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 i n 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 no dules (nodulation) may serve for ecotoxicological evaluation of contam inated soil. Substances affecting the macro- and/or microsymbionts vit ality, such as certain heavy metals or polycyclic aromatic hydrocarbon s (PAHs), reduce nodulation before visible damage of the plant occurs. Tests were performed in petri dishes that were planted with alfalfa s eedlings and inoculated with Rhizobium meliloti. Mineral agar served a s substrate. Test substances were added to the mineral agar before pou ring it into petri dishes (application of heavy metals), or were spray ed as etheral solutions onto the surface of solidified medium (applica tion of PAHs). For investigation of contaminated soil, the soil materi al 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 substr ate. A dose-responsive decrease in nodulation was found after applicat ion of CdCl2, NaAsO2, fluoranthene, and other PAHs. PAH-contaminated s oil (75 mg/kg) caused only a slight reduction in nodulation of alfalfa . But when PAHs were extracted and restored into the extracted soil ma terial, 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.