Can liming reduce cadmium (Cd) accumulation in rice (Oryza sativa) in slightly acidic soils? A contradictory dynamic equilibrium between Cd uptake capacity of roots and Cd immobilisation in soils

Discriminative algorithm approach to forecast Cd threshold exceedance probability for rice grain based on soil characteristics

The relationship between cadmium (Cd) concentration in rice grains and the soil that they are cultivated in is highly uncertain due to the influence of soil properties, rice varieties, and other undetermined factors. In this study, we introduce the probability of exceeding the threshold to characterize this uncertainty and then, build a probabilistic forewarning model. Additionally, a number of associated factors have been used as parameters to improve model performance. Considering that the physicochemical properties and Cd concentration in the soil (Cd_soil) do not follow a normal distribution, and are not independent of each other, a discriminative algorithm, represented by a logistic regression (LR), performed better than generative algorithms, such as the naive Bayes and quadratic discriminant analysis models. The performance of the LR based model was found to be 0.5% better in the case of the univariate model (Cd_soil) and 4.1% better with a multivariate model (soil properties used as additional factors) (p < 0.01). The output of the LR based model predicted probabilities that were positively correlated to the true exceedance rate (R² = 0.949,p < 0.01), within an exceedance threshold range of 0.1-0.4 mg kg^-1 and a mean deviation of 5.75%. A sensitivity analysis showed that the effect of soil properties on the exceedance probability weakens with an increase in Cd concentration in rice grains. When the threshold is below 0.15 mg kg^-1, soil pH strongly influences the exceedance probability. As the threshold increases, the influence of pH on the exceedance probability is gradually superseded. By quantifying the uncertainty regarding the relationship between Cd concentration in rice grains and soil, the discriminative algorithm-based probabilistic forecasting model offers a new way to assess Cd pollution in rice grown in contaminated paddy fields.

Spent mushroom substrate combined with alkaline amendment passivates cadmium and improves soil property

As an extremely toxic metal, cadmium (Cd) is readily taken up by most plants. In situ Cd passivation is of great importance to reduce Cd availability in soil. In this experiment, two alkaline amendments, lime (L) (at a dosage of 0.02%, 0.04%, or 0.08%) and biochar (B) (at a dosage of 0.5%, 1%, or 2%), were used to improve Cd passivation by spent mushroom substrate (SMS) in a simulating Cd-contaminated soil (0.6 mg kg^-1). Results showed that the application of SMS alone reduced Cd bioavailability by 44.80% and EC by 9.71% and increased soil pH by 0.61 units, CEC by 25.32%, and soil enzymes activities by 17.11% to 21.10% compared with non-amendment Cd-contaminated soil. Biochar combination enhanced the efficiency of SMS on Cd reduction by 48.32-66.58% and pH increased by 0.17 to 0.59 units and enzymes activities elevation by 5.74% to 47.29% in a dose-dependent manner. Lime also facilitated SMS to passivate Cd by decreasing bioavailable Cd by 63.10%-66.47% and increasing soil pH by 0.25-0.72 units and enzymes activities by 3.28% to 37.86% compared to those of SMS. Among six combined amendments, SMSB3 (0.5% SMS + 2% B) performed best in reducing bioavailable Cd (39.46% higher than SMS), increasing organic matter content (28.54% higher than SMS) and soil enzyme activities (25.82%, 47.29%, and 26.23% higher than that of SMS for catalase, urease, and invertase, respectively). Both biochar and lime can assist SMS to passivate Cd and improve soil property, and biochar is more efficient than lime in reducing cadmium content and increasing enzyme activity and organic matter.

Inconsistent effects of limestone on rice cadmium uptake: Results from multi-scale field trials and large-scale investigation

