Screening the phytotoxicity of micro/nanoplastics through non-targeted metallomics with synchrotron radiation X-ray fluorescence and deep learning: Taking micro/nano polyethylene terephthalate as an example

Microplastics (MPs) and nanoplastics (NPs) are global pollutants with emerging concerns. Methods to predict and screen their toxicity are crucial. Elemental dyshomeostasis can be used to assess toxicity of environmental pollutants. Non-targeted metallomics, combining synchrotron radiation X-ray fluorescence (SRXRF) and machine learning, has successfully differentiated cancer patients from healthy individuals. The whole idea of this work is to screen the phytotoxicity of nano polyethylene terephthalate (nPET) and micro polyethylene terephthalate (mPET) through non-targeted metallomics with SRXRF and deep learning algorithms. Firstly, Seed germination, seedling growth, photosynthetic changes, and antioxidant activity were used to evaluate the toxicity of mPET and nPET. It was showed that nPET, at 10 mg/L, was more toxic to rice seedlings, inhibiting growth and impairing chlorophyll content, MDA content, and SOD activity compared to mPET. Then, rice seedling leaves exposed to nPET or mPET was examined with SRXRF, and the SRXRF data was differentiated with deep learning algorithms. It was showed that the one-dimensional convolutional neural network (1D-CNN) model achieved 98.99% accuracy without data preprocessing in screening mPET and nPET exposure. In all, non-targeted metallomics with SRXRF and 1D-CNN can effectively screen the exposure and phytotoxicity of nPET/mPET and potentially other emerging pollutants. Further research is needed to assess the phytotoxicity of different types of MPs/NPs using non-targeted metallomics.

Soil parameters including organic matter affecting pretilachlor leaching in different paddy fields

Finding a method, which may indicate the contribution of soil parameters including organic matter, pH and clay to pretilachlor leaching (persistence) in the soil, with the use of a suitable indicator, which indicates pretilachlor presence in the soil is of significance. Accordingly, using undisturbed soil columns, four paddy fields (A, B, C, D) in the suburbs of Babol city (Mazandaran province, northern Iran) were sampled before the preparation and irrigation of the fields in April 2021. Soil samples were transferred into PVC pipes (divided into 2 cm layers) measuring 12 (height) × 10 cm (diameter), and were injected with pretilachlor at the recommended (1.75 L/ha) and high doses (3.5 L/ha). The surface layers of all fields had a higher concentration of pretilachlor and organic matter with the highest contribution to pretilachlor persistence followed by clay and pH. In the 0-4 cm depth, herbicide concertation was the lowest in field A (139 mg/kg) and the highest in field C (161 mg/kg). The corresponding values for organic matter were equal to 1.88 and 5.68%, respectively. The bioassay of rice (the indicator plant), with a significant correlation with chemical analysis results, indicated field A and field C had the pretilachlor infiltration of 6 and 4 cm, respectively. Accordingly, rice is a suitable plant indicator for measuring the presence of pretilachlor as examining its shoot length is a good criterion for bioassay. Additionally, changes in the amount of organic matter in different soil layers can be used to predict the leaching level of pretilachlor.

Synergistic impacts of ferromanganese oxide biochar and optimized water management on reducing Cd accumulation in rice

