Effects of polyaromatic hydrocarbons on photosystem II activity in pea leaves.

The acute effects of three typical polyaromatic hydrocarbons (PAHs): naphthalene (Naph), phenanthrene (Phen) and fluoranthene (Flu) on photochemical activity of photosystem II (PSII) in detached leaves of 3-week-old pea plants were studied. The leaves were exposed in water with PAHs under white light for 0.5-72 h. The activity of PSII was examined by prompt and delayed chlorophyll a (Chl a) fluorescence. The effects of PAHs depended on their concentration and exposure time. This dependency was more significant in the presence of chemical stressors (Triton X-100 or acetone) or under high intensity irradiance. Increased content of PAHs and long-term exposure (24-72 h) led to significant reduction of the maximum photochemical quantum efficiency (Fv/Fm) of PS II, changes in the polyphasic fluorescence induction (OJIP), and to decreasing amplitudes of fast and slow components of delayed Chl a fluorescence. The damage of PSII depended on water solubility of a given type of PAHs, their concentration and exposure time. During short-time exposure the compound with highest water-solubility - naphthalene - revealed the strongest effect. During long-time exposure the compounds with low water-solubility -Phen, Flu-revealed the strongest effect as the corresponding PAH accumulates in the thylakoids especially when the solution is oversaturated containing a solid phase. The reduction of PSII activity at the presence of naphthalene (30 mg L(-1)) was accompanied by transient generation of H2O2 as well as swelling of thylakoids and distortion of cell plasma membranes, which was indicated by electron microscopy images. Distortion of thylakoid membranes due to accumulation of PAHs as well as the development of oxidative stress seems to be the main pathways of PAHs influencing the photochemical activity of PS II.