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A clear description of the certain mechanisms of cadmium (Cd) uptake and translocation in rice (Oryza sativa L.) may help to reduce Cd accumulation in rice grain. Hydroponic experiments were carried out to determine the effects of cultivation conditions (aerated and stagnant) on the uptake, translocation and subcellular distribution of Cd in relation to the morphology and anatomy of roots in two rice genotypes with different Cd accumulations in grains. Marked differences in morphology and anatomy were observed between these two genotypes under different cultivation conditions. Genotypes with low Cd accumulation in grains tended to develop fewer root tips per root surface area, larger root porosity and more mature apoplastic barriers. The stagnant cultivation condition decreased the number of root tips per root surface area but increased root porosity and accelerated apoplastic barrier formation in root tissues. Correlative Cd uptake studies revealed that rice plants with fewer number of root tips per root surface area reduced root Cd uptake ability, while mature apoplastic barriers increased root Cd retention in cell walls and the symplast. Thus, the fewer number of root tips per root surface area and the earlier formation of mature apoplastic barriers led to lower Cd uptake and translocation. The results indicated that the morphology and anatomy of roots could play important roles in Cd uptake and translocation in rice, and could be influenced by both genotype and cultivation conditions. The present results would be useful in screening and planting rice plants with low Cd accumulation., (Copyright © 2018. Published by Elsevier B.V.)

In order to help mitigate widespread cadmium (Cd) and arsenic (As) co-contamination in paddy soils in China, screening and breeding of low-accumulating rice (Oryza sativa L.) cultivars (excluders) have been widely adopted. However, the performance of rice cultivars for grain Cd and As accumulation may vary in different growing environments. The inability to identify stable low-accumulating cultivars has largely hindered their application. In this study, 51 rice cultivars were evaluated at four Cd- and As-contaminated paddy sites in two crop seasons in northern Guangdong Province, China. The aim was to investigate the effects of cultivar, environment and their interactions in determining grain Cd and As concentrations, and so to identify stable low-accumulating cultivars. Results showed that environment effects dominated the Cd and As concentrations in rice grains, explaining 87% of the total variations. The crop season played a vital role; compared to early season, grain Cd levels increased and As levels lowered significantly in late season. Large variations in grain Cd, total As, inorganic As concentrations and the percentage of inorganic As were observed between different cultivars. Conventional japonica cultivars exhibited lower Cd levels but higher As levels in the grains than did indica cultivars. The cultivar × environment interaction (CEI) was significant, and its importance was comparable to the cultivar effect. By measuring and interpreting such an interaction, stable Cd and As excluder cultivars were identified based upon the yield, grain Cd and As levels as well as the stabilities of cultivars across the trial environments. Two stable Cd and As co-excluders were found among the hybrid indica cultivars. These results demonstrated that the variations in grain Cd and As concentrations could mainly be attributed to the environment effects and cultivar selection practices should include the analysis of CEI to identify stable low-accumulating rice cultivars., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Contamination of rice (Oryza sativa) by Cd is of great concern. Steel slag could be used to amend Cd-contaminated soils and make them safe for cereal production. This work was conducted to study the effects of steel slag on Cd uptake and growth of rice plants in acidic and Cd-contaminated paddy soils and to determine the possible mechanisms behind these effects. Pot (rhizobag) experiments were conducted using rice plants grown on two acidic and Cd-contaminated paddy soils with or without steel slag amendment. Steel slag amendment significantly increased grain yield by 36-45% and root catalase activity, and decreased Cd concentrations in brown rice by 66-77% compared with the control, in both soils. Steel slag amendment also markedly decreased extractable soil Cd, Cd concentrations in pore-water and Cd translocation from roots to above-ground parts. It also significantly increased soil pH, extractable Si and Ca in soils and Ca concentrations in roots. Significant positive correlations were found between extractable soil Cd and Cd concentrations in rice tissues, but it was negatively correlated with soil pH and extractable Si. Calcium in root tissues significantly and negatively correlated with Cd translocation factors from roots to straw. Overall, steel slag amendment not only significantly promoted rice growth but decreased Cd accumulation in brown rice. These benefits appear to be related to improvements in soil conditions (e.g. increasing pH, extractable Si and Ca), a reduction in extractable soil Cd, and suppression of Cd translocation from roots to above-ground parts., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Paddy soils and rice (Oryza sativa L.) contaminated by mixed heavy metals have given rise to great concern. Field experiments were conducted over two cultivation seasons to study the effects of steel slag (SS), fly ash (FA), limestone (LS), bioorganic fertilizer (BF), and the combination of SS and BF (SSBF) on rice grain yield, Cd, Pb, and Zn and nutrient accumulation in brown rice, bioavailability of Cd, Pb, and Zn in soil as well as soil properties (pH and catalase), at two acidic paddy fields contaminated with mixed heavy metals (Cd, Pb, and Zn). Compared to the controls, SS, LS, and SSBF at both low and high additions significantly elevated soil pH over both cultivation seasons. The high treatments of SS and SSBF markedly increased grain yields, the accumulation of P and Ca in brown rice and soil catalase activities in the first cultivation season. The most striking result was from SS application (4.0 t ha -1 ) that consistently and significantly reduced the soil bioavailability of Cd, Pb, and Zn by 38.5-91.2 % and the concentrations of Cd and Pb in brown rice by 20.9-50.9 % in the two soils over both cultivation seasons. LS addition (4.0 t ha -1 ) also markedly reduced the bioavailable Cd, Pb, and Zn in soil and the Cd concentrations in brown rice. BF remobilized soil Cd and Pb leading to more accumulation of these metals in brown rice. The results showed that steel slag was most effective in the remediation of acidic paddy soils contaminated with mixed heavy metals.

