This study was aimed to get deeper insights into the conditions that induce electron transfer from photosystem II (PS II) to exogenous acceptor in thylakoids isolated from leaves of spinach (Spinacia oleracea L. cv. Matador) plants. The pH-dependent effect of the b6/f-cytochrome complex inhibitors NQNO (2-n-nonyl-4-hydroxyquinoline-N-oxide) and DNP-INT (2,4-dinitrophenyl ether of 2-iodo-4-nitrothymol) on linear electron flow was studied in pH range from 4.5 to 8.0. It was shown that the extent of the inhibition positively correlates with pH value and at pH lower than 5.0 the inhibitors decrease the rate of light-dependent oxygen evolution in chloroplasts by 50% only. It is inferred that significant increase of the transmembrane proton gradient or decrease of pH lower than 5.0 induces alternative terminal acceptor reduction reaction at the level of PS II, without the participance of photosystem I. Photosynthetic photon flux density of actinic light also influenced this reaction: while saturating light conditions stimulated, limiting light suppressed this autonomous functioning mode of PS II. Similar redox reactions in PS II complex are also induced in the presence of some heavy metal ions in chloroplasts suspension medium. So, zinc and copper ions at sub-millimolar concentrations inhibited total linear electron transfer by no more than 25%, while light-dependent proton uptake, indicative of plastoquinone reduction at the QB-site of photosystem II, was inhibited by 90 % in the presence of 80 mikroМ of copper (II) ions and by 70 % in the presence of 200 mikroМ of zinc (II) ions. At the same time, about 50% of control value of transmembrane proton gradient and up to 75 % of control value of light-dependent oxygen evolution remained. This indicates that linear electron flow took place which was not accompanied by plastoquinone reduction, that is, exogenous acceptor was being reduced at the level of PS II, before the QB-site. In order to test the binding capability and effect of different heavy metals at the acceptor side of PS II, we studied changes in variable chlorophyll fluorescence relaxation kinetics after excitation of photosystems by white multiple-turnover saturating flash (600 ms), which reflects dark oxidation of the primary quinone acceptor in PS II - QA. Three components of the fluorescence decay were revealed, that indicate different ways/mechanisms of QA oxidation. Addition of copper (II), zinc (II), cadmium (II), or lead (II) ions in concentrations of 50 and 200 ?М, or mercury (II) ions in concentrations of 5 and 20 ?М into suspension medium led to changes in all components. Copper ions affected fluorescence decay in a similar manner to mercury ions, accelerating electron transfer at the Fe-QA level, probably by accepting electrons, in contrast to zinc, cadmium and lead ions, which inhibited electron transfer at the acceptor side of PS II by no more than 30 %. In addition, the relative content of QB-non-reducing PS II reaction centers was determined by analysis of fluorescence induction data, and weak inhibitory effect of the heavy metal ions at the acceptor side of PS II was confirmed. These results suggest that induction of alternative redox reactions in PS II, when QB reduction is suppressed due to overreduction of the photosynthetic electron-transport chain or binding of copper (II) or zinc (II) ions, is one of the possible mechanisms that play an important role in photoprotection
