Genotoxic effects of heavy metals: comparative investigation with plant bioassays

The potential use of micronucleus assays in plants for the detection of genotoxic effects of heavy-metal ions was investigated. Three different plant systems were comparatively investigated in micronucleus tests with Tradescantia pollen mother cells (Trad MCN) and micronucleus tests with meristematic root tip cells of Allium cepa and Vicia faba (Allium/ Vicia MCN). As3+, Pb2+, Cd2+, Zn2+ caused a dose-dependent increase of MCN frequencies in all three test systems. Cu2+ gave consistently negative responses in all three tests; Zn2+ caused only a moderate, statistically not significant increase of MCN frequencies in Vicia. The ranking of genotoxic potencies in all three tests was in the descending order: As3+ > Pb2+ > Cd2+ > Zn2+ Cu2+. In experiments with Tradescantia, induction of MCN was observed in a concentration range between 1 and 10 mM, whereas in tests with root tip cells, higher concentrations (10-1,000 mM) were required to show significant effects. Further increase of the exposure levels caused toxic effects (reduction of root growth), cell division delays, and a decrease of MCN frequencies. Comparisons by linear regression analyses indicated that the sensitivity of the three bioassays for heavy metals decreases in the order: Trad MCN > Vicia root MCN > Allium root MCN. In further experimental series, a soil sample which contained high concentrations of the five metals and a control soil were investigated. Aqueous soil extracts induced only weak effects in Trad MCN tests and no effects in the root tip assays, whereas cultivation of the plants in the soils resulted in a pronounced induction of MCN in the Tradescantia system and moderate effects in Vicia and Allium. In conclusion, the results of the study indicate that the Trad MCN assay detects the genotoxic effects of heavy metals and can be used for biomonitoring metal-contaminated soils.