Copper oxide nanoparticles and arsenic interact to alter seedling growth of rice (Oryza sativa japonica)

Harnessing the potential of copper-based nanoparticles in mitigating abiotic and biotic stresses in crops

The demand for crops production continues to intensify with the rapid increase in population. Agricultural crops continue to encounter abiotic and biotic stresses, which can substantially hamper their productivity. Numerous strategies have been focused to tackle the abiotic and biotic stress factors in various plants. Nanotechnology has displayed great potential to minimize the phytotoxic impacts of these environmental constraints. Copper (Cu)-based nanoparticles (NPs) have displayed beneficial effects on plant growth and stress tolerance. Cu-based NPs alone or in combination with plant growth hormones or microorganisms have been documented to induce plant tolerance and mitigate abiotic or biotic stresses in different plants. In this review, we have comprehensively discussed the uptake and translocation of Cu-based NPs in plants, and beneficial roles in improving the plant growth and development at various growth stages. Moreover, we have discussed how Cu-based NPs mechanistically modulate the physiological, biochemical, metabolic, cellular, and metabolic functions to enhance plant tolerance against both biotic (viruses, bacterial and fungal diseases, etc.) and abiotic stresses (heavy metals or metalloids, salt, and drought stress, etc.). We elucidated recent advancements, knowledge gaps, and recommendations for future research. This review would help plant and soil scientists to adapt Cu-based novel strategies such as nanofertilizers and nanopesticides to detoxify the abiotic or biotic stresses. These outcomes may contribute to the promotion of healthy food production and food security, thus providing new avenues for sustainable agriculture production.

Copper oxide nanoparticles mitigate cadmium toxicity in rice seedlings through multiple physiological mechanisms

Heavy metal pollution poses a serious threat to crops growth and yield. Recently, nanoparticles (NPs) have emerged as a promising strategy to mitigate the negative effect of heavy metal on crop growth. This study investigated the beneficial effects of copper oxide nanoparticles (CuO NPs) on the morphological and physiological-biochemical traits of rice seedlings (Oryza sativa L.) under cadmium (Cd) stress. The results demonstrated that the application of CuO NPs increased the contents of nutrition elements in shoots and roots as well as photosynthetic pigments, consequently improving the growth of rice seedlings under Cd stress, especially at low level of Cd stress. Meanwhile, CuO NPs obviously decreased the Cd accumulation in the rice seedlings and immobilized Cd in less toxic chemical forms and subcellular compartments. Moreover, CuO NPs modulated the antioxidant system, ameliorating oxidative damage and membrane injury caused by Cd. Multivariate analysis established correlations between physio-biochemical parameters and further revealed the mitigation of Cd damage to rice seedlings by CuO NPs was associated with inhibition Cd accumulation, altering Cd chemical form and subcellular distribution, increasing the contents of mineral nutrients, photosynthetic pigments and secondary metabolites and antioxidant enzyme activities, and reducing oxidative damage. Overall, the present study indicated that CuO NPs could effectively reduce the Cd toxicity to rice seedlings, demonstrating their potential application in agricultural production.

Unraveling the contribution of copper seed priming in enhancing chromium tolerance in wheat by improving germination, growth, and grain yield

Exposure to chromium (Cr) on farmlands drastically restricts the growth and productivity of cereal crops, including wheat (Triticum aestivum L.). Utilizing micronutrients, the seed-priming strategy is crucial to preventing the adverse consequences of Cr-stress. Nevertheless, additional investigation needs to be conducted to figure out whether Cu-priming remedies are beneficial for wheat experiencing Cr-stress. The objective of this study was to ascertain the contribution of Cu-treated seed priming in the mitigation of detrimental impacts of Cr-stress on wheat germination, growth, and production. Two wheat cultivars, Dilkash-20 and Subhani-21, were subjected to seed priming treatments (0 mg/L, 0.1 mg/L, and 1.0 mg/L) of Cu under Cr-stress levels (200 mg/kg) in two successive experiments, respectively, petri-dish and soil-filled pot experiments. The Cu-priming significantly enhanced the wheat seed germination, plant growth, and grain yield under Cr-stress. Cu priming improved enzyme activities such as glutathione peroxidase (14.60, 16.30%), superoxide dismutase (62.55, 115.21%), peroxidase, catalase (78.39, 80.23%), ascorbate peroxidase(17.72, 20.32%), and key primary and secondary metabolites such as proline (54.19, 81.27%), glycine betaine (40.13, 79.39%), total soluble proteins (47.92, 51.58%), phenolics (40.05, 18.61%), and flavonoids (56.90, 113.46%), respectively, of Dilkash-20 and Subhani-21 under Cr-stress. The outcome of our investigation underscored the efficacy of Cu-priming treatments (0.1 mg/L and 1.0 mg/L) in Cr-stress circumstances to augment wheat germination, growth, and grain yield.

