The Early Stages of Photosystem II Assembly Monitored by Measurements of Fluorescence Lifetime, Fluorescence Induction and Isoelectric Focusing of Chlorophyll-Proteins in Barley Etiochloroplasts

Abstract

The relationship between functional and structural aspects of PSII formation during greening of etiolated barley leaves has been investigated using fluorescence lifetime measurements, fluorescence kinetics analysis and analysis of chlorophyll-protein complexes by IEF-PAGE. Two phases of different character could be distinguished in the course of the greening process in dark-grown plants. An early phase covering the first 3-4 h after the onset of illumination and a late phase covering the subsequent greening. During the first phase the formation of PSII reaction centers and their minor antenna components was observed as manifested by the IEF-PAGE polypeptide pattern. This was accompanied by shortening of the slow and middle components of the fluorescence lifetime, as well as by the rapid drop in the amplitude of the slow component. A room temperature emission band at 676 nm was associated with uncoupled chlorophyll and with the slow fluorescence lifetime component during the first hours of greening. During the late greening phase peripheral light-harvesting complexes of PSII were formed concomitantly to an increase in lifetime and amplitude of the fast component and to a further decrease in the lifetime of the middle component. The gradual increase in PSII complexity during both phases of greening was also manifested by changes in proportion and kinetic properties of PSIIα and PSIIβ units. Similar changes in fluorescence lifetime components as in the late greening of dark-grown plants were also observed in intermittentlight plants during continuous greening associated with the development of PSII antenna. The relationships between fluorescence lifetime characteristics and development of PSII are discussed in terms of a stepwise mechanism invoving a first step of Chl integration into small size PSII units followed by progressive increase of antenna size.