Adding limestone into acidic paddy soils might reduce cadmium (Cd) accumulation in rice plants and the harvested grains but with inconsistent results in the field practice. We conducted three experiments of different field scales, including small-plots, multi-location trial, and large-scale field samplings of rice grown in a major production region of southern China, to investigate whether liming could sustainably limit the Cd phytoavailability to rice. Forty-eight physical, chemical, and biological attributes associated with paired soils and plants were collectively analyzed. Rice Cd uptake was significantly reduced when moderate dosage (2.25-3 t ha^-1) of liming was present in the soils. The limes decreased rice Cd uptake by reducing the Cd concentrations of soil solution phase and regulating Ca²⁺ and Cd²⁺ competitions for absorption sites at root surfaces. Soil Zn hardly any effect on rice Cd uptake. Rice Cd uptake was suppressed at the higher rates of liming (4.5-9 t ha^-1) due to the heavy loss of soil labile Mn. The tendencies of over compensating were soil-, plant-, and climate-dependent and were estimated by a transfer function and the risks were characterized using probabilistic analysis. The sustainable doses of limestone that reduced grain Cd accumulation, but did not compromise yield, or disrupt the rice rhizosphere was 3 t ha^-1 annually incorporated two weeks before the seedlings were transplanted.

Regulatory mechanisms of nitrogen (N) on cadmium (Cd) uptake and accumulation in plants: A review

Cadmium (Cd) is a heavy metal that is toxic to plants and animals. Nitrogen (N), the most significant macro-nutrient and a common input for crop production, is often excessively applied than plants' demands by farmers to obtain more economic benefits. Understanding the regulatory mechanisms of N that control Cd uptake, translocation, and accumulation may enable the development of solutions regarding Cd pollution in the trophic chain, a major and global threat to agricultural sustainability and human health. In this review, we clarified that an increased amount of N, regardless of its form, enhances Cd uptake, translocation, and accumulation in plants, and nitrate promotes Cd uptake more than any other N form. We also described that N fertilizer alters the Cd exchange capacity and the bio-available Cd content in soil; regulates nitric oxide induced divalent cation gene expression of Nramp1, HMA2, and IRT1; and changes cell wall isolation, chelation capacity, and oxidative resistance to regulate Cd accumulation in plants. By revealing the integrated interaction effects between Cd accumulation and N fertiliser use, we propose new challenges to investigate the functions and mechanisms of N in Cd-contaminated croplands and develop suitable N-fertilisation protocols to practically reduce food health risks in agricultural food production.

Efficiency of lime, biochar, Fe containing biochar and composite amendments for Cd and Pb immobilization in a co-contaminated alluvial soil

Present study reports the laboratory and field scale application of different organic and inorganic amendments to immobilize cadmium (Cd) and lead (Pb) in a co-contaminated alluvial paddy soil. For that purpose, lime, biochar, Fe-biochar and two composite amendments (CA) composed of biochar, lime, sepiolite and zeolite (CA1: composite amendment 1) and manure, lime and sepiolite (CA2: composite amendment 2) were firstly tested in an incubation experiment to ameliorate Cd and Pb co-contaminated alluvial soil. It was observed that liming and CA2 elevated the soil pH and reduced DTPA extractable Cd and Pb in the incubated soil leading to higher metal immobilization. Therefore, efficiency of lime and CA2 was further investigated in field conditions with mid rice as the test crop to evaluate field scale immobilization and precise application rate for the tested soil type. DTPA and CaCl₂ extractable Cd (46 and 51%) and Pb (68 and 70%) in field soil were decreased with applied treatments. Speciation of Cd and Pb also promoted conversion of metal exchangeable contents to less-available forms. Activated functional groups on amendments' surface (^_OH bonding, C^_O and CO, -O-H, Si-O-Si, carboxylic and ester groups) sequestered metals by precipitation, adsorption, ion exchange or electro static attributes. Application of lime at 2400 kg/acre (T4) and CA2 at 1200 kg/acre was more effective in reducing rice shoot and grains metal contents. Moreover, obtained results in terms of pH, extractable content, speciation and yield, and microanalysis of amendments highlights the remarkable efficiency of lime and composite amendment to sorb Cd and Pb providing the key evidence of these amendments for metals immobilization and environmental remediation. Considering these results, lime and CA2 are potential amendments for co-contaminated rice field especially in context of alluvial soil.

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.

