Cd Accumulation In Rice Plants Research Articles

Straw removal reduces Cd availability and rice Cd accumulation in Cd-contaminated paddy soil: Cd fraction, soil microorganism structure and porewater DOC and Cd

The impacts of straw removal on rice Cd absorption, behaviour of Cd and microbial community in rhizosphere soil were investigated in paddy fields over two consecutive seasons. The results of the experiments in two fields revealed that straw removal promoted the transformation of soil Cd from acid-extractable and oxidisable fraction to residual fraction and reduced soil DTPA-Cd content with the reduction in DOC and Cd ions in soil porewater, thereby decreasing Cd content in rice. Specifically, the Cd content in brown rice of early rice was below 0.2mg·kg−1 when all rice straw and roots were removed in the slightly Cd-contaminated soils. The α-diversity of soil microbial communities was less influenced by continuous straw removal, β-diversity was altered and the relative abundances of Anaeromyxobacter, Methylocystis and Mycobacterium microbes were increased. Redundancy analysis and network analysis exhibited that soil pH predominantly influenced the microbial community. Path analysis revealed that the Cd content in brown rice could be directly influenced by the soil Total-Cd and DTPA-Cd, as well as soil pH and OM. Straw removal, including root removal, is an economical and effective technique to reduce Cd accumulation in rice plants. ENVIRONMENTAL IMPLICATIONHeavy metal Cd pollution in paddy fields is still the primary problem affecting the safe production of rice in Asian countries, especially in China. Currently, straw removal is a significant technique and holds promise as a viable method for remediating Cd-contaminated soil in southern China. We analysed the effects of straw removal from putting the issues into perspective. It was found that straw removal reduced the contents of soil Total-Cd and DTPA-Cd by changing soil pH and OM, hindered the migration of Cd from soil to rice, and affected the microbial community and Cd accumulation in rice.

Cadmium-resistant bacterium Ralstonia sp. YDR alleviated Cd toxicity in rice seedlings by enhancing antioxidant defense and inhibiting Cd2+ influx and H+ efflux

Cadmium (Cd) accumulation in rice (Oryza sativa L.) and its potential risks for human health have attracted great attention. Several Cd-resistant bacteria can promote plant growth and alleviate Cd toxicity to rice, but how they regulate Cd2+ flux in rice roots is still poorly understood. Here, a new Cd-resistant endophytic bacterium Ralstonia sp. YDR was isolated and used to evaluate its role in reducing rice Cd accumulation. Ralstonia sp. YDR tolerated 80 mg L−1 Cd and removed amounts of Cd via bio-adsorption and bio-absorption. Ralstonia sp. YDR also harbored good alkalizing ability and exhibited typical plant growth-promoting characteristics. Hydroponic experiment showed that inoculation with Ralstonia sp. YDR not only significantly promoted rice growth but also decreased rice Cd by 14.4–41.3%. Further analyses verified the involvement of Ralstonia sp. YDR in increasing the activities of catalase, glutathione, peroxidase, and superoxide dismutase by 27.7–77.3%, 1.7–57.6%, 57.4–100.8%, and 5.31–60.2%, and decreasing the activity of malonaldehyde by up to 56% in rice seedlings as compared with control. Moreover, a significant reduction in Cd2+ influx and H+ efflux occurred in rice roots after inoculating with Ralstonia sp. YDR. Collectively, bacterial adsorption and absorption of Cd, bacteria-aided antioxidation, and bacterial regulation of Cd2+ and H+ fluxes all contributed to a low Cd accumulation in rice. Our study provides new clues to better understand microbial mechanisms underlying the low Cd accumulation in rice plants, which helps to enhance the production of low Cd rice in agricultural practice.

Microcystis-dominated paddy crusts enhancing the cadmium accumulation in rice plant (Oryza sativa L.)

