EVIDENCE FOR THE INVOLVEMENT OF CYCLIC ELECTRON-TRANSPORT IN THE PROTECTION OF PHOTOSYSTEM-II AGAINST PHOTOINHIBITION - INFLUENCE OF A NEW PHENOLIC COMPOUND

Organisms that perform oxygenic photosynthesis are subjected to inhibi tion of their photosynthetic functions when they are exposed to excess ive illumination. Photoinhibition occurs mainly at the level of photos ystem II, where a cyclic electron transport has often been proposed to be involved in photoprotection. However, a demonstration of direct pr otection by cyclic photosystem II against photoinhibitory damage has b een lacking. In this report, we used the newly characterized compound 4-[methoxybis(trifluoromethyl)methyl] -2,6-dinitrophenylhydrazine meth yl ketone (K-15), known to stimulate cyclic electron transport between the acceptor and donor sides of the photosystem [Klimov, V. V., Zharm ukhamedov, S. K., Allakhverdiev, S. I., Kolobanova, L. P., & Baskakov, Y. A. (1993) Biol. Membr. 6, 715-732], to verify if photosystem LI is significantly protected by cyclic electron transport against aerobic and anaerobic photoinhibitory damage. The photoinhibitory quenching of the maximal level of fluorescence and the decrease of the absorbance change at 685 nm related to pheophytin photoreduction observed during photoinhibitory illumination of untreated or Mn-depleted photosystem I I submembrane fractions are significantly attenuated in the presence o f K-15. The photodegradation of cytochrome b559 and the photobleaching of beta-carotene and chlorophyll-670 measured in Mn-depleted photosys tem II preparations are also strongly retarded when K-15 is present. T he detection, by photoacoustic spectroscopy, of the energy stored duri ng the cyclic electron transport is also reported in Mn-depleted photo system II submembrane fractions and in photosystem II reaction center complexes. This reaction is also gradually photoinhibited due to the p rogressive photodegradation of the required electron transport interme diates but is significantly more stable in the presence of K-15. It is deduced that cyclic electron transport around photosystem LI constitu tes an effective protective mechanism against photoinhibitory damage.