Ferromanganese oxide biochar composite (FMBC) is an efficient remediation material for cadmium -contaminated soils. However, the effect of FMBC under varied water managements on the remediation of Cd-polluted soil is unclear. In this study, we conducted both incubation and field experiments to investigate the combined effects of corn-stover-derived biochar modified with ferromanganese on the immobilization and uptake of Cd by rice under continuous aerobic (A), aerobic-flooded (AF), and flooded-aerobic (FA) water management regimes. The results showed that loading iron-manganese significantly increased the maximum sorption capacity (Qm) of Cd on FMBC (50.46 mg g^-1) due to increased surface area, as compared to the pristine biochar (BC, 31.36 mg g^-1). The results revealed that soil Eh and pH were significantly affected by FMBC and it's synergistic application with different water regimes, thus causing significant differences in the concentrations of DTPA-extractable Cd under different treatments. The lowest DTPA-extractable Cd content (0.28-0.46 mg^-1) was observed in the treatment with FMBC (2.5 %) combined FA water amendment, which reduced the content of available Cd in soil by 2.63-28.4 %. Moreover, the treatments with FMBC-FA resulted the proportion of residual Cd increased by 22.2 % compared to the control. Variations in the content and fraction of Cd had a significant influence on its accumulation in the rice grains. The FMBC-FA treatments reduced the Cd concentration in roots, shoots and grains by 37.97 %, 33.98 %, and 53.66 %, respectively, when compared with the control. Predominantly because of the reduction in Cd biological toxicity and the improved soil nutrient content, the combined application increased the biomass and yield of rice to some extent. Taken together, the combination of the Fe-Mn modified biochar and flooded-aerobic water management may potentially be applied in Cd-polluted soil to mitigate the impacts of Cd on rice production.

Soil gallium speciation and resulting gallium uptake by rice plants

Gallium (Ga) is widely used in high-tech industries and is an emerging contaminant in the environment. This study aimed to determine Ga speciation in soils and Ga accumulation in rice plants (Oryza sativa L.) grown in three Ga-contaminated soils. The results showed that, among the soils, the acidic soil with a coarse texture had the highest soil Ga availability, which enhanced Ga uptake by rice roots. The Ga K-edge X-ray absorption near edge structure and sequential extraction results of the soils showed that the predominant species of Ga associated with iron hydroxides transformed to Ga(OH)₃ precipitates, and the residue fraction increased with rice-growing time, resulting in lower Ga uptake by rice roots in the second half period of rice cultivation. A large fraction of Ga was accumulated in the rice roots, with only a small portion of Ga was transferred to the shoots and then to the rice grains. This study revealed that Ga speciation in soil-rice plant systems varied during rice cultivation and determined soil Ga availability to rice plants. Gallium accumulated in rice grains is distributed homogenously in the endosperm of the grains, suggesting a potential risk to public health via the intake of rice grains harvested from Ga-contaminated paddy fields.

Unravelling the interactive effect of soil and atmospheric mercury influencing mercury distribution and accumulation in the soil-rice system

Mercury (Hg) accumulation in rice is an emerging health concern worldwide. However, sources and interactions responsible for Hg species accumulation in different rice tissues are still uncertain. Four experimental plots were carefully designed at an artisanal Hg mining site and a control site to evaluate the effect of atmospheric and soil Hg contents on Hg accumulation in rice. We showed that inorganic Hg (IHg) contents in rice tissues grown either in contaminated or control site soil (non-contaminated soil) were higher at Hg artisanal mining site than those at the control site. Elevated total gaseous mercury (TGM) levels in ambient air were the predominant source of IHg to rice at the Hg mining area. Methylmercury (MeHg) concentrations in rice plant tissues increased in proportionality with MeHg contents in paddy soil. Our results suggest that both atmosphere and soil Hg sources have been impacted the IHg accumulation in rice. Above ground rice tissues, grains, leaves, and stalk accumulated IHg from both atmosphere and soil to varying degrees. Nonetheless, the study also provides the first direct evidence that atmospheric Hg accumulated by above-ground rice tissues could be translocated to below-ground tissues (roots). However, the extent to which atmosphere or soil Hg contributes to IHg in rice tissues may vary with each source's concentration gradient at the given site. No evidence of in planta Hg methylation was found during the current study. Hence, paddy fields are potential MeHg production sites, whereas paddy soil is a unique MeHg accumulation source in rice plants. This study expands and clarifies the contribution of various sources involved in Hg accumulation in the soil rice system. The findings here provide the basis for future research strategies to deal with the global issue of Hg contaminated rice.

