Arsenic accumulation and speciation in rice grains influenced by arsenic phytotoxicity and rice genotypes grown in arsenic-elevated paddy soils

Journal of Hazardous Materials 286 (2015) 179–186

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Arsenic accumulation and speciation in rice grains influenced by arsenic phytotoxicity and rice genotypes grown in arsenic-elevated paddy soils Chien-Hui Syu, Chia-Chen Huang, Pei-Yu Jiang, Chia-Hsing Lee, Dar-Yuan Lee ∗ Department of Agricultural Chemistry, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan

h i g h l i g h t s • As concentrations in grains of different rice genotypes were reduced by As toxicity. • Indica have equal or higher As concentrations than japonica in grains in this study. • The concentrations of DMA, instead of AsIII , increased with total As in rice grains.

a r t i c l e

i n f o

Article history: Received 27 September 2014 Received in revised form 9 December 2014 Accepted 27 December 2014 Keywords: Arsenic Arsenic species Rice Genotypes Phytotoxicity Paddy soil

a b s t r a c t Rice consumption is a major route of As exposure to human for the population of worldwide. This study investigates the effect of phytotoxicity and rice genotypes on the content and speciation of As in rice grains grown in different levels of As-elevated paddy soils from Taiwan. Three levels of As-elevated soils and six rice genotypes commonly planted in Taiwan were used for this study. The results indicate that As contents in grains of rice is not proportional to soil As concentrations and they were equal or higher in indica genotypes than japonica genotypes used in this study. It was also found that the As phytotoxicity not only reducing the grain yields but also the As concentrations in grain of rice. The predominant As species found in rice grains were dimethylarsinic acid (DMA) and arsenite. The concentrations of DMA increased with total As concentrations, wherggeas the arsenite remained in a narrow range from 0.1 to 0.3 mg kg−1 . Because of the lower toxicity of DMA than inorganic As species, the health risks may not be increased through consumption of rice even when total As content in the grains is increased. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Inorganic arsenic (As) is identified as a non-threshold, class 1 human carcinogen, and the intake of As through rice consumption may lead to serious health effects such as bladder and skin cancers [1,2]. The sources of As in rice paddy fields include natural (biogeochemical process) and anthropogenic (As-containing irrigation water, metal mining activity, arsenical pesticides and fertilizer applications) pathways [3,4]. Rice is the dietary staple for about half of the world’s population and unfortunately, rice consumption has been the main arsenic exposure route in recent years [1,5]. This is due to the high bioavailability and mobility of As in flooding

∗ Corresponding author. Tel.: +886 2 33664811; fax: +886 2 23638192. E-mail address: [email protected] (D.-Y. Lee). http://dx.doi.org/10.1016/j.jhazmat.2014.12.052 0304-3894/© 2014 Elsevier B.V. All rights reserved.

conditions, enhancing the uptake and accumulation of As by rice plants [6]. The concentration of As accumulated in rice grains is approximately 10-fold higher than other cereal crops [5,7]. Meharg and Rahman [8] found that the concentrations of As in rice grains grown in As-contaminated soils in certain parts of Bangladesh to be up to 1.8 mg kg−1 , resulting in serious As related risks to the local residents. Moreover, paddy rice grown in As-contaminated soils result in As phytotoxicity (inhibition of ATP formation and oxidative stress) which causes lower grain yields [9,10]. In pore water found in paddy soil, the concentration and speciation of As are controlled by the soil redox status and other soil properties such as soil pH, organic matter, aqueous chemistry [the concentration of phosphorus (P) and silicon (Si)], clay minerals and iron oxides content [4,11–13]. In submerged soils, the mobility of arsenite (iAsIII ; as H3 AsO3 ) is higher than arsenate (iAsV ; H3 AsO4 ) because of the reductive dissolution of iron (Fe) oxides/hydroxides

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Table 1 Total As concentrations in grains of rice grown in different countries. Country

Grain As (mg kg−1 )

Bangladesh China India Italy Spain Taiwan Thailand USA Vietnam

0.05–2.05 0.02–0.46 0.18–0.31 0.07–0.33 0.05–0.82 0.10–0.63 0.01–0.39 0.10–0.66 0.03–0.47

n 35 124 133 38 76 280 54 134 31

Reference Islam et al. [48] Meharg et al. [23] Meharg et al. [23] Meharg et al. [23] Meharg et al. [23] Lin et al. [49] Meharg et al. [23] Williams et al. [5] Phuong et al. [50]

