Several radionuclides (RN) and heavy metals (HM) are naturally present in the environment at trace level. If present at background levels they may be without discernable hazard or can act as essential nutrients (in case of HM) for plant growth. When present at high concentration, they can be toxic for the soil microflora and fauna, for plants and animals and for humans via the food chain. This study will concentrate on the potential biological effects induced following uptake of uranium (and cadmium) by plants. Large areas of land have been contaminated by radionuclides and fission-by products including uranium from nuclear weapons facilities, above ground nuclear testing, nuclear reactor operations, improper waste storage practices and nuclear accidents. Radioactive contamination of the environment surrounding facilities where uranium has been mined and processed has occurred in many countries. Uranium is reported to be the most frequent radionuclide contaminant in ground and surface water soils of the United States Department of Energy facilities (Entry et al., 1996). However, radioactive elements such as 238U, 226Ra and 232Th, and non-radioactive elements such as Cd, Zn, Cu, Ni, and As can simultaneously occur in a polluted area. This is for example the case at sites contaminated as a result of activities of industries involved in the extraction or processing of raw materials containing naturally occurring radionuclides (NOR). Examples of those industries are the phosphate industry, metal mining and smelting, coal mining and power generation from coal, petroleum industry, zirconium and ceramics' industry. The major contributor to environmental radioactive contamination in Belgium is most likely the phosphate industry. When evaluating the impact of a contamination at a site the multi-pollution and mixed nature of this contamination should not be neglected because an action decreasing the exposure to one contaminant possibly enhances the availability of other contaminants present. Apart from the obvious concern of public exposure following the ingestion of uranium containing food, the protection of the environment and sustainable development have presently become subject to public concern, the political world and the scientific community. There is an increasing interest for studying the effects of pollutants at the molecular, biochemical and genetic level. In plants, environmental adversity often leads to the increase in formation of highly reactive oxygen species (ROS). Under natural (non-stress) conditions ROS occur in the plant cell and therefore plants possess several antioxidative defense mechanisms to control the redox state of the cell which is essential for normal physiological and biochemical functioning. The defense systems comprise antioxidative enzymes (superoxide dismutases, peroxidases, catalases, glutathione reductase) and antioxidants (e.g. glutathione, ascorbate,Ö). HM toxicity results in an enhancement of the antioxidative defense system (Clijsters et al., 1999, Cuypers et al., 2002). Resistance to such conditions may be correlated with enzymes in oxygen detoxification (Bowler et al., 1991). Exposition to radionuclides and heavy metals may also result in direct or indirect (oxidative stress mediated) genotoxic effects. With respect to the effect of radiation dose rate at the molecular and genetic level, most studies were performed on animal cells or cell culture used as a model for studying the effect of radiation dose to man. Studies of the radiation dose on the plant and microbiota community is scant. If data are available they are mostly obtained under acute high external dose (Unscear, 1996). Very few studies deal with the effect of low-chronic external exposure on the plant (e.g. Okamoto and Tatara, 1995; Zaka et al., 2002a,b; Ptacek et al., 2002). Hardly any studies exist on the biological effects induced following incorporation of radionuclides. Appropriate prediction of potential environmental impact of uranium contamination requires the understanding of the mechanisms ruling uranium mobility and uptake and distribution in the plant. The geochemical behaviour of uranium has been investigated to an important extent (e.g. Fenton and Waite, 1996; Schultz et al., 1998; Roh et al., 2000). The bioavailability of this metal has been insufficiently explored in the past, however. There is limited information on how soil physico-chemical characteristics affect U-speciation and U bioavailability or on how processes in the root environment affect U availability. Often an increased uranium solubility with increasing pH is observed linked with the formation of highly soluble negatively charged carbonate complexes (Ebbs, 1997, Tyler and Olsson, 2001; Shahadeh and Hossner, 2002; Vandenhove et al. 2005). Vandenhove et al. (2005) demonstrated the importance of uranium speciation on the soil-to-shoot transfer of uranium. Uranium speciation may also affect the partitioning of uranium between roots and shoots. This was demonstrated by Ebbs (1997) who reported that peas exposed to uranium at pH 5, 6, 8, accumulated most uranium in the roots (and total plant) at pH 6 and 8 but shoot concentrations were highest at pH 5. It is hence clear that environmental conditions will affect uranium uptake by and repartitioning within the plants and hence the potential biological effects induced.
created:2011-12-14 14:18:59 UTC, source:web