Iron plaque reduces cerium uptake and translocation in rice seedlings (Oryza sativa L.) exposed to CeO2 nanoparticles with different sizes

STOTEN-30495; No of Pages 11 Science of the Total Environment xxx (xxxx) xxx

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Iron plaque reduces cerium uptake and translocation in rice seedlings (Oryza sativa L.) exposed to CeO2 nanoparticles with different sizes Yanyu Bao a,⁎, Chengrong Pan a, Weitao Liu a, Yunxia Li a, Chuanxin Ma b, Baoshan Xing c a Key Laboratory of Pollution Processes and Environmental Criteria, Ministry of Education, Tianjin Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300350, PR China b Department of Analytical Chemistry, The Connecticut Agricultural Experiment Station, New Haven, CT 06504, USA c Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, USA

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• IP significantly reduced Ce accumulation in rice under CeO2 NPs exposure. • Ce accumulation in rice decreased with increasing IP amounts. • IP formation weakened the interactive attraction of NPs and root surface.

a r t i c l e

i n f o

Article history: Received 25 November 2018 Received in revised form 11 January 2019 Accepted 14 January 2019 Available online xxxx Editor: Jay Gan Keywords: CeO2 nanoparticles (NPs) Iron plaque (IP) Accumulation Translocation Rice

a b s t r a c t This study aims to assess the role of iron plaque (IP) on cerium (Ce) uptake and translocation by rice after CeO2 nanoparticles (NPs) exposure over a 4 days period. A hydroponic experiment was performed under two IP levels (low and high) combined with two CeO2 NPs size (14 nm and 25 nm). It was found that CeO2 NPs as the main form was absorbed by rice due to limited NPs dissolution in hydroponic solution. IP significantly reduced surfaceCe, root-Ce and shoot-Ce accumulation, irrespective of CeO2 NPs sizes. The reduced uptake of Ce was more obvious in NP25 than NP14. Ce accumulations decreased with increasing IP amounts. In IP treatments, the interactive attraction between NPs and root surface was weakened through the enhancement of hydrodynamic diameters and the reduction of ζ-potential of CeO2 NPs in solution, as well as the reduction of |ζ| values of rice root, which reduced the Ce bioaccumulation in rice. PCA indicated the negative correlation between surface-Ce (IP-C-Ce and IP-A-Ce) and NPs size, and between shoot-Ce/root-Ce and IP-Fe/tissue-Fe. IP also decreased Ce translocation from root to shoot. A full life study indicated the reduction effect of IP on surface-Ce, root-Ce, shoot-Ce and grain-Ce accumulations. These findings are significant as they imply that the IP formation is a promising approach for preventing Ce accumulation in rice, which would regulate Ce uptake by rice in the following growth stages and decrease the health risk of CeO2 NPs exposure in agricultural environment. © 2019 Elsevier B.V. All rights reserved.

1. Introduction ⁎ Corresponding author at: College of Environmental Science and Engineering, Nankai University, Tianjin 300350, PR China. E-mail address: [email protected] (Y. Bao).

The use of CeO2 nanoparticles (NPs) in consumer products is everincreasing as fuel additives, polishing agents, additive in glass, solid-

https://doi.org/10.1016/j.scitotenv.2019.01.181 0048-9697/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Y. Bao, C. Pan, W. Liu, et al., Iron plaque reduces cerium uptake and translocation in rice seedlings (Oryza sativa L.) exposed to CeO2 nanoparticles with different sizes..., Science of the Total Environment, https://doi.org/10.1016/j.scitotenv.2019.01.181

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state fuel cells in automotive industry, UV protection additives in cosmetics, potential antioxidant agent, etc. (Zhou et al., 2018; Zhang et al., 2017; Dale et al., 2017; Giri et al., 2013). Consequentially, they will be released into the environment. Given the high possibility of its reaching agricultural environment through direct or indirect pathways, its interaction with crops is inevitable in the environment. Researchers have found that CeO2 NPs were taken up by plant roots and even translocated from the roots to shoots such as tomato (Wang et al., 2012), cucumber (Zhao et al., 2013) and carrot (Ebbs et al., 2016), which imposes risks to human health via food chains. Furthermore, CeO2 NP exhibited significant toxicity to ryegrass, alfalfa, mesquite, lettuce, corn, cilantro, and cucumber (Zhao et al., 2013). Rice (Oryza sativa L.), as one of the most vital food crops, is a staple cereal food for about 3 billion people (Cheng et al., 2013). Once released into the environment, the CeO2 NPs can be absorbed, translocated and accumulated in crop, followed by growth effect and even being moved to trophic chain and even generating biomagnifications (Rico et al., 2014). Rico et al. (2013b) found that CeO2 NP exposure compromised the quality of rice through decreasing Fe, S, prolamin, glutelin, lauric acid, valeric acid and starch in rice grains, and reduced antioxidant values in grains. The results from Rico et al. (2013a) also showed CeO2 NP exposure affected the oxidative stress and antioxidant defense system in rice seedlings. Thus, it is important to search for strategies to reduce rice CeO2 NPs accumulation and so minimize the above adverse influence under their exposure. However, little is known how to decrease its bioaccumulation in rice. In Fe2+-rich environment, iron plaque (IP) can be formed on the root surface of hydrophytes. Fe2+ is oxidized by the root-released oxygen and oxidants to Fe3+ that subsequently precipitates as iron oxides or hydroxides on the root surface as IP (Amaral et al., 2017; Liu et al., 2006). IP may undergo various interactions with contaminants, thereby affecting their uptake from solution to hydrophyte. IP acts as a “barrier” of heavy metals such as As (Amaral et al., 2017) and Cd (Chen et al., 2017) on rice root surface. Li et al. (2016) found that IP greatly decreased the uptake of both IHg and MeHg in rice seedlings. Koster van Groos et al. (2016) reported that U was observed in more oxidized states (U(VI) or U(V)) together with IP on plant roots, suggesting that plant associated U in wetlands was less susceptible to remobilization during episodic reoxidation events than if U was sequestered as U(IV). Our previous studies (Bao et al., 2018; Yan et al., 2017) showed that IP can alleviate two antibiotics (oxytetracycline and norfloxacin) toxicity and reduce antibiotics accumulation in rice under high antibiotics exposure levels. However, the comprehensive influence of IP formation on cerium (Ce) uptake and translocation in rice under CeO2 NPs exposure has been rarely studied. Furthermore, the mechanism by which IP formation affecting the Ce uptake by rice remains poorly understood. Therefore, it is necessary to analyze the uptake and translocation of Ce in environment-rice systems under CeO2 NPs exposure in order to obtain a more complete understanding of the influence process and mechanism of IP. To achieve these objectives, a hydroponic culture was conducted to measure the changes in surface-Ce, root-Ce and shoot-Ce concentrations under two CeO2 NPs exposure (14 and 25 nm size) when IP was present or absent. NPs' characteristics (containing hydrodynamic diameter, ζ-potential and dissolution), as well as root samples' ζ-potential were measured to explore the main factors affecting rice-Ce levels. Meanwhile, the Fe concentrations in the plant tissues and solution pH values were analyzed to understand the effects of Fe accumulation on the translocation of Ce in rice. SEM, XPS and TEM were also used to characterize the effect of IP formation on CeO2 NPs properties on root surface and inside root. 2. Materials and methods 2.1. Exposure assay Rice seeds (Oryza sativa L., Jinyuan-E28) were obtained from Tianjin Academy of Agricultural Sciences (Tianjin, China). The seeds were