and consequent reduction of iAsV to iAsIII [14]. The uptake and translocation of As in rice plants and the accumulation in rice grains are strongly dependent on the As species that exist in the rhizosphere [15]. In addition, the presence of P and Si in soils also impact the uptake of As by paddy rice [16–18]. There are two main mechanisms involved in As uptake into the roots of rice plants. The first route is through the phosphate transport pathway since arsenate is an analogue of phosphate [19]. The second route is when arsenite (silicic acid analog) and undissociated methylated As species (dimethylarsinic acid (DMA) and monomethylarsonic acid (MMA)) are taken up into the root by aquaporin channels [20,21]. Raab et al. [22] indicated that the uptake efficiency of methylated As species (DMA and MMA) into the root was much lower than inorganic arsenic species (iAsIII and iAsV ), but the translocation efficiency in the rice plant of methylated As species was much higher than inorganic arsenic species. It is generally believed that the toxicity of inorganic As is much higher than the pentavalent methylated As species [23,24]. Since the As species distribution in rice grains governs human toxicity, therefore, understanding the As species in rice grains is important for evaluating the As toxicity of rice to humans. In our previous studies, we found that the translocation of As in rice plants impacts on the accumulations of As in rice shoots among different rice genotypes in As-contaminated soils [25]. In this study, we intend to further investigate the effects of rice genotypes on As accumulation and speciation in rice grains grown in As-elevated paddy soils from the Guandu Plain of northern Taiwan. In general, DMA and iAs are the predominant As species in rice grains [26] and their percentages varied widely. Many studies have shown the effect of rice genotypes and environmental factors on the accumulation and speciation of As in rice grains [27–29]. Table 1 shows total As concentrations in grains of rice grown in different countries. It indicated that the maximum As concentrations in grains differed by 6–7 folds in rice grown in different countries. It also discovered that grain As concentrations differed by about 40-folds in rice produced in the same country, indicating that the genotypes and environmental factors play an important role in grain As accumulation. Norton et al. [28] and Ahmed et al. [27] reported that the environment was the main controlling factor in grain As concentrations. Norton et al. [29], and Pillai et al. [30] indicated that there were variation in grain As concentrations among different cultivars, and there were significant genotype effects on the As speciation in rice grains. In addition, As phytotoxicity also impacts on the accumulation and speciation of As in rice grains [31]. Many studies have investigated rice As uptake and accumulation in soils of As concentrations less than 100 mg kg−1 [27–30,32]. Norton et al. [28] evaluated the accumulation of As in grains of rice grown at two field sites in Bangladesh (soil As: 10.3 ± 2.2 and 29.6 ± 7.2 mg kg−1 ), India (soil As: 6.3 ± 1.3 and 17.9 ± 4.0 mg kg−1 ) and China (soil As: 64.6 ± 4.7 and 65.6 ± 2.5 mg kg−1 ), respectively. In addition, Hsu et al. [32] also investigated As concentrations in grains of rice grown in paddy soils in southwestern Taiwan (soil As: 52.2 ± 27.1 mg kg−1 ). However, There are only few studies that have investigated rice As uptake and accumulation in high level

Table 2 The basic properties of the three levels of As-contaminated soils of Guandu Plain. pH Org. C(%) Texture Fed Test soils As-L 6.6 3.6 As-M 5.1 3.6 As-H 4.9 2.1

Clay Clay Clay

g kg−1 20.0 21.9 36.6

Ald

Feo

Alo

g kg−1 3.7 2.6 4.3

g kg−1 10.5 21.7 25.3

g kg−1 mg kg−1 mg kg−1 1.9 8.3 16.3 2.2 257.4 343.3 3.0 334.1 512.3

Aso

Astotal

Fed dithionite–citrate–bicarbonate extractable Fe; Ald dithionite–citrate–bicarbonate extractable Al; Feo ammonium oxalate extractable Fe; Alo ammonium oxalate extractable Al; Aso ammonium oxalate extractable As.

of As-contaminated soils (> 200 mg As kg−1 ). The As accumulation and speciation in rice grain among different rice genotypes grown in different levels of As-contaminated soils is worthy to be studied. Therefore, the aim of this study is to investigate the effect of As phytotoxicity and rice genotypes on the content and speciation of As in rice grains. 2. Material and methods 2.1. Soil collection and characterization Three levels of As-contaminated soils (16.3 (As-L), 343.3 (As-M) and 512.3 (As-H) mg As kg−1 ) used in this study were collected from the surface soil (0–30 cm) of paddy fields in the Guandu Plain, Taipei, Taiwan (Table 2). The As concentrations of As-M and AsH soils are higher than farmland control standard (60 mg kg−1 ) of Taiwan’s Environment Protection Administration. Basic properties of tested soil are presented in Table 2. 2.2. Pot experiment of paddy rice Pot experiments were performed in the phytotron at controlled temperature (20/25 ◦ C, night/day) and relative humidity (70–95%) under sunlight. Six rice genotypes commonly planted in Taiwan including three japonica rice (TK 9, TC 192, TK 139) and three indica rice (TCN 1, TCSW 1, TCSY 837) were used in this study. There were three replicates for each rice genotype grown in three levels of As-contaminated soils. Rice seeds were sterilized in a solution containing 1% sodium hypochlorite (NaClO) solution and 1 drop of Tween 20 for 30 min, washed with deionized water and then germinated in Petri dishes containing moist tissue paper for three days. After germination, rice seedlings were transferred to a 0.6-L beaker and grown in half-strength modified Kimura B nutrient solution (pH was adjusted to 4.8–5.0 and the solution was renewed every two days) for 2 weeks. Two seedlings were transplanted into each pot filled with 2.3 kilograms of tested soil. The soils were saturated with water and the water level was maintained at about 3–5 cm above the soil surface during the whole period of plant growth. The soils were supplemented with 0.26 g N kg−1 as NH4 2 SO4 , 0.039 g P2 O5 kg−1 as KH2 PO4 and 0.054 g K2 O kg−1 as K2 SO4 as basal fertilizers. The application of 0.13 g N kg−1 as a top dressing was done 15 days and 60 days after transplantation respectively. Mature rice was harvested nearly 130 days after transplantation. The harvested rice plants were separated into grain, flag leaf, straw and root. These samples were rinsed with tap water and then with deionized water. In order to avoid changes of As species in plant tissues, a portion of plant samples were stored at −20 ◦ C till As species analysis. The grain yield, biomass of rice plants and lengths of each shoot and root were measured. 2.3. Soil pore water collection and analysis Soil pore water in the pots was collected by Rhizon soil moisture samplers (Rhizosphere Research Products) inserted into the