Paddy crusts (PCs) influence seriously on the heavy metal migration and transformation in the paddy field system. However, their quantitative effect on heavy metal accumulation in rice plant has been rarely reported. In this study, the influence of microcystis-dominated PCs on Cd content and physicochemical properties of the surface soil, water and rice plants (Oryza sativa L.) during PCs' overall life cycle was investigated. During the PC growth period (0–15 d), the Cd content in the surface soil (0–10 cm) and water decreased by 10–20% due to the accumulation effect. However, during the perish period (15–35 d), the Cd content in the surface soil and water increased by 20–30%, as Cd stored in the PCs was re-released. Moreover, the decomposition of PCs produced a large amount of low-molecular-weight organic substances (<500 Da), significantly increasing the bioavailable acid-soluble Cd in the surface soil. Finally, the PCs increase 25–80% and 60–85% Cd content in rice root and shoot, respectively. A structural equation model demonstrated that the contribution to the Cd in rice roots in the sequence: Cd in surface water > PC > surface soil. Potted experiments using sandy, clay, sandy loam soil containing 0.8–3.5 mg/kg of Cd collected from six provinces’ paddy field in southern China confirmed that PCs increased the Cd accumulation in rice plants by as much as 25–80%. Consequently, the presence of PCs can enhance the Cd concentration in rice plants, and the removing PCs could be a potential effectively approach to reducing Cd content in rice plants.

Study of ceramsite-supported iron and manganese oxides for enhancing soil immobilization and reducing rice plants uptake of cadmium

The use of metal oxides functionalized clay ceramsite as a soil amendment for immobilizing trace metals in the soil gives some novel insights and has gained extensive attention. The clay ceramsite functionalized with Fe/Mn oxides was controllably synthesized, including clay ceramsite (NCs), iron oxide ceramsite (FeOx-NCs), manganese oxide ceramsite (MnOx-NCs), and iron-manganese oxide ceramsite (FeOx/MnOx-NCs). The underlying mechanisms of these materials on Cd migration and transformation in soil, and bioaccumulation in rice plants were studied. The results revealed that compared to the control (without adding any materials), all the amendments increased soil pH, cation exchange capacity (CEC), and dissolved organic carbon (DOC). DTPA-Cd contents decreased by 7.15%, 4.14%, 34.21%, and 30.45% in the treatments of NCs, FeOx-NCs, MnOx-NCs, and FeOx/MnOx-NCs after 48 days, respectively. Exchangeable Cd fraction was mainly transformed into carbonate-bound and Fe/Mn-bound fractions. The concentrations of DCB-Fe, DCB-Mn, and DCB-Cd on the root surface of rice increased by 2.9%− 79.6%, 5.1%− 116.9%, and 56.7%− 84.4%, respectively. Obviously, the contents of root-Cd, stem-Cd, and leaf-Cd decreased by 26.37%− 49.22%, 45.39%− 85.73%, and 43.41%− 73.26%, respectively. Principal components analysis (PCA) and correlation analysis explained the migration pathways of Cd in the soil-rice-material system, which helped us to further explore the remediation mechanism of soil. All the amendments improved soil physicochemical properties, and decreased the Cd accumulation in rice plants. Meanwhile, the materials structural characterization pointed out that the chemical precipitation and surface complexation of oxygen-containing functional groups were main Cd immobilization mechanisms. Our results provided valuable references for Cd immobilization in the soil by using Fe/Mn oxides functionalized clay ceramsite.

Selenium bio-nanocomposite based on extracellular polymeric substances (EPS): Synthesis, characterization and application in alleviating cadmium toxicity in rice (Oryza sativa L.)

Selenium nanoparticles (SeNPs) have gained significant attention owing to their favorable bioavailability and low toxicity, making them widely applications in the fields of medicine, food and agriculture. In this study, bacterial extracellular polymeric substances (EPS) were used as a novel stabilizer and capping agent to prepare dispersed SeNPs. Results show that EPS-SeNPs presented negative potential (−38 mV), spherical morphologies with average particle size about 100–200 nm and kept stable at room temperature for a long time. X-ray diffraction (XRD) analysis demonstrated that the synthesized nanoparticles were pure amorphous nanoparticles, and X-ray photoelectron spectroscopy (XPS) spectrum showed a spike at 55.6 eV, indicating the presence of zero-valent nano‑selenium. Fourier-transform infrared spectroscopy (FTIR) and three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy analysis confirmed proteins and polysaccharides in EPS played a crucial role in the synthesis of EPS-SeNPs. Compared to EPS or sodium selenite (Na2SeO3), EPS-SeNPs showed a relatively moderate result in terms of scavenging free radicals in vitro. In contrast, EPS-SeNPs demonstrated lower toxicity to rice seeds than Na2SeO3. Notably, the exogenous application of EPS-SeNPs effectively alleviated the growth inhibition and oxidative damaged caused by cadmium (Cd), and significantly reduced Cd accumulation in rice plants.

