A plant life-cycle bioassay for contaminated soil,with comparison to other bioassays: Mercury and zinc

Bioassays using rapid-cycling plants allow measurement of multiple endpoints and assessment of impacts on both growth and reproduction. Selections of Brassica rapa develop rapidly in a broad range of soils and are very consistent in production of flower and seed. Their sensitivity to variation in growth conditions was investigated to define the variables that most affect performance. Yield differences between soils were substantial, indicating the need for careful selection and use of control treatments. The sensitivity to contaminants was investigated with applications of mercury (Hg) and zinc (Zn) to three soils. In a sand soil, bloom initiation was slowed by <10 mg Hg kg^–1 soil and <50 mg Zn kg^–1 soil. In contrast, lettuce emergence and earthworm survival were less sensitive to these metals in this soil. Survival of Daphnia magna and the Microtox^® assay in soil extracts were more sensitive to Hg than bloom initiation, but less sensitive to Zn. A similar relationship among the bioassays was observed for two finer-textured soils, although for these, effects were usually apparent only at soil metal concentrations >200 mg kg^–1. Enzyme assays were included for comparison, but were not sensitive to Hg contamination. Rapid-cycling B. rapa selections are suitable for routine bioassays, and are representative of several widely distributed and utilized species.