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soil near the middle of the container throughout the plant growth period. In order to prevent the precipitation of Fe ions and the change of As species, a portion of the solution was taken and preserved immediately in 5% HNO3 and 0.01 M H3 PO4 [33], respectively. The concentrations of Fe, As (As species) and dissolved organic carbon (DOC) were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES, Optima 2000 DV, Perkin Elmer), high pressure liquid chromatography-inductively coupled plasma-mass spectrometry (HPLC-ICP-MS, LC 1200 and ICP-MS 7700×, Agilent Technologies) and total organic carbon analyzer (TOC analyzer, Aurora 1030 W), respectively. The changes of soil pH and redox potential (Eh) were also monitored with flooding incubation. Five hundred grams of tested soils and 500 mL deionized water were put into plastic containers and incubated at room temperature (25 ± 2 ◦ C). A platinum electrode and a Ag/AgCl electrode (reference electrode) were used to measure the soil Eh during the flooding incubation period.

2.4. Plant digestion and analysis The rice grain (bran and polished grain separately) were oven dried at 70 ◦ C for 48 h, and ground to a fine powder. Dried grain samples (0.5–1.0 g) were digested in concentrated HNO3 /H2 O2 in heating blocks [8]. The volume of the digests was brought up to 50-

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mL with deionized water and filtered through a 0.45 ␮m filter and stored in plastic bottles for subsequent analysis. The concentrations of As was determined by ICP-MS. Certified reference materials of the plant sample (NSC DC73349) and rice flour (NIST 1568a) were used to verify the recovery of the digestion methods and elements analysis.

2.5. Determination of arsenic species in rice grains Rice grain samples were extracted with 10 mL 0.28 M HNO3 placed in heating blocks at 95 ◦ C for 90 min [34]. The reference material ERM BC-211 rice flour was used to check the extraction method. Arsenic species analysis standards used in this study including sodium meta-arsenite (NaAsO2 , J.T. Baker), sodium arsenate dibasic heptahydrate (Na2 HAsO4 ·7H2 O, Sigma), Monosodium acid methane arsonate sesquihydrate (MMA, CH4 AsNaO3 ·1.5H2 O, Chem Service) and dimethylarsinic acid (DMA, C2 H7 AsO2 , Chem Service). The matrix match standards and speciation extract were determined by HPLC coupled to an ICP-MS. An anion-exchange column (PRP-X100, 250 × 4.1 mm, 10 ␮m, Hamilton Company, Reno, NV, USA) was used. The mobile phase was 20 mM NH4 H2 PO4 , pH adjusted to 5.6 with NH4 OH. The injection volume was 50 ␮L, the flow rate was 1.5 mL min−1 and the temperature set at ambient temperature. In order to avoid changes of As in the extracts, the

Fig. 1. The soil (a) As and (b) As species percentage in pore water of the three levels of As contaminated soils of Guandu Plain during the growth period. Data means ± standard deviation. (As-L: 16.3 mg As kg−1 , As-M: 343.3 mg As kg−1 , As-H: 512.3 mg As kg−1 ).

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extracts were stored at 4 ◦ C in the dark, and all of the analyses were completed in 48 h. This analysis method can effectively separate four arsenic species in 10 min. The extraction recovery of ERM BC211 was 96.5%, and the HPLC–ICP-MS analysis recovery was 102.9% and 90.1% for inorganic As and DMA. The analysis recovery of tested rice samples was 95.5 ± 5.2%, and there were significant correlation between the concentrations of total digested As and the sum of As species in rice grains (R2 = 0.9903, P < 0.001). 2.6. Data analyses Data presented in this study are means (n = 3) plus standard deviations (SD). Analysis of variance (ANOVA) was used to test the effect of soil As levels and rice genotypes on As accumulation in rice plants. For comparing the differences between treatments (soils or genotypes), we used the least significant difference (LSD) test at the level of P = 0.05. ANOVA and LSD tests were performed using the SAS 9.2 software package. Regression analyses were conducted using Excel for Windows. 3. Results and discussion 3.1. Dynamics of As in soil pore water and rice plant growth The concentrations of As increased markedly in soil pore water with growth time (Fig. 1a), and the As concentrations in As-M and As-H soils were higher than 3000 ␮g L−1 , whereas the As concentrations in As-L soil was below 200 ␮g L−1 . Because soil Eh decreased markedly with flooding time (Fig. 2b) and high concentrations of dissolved organic carbon (DOC, 150–250 mg L−1 ) were released in pore water (Fig. 2 c), the reductive dissolution of Fe oxides/hydroxides and iAsV reduction were enhanced, leading to increased concentrations of As in pore water [14]. High concentra-

