toxicity by decreasing Cd uptake and accumulation. We advocate for supplement of Cd-contaminated soil with Si fertilizers and treatment of crops with NO as a practical approach to alleviating Cd toxicity.

Silicon application improved the yield and nutritional quality while reduced cadmium concentration in rice

Silicon (Si) is an essential nutrient for rice, but its effects on the yield and quality of rice under heavy metal stress remain uncertain. In this study, two typical paddy soils (acidic and calcareous purple soils) in the western region of Chongqing were selected for field plot experiment, with the purpose of understanding the effects of Si implementation methods on grain yields and cadmium (Cd) uptake, transport, and accumulation in the grain of a hybrid rice (Oryza sativa L, Changliangyou 772). Four treatments were set for the purposes including soil-based Si application, foliar spray of Si alone, foliar spray of selenium (Se)-containing Si fertilizer, and a control without Si application, respectively. The results indicated that the Si applications reduced Cd contents in brown rice by 11.45~51.85% in the slightly Cd-contaminated acidic purple soil (pH = 4.77, soil total Cd content 0.413 mg kg^-1) and 26.93~43.77% in the purple calcareous paddy soil (pH = 7.77) with similar Cd-polluting levels. It is worth noting that the Cd content of conventional fertilized rice exceeds the Chinese National Food Safety Standard limit (0.2 mg kg^-1, GB2762-2017) in the slightly Cd-contaminated acidic purple soil, and foliar spray treatments showed most effective effects that meets the safety threshold standard. Soil-based Si application reduced Cd accumulation in rice grains mainly by inhibiting the translocation of Cd from stem to the rice grain or root to stem, while foliar sprays of Si mainly by inhibiting the translocation of Cd from stem to brown rice. Si applications increased the rice yield by 17.15 to 25.45% in calcareous paddy soil with foliar spray being the best, while no significant yield increase was found in acidic paddy soil. Si and Se-containing Si fertilizer improved the nutritional quality of rice grain as indicated by the increases of Se, Si, and protein contents and the significant decreases of Cd contents in the rice grains. The comprehensive effects in improving the rice quality follow the order of foliar spray of Se-containing Si fertilizer > foliar spray of Si alone > soil-based Si application. Thus, foliar spray Si-containing fertilizer could be helpful in increasing rice yield while reducing the Cd uptake in rice grains, which might be a feasible approach in controlling Cd entry into the human body via crops.

Efficiency of various silicon rich amendments on growth and cadmium accumulation in field grown cereals and health risk assessment

Cadmium (Cd) contamination of arable soils and its subsequent accumulation in food is one of the global issues which needs urgent attention. Field experiments were conducted to explore the impacts of ten silicon (Si) rich amendments on Cd bioavailability and accumulation by maize and wheat irrigated with sewage effluents. Results depicted that applied amendments decreased the total Cd accumulation in shoots and grains of both crops with and the maximum decrease was observed in rice husk biochar (RHB) treatment. The RHB was able to significantly decrease the translocation factor, Cd harvest and health risk indexes. All amendments differentially affected the soil pH, EC, CaCl₂-extractable Si, and decreased the AB-DTPA-extractable soil Cd. Overall, suitable Si rich amendments (like RHB and CSB etc.) can be employed to mitigate the health risks associated with dietary Cd in untreated sewage irrigated fields. However, the cost-benefit analysis such Si rich amendments should be considered before final recommendations.

Residual effects of frequently available organic amendments on cadmium bioavailability and accumulation in wheat

Wheat (Triticum aestivum L.) cultivation in cadmium (Cd) polluted soil is a core concern to food quality and food security all over the world. Cadmium toxicity is mainly associated with a Cd influx from contaminated soils to human via grain consumption. Organic amendments are widely used for Cd immobilization and enhancement in plant growth, but the residual effects of these amendments are mostly unknown. The present study addressed the long-term effects of organic amendments in contaminated soils by evaluating their residual effects on 3rd crop (wheat) in the sequence. Initially six organic amendments viz. rice husk biochar (RHB), wheat-straw biochar (WSB), cotton-stick biochar (CSB), poultry manure (PM), press mud (PrMd) and farm manure (FM) were applied once at a rate of 2% in Cd (50 mg kg^-1) contaminated soil with wheat-rice rotation. After the harvest of wheat and rice crops, wheat (Var. Galaxy) was again grown in the same pots. Results revealed that plants grown under Cd stress (without any amendment) contain more tissue (root, shoot and grain) and soil AB-DTPA extractable Cd. The soil amended with RHB has shown lowest AB-DTPA extractable Cd (69% lower than control). Similarly, RHB application has significantly reduced wheat root, shoot and grain Cd concentrations compared to control and other amendments. Results have confirmed the effectiveness of RHB residual contents as an active amendment for restriction of Cd in non-bioavailable pool of soil and better growth and yield of wheat.

