PSI becomes more tolerant to fluoranthene through the initiation of cyclic electron flow

Environmental pollution by organic compounds such as polycyclic aromatic hydrocarbons (PAHs) poses a potential ecological risk to photosynthetic organisms. In the present study, the toxic effects of fluoranthene (FLT) on the energy conversion of PSI and PSII in wheat (Triticum aestivum L.) plants were studied. By evaluating the performance of both PSI and PSII, which act as an internal environmental sensor, it was revealed that activity of both photosystems was negatively affected by FLT treatment. However, the quantum yield of PSII, Y(II), was reduced at 5 mu M FLT, whereas the quantum yield of PSI, Y(I), significantly decreased at 25 mu M FLT. The decline in Y(II) was accompanied by an increase in nonregulated energy dissipation, Y(NO). The decrease in Y(I) induced by FLT was caused by donor-side, and acceptor side limitation of PSI. Cyclic electron flow (CEF) was activated only at higher concentrations and was associated with the inhibition of linear electron flow (LEF) after exposure to a higher concentration of FLT. The inhibition of LEF and induction of CEF seems to be essential for the tolerance of PSI to FLT toxicity.