Magnesium oxide nanoparticles alleviate arsenic toxicity, reduce oxidative stress and arsenic accumulation in rice (Oryza sativa L.)

Magnesium oxide nanoparticles (MgO NPs) have been attracted by the scientific community for their combating action against heavy metal stress in plants. However, their role towards the mitigation of arsenic (As) induced toxicity is still obscure. In the present study, MgO NPs were synthesized through the green route and assessed their efficacy towards the reduction of As accumulation and phytotoxicity in As-stressed rice cultivar MTU-1010 under laboratory conditions. Initially, rice seedlings were grown under separate and combined applications of As (10 mg/L) and MgO NPs (0, 10, 50, and 100 mg/L) and further analyzed plant growth attributes and As accumulation in rice seedlings. Characterization of biosynthesized MgO NPs by UV-Vis spectrophotometer, transmission electron microscopy (TEM), scanning electron microscopy with energy-dispersive X-ray analysis (SEM-EDX), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analysis showed the cubic in shape, and crystalline nature (73.10%) with average size ranges from 17-23 nm. The growth experiment showed a significant (p < 0.05) increase in seed germination, seedling growth, photosynthetic and other pigments content, and biomass accumulation in rice seedlings under the combined application of As (10 mg/L) and MgO NPs (50 mg/L) as compared to only As (10 mg/L) treatment. Additionally, As exposure resulted in declined primary metabolites such as soluble sugars and protein. However, the application of MgO NPs exhibited the alleviation of As toxicity through significant (p < 0.05) reduction of As accumulation by 34 and 53% in roots and 44 and 62% in shoots of rice seedlings under 50 and 100 mg/L MgO NPs supplementations, respectively and restored the accumulation of the primary metabolites. Furthermore, MgO NPs demonstrated the ability to scavenge reactive oxygen species (ROS) like hydrogen peroxide (H2O2) and superoxide anion (O2•-), through significant (p < 0.05) promotion of non-enzymatic (carotenoid, anthocyanin, flavonoid, and proline) and enzymatic (CAT, POD, and SOD) antioxidant defence under As stress. These findings highlighted the potential of green synthesized MgO NPs towards the mitigation of As contamination in rice plants. However, future study is necessary to unfold the actual mechanisms responsible for the protective effects of MgO NPs and to screen out the optimal dose to be used to formulate a potent nanofertilizer for sustainable rice production in metal-contaminated soils.

Potential Effects of Metal Oxides on Agricultural Production of Rice: A Mini Review

The extensive usage of metal oxide nanoparticles has aided in the spread and accumulation of these nanoparticles in the environment, potentially endangering both human health and the agroecological system. This research describes in detail the hazardous and advantageous impacts of common metal oxide nanomaterials, such as iron oxide, copper oxide, and zinc oxide, on the life cycle of rice. In-depth analyses are conducted on the transport patterns of nanoparticles in rice, the plant's reaction to stress, the reduction of heavy metal stress, and the improvement of rice quality by metal oxide nanoparticles, all of which are of significant interest in this subject. It is emphasized that from the perspective of advancing the field of nanoagriculture, the next stage of research should focus more on the molecular mechanisms of the effects of metal oxide nanoparticles on rice and the effects of combined use with other biological media. The limitations of the lack of existing studies on the effects of metal oxide nanomaterials on the entire life cycle of rice have been clearly pointed out.

Prediction models on biomass and yield of rice affected by metal (oxide) nanoparticles using nano-specific descriptors

The use of in silico tools to investigate the interactions between metal (oxide) nanoparticles (NPs) and plant biological responses is preferred because it allows us to understand molecular mechanisms and improve prediction efficiency by saving time, labor, and cost. In this study, four models (C5.0 decision tree, discriminant function analysis, random forest, and stepwise multiple linear regression analysis) were applied to predict the effect of NPs on rice biomass and yield. Nano-specific descriptors (size-dependent molecular descriptors and image-based descriptors) were introduced to estimate the behavior of NPs in plants to appropriately represent the wide space of NPs. The results showed that size-dependent molecular descriptors (e.g., E-state and connectivity indices) and image-based descriptors (e.g., extension, area, and minimum ferret diameter) were associated with the behavior of NPs in rice. The performance of the constructed models was within acceptable ranges (correlation coefficient ranged from 0.752 to 0.847 for biomass and from 0.803 to 0.905 for yield, while the accuracy ranged from 64% to 77% for biomass and 81% to 89% for yield). The developed model can be used to quickly and efficiently evaluate the impact of NPs under a wide range of experimental conditions and sufficient training data.

