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Cadmium accumulation in and tolerance of rice (Oryza sativa L.) varieties with different rates of radial oxygen loss
Environmental Pollution 159 (2011) 1730e1736
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Cadmium accumulation in and tolerance of rice (Oryza sativa L.) varieties with different rates of radial oxygen loss M.Y. Wang a, A.K. Chen b, M.H. Wong c, R.L. Qiu d, *, H. Cheng a, Z.H. Ye a, * a
School of Life Sciences, Sun Yat-sen University, Guangzhou 510006, PR China Department of Biology, Guangdong University of Education, Guangzhou 510303, PR China c Croucher Institute for Environmental Sciences, and Department of Biology, Hong Kong Baptist University, Hong Kong SAR, PR China d School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, PR China b
Rice cultivars with high rates of ROL tended to accumulate low Cd in grains.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 October 2010 Received in revised form 18 January 2011 Accepted 11 February 2011
Cadmium (Cd) uptake and tolerance were investigated among 20 rice cultivars based on a field experiment (1.2 mg Cd kg1 in soil) and a soil pot trial (control, 100 mg Cd kg1), and rates of radial oxygen loss (ROL) were measured under a deoxygenated solution. Significant differences were found among the cultivars in: (1) brown rice Cd concentrations (0.11e0.29 mg kg1) in a field soil, (2) grain Cd tolerance (34e113%) and concentrations (2.1e6.5 mg kg1) in a pot trial, and (3) rates of ROL (15e31 mmol O2 kg1 root d.w. h1). Target hazard quotients were calculated for the field experiment to assess potential Cd risk. Significant negative relationships were found between rates of ROL and concentrations of Cd in brown rice or straw under field and greenhouse conditions, indicating that rice cultivars with higher rates of ROL had higher capacities for limiting the transfer of Cd to rice and straw. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Cadmium Radial oxygen loss (ROL) Rice (Oryza sativa L.) Tolerance Uptake
1. Introduction Owing to its high toxicity and solubility in water and soils, cadmium (Cd) is one of the most important metals in terms of foodchain contamination (Liu et al., 2005b; Zhou et al., 2007). It is readily transferred from soil to plant and accumulates in edible plant parts, leading to yield reduction and a wide variety of acute and chronic toxic effects on mammalian tissues including kidneys, liver and lungs (Nakadaira and Nishi, 2003) when eaten. Areas of agricultural soils contaminated by Cd have been widely increasing in many countries as a result of anthropogenic activities, such as disposal of industrial effluent, mining waste, and sewage sludge, and application of phosphate fertilizers (Ye et al., 2000; Williams et al., 2009). In China, large areas of agricultural soils and many tons of crops such as rice (Oryza sativa L.) have been highly polluted by Cd in some provinces, including Hunan (Zhu et al., 2008; Williams et al., 2009), Guizhou (Huang et al., 2009), Guangdong (Zhou et al., 2007; Zhuang et al., 2009), and Jiangsu (Huang et al., 2007; Hang et al., 2009). This imposes a long-term health risk on
* Corresponding authors. E-mail addresses: [email protected] (R.L. Qiu), [email protected] (Z.H. Ye). 0269-7491/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2011.02.025
local populations. Wang (2002) reported that 1.46 108 kg of agricultural products (including 5.0 107 kg of rice) was polluted by Cd every year in China. A widespread provincial survey of rice from mine-despoiled paddies in Hunan found that 65% of the field rice exceeded the national food standard for Cd (Williams et al., 2009). Efforts have been made to remediate contaminated soils to allow production of safe crops with low heavy metal contents but traditional methods are extremely environmentally disruptive and expensive (Chaney et al., 2004). Phytoremediation, which is considered to be an effective and environmentally-friendly restorative method of removing heavy metals from soils (Brown et al., 1995), is still impractical because of the time required for it to have a meaningful effect (Eebbs et al., 1997; Keller and Hammer, 2004). However, in Japan, phytoextraction using a rice cultivar (Chokoukoku) with high Cd accumulation ability has been used to remove Cd from paddy fields contaminated with low to moderate levels of Cd (Murakami et al., 2008, 2009). Considerable variation in Cd uptake and accumulation among rice cultivars has been reported recently (Cheng et al., 2005; Li et al., 2005). Therefore, it is possible to develop and breed rice cultivars with a low Cd content in the edible parts (Liu et al., 2005b; Yu et al., 2006). Root radial oxygen loss (ROL) is a crucial factor for plants grown in wetland environments (Armstrong et al., 1994), which have an oxidizing environment in the rhizosphere and
M.Y. Wang et al. / Environmental Pollution 159 (2011) 1730e1736
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‘protect’ roots against soil-derived toxins (Armstrong, 1979; Colmer, 2003b). Rice cultivars with higher rates of ROL have higher capacities for limiting the transfer of arsenic to aboveground tissues (Mei et al., 2009). These results suggest that ROL may play an important role in heavy metal (e.g., Cd) uptake, accumulation and tolerance. However, so far the effects of variation in ROL on Cd accumulation and tolerance by rice are uncertain. Therefore, the aims of this study were to study: (1) Cd uptake and health risks of 20 different rice cultivars grown in a Cd-contaminated paddy field, (2) the relationships between the rates of ROL, levels of Cd accumulation in above-ground plant parts, and Cd tolerance of the different rice cultivars in a field experiment and two greenhouse pot trials.
washed thoroughly with tap water and then with deionized water. The straw and grain were separated and oven-dried at 60 C to constant weight.
2. Materials and methods
2.3.1. Plant sample preparation and analysis All plant samples including brown rice, grain, and straw were weighed separately for biomass, and then crushed in an analytic mill (IKA A11 basic; IKA-Werke GmbH, Germany), to pass through a 2 mm sieve. Samples were acid-digested with HNO3 at 190 C for 40 min in a microwave oven (MARS-X; CEM, USA). Cd concentrations were determined by Inductively-Coupled Plasma Optical Emission Spectrometry (ICP-OES, Optima 2000 DV, Perkin Elmer, USA). Blanks and tea standard material (GBW-08303) (China Standard Materials Research Center, Beijing, P.R. China) were used for quality control. The Cd recovery rates were 90 10%.
2.1. Field experiment A field experiment was conducted in a paddy field at Shangba village, Shaoguan, Guangdong Province, south China (113 470 E, 24 280 N). Most paddy fields at Shangba village have been contaminated with heavy metals (especially Cd) by irrigation water polluted by acid mine drainage from the nearby Dabaoshan multi-metal mine. Shangba village is a recognized ‘cancer village’ in China with more than 250 deaths from cancer in the last 20 years. The area has a humid subtropical climate with an average annual temperature of 20.6 C and an average annual rainfall of 1760 mm. Soil samples were collected at the top (0e20 cm depth) of the soil profile. After being air-dried and passed through a 2 mm sieve, soil physicochemical properties were measured. The summarized results are shown in Table 1. Twenty rice cultivars of four types were provided by the Rice Research Institute of the Guangdong Academy of Agricultural Sciences (Table 2). The field site was divided into 80 1 m 1 m sub-plots with a 20 cm buffer zone between each. Four replicates of each of the 20 rice cultivars were represented in the final planting design. Within each sub-plot, two of four-week-old seedlings were planted into 15 15 cm raised subunits. Growth progressed for three months after seedling emergence. Normal field management practices were used, i.e., plots were submerged during the growth stage and drained during the grain-filling period. The above-ground biomass was harvested at maturity in late July, 2008 and the straw and grain were separated. Both were oven-dried at 65 C to constant weight. The chaff of the grain was removed with a chaff-removing machine (IKA A11 basic; IKA-Werke GmbH, Germany).
2.2.2. Pot trial under deoxygenated nutrient conditions The same seeds were used in this experiment. Two-week rice seedlings were selected and transplanted into PVC trays (38 28 14 cm; breadth width height) filled with deoxygenated 0.25-strength Hoagland nutrient solution (Hoagland and Arnon, 1938) containing 0.1% (w/v) agar. Dilute agar prevents convective movements in the solution so that it mimics the changes in gas composition found in waterlogged soil (decreased O2 and increased ethylene) better than other methods used to impose root-zone O2 deficiency in solution culture (Wiengweera et al., 1997). The pots were arranged in a fully randomized design in the greenhouse and the Hoagland solutions were renewed every 6 days. After 45 days, ROL was measured as described by Mei et al. (2009). 2.3. Sample analysis
2.3.2. Soil analysis (soil physical and chemical characteristics) Soil pH was measured using a pH meter at a soil to deionized water ratio of 1:2.5. Soil was digested in an HCl/HNO3 mixture (3:1 w/v) at 180 C for 40 min in the microwave oven, and metal concentrations in the digests were determined by ICPOES. Blanks and soil standard material (GBW-07401) (China Standard Materials Research Center, Beijing, P.R. China) were used for quality control. The Cd recovery rates were again 90 10%. 2.4. Statistical analyses 2.4.1. Heath risk assessment The health risk of Cd through consumption of rice grown at Shangba village was assessed based on the target hazard quotient (THQ) (US EPA, 2000). A THQ < 1 means that the exposed population is assumed to be safe. The THQ value of Cd was determined by the following equation; THQ ¼
2.2. Pot trials 2.2.1. Pot trial under waterlogged soil conditions A pot trial was carried out in the greenhouse of Sun Yat-sen University (113 230 E, 23 300 N). The soil was collected from a paddy field (0e20 cm depth) at the campus of South China Agricultural University. Soil properties are presented in Table 1 (Huanong). After being air-dried and passed through a 2 mm sieve, 1 kg of soil was weighed into each pot (7.5 cm diameter and 14 cm high). The soil was then used without Cd (control) and amended with 100 mg Cd kg1 as CdCl2$H2O, mixed thoroughly and submerged for two weeks with 2 cm of water above the soil surface. Seeds of the 20 rice cultivars were disinfected in 30% H2O2 (w/v) solution for 15 min, washed thoroughly with deionized water, and then germinated in acidwashed quartz sand for two weeks. Uniform seedlings were selected and transplanted into the pots, with two seedlings per pot and four replications per cultivar. The potted soils were placed randomly in the greenhouse and maintained under flooded conditions during the whole rice growth period. At harvest, the aboveground portion of each rice plant was harvested by cutting 3 cm above the soil, and
EF ED FIR C 103 RFD WAB TA
where EF is exposure frequency (365 days year1); ED is the exposure duration (70 years); FIR is the food (rice) ingestion rate (g person1 day1); C is the metal (Cd) concentration in food (mg kg1); RFD is the oral reference dose (mg kg1 day1); WAB is the average body weight (kg), and TA is the averaged exposure time for noncarcinogens (365 days year1, number of exposure years assumed as 70). Oral reference doses for Cd were based on 1 103 mg kg1 day1 (US EPA, 2000). 2.4.2. Statistical analyses All results are presented as arithmetic means with standard errors attached. Data were analyzed and evaluated using SPSS 10.0 and Excel 2003 for Windows. Two significance levels (p < 0.05 and 0.01) were used in presentation and interpretation of the results.
