Cd In Rice Grains Research Articles

A combined management scheme to simultaneously mitigate As and Cd concentrations in rice cultivated in contaminated paddy soil

Paddy soils in southern China are heavily co-polluted by arsenic (As) and cadmium (Cd). The accumulation of these contaminants in rice grains may pose a high health risk. We evaluated the impact of adjusted water management practice (i.e., conventional irrigation and aerobic treatment after heading stage) and the application of two immobilization agents (i.e., CaO and Fe2O3) on the accumulation of As and Cd in rice grains of three rice varieties (i.e., Jinyou-463, Jinyou-268, and Mabayouzhan). The different schemes were tested via conducting a field experiment in paddy soil in Shaoguan, Guangdong Province, China. The results showed that the combined scheme (selecting Jinyou-268, aerobic water management after the heading stage, and 0.09% CaO and 0.5% Fe2O3 amendments) exhibited the best performance in the reduction of As and Cd accumulation in rice grains. This combined scheme decreased the grain As concentration by 26.19% and maintained the Cd at a low level (0.056 mg/kg) as compared to the use of local conventional irrigation patterns. Moreover, health risk assessment demonstrated that by applying the optimal scheme, neither As nor Cd content in rice had carcinogenic risk. However, the grain As remains at a high non-carcinogenic risk. We suggest that future field study design should fully incorporate the uncertainty of the natural environment to make the research conclusions more feasible for popularization and utilization. This study demonstrated an approach of utilizing the synergy effects of various measures for safe rice production in fields subjected to As and Cd contaminations.

Sprinkler irrigation in the production of safe rice by soils heavily polluted by arsenic and cadmium

Among the factors affecting the bioaccumulation of As and Cd in rice, a key role is played by the irrigation methods. The sprinkler irrigation (SP), optimized for rice in Sardinia, Italy, applied to several rice genotypes over many years has produced no differences in yields in comparison to what observed using the traditional continuous flooding irrigation method (CF). Because all the previous SP trials have been performed just on one, unpolluted soil, the principal aim of this study is to ascertain the effectiveness of SP to simultaneously minimize the bioaccumulation of As and Cd in rice grain even in soils severely polluted by As and/or Cd. Hence, a Carnise rice genotype was cultivated in an open field in: i) an unpolluted soil; ii) a soil polluted with 55 mg kg−1 of As; iii) a soil polluted with 40 mg kg−1 of Cd; iv) a soil polluted with 50 mg kg−1 of As and 50 mg kg−1 of Cd. In the worst condition of pollution, the amounts of total As and Cd measured in the kernels using a fully validated ICP-MS method is 90 ± 10 μg kg−1 and 50 ± 20 μg kg−1, respectively, i.e. less than 50% and the 25% of the maximum concentration set for these elements in rice by the European Community (200 μg kg−1 for the inorganic As and the total amount of Cd, respectively). SP might represent a simple and valuable tool able to produce safe rice also from soils where the traditional irrigation might produce inedible rice only.

Alleviating Cd translocation and accumulation in soil–rice systems: Combination of foliar spraying of nano‐Si or nano‐Se and soil application of nano‐humus

AbstractCadmium (Cd) contamination in farmland is extensive in China. Thus, exploring a cost‐effective and practical approach to control Cd contamination and ensure the sustainable use of soil is essential. Agronomic management practices, such as foliar spraying of nano‐silicon (nano‐Si) or nano‐selenium (nano‐Se), and soil amendments are very effective in soil contamination control and ensuring agricultural security. In this study, foliar application of nano‐Si (or Se) and soil application of nano‐humus were combined and applied to Cd‐contaminated soil (1.9 mg kg‐1) sown with rice. The application of nano‐humus in soil mainly reduced the proportion of oxidizable Cd and inhibited Cd migration from soil to rice. Cd translocation to the grain in rice was prevented by foliar application of nano‐Si or nano‐Se. Importantly, foliar application of 0.025 mM nano‐Se decreased Cd in rice grain to around 72% of that in the control, leading to a Cd content lower than the Cd standard limit in rice (0.2 mg kg‐1). These findings indicate that foliar spraying and soil treatment can improve the tolerance of rice to Cd and prevent Cd migration into the grain.

