On the origin of the `35-μs kinetics' of P680 +⋅ reduction in photosystem II with an intact water oxidising complex

Abstract

The origin of the `35-μs kinetics' of P680 +⋅ reduction in photosystem II (PS II) with an intact water oxidising complex has been analysed by comparative measurements of laser flash induced changes of the 830-nm absorption and the relative quantum yield of chlorophyll (Chl) fluorescence. The latter parameter was monitored at a time resolution of 500 ns by using newly developed home built equipment [Reifarth, F., Christen, G. and Renger, G. (1997) Photosynth. Res. 51, 231–242]. It was found that: (i) the amplitudes of the unresolved ns-kinetics of both 830-nm absorption changes and the rise of fluorescence yield exhibit virtually the same period four oscillation pattern when dark adapted samples are excited with a train of saturating laser flashes; (ii) the corresponding oscillation patterns of the normalised extent of the 35-μs kinetics under identical excitation conditions are strikingly different with maxima after the 3rd and 5th flash for the 830-nm absorption changes vs. pronounced maxima after the 4th and 8th flash for the rise of the fluorescence yield. The period four oscillations unambiguously show that the `35-μs kinetics' of P680 +⋅ reduction are characteristic for reactions in PS II entities with an intact water oxidising complex. However, the disparity of the oscillation patterns of (ii) indicates that in contrast to the ns components of P680 +⋅ reduction the 35-μs kinetics do not reflect exclusively an electron transfer from Y Z to P680 +⋅. It is inferred that a more complex reaction takes place which comprises at least two processes: (a) P680 +⋅ reduction by Y Z and (b) coupled and/or competing reaction(s) which give rise to additional changes of the chlorophyll fluorescence yield.