3. Results 3.1. Field experiment
Table 1 Physicochemical properties of soils used in the field experiment and pot trial. Soil properties
Shangba
Huanong
Soil texture Sand (%) Silt (%) Clay (%) pH OM (%) Total N (g kg1) Total P (g kg1) Total K (g kg1) Cd (mg kg1) DTPA Cd (mg kg1)
37 40 23 3.2 9.8 1.2 0.75 0.43 1.2 0.40
31 36 33 5.2 8.1 1.3 1.1 0.61 0.18 nd*
*nd: not detectable.
3.1.1. Grain yield and straw biomass Grain yields and straw biomass of the 20 rice cultivars grown in the paddy field soil at Shangba village with a soil Cd content of 1.2 mg kg1 are presented in Table 1. The grain yield under field conditions ranged from 0.56 to 1.17 kg m2, with an average of 0.78 kg m2; and the straw biomass ranged from 0.46 to 0.75 kg m2, with an average of 0.58 kg m2. The grain yields were higher than that of straw biomass among the 20 rice cultivars. 3.1.2. Cd concentrations in brown rice, chaff and straw Concentrations of Cd in different tissues (brown rice, chaff and straw) are presented in Table 2. After growing in the Cd-contaminated paddy field soil, Cd concentrations in brown rice ranged from
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M.Y. Wang et al. / Environmental Pollution 159 (2011) 1730e1736
Table 2 Grain and straw biomass (kg m2, dry wt.), Cd concentrations (mg kg1, dry wt.) in brown rice, chaff and straw, and THQ values of Cd for local people based on consumption of 20 rice cultivars grown in a paddy field soil in Shangba village contaminated with Cd (mean SE, n ¼ 4). Types/Cultivars
Biomass Grain
Hybrid/ Fengyousimiao Tianyou 116 Tianyou 2168 Tianyou 998 Wufengyou 128 Wufengyou 2168 Youyou 998 Yueza 889 Conventional/ Fengerzhan Fengfuzhan Fengxiuzhan Huanghuazhan Huanglizhan Huangsizhan Huangxinzhan Wushanyouzhan Glutinous/ Hangxiangnuo Nanfengnuo Nuo DBS Red rice/ Ruanhongmi
0.78 0.87 1.17 0.91 0.91 0.93 0.95 0.82
0.04 0.07 0.14 0.03 0.06 0.10 0.03 0.08
Cd concentrations Straw 0.60 0.59 0.75 0.61 0.71 0.60 0.58 0.58
0.04 0.02 0.06 0.02 0.04 0.05 0.04 0.05
THQ of Cd
Brown rice 0.21 0.23 0.29 0.22 0.29 0.29 0.18 0.24
Chaff
0.12 0.03 0.03 0.07 0.08 0.12 0.05 0.07
0.24 0.23 0.29 0.18 0.38 0.36 0.21 0.18
Straw
Adults
Children
0.11 0.03 0.04 0.07 0.18 0.15 0.05 0.03
1.4 0.6 1.3 0.1 1.6 0.2 1.1 0.2 1.8 0.6 1.7 1.1 1.0 0.2 1.2 0.5
1.4 0.8 1.5 0.2 2.0 0.2 1.5 0.5 1.9 0.5 2.0 0.8 1.2 0.4 1.6 0.5
1.3 0.8 1.5 0.2 1.9 0.2 1.4 0.4 1.8 0.5 1.9 0.8 1.2 0.4 1.5 0.4
0.67 0.06 0.71 0.08 0.74 0.07 0.61 0.07 0.74 0.09 0.70 0.07 0.71 0.03 0.60 0.07
0.47 0.04 0.61 0.04 0.47 0.07 0.46 0.02 0.65 0.05 0.59 0.05 0.54 0.03 0.50 0.07
0.27 0.03 0.29 0.04 0.26 0.04 0.25 0.08 0.24 0.03 0.29 0.09 0.25 0.05 0.20 0.06
0.21 0.03 0.20 0.03 0.23 0.04 0.18 0.05 0.18 0.05 0.24 0.04 0.17 0.05 0.13 0.02
2.0 0.6 1.7 0.5 1.4 0.5 1.4 0.1 1.1 0.2 1.0 0.2 1.4 0.1 1.4 0.2
1.8 0.2 2.0 0.2 1.7 0.3 1.7 0.5 1.6 0.2 2.0 0.6 1.7 0.4 1.3 0.4
1.7 0.2 1.9 0.2 1.6 0.3 1.6 0.5 1.5 0.2 1.9 0.5 1.6 0.3 1.3 0.4
0.68 0.04 0.74 0.05 0.83 0.08
0.57 0.05 0.59 0.10 0.55 0.09
0.17 0.03 0.11 0.03 0.27 0.04
0.16 0.05 0.19 0.03 0.22 0.06
0.8 0.3 0.7 0.2 1.3 0.4
1.2 0.2 0.7 0.1 1.8 0.3
1.1 0.2 0.7 0.1 1.7 0.3
0.56 0.05
0.50 0.04
0.23 0.02
0.22 0.06
1.1 0.3
1.5 0.1
1.4 0.1
Limit of Cd concentration in brown rice permitted by Chinese Standard is 0.2 mg kg1.
a
120 100 80 60 40
ou 99 8 ou 21 Hu 68 ax inz Hu ha an n gh ua zh Ru an an W h on ufe gm ng i yo Fe u 216 ng xiu 8 W zh ufe an ng yo u Yu 128 eza 8 Nu 89 oD Fe BS ng fuz Ha ha ng n xia W n ush an gnuo yo u Tia zhan Fe nyou ng 99 yo 8 us Hu imia o an g Na lizha n nfe n Tia gnu o ny o u Hu an 116 gs Fe izhan ng erz ha n
20
Tia
ny
Grain biomass (% of Control)
140
Yo
3.1.3. Health risk assessment (THQ) An oral reference dose of 1 103 mg kg1 day1 for Cd (US EPA, 2000), average daily rice ingestion rates for adults and children of 376 and 209 g person1 day1, respectively (Zou et al., 2008), and average body weights, as used in a previous study by Wang et al. (2005), of 56 kg for adults and 33 kg for children were used to calculate THQ values of Cd for the local people of Shangba village consuming brown rice from the 20 cultivars. Results are presented in Table 2. THQ values for adults consuming the rice of the 20 cultivars ranged from 0.73 to 2.0, while for children, values ranged from 0.69 to 1.9. The results showed that 19 of the 20 THQ values for both children and adults exceeded 1, with only one glutinous rice cultivar (Nanfengnuo) being less than 1.
are presented in Table 3. Grain Cd concentrations ranged from 2.1 to 6.5 mg kg1, with an average of 4.3 mg kg1. Straw Cd concentrations ranged from 9.3 to 26 mg kg1, with an average of 17 mg kg1. Grain Cd accumulation ranged from 5.5 to 13 mg pot1, with an average of 7.8 mg pot1 and straw Cd accumulation ranged from 33 to 139 mg pot1, with an average of 61 mg pot1.
uy
0.11 to 0.29 mg kg1, with an average of 0.24 mg kg1, and Cd concentrations in chaff ranged from 0.13 to 0.38 mg kg1, with an average of 0.23 mg kg1, while straw Cd concentrations ranged from 0.66 to 2.0 mg kg1, with an average of 1.3 mg kg1.
120
b
100 80 60 40
Ru
uy ou 998 an ho ng Fe mi n Hu gfuzh an an g Wu huaz ha fen n gyo u1 Tia 28 ny ou Ha 116 ng xia Tia ngnu o ny ou 216 8 Nu Wu oD fen BS gyo u2 168 Fe ng xiu zha n Yu eza 889 Hu axi nzh Tia an ny ou 998 Na nfe ng Fe nu ng o you Wu sim sha iao ny ou zha Hu n an gli zha Hu n an gsi zha Fe n ng erz ha n
20
Yo
3.2.1. Growth and Cd tolerance of rice Cadmium tolerance of the rice cultivars with the 100 mg Cd kg1 soil treatment (expressed as % biomass of the control) are shown in Fig. 1. Compared with rice grown in the control soil, most grain and straw biomass was reduced under the added Cd treatment. Cd tolerance indices were significantly different among the 20 rice cultivars, ranging from 34 to 113% for grains, and from 34 to 103% for straw biomass. Eighteen of the 20 rice cultivars exposed to Cd treatment had decreased grain biomass, with 8 of the 18 significantly reduced (p < 0.05) (Fig. 1a). Nineteen cultivars had decreased straw biomass with 10 of the 19 significantly reduced (p < 0.05) (Fig. 1b).