Comparative analysis of the seed germination of pakchoi and its phytoremediation efficacy combined with chemical amendment in four polluted soils

The seed germination plant growth parameters and level of heavy metal accumulation were investigated in pakchoi cultured in four contaminated soils with different levels of heavy metals supplemented with citric acid (CA) or calcium phosphate (CP). Results showed that the seed germination energy, germination percentage and germination index parameters were similar, while the seed vigor (SV) significantly (p < 0.05) decreased as the soil pollution level increased. The lengths of the shoots and roots presented the same trend as SV. All the seedlings grew in heavily polluted soil without any amendments before harvesting; therefore, no plant material was available for subsequent analyses. The photosynthesis parameters of pakchoi cultured in lightly polluted soil without amendment (LPS), lightly polluted soil with CA (LPSA) and moderately polluted soil with CP (MPSP) were similar. The concentrations of Pb, Zn, Mn, Cu and Cd in the shoots, roots and whole plants were in the order of MPSP > LPSA > LPS. Pakchoi cultured in MPSP showed the most promising results in terms of plant height, biomass and heavy metal accumulation. Pakchoi presented the highest translocation and bioaccumulation factors for Cd and the lowest for Pb.HighlightsSoil pollution and the type of chemical amendment had no effect on the seed germination of pakchoi.Citric acid addition in lightly polluted soil improved pakchoi growth and heavy metal extraction.Pakchoi cultured in moderately polluted soil with calcium phosphate amendment presented the highest biomass and heavy metal concentration.

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

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

The paradox in accumulation behavior of cadmium and selenium at different planting times in rice

The synergistic effects of trace elements selenium (Se) and cadmium (Cd) are well known. But the reasons for the trending accumulation behavior in both trace elements are under debate in the scientific community. The present investigation was conducted to evaluate the impact of heat units on the accumulation behavior in two environments. Se and Cd were applied in three groups (T₀; 0:0, T₁; 0.4:1, T₂; 1:2) mg kg^-1. As the time of planting and heat units consumed by the crop to attain its physiological maturity appears to be different. The sunlight may contribute as an important manipulating factor for the accumulation of heavy metals in the plant. The results of the present study indicated that the behavior in the accumulation pattern of both elements appears to be distinct in the same material. The increased fertilizer treatment in soil bulk linearly increased the metal contents in rice grain. The studies between different plant parts disclosed panicle as the primary reservoir for Se and Cd accumulation. The plant sown earlier accumulates more Se than Cd while the plants uptake more Cd when sown 1 month later. The plant completes the critical developmental phases (tillering, heading, and flowering) within 20-30 days interval depending on the variety. But the late-sown plants complete these transitional phases within 9-10 days interval ultimately result in less utilization of heat units. These quick transitional changes may lead to the uptake of an ample amount of Cd contents in rice grain even at a low level of Cd stress (1 and 2 mg kg^-1), making rice unsafe for edible purposes. The proper time of planting could be effective in timely acclimatization of Se and Cd sequestration and translocation in rice various components. Graphical Abstract.

The accumulation of cadmium in wheat (Triticum aestivum) as influenced by zinc oxide nanoparticles and soil moisture conditions

Both cadmium (Cd) contamination in agricultural soils and drought stress pose a serious problem for crop quality and human health. Owing to the specific physical and chemical characteristics, zinc oxide (ZnO) nanoparticles (NPs) can be used in agriculture as a nanofertilizer but their impact on Cd accumulation in wheat (Triticum aestivum) grains under normal and limited water conditions remains insufficient. In this study, the efficiency of ZnO NPs on Cd intake by wheat was investigated under normal and water-limited conditions grown in Cd-contaminated soil for 125 days after seed sowing. The lower biomass and higher oxidative stress were observed in the tissues of the control and drought stress further decreased the plant biomass and caused oxidative stress. Zinc oxide NP treatments increased the tissue dry weight and minimized the oxidative stress either Cd stress alone or combined with drought. Drought stress enhanced the Cd contents in wheat tissues and grains, while ZnO NPs significantly reduced the Cd accumulation in tissues and grains by reducing the soil bioavailable Cd and its accumulation by roots. These findings depicted that NP application to contaminated soils can promote wheat productivity and effectively alleviate soil Cd contamination either alone or under water-limited conditions. The baseline data demonstrated in this study provide insights that pave the way towards safer wheat production under combined drought and metal stress. However, the application of NPs at field levels with numerous crops and climatic conditions needs to be investigated before final recommendation.

