IMPACTS OF UV-RADIATION AND PHOTOMODIFICATION ON THE TOXICITY OF PAHSTO THE HIGHER-PLANT LEMNA-GIBBA (DUCKWEED)

The toxicity of polycyclic aromatic hydrocarbons (PAHs) can be enhance d by both biotic and abiotic processes. This is exemplified by light, which, by virtue of the extensive pi-orbital systems of PAHs, can be a major factor in PAH toxicity. Light activation of PAHs is known to oc cur via photosensitization reactions (generation of singlet oxygen and superoxide) and potentially by photomodification of the chemicals (ph otooxidation and/or photolysis) to more toxic species. To examine the modes of PAH action in the light and determine if the photomodified co mpounds are hazardous, we investigated the photoinduced toxicity of an thracene, phenanthrene and benzo[a]pyrene to the aquatic higher plant Lemna gibba (a duckweed). Toxicity end points were inhibition of growt h and extent of chlorosis. Light did indeed activate the phytotoxicity of PAHs, with UV radiation more effective than visible light. Dose-re sponse curves based on chemical concentration and light intensity reve aled the order of phytotoxic strength to be anthracene > phenanthrene > benzo[a]pyrene. To explore whether photomodified PAHs were contribut ing to toxicity, the chemicals were irradiated before toxicity testing . The rates of photomodification of the three PAHs were rapid (half-li ves in hours), and the relative velocities were coincident with the or der of toxic strength. Furthermore, the photomodified PAHs were more h azardous to Lemna than the intact compounds. Because interpretations o f the potential impacts of PAHs in the environment are based mostly on measurements of the structurally intact chemicals, the severity of PA H hazards is possibly underestimated.