Straw biomass (% of Control)
3.2. Pot trials under waterlogged soil conditions
Cultivars
3.2.2. Cd concentrations and accumulation in grain and straw Concentrations of Cd in grain and straw of the 20 rice cultivars grown in the Huanong (control) soil with addition of 100 mg Cd kg1
Fig. 1. Grain and straw Cd tolerance (expressed as % biomass of the control) of 20 rice cultivars grown under greenhouse conditions in a soil (Huanong) amended with 100 mg Cd kg1 (mean SE, n ¼ 4).
M.Y. Wang et al. / Environmental Pollution 159 (2011) 1730e1736 Table 3 Cd concentrations (mg kg1, dry wt.) and accumulations (mg per pot, dry wt.) in grain and straw of 20 rice cultivars grown in a soil (Huanong) amended with 100 mg Cd kg1, and rates of root ROL (mmo1 O2 kg1 root d.w. h1) grown in 0.1% agar deoxygenated nutrient solution (mean SE, n ¼ 4). Types/Cultivars
Hybrid/ Fengyousimiao Tianyou 116 Tianyou 2168 Tianyou 998 Wufengyou 128 Wufengyou 2168 Youyou 998 Yueza 889 Conventional/ Fengerzhan Fengfuzhan Fengxiuzhan Huanghuazhan Huanglizhan Huangsizhan Huangxinzhan Wushanyouzhan Glutinous/ Hangxiangnuo Nanfengnuo Nuo DBS Red rice/ Ruanhongmi
Cd concentration
Cd accumulation
Rate of ROL
Grain
Grain
Root
Straw
Straw
1733
and straw Cd concentrations (R ¼ 0.55, p < 0.05; Fig. 3b), and brown rice and chaff Cd concentrations (R ¼ 0.53, p < 0.05; Fig. 3c). In the pot trial, a significant positive relationship was found between Cd concentrations in the grain and straw (R ¼ 0.45, p < 0.05; Fig. 3d). 4. Discussion 4.1. Cd uptake and tolerance by rice of different cultivars
4.6 6.5 4.1 2.8 5.2 6.3 4.6 3.6
1.8 0.8 0.5 0.7 1.0 1.8 0.9 1.3
20 21 17 15 25 17 19 16
4.7 2.8 2.0 1.8 3.2 1.3 1.3 2.3
7.9 14 7.8 5.2 7.5 13 9.9 6.4
3.1 1.7 0.9 1.3 1.5 3.9 2.0 2.3
61 86 73 53 124 80 91 60
14 5.6 8.1 6.3 15 6.1 6.2 8.2
20 17 17 20 23 22 28 21
1.7 2.7 2.6 1.8 3.4 3.0 3.4 2.4
4.0 5.6 4.0 2.1 4.2 5.6 3.1 5.1
0.2 1.0 0.3 0.2 0.8 0.7 0.5 0.2
14 11 13 9.3 18 17 16 13
1.4 0.8 2.0 1.4 2.0 2.3 0.7 2.3
6.1 7.1 9.5 5.5 7.3 8.0 5.8 12
0.2 1.1 3.1 0.5 1.3 0.9 0.9 0.5
43 52 53 39 45 44 57 43
4.0 3.7 8.5 5.8 4.9 6.2 2.7 7.9
31 20 26 27 26 28 22 32
2.7 2.6 3.4 1.9 3.6 1.6 2.1 3.2
3.4 0.1 4.0 0.7 4.7 0.3
10 0.7 17 2.3 13 0.4
8.1 0.1 11 2.0 14 0.8
43 3.3 64 8.8 61 1.8
26 2.4 34 2.0 22 1.3
4.7 0.1
15 2.2
11 0.2
62 9.1
31 5.3
3.3. Rate of ROL in deoxygenated nutrient condition Rates of ROL of the 20 rice cultivars grown in 0.1% agar deoxygenated nutrient solution for 45 days are presented in Table 3. Rates of ROL were significantly different among the rice cultivars, ranging from 17 to 34 mmol O2 kg1 root d.w. h1 with an average of 24 mmol O2 kg1 root d.w. h1. The highest value was more than twice the lowest. Rates of ROL of hybrid rice cultivars were usually lower than those of the conventional and glutinous rice cultivars. 3.4. Correlations between rates of ROL and Cd concentrations in grain and straw in the pot trial or in brown rice, chaff and straw in the field experiment The results of correlation analysis between rates of ROL and Cd concentrations in brown rice, chaff and straw of rice grown in the Cd-contaminated paddy field soil are shown in Fig. 2. Significant negative correlations were observed between rates of root ROL and Cd concentrations in brown rice (R ¼ 0.46, p < 0.05; Fig. 2a), and straw (R ¼ 0.49, p < 0.05; Fig. 2b). There were no significant correlations between rates of root ROL and chaff Cd concentrations. Results of correlation analysis between rates of root ROL and Cd concentrations in grain and straw in the pot trial are presented in Fig. 2. A significant negative correlation (R ¼ 0.50, p < 0.05; Fig. 2d) was found between rates of ROL and straw Cd concentrations. Grain Cd concentrations also showed a negative relationship with rates of ROL but it was not significant (R ¼ 0.24, p > 0.05; Fig. 2c). 3.5. Correlations between Cd concentrations in grain and straw in pot trial and in rice, chaff and straw in the field experiment The results of correlation analysis between Cd concentrations of brown rice, chaff and straw in the field experiment are presented in Fig. 3. There were significant positive correlations between brown rice and straw Cd concentrations (R ¼ 0.79, p < 0.001; Fig. 3a), chaff
Cadmium is one of the most toxic of the heavy metals, and enters the environment mainly through anthropogenic activities. Zhuang et al. (2009) reported that the average Cd concentration in paddy soils in the vicinity of the Dabaoshan mine in south China was 3.9 mg kg1, with grain Cd concentrations of 0.82 mg kg1, both considerably exceeding the maximum allowable concentrations for agricultural soils and food in China. The Cd concentration in the soil collected from the paddy field studied, which was located 16 km downstream from the large-scale Dabaoshan mine, was 1.2 mg kg1, which is approximately 6 times the maximum allowable concentration and would be expected to pose a human health risk through consumption of Cd-contaminated food. Reduction of grain and straw biomass in response to Cd exposure has been reported in previous studies (Liu et al., 2005a, 2007b; Zhuang et al., 2009). Results in the present study showed that most of the straw and grain biomasses were reduced under the same Cd exposure. Significant differences in Cd tolerances were found among the 20 rice cultivars based on biomass reduction following exposure to 100 mg kg1 of added Cd, suggesting that, under the same degree of soil Cd contamination, the sensitivity and tolerance was cultivar-dependent. Liu et al. (2005a) also reported that rice tillering, plant height, leaf area, dry matter accumulation and grain yield are highly reduced under exposure to 100 mg kg1 of added Cd. Cadmium is highly soluble in the soil solution and can therefore easily transfer from soils to plants and accumulate therein. Our results showed that Cd concentrations in brown rice ranged from 0.11 to 0.29 mg kg1, with an average of 0.24 mg kg1 when grown in a Cd-contaminated paddy field soil (Table 2). This result supports those reported by He et al. (2006) who found average Cd concentrations in unpolished rice of 0.24 mg kg1 when rice was grown in soil continuously irrigated with untreated mine wastewater in Lechang, China, and that Cd concentrations in most rice samples exceeded the maximum Cd limit of 0.1 mg kg1 in brown rice proposed by FAO/WHO. In the pot trial, grain Cd concentrations ranged from 2.1 to 6.5 mg kg1, and straw Cd concentrations ranged from 9.3 to 26 mg kg1. Liu et al. (2005b) found that Cd concentrations in 52 brown rice cultivars ranged from 0.22 to 2.9 mg kg1 when rice was grown in soils spiked with 100 mg Cd kg1 in a pot trial. Significant differences were also found in straw Cd concentrations among the four rice types in the pot trial when grown with 100 mg of added Cd kg1 soil (data not shown). Straw of hybrid cultivars accumulated significantly higher Cd than that of conventional and glutinous-type cultivars. A similar result was also found for grain Cd accumulation. Liu et al. (2007b) reported that the indica cultivars accumulated significantly higher Cd than japonica cultivars. In another pot experiment, He et al. (2006) found that Cd accumulation in brown rice in the indica group of cultivars was 1.54 times higher than that in the japonica group. 4.2. Rates of ROL and their relationship with Cd concentrations in different tissues To adapt to waterlogging or flooded environments, wetland plants (include rice) have evolved an efficient strategy in which the
M.Y. Wang et al. / Environmental Pollution 159 (2011) 1730e1736
a
b
0.30
Straw Cd conc. (mg kg-1 )
Brown rice Cd conc. (mg kg-1 )
1734
0.25 0.20 0.15
R = -0.46, p < 0.05
0.10
1.6 1.4 1.2 1.0 0.8 0.6 -1
-1
Rate of ROL (mmol O2 kg root d.w. h )
c
d
22 Straw Cd conc. (mg kg )
7
-1
R= -0.24, p > 0.05
-1
R = -0.49, p < 0.05
16 18 20 22 24 26 28 30 32 34 36
16 18 20 22 24 26 28 30 32 34 36 -1 -1 Rate of ROL (mmol O2 kg root d.w. h )
Grain Cd conc. (mg kg )
1.8
6 5 4 3 2
-1
Rate of ROL (mmol O2 kg root d.w. h )
R= -0.50, p < 0.05
18 16 14 12 10 8
16 18 20 22 24 26 28 30 32 34 -1
20
16 18 20 22 24 26 28 30 32 34 -1
-1
Rate of ROL (mmol O2 kg root d.w. h )
Fig. 2. Correlations between rates of root ROL (mmo1 O2 kg1 root d.w. h1) grown in 0.1% agar deoxygenated nutrient solution and Cd concentrations (mg kg1) in above-ground tissues of 20 rice cultivars grown in a paddy field soil from Shangba village contaminated with Cd (a, b) or in a pot trial under greenhouse conditions using a soil amended with 100 mg Cd kg1 (c, d) (n ¼ 20).