Microbe-assisted phytoextraction shows a potential for the remediation of metal-contaminated soils. The aim of this study was to isolate, characterize, and evaluate the potential of endophytic bacteria in improving plant growth and metal uptake by Cd-hyperaccumulators-Amaranthus hypochondriacus and Amaranthus mangostanus. An endophytic bacterial strain JN27 isolated from roots of Zea mays displayed high tolerance and mobilization to Cd, and was identified as Rahnella sp. based on 16S rDNA sequencing. The strain also exhibited multiple plant growth beneficial features including the production of indole-3-acetic acid, siderophore, 1-aminocyclopropane-1-carboxylic acid deaminase and solubilization of insoluble phosphate. Subsequently, a pot trial was performed to elucidate the effects of inoculation with JN27 on plant growth and Cd uptake by A. hypochondriacus, A. mangostanus, Solanum nigrum and Z. mays grown in soils with different levels of Cd (25, 50, 100 mg Cd kg(-1)). The results revealed that inoculation with JN27 significantly increased the biomasses of all the tested plants and the Cd concentrations of all the tested plants except Z. mays in both above-ground and root tissues. Moreover, strain JN27 could successfully re-colonized in rhizosphere soils of all the tested plants and root interior of A. hypochondriacus and Z. mays. The present results indicated that the symbiont of A. hypochondriacus (or A. mangostanus) and strain JN27 can effectively improve the Cd uptake by plants and would be a new strategy in microbe-assisted phytoextraction for metal-contaminated soils., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