Source attributions of Cadmium contamination in rice grains by Cadmium isotope composition analysis: A field study

Cd contamination in rice grains has become a topic of great concern because of the high health risks associated with the long-term consumption of Cd-contaminated rice. Identification of Cd sources in rice grains by scientific methods is important for controlling heavy metal pollution and protecting human health. Here, the Cd concentrations and Cd isotopic compositions of rice plants (root, stem, leaf, and grain) and topsoil, and possible pollution sources (agricultural fertilizers, industrial dust, and automobile exhaust) were analyzed using an instrument of inductively coupled plasma mass spectrometry (ICP-MS) and multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The results showed variations in the Cd isotopes of different components of rice plants and the fractionation coefficient of rice grains relative to topsoil (Δ114/110Cdrice grains−topsoil = 0.25‰). The contributions of pollution sources to rice grains were realized by combining the Cd isotopic composition with an isotopic mixing model (Isosource). The analysis showed that all three possible pollution sources contributed to the Cd in the rice grains in the field, the average Cd contribution of industrial dust, agricultural fertilizers and automobile exhaust was 87%, 9%, and 4%, respectively. Our study provides a feasible method for the identification of pollution sources of Cd in rice grains at the field scale and demonstrates that Cd isotopic composition is one of the powerful tools to trace the pollution sources of Cd in crops.

Effects of Ferrous Sulfate and Ferric Nitrate on Cadmium Transportation in the Rhizosphere Soil-Rice System

Cadmium (Cd) contamination in the agricultural soils of China is a serious and growing environmental problem that urgently needs to be controlled and completely remediated. The biogeochemical cycles of nitrogen (N), sulfur (S), and iron (Fe), and the coupled cycles of Fe-N and Fe-S have been reported to control Cd transportation in the soil-rice system. Exploring practical remediation strategies for Cd from the perspective of the application of nutrients such as N, S, and Fe for rice growth is expected to obtain farm-specific and state-of-the-art technologies and products to reduce the accumulation of Cd in rice grains. Using our earlier study as a basis, the rhizosphere bag-pot experiment with ferrous sulfate (FeSO4) and ferric nitrate[Fe(NO3)3] treatments was conducted to investigate Cd bioavailability in rhizosphere soil and Cd translocation in rice plants, and to highlight some possible factors and mechanisms controlling Cd accumulation in rice grains. The results showed that both FeSO4 and Fe(NO3)3 treatments reduced the bioavailable Cd (NH4Ac-Cd) content in rhizosphere soil, with the decreasing extent being significantly lower in the former (55.6%) than in the latter (76.0%). Both FeSO4 and Fe(NO3)3 treatments changed the distribution characteristics of Cd in rice tissues, and the FeSO4 treatment increased the Cd content in brown rice (0.6 mg·kg-1), but the Fe(NO3)3 treatment decreased the Cd content in brown rice (0.1 mg·kg-1). Adsorption or co-precipitation of Cd by iron plaque, increased accumulations of Cd in root, stem, and leaf, and enhanced translocations of Cd from root, stem, and nodule to brown rice occurred with the increased Cd content in brown rice of the FeSO4 treatment. However, the decreased Cd content in brown rice with the Fe(NO3)3 treatment was ascribed to adsorption or co-precipitation of Cd by poorly crystalline Fe oxides and solid Fe sulfides, decreased accumulations of Cd in stem and nodule, and weakened translocations of Cd from root, leaf, and nodule to brown rice. These findings provide a scientific basis for the exploration and application of nutritive soil amendment, and will have significance in regards to the remediation of Cd-contaminated agricultural soils in China.

Study on safe usage of agricultural land in karst and non-karst areas based on soil Cd and prediction of Cd in rice: A case study of Heng County, Guangxi