Identification of zinc pollution in rice plants based on two characteristic variables

Traditional chemical methods used to measure the zinc content in rice plants are time-consuming, laborious, requires reagents, and have a limited monitoring range, while the Raman spectroscopy method has the advantage of being fast, non-destructive, and requires no reagents. Unfortunately, the identification accuracy of the Raman partial least squares (PLS) model based on principal components is only 53.33%. To boost this, a One-Way ANOVA method was used to extract the characteristic variables in the Raman spectra. Based on these Raman variables, a model for identifying zinc stressed samples was established. The identification accuracy was improved to 70% but still fell short of the measurement requirements. To further enhance these results, the Raman spectrum was decomposed into components based on the Hilbert Vibration Decomposition (HVD) method. Using characteristic variables of the Raman spectrum and its HVD components to establish a PLS model, the identification accuracy of the test set is raised to 90.25%. These results are a significant improvement from those obtained using a model solely based on the Raman spectral characteristic variables, revealing that HVD components provide highly effective identification information. A Raman modeling method based on the characteristic variables of the HVD component is an innovative way for improving the accuracy of Raman detection, especially for the measurement of trace substances.

Depletion of bioavailable As/Cd with rice plant from paddy soils of high contamination risk

Co-uptake and high accumulation of As and Cd by rice is an outstanding issue threatening public health. From the viewpoint of soil cleanup, however, efficient As/Cd extraction by this paddy-adapted plant, followed by biomass removal, could provide a major pathway depleting As/Cd accumulation in paddy soils and thus inhibiting their transfer in food chain. Here a field trial was performed to identify the significance of As/Cd cleanup from paddy soil by rice. 88 % and 51 % of total extracted As and Cd were retained in root. To eliminate specifically rice-available As/Cd pool and obstruct their cycling back to soil, one crop of rice root was removed, leading to the depletion of 46 % and 69 % of plant available As (soluble & exchangeable) and Cd (exchangeable & carbonate-bound), respectively. In the following cultivation on the post-cleanup field, polished rice As fell from 0.23 mg kg^-1 to 0.12 mg kg^-1, markedly lower than the Chinese (WHO) limit (0.2 mg kg^-1). Meanwhile, white rice Cd decreased by 24 %. This field work identified that As/Cd co-extraction by paddy-adapted rice plant, followed by root removal, as a primary step toward rice safety in areas with high contamination risk but little reserved paddy resources.

Expression Alteration of Candidate Rice MiRNAs in Response to Sheath Blight Disease

Background:

MicroRNAs, as small non-coding RNAs, are recently reported to be involved in plant defense system against pathogens including fungi.

Objective:

In this research, it was intended to investigate candidate susceptible rice (Oryza Sativa) Osa-miRNA expression alteration following the infection by Rhizoctonia solani.

Materials and Methods:

To this aim, literature review suggested eight conserved plant miRNAs that are involved in other plant-pathogen interactions. Then, sixty days old rice plants (Hashemi, susceptible cultivar) were inoculated with R. solani and candidate miRNA expression alterations were investigated 2 hpi (hours post inoculation), 2 dpi (days post inoculation) and 6 dpi.

Results:

RT-qPCR analysis suggested four subgroups of candidate miRNAs based on the time of their responses to the pathogenesis of R. solani. While Osa-miR-156 was early-responsive, Osa-miR159 was the last-responsive and Osa-miR167, Osa-miR171, Osa-miR408, and Osa-miR444 were late responsive to R. solani infection. Osa-miR166 and Osa-miR393 were non-responsive to this infection, compared to the mock-inoculated control group. Consistently, Os-SPL3 and Os-MADS known target genes were expressed in reverse correlation to Osa-miR156 and Osa-miR444, respectively.

Conclusions:

From these data, it is suggested that both early (Osa-miR-156) and late (Osa-miR167, Osa-miR171, Osa- miR408, Osa-miR444) responsive miRNAs might be involved in R. solani infection in rice plants.