sterilized in 3% H2O2 for 30 min, thoroughly washed three times with deionized water, and then soaked in deionized water for 24 h. The seeds were germinated on moist gauze in chamber with a light-todark cycle of 12:12 h at 25 °C for 10 days. Uniform seedlings were selected and transferred into the flasks containing 140 ml one-fourth strength international rice nutrient solution (pH 5) (Bao et al., 2018). The flasks were wrapped in aluminum foil. Rice seedlings acclimatized in the above solution for 6 d as pre-culture in order to adapt the solution environment, and the solution was replaced every 3 d. Rice seedlings were prepared with iron plaque (IP) and without IP (CK). For CK, seedlings grew up in above solution for 10 d, and there wasn't obvious IP formation. For IP treatments, the 25 mg·L−1 and 150 mg·L−1 Fe2+ from FeSO4 were added in above solution without FeCl3 nutrient, respectively, in order to induce different IP levels on root surface of rice. Our preliminary results showed that 25 mg·L−1 and 150 mg·L−1 Fe2+ didn't exhibit the toxicity to rice. After 10 d, IP was formed with IPlow and IPhigh on the root surface of rice, respectively. 14 nm (NP14) and 25 nm (NP25) CeO2 NPs used in the study were obtained from Sigma Aldrich (USA) and Shanghai Dingguo Biotechnology Co., LTD, respectively. X-Ray Diffraction (XRD, Ulitma, Japan) imaging revealed a primary particle size of 13.5 ± 1.2 nm and 24.8 ± 1.1 nm, respectively. The NP suspensions were prepared by suspending an appropriate amount of NPs in ultrapure water by ultrasonication for 1 h, to a final 25 mg·L−1 Ce concentration. Rice seedlings were treated by 0 and 25 mg·L−1 Ce from NP14 and NP25. Initial pH was adjusted to 5.0 in all hydroponic solution. All experiments were conducted in triplicate, and ten rice seedlings were included in each replicate. Deionized water was added in order to maintain initial solution volume every day during the entire exposure period (4 d). The plants were grown in incubator with a condition as described above. At harvest, plants were carefully rinsed with deionized water to remove NPs particles on root surfaces. Then, plants were then divided into roots and shoots. In order to observe the effect of IP on Ce accumulation in rice during a full life cycle, uniform seedlings were selected and transferred into the pots containing 1.50 kg paddy soil. There were two treatments containing 25 mg·kg−1 CeO2 NP with 25 nm (NP25) and 25 mg·kg−1 CeO2 NP + 25 mg·kg−1 Fe from FeSO4 (NP25 + IP). All experiments were conducted in triplicate, and two rice seedlings were included in each replicate. Deionized water was then added to full saturation with 1 cm of standing water above the soil surface. Rice seedlings grew up in greenhouse condition (25 °C) for 4.5 month from June to October. During this period, deionized water was added every day in order to keep waterlogging. The plants were harvested after 135 days. Ce concentration was determined in IP, root, shoot and grain.

2.2. Extraction of IP on root surface Ammonium oxalate (0.175 M) - oxalic acid (0.10 M) buffer was firstly used to extract amorphous fraction of IP from the root surface. Then, the sodium dithionite (Na2S2O4)‑sodium citrate (Na3C6O7H5)‑sodium bicarbonate (NaHCO3) (DCB) mixture solution was used to extract crystalline fraction of IP (Bao et al., 2018; Hu et al., 2015). The Fe and Ce concentrations of the samples extracted by ammonium oxalate buffer and DCB mixture were separately analyzed by Flame Atomic Absorption Spectrophotometer (FAAS) and ICP-MS, respectively. In order to avoid environmental Ce and Fe contamination, deionized water without Ce and Fe was selected, and reagent blank was also set up. About 0.006 mg·L−1 Ce and 0.009 mg·L−1 Fe were found in reagent blank. And all solutionCe and solution-Fe concentrations in the study subtracted their blank values, respectively. After two-step extraction, the roots were thoroughly rinsed with deionized water and then were freeze-dried at −65 °C for 24–48 h until analysis. The amount of IP and the Ce concentration in the IP are presented as the mass fraction of Fe and Ce in extractions to the dry weight of root after extraction (mg·kg−1 DW), respectively.

Please cite this article as: Y. Bao, C. Pan, W. Liu, et al., Iron plaque reduces cerium uptake and translocation in rice seedlings (Oryza sativa L.) exposed to CeO2 nanoparticles with different sizes..., Science of the Total Environment, https://doi.org/10.1016/j.scitotenv.2019.01.181

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2.3. Fe and Ce concentration analyses in rice tissues After IP extraction, the shoots and roots samples were freeze-dried at −65 °C for 24–48 h and then weighed up. A microwave-accelerated reaction system was used to digest the root or shoot samples with a mixture of HNO3 and H2O2 (V/V 5:3) at 190 °C for 40 min. The Ce and Fe concentrations in digest solution were analyzed by ICP-MS and FAAS, respectively.

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where, surface-Ce, root-Ce, shoot-Ce was Ce concentrations on root surface, inside root, shoot, respectively. 2.8. Statistical analysis The figures were made by Origin 8.5. All statistical analyses were performed with software packages IBM SPSS Statistics 20 with LSD test, withthe significant differences declared at p b 0.05.