Micronutrient seed priming: new insights in ameliorating heavy metal stress

Plants need to survive with changing environmental conditions, be it different accessibility to water or nutrients, or attack by insects or pathogens. Few of these changes, especially heavy metal stress, can become more stressful and needed strong countermeasures to ensure survival of plants. Priming, a pre-sowing hydration treatment, involves pre-exposure of plants to an eliciting component which enhance the plant's tolerance to later stress events. By considering the role of micronutrients in aiding plants to cope up under adverse conditions, this review addresses various aspects of micronutrient seed priming in attenuating heavy metal stress. Priming using micronutrients is an adaptive strategy that boosts the defensive capacity of the plant by accumulating several active or inactive signaling proteins, which hold considerable importance in signal amplification against the triggered stimulus. Priming induced 'defence memory' persists in both present generation and its progeny. Therefore, it is considered a promising approach by seed technologist for commercial seed lots to enhance the vigour in terms of seed germination potential, productivity and strengthening resistance response against metalloid stress. The present review provides an overview regarding the potency of priming with micronutrient to ameliorate harmful effects of heavy metal stress, possible mechanism how attenuation is accomplished, role of priming in enhancing crop productivity and inducing defence memory against the metalloid stress stimulus.

Effects of Silicon and Iron Application on Arsenic Absorption and Physiological Characteristics of Rice (Oryza sativa L.)

This study investigated the effects of silicon and iron on arsenic absorption, as well as the changes in the physiological indices of rice under arsenic stress and how these indices respond to silicon and iron. We found that application of silicon and iron reduces arsenic absorption in rice; co-application of silicon and iron reduced arsenic content by 25.6%-27.4%. The antioxidant enzyme activities of rice treated with silicon and iron were significantly lower than those of untreated rice, with the biggest decreases observed under co-application treatments. Iron significantly increased osmoregulatory substances, while silicon increased soluble sugar; Si1Fe1 treatment (containing 1 mM silicon, 0.1 mM iron) had the highest content of osmoregulatory substances except CK. Membership function analysis suggested that applying silicon and iron alone alleviates the stress condition in rice, with the lowest stresses observed under Si1Fe1 treatment. These results show that silicon and iron co-application significantly inhibits arsenic uptake in rice, decreases the antioxidant enzymes activity, while non-enzymatic substances in rice can be regulated to further alleviate arsenic stress.

Nanofertilizer Possibilities for Healthy Soil, Water, and Food in Future: An Overview

Conventional fertilizers and pesticides are not sustainable for multiple reasons, including high delivery and usage inefficiency, considerable energy, and water inputs with adverse impact on the agroecosystem. Achieving and maintaining optimal food security is a global task that initiates agricultural approaches to be revolutionized effectively on time, as adversities in climate change, population growth, and loss of arable land may increase. Recent approaches based on nanotechnology may improve in vivo nutrient delivery to ensure the distribution of nutrients precisely, as nanoengineered particles may improve crop growth and productivity. The underlying mechanistic processes are yet to be unlayered because in coming years, the major task may be to develop novel and efficient nutrient uses in agriculture with nutrient use efficiency (NUE) to acquire optimal crop yield with ecological biodiversity, sustainable agricultural production, and agricultural socio-economy. This study highlights the potential of nanofertilizers in agricultural crops for improved plant performance productivity in case subjected to abiotic stress conditions.

