Mechanisms of inhibitory effects of polycyclic aromatic hydrocarbons in photosynthetic primary processes in pea leaves and thylakoid preparations

Inhibitory effects of polycyclic aromatic hydrocarbons (PAHs) on plants were studied in pea leaves in order to elucidate the mechanisms of action of PAHs such as naphthalene (Naph) and phenanthrene (Phen) on activity of photosystem II (PSII). The changes in different Chl fluorescence parameters were calculated on the basis of Chl fluorescence induction curves. H₂O₂ content was measured in leaf homogenates with the luminol-dependent chemiluminescence method. We demonstrated that following PAH treatment, total energy dissipation (DI₀/ABS) and amount of Q_B-non-reducing complexes of PSII significantly increased. Non-photochemical quenching (NPQ) also increased, when weak oxidative stress after PAH application developed. In leaves, a two-step increase in H₂O₂ was found with time of incubation in the presence of PAHs, which may be associated with damage to the lipid bilayer of the plasma membrane and then violation of lipid bilayer membranes of cell organelles. A hypothesis for the mode of action of PAHs is provided that involves the role of ROS, membrane permeability and associated functional changes in PSII.