Cd Accumulation In Rice Grains Research Articles

The Effects of Water Management, Foliar Fertilizers, and Lime Application on the Accumulation of Cd and As in Rice Grains Based on a Field Trial

The aim of this study was to explore the feasible measurement of the control of cadmium (Cd) and arsenic (As) accumulation in rice grains. A field experiment was carried out to research the effect of different treatments, including spraying silicon (Si)/selenium (Se) foliar fertilizers, the application of lime (CaO), water management (continuous flooding), and the co-application of foliar fertilizers and flooding, on Cd and As accumulation in rice grains in Guangxi Province. The results indicate that Cd accumulation in rice grains decreased under different treatments and Cd content in rice grains reached the threshold of 0.2 mg kg−1. In the single technical treatments, CaO application, flooding, spraying foliar Se fertilizer, and spraying foliar Si fertilizer decreased Cd content by 73.15%, 60.44%, 45.76%, and 36.07%, respectively. However, flooding and CaO amendment enhanced As accumulation in rice grains. The co-application of flooding and spraying foliar fertilizer can simultaneously reduce Cd and As in rice grains. In addition, they resulted in lower Cd content than the single technical treatments. Among the treatments, the lowest bioaccumulation factors of Cd and As were found after the co-application of flooding and Si foliar fertilizer, which decreased these factors by 74.02% and 22.72%, respectively. These results suggest that spraying foliar Si fertilizer combined with flooding may be a promising method to synchronously inhibit the accumulation of Cd and As in rice.

The transcription factor OsNAC5 regulates cadmium accumulation in rice

Cadmium (Cd) is a hazardous heavy metal that threatens human health through the consumption of contaminated rice. To mitigate Cd accumulation in rice grains, it is crucial to reduce Cd uptake. Nevertheless, the transcriptional mechanisms governing Cd uptake in rice remain largely unknown. This research identifies the transcription factor OsNAC5 in Oryza sativa as a positive regulator of the Cd transporter gene OsNRAMP1, thereby influencing Cd uptake. OsNAC5 is predominantly expressed in the roots, resides in the nucleus, and is upregulated by Cd-induced hydrogen peroxide (H2O2). Knocking out OsNAC5 results in lower Cd concentrations in both shoots and roots and heightens sensitivity to Cd. The expression of OsNRAMP1, enhanced by Cd stress, is dependent on OsNAC5. OsNAC5 binds to "CATGTG" motifs in the OsNRAMP1 promoter, activating its expression. The loss of OsNAC5 function leads to reduced Cd accumulation in rice grains. Our findings provide insights into the transcriptional regulation of Cd stress response in rice and propose biotechnological strategies to lower Cd uptake in crops.

Exogenous selenium mitigates cadmium uptake and accumulation in two rice (Oryza sativa L.) varieties in cadmium-contaminated soil

Rice grown in cadmium (Cd)-contaminated soil, is a potential threat to human health, but exogenous selenium (Se) application on rice can mitigate Cd toxicity. However, the mechanisms underlying Se mitigation of Cd stress in ratoon rice (RR) are still poorly understood. We conducted a pot experiment with moderate Cd-contaminated yellow–brown paddy soil on two rice varieties ‘Taoyouxiangzhan’ (TX) and ‘Liangyou 6326’(LY). For all treatments, 1.0 mg kg−1 sodium selenite solution was added to soil. Treatment T1 was sodium selenite only, and in the other treatments 100 mg L−1 Se solution was sprayed on the leaves at seedling stage (T2), at tillering stage (T3), and in early anthesis stage (T4). Se treatments decreased Cd accumulation in rice grains and herbage. Under foliar spraying 100 mg L−1 Se at the seedling + 1.0 mg kg−1 Se in soil (T2), leaf Cd content decreased 16.95% in the current season and grains content decreased 46.67% in the subsequent season. Furthermore, grain Se content increased 0.94 mg kg−1 for the TX variety combined with the analysis of Cd bio-accumulation factor in grains, and Se treatments effectively decreased Cd grain concentrations due to reduced Cd translocation from roots to grains. TX variety rice showed a more pronounced response to Se treatments than LY.