tions of Fe in pore water during growth time (100–400 mg L−1 ) were also observed (Fig. 2d). In addition, the DOC may have competed with As for the adsorption sites of soil minerals or the formation of DOC-As complexes which can enhance the release of As into soil pore water [13,35]. Fig. 1b shows that the predominant As species in pore water is iAsIII and only a small proportion of As are in the form of iAsV , DMA, and MMA. The results are consistent with those found in our previous study [25]. Arsenic phytotoxicity was observed in rice plants grown in As-M and As-H soils. There were significant differences in the root length and shoot height of rice among the tested soils (root length: P < 0.01; shoot height: P < 0.01) and genotypes (root length: P < 0.001; shoot height: P < 0.001). The root length and shoot height of rice grown in As-L soils was greater than As-M and As-H soils, except for the root length of the KS 145 and TK 9 genotypes (Fig. 3a and b). There were no uniform trends in the biomass of straw among rice genotypes and soils (data not shown). The grain yields of six tested rice genotypes grown in As-L soils were also higher than those in AsM and As-H soils (Fig. 3c), and there were significant differences between the three tested soils (P < 0.01). Compared with the As-L soils, the grain yields for As-H soils was reduced 37.0–63.1%. However, there were no significant differences in grain yields among rice genotypes grown in As-M and As-H soils. This result indicates that As phytotoxicity causes grain yield loss in As-M and As-H soils, which was in agreement with previous studies [10,31,36,37]. Khan et al. [31] reported that rice grain yields decreased more than 66% when the soil As concentrations increased from 10 to 70 mg As kg−1 . In addition, it was also found that the Fe concentrations in As-M and As-H soils were higher than 200 mg L−1 (Fig. 2d), it may lead to Fe phytotoxicity and further lower grain yields. Vromman et al. [37] reported that the Fe stress (125 mg L−1 ) and combined stresses (Fe + As) reduced the grain yields.

Fig. 2. The soil (a) pH, (b) Eh, the concentrations of (c) dissolved organic carbon (DOC) and (d) Fe in pore water of the three levels of As contaminated soils of Guandu Plain during the growth period. Data means ± standard deviation. (As-L:16.3 mg As kg−1 , As-M: 343.3 mg As kg−1 , As-H: 512.3 mg As kg−1 ).

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Fig. 4. The concentrations of (a) grain (polished grain) and (b) bran of six tested rice genotypes grown in the three levels of As contaminated soils of Guandu Plain. Data means ± standard deviation (n = 3). Different small and capital letters above the bars indicate significant difference in the values among the tested soils and rice genotypes respectively based on the LSD test (P < 0.05). (As-L: 16.3 mg As kg−1 , As-M: 343.3 mg As kg−1 , As-H: 512.3 mg As kg−1 ).

Fig. 3. The (a) root length, (b) shoot height and (c) grain yield of six tested rice genotypes grown in the three levels of As contaminated soils of Guandu Plain. Data means ± standard deviation (n = 3). Different small and capital letters above the bars indicate significant difference in the values among the tested soils and rice genotypes respectively based on the LSD test (P < 0.05). (As-L: 16.3 mg As kg−1 , As-M: 343.3 mg As kg−1 , As-H: 512.3 mg As kg−1 ).

3.2. Influence of As phytotoxicity on the As accumulation in rice grains The As concentrations in grains (polished) and bran grown in As-L soils was higher than As-M and As-H soils and there were significant differences among the three tested soils. The only exception was the As in bran of TCSW 1 genotypes (Fig 4). The grain As concentrations of the six tested genotypes grown in As-M and As-H soils was reduced 40.4% and 35.0% respectively compared with AsL soils (Fig. 4a). The lower As concentrations in the grains of the six tested rice genotypes grown in As-M and As-H soils compared to As-L soils may result from the As phytotoxicity of rice plants grown in high As concentration soils (As-M and As-H soils), as shown in Fig. 3. Khan et al. [31] and Panaullah et al. [10] also found that

the phytotoxicity of As in rice plants resulted in the reduction of As accumulation in the rice grains. Khan et al. [31] reported that the As concentrations in rice grains (0.24–1.09 mg kg−1 ) decreased with soil As levels (4.0–137.9 mg kg−1 , tested soils collected from Bangladesh, China and UK). However, there are some studies indicating that the grain As concentrations increases with soil and pore water As concentrations. [36,38]. Most of studies indicate the reduction of rice grain yields under As-contaminated soils [10,31,36], but As concentrations in grains are varied and are influenced by soil properties, As bioavailability, As translocation and tolerance of rice among the various studies [15,27–29,39]. In this study, the soil and pore water As concentrations of As-L soils were 16.3 mg kg−1 and about 150 ␮g L−1 respectively, which were much lower than those in the As-M and As-H soils. However, the grain As concentrations of the six rice genotypes grown in As-L soils were 0.68–0.92 mg kg−1 , which were significant higher than those in AsM and As-H soils (Fig. 4a). This result indicates a higher As uptake and translocation efficiency in rice grains grown under normal (low As toxicity) growth conditions compared to those in high As toxicity soils. This observation is similar to those found in the study of Khan et al. [31] who reported that high concentrations of As accumulated in rice grains grown in low As concentration Bangladeshi soils (< 20 mg kg−1 ). In our pot experiment, the soils were flooding throughout the entire growth period. This can also enhance As release from soils, and further lead to the accumulation of As in rice

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Table 3 Total As concentrations in grains of 20 different genotypes of rice grown in soils regularly irrigated with 0.4 mg L−1 As-contaminated water. (compiled from Wu et al. [41]). Subspecies

Genotype

Origin

Type

Grain As (mg kg−1 )

Indica

Erjiufeng Xiushui 11 Guinongzhan Yingjingruanzhan Yuxiangyouzhan TNAU 98 CNT87059–3 TD71 IR72 PSB RC 70 IAPAR9 Dongnong413 Hejiang16 Nanyangzhan Handao Handao3 Handao1 Xijing7 Kinmaze TORO 2

