Summarg1. Photoinduced changes in absorbancy and fluorescence yields were studied in Chromatium minutissium and Ectothiorhodospira Shaposhnikovii suspensions. These parameters were examined as functions of light intensity in the region of 750-950 nm under aerobic and anaerobic conditions. 2. The fluorescence was shown to consist of two spectrally indistinguishable emissions : "photosynthetic" emission with a yield correlating with the state of reaction centres (P890) and "background" emission with constant yield.3. The "photosynthetic" fluorescence dependence on the portion of nonactive Psg0 was consistent with the multicentral (statistic) model of photosynthetic unit organization.4. In anaerobic conditions the amplitude ofthe photoinduced absorbancy increase around 910 nm was proportional to the portion of nonactive P890. 5. A precise method for determinating the quantum yield of the primary electron donation was proposed. For Chr. minutissimum and E. shaposhnikovii under aerobic conditions these yields were found to be equal to 0.91 • 0.02 and 0.93 J= 0.02 respectively.

Quinones caused quenching of Chl a fluorescence in native and model systems. Menadione quenched twofold the fluorescence of Chl a and BChl a in pea chloroplasts, chromatophores of purple bacteria, and liposomes at concentrations of 50‐80 μM. To obtain twofold quenching in Triton X‐100 micelles and in ethanol, the addition of 1.3 mM and 11 mM menadione was required, respectively. A proportional decrease in the lifetime and yield of Chl a fluorescence in chloroplasts, observed as the menadione concentration increased, is indicative of the efficient excitation energy transfer from bulk Chl to menadione. The decrease in the lifetime and yield of fluorescence was close to proportional in liposomes, but not in detergent micelles. The insensitivity of the menadione quenching effect to DCMU in chloroplasts, and similarity of its action in chloroplasts and liposomes indicate that menadione in chloroplasts interacts with antenna Chl, i. e., nonphotochemical quenching of fluorescence occurs.

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We show that CCCP, known as an uncoupler of photophosphorylation and an ADRY agent, inhibits FeCy photoreduction and coupled 0, evolution by isolated chloroplasts equally (I,, -2 PM), but is practically without effect on the O2 evolution coupled with SiMo reduction within the 0.2-10pM concentration range. CCCP has no effect on the nanosecond chlorophyll fluorescence in chloroplasts incubated at low light intensity, but decreases it at high light intensity. The electron transfer from reduced TMPD or duroquinol to methylviologen IS resistant to CCCP. The efficiency of the CCCP inhibitory action on the FeCy photoreduction depends on the rate of electron flow, which is controlled by the light intensity. The data obtained show that CCCP is oxidized by the photosystem II donor side and is reduced by Qr. competing for electrons with FeCy and the cytochrome blfcomplex.

The protonophoric uncouplers carbonyl cyanide m-chlorophenylhydrazone (CCCP), 2,3,4,5,6-pentachlorophenol (PCP) and 4,5,6,7-tetrachloro-2-trifluoromethylbenzimidazole (TTFB) inhibited the Hill reaction with K3[Fe(CN)6] (but not with SiMo) in chloroplast and cyanobacterial membranes (the I50 values were approx. 1-2, 4-6 and 0.04-0.10 μM, respectively). The inhibition is due to oxidation of the uncouplers on the Photosystem II donor side (ADRY effect) and their subsequent reduction on the acceptor side, ie. to the formation of a cyclic electron transfer chain around Photosystem II involving the uncouplers as redox carriers. The relative amplitude of nanosecond chlorophyll fluorescence in chloroplasts was increased by DCMU or HQNO and did not change upon addition of uncouplers, DBMIB or DNP-INT; the HQNO effect was not removed by the uncouplers. The uncouplers did not inhibit the electron transfer from reduced TMPD or duroquinol to methylviologen which is driven by Photosystem I. These data show that CCCP, PCP and TTFB oxidized on the Photosystem II donor side are reduced by the membrane pool of plastoquinone (Qp) which is also the electron donor for K3 [Fe(CN)6] in the Hill reaction as deduced from the data obtained in the presence of inhibitors. Inhibition of the Hill reaction by the uncouplers was maximum at the pH values corresponding to the pK of these compounds. It is suggested that the tested uncouplers serve as proton donors, and not merely as electron donors on the oxidizing side of Photosystem II.