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, especially ultraviolet (UV) radiation. To examine a potential mechanism(s) of photoinduced toxicity of PAHs to plants, the effects of anthracene and its photoproducts on photosynthesis were investigated using the aquatic higher plant Lemna gibba L. G-3 (duckweed). Photosynthetic activity was monitored both in vivo and in vitro by measuring chlorophyll a (Chi a) fluorescence, carbon fixation, and electron transport. In simulated solar radiation (a light source with a visible light:UV-A:UV-B ratio similar to sunlight), inhibition of photosynthesis was more rapid with photomodified anthracene than with intact anthracene, and intact anthracene appeared to only inhibit photosynthesis following its photomodification. The primary site of action of photomodified anthracene was found to be electron transport at or near photosystem I (PSI). This was followed by inhibition of photosystem II (PSII), probably due to excitation pressure on PSII once the downstream electron transport through PSI was blocked. Accordingly, higher chemical concentrations 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 can lead to diminished primary productivity. A linkage between inhibition of photosynthesis and inhibition of plant growth was established in terms of the initial site of action (PSI) and primary productivity (carbon fixation), which suggested that Chi a fluorescence can be used as a bioindicator of PAH impacts on plants.