China China China China China India Thailand Thailand Philippines Philippines Brazil China China China China China China China Japan USA

Paddy Paddy Paddy Paddy Paddy Paddy Paddy Paddy Paddy Paddy Upland Paddy Paddy Paddy Upland Upland Upland Upland Paddy Paddy

0.22 0.78 0.22 0.48 0.27 0.19 0.70 0.46 0.61 1.17 0.28 0.22 0.22 0.70 0.34 0.22 0.47 0.79 0.40 0.30

Japonica

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.05 0.04 0.05 0.08 0.06 0.05 0.26 0.11 0.33 0.25 0.07 0.06 0.08 0.05 0.03 0.01 0.09 0.08 0.04 0.07

grains [40]. Due to the high amounts of Fe in the soils (Table 2) and pore water of As-M and As-H soils (Fig. 2d), it may lead to reduced grain yields of rice (Fig. 3c). However, in the study of Vromman et al. [37] indicated that the Fe stresses had no insignificant impact on the As concentrations in rice grains under hydroponic culture. Therefore, the As phytotoxicity might be the main reason resulted in the reduction of the As accumulation in rice grains grown in As-M and As-H soils.

ond, As prefers to accumulate in the protein-rich tissues such as the aleurone layer and embryo. In addition, it was also found that there was significant positive correlation between the concentrations of As in rice bran and grain of six tested genotypes (R2 = 0.4656, P < 0.01, data not shown). 3.4. As species distribution in rice grains of different genotypes Since As species distribution in rice grains govern the toxicity to humans, and the maximum level for inorganic arsenic in polished rice (0.2 mg kg−1 ) has been adopted by Codex Alimentarius Commission [43]. Therefore, it is important to know the distribution of As species in rice plants. Fig. 5 shows As species distributed in polished grains and bran. Fig. 5a shows that the predominant As species in the polished grains were DMA (45.6–80.2%) and iAsIII (19.8–54.4%). This result is in agreement with previous studies [24,31]. Arao et al. [15] and Lomax et al. [44] found that the uptake and translocation of As species in rice plants and the accumulation of As species in rice plants tissues and grains may originate from media such as soil and nutrient solutions. However, in the present study, the high proportions of DMA found in rice grains does not correspond to the low proportion of DMA among As species in soil pore water (Fig 1b). This may be due to the fact that methylated As species such as DMA are elevated in rhizosphere [45] and cannot be effectively collected by soil water samples used in this study. Some studies also proposed that the differences in As methylation ability among the various rice genotypes [5,24,46]. It was also discovered that the concentrations of DMA in grains increased with total As concentrations of grains of the six tested rice genotypes grown in the three As level soils

3.3. Influence of rice genotypes on the As concentrations of rice grains The grain As concentrations of indica genotypes (0.88 ± 0.04 mg kg−1 ) were higher than japonica genotypes (0.71 ± 0.02 mg kg−1 ) grown in As-L soils. TCSW1 (0.92 ± 0.04 mg kg−1 ) and KS 145 (0.68 ± 0.01 mg kg−1 ) had the highest and lowest concentrations of As in rice grains, respectively (Fig. 4a). The results indicate that the As uptake and translocation capability of the indica genotypes are higher than japonica genotypes grown in low As-contaminated soils. However, there is no significant difference in grain As concentrations between indica genotypes and japonica genotypes grown in As-M and As-H soils. This possibly results from As uptake and translocation capabilities in rice plants being affected by As phytotoxicity. In the study of Wu et al. [41], they investigated the As concentrations in grains of 20 different genotypes including 10 indica and 10 japonica originated from different countries and types (paddy or upland rice) grown in soils regularly irrigated with 0.4 mg L−1 As-contaminated water (Table 3), They also found that there were no significant differences in As concentrations of grains between indica and japonica genotypes. There were significant differences in the As concentrations of bran among the tested genotypes grown in As-M and As-H soils, but there were no significant differences in As-L soils (Fig. 4 b). We also observed that the As concentrations in bran was 1.43–6.54 mg kg−1 , which was higher than those in the grains (0.34–0.92 mg kg−1 ) of the six tested genotypes. The rice bran is composed of maternal tissues such as ovular vascular system and filial tissues including the aleurone layer, embryo and parts of the endosperm. Lombi et al. [42] indicated that the reasons that the As concentrations in bran is higher than in polished grain are as follows: first, the rice bran may be a physiological barrier in As translocation in rice grain. Sec-

Fig. 5. The percentage of As species in (a) grain (polished grain) and (b) bran of six tested rice genotypes grown in the three levels of As contaminated soils of Guandu Plain. (As-L: 16.3 mg As kg−1 , As-M: 343.3 mg As kg−1 , As-H: 512.3 mg As kg−1 ).