Cadmium Minimization in Grains of Maize and Wheat Grown on Smelting-Impacted Land Ameliorated by Limestone.

Cadmium (Cd) contamination in agricultural soils has emerged as a significant concern, particularly due to its potential impact on plant-based food. Soil pH reductions can exacerbate Cd mobility, leading to excessive accumulation in crops. While liming has been demonstrated as an effective method to mitigate Cd accumulation in rice grains in acid soils of southern China, its efficacy in remediating acid soils in northern China remains unclear. In this study, a multi-year field experiment was conducted on farmland impacted by zinc ore smelting at coordinates of 33.92° N 112.46° E to investigate the use of limestone for controlling Cd accumulation in wheat and maize grains. The results indicated that applying 7.5 t ha-1 of limestone significantly raised the soil pH from 4.5 to 6.8 as anticipated. Different rates of limestone application (2.25, 4.45, and 7.50 t ha-1) reduced Cd bioavailability in the soil by 20-54%, and Cd accumulation in wheat grains by 5-38% and maize grains by 21-63%, without yield penalty. The remediation effects were sustained for at least 27 months, highlighting limestone as a promising ameliorant for smelting-affected farmland in northern China.

Design of rice with low cadmium accumulation in grain using single segment substitution line

Rice (Oryza sativa L.) is a major dietary source of cadmium (Cd). Developing rice varieties with reduced Cd levels in the grain is a cost-effective and practical approach to enhance food safety, particularly in regions with high Cd contamination. However, the genetic mechanisms underlying Cd accumulation in rice grains are not fully understood. In this study, we identified eight quantitative trait loci (QTLs) associated with Cd accumulation in rice grains through substitution mapping using single segment substitution lines (SSSLs). These QTLs, named qCd‐2‐1, qCd‐3‐1, qCd‐3‐2, qCd‐5‐1, qCd‐6‐1, qCd‐7‐1, qCd‐8‐1, and qCd‐11‐1, are distributed across seven chromosomes. Notably, the qCd‐5‐1 and qCd‐6‐1 loci are reported for the first time. We performed a detailed haplotype analysis of candidate genes related to heavy metal metabolism, specifically focusing on Cd accumulation. All SSSLs carrying alleles from donor parents exhibited a significant reduction in Cd accumulation, with additive effects ranging from −0.061 to −0.105. To further develop rice varieties with lower Cd accumulation in the grain, we developed six pyramided lines through crossing and marker-assisted selection. These pyramided lines showed significantly reduced Cd content in the grain compared to the elite indica recurrent parent, Huajingxian74 (HJX74). Importantly, most agronomic characteristics of the pyramided lines were similar to those of HJX74. In conclusion, this study demonstrates that identifying and pyramiding QTLs associated with reduced Cd accumulation is an effective strategy for developing rice varieties with lower Cd content in the grain.

High-efficiency remediation of Hg and Cd co-contaminated paddy soils by Fe–Mn oxide modified biochar and its microbial community responses

Fe–Mn oxide modified biochar (FMBC) was produced to explore its potential for remediation of Hg–Cd contaminated paddy soils. The results showed that the application of FMBC decreased the contents of bioavailable Hg and Cd by 41.49–81.85% and 19.47–33.02% in contrast to CK, while the amount of labile organic carbon (C) fractions and C-pool management index (CPMI) was increased under BC and FMBC treated soils, indicating the enhancement of soil C storage and nutrient cycling function. Dry weight of different parts of Oryza sativa L. was enhanced after the addition of BC and FMBC, and the contents of Fe and Mn in root iron–manganese plaques (IMP) were 1.46–2.06 and 6.72–19.35 times higher than those of the control groups. Hg and Cd contents in brown rice under the FMBC treatments were significantly reduced by 18.32–71.16% and 59.52–72.11% compared with the control. FMBC addition altered the composition and metabolism function of soil bacterial communities, especially increasing the abundance of keystone phyla, including Firmicutes, Proteobacteria and Actinobacteria. Partial least squares path modelling (PLSPM) revealed that the contents of Na2S2O3–Hg, DTPA–Cd and IMP were the key indicators affecting Hg and Cd accumulation in rice grains. These results demonstrate the simultaneous value of FMBC in remediation of Hg and Cd combined pollution and restoring soil fertility and biological productivity.Graphical

