Analysis of the Chl- a+II reduction kinetics with nanosecond time resolution in oxygen-evolving Photosystem II particles from Synechococcus at 680 and 824 nm

Abstract

The kinetics of Chl- a + II (P-680 +) reduction have been measured under repetitive excitation with nanosecond time resolution by monitoring the time-course of the absorption changes at 680 and 824 nm. O 2-evolving Photosystem II particles from Synechococcus sp. have been used because of the following advantages: (i) small ratio of antenna chlorophylls to Chl- a II (approx. 100), (ii) low content of PS I and (iii) reduced light scattering. 1. Identical time-courses of the absorption changes at 680 and 824 nm were found in these PS II particles. 2. For the most part (approx. 70%), Chl- a + II is reduced in the nanosecond time range. The remaining Chl- a + II (approx. 30%) is reduced with half-life times in the microsecond time range. 3. The decay in the nanosecond time range is multiphasic. This multiphasic reduction can be explained quantitatively by a superposition of different reduction kinetics reported for the first single turnover flashes in dark-adapted samples (Brettel, K., Schlodder, E. and Witt, H.T. (1984) in Advances in Photosynthesis Research, Vol. 1 (Sybesma, C., ed.), pp. 295–298, Martinus Nijhoff/Dr. W. Junk Publishers, The Hague). It results a description of the nanosecond decay (approx. 70%) by three exponential phases with t 1 2 ≈ 20 ns (approx. 33%), ≈ 50 ns (approx. 19%) and 300 ns (approx 19%). 4. The nanosecond kinetics are attributed to electrons supplied by the oxygen-evolving complex via a secondary donor, D. For the microsecond kinetics, different suggestions are discussed. 5. The difference spectrum of the absorption changes decaying in the nanosecond time range resembles that attributed to Chl- a + II minus Chl- a II. 6. In the absence of electron acceptors, a fast absorption change transient ( t < 6 ns) is observed, possibly due to a charge separation and recombination between Chl- a II and pheophytin. 7. Additional absorption changes observed at supersaturating flash intensities are described.