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in bran is higher than in polished grain, because inorganic As is difficult to be transported from bran to polished grain. Thus the consumption of brown rice grown in As-contaminated soils is not recommended. 4. Conclusion The results of this study indicate that due to As phytotoxicity, lower As levels accumulate in polished grain grown in elevated Ascontaminated Guandu Plain soils (As-M and As-H). In addition, the concentrations of As in rice grains of indica genotypes are equal or higher than japonica genotypes used in this study, suggesting that japonica genotypes are recommended for planting in As-elevated paddy soils. The predominant As species found in rice grains are DMA and iAsIII , and the percentage of DMA increases with total As concentrations in rice grains. Because the toxicity of DMA is lower than that of inorganic As species, the health risk may not be increased through consumption of rice even as the total As content in grains is increased. Since the intermediate contamination levels of soil As (such as 50, 100, or 200 mg kg−1 ) were not tested in this study, it should be noted that it is uncertain if the reduction of rice growth will occur, and what the As concentration and speciation in grain and bran will be, and if it could be more dangerous to human health when consuming grains of rice grown in an intermediate As-contaminated soil (which may have higher As uptake and accumulation and percentage of As (III) than those in As-M or As-H soils, and higher As availability than in As-L soil). Acknowledgements The financial support from the National Science Council, Executive Yuan, Taiwan (grant no. NSC-101-2313-B-002-012-MY3) is sincerely appreciated. Fig. 6. Regression between (a) the concentrations of total As and As species in grains (polished grain) and (b) the concentrations of total As and As species percentage in grains of six tested rice genotypes grown in the three levels of As contaminated soils of Guandu Plain.

(R2 = 0.9091, P < 0.001), but the concentrations of iAsIII remained in the narrow range of 0.1 to 0.3 mg kg−1 (Fig. 6a). Fig. 6b also shows that the percentage of DMA increases with total As concentrations of grains (R2 = 0.5911, P < 0.001), and conversely, the iAsIII concentration decreases (R2 = 0.5560, P < 0.001). These findings indicate that the translocation and accumulation of iAsIII into grains may be restricted while the total grain As concentrations increase. The results are similar to the study of Khan et al. [31], but differing from Meharg et al. [23] who found that the inorganic As in rice grains increases with total As concentrations of grains. The different results of the various studies may result from environmental factors and rice genotypes [27,28]. Carey et al. [46] also indicate that the translocation of DMA from shoot to grain is much more efficient than inorganic As. Therefore, from our results in this study, it reveals that the accumulation of As in polished rice grains was controlled by growth conditions (such as As phytotoxicity) and rice genotypes (the higher uptake capability in indica than japonica genotypes). However, according to the result of Fig. 6a, the distribution of As species in polished rice grains of different genotypes was highly depending on the concentrations of As accumulated in polished rice grains. It should be noted that there were higher concentrations of As (Fig. 4b) and larger proportion of iAsIII in the rice bran (72.2–98.3%, Fig. 5b) of all tested genotypes grown in the three As level soils compared with those in polished grains, which is in agreement with the study of Sun et al. [47] who found that the inorganic arsenic in rice bran was an order of magnitude higher than in bulk grain. Lombi et al. [42] also reported that the proportion of inorganic As

References [1] D. Mondal, D.A. Polya, Rice is a major exposure route for arsenic in Chakdaha block Nadia district, West Bengal India: a probabilistic risk assessmentm, Appl. Geochem. 23 (2008) 2987–2998. [2] B.P. Tchounwou, J.A. Centeno, A.K. Patlolla, Arsenic toxicity, mutagenesis, and carcinogenesis–a health risk assessment and management approach, Mol. Cell Biochem. 255 (2004) 47–55. [3] P. Bhattacharyya, A.K. Ghosh, A. Chakraborty, K. Chakrabrti, S. Tripathy, M.A. Powell, Arsenic uptake by rice and accumulation in soil amended with municipal solid waste compost, Comm. Soil Sci. Plant Anal. 34 (2003) 2779–2790. [4] P.L. Smedley, D.G. Kinniburgh, A review of the source: behavior and distribution of arsenic in natural waters, Appl. Geochem. 17 (2002) 517–568. [5] P.N. Williams, A. Raab, J. Feldmann, A.A. Meharg, Market basket survey shows elevated levels of as in South Central US processed rice compared to California: consequences for human dietary exposure, Environ. Sci. Technol. 41 (2007) 2178–2183. [6] X.Y. Xu, S.P. McGrath, A.A. Meharg, F.J. Zhao, Growing rice aerobically markedly decreases As accumulation, Environ. Sci. Technol. 42 (2008) 5574–5579. [7] P.N. Williams, A. Villada, C. Deacon, A. Raab, J. Figuerola, A.J. Green, J. Feldmann, A.A. Meharg, Greatly enhanced arsenic shoot assimilation in rice leads to elevated grain levels compared to wheat and barley, Environ. Sci. Technol. 41 (2007) 6854–6859. [8] A.A. Meharg, M. Rahman, Arsenic contamination of Bangladesh paddy field soils: implications for rice contribution to arsenic consumption, Environ. Sci. Technol. 37 (2003) 229–234. [9] A.A. Meharg, J. Hartley-Whitaker, Arsenic uptake and metabolism in arsenic resistant and non-resistant plant species, New Phytol. 154 (2002) 29–43. [10] G.M. Panaullah, T. Alam, M.B. Hossain, R.H. Loeppert, J.G. Lauren, C.A. Meisner, Z.U. Ahmed, J.M. Duxbury, Arsenic toxicity to rice (Oryza sativa L.) in Bangladesh, Plant Soil 317 (2009) 31–39. [11] K. Bogdan, M.K. Schenk, Evaluation of soil characteristics potentially affecting arsenic concentration in paddy rice (Oryza sativa L.), Environ. Pollut. 157 (2009) 2617–2621. [12] S. Dixit, J.G. Hering, Comparison of arsenic(V) and arsenic(III) sorption onto iron oxide minerals: implications for arsenic mobility, Environ. Sci. Technol. 37 (2003) 4182–4189. [13] P.N. Williams, H. Zhang, W. Davison, A.A. Meharg, M.H. Sumon, G.J. Norton, H. Brammer, M.R. Islam, Organic matter-solid phase interactions are critical for