Biochar impacts on carbon dioxide, methane emission, and cadmium accumulation in rice from Cd-contaminated soils; A meta-analysis

Climate change and cadmium (Cd) contamination pose severe threats to rice production and food security. Biochar (BC) has emerged as a promising soil amendment for mitigating these challenges. To investigate the BC effects on paddy soil upon GHG emissions, Cd bioavailability, and its accumulation, a meta-analysis of published data from 2000 to 2023 was performed. Data Manager 5.3 and GetData plot Digitizer software were used to obtain and process the data for selected parameters. Our results showed a significant increase of 18% in soil pH with sewage sludge BC application, while 9% increase in soil organic carbon (SOC) using bamboo chips BC. There was a significant reduction in soil bulk density (8%), but no significant effects were observed for soil porosity, except for wheat straw BC which reduced the soil porosity by 6%. Sewage sludge and bamboo chips BC significantly reduced carbon dioxide (CO2) by 7–8% while municipal biowaste reduced methane (CH4) emissions by 2%. In the case of heavy metals, sunflower seedshells-derived materials and rice husk BC significantly reduced the bioavailable Cd in paddy soils by 24% and 12%, respectively. Cd uptake by rice roots was lowered considerably by the addition of kitchen waste (22%), peanut hulls (21%), and corn cob (15%) based BC. Similarly, cotton sticks, kitchen waste, peanut hulls, and rice husk BC restricted Cd translocation from rice roots to shoots by 22%, 27%, 20%, and 19%, respectively, while sawdust and rice husk-based BC were effective for reducing Cd accumulation in rice grains by 25% and 13%. Regarding rice yield, cotton sticks-based BC significantly increased the yield by 37% in Cd-contaminated paddy soil. The meta-analysis demonstrated that BC is an effective and multi-pronged strategy for sustainable and resilient rice cultivation by lowering greenhouse gas emissions and Cd accumulation while improving yields under the increasing threat of climate change.

Effects of foliar application of Zn combined with organic matters on Cd accumulation and its chemical forms in rice.

Rice consumption is a key Cd exposure pathway, which poses a health risk to humans. Reducing cadmium (Cd) concentrations in rice remains challenging. In this study, a pot experiment was conducted to examine the effects of foliar spray of Zn combined with organic matters (including Zn-lysine (Zn-Lys), Zn-fulvic acid (Zn-FA), Zn-amino acid (Zn-AA), and Zn combined with glutathione (Zn + GSH)) on Cd accumulation in rice grains. Compared with the control group, all treatment groups exhibited reduced Cd concentration in rice grains, while improving plant growth, and reducing Cd transport from other tissues to the grains. Zn-FA was found to be the most effective fertilizer, which considerably reduced Cd concentrations in grains from 0.77 ± 0.068 to 0.14 ± 0.021mg/kg and yielded reductions of up to 81%, which is within the Chinese food maximum tolerable limit of 0.2mg/kg. Furthermore, the analysis of the chemical forms of Cd of rice tissues indicated that the treatment groups had increased proportions of integrated with pectates and protein in the stems. Except for the group treated with Zn-Lys spray, the percentages of undissolved Cd phosphate in the leaves were increased in all treatment groups, which reduced Cd toxicity to rice plants. The foliar application of Zn combined with organic matters may be a promising strategy to decrease Cd concentration in rice grains cultivated in severely Cd-contaminated agricultural soil, particularly in the karst area in southwest China with limited available cultivable agricultural land.