Melatonin alleviates copper stress to promote rice seed germination and seedling growth via crosstalk among various defensive response pathways

Copper (Cu) contamination dramatically affects crop growth and thus threatens crop production; while applications of melatonin (MT) serve as an effective way to tolerate Cu stress for plant development, the underlying mechanism remains largely unknown in rice. Here, we found that Cu toxicity remarkably decreased germination rates and seedling growth compared to the untreated control (CK), while seed priming with a solution of 100 μM MT significantly alleviated the adverse effects on Cu-stressed seeds. In addition, the MT treatment decreased the accumulation of Cu in seedlings at 7 days after imbibition (DAI), possibly through enhanced Cu sequestration, and improved reserve mobilization through the promoted activity of α-amylase and protease in seeds under Cu stress. Interestingly, gibberellin (GA) synthesis was restored to or even exceeded the CK levels in the MT presoaking treatment, while the abscisic acid (ABA) content decreased compared to those of the Cu-stressed seeds, suggesting crosstalk between MT and other phytohormones, e.g., GA and ABA. More importantly, MT pretreatment also significantly promoted the growth of postgermination seedlings. This was largely ascribed to the MT-ameliorated antioxidant system, which consequently reduced the accumulation of Cu stress-induced oxidative products, e.g., hydrogen peroxide (H₂O₂), malondialdehyde (MDA), and superoxide (O₂·^_). Collectively, these results demonstrate that seed priming with MT could greatly mitigate the adverse effects of Cu stress on seed germination and subsequent postgermination growth through crosstalk among various defensive response pathways. This study provides vital guidance for applications of MT in agronomic production.

Nanoparticles as a potential protective agent for arsenic toxicity alleviation in plants

Aggrandized technological and industrial progression in past decades have occasioned immense depreciation in the quality of environment and ecosystem, majorly due to augmentation in the number of obnoxious pollutants incessantly being released in soil, water or air. Arsenic (As) is one such hazardous metalloid contaminating the environment which has the potential to detrimentally affect the life on earth. Even in minute quantity, As is known to cause various critical diseases in humans and toxicity in plants. Recent studies on nanoparticles (NPs) approve of their ability to qualify the criterion of becoming a potent tool for mitigating As-induced phytotoxicity. Nanoparticles are reported to promote plant growth under As-stress by stimulating various alterations at physiological, biochemical, and molecular levels. In this review, we provide an up-to-date compilation of research that has been carried out in comprehending the mechanisms utilized by nanoparticles including controlled As uptake and distribution in plants, maintenance of ROS homeostasis during stress and chelation and vacuolar sequestration of As so as to reduce the severity of toxicity induced by As, and potential areas of research in this field will also be indicated for future perspectives.

Impact of engineered nanomaterials on rice (Oryza sativa L.): A critical review of current knowledge

After use, a large number of engineered materials (ENMs) are directly or indirectly released into the environment. This may threaten the agricultural ecosystem, especially with crops under high demand for irrigation water, such as rice (Oryza sativa L.), a crop that feeds nearly half of the world's population. However, consistent and detailed information on the effects of nanoparticles in rice is limited. This review is a systematic exploration of the effects of ENMs on rice, with a critical evaluation of the mechanisms reported in the literature by which different nanomaterials cause toxicity in rice. The physiological and biochemical effects engendered by the nanoparticles are highlighted, focusing on rice growth and development, ENMs uptake and translocation, gene expression changes, enzyme activity modifications, and secondary metabolite alterations.

Recent advances in responses of arbuscular mycorrhizal fungi - Plant symbiosis to engineered nanoparticles

The application of engineered nanomaterials (ENMs) is increasing in all walks of life, inevitably resulting in a high risk of ENMs entering the natural environment. Recent studies have demonstrated that phytoaccumulation of ENMs in the environment may be detrimental to plants to varying degrees. However, plants primarily assimilate ENMs through the roots, which are inevitably affected by rhizomicroorganisms. In this review, we focus on a group of common rhizomicroorganisms-arbuscular mycorrhizal fungi (AMF). These fungi contribute to ENMs immobilization and inhibition of phytoaccumulation, improvement of host plant growth and activation of systematic protection in response to excess ENMs stress. In present review, we summarize the biological responses of plants to ENMs and the modulatory mechanisms of AMF on the immobilization of ENMs in substrate-plant interfaces, and indirectly regulatory mechanisms of AMF on the deleterious effects of ENMs on host plants. In addition, the information of feedback of ENMs on mycorrhizal symbiosis and the prospects of future research on the fate and mechanism of phyto-toxicity of ENMs mediated by AMF in the environment are also addressed. In view of above, synergistic reaction of plants and AMF may prove to be a cost-effective and eco-friendly technology to bio-control potential ENMs contamination on a sustainable basis.