186

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

C.-H. Syu et al. / Journal of Hazardous Materials 286 (2015) 179–186 predicting arsenic release and plant uptake in Bangladesh paddy soils, Environ. Sci. Technol. 45 (2011) 6080–6087. Y. Takahashi, R. Minamikawa, K.H. Hattori, K. Kurishima, N. Kihou, K. Yuita, Arsenic behavior in paddy fields during the cycle of flooded and non-flooded periods, Environ. Sci. Technol. 38 (2004) 1038–1044. T. Arao, A. Kawasaki, K. Baba, S. Matsumoto, Effects of arsenic compound amendment on arsenic speciation in rice grain, Environ. Sci. Technol. 45 (2011) 1291–1297. K. Bogdan, M.K. Schenk, Arsenic in rice (Oryza sativa L.) related to dynamics of arsenic and silicic acid in paddy soils, Environ. Sci. Technol. 42 (2008) 7885–7890. A.L. Seyfferth, S. Fendorf, Silicate mineral imparts on the uptake and storage of arsenic and plant nutrients in rice (Oryza sativa L.), Environ. Sci. Technol. 46 (2012) 13176–13183. A.S.M.H.M. Talukder, C.A. Meisner, M.A.R. Sarkar, M.S. Islam, K.D. Sayre, J.G. Duxbury, J.G. Lauren, Effect of water management: arsenic and phosphorus levels on rice in a high-arsenic soil-water system: II. Arsenic uptake, Ecotox. Environ. Saf. 80 (2012) 145–151. Z.C. Wu, H.Y. Ren, S.P. McGrath, P. Wu, F.J. Zhao, Investigating the contribution of the phosphate transport pathway to arsenic accumulation in rice, Plant Physiol. 157 (2011) 498–508. R.Y. Li, Y. Ago, W.J. Liu, N. Mitani, J. Feldmann, S.P. McGrath, J.F. Ma, F.J. Zhao, The rice aquaporin Lsi1 mediates uptake of methylated arsenic species, Plant Physiol. 150 (2009) 2071–2080. J.F. Ma, N. Yamaji, N. Mitani, X.Y. Xu, Y.H. Su, S.P. McGrath, F.J. Zhao, Transporters of arsenite in rice and their role in arsenic accumulation in rice grain, Proc. Natl. Acad. Sci. U. S. A. 105 (2008) 9931–9935. A. Raab, K. Ferreira, J. Feldmann, A.A. Meharg, Can As-phytochelatin complex formation be used as an indicator for toxicity in Helianthus annuus? J. Exp. Bot. 58 (2007) 1333–1338. A.A. Meharg, P.N. Williams, E. Adomako, Y.Y. Lawgali, C. Deacon, A. Villada, R.C.J. Gambell, G.X. Sun, Y.G. Zhu, J. Feldmann, A. Raab, F.J. Zhao, R. Islam, S. Hossain, J. Yanai, Geographical variation in total and inorganic arsenic content of polished (white) rice, Environ. Sci. Technol. 43 (2009) 1612–1617. Y.J. Zavala, R. Gerads, H. Gurleyuk, J.M. Duxbury, Arsenic in rice: II: arsenic speciation in USA grain and implications for human health, Environ. Sci. Technol. 42 (2008) 3861–3866. C.H. Syu, C.H. Lee, P.Y. Jiang, M.K. Chen, D.Y. Lee, Comparison of As sequestration in iron plaque and uptake by different genotypes of rice plants grown in As-contaminated paddy soils, Plant Soil 374 (2014) 411–422. F.J. Zhao, S.P. McGrath, A.A. Meharg, Arsenic as a food chain contaminant: mechanism of plant uptake and metabolism and mitigation strategies, Annu. Rev. Plant Biol. 64 (2010) 535–559. Z.U. Ahmed, G.M. Panaullah, H. Gauch, S.R. McCouch, W. Tyagi, M.S. Kabir, J.M. Duxbury, Genotype and environment effects on rice (Oryza sativa L.) grain arsenic concentration in Bangladesh, Plant Soil 338 (2011) 367–382. G.J. Norton, G.L. Duan, T. Dasgupta, M.R. Islam, M. Lei, Y.G. Zhu, C.M. Deacon, A.C. Moran, S. Islam, F.J. Zhao, J.L. Stroud, S.P. McGrath, J. Feldmann, A.H. Price, A.A. Meharg, Environmental and genetic control of arsenic accumulation and speciation in rice grain: comparing a range of common cultivars grown in contaminated sites across Bangladesh, China, and India, Environ. Sci. Technol. 43 (2009) 8381–8386. G.J. Norton, M.R. Islam, C.M. Deacon, F.J. Zhao, J.L. Stroud, S.P. McGrath, S. Islam, M. Jahiruddin, J. Feldmann, A.H. Price, A.A. Meharg, Identification of low inorganic and total grain arsenic rice cultivars from Bangladesh, Environ. Sci. Technol. 43 (2009) 6070–6075. T.R. Pillai, W.G. Yan, H.A. Agrama, W.D. James, A.M.H. Ibrahim, A.M. McClung, T.J. Gentry, R.H. Loeppert, Total grain-arsenic and arsenic-species concentrations in diverse rice cultivars under flooded conditions, Crop Sci. 50 (2010) 2065–2075.