Co-foliar application of zinc and nano-silicon to rice helps in reducing cadmium exposure risk: Investigations through in-vitro digestion with human cell line bioavailability assay

Foliar application of zinc (Zn) or silicon nanoparticles (Si-NPs) may exert regulatory effects on cadmium (Cd) accumulation in rice grains, however, their impact on Cd bioavailability during human rice consumption remains elusive. This study comprehensively investigated the application of Zn with or without Si-NPs in reducing Cd accumulation in rice grains as well to exactly evaluate the potential risk of Cd exposure resulting from the rice consumption by employing field experiment as well laboratory bioaccessibility and bioavailability assay. Sole Zn (ZnSO4) or in combination with Si (ZnSO4 +Si and ZnO+Si) efficiently lowered the Cd concentration in rice grains. However, the impact of bioaccessible (0.1215–0.1623 mg kg−1) and bioavailable Cd (0.0245–0.0393 mg kg−1) during simulated human rice consumption depicted inconsistent trend. The straw HCl-extractable fraction of Cd (FHCl-Cd) exhibited a significant correlation with total, bioaccessible, and bioavailable Cd in grains, indicating the critical role of FHCl-Cd in Cd accumulation and translocation from grains to human. Additionally, foliar spraying of Zn+Si raised the nutritional value of rice grains, leading to increased protein content and reduced phytic acid concentration. Overall, this study demonstrates the potential of foliar application of ZnSO4 +Si in mitigating the Cd levels in rice grains and associated health risks upon consumption.

Regulation Effects of Humus Active Components on Soil Cadmium Availability and Critical Threshold for Rice Safety

Organic materials containing humic acids (HAs) play important roles in regulating the bioavailability of cadmium (Cd) in soils and thus its accumulation in crops. The effects of the two active components of HAs, humic acid (HA) and fulvic acid (FA), in organic materials and their different ratios (HA/FA) on Cd uptake and accumulation in rice were investigated using a field plot experiment, and their relationships with the Cd fractions and availability in paddy soil as influenced by the use of these organic materials were analyzed in combination with the fractionation method of chemical continuous extraction. The results showed that the effects of HAs on Cd availability in soil and Cd accumulation in rice grains were controlled by the ratios of the active components in the organic materials. The treatments with an HA/FA ratio ≥ 4/6 had a passivating effect on soil Cd, resulting in a significant reduction in Cd availability. Compared with that in the control without the application of HAs (CK), rice grain Cd concentration was reduced by 15.2%-33.3%, whereas those with an HA/FA ratio ≤ 2/8 activated Cd in soil, and the available Cd content was significantly increased. Compared with that in CK, rice grain Cd concentration was increased by 24.2%-42.4%. The ratios of HA/FA in HAs affected the morphological transformation of soil Cd. Compared with the CK treatment, the treatments with ratios of HA/FA ≥ 4/6 promoted the transformation of soil Cd from the exchangeable form (EX-Cd) with high activity to the carbonate bound form (CA-Cd) and Fe and Mn oxide-bound forms (FM-Cd) with low activity, whereas those with ratios of HA/FA ≤ 2/8 showed the opposite effects. The effects of HA and FA on soil pH and available sulfur concentration differed. Soil pH had a significant positive correlation with HA addition but a negative correlation with FA addition, and soil available sulfur content had a significant positive correlation with FA addition at the rice tillering stage. Therefore, to ensure the quality and safety of rice, organic materials with an HA/FA ratio ≥ 4/6 should be selected. The results provided a scientific basis for the directed utilization of organic materials containing HAs.

Spatial and variety distributions, risk assessment, and prediction model for heavy metals in rice grains in China.