Regulatory Mechanism of Copper Oxide Nanoparticles on Uptake of Different Species of Arsenic in Rice

Copper oxide nanoparticles (CuO NPs) are widely used as a fungicide in agriculture. The application of CuO NPs in agriculture affects the growth of rice and metal accumulation in rice. However, the mechanism of CuO NPs on arsenic (As) accumulation in rice remains unclear. In this study, a hydroponic culture was produced to investigate the mechanism of the effect of 50 and 100 mg L-1 CuO NPs on As accumulation in rice. Our results showed that CuO NPs decreased As(III/V) accumulation in the roots and shoots by adsorbing As(III/V), oxidizing of As(III) on the surface, and thickening the root cell wall. The addition of CuO NPs regulated the expression of the OsNIP1;1, OsHAC1;1, and OsHAC4 genes, which decreased As(III) transport and promoted As(V) reduction in the roots. Moreover, when CuO NPs were co-exposed to As, a negative correlation between the concentration of Cu and As in rice was also found in our study. However, CuO NPs significantly increased Cu accumulation in rice and constrained the rice growth. In conclusion, CuO NPs might be a promising way to decrease As accumulation in rice, but the negative effects such as growth inhibition should be further considered. Therefore, the application of CuO NPs in rice plants should take a more restrained approach.

Mobility of arsenic in the growth media of rice plants (Oryza sativa subsp. japonica. 'Koshihikari') with exposure to copper oxide nanoparticles in a life-cycle greenhouse study

The increasing arsenic (As) concentration in agriculture media poses increasing risks to both environment and human health. Arsenic mobility determines its bioavailability and entry into the food chain. Nanoparticle application may help to control As mobility in crop cultivation media, and thus decreasing As bioavailability for plants. This research studied the adsorption kinetics of As(V) on copper oxide nanoparticles (nCuO) and nCuO dissolution in a hydroponic solution, and the effects of nCuO on As mobility in a greenhouse system exposed to As(V) addition of 10 mg/kg and nCuO at 0.1-100 mg/L for a life-cycle growth of rice. Arsenic adsorption was dependent on both the total mass and the concentration of nCuO as well as the initial concentration of As(V), while nCuO dissolution was mainly dependent on nCuO concentration regardless of As(V). Arsenic in the simulated paddy was quickly mobilized from soil to aqueous phase during week 1, and further interacted with components in water phase, sediment-water interfacial transition and rice plants. Copper (Cu) and As speciation in the soil were observed by X-Ray Absorption Near Edge Spectrometry. Dissolved Cu was complexed with organic ligands. As(V) was adsorbed to kaolinite, or reduced to As(III) and adsorbed to ferrihydrite. Percent As removal from water phase in the growth container was determined by both nCuO application and As(V) initial concentration. Based on our previous finding that As accumulation in rice grains was significantly decreased by nCuO at 50 mg/L and the results of this study on As adsorption capacity of nCuO and As removal from water due to nCuO application, nCuO at 50 mg/L was proposed to be an appropriate application in rice paddy to immobilize As. Further research is needed in actual agriculture to verify the appropriate nCuO application and get an integrated beneficial effect for rice plants and humans.

Copper: uptake, toxicity and tolerance in plants and management of Cu-contaminated soil

Copper (Cu) is an essential mineral nutrient for the proper growth and development of plants; it is involved in myriad morphological, physiological, and biochemical processes. Copper acts as a cofactor in various enzymes and performs essential roles in photosynthesis, respiration and the electron transport chain, and is a structural component of defense genes. Excess Cu, however, imparts negative effects on plant growth and productivity. Many studies have summarized the adverse effects of excess Cu on germination, growth, photosynthesis, and antioxidant response in agricultural crops. Its inhibitory influence on mineral nutrition, chlorophyll biosynthesis, and antioxidant enzyme activity has been verified. The current review focuses on the availability and uptake of Cu by plants. The toxic effects of excess Cu on seed germination, plant growth and development, photosynthesis, and antioxidant response in plants are discussed. Plant tolerance mechanisms against Cu stress, and management of Cu-contaminated soils are presented.

Graphitic Carbon Nitride (C3N4) Reduces Cadmium and Arsenic Phytotoxicity and Accumulation in Rice (Oryza sativa L.)

