Metal bioavailability and phytotoxicity study on Vicia faba L. grown on natural and spiked contaminated soils.

In soil, metals are associated with several fractions: 1) in soil solution, as free metal ions and soluble metal complexes, 2) adsorbed to inorganic soil constituents at ion exchange sites, 3) bound to soil organic matter, 4) precipitated such as oxides, hydroxides, carbonates, and 5)embedded in structure of the silicate minerals. Bioavailability depends on metal solubility in soil solution. Only metals associated with fractions 1 and 2 are readily available for plant uptake. Soil properties affect metal bioavailability to ecological receptors. Therefore contaminant bioavailability varies among soil types and may depend on one or a combination of soil properties and contaminants characteristics. A single total contaminant level can result in multiple contaminant exposure doses across different soils due to modification by soil properties or in situ remediation. In this work different soils contaminated by boron (B) were studied in relation to plant bioavailability and toxicity. B is an essential micronutrient for plants and generally it is absorbed from soil in the form of boric acid. Previous studies demonstrated that both excess or deficiency of B can affect normal plant development. Natural B polluted soils and artificially B spiked soils, with B total concentrations ranged between 20 and 100 mg/Kg, were investigated. Vicia faba L., plant commonly used for detecting the genotoxic effects of environmental pollutants grown on soils. Initially developmental and physiological parameters were analyzed during plant growth in the B contaminated soils. The B content in different plant tissues was determined; a direct correlation between B concentration in soil and B content in plants was found. Interestingly bioavailability of B, and its content in the different plant organs, resulted higher in spiked soils when compared with the same B concentrations in natural soils. Moreover cytological analysis was carried out on root tip meristems of Vicia faba, after 3 days of seed germination in the different B polluted soils; Mitotic Index (MI) and micronucleus assay (MNC) were determined for genotoxicity evaluation. A close correlation between high B concentration in roots and major level of cytogenetic defects was found. These results confirmed plant genotoxicity of B polluted soils and showed clear evidences between soil properties, B bioavailability and phytotoxicity.