Silicon nanoparticles enhanced the growth and reduced the cadmium accumulation in grains of wheat (Triticum aestivum L.)

The application of silicon (Si) under heavy metal stress is well known, but the use of Si nanoparticles (NPs) under metal stress in not well documented. Thus, the experiments were performed to investigate the impacts of soil and foliar applied Si NPs on wheat (Triticum aestivum L.) growth and cadmium (Cd) accumulation in grains under Cd toxicity. The plants were grown under natural environmental conditions and were harvested after physiological maturity (124 days after sowing). The results demonstrated that Si NPs significantly improved, relative to the control, the dry biomass of shoots, roots, spikes and grains by 24-69%, 14-59%, 34-87%, and 31-96% in foliar spray and by 10-51%, 11-49%, 25-69%, and 27-74% in soil applied Si NPs, respectively. The Si NPs enhanced the leaf gas exchange attributes and chlorophyll a and b concentrations, whereas diminished the oxidative stress in leaves which was indicated by the reduced electrolyte leakage and enhancement in superoxide dismutase and peroxidase activities in leaf under Si NPs treatments over the control. When compared with the control, the foliar spray of Si NPs reduced the Cd contents in shoots, roots, and grains by 16-58%, 19-64%, and 20-82%, respectively, whereas soil applied Si NPs reduced the Cd concentrations in shoots, roots, and grains by 11-53%, 10-59%, and 22-83%, respectively. In comparison with the control, Si concentrations significantly (p ≤ 0.05) increased in the shoots and roots in both foliar and soil supplementation of Si NPs. Our results suggested that Si NPs could improve the yield of wheat and more importantly, reduce the Cd concentrations in the grains. Thus, the use of Si NPs might be a feasible approach in controlling Cd entry into the human body via crops.

Cross-Talk between Cadmium and Selenium at Elevated Cadmium Stress Determines the Fate of Selenium Uptake in Rice

Cadmium (Cd) is a well-known metal imposing threats to human health, and it can be accumulated in polished rice over the permitted range of 0.2 mg kg-1 (GB 2762-2017). It has been reported that selenium (Se) application decreases Cd uptake. Se-rich diets have gained attention recently, but the potential of Se-rich rice in mitigating Cd stress needs further investigation. In this study, a pot experiment in the field was conducted to assess the influence of environmental factors and exogenous split application of Se on the nutritional status of rice under Cd stress. The results indicated that the increased fertilizer treatment in soil bulk linearly increased the metal content in rice grains. Approximately 50-70% of metal was recovered in rice tissues, while 5-20% of the metal that was applied leached down into the soil. A Se concentration of 0.4 mg kg-1 could significantly improve the total Se content in grain and mitigate Cd toxicity (1 mg kg-1) below the permitted range. Panicles and roots were more active for total Se accumulation in Se-rich and non-Se-rich rice, respectively. Polishing and milling operations can significantly reduce the Cd content, as rice bran in rice tissues accumulated most of the metal's residues. The late matured rice cultivars consumed more heat units, and more metal contents were found in them. Collectively, it was found that Se can mitigate Cd toxicity, but the rice cultivation at T2 (high Cd; 2 mg kg-1 and Se; 1 mg kg-1) increased the metal uptake capability and health-risk index in polished rice, with its Se content heightened over permitted range of 0.04 to 0.30 mg kg-1 (GB/T 22499-2008). However, further molecular studies are required, in order to completely access the inverted Se accumulation behavior in rice tissues at high Cd soil stress.

Opportunities and challenges in the remediation of metal-contaminated soils by using tobacco (Nicotiana tabacum L.): a critical review

The successful phytoextraction of potentially toxic elements (PTEs) from polluted soils can be achieved by growing non-food and industrial crops. Tobacco (Nicotiana tabacum L.) is one of the main industrial crops and is widely grown in many countries. Tobacco can uptake high concentrations of PTEs especially in aboveground biomass without suffering from toxicity. This review highlighted the potential of tobacco for the phytoextraction of heavy metals and tolerance mechanisms under metal stress. Different management practices have been discussed which can enhance the potential of this plant for metal extraction. Finally, suitable options for the management/disposal of biomass enriched in excess metal have been elaborated to prevent secondary pollution.