aerenchyma of roots can release extra oxygen to the rhizosphere with about 30e40% of the O2 supplied via the root aerenchyma being lost to the soil by ROL (Armstrong, 1979). ROL is determined by anatomical, morphological, and physiological characteristics and demand for oxygen by the rhizosphere, and is highly related to plant species and cultivars (Colmer, 2003a; Peter et al., 2005). It was reported that ROL or rates of ROL from root zones differ markedly among wetland species and between different genotypes within a species (Colmer, 2003b; Yang et al., 2010). Yang et al. (2010) measured ROL in four wetland species, Aneilema bracteatum, Cyperus alternifolius, Ludwigia hyssopifolia and Veronica serpyllifolia, and found that ROL of the these species were range from 1.2 mmol O2 plant1 h1 (A. bracteatum) to 3.1 mmol O2 plant1 h1 (V. serpyllifolia). Variation in rates of ROL has been reported among rice cultivars (Colmer, 2003a; Mei et al., 2009). Mei et al. (2009) reported that the rates of ROL ranged from 7.4 to 13 mmol O2 kg1 root d.w. h1 with an average value of 10 mmol O2 kg1 root d.w. h1 among 25 rice cultivars. Our study has shown that the rates of ROL differ greatly among the 20 rice cultivars under deoxygenated nutrient solution conditions (Table 3). The highest ROL (Fengerzhan, 31 mmol O2 kg1 root d.w. h1) was about twice that of the lowest value (Tianyou 2168, 15 mmol O2 kg1 root d.w. h1). Peter et al. (2005) suggested that the rates of ROL in different wetland species were mainly affected by root respiration, the presence of root aerenchyma and a barrier in the basal root zones. ROL may protect the apex and laterals from chemically reduced soil toxins (Armstrong, 1979; Colmer, 2003b). It has been reported that ROL can reduce the concentration of Fe2þ and toxin productions in soils (Begg et al., 1994). Mei et al. (2009) found that rice cultivars with higher rates of ROL tended to limit the transfer of arsenic to above-ground tissues. Our results showed negative correlations between rates of ROL and grain or straw Cd concentrations in the
Cd-contaminated soil both in the paddy field and pot trial soils (Fig. 2), which suggests that rice cultivars with higher rate of ROL trend to inhibit Cd accumulation in the grain and straw of rice. A common feature of wetland plants (e.g., rice) is the formation of iron (Fe) plaque on the root surface and rhizosphere (Crowder and St-Cyr, 1991). Both biological and physicochemical factors can control the existence and extent of Fe plaque formation. ROL from plant roots is considered the most important physiological factor controlling Fe plaque formation (Mendellsohn et al., 1995). Wetland plants with high rates of ROL tend to form more iron plaque on root surfaces and in the rhizosphere (Li et al., 2011). An enhancement of Fe plaque could then increase the sequestration of Cd in the rhizosphere and on the root surfaces of rice, providing a means of external exclusion of soil Cd (Liu et al., 2008, 2010). Liu et al. (2008) found that Cd concentrations in shoots and roots of rice seedlings with a higher extent of Fe plaque on root surfaces were significantly lower than those of seedlings with a lower extent of Fe plaque. The formation of Fe plaque on rice roots and in the rhizosphere could act as a barrier to soil Cd toxicity, and may be a ‘buffer’ or a ‘reservoir’ which could reduce Cd uptake into rice roots. Furthermore, the plaque may contribute, to some extent, to the genotypic differences between rice cultivars in Cd uptake and tolerance (Liu et al., 2010). In a previous study, we found that Fe plaque may act as an effective Fe reservoir to increase Fe concentration in active cells and then ameliorate metal toxicity (Ye et al., 1998). Shao et al. (2007) reported that the occurrence of oxidative stress in plants exposed to Cd stress is mediated by Fe nutrition, with high Fe levels in the rhizosphere causing a marked reduction in Cd content in both leaves and roots. Liu et al. (2007a) also reported that uptake and translocation of Cd appear to be related Fe nutritional levels in the plants, and high Fe concentration in plants may help, to some extent, to minimize additional Cd uptake by plants. Formation of Fe plaque, therefore, may act as both a Cd ‘buffer’ which can
M.Y. Wang et al. / Environmental Pollution 159 (2011) 1730e1736
b 0.40
0.28 0.24 0.20 0.16 R = 0.79, p < 0.001 0.12 0.6
0.8
1.0
1.2
1.4
1.6
Chaff Cd conc. (mg kg -1)
Brown rice Cd conc. (mg kg-1)
a
R = 0.55, p < 0.05 0.35 0.30 0.25 0.20 0.15 0.10
1.8
-1
0.8
1.0
1.2
1.4
1.6
1.8
Straw Cd conc. (mg kg )
c
d 7 -1
Grain Cd conc. (mg kg )
-1
0.30
0.6
-1
Straw Cd conc. (mg kg )
Brown rice Cd conc. (mg kg )
1735
0.25 0.20 0.15 0.10 0.10
R = 0.53, p < 0.05
0.15 0.20 0.25 0.30 0.35 -1 Chaff Cd conc. (mg kg )
0.40
6 5 4 3 R = 0.45, p < 0.05 2 8
10 12 14 16 18 20 22 24 26 -1 Straw Cd conc. (mg kg )
Fig. 3. Correlations between the Cd concentrations of different tissues grown in a paddy field soil from Shangba village contaminated with Cd (aec) and in a pot trial under greenhouse conditions using a soil amended with 100 mg kg1 Cd (d).
sequestrate Cd on root surfaces and rhizoshpere, and a Fe ‘reservoir’ which can increase the Fe nutritional level in rice, to reduce Cd toxicity, uptake and translocation from roots to shoots. Further research is clearly needed using more rice cultivars to explain the detailed relationships among rates of ROL and Fe plaque and Cd tolerances and accumulation. 4.3. THQs of Cd and rice cultivars selection THQs of Cd applicable to consumption by the residents (adults and children) of Shangba village of the 20 rice cultivars studied are presented in Table 2. Only one THQ value (for the Nanfengnuo cultivar) was less than 1, which means that only the planting of this cultivar out of the 20 rice cultivars studied can ensure residents’ safety from a high Cd THQ through rice consumption. THQ values of the remaining 19 cultivars are greater than 1, which implies that residents are at risk of Cd toxicity, assuming consumption of rice only. In fact, the potential health risk is far greater than our calculations show. The residents are also considerably exposed to Cd and other heavy metal intakes through other foods such as vegetables, meat, fish, water, and fruit. Shangba village is a recognized ‘cancer village’ with more than 250 people having died of cancer in the last 20 years. High levels of toxic heavy metals, especially Cd, have been detected in water, soil and food there, and a direct connection between incidences of cancer and heavy metal pollution in this village is likely. Rice is a staple food and is also the main pathway by which Cd enters the human body especially in Asian countries. How to reduce Cd concentrations in rice is of continuing concern. It has been well documented that the tolerance and uptake of Cd varies greatly among different rice cultivars under soil Cd stress (Liu et al., 2005b, 2007b; He et al., 2006; Yu et al., 2006; Zeng et al., 2008). In a slightly contaminated soil (1.09 mg Cd kg1 soil), the
absolute difference in grain Cd concentrations among 138 rice cultivars was 9.1-fold (Zeng et al., 2008). In the present study, Cd tolerances and concentrations in both grain and straw varied greatly among the 20 rice cultivars under an application of 100 mg Cd kg1 soil (Fig. 1, Table 3). The same result was also found in brown rice, grain chaff and straw in the field experiment (Table 2). The differences in Cd tolerances and concentrations among rice cultivars indicated that it is possible to reduce Cd accumulation while ensuring yield in Cd-contaminated fields through cultivar selection and breeding. For instance, in the present study in Shangba village, cv. Nanfengnuo would be the most suitable rice cultivar of those studied for local planting, in terms of protecting the residents from excess Cd intake through rice consumption. However, the yield of cv. Nanfengnuo is relatively low and the type is not popular with farmers. The present study has shown that rice cultivars with a high rate of ROL tend to inhibit Cd accumulation in above-ground tissues, though the mechanism by which this is mediated is still not clear. ROL is potentially an important index for selection of high Cd tolerance and low Cd accumulation rice cultivars.
5. Conclusion There were significant differences in Cd tolerance and concentrations in grain and straw of the 20 rice cultivars grown in Cdcontaminated soils under both field and greenhouse conditions. The results of THQ values based on the field experiment suggest that selection of rice cultivars to ensure residents’ health is possible in certain Cd-contaminated areas. This is the first study showing that rice cultivars with higher rates of ROL tend to result in lower Cd concentrations both in grain and straw. Mechanisms of this effect require further investigation.