It is well-known that Cd concentration in the soil, Cd bioavailability, and Cd bioaccumulation in rice (Oryza sativa L.) grains vary greatly in different soil parent materials. Therefore, the classification of agricultural land environmental quality and agricultural land safe usage based on the Cd content in the soil has a wide deviation. By the application of systematic sampling and analysis of soil and rice samples, and in comparison with the different soil parent materials in Heng County, it was found that soil derived from carbonate rocks has higher total Cd and lower mobile Cd proportion than soil from the non-karst areas. This result indicated that soil carbonates raised soil pH and the adsorption of Fe/Mn oxide/hydroxide on Cd significantly reduced the bioavailability of Cd in karst areas. In contrast, acidic soils with relatively lower CaO, TFe2O3, Mn and total Cd contents, the grown rice plants accumulated higher Cd in their grains. Further research confirmed significant differences in Cd bioaccumulation abilities in the soil between karst and non-karst areas. On this basis, the bioaccumulation factor prediction models of Cd in rice grains were developed in karst and non-karst areas. According to the total concentration of Cd in topsoil samples obtained from the area survey and the predicted Cd content in rice grains, an agricultural land safe usage scheme was put forward. The results showed that the agricultural land classification method based on Cd concentration in the soil and rice grains was more accurate and scientific than that based on the Cd contents in the soil alone.

Phytoavailability and human risk assessment of heavy metals in soils and food crops around Sutlej river, India

We assessed the phytoavailability of heavy metals in wheat and rice grown on soils along flood plains of Sutlej river (India) and its effects on human health. Surface soil samples along with wheat (pre-monsoon season) and rice (post-monsoon season) grain samples were collected during the years 2017 and 2018. Soil samples were analysed for total and DTPA-extractable metals and, wheat and rice grains samples for total metals. There were no significant (p < 0.05) changes in total metal concentration during both the years, but the concentration of DTPA-extractable metals was higher during pre-monsoon than post-monsoon season. The concentration of Pb, Cd, and Co in wheat grains, whereas the concentration of Cr, Co, Pb and Cd in rice grains surpassed their safe limits. Both DTPA-extractable metals (water soluble plus exchangeable) and non-exchangeable fractions explained more than 80% of the variations in metal uptake by plants. The bioaccumulation factor for DTPA-extractable metals (BCF) was above one, but Pollution Load Index for these metals (PLI) was less than one. The carcinogenic risks due to ingestion of wheat and rice grains were higher for Cd. The BAF, PLI and health risks were higher along transboundary of the river. These results suggest that exchangeable and non-exchangeable fractions are contributing towards metals uptake by wheat and rice along flood plain soils of the Sutlej river and the present study may act as a model for carrying out similar studies to find out the risks of heavy metals and it effect on human health in future.

Chemical Speciation and Distribution of Cadmium in Rice Grain and Implications for Bioavailability to Humans.

Consumption of rice (Oryza sativa) is the major dietary source of cadmium (Cd) for populations with rice as the staple. Little is known about the distribution and chemical speciation of Cd in rice grain, which is critical in determining the bioavailability of Cd to humans. We used synchrotron-based techniques for analyses of the speciation and distribution of Cd in rice grain. The majority of the Cd in rice grain was present as Cd-thiolate complexes (66-92%), likely in the form of Cd bound with thiol-rich proteins. The remainder was present as Cd-carboxyl compounds and Cd-histidine. Elemental mapping showed two different patterns of Cd distribution, one with an even distribution throughout the entire grain and the other with a preferential distribution in the outer tissues (aleurone layer and outer starchy endosperm). The distribution pattern is important as it affects the removal of Cd during milling. On average, milling reduced grain Cd concentrations by 23.5% (median of 27.5%), although the range varied widely from a 64.7% decrease to a 22.2% increase, depending upon the concentration of Cd in the bran. We found that the variation in the distribution pattern of Cd in the rice grain was due to a temporal change in the supply of Cd from the soil porewater during grain filling. These results have important implications for Cd bioavailability in human diets.

Limestone dosage response of cadmium phytoavailability minimization in rice: A trade-off relationship between soil pH and amorphous manganese content

Limestone shows great potential to reduce the production of cadmium (Cd)-contaminated rice in acidic paddy soils, but has generated uncertainty effects. We conducted batch sorption and greenhouse experiments to investigate optimal conditions of pH and amorphous manganese content (Mnox) in limestone treated-soil for suppressing the Cd uptake by rice plants. The adsorption/desorption behavior of Cd in a soil/limestone mixture was dominated by the composition and density of sorption sites, followed by sorption conditions, which were mainly influenced by soil pH and exchangeable Ca2+. Interactions among soil factors were influenced both by the limestone effects and plant responses. The Cd uptake of rice plants did not matched to the doses of limestone applied. The increase in pH and decrease in Mnox following higher dosages of limestone treatment might produce contradictory effects on rice Cd uptake. We proposed a trade-off model to demonstrate how did the interactions of soil pH and Mnox affect the rice Cd uptake. To minimize the accumulation of Cd in rice grain harvested from acidic paddy soils, limestone was applied at 0.25 % to achieve an optimal pH of 6.5 and a Mnox of 95 mg kg−1.