Kinetin-mediated reduction of cadmium accumulation in rice (Oryza sativa L.) via modulation of cell wall binding capacity in a NO-dependent manner

Cadmium (Cd) is a heavy metal that is toxic to both plant growth and human health. Kinetin, a precursor of cytokinin, plays crucial roles in the response of plants to abiotic stress. However, the mechanism underlying kinetin detoxification in Cd-induced stress remains poorly understood. In this study, we observed a significant increase in endogenous kinetin levels in both rice shoots and roots under Cd stress. Furthermore, the application of exogenous kinetin resulted in a reduction in hemicellulose content within cell walls, thereby reducing the cell wall's capacity to bind Cd and limit its entry into cells. In addition, exogenous kinetin downregulated the transcript levels of genes associated with Cd uptake and root-to-shoot translocation while simultaneously upregulating the transcript levels of genes involved in Cd efflux from cells and sequestration into vacuoles, consequently leading to reduced Cd accumulation in rice plants. Moreover, kinetin significantly decreased nitric oxide (NO) accumulation in response to Cd stress, potentially facilitating detoxification during Cd-induced stress through a NO-dependent mechanism. Overall, our findings suggest that kinetin enhances rice's tolerance to Cd by increasing the cell wall's ability to bind Cd and regulating gene expression related to Cd accumulation. These mechanisms may be NO-dependent and could potentially inform the development of new strategies for mitigating Cd toxicity in agricultural systems.

Effect of amendments on bioavailability of cadmium in soil-rice system: a field experiment study.

The field experiment study investigated the effect of lime (L), manure compost (M), combination of lime and manure (LM), and combinations of lime with four kinds of passivators (LP1, LP2, LP3, and LP4) on the bioavailability of cadmium (Cd) in soil and Cd accumulation in rice plants. These four passivating products were composed of organic and inorganic compounds such as silicon-sulfhydryl group, CaO, SiO2, and so on. The results indicated that the application of these amendments improved soil pH, organic matter content, and cation exchange capacity (CEC) by 0.19-0.73 unit, 0.6-8.2%, and 5.7-38.9%, respectively; meanwhile, decreased soil acid-extractable Cd by 4.0-13.9% compared with before remediation. Alleviating Cd stress to rice also resulted in a significant increase in rice grains yield, whereas the LP4 showed an increment of 15.8-27.6%. Among these amendments, LP4 had a relatively high effectiveness, it promoted the decrease of extractable Cd by 13.9% and the increase of residual Cd by 8.1%; meanwhile, the bioconcentration factor of rice grain in LP4 decreased by 71.3%. The high pH, CEC, and rich functional groups in amendments might cause soil Cd transform from mobile fraction to residual fraction, and the cation ions in amendments also competed with Cd ions due to the antagonism. Taken all of these effects, the amendments alleviated Cd pollution in soil-rice system, decreasing Cd migration from soil to grain. In future, the long-term field experiment will need to be done for verify the long-term effect of soil amendments.

A Glucuronic Acid-Producing Endophyte Pseudomonas sp. MCS15 Reduces Cadmium Uptake in Rice by Inhibition of Ethylene Biosynthesis.

Dynamic regulation of phytohormone levels is pivotal for plant adaptation to harmful conditions. It is increasingly evidenced that endophytic bacteria can regulate plant hormone levels to help their hosts counteract adverse effects imposed by abiotic and biotic stresses, but the mechanisms underlying the endophyte-induced stress resistance of plants remain largely elusive. In this study, a glucuronic acid-producing endophyte Pseudomonas sp. MCS15 alleviated cadmium (Cd) toxicity in rice plants. Inoculation with MCS15 significantly inhibited the expression of ethylene biosynthetic genes including OsACO3, OsACO4, OsACO5, OsACS2, and OsACS5 and thus reduced the content of ethylene in rice roots. In addition, the expression of iron uptake-related genes including OsIRT1, OsIRT2, OsNAS1, OsNAS2 and OsYSL15 was significantly downregulated in the MCS15-inoculated roots under Cd stress. Similarly, glucuronic acid treatment also remarkably inhibited root uptake of Cd and reduced the production of ethylene. However, treatment with 1-aminocyclopropyl carboxylic acid (ACC), a precursor of ethylene, almost abolished the MCS15 or glucuronic acid-induced inhibition of Cd accumulation in rice plants. Conversely, treatment with aminoethoxyvinyl glycine (AVG), an inhibitor of ethylene biosynthesis, markedly reduced the Cd accumulation in plants. Taken together, our results revealed that the endophytic bacteria MCS15-secreted glucuronic acid inhibited the biosynthesis of ethylene and thus weakened iron uptake-related systems in rice roots, which contributed to preventing the Cd accumulation.