A post-harvest experiment was conducted further to our previous greenhouse pot study on upland kangkong (Ipomoea aquatica Forsk.) and Alfred stonecrop (Sedum alfredii Hance) intercropping system in Cd-contaminated soil inoculated with arbuscular mycorrhizal (AM) fungi. Previously, four treatments were established in the intercropping experiment, including monoculture of kangkong (control), intercropping with stonecrop (IS), and IS plus inoculation with Glomus caledonium (IS+Gc) or Glomus versiforme (IS+Gv). Both kangkong and stonecrop plants were harvested after growing for 8 weeks. Then, the tested soils were reclaimed for growing post-harvest kangkong for 6 weeks. In the post-harvest experiment, there were no significant differences between the IS and control treatments, except for a significantly decreased (p<0.05) soil available P concentration with IS treatment. Compared with IS, both IS+Gc and IS+Gv significantly decreased (p<0.05) soil DTPA-extractable (phytoavailable) Cd concentrations, but not total Cd, by elevating soil pH, causing significantly lower (p<0.05) Cd concentrations in both the root and shoot of kangkong. In addition, both Gc and Gv significantly increased (p<0.05) soil acid phosphatase activities and available P concentrations and hence resulted in significantly higher (p<0.05) plant P acquisitions. However, only Gv significantly increased (p<0.05) kangkong yield, while Gc only significantly elevated (p<0.05) the shoot P concentration. It suggested that AM fungi have played key roles in Cd stabilization and P mobilization in the intercropping system, and such positive responses seemed to be sustainable and valuable in post-harvest soils.

Temporal variations and correlations between radial oxygen loss (ROL), iron (Fe) plaque formation, cadmium (Cd) and arsenic (As) accumulation were investigated in two rice cultivars at four different growth stages based upon soil pot and deoxygenated solution experiments. The results showed that there were significant differences in ROL (1.1-16 μmol O(2) plant(-1) h(-1)), Fe plaque formation (4,097-36,056 mg kg(-1)), Cd and As in root tissues (Cd 77-162 mg kg(-1); As 49-199 mg kg(-1)) and Fe plaque (Cd 0.4-24 mg kg(-1); As 185-1,396 mg kg(-1)) between these growth stages. ROL and Fe plaque increased dramatically from tillering to ear emergence stages and then were much reduced at the grain-filling stage. Furthermore, significantly positive correlations were detected between ROL and concentrations of Fe, Cd and As in Fe plaque. Our study indicates that increased Fe plaque forms on rice roots at the ear emergence stage due to the increased ROL. This stage could therefore be an important period to limit the transfer and distribution of Cd and As in rice plants when growing in soils contaminated with these toxic elements.

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. • The cultivar-water condition interaction (CWI) resulted in significant variations in the Cd-grains and Cd-roots. • CWI derived from cultivar-dependent responses of radial oxygen loss and root Cd uptake to water regimes. • The radial oxygen loss (ROL) affected soil properties only under flooding, which resulted in the CWI effect. • The low Cd translocation and ROL were key factors for screening stable low-Cd cultivars. [ABSTRACT FROM AUTHOR]

Large areas of paddy fields in mining areas in south China have been contaminated by mixed Cd and As. To test the possibility of using organic manure (OM) as a method of reducing Cd and As accumulation in brown rice and increase the grain yield in such paddy fields, two rice cultivars [Jianyou G2 (JY) and Fengyousimiao (FY)] and three amounts of OM (0, 0.5, 1.5 kg/m) were examined in three Cd-As-contaminated paddy fields (Fogang, Dabaoshan, Fankou) in Guangdong Province. The results show that the application of OM can effectively increase the grain yield and reduce Cd. However, the As concentration was found to increase in brown rice, especially when high levels of OM were used. The addition of OM increased the soil pH and organic matter content in the soils and enhanced the activities of superoxide dismutase (SOD) and catalase (CAT) in plant tissues. Our results suggest that OM can be used as an amendment to effectively decrease Cd accumulation in brown rice in Cd-contaminated paddy fields. However, it is necessary to consider the negative effects of OM amendments when adding OM to As-contaminated paddy fields. [ABSTRACT FROM AUTHOR]