2.4. Hydroponic solution properties

3. Results

During the experiment, solution pH values were determined at 2, 3 and 4 d. In the end of experiment, the Fe2+ concentrations in culture solution were determined by the 1,10-phenanthroline (Phe) colorimetric method (Gupta, 1968). Phe is known to react extremely rapidly and selectively with Fe2+ to form a stable orange-red complex with a maximum absorbance at 510 nm by UV − vis spectroscopy in HAc-NaAc buffer (pH 4.6) and it does not bind with Fe3+. The total iron ion concentrations were determined using FAAS. The difference between total iron ion and Fe2+ was Fe3+.

3.1. Effect of iron plaque (IP) formation and CeO2 NPs exposure on plant biomass

2.5. SEM, TEM imaging, X-ray photoelectron spectroscopy (XPS) and ζpotential analysis of root samples To observe the effects of particle size and IP on CeO2 NPs accumulation on root surface, the root surfaces were imaged using a scanning electron microscope (SEM, S-3500 N, High-Technologies Corporation, Tokyo, Japan). Root surface elements speciation was determined by Energy Dispersive Spectrometer (EDS). The root apexes were cut and fixed in 2.5% glutaraldehyde solution. Then the tissues were dehydrated in a graded acetone series and embedded in Spurr's resin. Ultrathin sections of 70 nm were cut by an UC7 ultramicrotome (Leica, Germany) with a diamond knife and collected on copper grids. Sections were observed under a Hitachi HT7700 (Japan) Transmission Electron Microscope operating at 80 kV. Root samples were grinded into powder, and then were determined using XPS (Thermo Scientific ESCALAB 250Xi, USA) to observe the Ce valence states. Four treatments were prepared for SEM, TEM imaging and XPS containing NP14, NP25, NP14 + IPhigh, NP25 + IPhigh. For XPS, initial CeO2 NPs were as the control. The ζ-potential |ζ| values of rice root samples were determined when IP was present or not. IP treatments contained IPlow and IPhigh. 2.6. CeO2 NPs characterization changes and its dissolution in solution Hydrodynamic diameter and ζ-potential of NPs suspensions (25 mg·L−1) in root exudate solution at pH 5 were determined by dynamic light scattering using a Malvern Zeta Sizer (3000HS, Worcestershire, UK) to study the effect of different Fe2+ levels (10 mg·L−1 and 25 mg·L−1) from FeSO4. Root exudates were collected after rice seedlings growth for 24 h. The dissolution as Ce ions from CeO2 NPs was investigated in root exudate solution at different time points when Fe2+ was present or not. 2.7. Data analysis Translocation factor (TF) indicated the translocation ability of Ce from root surface to root (TF surface-root ), or root to shoot of rice (TFroot-shoot). TF was calculated as follow:

TFsurface−root ¼

TFroot−shoot ¼

root–Ce surface–Ce

shoot–Ce root–Ce

Rice plants treated “low” (IPlow) and “high” (IPhigh) IP amounts showed insignificant effect (p N 0.05) on root dry weights compared to the treatment without IP formation, irrespective of CeO2 NPs sizes (Fig. 1A). In no IP and IPlow treatments, 25 mg·L−1 Ce from 14 nm (NP14) and 25 nm (NP25) CeO2 NPs showed insignificant effect (p N 0.05) on root dry weights compared to no NPs treatment. However, CeO2 NPs (14 nm and 25 nm) increased significantly the root dry weights in IPhigh treatments compared to no NPs treatments (Fig. 1A). Fig. 1B shows IP formation had no impact (p N 0.05) on rice shoot weights compared to the treatment without IP formation, irrespective of CeO2 NPs sizes. Also, 14 nm and 25 nm CeO2 NPs had insignificant effect (p N 0.05) on shoot dry weights compared to no NPs treatments, in no IP and IP treatments. Hence, IP formation and two CeO2 NPs exposure exhibited no negative toxicity effects on rice seedling growth. 3.2. Ce uptake and accumulation in rice For NP14 and NP25, IP formation significantly (p b 0.05) reduced the concentrations of surface-Ce (Fig. 2A), root-Ce (Fig. 2B) and shoot-Ce (Fig. 2C), as compared with CK. The dose-response relationships were observed between the reduction effect above and IP amounts. SurfaceCe, root-Ce and shoot-Ce concentrations decreased with increasing IP amounts from “low” to “high” (p b 0.05), except that there wasn't obvious difference of shoot-Ce concentrations between NP14 + IPlow and NP14 + IPhigh treatments. In Fig. 2A, IP formation significantly decreased Ce concentration in crystalline (IP-Cr-Ce) and amorphous (IP-Am-Ce) fractions (p b 0.05) as compared with no IP treatment. IP-Cr-Ce and IP-Am-Ce concentrations decreased significantly with increasing IP amounts (p b 0.05), except that the decrease of IP-Am-Ce concentrations was insignificant between NP25 + IPlow and NP25 + IPhigh treatments (p N 0.05). These results indicated that IP formation inhibited surface-Ce accumulation through reducing IP-Cr-Ce and IP-Am-Ce concentrations. As shown in Fig. 2A, Ce was accumulated more readily on root surface in NP14 treatments compared to NP25 treatments, irrespective of IP levels. Similar results were found in IP-Cr-Ce and IP-Am-Ce accumulation, suggesting that high surface-Ce concentrations in NP14 treatments were mainly from high IP-Cr-Ce or IP-Am-Ce accumulation compared to NP25 treatments. As shown in Fig. 2B, root-Ce concentration was significantly lower (p b 0.05) in NP14 treatment than that in NP25 treatment when no IP was formed. However, in IPlow and IPhigh treatments, root-Ce concentration was significantly higher (p b 0.05) in NP14 treatment than one in NP25 treatment. As shown in Fig. 2C, shoot-Ce concentrations in NP14 treatments were always significantly lower than ones in NP25 treatments (p b 0.05), irrespective of IP formation. After 4 d exposure to NPs, Ce distribution in solution-rice system was analyzed (Fig. 2D). In all treatments, Ce was still remained mainly in solution, accounting for 95.8–98.4% of total Ce added as CeO2 NPs. In rice, Ce accumulated predominantly in roots in all treatments, accounting for 1.44–3.54%. Surface-Ce and shoot-Ce accounted for only 0.11–0.47% and 0.04–0.30%, respectively. For each NPs treatment, the

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Fig. 1. Dry weights of roots and shoots of rice at the end of experiment. Different letters for each column indicated a significant difference at p ≤ 0.05. The same was as below.

