Alterations in Photosystem II electron transport as revealed by thermoluminescence of Cu-poisoned chloroplasts

Abstract

The thermoluminescence characteristics of functionally intact thylakoids and TRIS-washed BBY particles were studied under Cu(II) poisoned conditions. In thylakoids, both the A and B thermoluminescence bands corresponding to S3QA- and S2S3QB- charge recombinations, respectively showed specific responses to Cu(II) treatment. The amplitude of the B band was gradually decreased, which corresponds to the Cu(II) induced inactivation of TyrZ. The simultaneous stepwise shift in the peak position of the B band indicated, however, that S3QB- charge recombination is more resistant to Cu(II) poisoned conditions. The shifted peak position of the A band toward the higher temperature in Cu(II) treated thylakoids also showed a change in the redox span between the recombination partners generating the A band of the glow curve. The AT band due to the His+QA- recombination in TRIS-washed BBY particles was insensitive to Cu(II) addition indicating that Cu(II) did not affect either His+ or QA-. The unaffected intensity of the A and AT bands when Cu(II) inhibits TyrZ function favours the assumption of an alternative pathway in which functional TyrZ is not required. In addition, Cu-induced changes of the TL bands were compared to those produced by the Tyr and His modifiers NBD and DEPC, respectively. We obtained very similar results regarding TL bands by either adding NBD or Cu-poisoning in functional thylakoids. Regarding DEPC, the A and AT bands were abolished by increasing concentrations of the His modifier. This effect was associated with the decrease of the B band and its replacement by the Q band at around 0 °C. Comparing our data obtained by Cu, NBD and DEPC treatments, we have found a strong interrelation between His+ and S3 state. We assume that in some inhibitory conditions in the S3 state His is oxidized in place of Mn and this alternative pathway does not require functional TyrZ.