A post-harvest experiment was conducted further to our previous greenhouse pot study on upland kangkong ( Ipomoea aquatica Forsk.) and Alfred stonecrop ( Sedum alfredii Hance) intercropping system in Cd-contaminated soil inoculated with arbuscular mycorrhizal (AM) fungi. Previously, four treatments were established in the intercropping experiment, including monoculture of kangkong (control), intercropping with stonecrop (IS), and IS plus inoculation with Glomus caledonium (IS +  Gc) or Glomus versiforme (IS +  Gv). Both kangkong and stonecrop plants were harvested after growing for 8 weeks. Then, the tested soils were reclaimed for growing post-harvest kangkong for 6 weeks. In the post-harvest experiment, there were no significant differences between the IS and control treatments, except for a significantly decreased ( p < 0.05) soil available P concentration with IS treatment. Compared with IS, both IS +  Gc and IS +  Gv significantly decreased ( p < 0.05) soil DTPA-extractable (phytoavailable) Cd concentrations, but not total Cd, by elevating soil pH, causing significantly lower ( p < 0.05) Cd concentrations in both the root and shoot of kangkong. In addition, both Gc and Gv significantly increased ( p < 0.05) soil acid phosphatase activities and available P concentrations and hence resulted in significantly higher ( p < 0.05) plant P acquisitions. However, only Gv significantly increased ( p < 0.05) kangkong yield, while Gc only significantly elevated ( p < 0.05) the shoot P concentration. It suggested that AM fungi have played key roles in Cd stabilization and P mobilization in the intercropping system, and such positive responses seemed to be sustainable and valuable in post-harvest soils. [ABSTRACT FROM AUTHOR]

Cadmium (Cd) and arsenic (As) contamination in paddy soils poses serious health risks to humans. The accumulation of Cd and As in rice (Oryza sativa L.) depends on their bioavailability, which is affected by soil physicochemical properties and soil microbial activities. However, little is known about the intricate interplay between rice plants and their rhizosphere microbes during the uptake of Cd and As. In this study, different bacterial communities were established by sterilizing paddy soils with γ-radiation. A pot experiment using two paddy soils with different levels of contamination was conducted to explore how the bacterial community composition affects Cd and As accumulation in rice plants. The results showed that the sterilization treatment substantially changed the bacterial composition in the rhizosphere, and significantly increased the grain yield (by 33.5–38.3%). The sterilization treatment resulted in significantly decreased concentrations of Cd (by 18.2–38.7%) and As (by 20.3–36.7%) in the grain, straw, and root of rice plants. The accumulation of Cd and As in rice plants was negatively correlated with the relative abundance of sulfate-reducing bacteria and iron-oxidizing bacteria in the rhizosphere. Other specific taxa associated with the accumulation of Cd and As in rice plants were also identified. Our results suggest that regulating the composition of the rhizosphere bacterial community could simultaneously reduce Cd and As accumulation in rice grain and increase the grain yield. These results would be useful for developing strategies to cultivate safe rice crops in areas contaminated with Cd and As. [Display omitted] • Soil sterilization helped reduce Cd and As accumulation in rice. • The bacterial composition rather than diversity affected rice Cd and As accumulation. • The rhizosphere SRB and FeOB negatively correlated with rice Cd and As accumulation. • Other specific taxa associated with rice Cd and As accumulation were also found. [ABSTRACT FROM AUTHOR]