In this study, 6229 brown rice grains from three major rice-producing regions were collected to investigate the spatial and variety distributions of heavy metals in rice grains in China. The potential sources of heavy metals in rice grains were identified using the Pearson correlation matrix and principal component analysis, and the health risks of dietary exposure to heavy metals via rice consumption were assessed using the hazard index (HI) and total carcinogenic risk (TCR) method, respectively. Moreover, 48 paired soil and rice samples from 11 cities were collected to construct a predicting model for Cd accumulation in rice grains using the multiple linear stepwise regression analysis. The results indicated that Cd and Ni were the main heavy metal pollutants in rice grains in China, with approximately 10% of samples exceeding their corresponding maximum allowable limits. The Yangtze River basin had heavier pollution of heavy metals than the Southeast Coastal Region and Northeast Plain, and the indica rice varieties had higher heavy metal accumulation abilities compared with the japonica rice. The Cu, Pb, and Cd mainly originated from anthropogenic sources, while As, Hg, Cr, and Ni originated from both natural and anthropogenic sources. The mean HI and TCR values of dietary exposure to heavy metals via rice consumption ranged from 2.92 to 4.31 and 9.74 × 10-3 to 1.44 × 10-2, respectively, much higher than the acceptable range, and As and Ni were the main contributor to the HI and TCR for Chinese adults and children, respectively. The available Si (ASi), total Cd (TCd), available Mo (AMo), and available S (AS) were the main soil factors determining grain Cd accumulation. A multiple linear stepwise regression model was constructed based on ASi, TCd, AMo, and AS in soils with good accuracy and precision, which could be applied to predict Cd accumulation in rice grains and guide safe rice production in contaminated paddy fields.

Biochar-mediated Cd accumulation in rice grains through altering chemical forms, subcellular distribution, and physiological characteristics

Biochar can change the availability and morphology of soil Cd. However, the influence of biochar on Cd chemical form and subcellular fraction in rice is poorly understood, particularly under different irrigation methods. A pot experiment of biochar application combined with two irrigation methods (continuous flooding and intermittent irrigation, CF and II) was conducted. The Cd accumulation, chemical form and subcellular fraction in rice organs and the associated physiological responses were examined. Biochar significantly reduced soil available Cd (30.85–47.26% and 32.35–52.35%) under CF and II but increased the Cd content (30.4–63.88% and 13.03–18.59%) in brown rice. Additionally, the Cd content in shoots/grains under II was higher than that under CF. Biochar elevated the Cd soluble fraction in roots while lowered the cell wall fraction under both irrigation methods, whereas the opposite result was observed in leaves. Biochar increased water-, ethanol-, and NaCl-extractable Cd in roots meanwhile increased ethanol-extractable Cd in leaves under both irrigation methods. Moreover, the total amount of water-, ethanol-, and NaCl-extractable Cd in rice roots was higher under II than under CF. Related hormones and antioxidant enzymes may also be involved in biochar-mediated Cd accumulation in rice grains. Thus, changes in Cd chemical form and subcellular fraction in the root and leaf are the main mechanisms of biochar-induced rice grains Cd accumulation.Graphical

A small extent of seawater intrusion significantly enhanced Cd uptake by rice in coastal paddy fields

Paddy fields located around estuaries suffer from seawater intrusion, and how and to what extent salinity levels influence Cd accumulation in rice grains is still unclear. Pot experiments were carried out by cultivating rice under alternating flooding and drainage conditions with different salinity levels (0.2‰, 0.6‰ and 1.8‰). The Cd availability was greatly enhanced at 1.8‰ salinity due to the competition for binding sites by cations and the formation of Cd complexation with anions, which also contributed to Cd uptake by rice roots. The soil Cd fractions were investigated and found that the Cd availability significantly decreased during flooding stage, while it rapidly increased after soil drainage. During drainage stage, Cd availability was greatly enhanced at 1.8‰ salinity mainly attributed to the formation of CdCln2−n. The kinetic model was established to quantitatively evaluate Cd transformation, and it found that the release of Cd from organic matter and Fe-Mn oxides was greatly enhanced at 1.8‰ salinity. The results of pot experiments showed that there was a significant increase in Cd content in rice roots and grains in the treatment of 1.8‰ salinity, because the increasing salinity induced an increase in Cd availability and upregulation of key genes regulating Cd uptake in rice roots. Our findings elucidated the key mechanisms by which high salinity enhanced Cd accumulation in rice grains, and more attention should be given to the food safety of rice cultivated around estuaries.