Reduced Cd, Pb, and As accumulation in rice (Oryza sativa L.) by a combined amendment of calcium sulfate and ferric oxide

A combined amendment (CF) consisting of 90% calcium sulfate (CaSO₄) and 10% ferric oxide (Fe₂O₃) was used to investigate the feasibility, active principles, and possible mechanisms of the immobilization of heavy metals in paddy soil. A soil incubation experiment, two consecutive pot trials, and a field experiment were conducted to evaluate the effectiveness and persistence of CF on metal(loid) immobilization. Soil incubation experiment results indicated that the application of CF significantly decreased the concentrations of cadmium (Cd), lead (Pb), and arsenic (As) in soil solution. CF treatments simultaneously reduced the accumulation of Cd, Pb, and As in two consecutive pot trials. The total Cd, Pb, and As concentrations in the rice grains were respectively 0.02, 2.08, and 0.62 mg kg^-1 in the control treatment in the second year, which exceeded the safety limits of contaminants in food products in China. However, a high amount of CF amendment (CF-H, 0.3%) effectively decreased Cd, Pb, and As by 75.0%, 75.5%, and 46.8%, respectively. Further, with the CF amendment, the bioavailable Cd and Pb in the soil and the accumulation of Cd, Pb, and As in rice grain in the field experiment were also significantly decreased. The concentrations of Cd, Pb, and As in grains were respectively 0.02, 0.03, and 0.39 mg kg^-1 in the control treatment in the field experiment, which decreased to 0.01, 0.01, and 0.22 mg kg^-1 with CF addition, suggesting that grains produced in the field could pose less health risk. In conclusion, these results implied that CF was an effective and persistent combined amendment to immobilize heavy metals in soil and thereby can reduce the exposure risk of metal(loid)s associated with rice consumption.

Polyaspartate and liquid amino acid fertilizer are appropriate alternatives for promoting the phytoextraction of cadmium and lead in Solanum nigrum L

Traditional metal chelators, such as ethylenediaminetetraacetic acid (EDTA), have been gradually replaced due to their poor biodegradability in soil and high risk of heavy metal leaching into groundwater, which pose high environmental risks to the health of humans and animals. In this study, a liquid amino acid fertilizer (LAAF, waste proteins from hydrolysates of animal carcasses) and polyaspartate (PASP) were used as additives to enhance the phytoextraction of cadmium (Cd) and lead (Pb) from contaminated soil. We conducted pot experiments to investigate the phytoextraction capacity of Solanum nigrum, a Cd accumulator, grown on soil highly contaminated with Cd and Pb in the absence (as controls) or presence of PASP and LAAF. Both PASP and LAAF significantly improved plant growth, Cd accumulation, and total Cd and Pb content in S. nigrum shoots and roots. PASP and LAAF application promoted Cd translocation from roots to shoots in S. nigrum and Cd bio-accessibility in rhizosphere soils, but this was not the case for Pb. Both PASP and LAAF increased Cd and Pb phytoextraction by S. nigrum plants, and Cd phytoextraction was more effective in LAAF-assisted S. nigrum than in PASP-assisted S. nigrum. These findings demonstrate that the low cost and ecofriendly features of recycled waste proteins make them good candidates for chelant-enhanced phytoextraction from heavy metal-contaminated soils.

Rice vegetative organs alleviate cadmium toxicity by altering the chemical forms of cadmium and increasing the ratio of calcium to manganese

Altering Cd chemical form is one of the mechanisms to alleviate Cd toxicity in rice plant. Field experiments were carried out in this study to investigate the potential of rice vegetative organs in altering Cd into insoluble chemical forms in the natural environment. Experimental results showed that more than 80% of Cd in rice roots existed in the insoluble forms. Uppermost nodes altered Cd into insoluble form preferentially and generally had higher content of insoluble Cd than other organs. Rachises displayed a slow increasing trend in soluble Cd when total Cd in roots was less than 1.8 mg kg^-1. However, when Cd content in roots exceeded 2.8 mg kg^-1, the ratio of insoluble to soluble Cd remained stable at 85:15 in rachises and roots, and at 75:25 in uppermost nodes and flag leaves. Cd concentration in grains was greatly lower than that in vegetative organs, and closely correlated with the content of soluble Cd in rachises (r = 0.991**) as well as in uppermost nodes. Soluble Cd in the uppermost nodes displayed a much lower mobility than that in other organs. Accumulation of soluble Cd was always companied by decrease of Ca and increase of Mn in roots, uppermost nodes and rachises. A small increase of soluble Cd from 0.05 to 0.1 mg kg^-1 caused a sharp decline of Ca:Mn ratio in roots and rachises. Roots and nodes had much higher Ca:Mn ratio than rachises when soluble Cd was less than 0.5 mg kg^-1 in them. These results indicate that vegetative organs have a great potential to alter more than 75% Cd into insoluble forms and increasing Ca:Mn ratio may be another way to alleviate Cd toxicity by establishing new ionic homeostasis in rice plants.