Fine mapping and characterization of two novel quantitative trait loci for early seedling growth in rice

Two novel QTLs for early seedling growth in rice were fine mapped, with one of which to a 4-kb identical to the known GW6a gene, and another one to a 43-kb region that contains six candidate genes. Leaves are extremely important for plant photosynthesis: the size and shape of which determine the rate of transpiration, carbon fixation and light interception, and their robust growth at seedling stage endow crops with the ability to compete with weeds. So far, many genes for the traits have been cloned with mutants; however, identification of those quantitative trait loci (QTLs) that control early seedling growth has seldom been reported. In this study, we report the identification of two QTLs, qLBL1 and qLBL2 on the rice chromosome 6 for leaf blade length at early seedling stage. Fine mapping revealed that qLBL1 was placed into a 4-kb, and qLBL2 was delimited to a 43-kb genomic interval. We further found that LBL1 was equivalent to the known grain-size gene GW6a and the qLBL2 region contains 6 candidate genes. Genetic analysis using nearly isogenic lines and transgenic rice plants revealed that both genetic factors were positive regulators. The genetic effects were mainly due to alterations of cell division by cytological observations. RT-qPCR results showed that LBL1 was preferentially expressed in leaf blades, and consistently, histochemical staining of pGW6a::GUS plants showed that GUS signal was strong in the vascular tissues of leaf blade of seedlings. Thus, we fine mapped and characterized two QTLs for early seedling growth and provided useful information to improve crop breeding.

Mercury in rice paddy fields and how does some agricultural activities affect the translocation and transformation of mercury - A critical review

Human exposure to methylmercury (MeHg) through rice consumption is raising health concerns. It has long been recognized that MeHg found in rice grain predominately originated from paddy soil. Anaerobic conditions in paddy fields promote Hg methylation, potentially leading to high MeHg concentrations in rice grain. Understanding the transformation and migration of Hg in the rice paddy system, as well as the effects of farming activities, are keys to assessing risks and developing potential mitigation strategies. Therefore, this review examines the current state of knowledge on: 1) sources of Hg in paddy fields; 2) how MeHg and inorganic Hg (IHg) are transformed (including abiotic and biotic processes); 3) how IHg and MeHg enter and translocate in rice plants; and 4) how regular farming activities (including the application of fertilizer, cultivation methods, choice of cultivar), affect Hg cycling in the paddy field system. Current issues and controversies on Hg transformation and migration in the paddy field system are also discussed.

Identification of qLG2, qLG8, and qWG2 as novel quantitative trait loci for grain shape and the allelic analysis in cultivated rice

Three novel QTLs for grain shape were genetically fine mapped, with two of which to a 250-kb target interval on rice chromosome 2 that contains fourteen candidate genes. Grain shape (grain length, width, and thickness) determines crop yield and grain quality. However, the trait is regulated by numerous naturally occurring quantitative trait loci (QTLs) and the underlying mechanism remains largely unknown. Here, we report the genetic mapping of three new QTLs, qLG2, qWG2, and qLG8 that each exerts a semi-dominant effect on grain shape in cultivated rice. These QTLs were validated using populations derived from the corresponding chromosome segment substitution lines (CSSLs), and were further delimited to small genomic intervals in progeny testing experiments. Especially, qLG2/qWG2 was placed into an about 250-kb genomic candidate region, and 14 predicted ORFs localized within the interval. We also evaluated the individual and pyramiding genetic effect(s) of these QTL(s) using the corresponding nearly isogenic lines, and found that they have additive effects on the traits. Collectively, these findings provided useful information as a tool to improve grain shape in crop breeding programs and established foundations for future QTL cloning.

Reducing arsenic in rice grains by leonardite and arsenic-resistant endophytic bacteria