Ce distribution was always in the order root N root surface N shoot. The results indicated that IP formation decreased Ce distribution in rice through remaining more CeO2 NPs in solution. TFsurface-root of Ce was 6.56, 6.63, 6.94, respectively, for NP14, NP14 + IPlow, NP14 + IPhigh treatments, which revealed no significant differences (p N 0.05) between above treatments. TFsurface-root was 9.44, 9.59, 12.7, respectively, for NP25, NP25 + IPlow, NP25 + IPhigh treatments, which showed that high IP level increased TFsurface-root significantly (p b 0.05), but not for low IP level. TFroot-shoot of Ce was 0.12, 0.03, 0.03, respectively, for NP14, NP14 + IPlow, NP14 + IPhigh treatments. And, TFroot-shoot was 0.13, 0.09, 0.08, respectively, for NP25, NP25 + IPlow, NP25 + IPhigh treatments. Above results revealed that, in both NP14 and NP25 treatments, IP formation reduced Ce translocation from root to shoot in. For big particle size – NP25, IP formation decreased the TFroot-shoot of Ce by 32.8% and 34.4%, respectively, in IPlow and IPhigh compared with the absence of IP. For small particle size – NP14, IP formation decreased the TFroot-shoot by 72.8% and 73.6%, respectively, in IPlow and IPhigh. These results revealed that 25 nm NPs caused higher TFsurface-root and TFroot-shoot of Ce compared to 14 nm NPs, irrespective of IP formation. Thus, the IP formation prevented the Ce uptake by rice mainly through reducing Ce translocation from root to shoot. For the smaller particle size of NPs, the influence of IP was more obvious. In the same NPs treatments, no insignificant differences were observed between IPlow and IPhigh treatments. A full life cycle study was also conducted (Fig. 3). Similar to short period, IP presence reduced surface-Ce, root-Ce, root-Ce and grain-Ce accumulations in NP25 + IP compared to NP25. A significant effect was observed for surface-Ce, root-Ce and grain-Ce (p b 0.05). 3.3. Fe accumulation in rice In IPlow and IPhigh treatments, the red IP was visible on the root surface. No red color was observed on the root surface in CK due to limited amounts of Fe oxides precipitation. CeO2 NPs exposure decreased significantly (p b 0.05) surface-Fe (IP) concentrations in comparing with the absence of NPs (Fig. 4A). This was mainly due to the Fe amounts decrease in crystalline fraction of IP (IP-Cr-Fe), not that in amorphous fraction of IP (IP-Am-Fe). It was also found that the decrease effect of surface-Fe concentrations in NP25 was more obvious than that in NP14. In IP treatments, NPs exposure decreased significantly (p b 0.05) rootFe concentrations (Fig. 4B), while increased significantly (p b 0.05) shoot-Fe concentrations (Fig. 4C), except in NP14 + IPlow where an insignificant increase of shoot-Fe with respect to NP14 was observed. In

no IP treatments, compared with CK, two NPs exposure didn't affect surface-Fe, root-Fe and shoot-Fe accumulation (Fig. 4). In all treatments, the Fe accumulation in rice followed the order as: root surface N shoot N root, which revealed that Fe was mainly accumulated on root surface. 3.4. Morphology characteristic of rice root after CeO2 NPs exposure when IP was present or not Scanning electron microscope equipped with energy-dispersive spectroscopy (SEM − EDS) was used to characterize the root surface of rice (Fig. S1). SEM images showed the obvious CeO2 NPs aggregation on root surface in NP14 and NP25 treatments (Fig. S1A and S1B), with more NPs aggregations in NP14 treatment than ones in NP25 treatment. This is consistent with the results obtained by higher surface-Ce accumulation in NP14 treatment than one in NP25 treatment (Fig. 2A). The EDS spectrum also supported the presence of Ce on root surface treated with 14 nm and 25 nm CeO2 NPs (Fig. S1I and S1II). In NP + IP treatments (Fig. S1C and S1D), there weren't obvious NPs aggregations on root surface. The EDS spectrum detected the presence of Ce and Fe on root surface in NP14 + IPhigh (Figure S1III), whereas Ce and Fe signal weren't detected at the same time in NP25 + IPhigh (Figure S1IV and S1V), suggesting that nanoparticles of the larger size (25 nm CeO2 NPs) could cover easily the IP, but the smaller size (14 nm CeO2 NPs) couldn't. Transmission electron microscope (TEM) imaging was used to characterize NPs distribution inside rice root (Fig. S2). TEM observation showed the obvious NPs aggregations in root cell whether IP was present or not. In all treatments, NPs and their aggregates were mainly accumulated in vacuoles of root cell, rather than in other parts of root cell, such as intercellular, cytoplasm, etc. The Ce valence states were determined in root samples after two CeO2 NPs exposure when or not IP was present (Fig. S3). In pristine 14 nm and 25 nm CeO2 NPs, the major peaks of Ce4+ (3d3/2) and Ce4+ (3d5/2) were observed at binding energies of 916.7 and 901.1, 898.2 and 882.5 eV, respectively. In root samples from NP and NP + IP treatments, Ce4+ valence was still main state. The peaks of Ce4+ (3d3/2) and Ce4+ (3d5/2) were located at 901.1, 898.2 and 882.5 eV in NP14 and NP14 + IP (Fig. S3A). And, the peaks were located at 916.3 and 882.5 eV in NP25 and NP25 + IP (Fig. S3B). The peaks of Ce3+ (3d3/2) and Ce3+ (3d5/2) were located at 903.7 and 885.3 eV for NP14 and NP25 + IPhigh treatments, which showed Ce3+ valence presence in root samples after CeO2 NPs exposure. In pristine 25 nm CeO2 NPs, there were no peaks of Ce3+ (3d3/2) and Ce3+ (3d5/2). However, in NP25 + IPhigh

Please cite this article as: Y. Bao, C. Pan, W. Liu, et al., Iron plaque reduces cerium uptake and translocation in rice seedlings (Oryza sativa L.) exposed to CeO2 nanoparticles with different sizes..., Science of the Total Environment, https://doi.org/10.1016/j.scitotenv.2019.01.181

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Fig. 2. Ce concentrations in the IP on root surface (A), roots (B) and shoots (C) of rice and Ce distribution in the solution-rice system (D) at the end of the exposure experiment. In Fig. 2A, different lower and upper case letters for each column indicated a significant difference at p ≤ 0.05, respectively, for crystalline and amorphous fraction of IP. The same was as below.

(Fig. S3B), the obvious peak of Ce3+ (3d3/2) was located at 903.7 eV, which revealed the reductive dissolution of CeO2 NPs during their uptake by rice root. 3.5. CeO2 NPs characteristic in hydroponic solution

Fig. 3. Ce concentrations on root surface, roots and shoots of rice at the end of the life cycle. * indicated a significant decrease at p ≤ 0.05 compared to NP25.

