ECOTOXICOLOGICAL EVALUATION OF CONTAMINATED SOIL USING THE LEGUME ROOT-NODULE SYMBIOSIS AS EFFECT PARAMETER

The legume root nodules are the site of biological nitrogen fixation in the legume-rhizobium symbiosis. They are a structure unique to this symbiosis and morphologically as well as physiologically distinct from other plant organs. We will present evidence that the formation of nodules (nodulation) may serve for ecotoxicological evaluation of contaminated soil. Substances affecting the macro- and/or microsymbionts vitality, such as certain heavy metals or polycyclic aromatic hydrocarbons (PAHs), reduce nodulation before visible damage of the plant occurs. Tests were performed in petri dishes that were planted with alfalfa seedlings and inoculated with Rhizobium meliloti. Mineral agar served as substrate. Test substances were added to the mineral agar before pouring it into petri dishes (application of heavy metals), or were sprayed as etheral solutions onto the surface of solidified medium (application of PAHs). For investigation of contaminated soil, the soil material was first mixed with mineral agar and then poured into petri dishes. Appropriate controls were made by extracting an aliquot of soil with methylene chloride to remove hazardous substances. Data from stressed plants were taken after 15 days of cultivation on contaminated substrate. A dose-responsive decrease in nodulation was found after application of CdCl2, NaAsO2, fluoranthene, and other PAHs. PAH-contaminated soil (75 mg/kg) caused only a slight reduction in nodulation of alfalfa. But when PAHs were extracted and restored into the extracted soil material, toxicity and uptake of PAHs into roots and shoots dramatically increased. Toxicity and uptake of PAHs were also enhanced by reducing the amount of humic matter added to the test system. (C) 1995 by John Wiley and Sons, Inc.