Preparation and characterization of carboxymethyl starch from cadmium-contaminated rice

Cadmium-contaminated rice has been a serious food safety issue in China. In this research, carboxymethyl rice starch (CMS) from cadmium-contaminated rice and native rice was prepared to remove the cadmium in rice. The preparation of native rice starch (NRS) and starch from cadmium-contaminated rice (Cd-CRS) was similar, and carboxymethyl starch was prepared following the same steps. A single factor experiment was performed to obtain the carboxymethyl starch prepared under the optimal conditions. Cadmium content was reduced from 0.38 mg/kg to 0.04 mg/kg after alkalization. The physical properties, including particle size, degree of crystallinity, water absorption and freeze-thaw stability, of NRS and Cd-CRS and their carboxymethyl starches were studied. The results showed that the cadmium was significantly removed after extracting starch from cadmium-contaminated rice by alkalization and carboxymethylation. Then, starch samples and carboxymethyl starch samples were characterized. All results showed no obvious difference between Cd-CMS and NCMS, indicating that carboxymethyl starch from cadmium-contaminated rice could be widely used in both food and nonfood industries.

Phosphate addition diminishes the efficacy of wollastonite in decreasing Cd uptake by rice (Oryza sativa L.) in paddy soil

Cadmium (Cd) contamination in paddy soils poses food security risks and public health concerns. Exploring effective strategies to reduce rice grain Cd is an urgent need. In this study, field plot experiments were conducted to evaluate the effects of wollastonite application with or without phosphate (P) addition on Cd accumulation in rice (Oryza sativa L.). Co-application of P and wollastonite showed greater efficiency than wollastonite amendments alone in raising soil pH and CEC and decreasing soil Cd availability. Cd concentration in brown rice was decreased by 71% under the wollastonite treatment alone, but was decreased by only 29-39% when wollastonite was coupled with different P amendments. This seeming contradiction could be ascribed to the dramatic decline in the phytoavailability of manganese (Mn) and the increase in molar ratio of iron (Fe) to Mn (Fe/Mn) in Fe plaques on root surfaces in the presence of P additions. Significant negative correlations between Mn and Cd in rice plants and positive correlations between Fe/Mn in Fe plaque and Cd in rice plants indicated that P-induced soil Mn deficiency and reduced Mn in Fe plaque impeded the alleviation of Cd accumulation in rice. Application of wollastonite in Si-deficient paddy soils was effective in reducing rice Cd accumulation while boosting rice yield, but co-application of P and wollastonite was counterproductive and should be avoided. This work emphasized that a better understanding of the relationships between Cd and related mineral nutrient uptake would be helpful in developing more efficient measures to reduce rice grain Cd.

Soil liming effects on CH4, N2O emission and Cd, Pb accumulation in upland and paddy rice

Keeping in view the expanding environmental pollution and irrigation water deficit, a pot experiment was performed for the upland (Huyou2, Hanyou737) and paddy rice cultivars (Taigeng8; Yixiang2292), to study soil liming effects on methane (CH₄) and nitrous oxide (N₂O) emission, bioavailability and accumulation of Cd, Pb in upland and paddy rice. Upland rice reduced 90% of soil CH₄ emission as compared to paddy conditions. Soil CH₄ emission decreased by 45% and 39% with dolomite, and it reduced by 35% and 33% with lime treatment both in upland and paddy conditions, respectively. Soil N₂O emission decreased by 44% and 52% with dolomite, and with the lime application, it was reduced by 37% and 44% for both upland and paddy conditions respectively. Reduction in soil DTPA-extractable Cd was between 37-53% and 43-80% with dolomite and 16-37% and 24-72% Cd decreased with lime application in upland and paddy conditions respectively. Soil DTPA-extractable Pb reduced by 27-44% and 25-53% with dolomite and 16-40% and 11-42% with soil-applied lime in upland and paddy conditions, respectively. Cd accumulation in rice grain was decreased by 47-88% and 62-79% with dolomite and 31-86% and 45-52% reduction by lime application in upland and paddy rice respectively. Rice grain Pb reduced by 58-91% and 66-78% with dolomite application and 32-71% and 44-71% with lime in upland and paddy rice, respectively. Our results showed that soil liming significantly reduced soil N₂O and CH₄ emission and Cd, Pb accumulation in rice grain, but dolomite was more effective as compared to lime. Altogether, results of this study suggest that upland rice can be cultivated in CdPb polluted soils with least soil CH₄ emission. Cd and Pb toxicity, accumulation, and N₂O emission in upland rice can be minimized by soil liming of 3 g kg^-1 and optimizing the nutrients composition of the soil.

