Arsenic uptake and speciation in rice plants grown under greenhouse conditions with arsenic contaminated irrigation water
Introduction
Arsenic (As) contamination of groundwater has been reported in many countries throughout the world. The most severe incidences of human health issues have been reported in Bangladesh (Nickson et al., 1998, Chowdhury et al., 1999), West Bengal, India (Chowdury et al., 2000) and China (Lu et al., 2001). In Bangladesh, the British Geological Survey identified that 35% of groundwater samples analysed (n = > 2000) were above the Bangladesh drinking water standard of 0.05 mg As l− 1 and 51% above the 0.01 mg As l− 1 World Health Organisation standard (British Geological Survey, 2000). Groundwater is the main source of drinking water in Bangladesh and West Bengal, India, however, groundwater is also used as a major source of water for crop irrigation.
It is estimated that 83% of the total irrigated area in Bangladesh is used for rice (Oryza sativa L.) cultivation (Dey et al., 1996, Abedin et al., 2002d). As a consequence of rice cultivation with As contaminated groundwater, rice may contain elevated concentrations of As which may represent a significant potential As exposure pathway for humans (Meharg, 2004, Meharg and Rahman, 2003, Alam et al., 2002). Alam et al. (2002) reported elevated As concentrations in rice grain samples collected from Samta village in the Jessore district of Bangladesh with As concentrations ranging from 0.16 to 0.58 mg kg− 1 (mean 0.35 mg kg− 1). Meharg and Rahman (2003), however, reported that rice grown in some regions of Bangladesh contained considerably higher As concentrations in the grain, in some cases in excess of 1.7 mg kg− 1. Rice grain samples (n = 20) collected by researchers as part of an Australian Centre for International Agricultural Research grant (grant number LWR1/1998/003) varied considerably and ranged from < 0.1 to 0.5 mg As kg− 1 (ACIAR 2004 Yearly Report). However, elevated As concentrations in rice grain were generally observed in plants which were irrigated with groundwater containing elevated As for extended periods of time. Abedin et al., 2002b, Abedin et al., 2002c) reported that the mean As concentration in rice grain grown under greenhouse conditions ranged from 0.26 mg kg− 1 to 0.74 mg kg− 1 depending on the concentration of As supplied in the irrigation water. The aforementioned examples indicate that under suitable field or greenhouse conditions, As may accumulate to elevated levels in the rice grain.
The extent to which As in rice contributes to As related diseases is under considerable debate in the scientific literature. In some countries such Bangladesh and the province of West Bengal, India, rice is the main staple of the village diet with up to 0.45 kg of rice being ingested per day (Alam et al., 2002, Correll et al., 2006). The ingestion of As via the food exposure pathway may represent a considerable contribution to the dietary As intake. Meharg (2004) estimated that if drinking water As concentrations were at 0.01 mg l− 1 and rice contained 0.05 mg As kg− 1 then the consumption of rice will contribute approximately 60% to the human dietary As exposure. Similarly Correll et al. (2006) estimated that As contaminated rice contributed between 11 and 32% to the Bangladeshi dietary intake. The percentage contribution of As exposure from rice was estimated from dietary intake information collected in 3 Bangladesh villages where As concentrations in drinking water ranged from 0.22 ± 0.21 to 0.33 ± 0.13 mg l− 1 and rice contained between < 0.1 to 0.5 mg As mg kg− 1.
A major assumption when estimating the potential dietary intake of As is that 100% of the As is present in the inorganic form and is absorbed following consumption (i.e. 100% bioavailable). However, when considering human health risk assessment, As speciation and bioavailability are critical as As species vary in their toxicity and bioavailability influences the dose that reaches systemic circulation (Laparra et al., 2005). Few studies have focused on As speciation in the rice grain and its influence on As dietary intake values. The aim of this study was to investigate the uptake of As by rice plants and the accumulation of As in various rice plant compartments (roots, shoots, leaves and grain). Qualification of As speciation in rice plant compartments was also determined as As speciation has important toxicological implications once ingested (Tamaki and Frankenberger, 1992).
Section snippets
Rice experimental methodology
Paddy rice (O. sativa Quest) was selected for this study as it was easily obtainable in Australia due to strict quarantine conditions imposed by the Australian Quarantine Department. Rice seeds were germinated in moist compost and after 3 weeks, plants of uniform size were transferred into 3 pools containing a slow release fertiliser mixed with washed sand (pH 7.5). The slow release of fertiliser was applied at a rate consistent with that applied in field conditions (70 kg ha− 1). After
Arsenic concentration in plant material
Rice grown in a greenhouse under paddy field conditions accumulated marked concentrations of As in the roots, stem, leaf and grain (Fig. 1). Arsenic concentrations in the rice plants were highest in the root material at 248 ± 65 mg As kg− 1 and lowest in the grain material at 1.25 ± 0.23 mg As kg− 1. The As concentrations observed in various rice tissues is within the range reported by Abedin et al., 2002a, Abedin et al., 2002b, Abedin et al., 2002c who grew paddy rice under greenhouse conditions
Conclusions
Analysis of the distribution of As within rice showed that As accumulated in plant tissues in the order root>leaf>grain. Up to 248 ± 65 mg As kg− 1 was detected in root tissue where as 1.25 ± 0.23 mg As kg− 1 was detected in the grain. Speciation studies demonstrated that root, shoot and leaf tissue contained mainly inorganic AsIII and AsV species while the rice grain contained predominantly DMA (85 to 94%) and AsIII.
Acknowledgements
The authors would like to acknowledge the support of the Australian Centre for Industry and Agricultural research funding (LWR/1998/003/) for partial support of Dr Euan Smith, the Australian Research Council Linkage Grant Scheme with the support of IPOH Pacific Ltd (Grant number LP0347301) and the generous support of the University of South Australia.
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