Influence of Iron Plaque on Accumulation and Translocation of Cadmium by Rice Seedlings

This study investigated the impact of soil type and rice cultivars on variations in the iron plaque formation and cadmium (Cd) accumulation by different portions of rice seedlings under the influence of Fe amendment. The experiments were performed in pots under glasshouse conditions using two typical paddy soils. Rice seedlings were exposed to three concentrations of Cd (0, 1 and 3 mg kg−1 soil) and Fe (0, 1.0 and 2.0 g kg−1 soil). The results revealed that shoot biomass decreased by 12.2–23.2% in Quest and 12.8–30.8% in Langi in the Cd1.0 and Cd3.0 treatments, while shoot biomass increased by 11.2–19.5% in Quest and 26–43.3% in Langi in Fe1.0 and Fe2.0 as compared to the Fe control. The Cd concentration in the roots and shoots of rice seedlings were in the order of Langi cultivar &gt; Quest cultivar, but the Fe concentration in rice tissues showed the reverse order. Fe plaque formations were promoted by Fe application, which was 7.8 and 10.4 times higher at 1 and 2 g kg−1 Fe applications compared to the control Fe treatment. The Quest cultivar exhibited 13% higher iron plaque formation capacity compared to the Langi cultivar in both soil types. These results indicate that enhanced iron plaque formation on the root surface was crucial to reduce the Cd concentration in rice plants, which could be an effective strategy to regulate grain Cd accumulation in rice plants.

Salicylic acid reduces cadmium (Cd) accumulation in rice (Oryza sativa L.) by regulating root cell wall composition via nitric oxide signaling

The effects of salicylic acid (SA) on cadmium (Cd) accumulation, Cd subcellular distribution, cell wall composition and Cd adsorption in Cd-stressed rice seedlings were examined. The interaction between SA and nitric oxide (NO) signaling in regulating cell wall composition under Cd exposure was also investigated. Our results showed that 5 μmol·L−1 Cd treatment significantly decreased plant height, root length and plant dry weight by 40.1%, 46.1% and 21.3% (p < 0.05), respectively, and the inhibitory effects of Cd on the growth parameters were alleviated by exogenous SA. Application of SA remarkably decreased Cd concentrations in roots and shoots of rice seedlings by 48.0% and 19.6%, respectively, and increased the distribution ratio of Cd in the root cell wall fraction (from 35.7% to 40.6%) compared with Cd treatment alone. The reduced Cd accumulation in rice plants could be attributed to that SA application promoted pectin synthesis and demethylesterification, thereby increasing Cd deposition in the root cell wall. Moreover, SA application promoted lignin biosynthesis to strengthen the cell wall and prevent Cd from entering the root cells. In addition, NO might be involved in SA-induced pectin synthesis, pectin demethylesterification and lignin biosynthesis as a downstream signaling molecule, contributing to reduced Cd accumulation in Cd-stressed rice seedlings. The results provide deep insights into the mechanisms of exogenous SA action in reducing Cd accumulation in rice plants.

Influence of elemental sulfur on cadmium bioavailability, microbial community in paddy soil and Cd accumulation in rice plants

Cadmium (Cd) is highly toxic to living organisms and the contamination of Cd in paddy soil in China has received much attention. In the present study, by conducting pot experiment, the influence of S fertilizer (S0) on rice growth, iron plaque formation, Cd accumulation in rice plants and bacterial community in rice rhizosphere soil was investigated. The biomass of rice plants was significantly increased by S0 addition (19.5–73.6%). The addition of S0 increased the formation of iron plaque by 24.3–45.8%, meanwhile the amount of Cd sequestered on iron plaque increased. In soil treated with 5 mg/kg Cd, addition of 0.2 g/kg S0 decreased the diffusive gradients in thin films (DGT) extractable Cd by 60.0%. The application of S0 significantly decreased the concentration of Cd in rice grain by 12.1% (0.1 g/kg) and 36.6% (0.2 g/kg) respectively. The addition of S0 significantly increased the ratio of Acidobacteria, Bacteroidetes in rice rhizosphere soil. Meanwhile, the ratio of Planctomycetes and Chloroflexi decreased. The results indicated that promoting Fe- and S-reducing and residue decomposition bacterial in the rhizosphere by S0 may be one biological reason for reducing Cd risk in the soil-rice system.