Immobilization of cadmium and improvement of bacterial community in contaminated soil following a continuous amendment with lime mixed with fertilizers: A four-season field experiment

The effects of the continuous amendments with lime (L), lime mixed with organic manure (LO), or phosphate fertilizer (LP) on the soil bacterial community, soil available cadmium (Cd) content, and Cd accumulation in rice planted in a Cd contaminated paddy soil were determined through a four-season field experiment. The results showed that with continuous application of amendments during the four seasons, the soil pH increased significantly compared with the control, while the soil available Cd content significantly decreased by 12.9-18.2%, 13.1-17.3% and 0.09-23.2% under the L, LO, or LP treatments, and the Cd content of rice was significantly reduced by 28.5-56.2%, 37.6-53.4%, and 31.2-44.6%, respectively. The rice Cd content in each season at amendment treatments was lower than the National Food Safety Standard of China (maximum level of Cd in grains is 0.2 mg/kg). The diversity and richness of soil bacteria significantly increased after the continuous amendments in soil for four-season cropping. Soil pH and available Cd content were important factors for soil bacterial community. Lime mixed with phosphate fertilizer or organic manure had been characterized by a significant increase of Proteobacteria, Nitrospirae, and Chloroflexi and a decrease of Acidobacteria based on an Illumina Miseq sequencing analysis. The results indicate that the continuous application of lime mixed with organic manure or phosphate fertilizer is a very important measure to ensure the quality safety of rice and improve soil quality in a Cd-contaminated paddy.

Cadmium in rice grains from a field trial in relation to model parameters of Cd-toxicity and -absorption in rice seedlings

Selecting rice varieties that absorb less Cd from soil will reduce human health risks posed by Cd through rice consumption. Nine rice cultivars that are commonly grown in Taiwan were used for investigating genotypic differences in Cd tolerance and absorption. Hydroponic testing with Cd treatments of 5, 10, and 50 μM CdCl₂ for 7-day exposure was conducted for the cultivars. The reductions in plant growth by Cd treatments were fitted to a dose-response curve; the modeling parameters, that is, the effective Cd concentration resulting in 50% reduction (EC₅₀), were obtained. The Cd concentrations in plant were expressed by a Michaelis-Menten kinetic model and the uptake rate parameters (M/k) were obtained. A field experiment was also conducted in farmland with Cd ~0.2 mg kg^-1 in soil. For the rice cultivars used in hydroponics, Cd distributions and physiological traits (CAT, H₂O₂, and MDA) in seedlings were related to their tolerances to Cd toxicity. Modeling parameters, EC₅₀ and M/k, correspond to the Cd concentrations in rice plant. In the field experiment, the Cd concentrations in brown rice of the indica cultivars (i.e., TCS10, TCS17, and TNGS22) were 0.6 mg kg^-1; these were significantly higher than those of the japonica cultivars (i.e. TY3, TK9, TNG71, KH145, TKW1, and TKW3). By contrast, the three cultivars, KH145, TKW1, and TKW3, whose Cd concentrations in brown rice were lower than 0.3 mg kg^-1 were considered safe relative to the permissible level of 0.4 mg kg^-1. In addition, for the used cultivars, Cd concentrations in brown rice were well expressed (i.e., r² = 0.95) as a function of EC₅₀, M/k, and MDA by using multiple regression. Newly bred cultivars could be screened rapidly with hydroponic testing to predict their Cd concentrations in brown rice when grown in the field.

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.