As shown in Fig. S4, Fe2+ and Fe3+ ions were present in hydroponic solution after experiment. In IP treatments, solution Fe2+ ion was main ion due to the reductive dissolution and release of IP, which occupied N95% of total Fe ions (Fe2+ + Fe3+) in solution. During 4 d experiment, solution pH values were around 5 in NP, IPlow and NP + IPlow treatments (Fig. S5). However, in IPhigh and NP + IPhigh treatments, solution pH values decreased over time due to more Fe2+ release in solution from IPhigh. Herein, it was investigated for the effect of Fe2+ on two CeO2 NPs' characteristics. Hydrodynamic diameters (Fig. 5A) and ζ-potential (Fig. 5B) of CeO2 NPs were determined in rhizosphere solution. The DLS measurements (Fig. 5A) indicated that the hydrodynamic diameters of NP14 were always lower significantly (p b 0.05) than ones of NP25 whether the Fe2+ was present or not. The results revealed that NP14 used in this study was more resistant to aggregation than NP25 in root exudates solution. For two NPs, the presence of Fe2+ increased

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Fig. 4. The effect of CeO2 NP exposure on surface-Fe concentrations (A), root-Fe (B) and shoot-Fe (C) of rice in hydroponic culture.

significantly (p b 0.01) their hydrodynamic diameters. Hydrodynamic diameters increased with increasing Fe2+ concentration, suggesting the presence of Fe2+ promoted the aggregation of NPs. In the study, ζpotential |ζ|values of two NPs were both positive. ζ-potential values of pristine NP14 was higher than that of pristine NP25 (Fig. 5B). The presence of Fe2+ decreased positive charges amounts of two NPs, although all ζ-potential values were still positive. Meanwhile, ζ-potential |ζ|of rice root samples was determined when or not IP was formed (Fig. 5C). All ζ-potential |ζ| values of rice root were negative. Moreover, IP presence decreased significantly (p b 0.05) |ζ| values of root. Soluble Ce ions from CeO2 NPs dissolution were measured in root exudates (Fig. 5D). Ce ions concentrations increased over time whether Fe2+ was present or not. No significant differences (p N 0.05) were observed between two CeO2 NPs when Fe2+ was absent. Fe2+ plays a positive role in promoting CeO2 dissolution. The dissolution increased with the increase of Fe2+ concentrations. Fe2+ induced greater 14 nm NP dissolution than 25 nm NP, meaning that smaller size CeO2 NP was more easily dissolved than large size of NP. 3.6. The principal component analysis (PCA) PCA (Fig. 6) was performed with Ce and Fe accumulation in amorphous, crystalline fractions of IP, root and shoot of rice, selected NP

properties (size, dissolution, ζ-potential and hydrodynamic diameter), and pH of culture solution in order to analyze the relationships among these indices and identify the main factors affecting Ce accumulation in rice. The two first principal components PC1 and PC2 accounted for 90.60% and 5.90% of the variability, respectively. Score plot from PCA (Fig. 6A) showed that there was obvious cluster for two NPs samples when no IP was present on root surface, and they grouped together on the band of the negative axis of PC1 and PC2. However, in IP treatments, they were separated on the positive and negative axis of PC2, respectively. With the increase of IP amounts, the dispersion of these samples obtained moved to the band of the positive axis of PC1. The dispersion of these samples obtained shared the band of the positive axis of PC1 when high amounts of IP were formed. In IP treatments, dispersion of NP14 samples distributed on the band of the negative axis of PC2, and one of NP25 always distributed on the band of the positive axis of PC2. These findings suggested that the IP presence and its amounts had a considerable effect on the Ce accumulation in rice. Moreover, the effect degree was enhanced with increasing IP amounts. As shown in loading plot (Fig. 6B), the significant negative relationship between IP-Cr-Ce/IP-Am-Ce/root-Ce and NPs size revealed that the larger NPs size was, the more Ce was accumulated on root surface and inside root of rice. The IP-Cr-Ce, IP-Am-Ce, root-Ce and shoot-Ce were close to each other, which revealed significant positive relationships between

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Fig. 5. The hydrodynamic diameter (A) and ζ-potential (B) of CeO2 NP in the solution with pH 5 as the effect of NP size and Fe2+ addition. ζ-potential of rice root samples with or without IP (C). And, the dissolution of Ce (D) from CeO2 NP in root exudates with pH 5.

them. However, the above four rice-Ce had significant negative relationship with Fe in root, shoot and amorphous and crystalline fractions of IP. For shoot-Ce the high negative loading was recorded for IP-Am-Fe and IPCr-Fe (r = −0.787 and − 0.876; p b 0.05). For root-Ce, the negative loading was recorded for IP-Am-Fe and IP-Cr-Fe (r = −0.743 and −0.736; p b 0.10). This further verified that a rise of Fe in the IP and rice tissues more effectively decreased the accumulation of Ce in rice shoot than root. The high negative relationship was observed between Ce in IP/root and Fe in root/shoot. Root-Fe had significantly negative relationship with shoot-Ce (−0.925, p b 0.005), then was root-Ce (−0.882, p b 0.01) and IP-Am-Ce (−0.810, p b 0.05). Shoot-Fe had significantly negative relationship with root-Ce (−0.891, p b 0.01), then was shoot-Ce (−0.813, p b 0.05) and IP-Ar-Ce (−0.771, p b 0.05). The results showed that the reduction of IP-Ce and root-Ce accumulation depended on the enhancement of root-Fe and shoot-Fe accumulation. In IP treatments, Fe2+ was present in culture solution due to the release and dissolution of IP during 4d experiment (Fig. S4). Fe2+ increased hydrodynamic diameter (Fig. 5A) and decreased ζ-potential (Fig. 5B) of two NPs. ζ-potential was positively correlated with IP-CrCe, IP-Am-Ce, root-Ce and shoot-Ce (Fig. 6B), which showed that the decrease of ζ-potential inhibited Ce accumulation in IP/root/shoot.

