PHOTOREDUCTION OF THE SECONDARY PHOTOSYSTEM-I ELECTRON-ACCEPTOR VITAMIN-K-1 IN INTACT SPINACH-CHLOROPLASTS AND CYANOBACTERIA IN-VIVO

The light-induced EPR changes in the g = 2 region of intact spinach ch loroplasts and of marine cyanobacteria Synechococcus PCC 7002 at room temperature have been analysed. Illumination after dark adaptation res ults in the well known transient oxidation of P700 and its subsequent re-reduction during the first 100 ms of the illumination, as monitored by EPR Signal I. Here we show, that after a distinct lag phase, coinc iding with the re-reduction of P700, a second EPR signal is generated. This signal could be identified as the reduction signal of phylloquin one electron acceptor A(1) by its asymmetric spectrum (g = 2.0049, Del ta H-ptp = 1.00 mT) and by its sensitivity to conditions which suppres s limitation of electron transport at the acceptor side of PS I during illumination. Since steady state reduction of A(1) has to date been f ound only by photoaccumulation in prereduced PS I particles at cryogen ic temperatures, this is the first observation of the species under ph ysiological conditions. A(1) reduction is shown to saturate with light intensity at a level of 0.5 per P700. The observation of substantial levels of reduced A(1) in vivo should provide new insights into the me chanisms of energy dissipation and light regulation of the reaction ce ntre of PS I under acceptor limited conditions.