Root iron plaque alleviates cadmium toxicity to rice (Oryza sativa) seedlings

Iron plaque (IP) on root surface can enhance the tolerance of plants to environmental stresses. However, it remains unclear the impact of Fe²⁺ on cadmium (Cd) toxicity to rice (Oryza sativa) seedlings. In this study, the effects of different Fe²⁺ and Cd²⁺ concentration combinations on rice growth were examined hydroponically. Results indicated that Fe²⁺ concentration up to 3.2 mM did not damage rice roots while induced IP formation obviously. Cd²⁺ of 10 μM repressed rice growth significantly, while the addition of 0.2 mM Fe²⁺ to 10 μM Cd²⁺ solution (Cd+Fe) did not damage rice roots, indicating that Fe²⁺ could ameliorate Cd toxicity to rice seedlings. Microstructure analysis showed Cd+Fe treatment induced the formation of IP with dense and intricate network structure, Cd adsorption on the root surface was reduced significantly. Cd concentration of rice roots and shoots and Cd translocation from roots to shoots with Fe+Cd treatment were reduced by 34.1%, 36.0% and 20.1%, respectively, in comparison to a single Cd treatment. Noteworthy, the removal of IP resulted in a larger loss of root biomass under Cd treatment. In addition, Cd+Fe treatment increased the activities of root superoxide dismutase and catalase by 105.5% and 177.4%, and decreased H₂O₂ and O₂·^- accumulation of rice roots by 56.9% and 35.9%, and recovered Cd-triggered electrolyte leakage obviously, when compared with a single Cd treatment. The results from this experiment indicated that the formed dense IP on rice roots decreased Cd absorption and reactive oxygen species accumulation, and Fe²⁺ supply alleviated Cd toxicity to rice seedlings.

Evolution, and functional analysis of Natural Resistance-Associated Macrophage Proteins (NRAMPs) from Theobroma cacao and their role in cadmium accumulation

The presence of the toxic metal cadmium (Cd²⁺) in certain foodstuffs is recognised as a global problem, and there is increasing legislative pressure to reduce the content of Cd in food. The present study was conducted on cacao (Theobroma cacao), the source of chocolate, and one of the crops known to accumulate Cd in certain conditions. There are a range of possible genetic and agronomic methods being tested as a route to such reduction. As part of a gene-based approach, we focused on the Natural Resistance-Associated Macrophage Proteins (NRAMPS), a family of proton/metal transporter proteins that are evolutionarily conserved across all species from bacteria to humans. The plant NRAMP gene family are of particular importance as they are responsible for uptake of the nutritionally vital divalent cations Fe²⁺, Mn²⁺, Zn²⁺, as well as Cd²⁺. We identified the five NRAMP genes in cacao, sequenced these genes and studied their expression in various organs. We then confirmed the expression patterns in response to variation in nutrient cation availability and addition of Cd²⁺. Functional analysis by expression in yeast provided evidence that NRAMP5 encoded a protein capable of Cd²⁺ transport, and suggested this gene as a target for genetic selection/modification.

Effectiveness and potential risk of CaO application in Cd-contaminated paddy soil

In this study, the accurate effectiveness of CaO in mitigating Cd bioavailability in paddy soil-rice system was investigated and moreover, the potential for reversibility of CaO liming process was provided. Increasing soil pH to ≥6.5 by CaO was determined to be the minimal threshold for minimizing Cd transfer into rice in historically contaminated soils across light to severe Cd levels, while an elevated CaO ratio was needed for soil with recent input of Cd. In CaO remediation treatment, a reduced pool of bioavailable Cd in rhizosphere soil coupled with an increased Cd retention by Fe plaque and an inhibited planta Cd transfer was determined consistently and proposed to be largely responsible for the significant reduction in brown rice Cd. Under continuous simulated acid precipitation test, a negligible level of Cd in eluate with ∼1 unit higher pH than control was recorded with CaO. Significant acidification, however, increased Cd solubilization in the limed soil than in unlimited control, notably at pH 5.5-6.5. As indicated by Visual MINTEQ, a higher Cd-carbonate solubility and much decreased Cd complexation on Fe-(hydr)-oxides across this pH range occurred as a result of elevated Ca input. This suggests that a high soil pH (≥6.5) needs to be permanently maintained once liming has been performed to minimize Cd mobilization. Therefore, amending Cd-contaminated soils with CaO is a cost-effective remedial measure for reducing Cd bioavailability to paddy rice, while a high soil pH need to be permanently maintained to sustain this beneficial effect.