[31] M.A. Khan, J.L. Stroud, Y.G. Zhu, S.P. McGrath, F.J. Zhao, Arsenic bioavailability to rice is elevated in Bangladeshi paddy soils, Environ. Sci. Technol. 44 (2010) 8515–8521. [32] W.M. Hsu, H.C. Hsi, Y.T. Huang, C.S. Liao, Z.Y. Hseu, Partitioning of arsenic in soil-crop system irrigated using groundwater: a case study of rice paddy soils in southwestern Taiwan, Chemosphere 86 (2012) 606–613. [33] H. Daus, H. Weiss, J. Mattusch, R. Wennrich, Preservation of arsenic species in water samples using phosphoric acid–limitations and long-term stability, Talanta 69 (2006) 430–434. [34] J.H. Huang, G. Ilgen, P. Fecher, Quantitative chemical extraction for arsenic speciation in rice grains, J. Anal. At. Spectrom. 25 (2010) 800–802. [35] L.P. Weng, W.H. Van Riemsdijk, T. Hiemstra, Effects of fulvic and humic acids on arsenate adsorption to goethite: experiments and modeling, Environ. Sci. Technol. 43 (2009) 7198–7204. [36] G.J. Norton, E.E. Adomako, C.M. Deacon, A.M. Carey, A.H. Price, A.A. Meharg, Effect of organic matter amendment, arsenic amendment and water management regime on rice grain arsenic species, Environ. Pollut. 177 (2013) 38–47. [37] D. Vromman, S. Lutts, I. Lefevre, L. Somer, O.D. Vreese, Z. Slejkovec, M. Quinet, Effects of simultaneous arsenic and iron toxicities on rice (Oryza sativa L.) development: yield- related parameters and As and Fe accumulation in relation to As speciation in the grains, Plant Soil 371 (2013) 199–217. [38] J.L. Stroud, G.J. Norton, M.R. Islam, T. Dasgupta, R.P. White, A.H. Price, A.A. Meharg, S.P. McGrath, F.J. Zhao, The dynamics of arsenic in four fields in the Bengal delta, Environ. Pollut. 159 (2011) 947–953. [39] X.Q. Mei, Z.H. Ye, M.H. Wong, The relationship of root porosity and radial oxygen loss on arsenic tolerance and uptake in rice grains and straw, Environ. Pollut. 157 (2009) 2550–2557. [40] T. Arao, A. Kawasaki, K. Baba, S. Mori, S. Matsumoto, Effects of water management on cadmium and arsenic accumulation and dimethylarsinic acid concentrations in Japanese rice, Environ. Sci. Technol. 43 (2009) 9361–9367. [41] C. Wu, Z.H. Ye, W.H. Shu, Y.G. Zhu, M.H. Wong, Arsenic accumulation and speciation in rice are affected by root aeration and variation of genotypes, J. Exp. Bot. 62 (2011) 2889–2898. [42] E. Lombi, K.G. Scheckel, J. Pallon, A.M. Carey, Y.G. Zhu, A.A. Meharg, Speciation and distribution of arsenic and localization of nutrients in rice grains, New Phytol. 184 (2009) 193–201. [43] Codex Alimentarius Commission, Proposed draft maximum levels for arsenic in rice (polished and husked), in: Reports of the 37th Session on Joint FAO/WHO Food Standards Programme, Geneva, Switzerland, 2014, p. 4. [44] C. Lomax, W.J. Liu, L. Wu, K. Xue, J. Xiong, J. Zhou, S.P. McGrath, A.A. Meharg, A.J. Miller, F.J. Zhao, Methylated arsenic species in plants originate from soil microorganisms, New Phytol. 193 (2012) 665–672. [45] Y. Jia, H. Huang, M. Zhong, F.H. Wang, L.M. Zhang, Y.G. Zhu, Microbial arsenic methylation in soil and rice rhizosphere, Environ. Sci. Technol. 47 (2013) 3141–3148. [46] A.M. Carey, K.G. Scheckel, E. Lombi, M. Newville, Y.S. Choi, G.J. Norton, J.M. Charnock, J. Feldmann, A.H. Price, A.A. Meharg, Grain unloading of arsenic species in rice, Plant Physiol. 152 (2010) 309–319. [47] G.X. Sun, P.N. Williams, A.M. Carey, Y.G. Zhu, C. Deacon, A. Raab, J. Feldmann, R.M. Islam, A.A. Meharg, Inorganic arsenic in rice bran and its products are an order of magnitude higher than in bulk grain, Environ. Sci. Technol. 42 (2008) 7542–7546. [48] M.R. Islam, M. Jahiruddin, S. Islam, Assessment of arsenic in the water–soil–plant systems in gangetic flood plains of Bangladesh, Asian. J. Plant. Sci. 3 (2004) 489–493. [49] H.T. Lin, S.S. Wong, G.C. Li, Heavy metal content of rice and shellfish in Taiwan, J. Food Drug. Anal. 12 (2004) 167–174. [50] T.D. Phuong, P. Van Chuong, D.T. Khiem, S. Kokot, Elemental content of Vietnamese rice – part 1. Sampling: analysis and comparison with previous studies, Analyst 124 (1999) 553–560.