Nano-scale zero valent iron modulates Fe/Cd transporters and immobilizes soil Cd for production of Cd free rice

In recent decades, nanoscale zero valent iron (nZVI) has been found to be a promising approach for heavy metal remediation. This study is the first report highlighting the role of nZVI to ameliorate Cadmium (Cd) stress in rice along with its effects in expressions of transporter genes, agronomic parameters and grain nutrient status. Initially, 3 concentration of Cd (10, 50, 250 μM) and nZVI (50, 100, 200 mg L−1) were selected. PCA analysis based on growth parameters, photosynthetic pigment contents and lipid peroxidation rate confirmed that 100 mg L−1 nZVI was most suitable for remediation of 10 μM Cd. It was evident that, nZVI can alleviate Cd-induced toxic effects by enhancing antioxidant defense mechanisms and other physiological processes in plants. nZVI treated rice seedlings also showed upregulation of phytochelatins which aided in Cd chelation within vacuoles. Study of root morphology with scanning electron microscopy and ROS imaging with confocal microscopy confirmed that nZVI could alleviate oxidative stress due to Cd uptake. In nZVI treated rice seedlings, gene expressions of iron (Fe) transporters (like, IRT1,IRT2,YSL2,YSL15) which are responsible for both Fe and Cd uptake were significantly down-regulated whereas, OsVIT1 and OsCAX4 genes were over expressed which lead to sequestration of Cd in vacuoles. Cd localization assay with leadmium proved that Cd translocation was reduced with nZVI treatment. To further validate our findings a pot experiment was carried out where it was found that nZVI could immobilize Cd in soil prevented accumulation of Cd in rice grains in addition to improving yield.

Predicting accumulation of Cd in rice (Oryza sativa L.) and soil threshold concentration of Cd for rice safe production

Rice contamination by cadmium (Cd) poses a serious threat to human health, which has attracted widespread concerns in China. It is imperative to determine major soil factors influencing the accumulation of Cd in rice and develop prediction models to derive the threshold concentration of Cd in soil for rice food safety. In this study, the bioavailability, accumulation, and transfer of Cd in the 18 typical paddy soil-rice systems with a wide range of soil properties was investigated using pot experiments. The regression-based models incorporated with total or extractable Cd and soil properties were constructed to predict Cd content of rice grain. Pot experimental results indicated that rice showed a high accumulation potential for Cd, while rice grains grown in acid soils displayed larger Cd contents than those in neutral and alkaline soils. The pH and MnO content were major soil factors influencing the Cd accumulation of rice. Multiple regression models based on the total Cd, extractable Cd, pH, and MnO content in soils could well describe the Cd content in rice grain. Measured Cd content of rice grains from field samples demonstrated that the empirical models could quantitatively predict the Cd content of rice grains. The threshold concentrations of Cd in soils for rice food safety could be back-calculated by both EDTA-extractable and total Cd contents in soils. The EDTA-extractable Cd in soils could use as an indication to derive the threshold concentrations of Cd for rice food safety. In conclusions, multiple regression models proved reliable and practical in predicting Cd accumulation in rice grain. These empirical models could well predict the content of Cd in rice grain and deduce soil Cd threshold criteria. These results could help to quantitatively evaluate the health risk of Cd accumulation in rice crop and provide a useful reference for safe production of rice.

