Inhibition of the Photosynthetic Electron Transport by Pyrethroid Insecticides in Cell Cultures and Thylakoid Suspensions from Higher Plants

Abstract

Synthetic pyrethroid insecticides with different molecular structures have been investigated with respect to their effect on photosynthetic electron transport reactions in chloroplast suspensions and cell cultures from higher plants. The fluorescence induction curves of tobacco (Nicotiana tabacum) leaves and tomato cells were substantially affected by permethrin and cypermethrin resulting in a strong increase of the maximum fluorescence. Application of different concentrations (0.3-1.2 mᴍ ) of the respective chemical abolishes virtually any kinetics of the normal Kautsky effect. Oxygen evolution from cell cultures from tomato (Lycopersicon peruvianum) was completely inhibited by cypermethrin. Analysis of partial reactions of the photosynthetic electron transport showed that both a methylviologen-mediated Mehler reaction and a ferricyanide-driven Hill reaction were quantitatively inhibited by e.g. fenvalerate. On the other hand, neither a silicomolybdate-driven Hill reaction nor a methylviologen- driven Mehler reaction using dichlorophenol indophenol/ascorbate as electron donors could be inhibited by the pyrethroid. The analyses suggest that pyrethroid insecticides interfere with the photosynthetic electron transport at the same site as urea-type herbicides do. Depending on the molecular structure and on the halogen compound in the molecule, however, different pyrethroids are more or less phytotoxic to the investigated photosynthetic membranes - cypermethrin with two Cl-substituents requires much higher concentrations to be applied for significant inhibition of the electron transport reactions than the Br-derivative deltamethrin does. Moreover, qualitative differences have to be taken into account. In the case of fenvalerate the effect seems to exist in a type of all-or-nothing reaction when the reaction centres are inhibited by the pyrethroid. None of the S-states nor the transition probabilities are specifically influenced by increasing concentrations of fenvalerate. In the case of deltamethrin, however, it was found that the overreduced state S-1 is significantly increased at the expense of both S1 and S0. Moreover, the miss parameter a is increased in the case of deltamethrin addition. The results and the significance of different substituents for the investigated pyrethroids are discussed