However, hydrodynamic diameter was negatively correlated with IPCr-Ce, IP-Am-Ce, root-Ce and shoot-Ce (Fig. 6B), which revealed that increase of hydrodynamic diameter in the study decreased effectively Ce accumulation in IP/root/shoot. In IPhigh treatments, low pH values were always observed in solution (Fig. S5). Only shoot-Ce had significantly positively relationship with pH in different experiment periods (Fig. 6B). However, no significant correlations were observed between rice-Ce (root-Ce, IP-Cr-Ce, IP-Am-Ce) and the pH values. The loading plot (Fig. 6B) showed that Ce accumulation in IP/root/shoot was strongly negatively correlated with the Ce concentration released from NPs at different experiment periods. The significant correlations (p b 0.05) were always observed between the Ce dissolution and root-Ce/IP-Cr-Ce/IP-Am-Ce. 4. Discussion 4.1. IP formation reducing surface-Ce accumulation under CeO2 NPs exposure Root is the only route for CeO2 NPs uptake by rice in the study. And, root surface is a necessary way of NPs uptake due to its inherent contact

Please cite this article as: Y. Bao, C. Pan, W. Liu, et al., Iron plaque reduces cerium uptake and translocation in rice seedlings (Oryza sativa L.) exposed to CeO2 nanoparticles with different sizes..., Science of the Total Environment, https://doi.org/10.1016/j.scitotenv.2019.01.181

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Fig. 6. The two first principal components PC1 and PC2 explained 90.60% and 5.90% of the variability, respectively. Score plots (A) showing clustering of the individuals according to NP size and IP amounts. The correlation loading plot (B) shows the relationships between the response variables in the space of the first two components. IP-Am and IP-Cr mean amorphous and crystalline fractions of IP, respectively.

between environmental media and plant roots. To assess the potential translocation of CeO2 NPs from hydroponic solution to rice roots, their accumulation on root surface were determined. It has been reported that CeO2 NPs could be strongly adsorbed on root surface of plants (Ma et al., 2015; Zhang et al., 2012). A large amount of mucilage (Parsons et al., 2010) on root surface was secreted from root and electro negativity of plant cell walls, including epidermis cells on root surfaces (Meychik and Yermakov, 2001), which promoted NPs adsorption on root surface due to the positive charges of NPs. IP presence inhibited surface-Ce accumulation, which decreased with increasing IP amounts (Fig. 2A). During the experiment, Ce4+ from CeO2 NPs will react with Fe2+ from IP dissolution, which reduced the amounts of IP. As shown in Fig. S4, Fe2+ was present in solution due to IP dissolution and release in all IP treatments after CeO2 NPs exposure. Although CeO2 NPs decreased slightly IP amounts, high amounts of IP were still remained on root surface in IPlow and IPhigh treatments (Fig. 2A). The Ce bioaccumulation in plants could be governed by key NPs physiochemical properties. In order to investigate the impact of IP formation on surface-Ce accumulation, the changes of NPs characteristics were determined containing initial size, ζ-potential, hydrodynamic diameter, dissolution concentrations in solution, solution pH and root surface properties when or not IP was present. In the study, the reason of thesurface-Ce accumulation reduction was as follows. First, Fe2+ release was observed from IP (Fig. S4), whose presence increased hydrodynamic diameters of two NPs (up to 2130–2692 nm under high Fe2+ concentration) (Fig. 5A). It was reported that root exudates (such as citric acid) reduced the hydrodynamic diameters through coating NPs (Trujillo-Reyes et al., 2013) compared to bare NPs. Maybe, Fe2+ addition reduced the coating through the complexation between Fe2+ and root exudates, which caused the increase of the hydrodynamic diameters compared to no Fe2+ presence. The increased hydrodynamic diameters suggested that individual NPs tended to aggregate when Fe2+ was present, which led to fewer individual NPs near to the root surface, and then reduced surface-Ce accumulation. Second, the presence of Fe2+ decreased ζpotential |ζ|values of two NPs (Fig. 5B). ζ-potential values were 2.02 and 1.55 mV for NP14 and NP25, respectively. After adding high Fe2+ concentration, ζ-potential decreased to only 0.63 and 0.67 mV, respectively. The reduction of NPs' ζ-potential |ζ| values weakened the interactive attraction of NPs and root surface. ζ-potential |ζ| values of rice root were always negative (Fig. 5C). IP presence decreased |ζ| values of root significantly (p b 0.05), and |ζ| values decreased with increasing IP amounts. Thus, root surface with the reduced negative charges further

weakened NPs adsorption on root surface in combination with the decrease of ζ-potential of NPs in IP treatments. Then, surface-Ce accumulation was further decreased. The reduction of Ce accumulation was observed in both crystalline and amorphous fractions of IP (Fig. 2A). IP-Cr-Ce and IP-Am-Ce decreased with increasing the amounts of crystalline and amorphous of IP, respectively (Fig. 2A and Fig. 4A). Above results implied that, in IP treatments, surface-Ce containing IP-C-Ce and IP-A-Ce decreased due to the hydrodynamic diameter's enhancement and the ζ-potential |ζ| value's reduction of NPs, as well as the reduction of |ζ| values of rice root. The effect of IP was observed in both NP14 and NP25 treatments. Similar to short period, IP presence significantly reduced surface-Ce at the end of full life cycle (Fig. 3). In IP treatments, Fe2+ presence promoted two CeO2 NPs dissolution (Fig. 5D) due to Fe2+ as a reducing agent promoting the reduction of Ce4+ from CeO2 NPs into Ce3+ and then promoting the release of Ce ions in solution. Compared to IPlow treatment, IPhigh treatment resulted in greater Fe2+ release in solution from IP, which promoted more CeO2 NPs dissolution (Fig. S4). The results agreed well with a similar finding that Fe2+ plays a positive role in promoting CeO2 NPs dissolution, and Ce concentration from dissolution of CeO2 NPs increased with increasing Fe2+ concentration (Liu et al., 2015). In the current study, particle size dependent dissolution was observed. In the same IP treatments, the increased dissolution of small particle (14 nm CeO2 NPs) was more significant than larger particle (25 nm CeO2 NPs) over a 4 days period (Fig. 5D). In addition, a prior study found that CeO2 dissolution was only significant at pH b 5 (Dahle et al., 2015). Herein, IPhigh treatments caused low pH values in solution ranging from 4.0 to 5.0 (Fig. S5), which further promoted Ce ions release. Although Fe2+ addition enhanced two NPs dissolution, limited Ce release with 0.005 to 0.916 mg·L−1 only accounted for b3.66% of total Ce from NPs in all treatments (Fig. 5D). Therefore, low Ce ions concentration in solution limited their uptake by rice. According to SEM observation (Fig. S1C and S1D), CeO2 NPs aggregation was always observed on root surface in NP + IP treatments. The EDS spectrum supported the presence of Ce and Fe on root surface of rice treated with NP14 + IPhigh (Fig. S1III), whereas Ce and Fe signals weren't detected simultaneously in NP25 + IPhigh (Figure S1IV and S1V), suggesting that nanoparticle of large size (25 nm CeO2 NPs) could cover easily the IP compared to small size (14 nm CeO2 NPs). It was possible that too low NP25 dissolution resulted in lower Ce ions precipitation on root surface (Fig. 5D), which reduced the possibility of simultaneous appearance of Ce and Fe. According to XPS spectra analysis (Fig. S3), Ce4+ valence was the main state in root

