Inhibition of Photosystem II (PS II) activity induced by continuous light or by saturating single turnover flashes was investigated in Ca2+-depleted, Mn-depleted and active PS II enriched membrane fragments. While Ca2+- and Mn-depleted PS II were more damaged under continuous illumination, active PS II was more susceptible to flash-induced photoinhibition. The extent of photoinactivation as a function of the duration of the dark interval between the saturating single turnover flashes was investigated. The active centres showed the most photodamage when the time interval between the flashes was long enough (32 s) to allow for charge recombination between S2 or S3 and Q(B)/- to occur. Illumination with groups of consecutive flashes (spacing between the flashes 0.1 s followed by 32 s dark interval) resulted in a binary oscillation of the loss of PS II-activity in active samples as has been shown previously [(Keren et al. (1995) J. Biol. Chem. 270: 806-814)]. Ca2+- and Mn-depleted PS II did not show this effect. The data are explained by assuming that charge recombination in active PS II results in a back reaction that generates P680 triplet and thence singlet oxygen, while in Ca2+- and Mn-depleted PS II charge recombination occurs through a different pathway that does not involve triplet generation. This correlates with an up-shift of the midpoint potential of Q(A) in samples lacking Ca2+ or Mn that, in term, is predicted to result in the triplet generating pathway becoming thermodynamically less favourable [(Johnson et al. (1995) Biochim. Biophys. Acta 1229: 201-207)]. The diminished susceptibility to flash-induced photoinhibition in Ca2+- and Mn-depleted PS II is attributed at least in part to this mechanism.
Bibliographical noteFunding Information:
We would like to thank Dr Alexander Berg (Jerusalem) and Dr Winfried Leibl (Saclay) for their help with using the laser facilities and Dr Giles Johnson for the critical reading of the manuscript. This work was supported by ISF the Israel Academy of Sciences and the Deutsche Forschungsgemein-schaft. N.K. and A.K. were supported by short term fellowships of the European Science Foundation. We would also like to thank Prof. W. Haehnel for supporting this work.
- Ca -depletion
- Charge recombination
- Photosystem II
- Q(A) midpoint potential