Using chlorophyll a fluorescence to detect the onset of anthracene photoinduced toxicity in Lemna gibba, and the mitigating effects of a commercial humic acid

The influence of a commercial humic acid (Aldrich; AHA) on the development of anthracene photoinduced toxicity to the duckweed Lemna gibba was examined using both room- and low-temperature (77°K) chlorophyll fluorescence assays. Plants were exposed to 2 mg liter -1 anthracene both with and without 6.2 mg liter -1 of AHA and grown under either visible light or simulated solar radiation that mimics the natural abundance of UV radiation. Exposure periods ranged from 1 to 48 h to examine temporal changes in chlorophyll degradation and chlorophyll a fluorescence induction in response to these light and HA treatments. The onset of anthracene photoinduced toxicity followed a definite sequence; chlorophyll a fluorescence induction parameters (F v /F m , and t 1 /2) responded earliest to anthracene exposure, with observable chlorophyll degradation requiring up to 24 to 48 h. Of these, t 1 /2 was the most sensitive, with significant inhibition apparent within I h of exposure. Throughout the entire 48-h exposure, 6.2 mg liter 1 AHA ameliorated the photoinduced toxicity of anthracene, in terms of both chlorophyll degradation and F v / F m inhibition. In contrast, AHA delayed the complete inhibition of t 1 /2 by only I to 24 h rather than permanently protecting the plants from anthracene damage to PS2. This suggests that AHA may slow, but not prevent, the entrance of either intact anthracene or its photooxidized byproducts under these exposure conditions.