Please cite this article as: Y. Bao, C. Pan, W. Liu, et al., Iron plaque reduces cerium uptake and translocation in rice seedlings (Oryza sativa L.) exposed to CeO2 nanoparticles with different sizes..., Science of the Total Environment, https://doi.org/10.1016/j.scitotenv.2019.01.181

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samples from NP and NP + IP treatments. As a result, surface-Ce accumulation was mainly from nanoparticle-Ce with Ce4+ valence, not from Ce ions with Ce3+ valence. The results revealed that surface-Ce accumulation in rice was independent on the NP dissolution. In all treatments, there was always more surface-Ce (IP-C-Ce and IPA-Ce) accumulation in NP14 treatment than the one in NP25 treatment (Fig. 2A). First, in the same treatment, NP14 used in this study was more resistant to aggregation than NP25 in root exudates solution (Fig. 5A), which would promote more nanoparticles moving to root surface and then precipitate as surface-Ce. Second, |ζ| value of 14 nm NPs in solution was significantly higher than that 25 nm NPs (Fig. 5B), except for Fehigh treatment with insignificant difference between them. The above-described reasons led to more surface-Ce accumulation in NP14 treatment compared to NP25 treatment (Fig. 2A) due to strong attractive electrostatic interactions between root surface and NP14. SEM imaging also revealed more NPs aggregations on root surface in NP14 treatment compared to NP25 treatment (Fig. S1A and S1B). In the current study, as compare with no IP, IP formation significantly reduced the surface-Ce accumulation with reduction rates of 22.8–41.9% and 62.5–77.8%, respectively, in NP14 and NP25 treatments (Fig. 2A), which revealed the effect of IP was more obvious in NP25 treatment compared to NP14 treatment. PCA was conducted to further analyze the effect of CeO2 NPs characteristics changes in solution on the surface-Ce accumulation when Fe2+ was present in solution. The results indicated that the reduction of ζpotential and the enhancement of hydrodynamic diameters of NPs strongly inhibited surface-Ce (containing IP-C-Ce and IP-A-Ce) accumulation (Fig. 6B), which were in good agreement with above discussion about the dependent of hydrodynamic diameters and ζ-potential. This was due to hydrodynamic diameter being inversely proportional to movement of NPs from solution to root surface as well as the interactive attraction of NPs and root surface. However, the interactive attraction of NPs and root surface was promoted with increasing ζ-potential. The increase of NPs dissolution was in agreement with the reduction of Ce bioaccumulation in IP treatments (Fig. 6B), which was probably caused by limited NPs dissolution over a 4-day short period (Fig. 5D). For example, Ce amounts in solution from NPs dissolution were measured after 4 d. Total Ce amounts in rice were measured containing surface-Ce, rootCe and shoot-Ce. The results indicated Ce from dissolution of NP14 and NP25 only accounted for 7.61% and 6.18% of total Ce in rice, respectively. The PCA also didn't reveal the positive effect of dissolution on surface-Ce accumulation. PCA indicated the negative correlation between surfaceCe (IP-C-Ce and IP-A-Ce) accumulation and NPs size (Fig. 6B), which was consistent with the results described above.

4.2. IP formation reducing Ce accumulation in rice tissues Zhang et al. (2011) found that most CeO2 NPs were adsorbed on the root surface of cucumber due to their physical adsorption, not inside root, which was different from the present study showing most NPs accumulated inside rice root, not root surface. In rice tissues, root-Ce concentration had a range between 0.845 and 2.87 mg·g−1 (Fig. 2B), while shoot-Ce concentration ranged only from 0.043 to 0.366 mg·g−1 (Fig. 2C). This suggested more Ce bioaccumulation in the root than that in the shoot. TEM imaging also observed NPs aggregations in root cell (mainly in vacuoles regions) whether IP was present or not (Fig. S2). Usually, Ce ions are more easily taken up by rice root compared to CeO2 NPs. In the study, Ce ions from NPs dissolution (4 d) only accounted for 8.68% and 6.79% of total Ce amounts in rice tissues, respectively, in NP14 and NP25. The result showed that Ce in rice tissue wasn't mainly from NPs dissolution. In all treatments, Ce4+ valence was still the main state in root samples (Fig. S3), which showed that CeO2 NPs as the main form was absorbed by rice tissues, not Ce ions. The bioaccumulation of pristine nanoparticle in rice could pose a adverse effect on rice growth.