The present study investigated the role of graphitic carbon nitride (C₃N₄) in alleviating cadmium (Cd)- and arsenic (As)-induced phytotoxicity to rice (Oryza sativa L.). A high-temperature pyrolysis was used to synthesize the C₃N₄, which was characterized by transmission electron microscopy, Fourier-transform infrared spectroscopy, and dynamic light scattering. Rice seedlings were exposed to C₃N₄ at 50 and 250 mg/L in half-strength Hoagland’s solution amended with or without 10 mg/L Cd or As for 14 days. Both Cd and As alone resulted in 26–38% and 49–56% decreases in rice root and shoot biomass, respectively. Exposure to 250 mg/L C₃N₄ alone increased the root and shoot fresh biomass by 17.5% and 25.9%, respectively. Upon coexposure, Cd + C₃N₄ and As + C₃N₄ alleviated the heavy metal-induced phytotoxicity and increased the fresh weight by 26–38% and 49–56%, respectively. Further, the addition of C₃N₄ decreased Cd and As accumulation in the roots by 32% and 25%, respectively, whereas the metal contents in the shoots were 30% lower in the presence of C₃N₄. Both As and Cd also significantly altered the macronutrient (K, P, Ca, S, and Mg) and micronutrient (Cu, Fe, Zn, and Mn) contents in rice, but these alterations were not evident in plants coexposed to C₃N₄. Random amplified polymorphic DNA analysis suggests that Cd significantly altered the genomic DNA of rice roots, while no difference was found in shoots. The presence of C₃N₄ controlled Cd and As uptake in rice by regulating transport-related genes. For example, the relative expression of the Cd transporter OsIRT1 in roots was upregulated by approximately threefold with metal exposure, but C₃N₄ coamendment lowered the expression. Similar results were evident in the expression of the As transporter OsNIP1;1 in roots. Overall, these findings facilitate the understanding of the underlying mechanisms by which carbon-based nanomaterials alleviate contaminant-induced phyto- and genotoxicity and may provide a new strategy for the reduction of heavy metal contamination in agriculture.

Copper uptake, essentiality, toxicity, detoxification and risk assessment in soil-plant environment

Copper (Cu) is an essential metal for human, animals and plants, although it is also potentially toxic above supra-optimal levels. In plants, Cu is an essential cofactor of numerous metalloproteins and is involved in several biochemical and physiological processes. However, excess of Cu induces oxidative stress inside plants via enhanced production of reactive oxygen species (ROS). Owing to its dual nature (essential and a potential toxicity), this metal involves a complex network of uptake, sequestration and transport, essentiality, toxicity and detoxification inside the plants. Therefore, it is vital to monitor the biogeo-physiochemical behavior of Cu in soil-plant-human systems keeping in view its possible essential and toxic roles. This review critically highlights the latest understanding of (i) Cu adsorption/desorption in soil (ii) accumulation in plants, (iii) phytotoxicity, (iv) tolerance mechanisms inside plants and (v) health risk assessment. The Cu-mediated oxidative stress and resulting up-regulation of several enzymatic and non-enzymatic antioxidants have been deliberated at molecular and cellular levels. Moreover, the role of various transporter proteins in Cu uptake and its proper transportation to target metalloproteins is critically discussed. The review also delineates Cu build-up in plant food and accompanying health disorders. Finally, this review proposes some future perspectives regarding Cu biochemistry inside plants. The review, to a large extent, presents a complete picture of the biogeo-physiochemical behavior of Cu in soil-plant-human systems supported with up-to-date 10 tables and 5 figures. It can be of great interest for post-graduate level students, scientists, industrialists, policymakers and regulatory authorities.

Removal of labile arsenic from flooded paddy soils with a novel extractive column loaded with quartz-supported nanoscale zero-valent iron

Efficient removal of labile arsenic (As) from paddy soil is a fundamental pathway mitigating As accumulation in rice from a long-term perspective. In this study, a porous and pencil-shaped column prepacked with quartz-supported nanoscale zero-valent iron (NZVI) was designed to extract elevated porewater As from paddy soil under flooded condition. With fine quartz as supporting medium in the core layer, only 0.07% out-migration of the loaded NZVI occurred in arsenite As(III) solution. At pH 5-9, removal of aqueous As(III) with NZVI-column was 73-78%, while silicic acid and phosphate at their environmentally realistic concentrations exhibited 27-30% and 14-17% inhibition on As(III) extraction, respectively. For two paddy soils with slight (S-As) and moderate (M-As) As contamination, four cycles of intermittent extraction with NZVI-column induced steady and marked decrease in porewater As. By the end of four successive extractions, profiles of DGT-labile As in S-As and M-As soils decreased by 22% and 29% on average with simultaneous decline of the most available fraction of soil As (including soluble and exchangeable fraction) by 26% and 17%, respectively. For the post-extracted two soils, As accumulation of rice seedlings declined by 29-57% than those in control. These results identify the effectiveness of NZVI-column in extracting elevated labile As from paddy soils with the aid of flooding. Targeting fast removal of high porewater As, column-extraction could serve as the first step in "remediation train" of paddy soils with relatively high As to shorten cleanup time by rendering much lowered soil As burden for the following phytoextraction and other measures.

