KINETIC RELATIONSHIPS BETWEEN STRUCTURAL-CHANGES IN THE THYLAKOID MEMBRANES AND PHOTOSYSTEMS ORGANIZATION

Events involved in cation-induced aggregation of thylakoid membranes and grana formation were studied in broken chloroplasts. Membrane stacking was evaluated through changes in light scattering while lateral interactions and movements of pigment-protein complexes were monitored through changes in steady-state, maximum and minimum chlorophyll fluorescence (F(s), F(m) and F(o) resp.). Changes in the imbalance between the photoactivities of PS I and PS II were derived from these fluorescence parameters and the rates of these changes were compared at room temperature and at 5-degrees-C. Particularly, a distinction was made between altered protein-protein interactions within the membrane (microstructural changes) and altered membrane-membrane interactions (macrostructural changes). following addition of MgCl2 (5 mM), the increase in F(s) and in the photosystems' imbalance lagged behind the increase in F(m), all of which lagged behind the increase in light scattering. This effect was mostly pronounced at the low temperature. One may conclude that membrane stacking (reflected mainly by the scattering increase) does not require and is not simultaneously coordinated with the lateral movements of pigment complexes (reflected by an increase in the imbalance) and the lateral organization of the photosystems (reflected by an increase in F(m)) but is rather a prerequisite for it. Such photosystem organization occurs in later stages of the cation-induced overall process. On the other hand, in the reverse processes, which were followed by depleting Mg2+ with EDTA, membrane destacking and grana disintegration lagged behind the decrease in light excitation imbalance in favor of PS II and thus are probably limited by lateral rearrangements of pigment complexes. The decrease in the imbalance was much faster than the decrease in F(m), the last one presumably reflecting an increased exciton interaction between PS I and PS II (i.e., quenching of PS II excitation by energy transfer to PS I - 'spillover' - when the two photosystems are sufficiently close, as the diffusional movement progresses). The difference between the kinetics of F(m) and the imbalance when they increase upon cation addition or decrease upon cation depletion implies that the changes in the imbalance of the photosystems and spillover are largely independent processes.