Influence of Clay Application and Water Management on Ability of Rice to Resist Cadmium Stress

A high percentage of available Cadmium (Cd) in acid rice fields in south China promoted Cd accumulation in rice plant and caused a grain Cd content above the national standard GB 2762-2012 guidelin...

Attapulgite and processed oyster shell powder effectively reduce cadmium accumulation in grains of rice growing in a contaminated acidic paddy field

Heavy-metal contamination is widespread in agricultural soils worldwide, especially paddy soils contaminated by Cd. Amendment-induced immobilization of heavy metals is an attractive and effective technique, provided that cost-effective materials are used. This field experiment compared three alkaline passivators (attapulgite, processed oyster shell powder, and mixed soil conditioner) at a rate of 2.25 t ha−1 for their effectiveness in decreasing Cd bioavailability in soils and accumulation in rice plants in a paddy field contaminated by Cd (0.38 Cd mg kg−1). The utilization of attapulgite and processed oyster shell powder decreased labile fractions but increased stable fractions of Cd in soils through ion exchange, precipitation and complexation. The addition of attapulgite decreased the concentration of bioavailable Cd in both bulk and rhizosphere soils, whereas the amendment of processed oyster shell powder decreased it only in bulk soil. The Cd accumulation in rice plants correlated significantly with acid-soluble and residual Cd fractions in the rhizosphere soil but not in the bulk soil. The addition of attapulgite and processed oyster shell powder decreased Cd accumulation in rice grains from 0.26 mg kg−1 to 0.14 and 0.19 mg kg−1, respectively, meeting the National Food Safety Standard (< 0.20 mg kg−1). However, the mixed soil conditioner did not decrease the Cd accumulation in rice shoots or grains. This study demonstrated that attapulgite and processed oyster shell powder were economic agents in reducing Cd accumulation in rice grains.

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 100mgL-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 (50mgL-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.

Effects of Exogenous Phosphorus on Rice Growth and Cadmium Accumulation and Transportation Under Cadmium Stress

In this study, we investigated the effects of exogenous phosphorus on the accumulation and transportation of cadmium in rice plants through a hydroponic experiment. In the experiment, the rice variety was Huanghuazhan, P solution concentrations were 10.0-45.0 mg ·L-1 that was made using NaH2PO4, and Cd solution concentrations were 0.1 mg ·L-1 and 0.2 mg ·L-1. The results showed that: ① the biomass in all parts of rice plants and contents of photosynthetic pigment (chlorophyll a, chlorophyll b, and carotenoid) increased gradually with an increase in exogenous P. ② Content of Cd in rice stems, leaves, husk, and brown rice increased gradually with an increase in the amounts of exogenous P. The content of Cd in brown rice increased by 2.8%-22.8% and 40.9%-61.8% when treated with Cd concentrations of 0.1 mg ·L-1 and 0.2 mg ·L-1 in hydroponic solutions, respectively. ③ Cd accumulation in rice plants was accelerated due to the application of exogenous P. Cd accumulating amounts increased from 395.1 μg ·plant-1 to 542.6 μg ·plant-1 and 639.6 μg ·plant-1 to 1082.0 μg ·plant-1 when treated with Cd concentrations of 0.1 and 0.2 mg ·L-1 in hydroponic solutions, respectively. ④ With an increase in the applied amounts of exogenous P, the P/Cd quality ratio in rice roots increased, while those in rice stems, leaves, husks, and brown rice decreased; meanwhile, the Cd transfer coefficients from root to stem (TFroot-stem) and stem to leaf (TFstem-leaf) increased. This showed that there was a certain synergistic effect between P and Cd in the rice parts. Finally, the application of exogenous P promoted the transfer of Cd from the rice root to other rice tissues, resulting in a synergistic effect on Cd accumulation and transportation in various rice tissues and increased Cd contents in brown rice.

