Reduction of the Mn cluster in oxygen evolving complex under dark-chilling condition

Abstract

The oxidizing side of photosystem II is inhibited under dark-chilling condition. When leaves of cucumber are treated at 0oC in the dark, the extrinsic proteins dissociate from Photosystem II, resulting in the decrease in oxygen evolution. This inhibition is reversible and oxygen-evolving activity recovers under moderate light at room temperature. In this study, we have investigated the redox properties of the Mn-cluster in dark-chilled leaves by means of thermoluminescence (TL) measurements. Cucumber leaves, which had been treated for 12, 24 and 36 hours at 0oC in the dark, were excited by a series of flashes, and then subjected to TL measurements. TL signal was hardly detected in 12-h treated leaves when excited by single flash. After two flashes, however, TL band with a peak at around 30oC was observed. This band arises from charge recombination between either the S2 or the S3 state of the Mn-cluster and QB-, the secondary quinone acceptor. This result indicates that the Mn-cluster of the leaves dark-chilled for 12 h is mostly in the S0 state. In 24-h treated leaves, TL signals appeared only after the excitation by four flashes. In 36-h treated leaves, excitation with five flashes was required for TL emissions. These results indicate that the over-reduced Mn-cluster such as the S-2 and the S-3 state may be formed during the treatment for 24 and 36 h, respectively. Thus, the cause of inhibition in oxygen evolution can be partly ascribed to the over-reduction of the Mn-cluster.