Arsenic contaminated in rice plants can cause many physiological, biochemical and productivity in rice. This also had a negative impact on human health. To decrease arsenic in grains, a combination of leonardite as amendment and arsenic-resistance endophytic bacteria was investigated. The results showed that 1% (w/v) leonardite (91.86 ± 2.04%) had the highest efficiency in adsorbing initial arsenic concentration of 2 mg L^-1, which was higher than bagasse fly ash (16.25 ± 3.97%), rice husk ash (10.36 ± 1.28%), and sawdust fly ash (63.00 ± 5.67%) under the same condition. This was due to the higher aluminium and iron contents of leonardite strongly binding to arsenic anions. Meanwhile, Bacillus pumilus had an ability to decrease arsenic accumulation in rice grains to levels below those achieved by Pseudomonas sp. and Bacillus thuringiensis. This was possibly due to B. pumilus producing higher siderophore. Interestingly, a combination of microbe and leonardite addition could decrease arsenic accumulation in grains to below the permissible limit (0.2 mg As kg^-1 for inorganic arsenic). It could also reduce oxidative stress and showed down-regulation of Lsi1, Lsi2 and OsPT4 at the heading stage, which coincided with low arsenic and high silicon accumulation in roots. Therefore, this result could be used to decrease arsenic accumulation in grains in arsenic-contaminated paddy fields, improved rice plants defense and endured of arsenic stress, and increased rice productivity.

The effect of biochar nanoparticles on rice plant growth and the uptake of heavy metals: Implications for agronomic benefits and potential risk

The interaction between biochar nanoparticles (nano-BC) and plant roots in the rhizosphere is largely unknown, although it is crucial for understanding the role of BC in plant growth and bioavailability of pollutants. The effect of nano-BC produced at a series of temperatures (300-600 °C) on alleviating the phytotoxicity of Cd²⁺ to rice plants was investigated from the aspects of biochemical changes and Cd uptake in this study. The kinetics of Cd²⁺ fluxes in different root zones in the presence of nano-BC were also measured using a scanning ion-selective electrode technique. We found that the high-temperature nano-BC could more significantly alleviate the phytotoxicity of Cd²⁺ than the low-temperature and bulk BCs as reflected by the higher increased biomass, root vitality, chlorophyll content, and decreased MDA content as well as relative electrical conductivity of rice plants, which is due to the high adsorption affinity of nano-BC for Cd²⁺. Also, for the first time we demonstrated that nano-BC could differentially affect the net flux of Cd²⁺ in different zones of the root tips. However, nano-BC (especially that produced at higher temperatures) more significantly increased the contents of antioxidative enzyme activities (e. g., SOD, POD, and CAT) and soluble protein than the treatment only with Cd²⁺ (5.0 mg/L), indicating that nano-BC could induce oxidative stress in the rice plants. These results indicate that nano-BC could greatly reduce the uptake and phytotoxicity of Cd²⁺, but its potential risk should not be overlooked during the environmental and agricultural applications of biochar.

Effects of clay combined with moisture management on Cd immobilization and fertility index of polluted rice field

A field-scale trial was conducted to investigate the remediation effects of sepiolite on Cd-polluted paddy soils under different moisture managements, using a series of variables (pH and extractable Cd of soil, plant Cd concentration, plant nutrition and enzyme activity in soil). Results revealed that soil pH increased significantly after sepiolite addition, which promoted the reduction in extractable Cd in soil. After applying 0.5-2.5% sepiolite into soil, due to higher pH and lower TCLP Cd concentration, brown rice Cd reduced by 17-67% under continuous flooding, 14-62% under conventional irrigation, and 13-61% under wetting irrigation (p < 0.05). The activities of phosphatase and invertase increased compared with unamended soil (p < 0.05). The available phosphorus in clay treated soil showed a remarkable raise, with a maximum increase by 14.5%, 16.9% and 18.1% under three moisture managements (p < 0.05). The increasing values of enzyme activity and then plant nutrition in soil revealed that clay application improved the ecological condition of Cd-contaminated paddy soil. The sepiolite application in combination with continuous flooding provided an efficient and safe remediation technology for Cd-polluted rice field.