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Current literatures have shown that the IP formation may act as a barrier to prevent the bioaccumulation of toxic contaminants in plant tissue (Li et al., 2016; Fresno et al., 2016). In the present study, IPlow and IPhigh were set up to observe the IP effect on Ce accumulation in rice. The root-Ce (Fig. 2B) and shoot-Ce (Fig. 2C) decreased significantly with the increase of IP amounts, meaning that IP formation prevented Ce accumulation in rice tissues with two NPs exposure. In all treatments, insignificant rice growth inhibition was observed compared to CK (Fig. 1), suggesting that the effect on Ce bioaccumulation was attributed to the formation of IP. IP formation weakened the interactive attraction of CeO2 NPs and root surface in the Section 4.1 discussed above, which caused the reduction of not only surface-Ce accumulation, but also root-Ce and shoot-Ce accumulation. The results of PCA also indicated the positive correlation between root/shoot-Ce and ζ-potential, and the negative correlation between root/shoot-Ce and hydrodynamic diameter (Fig. 6B). Similar to the experiment of short period, IP presence reduced root-Ce, shoot-Ce and grain-Ce at the end of full life cycle (Fig. 3). The decreased grain-Ce could pose a positive effect on food security. Therefore, the inhibition effect of IP was always present in both experiments of short and long period. Root-Ce accumulation (Fig. 2A and B) was lower in NP25 than NP14 due to high hydrodynamic diameter and low ζ-potential in NP25 compared to NP14 (Fig. 5A), except at NP treatments where a significant increase of root-Ce in NP25 with respect to NP14. The similar finding wasn't observed for shoot-Ce accumulation (Fig. 2C). Reversely, shoot-Ce accumulation was higher in NP25 than in NP14. The results of PCA also indicated the negative correlation between root-Ce and NPs size (Fig. 6B). In IP treatments, root-Ce decreased 25.4–38.4% and 61.9–70.6% compared to no IP treatments, respectively, in NP14 and NP25 treatments (Fig. 2B). It was observed that, in IP treatments, tissue-Ce (containing root and shoot) decreased 20.8–39.4% and 54.3–61.2% compared to no IP, respectively, in NP14 and NP25 treatments. Our results presented herein revealed that the inhibition effects of IP on Ce accumulation in root and whole rice tissue followed the trend NP25 N NP14 (Fig. 2B and C). Similar trend was also observed for inhibition effects of IP on surface-Ce (Fig. 2A). This suggested that IP formation inhibited more effectively Ce accumulation in rice tissues in large size of NPs than small size of NPs. IP formation caused more Fe accumulation in rice tissue (Fig. 4). With the increase of IP amounts, root-Ce and shoot-Fe increased. In IPlow and IPhigh treatments, CeO2 NPs exposure decreased surface-Ce, root-Fe accumulations, while increased shoot-Fe (Fig. 4). However, in no IP treatments, two CeO2 NPs exposure didn't affect surface-Fe, rootFe and shoot-Fe accumulations (Fig. 4), which was different from Ma et al. (2016) who found significant lower levels of Fe accumulation in Arabidopsis root tissues exposed to CeO2 NPs. In the present study, the effect of CeO2 NPs was only observed in IP treatments, not in no IP treatments. The results showed that the effect of CeO2 NPs exposure on Fe uptake depended on plant species and Fe amounts added. A different Fe distribution in rice (Fig. 4) was observed compared to Ce accumulation (Fig. 2). Fe accumulation followed the order as: surface N shoot N root. However, the highest Ce accumulation was always found in the root. For the same NP treatments, Ce accumulation in root/shoot decreased with the increase of Fe accumulation in root/shoot, suggesting that the Fe accumulation affected Ce accumulation in rice tissues. PCA also indicated the negative correlation between shoot-Ce/root-Ce and IP-Fe/tissue-Fe (Fig. 6B). A possible explanation was that more Fe accumulation on root surface as IP and in root and shoot of rice in such Fe enriched environment decreased CeO2 NPs movement to root surface and then reduced their accumulation in rice root and shoot. 4.3. IP formation affecting Ce translocation and Ce distribution in rice At present, there was limited work on how Ce was accumulated external and internal root of plants after CeO2 NPs exposure (Zhang et al., 2011; Zhao et al., 2013), which caused the difficult assessment to

Please cite this article as: Y. Bao, C. Pan, W. Liu, et al., Iron plaque reduces cerium uptake and translocation in rice seedlings (Oryza sativa L.) exposed to CeO2 nanoparticles with different sizes..., Science of the Total Environment, https://doi.org/10.1016/j.scitotenv.2019.01.181

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TFsurface-root of Ce. Further experimentation is necessary to separate Ce accumulation on root surface and inside root. The present study showed that IP formation had no significant effects on TFsurface-root of Ce in NP14 and NP25 treatments, except that IPhigh increased TFsurface-root of Ce in NP25. In rice, IP formation with different amounts (IPlow and IPhigh) reduced significantly TFroot-shoot of Ce in two CeO2 NPs exposure with 14 nm and 25 nm sizes. Similar finding was observed by Li et al. (2016) that IP decreased significantly the translocation of both IHg and MeHg in rice seedlings. Also, Fresno et al. (2016) reported that the presence of IP provoked a reduction in the As transfer to shoots. Herein, IP formation might act to inhibit Ce uptake by shoot, and decrease its food-chain toxicity risk in the future. IP formation decreased Ce distribution, accumulation and translocation from root to shoot through remaining more CeO2 NPs in solution (Fig. 2D), which will have potential applications on Ce accumulation in rice through IP presence to minimize Ce accumulation in rice. Whether IP was present or not, Ce distribution always followed the order as: root N root surface N shoot (Fig. 2D). This was different from what Zhang et al. (2017) found in romaine lettuce experiment after CeO2 NPs exposure, where Ce mostly distributed outside the roots, not inside root. It suggested that the Ce accumulation and distribution was dependent on crop species. This study indicated that Ce was easily accumulated in rice root, which could affect Ce uptake and translocation in rice tissues in the subsequent growth stages. IP presence contributed to the reduced uptake of Ce by rice compared to no IP.

5. Conclusion Upon exposure to 14 nm and 25 nm CeO2 NPs, IP formation had obvious inhibitory effects on Ce accumulation on root surface and tissue (root and shoot) of rice. CeO2 NPs as the main form was absorbed by rice due to limited NPs dissolution in hydroponic solution. In IP treatments, Fe2+ as the main iron ion was released from IP dissolution, which enhanced hydrodynamic diameter of CeO2 NPs and reduced their ζ-potential values. IP formation decreased |ζ| values of rice root as well. This weakened the interactive attraction of NPs and root surface in IP treatments compared to no IP treatments, which reduced surfaceCe accumulation, and further decreased root-Ce and shoot-Ce accumulations. The effect of IP on Ce accumulation in rice increased with increasing IP amounts. PCA indicated the negative correlation between surface-Ce (IP-C-Ce and IP-A-Ce) accumulation and NPs size, and between shoot-Ce/root-Ce and IP-Fe/tissue-Fe. Moreover, the reduction effect was more obvious in NP25 than NP14. It was also found that IP decreased Ce translocation from root to shoot. A full life study showed the reduction effect of IP on surface-Ce, root-Ce, shoot-Ce and grain-Ce accumulations. To the best of our knowledge, this is the first study describing the effect of IP formation on Ce accumulation in rice under CeO2 NPs exposure. Overall, this study offers new insight into preventing CeO2 NPs accumulation in rice through IP formation. However, further works are essential to confirm the above results, especially to characterize the effects of growth stages, culture time and rice species on reduced rice-Ce accumulation in IP treatments.

Acknowledgement This work is financially supported by the Tianjin Municipal Science and Technology Commission (Grant 16JCZDJC39200), by National Key R&D Program of China (2018YFD0800303) and by the National Natural Science Foundation of China (41471400). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2019.01.181.

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Please cite this article as: Y. Bao, C. Pan, W. Liu, et al., Iron plaque reduces cerium uptake and translocation in rice seedlings (Oryza sativa L.) exposed to CeO2 nanoparticles with different sizes..., Science of the Total Environment, https://doi.org/10.1016/j.scitotenv.2019.01.181