Efficiency and mechanisms of fermented horse manure, vermicompost, bamboo biochar, and fly ash on Cd accumulation in rice.

The aims of this study were to investigate the effectiveness of fermented horse manure (HM) over vermicompost (VC), bamboo biochar (BB), and fly ash (FA) in suppressing Cd accumulation in rice plants and further to elucidate the underlying mechanisms of HM. Greenhouse experiments were conducted to determine the Cd concentration in rice tissues after the application of amendments to different Cd-contaminated soils. Chemical properties of root Fe plaque and pore water were also determined, including the amount of Fe in Fe plaque, and the changes in redox potential (Eh) and concentration of Cd and Fe in pore water with cultivation time. The results showed that HM was the most effective of all amendments in suppressing Cd accumulation in rice (by at least 57%), and the effect increased with the application rate. Furthermore, the application of HM can decrease the concentration of Cd in soil pore water and promote the formation of root Fe plaque (a barrier to prevent Cd uptake by rice plants) by decreasing the Eh and increasing the Fe concentration in pore water. The decrease in Cd concentration in soil pore water and the increase in root Fe plaque were the main reasons for the considerable reduction in the uptake and accumulation of Cd in rice plants by HM.

Effects of 24-epibrassinolide and 28-homobrassinolide on iron plaque formation and the uptake of As and Cd by rice seedlings (Oryza sativa L.) in solution culture

Abstract The effects of 24-epibrassinolide (Br24) or 28-homobrassinolide (Br28) on the formation of iron plaque (IP) and the uptake of As and Cd by rice plant are poorly understood. Thus, the current work was performed to examine the impacts of Br24/Br28 (10−8 M) application on IP formation (50 mg Fe2＋ L−1 supply) and their interactions on As and Cd accumulation in rice seedlings exposed to As (III) (0.5 mg L−1) and/or Cd (0.5 mg L−1) stresses by a hydroponic experiment. The results showed that the combination of Br24/Br28 and IP induction decreased rice biomass, but markedly promoted IP formation and Fe concentrations in rice plant in comparison with single IP induction. Under Cd stress, IP induction significantly reduced Cd concentrations in shoots by 31% and root tissues by 51%, but the combinations of IP induction and Br24/Br28 promoted Cd concentrations in shoots, root tissues and root surface under Cd or As+Cd treatments. Under As stress, Br24 or IP induction did not affect As concentrations in root tissues and root surface, but markedly decreased that by 27% and 48% in shoots. Under As or As+Cd treatments, the interactions between IP induction and Br24/Br28 significantly enhanced As concentrations on the root surface and decreased that by 14%–50% in shoot. Our results demonstrate that Br24/Br28 application plays vital roles in promoting iron plaque formation and regulating As and Cd accumulation in rice plant.

Influence of silicon on cadmium availability and cadmium uptake by rice in acid and alkaline paddy soils

The aim of this research was to study the inhibiting effects of cadmium uptake into plants by exogenous silicon between acid and alkaline paddy soils, especially, the effects of silicon without the differentiation of pH and accompanying ions. Therefore, without the interference of pH and accompany ions (Na+), the mechanisms of soil chemistry on cadmium (Cd) transformation and accumulation by Si application in the soil-plant system were examined. The acid and alkaline soils for the experiment were collected from the upper soil layer (0~20 cm) from two paddy fields. The Cd solution as CdNO3 and different levels of SiO2 as Na2SiO3 solutions were applied in the potted soils. To eliminate differences of nitrate and sodium among the treatments, 6.8, 6.8, and 1.7 g sodium nitrate was added into the CK, Si0Cd5, and Si90Cd5 treatments, respectively. The soil redox potential and pH during the experiments were determined. After harvest, changes in Cd concentrations in rice grains, stems, and roots were measured. Soil samples in close proximity to the rice roots were collected at harvest. The sequential extraction was employed to estimate the chemical speciation of Cd in the treated soils. The Si applications reduced Cd accumulation in rice plants, thereby increasing the rice yield. With increase of the dosages, the ratios of Cd contents in root, stem, and grain decreased significantly. The Eh and pH of the Si treatments were higher than application of Cd only. In the alkaline soil, Cd contents in the soil solutions of the Si treatments were higher than the Si0Cd5 treatment. In contrast, in the acid soil, the Cd contents in the soil solution of the Si treatments were lower than that Si0Cd5 treatment. In the acid soil, Si application improved the Cd adsorption by soil and alleviated Cd uptake by plants. Therefore, the alkaline soil reduced Cd phytoavailability by forming Si-Cd water-soluble complex, which could not be taken up by rice plants. The Si application significantly increased yield of rice and decreased Cd concentrations in the roots, stems, and grains of rice plants. In the alkaline soil, Cd and Si formed the Si-Cd water-soluble complex, which could not be taken up by the rice plants. In both soils, Si reduced the concentrations of acid extractable Cd and reducible Cd while promoting the formation of oxidable Cd and residual Cd, resulting in low soil Cd bioavailability.

