The kinetics of chlorophyll photobleaching were followed in whole thylakoid membranes as well as in photosystem I and photosystem I1 submembrane fractions. The onset of photobleaching was characterized by a slow rate which indicated the presence of energy traps implicated in the photoprotection of the bulk pigments. The pigments in photosystem I submembrane fractions bleached at a faster rate than those in photosystem I1 counterparts, the latter being more sensitive towardsphotoinhibition. An analysis of the pigment-protein complexes isolated from whole thylakoid membranes during the course of a photobleaching experiment has shown that the core-antenna complexes, including CP29, are more sensitive to illumination than the peripheral complexes. The absorption spectra of the CPI and CP2Y complexes presented a blue shift of the red absorption maximum after partial photobleaching, indicative of a non-homogeneous bleaching of the holochromes in these complexes. An analysis of these data points towards the involvement of CP2Y in a photoprotection mechanism at the level of photosystem 11. The weaker resistance of photosystem I to photobleaching relative to photosystem I1 and its stronger resistance to photoinhibition is discussed in terms of an energy dissipation pathway in thylakoid membranes.
The inhibitory effect of the dye ruthenium red was studied in photosystem II-enriched submembrane fractions. A number of distinct types of interaction were found, which differed in their concentration range and required incubation time. Ruthenium red instantaneously quenches the initial chlorophyll a fluorescence level (F ) and the maximum fluorescence level (F ) by enhancing radiationless deactivation in the chlorophyll light harvesting complex. Associated with this quenching of fluorescence is an instantaneous decrease in the quantum yield of oxygen evolution. Ruthenium red also inhibited the light saturated rate of oxygen evolution and the variable fluorescence, monitored 80 µs after a saturating excitation-flash. These inhibitions increased with incubation time and became greater than 50% within 5 min. Although ruthenium red was known to affect Ca or Cl sites specifically, the inhibitory action was more pronounced than simple Ca or Cl depletion. Incubation with ruthenium red for 5 min blocks the Z P680 → Z P680 charge transfer reaction. Upon mixing with the photosystem II preparation, ruthenium red induced specific release of the extrinsic 16 kDa polypeptide associated with water-splitting without release of Mn. It is proposed that the inhibitor produces an ionic imbalance which alters the configuration of the donor side of photosystem II.
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