Effects of cultivar, water condition and their interactions on Cd accumulation in rice grains

Using low Cd accumulation cultivars and managing field water regimes are effective measures to mitigate Cd accumulations in rice grains. However, the effect of the cultivar-water condition interaction (CWI) on grain Cd accumulations has largely been ignored. To solve this problem, pot and hydroponic experiments were conducted using 14 rice cultivars and two contrasting water conditions. The results showed that CWI significantly affected Cd concentrations in rice grains and roots, explaining 8.8% and 22.8% of the total variance, respectively. These CWI effects were derived from cultivar-dependent variations in rhizosphere soil properties [Eh, pH and available Cd associated with root radial oxygen loss (ROL)] and root Cd uptake. In this context, cultivar HH61 exhibited low, stable Cd accumulations, owing to its stably lower translocation rate, root Cd uptake ability and available Cd in its rhizosphere than the other cultivars, which was induced by its lower ROL. Root-to-grain Cd translocation rates were vital in determining Cd accumulations in grain of different cultivars but were independent from CWI. These results indicated that CWI could play an important role in Cd accumulation in rice while stable low-Cd cultivar should possess low ROL under flooding and low root-to-grain Cd translocation rate. The results will provide novel theoretical basis for cultivar selection and hence benefit the extensive use of low-accumulation cultivars and public health.

Low pe+pH inhibits Cd transfer from paddy soil to rice tissues driven by S addition

Both soil irrigation and sulfur (S) are associated with the precipitation of cadmium (Cd)-sulfide in paddy soil, their interaction affecting on Cd solubility and extractability is still unknown. This study primarily discusses the effect of exogenous S addition on the bioavailability of Cd in paddy soil under unsteady pe + pH conditions. The experiment was treated with three different water strategies: continuous dryness (CD), continuous flooding (CF), and alternating dry-wet cycles for one cycle (DW). These strategies were combined with three different S concentrations. The results indicate that the CF treatment, particularly when combined with S addition, had the most significant effect on reducing pe + pH and Cd bioavailability in the soil. The reduction of pe + pH from 10.2 to 5.5 resulted in a decrease in soil Cd availability by 58.3%, and Cd accumulation in rice grain by 52.8%, compared to the other treatments. While it was more conducive to the formation of iron plaque on the root surface in DW treatment with S addition at rice maturing stage and enhanced the gathering of Fe/S/Cd. Structural equation model (SEM) analysis further confirmed a significant negative correlation (r = −0.916) between the abundance of soil Fer-reducing bacteria (FeRB) and sulfate-reducing bacteria (SRB) like Desulfuromonas, Pseudomonas, Geobacter, and the Cd content in rice grains. This study provides a basic mechanistic understanding of how soil redox status (pe + pH), S addition, and FeRB/SRB interacted with Cd transfer in paddy soil-rice tissues.