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Cadmium accumulation in and tolerance of rice (Oryza sativa L.) varieties with different rates of radial oxygen loss M.Y. Wang a, A.K. Chen b, M.H. Wong c, R.L. Qiu d, *, H. Cheng a, Z.H. Ye a, * a
School of Life Sciences, Sun Yat-sen University, Guangzhou 510006, PR China Department of Biology, Guangdong University of Education, Guangzhou 510303, PR China c Croucher Institute for Environmental Sciences, and Department of Biology, Hong Kong Baptist University, Hong Kong SAR, PR China d School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, PR China b
Rice cultivars with high rates of ROL tended to accumulate low Cd in grains.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 October 2010 Received in revised form 18 January 2011 Accepted 11 February 2011
Cadmium (Cd) uptake and tolerance were investigated among 20 rice cultivars based on a field experiment (1.2 mg Cd kg1 in soil) and a soil pot trial (control, 100 mg Cd kg1), and rates of radial oxygen loss (ROL) were measured under a deoxygenated solution. Significant differences were found among the cultivars in: (1) brown rice Cd concentrations (0.11e0.29 mg kg1) in a field soil, (2) grain Cd tolerance (34e113%) and concentrations (2.1e6.5 mg kg1) in a pot trial, and (3) rates of ROL (15e31 mmol O2 kg1 root d.w. h1). Target hazard quotients were calculated for the field experiment to assess potential Cd risk. Significant negative relationships were found between rates of ROL and concentrations of Cd in brown rice or straw under field and greenhouse conditions, indicating that rice cultivars with higher rates of ROL had higher capacities for limiting the transfer of Cd to rice and straw. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Cadmium Radial oxygen loss (ROL) Rice (Oryza sativa L.) Tolerance Uptake
1. Introduction Owing to its high toxicity and solubility in water and soils, cadmium (Cd) is one of the most important metals in terms of foodchain contamination (Liu et al., 2005b; Zhou et al., 2007). It is readily transferred from soil to plant and accumulates in edible plant parts, leading to yield reduction and a wide variety of acute and chronic toxic effects on mammalian tissues including kidneys, liver and lungs (Nakadaira and Nishi, 2003) when eaten. Areas of agricultural soils contaminated by Cd have been widely increasing in many countries as a result of anthropogenic activities, such as disposal of industrial effluent, mining waste, and sewage sludge, and application of phosphate fertilizers (Ye et al., 2000; Williams et al., 2009). In China, large areas of agricultural soils and many tons of crops such as rice (Oryza sativa L.) have been highly polluted by Cd in some provinces, including Hunan (Zhu et al., 2008; Williams et al., 2009), Guizhou (Huang et al., 2009), Guangdong (Zhou et al., 2007; Zhuang et al., 2009), and Jiangsu (Huang et al., 2007; Hang et al., 2009). This imposes a long-term health risk on
* Corresponding authors. E-mail addresses: [email protected] (R.L. Qiu), [email protected] (Z.H. Ye). 0269-7491/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2011.02.025
local populations. Wang (2002) reported that 1.46 108 kg of agricultural products (including 5.0 107 kg of rice) was polluted by Cd every year in China. A widespread provincial survey of rice from mine-despoiled paddies in Hunan found that 65% of the field rice exceeded the national food standard for Cd (Williams et al., 2009). Efforts have been made to remediate contaminated soils to allow production of safe crops with low heavy metal contents but traditional methods are extremely environmentally disruptive and expensive (Chaney et al., 2004). Phytoremediation, which is considered to be an effective and environmentally-friendly restorative method of removing heavy metals from soils (Brown et al., 1995), is still impractical because of the time required for it to have a meaningful effect (Eebbs et al., 1997; Keller and Hammer, 2004). However, in Japan, phytoextraction using a rice cultivar (Chokoukoku) with high Cd accumulation ability has been used to remove Cd from paddy fields contaminated with low to moderate levels of Cd (Murakami et al., 2008, 2009). Considerable variation in Cd uptake and accumulation among rice cultivars has been reported recently (Cheng et al., 2005; Li et al., 2005). Therefore, it is possible to develop and breed rice cultivars with a low Cd content in the edible parts (Liu et al., 2005b; Yu et al., 2006). Root radial oxygen loss (ROL) is a crucial factor for plants grown in wetland environments (Armstrong et al., 1994), which have an oxidizing environment in the rhizosphere and
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‘protect’ roots against soil-derived toxins (Armstrong, 1979; Colmer, 2003b). Rice cultivars with higher rates of ROL have higher capacities for limiting the transfer of arsenic to aboveground tissues (Mei et al., 2009). These results suggest that ROL may play an important role in heavy metal (e.g., Cd) uptake, accumulation and tolerance. However, so far the effects of variation in ROL on Cd accumulation and tolerance by rice are uncertain. Therefore, the aims of this study were to study: (1) Cd uptake and health risks of 20 different rice cultivars grown in a Cd-contaminated paddy field, (2) the relationships between the rates of ROL, levels of Cd accumulation in above-ground plant parts, and Cd tolerance of the different rice cultivars in a field experiment and two greenhouse pot trials.
washed thoroughly with tap water and then with deionized water. The straw and grain were separated and oven-dried at 60 C to constant weight.
2. Materials and methods
2.3.1. Plant sample preparation and analysis All plant samples including brown rice, grain, and straw were weighed separately for biomass, and then crushed in an analytic mill (IKA A11 basic; IKA-Werke GmbH, Germany), to pass through a 2 mm sieve. Samples were acid-digested with HNO3 at 190 C for 40 min in a microwave oven (MARS-X; CEM, USA). Cd concentrations were determined by Inductively-Coupled Plasma Optical Emission Spectrometry (ICP-OES, Optima 2000 DV, Perkin Elmer, USA). Blanks and tea standard material (GBW-08303) (China Standard Materials Research Center, Beijing, P.R. China) were used for quality control. The Cd recovery rates were 90 10%.
2.1. Field experiment A field experiment was conducted in a paddy field at Shangba village, Shaoguan, Guangdong Province, south China (113 470 E, 24 280 N). Most paddy fields at Shangba village have been contaminated with heavy metals (especially Cd) by irrigation water polluted by acid mine drainage from the nearby Dabaoshan multi-metal mine. Shangba village is a recognized ‘cancer village’ in China with more than 250 deaths from cancer in the last 20 years. The area has a humid subtropical climate with an average annual temperature of 20.6 C and an average annual rainfall of 1760 mm. Soil samples were collected at the top (0e20 cm depth) of the soil profile. After being air-dried and passed through a 2 mm sieve, soil physicochemical properties were measured. The summarized results are shown in Table 1. Twenty rice cultivars of four types were provided by the Rice Research Institute of the Guangdong Academy of Agricultural Sciences (Table 2). The field site was divided into 80 1 m 1 m sub-plots with a 20 cm buffer zone between each. Four replicates of each of the 20 rice cultivars were represented in the final planting design. Within each sub-plot, two of four-week-old seedlings were planted into 15 15 cm raised subunits. Growth progressed for three months after seedling emergence. Normal field management practices were used, i.e., plots were submerged during the growth stage and drained during the grain-filling period. The above-ground biomass was harvested at maturity in late July, 2008 and the straw and grain were separated. Both were oven-dried at 65 C to constant weight. The chaff of the grain was removed with a chaff-removing machine (IKA A11 basic; IKA-Werke GmbH, Germany).
2.2.2. Pot trial under deoxygenated nutrient conditions The same seeds were used in this experiment. Two-week rice seedlings were selected and transplanted into PVC trays (38 28 14 cm; breadth width height) filled with deoxygenated 0.25-strength Hoagland nutrient solution (Hoagland and Arnon, 1938) containing 0.1% (w/v) agar. Dilute agar prevents convective movements in the solution so that it mimics the changes in gas composition found in waterlogged soil (decreased O2 and increased ethylene) better than other methods used to impose root-zone O2 deficiency in solution culture (Wiengweera et al., 1997). The pots were arranged in a fully randomized design in the greenhouse and the Hoagland solutions were renewed every 6 days. After 45 days, ROL was measured as described by Mei et al. (2009). 2.3. Sample analysis
2.3.2. Soil analysis (soil physical and chemical characteristics) Soil pH was measured using a pH meter at a soil to deionized water ratio of 1:2.5. Soil was digested in an HCl/HNO3 mixture (3:1 w/v) at 180 C for 40 min in the microwave oven, and metal concentrations in the digests were determined by ICPOES. Blanks and soil standard material (GBW-07401) (China Standard Materials Research Center, Beijing, P.R. China) were used for quality control. The Cd recovery rates were again 90 10%. 2.4. Statistical analyses 2.4.1. Heath risk assessment The health risk of Cd through consumption of rice grown at Shangba village was assessed based on the target hazard quotient (THQ) (US EPA, 2000). A THQ < 1 means that the exposed population is assumed to be safe. The THQ value of Cd was determined by the following equation; THQ ¼
2.2. Pot trials 2.2.1. Pot trial under waterlogged soil conditions A pot trial was carried out in the greenhouse of Sun Yat-sen University (113 230 E, 23 300 N). The soil was collected from a paddy field (0e20 cm depth) at the campus of South China Agricultural University. Soil properties are presented in Table 1 (Huanong). After being air-dried and passed through a 2 mm sieve, 1 kg of soil was weighed into each pot (7.5 cm diameter and 14 cm high). The soil was then used without Cd (control) and amended with 100 mg Cd kg1 as CdCl2$H2O, mixed thoroughly and submerged for two weeks with 2 cm of water above the soil surface. Seeds of the 20 rice cultivars were disinfected in 30% H2O2 (w/v) solution for 15 min, washed thoroughly with deionized water, and then germinated in acidwashed quartz sand for two weeks. Uniform seedlings were selected and transplanted into the pots, with two seedlings per pot and four replications per cultivar. The potted soils were placed randomly in the greenhouse and maintained under flooded conditions during the whole rice growth period. At harvest, the aboveground portion of each rice plant was harvested by cutting 3 cm above the soil, and
EF ED FIR C 103 RFD WAB TA
where EF is exposure frequency (365 days year1); ED is the exposure duration (70 years); FIR is the food (rice) ingestion rate (g person1 day1); C is the metal (Cd) concentration in food (mg kg1); RFD is the oral reference dose (mg kg1 day1); WAB is the average body weight (kg), and TA is the averaged exposure time for noncarcinogens (365 days year1, number of exposure years assumed as 70). Oral reference doses for Cd were based on 1 103 mg kg1 day1 (US EPA, 2000). 2.4.2. Statistical analyses All results are presented as arithmetic means with standard errors attached. Data were analyzed and evaluated using SPSS 10.0 and Excel 2003 for Windows. Two significance levels (p < 0.05 and 0.01) were used in presentation and interpretation of the results.