The radial translocation of cadmium (Cd) from the root to the shoot is one of the major processes affecting Cd accumulation in rice (Oryza sativa L.) grains, but few studies have focused on Cd apoplastic transport in rice. The aim of this study was to determine how apoplastic barriers affect Cd translocation via the apoplastic pathway, Cd accumulation levels in upper parts (shoot and grains) of rice cultivars, and the possible mechanism involved. Hydroponic and soil pot trials were conducted to study the development and chemical constituents of apoplastic barriers and their permeability to bypass flow, and to determine Cd localization in the roots of rice cultivars with different Cd-accumulating characteristics. The Cd accumulation in upper parts was positively correlated with bypass flow in the root and the apparent Cd concentration in the xylem, indicating that the apoplastic pathway may play an important role in Cd root-shoot translocation in rice. Apoplastic barriers were deposited closer to the root tip and were thicker in low Cd-accumulating cultivars than in high Cd-accumulating cultivars. The amounts and rates of increase in lignin and suberin were significantly higher in ZD14 (a low Cd-accumulating cultivar) than in FYXZ (a high Cd-accumulating cultivar) under Cd stress, indicating that stronger barriers were induced by Cd in ZD14. The stronger and earlier formation of barriers in the low Cd-accumulating cultivar decreased bypass flow more efficiently, so that more Cd was retained in the root during apoplastic translocation. This was confirmed by localization analyses of Cd in root transverse sections. These results suggest that apoplastic barriers reduce Cd root-to-shoot translocation via the apoplastic pathway, leading to lower Cd accumulation in the upper parts of rice plants. Bypass flow may have the potential to be used as a rapid screening indicator for low Cd-accumulating rice cultivars. Image 1 • Apoplastic barriers reduced Cd transport into xylem via apoplastic pathway. • Cd low-accumulating cultivars had stronger and earlier-deposited barriers. • Stronger apoplastic barriers in low-accumulating cultivar retained more Cd in root. • Ability of develop barriers in response to stress also determined Cd accumulation. Root apoplastic barriers play an important role in Cd accumulation in rice. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fly ash and steel slag amendments mitigated Cd and As accumulation in rice grains. • Mitigation effects declined over time and lasted for one to three crop seasons. • Soil pH and labile Si were crucial in determining and sustaining amendment effects. • Steel slag significantly increased soil inorganic nutrients and some toxic elements. • Steel slag altered enzyme activities and bacterial community composition in soil. Fly ash and steel slag can potentially mitigate the cadmium (Cd) and arsenic (As) accumulation in rice grains but their long-term effectiveness and impact on soil health are unclear. By running a four-crop-season field trial, we found that the concentrations of Cd, As and inorganic As in rice grains were significantly reduced by steel slag and (consecutively applied) fly ash. For both amendments, decreased soil extractable Cd by increased pH was crucial in reducing grain Cd, but soil re-acidification diminished their effects. Increased soil extractable silicon played a key role in alleviating grain As accumulation. Steel slag had a more persistent effect on reducing grain Cd than fly ash but the sustainability of their effects on reducing grain As depended on rice cultivars. Steel slag improved soil fertility by increasing soil calcium, magnesium, manganese and zinc but chromium and nickel were also increased; it also enhanced the activities of soil urease and alkaline phosphatase, shifted soil bacterial community composition, and increased bacterial diversity. Fly ash had little effect on soil health. Our results indicated that steel slag had positive and sustainable effects on mitigating grain Cd and As accumulation but its potential negative impact on soil health requires in-depth monitoring. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal fungus (AMF) can enhance plant growth and resistance to toxicity produced by heavy metals (HMs), affect the bioavailability of HMs in soil and the uptake of HMs by plants, and thus has been emerged as the most prominent symbiotic fungus for contribution to phytoremediation. A greenhouse pot experiment was conducted to assess the effect of Glomus versiforme BGC GD01C (Gv) on the growth and Cd accumulation of Cd-hyperaccumulator Solanum nigrum in different Cd-added soils (0, 25, 50, 100 mg Cd kg −1 soil). Mycorrhizal colonization rates were generally high (from 71% to 82%) in Gv-inoculated treatments at all Cd levels. Gv colonization enhanced soil acid phosphatase activity, and hence elevated P acquisition and growth of S. nigrum at all Cd levels. Moreover, the presence of Gv significantly increased DTPA-extractable (phytoavailable) Cd concentrations in 25 and 50 mg Cd kg −1 soils, but did not affect phytoavailable Cd in 100 mg Cd kg −1 soil. Similarly, inoculation with Gv significantly increased Cd concentrations of S. nigrum in 25 and 50 mg Cd kg −1 soils, but decreased Cd concentrations of the plants in 100 mg Cd kg −1 soil. Overall, inoculation with Gv greatly improved the total Cd uptakes in all plant tissues at all Cd levels. The present results indicated that S. nigrum associated with Gv effectively improved the Cd uptake by plant and would be a new strategy in microbe-assisted phytoremediation for Cd-contaminated soils. [ABSTRACT FROM AUTHOR]