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Flash-induced absorption changes at 515 nm observed as a function of flash number are examined in relation to the flash-induced fluorescence yields in inside-out thylakoids. After partial dissipation of the delocalized transmembrane electric field by adding gramicidin, the analysis of period 4 oscillations and of the kinetics in the 10 ms-1 s range suggest that the variation of the absorption changes at 515 nm as a function of flash number is the result of at least two processes:1) an electric field increase related to the S2 state and 2) the fact that the field generated by the water protons inside the membrane decreases when these protons are released outside the membrane. The former field correlates with the flash-induced fluorescence yield increase induced by the donor side of Photosystem II. Both measurements show similar oscillations as a function of flash number, with maxima on the 1st, 5th and 9th flash. These oscillations, after a shift of two flashes, appear to be different from those of the O2 yield observed under similar conditions. It is proposed that, in a population of centers the electric field during the S2 state reflects the presence of a stabilized positive equivalent in the protein close to the Mn complex.

The presence of exogenous quinones PPBQ, DCBQ and DMQ in inside-out thylakoids alters the oscillation pattern with period 4 of the flash-induced fluorescence yield. The results can be interpreted by assuming that at ambient temperature PPBQ replaces PQ in 100% of the PS II centers detected by fluorescence, DMQ and DCBQ being bound to 4060% of these centers only. The evaluation of the percentage of double hits in a 3 ps flash allows estimation of the electron-transfer rate from Qi to Fd+, which varies from 9 ps (PPBQ) to 20 ps (DMQ).Oxygen evolution; Photosystem II; Chlorophyll fluorescence; Quinone; (Inside-out thylakoid)