Evaluation of chalcone derivatives as electron flow inhibitors on photosystem II

Chlorophyll a (Chl a) fluorescence measurement is an important tool for bioactive compounds evaluating as potential photosynthesis inhibitors. In this work, six synthetic chalcones were evaluated through Chl a fluorescence experiment on spinach discs, using Krebs medium with 0.05% of Tween. The leaf discs were maintained in a photoperiod of 12 h for leaf disc stability. Solutions of chalcones were added and the discs were kept for additional 6 h. Then, the discs were kept in the dark for 30 min. The maximum fluorescence yield from the sample was obtained by 2 s irradiation. DMSO was added in the control experiments in order to assess the solvent effect. The Chl a fluorescence induction curves were measured at room temperature with a portable Hansatech Fluorescence Handy PEA (plant efficiency analyzer) apparatus. Photosystem inhibitory activity was performed in a completely randomized design and data were analysed using the statistical program SPSS 11.0, including analysis of variance (ANOVA) for differences between means in all tests. Among the compounds evaluated, (E)-chalcone (1), (E)-3-(4-bromophenyl)-1-(4-nitrophenyl)prop-2-en-1-one (2) and (E)-3-(4-bromophenyl)-1-(pyridin-3-yl)prop-2-en-1-one (3) showed statistically significant variations when compared to the control (p < 0.01), reducing the PI(abs) parameter by 85, 51 and 30%, respectively.

Compounds 1, 2 and 3 decreased the electron transport chain by 52, 33 and 16 %, respectively, indicating the observed stress is attributed to the blockage of electron transport at the quinone pool level. Additionally, the reduction of quantum parameters such PHI(E0) and PSi0 and the increase of DI0/CSm corroborate the identification of the action of compounds in the electron flux directly preventing the redox process between QA-QB on PSII.