Exploring the spatially varying relationships between cadmium accumulations and the main influential factors in the rice-wheat rotation system in a large-scale area

Cadmium (Cd) accumulations in crops and the effects of the related soil factors on them are critical to developing precise soil management measures for food safety. Traditionally-used non-spatial multiple linear regression (MLR) cannot adequately model the spatially varying effects of the related soil properties on Cd accumulations in crop (or soil). Moreover, the traditionally-used methods for exploring the spatial accumulation characteristics (e.g., ordinary kriging) and the effects of other factors on Cd accumulations (e.g., MLR) are sensitive to outliers. In this study, robust geostatistics, enrichment index, and bioavailability index were first used to explore the spatial accumulation characteristics of Cd in wheat grain (wheat-Cd), Cd in rice grain (rice-Cd), and soil DTPA-extractable Cd (DTPA-Cd) in Jintan County, a typical rice-wheat rotation area in China. Then, robust geographically weighted regression (RGWR), established in geographic space rather than variable space, was used to explore the spatially varying relationships between Cd accumulations and the corresponding main influential factors determined by stepwise regression. Last, the modelling accuracy of RGWR was compared with those of basic GWR and MLR. Results showed that (i) outliers affected the spatial predictions of soil total Cd, soil DTPA-Cd, wheat-Cd, and rice-Cd and robust variograms should be used; (ii) the enrichment index of wheat grain was significantly higher than that of rice grain in almost the whole study area; (iii) the areas with the high bioavailability index of soil Cd mainly located in the southeast, southwest, and centre of the study area; (iv) RGWR acquired higher modelling accuracy than GWR and MLR; (v) the spatially varying relationships between Cd accumulations and the corresponding influential factors were revealed by RGWR, which cannot be determined by MLR. The methods suggested in this study provided more precise spatial decision support for soil management measures to guarantee main agricultural product safety in large-scale areas.

Evaluation of cadmium (Cd) transfer from paddy soil to rice (Oryza sativa L.) using DGT in comparison with conventional chemical methods: derivation of models to predict Cd accumulation in rice grains.

The consumption of rice contaminated with soil cadmium (Cd) threatens human health. It is essential to ensure the production of rice that meets food quality standards. Therefore, a large-scale field survey was conducted in Zhejiang province, southeastern China, to investigate the relationship between Cd accumulation in rice grains and Cd bioavailability in soil, and thus to establish a model to predict Cd contents in rice grains based on soil properties. For this purpose, a total of 156 paired rice and soil samples were collected. Pearson's correlation analysis revealed that Cd measurements obtained by diffusive gradient in thin films (DGT) had a higher correlation (r = 0.818, p < 0.001) with the Cd in rice grains as compared to the Cd measured by the DTPA, CaCl2, EDTA, and HCl extraction methods, which indicated that the DGT technique was a reliable method for the assessment of Cd bioavailability in soils. In addition, among the four extraction methods, the DTPA-extractable Cd showed the highest correlation with the Cd contents in rice grains. Therefore, we developed two predictive models (modelDGT and modelDTPA) to predict Cd levels in rice grains via Cubist multivariate mixed linear regression, using "soil DGT-measured Cd, pH, and oxide contents of Ca, Si, and Fe" or "soil DTPA-extractable Cd, pH, OM, and oxide contents of Ca and Fe" as explanatory variables, respectively. The overall modelDGT and modelDTPA had R2 values of 0.95 and 0.93, respectively, and relative error values of 0.30 and 0.33, respectively. Simple correlation analysis showed direct and close relationships between the measured Cd in rice grains and the Cd concentrations predicted by the Cubist modelDGT and modelDTPA, with R2 values of 0.979 and 0.922, respectively. Therefore, Cd levels in rice grains could be predicted very well based on the two prediction models, and thus, the two models derived in this study are effective in identifying soils in which the Cd in rice grains will exceed food safety standards, thereby helping to ensure safe rice production.

Simultaneous reduction of arsenic (As) and cadmium (Cd) accumulation in rice by zinc oxide nanoparticles

