Changes in phytotoxicity of polycyclic aromatic hydrocarbons in the course of microbial degradation

Phytotoxicity of six polycyclic aromatic hydrocarbons (PAHs) and their 16 oxidized derivatives that may be microbial metabolites arising in the course of PAH degradation was determined using an express test with the seedlings of sorghum (Sorghum bicolor L. Moench) and alfalfa (Medicago sativa L.). It was shown that germinating capacity is the least informative characteristic and the most useful parameter is development of seedlings during 3 days in the presence of compounds under investigation. Among unsubstituted compounds, toxicity in respect to seedlings decreased in the series fluorene > phenanthrene > anthracene. Chrysene, fluoranthene, and pyrene stimulated shoot development. It was found that some of the metabolites produced as a result of microbial degradation of phenanthrene (9,10-phenanthrenequinone, 1-hydroxy-2-naphthoic and benzoic acids) are more toxic for plants than starting PAH molecules. The obtained results are important for understanding rhizosphere processes associated with phytoremediation technique.