3. Results 3.1. Field experiment
Table 1 Physicochemical properties of soils used in the field experiment and pot trial. Soil properties
Shangba
Huanong
Soil texture Sand (%) Silt (%) Clay (%) pH OM (%) Total N (g kg1) Total P (g kg1) Total K (g kg1) Cd (mg kg1) DTPA Cd (mg kg1)
37 40 23 3.2 9.8 1.2 0.75 0.43 1.2 0.40
31 36 33 5.2 8.1 1.3 1.1 0.61 0.18 nd*
*nd: not detectable.
3.1.1. Grain yield and straw biomass Grain yields and straw biomass of the 20 rice cultivars grown in the paddy field soil at Shangba village with a soil Cd content of 1.2 mg kg1 are presented in Table 1. The grain yield under field conditions ranged from 0.56 to 1.17 kg m2, with an average of 0.78 kg m2; and the straw biomass ranged from 0.46 to 0.75 kg m2, with an average of 0.58 kg m2. The grain yields were higher than that of straw biomass among the 20 rice cultivars. 3.1.2. Cd concentrations in brown rice, chaff and straw Concentrations of Cd in different tissues (brown rice, chaff and straw) are presented in Table 2. After growing in the Cd-contaminated paddy field soil, Cd concentrations in brown rice ranged from
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M.Y. Wang et al. / Environmental Pollution 159 (2011) 1730e1736
Table 2 Grain and straw biomass (kg m2, dry wt.), Cd concentrations (mg kg1, dry wt.) in brown rice, chaff and straw, and THQ values of Cd for local people based on consumption of 20 rice cultivars grown in a paddy field soil in Shangba village contaminated with Cd (mean SE, n ¼ 4). Types/Cultivars
Biomass Grain
Hybrid/ Fengyousimiao Tianyou 116 Tianyou 2168 Tianyou 998 Wufengyou 128 Wufengyou 2168 Youyou 998 Yueza 889 Conventional/ Fengerzhan Fengfuzhan Fengxiuzhan Huanghuazhan Huanglizhan Huangsizhan Huangxinzhan Wushanyouzhan Glutinous/ Hangxiangnuo Nanfengnuo Nuo DBS Red rice/ Ruanhongmi
0.78 0.87 1.17 0.91 0.91 0.93 0.95 0.82
0.04 0.07 0.14 0.03 0.06 0.10 0.03 0.08
Cd concentrations Straw 0.60 0.59 0.75 0.61 0.71 0.60 0.58 0.58
0.04 0.02 0.06 0.02 0.04 0.05 0.04 0.05
THQ of Cd
Brown rice 0.21 0.23 0.29 0.22 0.29 0.29 0.18 0.24
Chaff
0.12 0.03 0.03 0.07 0.08 0.12 0.05 0.07
0.24 0.23 0.29 0.18 0.38 0.36 0.21 0.18
Straw
Adults
Children
0.11 0.03 0.04 0.07 0.18 0.15 0.05 0.03
1.4 0.6 1.3 0.1 1.6 0.2 1.1 0.2 1.8 0.6 1.7 1.1 1.0 0.2 1.2 0.5
1.4 0.8 1.5 0.2 2.0 0.2 1.5 0.5 1.9 0.5 2.0 0.8 1.2 0.4 1.6 0.5
1.3 0.8 1.5 0.2 1.9 0.2 1.4 0.4 1.8 0.5 1.9 0.8 1.2 0.4 1.5 0.4
0.67 0.06 0.71 0.08 0.74 0.07 0.61 0.07 0.74 0.09 0.70 0.07 0.71 0.03 0.60 0.07
0.47 0.04 0.61 0.04 0.47 0.07 0.46 0.02 0.65 0.05 0.59 0.05 0.54 0.03 0.50 0.07
0.27 0.03 0.29 0.04 0.26 0.04 0.25 0.08 0.24 0.03 0.29 0.09 0.25 0.05 0.20 0.06
0.21 0.03 0.20 0.03 0.23 0.04 0.18 0.05 0.18 0.05 0.24 0.04 0.17 0.05 0.13 0.02
2.0 0.6 1.7 0.5 1.4 0.5 1.4 0.1 1.1 0.2 1.0 0.2 1.4 0.1 1.4 0.2
1.8 0.2 2.0 0.2 1.7 0.3 1.7 0.5 1.6 0.2 2.0 0.6 1.7 0.4 1.3 0.4
1.7 0.2 1.9 0.2 1.6 0.3 1.6 0.5 1.5 0.2 1.9 0.5 1.6 0.3 1.3 0.4
0.68 0.04 0.74 0.05 0.83 0.08
0.57 0.05 0.59 0.10 0.55 0.09
0.17 0.03 0.11 0.03 0.27 0.04
0.16 0.05 0.19 0.03 0.22 0.06
0.8 0.3 0.7 0.2 1.3 0.4
1.2 0.2 0.7 0.1 1.8 0.3
1.1 0.2 0.7 0.1 1.7 0.3
0.56 0.05
0.50 0.04
0.23 0.02
0.22 0.06
1.1 0.3
1.5 0.1
1.4 0.1
Limit of Cd concentration in brown rice permitted by Chinese Standard is 0.2 mg kg1.
a
120 100 80 60 40
ou 99 8 ou 21 Hu 68 ax inz Hu ha an n gh ua zh Ru an an W h on ufe gm ng i yo Fe u 216 ng xiu 8 W zh ufe an ng yo u Yu 128 eza 8 Nu 89 oD Fe BS ng fuz Ha ha ng n xia W n ush an gnuo yo u Tia zhan Fe nyou ng 99 yo 8 us Hu imia o an g Na lizha n nfe n Tia gnu o ny o u Hu an 116 gs Fe izhan ng erz ha n
20
Tia
ny
Grain biomass (% of Control)
140
Yo
3.1.3. Health risk assessment (THQ) An oral reference dose of 1 103 mg kg1 day1 for Cd (US EPA, 2000), average daily rice ingestion rates for adults and children of 376 and 209 g person1 day1, respectively (Zou et al., 2008), and average body weights, as used in a previous study by Wang et al. (2005), of 56 kg for adults and 33 kg for children were used to calculate THQ values of Cd for the local people of Shangba village consuming brown rice from the 20 cultivars. Results are presented in Table 2. THQ values for adults consuming the rice of the 20 cultivars ranged from 0.73 to 2.0, while for children, values ranged from 0.69 to 1.9. The results showed that 19 of the 20 THQ values for both children and adults exceeded 1, with only one glutinous rice cultivar (Nanfengnuo) being less than 1.
are presented in Table 3. Grain Cd concentrations ranged from 2.1 to 6.5 mg kg1, with an average of 4.3 mg kg1. Straw Cd concentrations ranged from 9.3 to 26 mg kg1, with an average of 17 mg kg1. Grain Cd accumulation ranged from 5.5 to 13 mg pot1, with an average of 7.8 mg pot1 and straw Cd accumulation ranged from 33 to 139 mg pot1, with an average of 61 mg pot1.
uy
0.11 to 0.29 mg kg1, with an average of 0.24 mg kg1, and Cd concentrations in chaff ranged from 0.13 to 0.38 mg kg1, with an average of 0.23 mg kg1, while straw Cd concentrations ranged from 0.66 to 2.0 mg kg1, with an average of 1.3 mg kg1.
120
b
100 80 60 40
Ru
uy ou 998 an ho ng Fe mi n Hu gfuzh an an g Wu huaz ha fen n gyo u1 Tia 28 ny ou Ha 116 ng xia Tia ngnu o ny ou 216 8 Nu Wu oD fen BS gyo u2 168 Fe ng xiu zha n Yu eza 889 Hu axi nzh Tia an ny ou 998 Na nfe ng Fe nu ng o you Wu sim sha iao ny ou zha Hu n an gli zha Hu n an gsi zha Fe n ng erz ha n
20
Yo
3.2.1. Growth and Cd tolerance of rice Cadmium tolerance of the rice cultivars with the 100 mg Cd kg1 soil treatment (expressed as % biomass of the control) are shown in Fig. 1. Compared with rice grown in the control soil, most grain and straw biomass was reduced under the added Cd treatment. Cd tolerance indices were significantly different among the 20 rice cultivars, ranging from 34 to 113% for grains, and from 34 to 103% for straw biomass. Eighteen of the 20 rice cultivars exposed to Cd treatment had decreased grain biomass, with 8 of the 18 significantly reduced (p < 0.05) (Fig. 1a). Nineteen cultivars had decreased straw biomass with 10 of the 19 significantly reduced (p < 0.05) (Fig. 1b).
