DOES THE XANTHOPHYLL CYCLE TAKE PART IN THE REGULATION OF FLUIDITY OF THE THYLAKOID MEMBRANE

Upon strong illumination (30 min, 1400 W m-2) of intact pea leaves the spin label-monitored fluidity of the isolated thylakoid membranes decreases in the peripheral region of the hydrophobic core (as measured with the 5-doxylstearic acid spin label) but remains unchanged in the membrane interior (as measured with the 16-doxylstearic acid spin label). At the same time, as a consequence of the light driven de-epoxidation of violaxanthin to zeaxanthin, the molar ratio of these two pigments in the photosynthetic apparatus decreases from 6.25 to 0.64. The light-induced alterations of thylakoid fluidity have been attributed to the membrane-modifying effect of zeaxanthin. Such concept is supported by the results of analogous experiments in which the xanthophyll cycle-related zeaxanthin accumulation was partially blocked by the de-epoxidase inhibitor, dithiothreitol.