Microbiological mechanism for “production while remediating” in Cd-contaminated paddy fields: A field experiment

Security utilization measures (SUMs) for “production while remediating” in moderate and mild Cd-polluted paddy fields had been widely used. To investigate how SUMs drove rhizosphere soil microbial communities and reduced soil Cd bioavailability, a field experiment was conducted using soil biochemical analysis and 16S rRNA high-throughput sequencing. Results showed that SUMs improved rice yield by increasing the number of effective panicles and filled grains, while also inhibiting soil acidification and enhancing disease resistance by improving soil enzyme activities. SUMs also reduced the accumulation of harmful Cd in rice grains and transformed it into FeMn oxidized Cd, organic-bound Cd, and residual Cd in rhizosphere soil. This was partly due to the higher degree of soil DOM aromatization, which helped complex the Cd with DOM. Additionally, the study also found that microbial activity was the primary source of soil DOM, and that SUMs increased the diversity of soil microbes and recruited many beneficial microbes (Arthrobacter, Candidatus_Solibacter, Bryobacter, Bradyrhizobium, and Flavisolibacter) associated with organic matter decomposition, plant growth promotion, and pathogen inhibition. Besides, special taxa (Bradyyrhizobium and Thermodesulfovibrio) involved in sulfate/sulfur ion generation and nitrate/nitrite reduction pathway were observably enriched, which effectively reduced the soil Cd bioavailability through adsorption and co-precipitation. Therefore, SUMs not only changed the soil physicochemical properties (e.g., pH), but also drove rhizosphere microbes to participate in the chemical species transformation of soil Cd, thus reducing Cd accumulation in rice grains.

Foliar spraying with a mixture of transpiration inhibitor-rhamnolipid reduces the Cd content in rice grains

A field experiment was conducted to investigate the effectiveness and mechanisms of foliar spraying of transpiration inhibitor (TI) and different amounts of rhamnolipid (Rh) on the Cd content in rice grain. The contact angle of TI on the rice leaves was significantly reduced when it was combined with one critical micelle concentration of Rh. The Cd concentration in the rice grain in the presence of TI, TI + 0.5Rh, TI + 1Rh, and TI + 2Rh significantly decreased by 30.8 %, 41.7 %, 49.4 %, and 37.7 % respectively, compared with the control treatment. Specifically, the Cd content with TI + 1Rh was as low as 0.182 ± 0.009 mg/kg, which meets the national food safety requirements (< 0.2 mg/kg). The rice yield and plant biomass of TI + 1Rh were highest compared to the other treatments, possibly because of the alleviation of oxidative stress due to Cd. The hydroxyl and carboxyl concentrations in the soluble components in the leaf cells for the TI + 1Rh treatment were the highest compared to the other treatments. Our results demonstrated that the foliar spraying of TI + 1Rh is an efficient method to reduce Cd accumulation in rice grain. It holds potential for the future development of safe food production in soils polluted with Cd.

Combined Utilization of Chinese Milk Vetch, Rice Straw, and Lime Reduces Soil Available Cd and Cd Accumulation in Rice Grains

Cadmium (Cd) pollution poses a growing threat to rice production in acidic paddies. In south China, a common agricultural practice involves the combined utilization of Chinese milk vetch (M) and rice straw (R). However, it is unclear how the addition of lime to these amendments affects Cd bioavailability and accumulation in soil. Control (CK), chemical fertilizer (F), Chinese milk vetch + rice straw + chemical fertilizer (MRF), and Chinese milk vetch + rice straw + chemical fertilizer + lime (MRFL) treatments were applied to develop a kind of green, efficient, and practical amendment for acidic paddies. We conducted a microplot experiment to explore Cd immobilization in paddy soil and the Cd content in rice grains with these treatments. The results showed that compared with F, the rice Cd in the MRF and MRFL treatments were significantly decreased by 51.7% and 65.2% in early rice and 23.0% and 43.3% in late rice, respectively. Both the MRF and MRFL treatments significantly reduced soil available Cd and weak acid-extractable cadmium (Aci-Cd) concentrations and increased soil organic matter (SOM), exchangeable cation concentrations, and pH, which converted Cd into a stable form in soil. In addition, the MRF and MRFL treatments increased soil pH value by reducing soil exchangeable hydrogen ion concentration (E-H). Additionally, recombination of Cd forms was the primary factor in the reduction in available Cd concentration according to partial least squares path modeling (PLS-PM) analysis. The Cd concentration of rice grains was primarily associated with soil available Cd, soil pH value, and SOM. Overall, these results provide useful data and novel insights into reducing rice grain Cd in south China.