Exposure to Copper Oxide Nanoparticles and Arsenic Causes Intergenerational Effects on Rice (Oryza sativa japonica Koshihikari) Seed Germination and Seedling Growth

Offspring generation (F1) rice (Oryza sativa japonica Koshihikari) seed germination and seedling growth tests were conducted for 18 d to investigate intergenerational effects of arsenic (As) and copper oxide nanoparticles (nCuO), with seeds harvested from a life cycle study exposed to As (0 and 10 mg/kg) and nCuO (0, 0.1, 1.0, 10, 50, and 100 mg/L). Seed germination and seedling growth of F1 plants were influenced by treatments experienced by parent generation (F0) plants (p < 0.05). Seeds produced from plants in F0 treatment with nCuO 50 mg/L had the lowest germination percentage and shortest seedling shoot length and root length in F1 control (F1C) and As at 10 mg/kg (F1As) alone treatments (p < 0.05). The shoot length and root length were decreased, whereas the number of root branches was increased in F1As treatment compared with F1C (p < 0.001). Interaction of As and nCuO also caused differential seed germination and seedling growth at various nCuO concentrations in quasi-F0 treatment (seeds receiving the same exposure as F0 plants; p < 0.05). Copper and As uptake in F1C seedlings were not affected by seeds' F0 exposure; this indicated that the transgenerational effects on rice seedling growth were not dependent on total Cu or As uptake in seedlings. The enhanced effects on seedlings from quasi-F0 treatment were influenced by additional exposure to nCuO and As that also interacted to affect Cu and As uptake in seedlings. Environ Toxicol Chem 2019;38:1978-1987. © 2019 SETAC.

Liquid assisted pulsed laser ablation synthesized copper oxide nanoparticles (CuO-NPs) and their differential impact on rice seedlings

Hydroponic experiments were conducted to investigate impact of laser ablated copper oxide nanoparticles (CuO-NPs) on rice seedlings. The present work demonstrates that exposure of lower concentrations (5, 10, 20, and 50 µM) of CuO-NPs enhance growth (in terms of fresh and dry weight and length), of rice seedlings. However, at higher concentrations (100, 200, and 500 µM) of CuO-NPs, growth (in terms of length, fresh weight and dry weight) decreased significantly (P < 0.05). Further, photosynthetic pigments (total chlorophyll and carotenoids) and protein contents were also found to be in accordance with the results of growth. This had occurred due to enhanced level of CuO-NPs accumulation at higher doses which also enhanced the level of oxidative stress markers such as hydrogen peroxide (H₂O₂) and malondialdehyde (MDA). Chlorophyll a fluorescence parameters (F_v/F_m and qP and except NPQ) and amount of some minerals (Ca, Mg, Na, and K) increased at lower concentrations of CuO-NPs. In contrast, the levels of F_v/F_m and qP were significantly (P < 0.05) reduced at higher concentration of CuO-NPs, which might be due to enhanced accumulation of Cu and oxidative stresses markers. Our results showed that lower dosages of pulsed laser ablated CuO-NPs (5, 10, 20, and 50 µM) might be beneficial for growth and development of rice seedlings.

Aggregation, Sedimentation, and Dissolution of Copper Oxide Nanoparticles: Influence of Low-Molecular-Weight Organic Acids from Root Exudates

The rhizosphere is an essential pathway for the uptake of metal-based nanoparticles (MNPs) by plant roots. However, the interaction between root exudates and MNPs is still unclear. In this study, we initially identified the major low-molecular-weight organic acids (LMWOAs) in the rice root exudates using hydroponics. Then, the individual LMWOAs were added to CuO nanoparticle suspensions to investigate their effects on the environmental behavior of the MNPs. The results showed that both the variety and the concentration of LMWOAs impacted the aggregation, sedimentation, and dissolution of CuO nanoparticles (NPs). Almost all LMWOAs except succinic acid inhibited the aggregation of CuO NPs by enhancing the electrostatic repulsive force between NPs. The presence of citric and oxalic acids rather than lactic acid greatly improved the stability of CuO NP suspensions, but other acids showed a low promoting and high inhibiting effect on NP sedimentation. Moreover, all the LMWOAs from root exudates facilitated the dissolution of CuO NPs with a positive dose-dependent correlation, especially formic acid. Notably, citric acid, as the most abundant LMWOAs in rice root exudates, largely determined the aggregation, sedimentation, and dissolution of CuO NPs. This study provides a better understanding on NP-plant interactions in the rhizosphere.