Accumulation of arsenic (As) and cadmium (Cd) in rice grains is a serious food safety concern for populations consuming rice as a staple food worldwide. Some agricultural practices such as water and nutrient management strategies can exhibit significant impact on their fate and bioavailability in rice paddies due to the profound impact of these activities on paddy soil properties. However, due to the unique biogeochemistry of As and Cd in rice paddies, their bioavailability often goes opposite directions and thus simultaneous control of both As and Cd in rice grains has not been achieved. Several previous studies have showed that some engineered nanoparticles (ENPs) can substantially lower the accumulation of either As or Cd in rice grains. The primary goal of this study was to evaluate whether zinc oxide nanoparticles, a popular nanofertilizer, can simultaneous lower both As and Cd in rice tissues using a greenhouse setup. The results suggested that both zinc oxide nanoparticles (ZnONPs) and zinc ions (Zn2+) significantly reduced the total As in rice roots (−39.5% and −83.3%) and shoots (−60.2% and −80.0%) and the reduction was primarily due to the lowered inorganic As(III) and organic As species. However, only ZnONPs reduced the Cd accumulation in rice shoots. Zn2+ significantly increased Cd content in rice shoot by 26.8%, suggesting the differential impact of ZnONPs and Zn2+ on As and Cd accumulation in rice tissues. Only ZnONPs holds the potential to simultaneously reduce both As and Cd in rice grains in As and Cd co-contaminated rice paddies. The results provide new insights into the sustainable applications of nanotechnology in agriculture.

Arsenic and cadmium accumulation in rice and mitigation strategies

Arsenic (As) and cadmium (Cd) are two toxic elements that have a relatively high risk of transfer from paddy soil to rice grain. Rice is a major dietary source of these two elements for populations consuming rice as a staple food. Reducing their accumulation in rice grain is important for food safety and human health. We review recent progress in understanding the biogeochemical processes controlling As and Cd bioavailability in paddy soil, the mechanisms of their uptake, translocation and detoxification in rice plants, and strategies to reduce their accumulation in rice grain. Similarities and differences between the two elements are emphasized. Some knowledge gaps are also identified. The concentrations of As and Cd in rice grain vary by three orders of magnitude, depending on the bioavailability of the two elements in soil, rice genotype and growing conditions. The redox potential in paddy soil has a profound but opposite effect on As and Cd bioavailability, whereas soil pH affects Cd bioavailability more than As bioavailability. A number of key genes involved in As and Cd uptake, translocation, sequestration, and detoxification in rice have been characterized. Allelic variations of several genes underlying the variations in Cd accumulation have been identified, but more remains to be elucidated, especially for As. Two types of strategies can be used to reduce As and Cd accumulation, reducing their bioavailability in soil or their uptake and translocation in rice. Reducing the accumulation of both As and Cd in rice simultaneously remains a great challenge.

Evaluation of various approaches to predict cadmium bioavailability to rice grown in soils with high geochemical background in the karst region, Southwestern China

Evaluating the bioavailability of Cd to rice (Oryza sativa L.) was essential in the karst region, Southwestern China, where the soils have previously been shown to be anomalously enriched in Cd through geogenic processes. In this research, we examined the bioavailability of Cd to rice samples collected from 278 sites in Guangxi province, where rice is the most widely cultivated cereal crop that is responsible for the largest human dietary exposure to Cd. Both soil chemical extraction and soil-plant transfer modelling approaches were used to predict the bioavailability to rice. Some of the soil types were highly enriched in Cd, but their bioavailability was low, since the soil carbonates raised soil pH and remarkably reduced Cd bioavailability. In contrast, acidic soils (Ca was largely leached) with relatively low total Cd, the grown rice plants accumulated higher Cd in their grains. Results from CaCl2 extraction experiments provided good predictions for Cd in rice grain grown in soils of different types. Stepwise multiple regressions revealed soil pH and soil Ca content were the dominant factors that control the transfer of Cd from soil to rice. An extended Freundlich-type model and a polynomial surface model provided good prediction for Cd in rice grains. The diffusive gradients in thin films (DGT) technique gave the best estimation of soil Cd bioavailability, whereas water-extracted soil solution Cd provided relatively poor fits. Regional soil threshold that derived using the models, can avoid exceedance of Cd in rice and thereby enable local agricultural practitioners or authorities to develop appropriate management for croplands with high Cd background.

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.