Effects of Cd-resistant bacteria and calcium carbonate + sepiolite on Cd availability in contaminated paddy soil and on Cd accumulation in brown rice grains.

A pot experiment was conducted to evaluate the effects of combined application of cadmium (Cd)-resistant bacteria (J) and calcium carbonate+sepiolite (G) on both Cd bioavailability in contaminated paddy soil and on Cd accumulation in rice plants. Adding the mixture (J+G) to the soils significantly increased soil pH, decreased extractable Cd contents, and increased Fe/Mn-oxide Cd and organic-bound Cd contents. The applying of J+G, J and G decreased Cd contents in various rice tissues (roots, stems and leaves, husks, and brown rice grains) to different degrees. Compared with those of the CK, Cd contents decreased by 17.8%-53.3% in the roots, 12.3%-27.4% in the stems and leaves, 25.4%-44.6% in the husks, and 28.8%-55.7% in the brown rice grains for the application of J+G; Cd contents decreased by 8.2%-28.5% in the roots, 11.5%-32.0% in the husks, and 27.8%-45.9% in the brown rice grains for the application of J; Cd contents decreased by 12.9%-26.5% in the roots, in the stems and leaves decreased by 4.6%-34.1% in the stems and leaves, 60.2%-79.7% in the husks, and 35.7%-47.6% in the brown rice grains for the application of G. The alone application of bacteria (J) could reduce the bioavailability of Cd in soil and the contents of Cd in brown rice grains to some extent. Moreover, when the bacteria were applied in combination with mineral (J+G), it was a more effective method than the alone application of J or G to reduce the soil Cd bioavailability. Under all the tested conditions, applications of J4+G4 (320mLkg-1 of J+8gkg-1 of G) resulted in the greatest reduction in Cd contents in brown rice grains. Overall, the results indicated that the combination of Cd-resistant bacteria and mineral material could effectively reduce Cd bioavailability in paddy soils and inhibit Cd accumulation in brown rice grains.

Iron fractions responsible for the variation of Cd bioavailability in paddy soil under variable pe+pH conditions

Iron (Fe) in soil is closely related to cadmium (Cd) uptake by rice plants, and soil pe + pH significantly influences Fe redox behavior. This study aimed to explore the influential mechanisms of varying pe + pH conditions on the transformation of iron oxides in the rhizosphere and the subsequent effect on Cd accumulation in rice plants. A two-month pot experiment was conducted to investigate the effect of soil pe + pH on the fractions of iron oxides and formation of iron plaque (IP), as well as the effect of these changes on Cd uptake by rice plants (Oryza sativa L.). Different irrigation strategies, 70% water holding capacity (DY), continuous flooding (FL), and alternate flooding/drying weekly (AWD), were used to achieve various soil pe + pH levels. The results showed that low pe + pH conditions (under the FL and AWD treatments) were more beneficial to the transformation of crystalline iron oxides into amorphous forms in rhizosphere soil and the precipitation of IP on rice roots. The increase of amorphous iron oxides resulted in the reduction of Cd availability in rhizosphere soil by immobilizing more Cd on Fe oxides. Moreover, Cd adsorbed on rice root surfaces reacted with IP, inhibiting Cd soil-to-root transport. The two mechanisms combinatively functioned at decreasing Cd concentration in rice shoots by 14.1–33.1% at low pe + pH conditions compared to that of the high pe + pH (DY treatment). These results indicate that lowering soil pe + pH effectively reduced Cd accumulation in rice plants, probably through the immobilization of amorphous Fe oxides on Cd and sequestration of iron-plaque on Cd.