MECHANISMS OF PHOTOINDUCED TOXICITY OF PHOTOMODIFIED ANTHRACENE TO PLANTS - INHIBITION OF PHOTOSYNTHESIS IN THE AQUATIC HIGHER-PLANT LEMNA-GIBBA (DUCKWEED)

Polycyclic aromatic hydrocarbon (PAH) toxicity is enhanced by light, e specially ultraviolet (UV) radiation. To examine a potential mechanism (s) of photoinduced toxicity of PAHs to plants, the effects of anthrac ene and its photoproducts on photosynthesis were investigated using th e aquatic higher plant Lemna gibba L. G-3 (duckweed). Photosynthetic a ctivity was monitored both in vivo and in vitro by measuring chlorophy ll a (Chi a) fluorescence, carbon fixation, and electron transport. In simulated solar radiation (a light source with a visible light:UV-A:U V-B ratio similar to sunlight), inhibition of photosynthesis was more rapid with photomodified anthracene than with intact anthracene, and i ntact anthracene appeared to only inhibit photosynthesis following its photomodification. The primary site of action of photomodified anthra cene was found to be electron transport at or near photosystem I (PSI) . This was followed by inhibition of photosystem II (PSII), probably d ue to excitation pressure on PSII once the downstream electron transpo rt through PSI was blocked. Accordingly, higher chemical concentration s and/or longer exposures were required to inhibit PSII than PSI. Net photosynthesis (carbon fixation) was also inhibited, implying that the inhibition of electron transport in PSI by photomodified anthracene c an lead to diminished primary productivity. A linkage between inhibiti on of photosynthesis and inhibition of plant growth was established in terms of the initial site of action (PSI) and primary productivity (c arbon fixation), which suggested that Chi a fluorescence can be used a s a bioindicator of PAH impacts on plants.