Alterations in the excitation energy distribution in Synechococcus PCC 7942 due to prolonged partial inhibition of Photosystem II

Changes in the excitation energy distribution between the photosystems have been investigated in response to long-term partial inhibition of Photosystem II (PS II) activity by growing the cells of cyanobacterium Synechococcus PCC 7942 in presence of a PS II inhibitor, SANDOZ 9785 (a pyridazinone derivative, also known as BASF 13.338). Transfer of excitation energy from PS II to PS I decreased in the treated cells leading to an enhanced efficiency of energy utilization by PS II. The redistribution of energy did not seem to affect the ability of the SANDOZ 9785-treated cells to undergo state transitions. The effect of SANDOZ 9785 was also studied on R2TaqI, a mutant of Synechococcus PCC 7942 resistant to inhibition by many PS II inhibitors, to determine whether the observed changes are solely a consequence of PS II inhibition or a specific effect of SANDOZ 9785. The PS II activity of R2TaqI was lower as compared to the wild type; also, the energy distribution characteristics of R2TaqI were found to be remarkably similar to SANDOZ 9785-grown wild-type cells. Furthermore, growing the R2TaqI cells in the presence of SANDOZ 9785 did not alter these characteristics. It is therefore proposed that inhibition of PS II activity, irrespective of the manner by which it is effected, leads to redistribution of excitation energy in favour of PS II.