Effects of rapeseed residue organic fertilizer on cadmium toxicity in soil and its uptake and transfer by rice plants (Oryza sativa L.)

AbstractCadmium (Cd) contamination of rice fields is a global agro‐environmental issue. Rapeseed (canola) residue organic fertilizer (RROF) as a potential strategy to minimize Cd accumulation in rice planted in acidic soil. The results showed that application of 7.5–30.0 g kg−1 RROF increased the soil pH value by 0.01–0.55 units, increased soil organic matter (OM) by 7.8–25.0%, increased soil microbial biomass carbon (MBC) and nitrogen (MBN) by 0.0–89.7% and 0.0–47.5%, respectively, and increased soil dehydrogenase (DH), acid phosphatase (ACP), and urease (UA) activities by 14.3–344.0%, 0.0–127.1%, and 3.5–14.3%, respectively. The application of RROF treatments reduced the soil acid‐extractable Cd (ACI–Cd) content by 8.1–36.4% and increased the reducible Cd (RED–Cd) and oxidizable Cd (OXI–Cd) by 1.3–36.4% and 16.2–47.9%, respectively. The path analysis demonstrated that soil OM, ACP activity, Mn, and Ca ions had a significant negative indirect effect on the decrease in soil ACI‐Cd (rOM = −0.628, rACP = −0.582, rMn = −0.627, rCa = −0.341), while Zn ions showed a strong direct effect (bZn = 0.567) and indirect positive effect (rZn = 0.580). In rice plants, the contents of Cd, Fe, K, Mg, P, and Zn in rice tissues increased, and the Cd content in rice grains decreased by 3.1–31.3% under RROF application. The results of the correlation analysis found that K and Zn in rice husks and Mg in grains were significantly and negatively correlated with the husk–grain translocation factor of Cd (r = −0.88** for Zn, r = −0.81** for K, and r = −0.59* for Mg). In addition, a significant negative correlation of root–straw translocation factors of Cd with Ca and Fe ions in the roots was observed (r = −0.63* and r = −0.60* for Ca at the filling and maturity stages of rice, r = −0.61* for Fe at the tillering stage of rice). RDA showed that the distribution fraction of soil Cd controlled the accumulation of Cd in various parts of rice, with soil ACI–Cd content explaining 22.4% of the total eigen values at a significant level, followed by 14.1% for OXI–Cd. This means that ACI–Cd was the primary factor that controlled Cd accumulation in rice grain, followed by OXI–Cd.

The road toward Cd-safe rice: From mass selection to marker-assisted selection and genetic manipulation

Rice is an important dietary source of the toxic mineral cadmium (Cd) for populations in which rice is the main staple food. When grown in agricultural soils that are contaminated with Cd, rice often accumulates excessive Cd into the grains, which is a serious threat to agricultural sustainability and human health. To limit Cd accumulation in rice grains, studies on the genetic basis of Cd accumulation in rice have been carried out extensively, and some low-Cd rice varieties have also been developed in recent years. However, the challenges in low-Cd rice breeding still exist because the outcomes of the current genetic improvements for low-Cd rice cannot fully meet the requirements for the development of Cd-safe rice at present. In this review, we outline the progress in understanding the physiological mechanisms and the genetic nature of Cd accumulation in rice and summarize the strategies and outcomes of low-Cd rice breeding over the past decade. By graphing the physiological mechanism of Cd transport in the rice plant, three key steps and some underlying genes are summarized and discussed. Also, two genetic features of the natural variation in rice grain-Cd accumulation, the phenotypic plasticity and subspecies divergence, and the potential genetic explanations for these features are also discussed. Finally, we summarize and discuss current progress and the potential issues in low-Cd rice breeding using different breeding strategies. We hope to propose strategies for future success in the breeding of low-Cd rice varieties over the next decade.