Effects of an additive (hydroxyapatite-biochar-zeolite) on the chemical speciation of Cd and As in paddy soils and their accumulation and translocation in rice plants

A pot experiment was carried out to investigate the remediation of paddy soils contaminated with both Cd and As. The effects of a soil additive (HZB: hydroxyapatite + zeolite + biochar) on the chemical speciation of Cd and As in the soil and on the accumulation and translocation of these two elements in rice plants were studied. The application of HZB decreased the concentration of acid-extractable Cd in the soil by 7.3-32.6% by promoting the transformation of soil Cd from soluble to insoluble species; as a result, the concentration of organically bound Cd in the soil increased by 6.8-49.5%. The application of HZB also promoted the transformation of soil As from soluble to insoluble species, thus increasing the concentration of Ca-bound As by 34.1-93.4% and reducing the concentration of soil-exchangeable As by 12.2-55.1%. However, when the application rate of HZB was greater than 4.0 g kg^-1, the concentration of soil-exchangeable As increased again. It was found that the application of HZB decreased the Cd and As bioaccumulation capacity of rice root, and among various rice organs, rice husk was the highest in Cd transferring capacity, whereas rice root was the highest in As transferring capacity. When the amount of HZB applied was 0.05-0.2%, the Cd and As concentrations in the various parts of the rice plants decreased significantly.

Mitigating arsenic contamination in rice plants with an aquatic fern, Marsilea minuta

Dangers of arsenic contamination are well known in human civilization. The threat increases when arsenic is accumulated in food and livestock through irrigated crops or animal food. Hence, it is important to mitigate the effects of arsenic as much as possible. This paper discusses a process for reducing the level of arsenic in different parts of rice plants with an aquatic fern, Marsilea minuta L. A pot experiment was done to study the possibility of using Marsilea minuta as a phytoremediator of arsenic. Rice and Marsilea minuta were allowed to grow together in soils. As a control, Marsilea minuta was also cultured alone in the presence and absence of arsenic (applied at 1 mg/L as irrigation water). We did not find any significant change in the growth of rice due to the association of Marsilea minuta, though it showed a reduction of approximately 58.64% arsenic accumulation in the roots of rice grown with the association of fern compared to that grown without fern. We measured a bioaccumulation factor (BF) of > 5.34, indicating that Marsilea minuta could be a good phytoremediator of arsenic in rice fields.

Use of clay to remediate cadmium contaminated soil under different water management regimes

We examined in situ remediation of sepiolite on cadmium-polluted soils with diverse water regimes, and several variables including brown rice Cd, exchangeable Cd, pH, and available Fe/P. pH, available Fe/P in soils increased gradually during continuous flooding, which contributed to Cd absorption on colloids. In control group (untreated soils), compared to conventional irrigation, brown rice Cd in continuous flooding reduced by 37.9%, and that in wetting irrigation increased by 31.0% (p<0.05). In contrast to corresponding controls, brown rice Cd in sepiolite treated soils reduced by 44.4%, 34.5% and 36.8% under continuous flooding, conventional irrigation and wetting irrigation (p<0.05), and exchangeable Cd in amended soils reduced by 27.5-49.0%, 14.3-40.5%, and 24.9-32.8% under three water management regimes (p<0.05). Compared to corresponding controls, decreasing amplitudes of exchangeable Cd and brown rice Cd in sepiolite treated soils were higher in continuous flooding than in conventional irrigation and wetting irrigation. Continuous flooding management promoted soil Cd immobilization by sepiolite.

Risk assessment and vertical distribution of thallium in paddy soils and uptake in rice plants irrigated with acid mine drainage

The objective of this paper is to assess the influence of irritating paddy fields with acid mine drainage containing thallium (Tl) to rice plant-soil system and potential health risks for local residents. Vertical distribution of Tl, pH, organic matter (OM), and cation exchange capacity (CEC) in 24 paddy soil profiles around Yunfu pyrite mine area was investigated. Rice plant samples were collected from the corresponding soil sampling site. The results showed that Tl concentrations in paddy soils at 0-60 cm depth range from 3.07 to 9.42 mg kg^-1, with a mean of 5.74 mg kg^-1, which were significantly higher than the background value of soil in China (0.58 mg kg^-1). On the whole, Tl contents in paddy soil profiles increased quickly with soil depth from 0 to 30 cm and decreased slowly with soil depth from 30 to 60 cm. The soil Tl content was significant negatively correlated with soil pH. The mean content of Tl in the root, stem, leaf, and rice was 4.36, 1.83, 2.74, and 1.42 mg kg^-1, respectively, which exceeded the proposed permissible limits for foods and feedstuffs in Germany. The Tl content in various tissues of the rice plants followed the order root > leaf > stem (rice), which suggested that most Tl taken up by rice plants retained in the root, and a little migrated to the leaf, stem, and rice. Correlation analysis showed that Tl content in root was significant positively correlated with Tl content in leaf and rice. The ranges of hazard quotient (HQ) values were 4.08∼24.50 and 3.84∼22.38 for males and females, respectively. Males have higher health risk than females in the same age group. In childhood age groups (2 to <21 years) and adult age groups (21 to <70 years), the highest health risk level was observed in the 11 to 16 age group and 21 to 50 age group, respectively. The findings indicated that regular irrigation with Tl-bearing acid mine drainage led to considerable contamination of Tl in paddy soil and rice plant. Local government should take various measures to treat Tl contamination, especially the tailings.