Straw biomass (% of Control)
3.2. Pot trials under waterlogged soil conditions
Cultivars
3.2.2. Cd concentrations and accumulation in grain and straw Concentrations of Cd in grain and straw of the 20 rice cultivars grown in the Huanong (control) soil with addition of 100 mg Cd kg1
Fig. 1. Grain and straw Cd tolerance (expressed as % biomass of the control) of 20 rice cultivars grown under greenhouse conditions in a soil (Huanong) amended with 100 mg Cd kg1 (mean SE, n ¼ 4).
M.Y. Wang et al. / Environmental Pollution 159 (2011) 1730e1736 Table 3 Cd concentrations (mg kg1, dry wt.) and accumulations (mg per pot, dry wt.) in grain and straw of 20 rice cultivars grown in a soil (Huanong) amended with 100 mg Cd kg1, and rates of root ROL (mmo1 O2 kg1 root d.w. h1) grown in 0.1% agar deoxygenated nutrient solution (mean SE, n ¼ 4). Types/Cultivars
Hybrid/ Fengyousimiao Tianyou 116 Tianyou 2168 Tianyou 998 Wufengyou 128 Wufengyou 2168 Youyou 998 Yueza 889 Conventional/ Fengerzhan Fengfuzhan Fengxiuzhan Huanghuazhan Huanglizhan Huangsizhan Huangxinzhan Wushanyouzhan Glutinous/ Hangxiangnuo Nanfengnuo Nuo DBS Red rice/ Ruanhongmi
Cd concentration
Cd accumulation
Rate of ROL
Grain
Grain
Root
Straw
Straw
1733
and straw Cd concentrations (R ¼ 0.55, p < 0.05; Fig. 3b), and brown rice and chaff Cd concentrations (R ¼ 0.53, p < 0.05; Fig. 3c). In the pot trial, a significant positive relationship was found between Cd concentrations in the grain and straw (R ¼ 0.45, p < 0.05; Fig. 3d). 4. Discussion 4.1. Cd uptake and tolerance by rice of different cultivars
4.6 6.5 4.1 2.8 5.2 6.3 4.6 3.6
1.8 0.8 0.5 0.7 1.0 1.8 0.9 1.3
20 21 17 15 25 17 19 16
4.7 2.8 2.0 1.8 3.2 1.3 1.3 2.3
7.9 14 7.8 5.2 7.5 13 9.9 6.4
3.1 1.7 0.9 1.3 1.5 3.9 2.0 2.3
61 86 73 53 124 80 91 60
14 5.6 8.1 6.3 15 6.1 6.2 8.2
20 17 17 20 23 22 28 21
1.7 2.7 2.6 1.8 3.4 3.0 3.4 2.4
4.0 5.6 4.0 2.1 4.2 5.6 3.1 5.1
0.2 1.0 0.3 0.2 0.8 0.7 0.5 0.2
14 11 13 9.3 18 17 16 13
1.4 0.8 2.0 1.4 2.0 2.3 0.7 2.3
6.1 7.1 9.5 5.5 7.3 8.0 5.8 12
0.2 1.1 3.1 0.5 1.3 0.9 0.9 0.5
43 52 53 39 45 44 57 43
4.0 3.7 8.5 5.8 4.9 6.2 2.7 7.9
31 20 26 27 26 28 22 32
2.7 2.6 3.4 1.9 3.6 1.6 2.1 3.2
3.4 0.1 4.0 0.7 4.7 0.3
10 0.7 17 2.3 13 0.4
8.1 0.1 11 2.0 14 0.8
43 3.3 64 8.8 61 1.8
26 2.4 34 2.0 22 1.3
4.7 0.1
15 2.2
11 0.2
62 9.1
31 5.3
3.3. Rate of ROL in deoxygenated nutrient condition Rates of ROL of the 20 rice cultivars grown in 0.1% agar deoxygenated nutrient solution for 45 days are presented in Table 3. Rates of ROL were significantly different among the rice cultivars, ranging from 17 to 34 mmol O2 kg1 root d.w. h1 with an average of 24 mmol O2 kg1 root d.w. h1. The highest value was more than twice the lowest. Rates of ROL of hybrid rice cultivars were usually lower than those of the conventional and glutinous rice cultivars. 3.4. Correlations between rates of ROL and Cd concentrations in grain and straw in the pot trial or in brown rice, chaff and straw in the field experiment The results of correlation analysis between rates of ROL and Cd concentrations in brown rice, chaff and straw of rice grown in the Cd-contaminated paddy field soil are shown in Fig. 2. Significant negative correlations were observed between rates of root ROL and Cd concentrations in brown rice (R ¼ 0.46, p < 0.05; Fig. 2a), and straw (R ¼ 0.49, p < 0.05; Fig. 2b). There were no significant correlations between rates of root ROL and chaff Cd concentrations. Results of correlation analysis between rates of root ROL and Cd concentrations in grain and straw in the pot trial are presented in Fig. 2. A significant negative correlation (R ¼ 0.50, p < 0.05; Fig. 2d) was found between rates of ROL and straw Cd concentrations. Grain Cd concentrations also showed a negative relationship with rates of ROL but it was not significant (R ¼ 0.24, p > 0.05; Fig. 2c). 3.5. Correlations between Cd concentrations in grain and straw in pot trial and in rice, chaff and straw in the field experiment The results of correlation analysis between Cd concentrations of brown rice, chaff and straw in the field experiment are presented in Fig. 3. There were significant positive correlations between brown rice and straw Cd concentrations (R ¼ 0.79, p < 0.001; Fig. 3a), chaff
Cadmium is one of the most toxic of the heavy metals, and enters the environment mainly through anthropogenic activities. Zhuang et al. (2009) reported that the average Cd concentration in paddy soils in the vicinity of the Dabaoshan mine in south China was 3.9 mg kg1, with grain Cd concentrations of 0.82 mg kg1, both considerably exceeding the maximum allowable concentrations for agricultural soils and food in China. The Cd concentration in the soil collected from the paddy field studied, which was located 16 km downstream from the large-scale Dabaoshan mine, was 1.2 mg kg1, which is approximately 6 times the maximum allowable concentration and would be expected to pose a human health risk through consumption of Cd-contaminated food. Reduction of grain and straw biomass in response to Cd exposure has been reported in previous studies (Liu et al., 2005a, 2007b; Zhuang et al., 2009). Results in the present study showed that most of the straw and grain biomasses were reduced under the same Cd exposure. Significant differences in Cd tolerances were found among the 20 rice cultivars based on biomass reduction following exposure to 100 mg kg1 of added Cd, suggesting that, under the same degree of soil Cd contamination, the sensitivity and tolerance was cultivar-dependent. Liu et al. (2005a) also reported that rice tillering, plant height, leaf area, dry matter accumulation and grain yield are highly reduced under exposure to 100 mg kg1 of added Cd. Cadmium is highly soluble in the soil solution and can therefore easily transfer from soils to plants and accumulate therein. Our results showed that Cd concentrations in brown rice ranged from 0.11 to 0.29 mg kg1, with an average of 0.24 mg kg1 when grown in a Cd-contaminated paddy field soil (Table 2). This result supports those reported by He et al. (2006) who found average Cd concentrations in unpolished rice of 0.24 mg kg1 when rice was grown in soil continuously irrigated with untreated mine wastewater in Lechang, China, and that Cd concentrations in most rice samples exceeded the maximum Cd limit of 0.1 mg kg1 in brown rice proposed by FAO/WHO. In the pot trial, grain Cd concentrations ranged from 2.1 to 6.5 mg kg1, and straw Cd concentrations ranged from 9.3 to 26 mg kg1. Liu et al. (2005b) found that Cd concentrations in 52 brown rice cultivars ranged from 0.22 to 2.9 mg kg1 when rice was grown in soils spiked with 100 mg Cd kg1 in a pot trial. Significant differences were also found in straw Cd concentrations among the four rice types in the pot trial when grown with 100 mg of added Cd kg1 soil (data not shown). Straw of hybrid cultivars accumulated significantly higher Cd than that of conventional and glutinous-type cultivars. A similar result was also found for grain Cd accumulation. Liu et al. (2007b) reported that the indica cultivars accumulated significantly higher Cd than japonica cultivars. In another pot experiment, He et al. (2006) found that Cd accumulation in brown rice in the indica group of cultivars was 1.54 times higher than that in the japonica group. 4.2. Rates of ROL and their relationship with Cd concentrations in different tissues To adapt to waterlogging or flooded environments, wetland plants (include rice) have evolved an efficient strategy in which the
M.Y. Wang et al. / Environmental Pollution 159 (2011) 1730e1736
a
b
0.30
Straw Cd conc. (mg kg-1 )
Brown rice Cd conc. (mg kg-1 )
1734
0.25 0.20 0.15
R = -0.46, p < 0.05
0.10
1.6 1.4 1.2 1.0 0.8 0.6 -1
-1
Rate of ROL (mmol O2 kg root d.w. h )
c
d
22 Straw Cd conc. (mg kg )
7
-1
R= -0.24, p > 0.05
-1
R = -0.49, p < 0.05
16 18 20 22 24 26 28 30 32 34 36
16 18 20 22 24 26 28 30 32 34 36 -1 -1 Rate of ROL (mmol O2 kg root d.w. h )
Grain Cd conc. (mg kg )
1.8
6 5 4 3 2
-1
Rate of ROL (mmol O2 kg root d.w. h )
R= -0.50, p < 0.05
18 16 14 12 10 8
16 18 20 22 24 26 28 30 32 34 -1
20
16 18 20 22 24 26 28 30 32 34 -1
-1
Rate of ROL (mmol O2 kg root d.w. h )
Fig. 2. Correlations between rates of root ROL (mmo1 O2 kg1 root d.w. h1) grown in 0.1% agar deoxygenated nutrient solution and Cd concentrations (mg kg1) in above-ground tissues of 20 rice cultivars grown in a paddy field soil from Shangba village contaminated with Cd (a, b) or in a pot trial under greenhouse conditions using a soil amended with 100 mg Cd kg1 (c, d) (n ¼ 20).
