Effects of Ions and Gravity Forces on the Supramolecular Organization and Excitation Energy Distribution in Chloroplast Membranes

This study was designed to explore the possible relationship between chloroplast membrane stacking or particle aggregation in stacked membrane regions (or both) and excitation energy distribution between photosystems I and II. To this end we have quantitatively examined the effects of different concentrations of univalent ions on the above-mentioned parameters, using a combination of freeze-fracture and thin-section electron microscopy for structural analysis and chlorophyll fluorescence measurements to assay energy-transfer processes. Membrane stacking was found to saturate at about 15Om~-NaCl. Maximal EF_s-face particle density and chlorophyll fluorescence occurred at about 100m-MNaCl, although only 50% of the potential EF_s-face particles were located in stacked membrane regions at this salt concentration. Centrifugation (30000 g, 1 h) could significantly increase the amount of stacked membranes at salt concentrations between 20-and 60-mM-NaCl;in contrast, centrifugation had little effect on cation-regulation of chlorophyll fluorescence properties. These and other findings suggest that neither chloroplast membrane stacking nor the aggregation of EF-face particles into stacked regions is directly related to the mechanism of excitation energy distribution between the two photosystems (as measured by chlorophyll fluorescence changes) although both structural and functional changes may be mediated by the same membrane component. It is proposed that the salt-induced stacking of chloroplast membranes and the concomitant aggregation of EF-face particles is mediated by the screening of negative surface charges on the membrane pigment-protein subunits, by the establishment of specific interactions between light-harvesting pigment-protein complexes and by 'entropic ordering' forces.