Inhibition of Photosystem II activity by saturating single turnover flashes in calcium-depleted and active Photosystem II

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.