Effect of photosystem II reaction center closure on nanosecond fluorescence relaxation kinetics

Abstract

The fluorescence decay of chlorophyll in spinach thylakoids was measured as a function of the degree of closure of Photosystem II reaction centers, which was set for the flowed sample by varying either the preillumination by actinic light or the exposure of the sample to the exciting pulsed laser light. Three exponential kinetic components originating in Photosystem II were fitted to the decays; a fourth component arising from Photosystem I was determined to be negligible at the emission wavelength of 685 nm at which the fluorescence decays were measured. Both the lifetimes and the amplitudes of the components vary with reaction center closure. A fast (170-330 ps) component reflects the trapping kinetics of open Photosystem II reaction centers capable of reducing the plastoquinone pool; its amplitude decreases gradually with trap closure, which is incompatible with the concept of photosynthetic unit connectivity where excitation energy which encounters a closed trap can find a different, possibly open one. For a connected system, the amplitude of the fast fluorescence component is expected to remain constant. The slow component (1.7-3.0 ns) is virtually absent when the reaction centers are open, and its growth is attributable to the appearance of closed centers. The middle component (0.4-1.7 ns) with approximately constant amplitude may originate from centers that are not functionally linked to the plastoquinone pool. To explain the continuous increase in the lifetimes of all three components upon reaction center closure, we propose that the transmembrane electric field generated by photosynthetic turnover modulates the trapping kinetics in Photosystem II and thereby affects the excited state lifetime in the antenna in the trap-limited case.

Photoelectric study on the kinetics of trapping and charge stabilization in oriented PS II membranes

“…2. Our interpretation is in line with the conclusions of Keuper and Sauer (1989). It is also supported by fluorescence decay measurements of Scenedesmus obliquus in which the middle phase increases and the fast phase decreases upon the transition from state I to state II, the latter being connected with a phosphorylation of the lightharvesting protein on the acceptor side (Wendler and Holzwarth, 1987).…”

“…This is within the limit of the values reported for the fast (200-300ps) fluorescence decay attributed to the trapping time in unfractionated thylakoids with 200-250 Chl/P-680 and in the absence of artificial acceptors (Holzwarth et al 1985, Schatz and Holzwarth 1987, Keuper and Sauer, 1989. The same trapping time was also found by an indirect measurement of the effect of DNB on fluorescence and photovoltage (Kischkoweit et al 1988).…”

“…It has also been suggested, that the rate constants in the reversible scheme might not only be affected by the charge on QA, but also by other parameters like the membrane potential (Keuper and Sauer 1989). Although there is evidence for a reversible charge separation, the extent of this reversibility and how it is controlled in the intact system is still largely unknown.…”

A simple model relating photoinhibitory fluorescence quenching in chloroplasts to a population of altered Photosystem II reaction centers

“…Krause, unpublished). From recent fluorescence lifetime studies (Sehatz et al 1988, Keuper and Sauer 1989, see Holzwarth 1991 it was concluded that the primary photochemical reaction is not limited by exciton transfer from the antennae to the reaction centers, but rather by the rate of primary charge separation ('trap limitation'). In such 'exciton equilibrated' systems, thermal deactivation in the antennae cannot be discriminated from that occurring in the reaction centers by means of fluorescence analysis.…”

Chlorophyll a fluorescence induction: a personal perspective of the thermal phase, the J–I–P rise

“…8) is used; it includes: (a) trapping of the excitation energy; (b) primary charge separation, with the rate constant of forward and back reactions being influenced electrostatically by the negative charge on reduced Q A (however, only moderately; Renger et al 1995), and also by transmembrane DW and DpH (Schatz et al 1988;Meiburg et al 1983;Keuper and Sauer 1989); and (c) different energy dissipation reactions, which allow verification of the theory of Schreiber and Krieger (1996). (3) The '3-quencher' model of Steffen (2003) and Steffen et al (2001Steffen et al ( , 2005 In the model of Belyaeva and coworkers, the timedependent evolution of the fluorescence yield was calculated by multiplying the fraction of all fluorescent PSII states (hChl-P680i*) by k F /k L , where k F is the rate constant of fluorescence, and k L is the rate constant for excitation with light, which were assumed to be independent of the redox states of Q A and Q B , or the occupancy of Q B site.…”