Distribution and Speciation of Copper and Arsenic in Rice Plants ( Oryza sativa japonica 'Koshihikari') Treated with Copper Oxide Nanoparticles and Arsenic during a Life Cycle

A 6 × 2 factorial study was conducted to investigate the effects of copper oxide nanoparticles (nCuO, 0-100 mg/L), arsenic (As, 0-10 mg/kg), and their interaction on uptake, distribution, and speciation of Cu and As in rice plants ( Oryza sativa japonica 'Koshihikari'). Arsenic (in As-addition treatments) and Cu in seedling roots (SRs) were 1.45 and 1.58 times those in soil, respectively. Arsenic and Cu concentrations further increased in mature plant roots (MRs), which were 2.06 and 2.35 times those in soil, respectively. Arsenic and Cu concentrations in seedling shoots (SSs) were 79% and 54% lower than those in SRs, respectively. The mature stems, however, contained only 3% and 44% of As and Cu in SSs. Copper in flag leaves did not vary much compared to that in stems, whereas As was 14.5 times that in stems. Species transformations of Cu and As were observed in rice including reductions of Cu(II) to Cu(I) and As(V) to As(III). Arsenic in dehusked grains was negatively correlated with Cu and was lowered by nCuO below the WHO (World Health Organization) maximum safe concentration for white rice (200 ng/g). This may alleviate As adverse effects on humans from rice consumption.

Physiological Effects of Copper Oxide Nanoparticles and Arsenic on the Growth and Life Cycle of Rice ( Oryza sativa japonica 'Koshihikari')

A factorial study was conducted to evaluate the phytotoxicity of copper oxide nanoparticles (nCuO, 0.1-100 mg/L), arsenic (As, 0 and 10 mg/kg), and their interaction to rice plants ( Oryza sativa japonica 'Koshihikari') during the life cycle. No significant effect was observed on seed germination. The main effects of nCuO and As were observed on lengths and biomasses of seedling shoots and roots and on root branching. The interaction between nCuO and As also significantly influenced these parameters. nCuO addition increased Cu uptake in seedlings and generally improved seedling growth. With As addition, As was highly concentrated in roots and increased in shoots, and seedling growth was also inhibited. Additionally, nCuO and As had significant main and interaction effects on mature plant dry biomass, panicle number, total grain weight, average grain weight, and several other panicle parameters. Moreover, nCuO and As interacted to affect panicle emergence. nCuO also decreased As accumulation in dehusked grains. The accelerated heading stage by nCuO may help shorten the life cycle of rice plants, thereby reducing As accumulation in grains. This study is the first to examine the influence of nCuO in combination with As on the life cycle of rice plants.

Iron Plaque: A Barrier Layer to the Uptake and Translocation of Copper Oxide Nanoparticles by Rice Plants

The waterlogging environment generally results in the deposition of iron plaque on plant roots, which may impact the fate of metal-based nanoparticles. Here, we investigated the influence of iron plaque on the uptake, translocation, and transformation of copper oxide nanoparticles (CuO NPs) in rice plants. The results show that the presence of iron plaque dramatically reduced the Cu contents in roots and shoots by 89% and 78% of those without iron plaque under 100 mg/L CuO NP treatment. Meanwhile, the Cu accumulation in plants was negatively related to the amount of iron plaque. X-ray absorption near edge structure (XANES) analysis demonstrated lower percentage of CuO but higher proportion of Cu(I) in shoots exposed to CuO NPs with the formation of iron plaque. Furthermore, micro X-ray fluorescence (μ-XRF) combined with μ-XANES revealed that the iron plaque in the root epidermis and exodermis consisted of goethite and ferrihydrite, which hindered the uptake of CuO NPs by roots. However, a few CuO NPs were still absorbed by roots via root hairs or lateral roots, and further translocated to shoots. But eventually, more than 90% of total Cu(II) was reduced to Cu(I)-cysteine and Cu₂O in leaf veins of rice plants with iron plaque.