Glutamate alleviates cadmium toxicity in rice via suppressing cadmium uptake and translocation

Cadmium (Cd), a naturally occurring heavy metal, is toxic to animals and plants. Minimization of Cd in rice grain is important to human health since rice is the main source of Cd intake for human populations feeding on it as staple food. Glutamate (Glu) is reportedly involved in plant abiotic stress responses, whereas the underlying molecular mechanism remains poorly understood. In this study, we showed that supplement of Glu, but not glutamine, significantly alleviated Cd toxicity in hydroponically grown rice plants. Cd accumulation was reduced by 44.1% and 65.6% in root and shoot of rice plants respectively, after Glu supplementation (3 mM). Glu supplement restored chlorophyll biosynthesis and significantly ameliorated Cd-induced oxidative stress with reduced levels of H2O2, 1O2, MDA, and increased activities of major anti-oxidant enzymes, catalase, peroxidase and glutathione S-transferase. Levels of stress-associated free amino acids proline, arginine and γ-aminobutyric acid were also reduced after Glu supplement. We further demonstrated that Glu supplement suppressed the Cd-induced expression of metal transporter genes OsNramp1, OsNramp5, OsIRT1, OsIRT2, OsHMA2 and OsHMA3 in roots of Cd-treated plants. Taken together, our results suggest that Glu supplement could alleviate Cd toxicity in rice by suppressing Cd uptake and translocation.

Mutation at Different Sites of Metal Transporter Gene OsNramp5 Affects Cd Accumulation and Related Agronomic Traits in Rice (Oryza sativa L.).

OsNramp5 is a key gene involved in the control of the uptake of Cd, Mn, and other metal ions by rice root cells. The functional deficiency of this gene can significantly reduce the accumulation of Cd in rice grains, but the effects of its mutation on agronomic traits such as yield and quality have not been investigated comprehensively yet. In the present study, three Huanghuazhan-based OsNramp5 mutants [LCH1 (Low Cadmium Huanghuazhan 1), LCH2 (Low Cadmium Huanghuazhan 2), and LCH3 (Low Cadmium Huanghuazhan 3)] were obtained using clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) technology. The mutation-type analysis showed that LCH1, LCH2, and LCH3 encoded defective OsNramp5 protein sequences containing only 76aa, 176aa, and 266aa, respectively. The determination of metal content and the statistics of related agronomic traits revealed that the functionally deficient OsNramp5 not only significantly reduced the accumulation of Cd in the grains of the mutants but also affected rice yield and quality. However, with the decrease of OsNramp5 mutation degree, its effects on chlorenchyma Mn accumulation, yield, and quality were also diminished. Additionally, we also found that the increase in the concentration of Mn in the soil restored the phenotype of the declined yield and quality due to the functional deficiency of OsNramp5. Our findings provide novel insights into and new materials for breeding rice varieties with low Cd accumulation and excellent agronomic traits under severe Cd pollution environment.

Identification of cadmium bioaccumulation in rice (Oryza sativa L.) by the soil-plant transfer model and species sensitivity distribution.

Contamination of agricultural soil with cadmium (Cd) poses a severe threat to food safety and human health, especially for Cd in rice. It is very important to identify Cd bioaccumulation in rice in order to screen Cd-safe cultivars. In the present study, 183 pairs of rice and soil data collected from Cd-contaminated soil were used to investigate the differences of Cd bioaccumulation in grains among rice cultivars. The results showed that the adverse effect on grain Cd accumulation of japonica was less than that of indica under Cd exposure. The percentage of japonica with grain Cd concentration exceeding 0.2 mg/kg reduced 50.3% compared with indica. Partial correlation analyses suggested that lower pH contributed to Cd accumulation in grains, and a significant increase in grain Cd concentration was observed with increasing soil Cd concentration. The bioaccumulation factors (BCF) of Cd in rice grains could be divided into 5 grades by combining an empirical soil-plant transfer model with species sensitivity distribution (SSD). Grades with lower Cd bioaccumulation (grades 1 and 2) were dominated by japonica, and the intrinsic sensitivity index of Cd-enrichment (k value) and straw to grain transfer factors (TF) increased with ascending grades. Average k value and TF of cultivars in grade 5 were 1.4-7.9 and 1.5-5.7 times higher than those of cultivars in grades 1 to 4, which eventually caused the increase of Cd accumulation in grains. The lower level of Cd absorption and translocation contributed to reducing the bioaccumulation of Cd in rice grains had been proved by the classification of rice on Cd accumulation. Considering the influence of soil properties and intrinsic sensitivity of rice, cultivars with grain Cd bioaccumulation controlled at low levels to safe for human consumption could be identified on Cd-contaminated soils.