aerenchyma of roots can release extra oxygen to the rhizosphere with about 30e40% of the O2 supplied via the root aerenchyma being lost to the soil by ROL (Armstrong, 1979). ROL is determined by anatomical, morphological, and physiological characteristics and demand for oxygen by the rhizosphere, and is highly related to plant species and cultivars (Colmer, 2003a; Peter et al., 2005). It was reported that ROL or rates of ROL from root zones differ markedly among wetland species and between different genotypes within a species (Colmer, 2003b; Yang et al., 2010). Yang et al. (2010) measured ROL in four wetland species, Aneilema bracteatum, Cyperus alternifolius, Ludwigia hyssopifolia and Veronica serpyllifolia, and found that ROL of the these species were range from 1.2 mmol O2 plant1 h1 (A. bracteatum) to 3.1 mmol O2 plant1 h1 (V. serpyllifolia). Variation in rates of ROL has been reported among rice cultivars (Colmer, 2003a; Mei et al., 2009). Mei et al. (2009) reported that the rates of ROL ranged from 7.4 to 13 mmol O2 kg1 root d.w. h1 with an average value of 10 mmol O2 kg1 root d.w. h1 among 25 rice cultivars. Our study has shown that the rates of ROL differ greatly among the 20 rice cultivars under deoxygenated nutrient solution conditions (Table 3). The highest ROL (Fengerzhan, 31 mmol O2 kg1 root d.w. h1) was about twice that of the lowest value (Tianyou 2168, 15 mmol O2 kg1 root d.w. h1). Peter et al. (2005) suggested that the rates of ROL in different wetland species were mainly affected by root respiration, the presence of root aerenchyma and a barrier in the basal root zones. ROL may protect the apex and laterals from chemically reduced soil toxins (Armstrong, 1979; Colmer, 2003b). It has been reported that ROL can reduce the concentration of Fe2þ and toxin productions in soils (Begg et al., 1994). Mei et al. (2009) found that rice cultivars with higher rates of ROL tended to limit the transfer of arsenic to above-ground tissues. Our results showed negative correlations between rates of ROL and grain or straw Cd concentrations in the
Cd-contaminated soil both in the paddy field and pot trial soils (Fig. 2), which suggests that rice cultivars with higher rate of ROL trend to inhibit Cd accumulation in the grain and straw of rice. A common feature of wetland plants (e.g., rice) is the formation of iron (Fe) plaque on the root surface and rhizosphere (Crowder and St-Cyr, 1991). Both biological and physicochemical factors can control the existence and extent of Fe plaque formation. ROL from plant roots is considered the most important physiological factor controlling Fe plaque formation (Mendellsohn et al., 1995). Wetland plants with high rates of ROL tend to form more iron plaque on root surfaces and in the rhizosphere (Li et al., 2011). An enhancement of Fe plaque could then increase the sequestration of Cd in the rhizosphere and on the root surfaces of rice, providing a means of external exclusion of soil Cd (Liu et al., 2008, 2010). Liu et al. (2008) found that Cd concentrations in shoots and roots of rice seedlings with a higher extent of Fe plaque on root surfaces were significantly lower than those of seedlings with a lower extent of Fe plaque. The formation of Fe plaque on rice roots and in the rhizosphere could act as a barrier to soil Cd toxicity, and may be a ‘buffer’ or a ‘reservoir’ which could reduce Cd uptake into rice roots. Furthermore, the plaque may contribute, to some extent, to the genotypic differences between rice cultivars in Cd uptake and tolerance (Liu et al., 2010). In a previous study, we found that Fe plaque may act as an effective Fe reservoir to increase Fe concentration in active cells and then ameliorate metal toxicity (Ye et al., 1998). Shao et al. (2007) reported that the occurrence of oxidative stress in plants exposed to Cd stress is mediated by Fe nutrition, with high Fe levels in the rhizosphere causing a marked reduction in Cd content in both leaves and roots. Liu et al. (2007a) also reported that uptake and translocation of Cd appear to be related Fe nutritional levels in the plants, and high Fe concentration in plants may help, to some extent, to minimize additional Cd uptake by plants. Formation of Fe plaque, therefore, may act as both a Cd ‘buffer’ which can
M.Y. Wang et al. / Environmental Pollution 159 (2011) 1730e1736
b 0.40
0.28 0.24 0.20 0.16 R = 0.79, p < 0.001 0.12 0.6
0.8
1.0
1.2
1.4
1.6
Chaff Cd conc. (mg kg -1)
Brown rice Cd conc. (mg kg-1)
a
R = 0.55, p < 0.05 0.35 0.30 0.25 0.20 0.15 0.10
1.8
-1
0.8
1.0
1.2
1.4
1.6
1.8
Straw Cd conc. (mg kg )
c
d 7 -1
Grain Cd conc. (mg kg )
-1
0.30
0.6
-1
Straw Cd conc. (mg kg )
Brown rice Cd conc. (mg kg )
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0.25 0.20 0.15 0.10 0.10
R = 0.53, p < 0.05
0.15 0.20 0.25 0.30 0.35 -1 Chaff Cd conc. (mg kg )
0.40
6 5 4 3 R = 0.45, p < 0.05 2 8
10 12 14 16 18 20 22 24 26 -1 Straw Cd conc. (mg kg )
Fig. 3. Correlations between the Cd concentrations of different tissues grown in a paddy field soil from Shangba village contaminated with Cd (aec) and in a pot trial under greenhouse conditions using a soil amended with 100 mg kg1 Cd (d).
sequestrate Cd on root surfaces and rhizoshpere, and a Fe ‘reservoir’ which can increase the Fe nutritional level in rice, to reduce Cd toxicity, uptake and translocation from roots to shoots. Further research is clearly needed using more rice cultivars to explain the detailed relationships among rates of ROL and Fe plaque and Cd tolerances and accumulation. 4.3. THQs of Cd and rice cultivars selection THQs of Cd applicable to consumption by the residents (adults and children) of Shangba village of the 20 rice cultivars studied are presented in Table 2. Only one THQ value (for the Nanfengnuo cultivar) was less than 1, which means that only the planting of this cultivar out of the 20 rice cultivars studied can ensure residents’ safety from a high Cd THQ through rice consumption. THQ values of the remaining 19 cultivars are greater than 1, which implies that residents are at risk of Cd toxicity, assuming consumption of rice only. In fact, the potential health risk is far greater than our calculations show. The residents are also considerably exposed to Cd and other heavy metal intakes through other foods such as vegetables, meat, fish, water, and fruit. Shangba village is a recognized ‘cancer village’ with more than 250 people having died of cancer in the last 20 years. High levels of toxic heavy metals, especially Cd, have been detected in water, soil and food there, and a direct connection between incidences of cancer and heavy metal pollution in this village is likely. Rice is a staple food and is also the main pathway by which Cd enters the human body especially in Asian countries. How to reduce Cd concentrations in rice is of continuing concern. It has been well documented that the tolerance and uptake of Cd varies greatly among different rice cultivars under soil Cd stress (Liu et al., 2005b, 2007b; He et al., 2006; Yu et al., 2006; Zeng et al., 2008). In a slightly contaminated soil (1.09 mg Cd kg1 soil), the
absolute difference in grain Cd concentrations among 138 rice cultivars was 9.1-fold (Zeng et al., 2008). In the present study, Cd tolerances and concentrations in both grain and straw varied greatly among the 20 rice cultivars under an application of 100 mg Cd kg1 soil (Fig. 1, Table 3). The same result was also found in brown rice, grain chaff and straw in the field experiment (Table 2). The differences in Cd tolerances and concentrations among rice cultivars indicated that it is possible to reduce Cd accumulation while ensuring yield in Cd-contaminated fields through cultivar selection and breeding. For instance, in the present study in Shangba village, cv. Nanfengnuo would be the most suitable rice cultivar of those studied for local planting, in terms of protecting the residents from excess Cd intake through rice consumption. However, the yield of cv. Nanfengnuo is relatively low and the type is not popular with farmers. The present study has shown that rice cultivars with a high rate of ROL tend to inhibit Cd accumulation in above-ground tissues, though the mechanism by which this is mediated is still not clear. ROL is potentially an important index for selection of high Cd tolerance and low Cd accumulation rice cultivars.
5. Conclusion There were significant differences in Cd tolerance and concentrations in grain and straw of the 20 rice cultivars grown in Cdcontaminated soils under both field and greenhouse conditions. The results of THQ values based on the field experiment suggest that selection of rice cultivars to ensure residents’ health is possible in certain Cd-contaminated areas. This is the first study showing that rice cultivars with higher rates of ROL tend to result in lower Cd concentrations both in grain and straw. Mechanisms of this effect require further investigation.
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