ABSTRACT
       Excessive concentration of some heavy metals in soil and water is one of the most pressing problems in today’s world. This is due to the toxicity risk associated with their integration into the food chain.
       Uranium (U) naturally occurs in all soil types and in almost all natural sources of water. However, its concentration is higher in specific rocks and minerals, which can be exploited to obtain U for energy generation. The potential use of plants for the cleanup of soil and water contaminated with excessive uranium levels derived from its manipulation is an active research topic, and has already led to the installation of a pilot system for rhizofiltration using sunflowers.
       The main objective of this study is to improve our understanding of the process of U uptake by terrestrial plants and to characterise the physiological behaviour of these plants in U-contaminated environments. The research has been subdivided into two objectives: 1. To study the potential U uptake in situ by specific plant species of spontaneous emergence in a mining area; 2. To study the behaviour and tolerance of the red beet (Beta vulgaris) at various U concentrations in hydroponic conditions within a laboratory setting.
       The only plant species to be found in all soil types, including mine tailings, was the Stipa sp. (“corión”) which, within the media used, showed tolerance to U concentrations of up to 386 mgU kg-1. Tagetes minuta (“chin-chil or chil-chil”) was found in soils with U concentration up to 280 mgU kg-1. In contrast, the two species of “jarilla”, Larrea divaricata and Larrea nitida, were only present in soils with low levels of contamination. In all cases, the U accumulation and the soil-to-plant transfer factor or bioaccumulation factor (BF) were low.
       In hydroponic conditions, beets survived in nutrient solution with the addition of U concentrations in the range 0 - 5 mgU L-1 for at least 15 days.
       The tolerance to U could be linked to the presence of phosphate ion in the nutrient solution. Without this element, beets survived for only 7 days in a solution containing 3.3 mgU·L-1, a weaker concentration than the maximum tolerated (5 mgU L-1) by the beet for 15 days with phosphate present.

The accumulation of U in roots was found to be very high, with a BF average of 4,600. In contrast, no U bio-concentration was detected in the aerial organs (BF < 1).
       The restriction of U translocation from root to aerial organs was observed in both the plant species of spontaneous emergence in soils and in the beet cultivated under controlled conditions. In beet, the translocation was slightly increased when phosphate was removed from the nutrient solution. It is probable that the “coiron”, “chil-chil” and beet U tolerance is based on the U exclusion from the aerial organs, where U could interfere with vital physiological processes.
       In fact, the evapotranspiration, the relative water content and total chlorophyll content were drastically reduced in beet plants grown in the presence of U. The activity of the ascorbic peroxidase, an enzyme that operates in oxidative stress states, also increased in these conditions. After the beet plants had been exposed to U contamination, its removal from the medium was not sufficient to attain a full recovery of the plants.
       The use of the currently studied plant species in programmes of phytoremediation is restricted by the low accumulation of pollutants in their aerial organs, rendering these of little use for soil phytoextraction. However, the high capacity of colonization of polluted soil and the U tolerance of “coirón” and “chilchil” could be employed in the phytostabilization of U-contaminated substrates, thus preventing the dispersion of pollution through wind and water action. Nevertheless, the implementation of beet rhizofiltration systems to clean up U polluted waters appears to be feasible. However, it is still necessary to improve the operational conditions used in the laboratory and to adapt these to an industrial or semi-industrial scale for these procedures to be of general use.

Plant responses to Uranium stress. The accumulation of Uranium in plant species from mine areas and a study on the physiological changes in red beet (Beta vulgaris)
Silvia Concepción López





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