Impacts of C-uptake by plants on the spatial distribution of 14C accumulated in vegetation around a nuclear facility-Application of a sophisticated land surface 14C model to the Rokkasho reprocessing plant, Japan

The impacts of carbon uptake by plants on the spatial distribution of radiocarbon (¹⁴C) accumulated in vegetation around a nuclear facility were investigated by numerical simulations using a sophisticated land surface ¹⁴C model (SOLVEG-II). In the simulation, SOLVEG-II was combined with a mesoscale meteorological model and an atmospheric dispersion model. The model combination was applied to simulate the transfer of ¹⁴CO₂ and to assess the radiological impact of ¹⁴C accumulation in rice grains during test operations of the Rokkasho reprocessing plant (RRP), Japan, in 2007. The calculated ¹⁴C-specific activities in rice grains agreed with the observed activities in paddy fields around the RRP within a factor of four. The annual effective dose delivered from ¹⁴C in the rice grain was estimated to be less than 0.7 μSv, only 0.07% of the annual effective dose limit of 1 mSv for the public. Numerical experiments of hypothetical continuous atmospheric ¹⁴CO₂ release from the RRP showed that the ¹⁴C-specific activities of rice plants at harvest differed from the annual mean activities in the air. The difference was attributed to seasonal variations in the atmospheric ¹⁴CO₂ concentration and the growth of the rice plant. Accumulation of ¹⁴C in the rice plant significantly increased when ¹⁴CO₂ releases were limited during daytime hours, compared with the results observed during the nighttime. These results indicated that plant growth stages and diurnal photosynthesis should be considered in predictions of the ingestion dose of ¹⁴C for long-term chronic releases and short-term diurnal releases of ¹⁴CO₂, respectively.

Suppression of Magnaporthe oryzae and interaction between Bacillus subtilis and rice plants in the control of rice blast

Magnaporthe oryzae, the causative pathogen of rice blast, has caused extensive losses to rice cultivation worldwide. Strains of the bacterium Bacillus subtilis have been used as biocontrol agents against rice blast. However, little has been reported about the interaction between B. subtilis and the rice plant and its mechanism of action. Here, the colonization process and induced disease resistance by B. subtilis SYX04 and SYX20 in rice plants was examined. Strains of B. subtilis labeled with green fluorescent protein reached population of more than 5 × 10⁶ CFU/g after 20 days on mature rice leaves and were detected after 3 days on newly grown leaves. Results showed that SYX04 and SYX20 not only inhibited spore germination, germ tube length, and appressorial formation but also caused a series of alterations in the structures of hyphae and conidia. The cell walls and membrane structures of the fungus showed ultrastructural abnormalities, which became severely degraded as observed through scanning electron microscopy and transmission electron microscopy. The mixture of both B. subtilis and M. oryzae resulted in enhanced activity of peroxidase, and polyphenol oxidase while there was significantly more superoxide dismutase activity in plants that had been sprayed with B.subtilis alone. The present study suggests that colonized SYX04 and SYX20 strains protected rice plants and exhibited antifungal activity and induced systemic resistance, thus indicating their potential biological control agents.