Earlier experiments, using 31 P-NMR and time-resolved merocyanine fluorescence spectroscopy, have shown that isolated intact, fully functional plant thylakoid membranes, in addition to the bilayer phase, contain three non-bilayer (or non-lamellar) lipid phases. It has also been shown that the lipid polymorphism of thylakoid membranes can be characterized by remarkable plasticity, i.e. by significant variations in 31 P-NMR signatures. However, changes in the lipid-phase behaviour of thylakoids could not be assigned to changes in the overall membrane organization and the photosynthetic activity, as tested by circular dichroism and 77 K fluorescence emission spectroscopy and the magnitude of the variable fluorescence of photosystem II, which all showed only marginal variations. In this work, we investigated in more detail the temporal stability of the different lipid phases by recording 31 P-NMR spectra on isolated thylakoid membranes that were suspended in sorbitol- or NaCl-based media. We observed, at 5°C during 8 h in the dark, substantial gradual enhancement of the isotropic lipid phases and diminishment of the bilayer phase in the sorbitol-based medium. These changes compared well with the gradually increasing membrane permeability, as testified by the gradual acceleration of the decay of flash-induced electrochromic absorption changes and characteristic changes in the kinetics of fast chlorophyll a-fluorescence transients; all variations were much less pronounced in the NaCl-based medium. These observations suggest that non-bilayer lipids and non-lamellar lipid phases play significant roles in the structural dynamics and functional plasticity of thylakoid membranes.

Department of Physics Faculty of Science University of Ostrava Ostrava CZ 710 00 Czech Republic

Department of Physics Faculty of Science University of Ostrava Ostrava CZ 710 00 Czech Republic Global Change Research Institute Czech Academy of Sciences Brno 603 00 Czech Republic

Institute of Plant Biology Biological Research Center Hungarian Academy of Sciences Szeged H 6726 Hungary

Institute of Plant Biology Biological Research Center Hungarian Academy of Sciences Szeged H 6726 Hungary Department of Physics Faculty of Science University of Ostrava Ostrava CZ 710 00 Czech Republic

Slovenian NMR Center National Institute of Chemistry Ljubljana Slovenia

Slovenian NMR Center National Institute of Chemistry Ljubljana Slovenia EN FIST Center of Excellence Ljubljana Slovenia

Slovenian NMR Center National Institute of Chemistry Ljubljana Slovenia EN FIST Center of Excellence Ljubljana Slovenia Faculty of Chemistry and Chemical Technology Ljubljana Slovenia

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