THYLAKOID MEMBRANE FLUIDITY AND THERMOSTABILITY DURING THE OPERATION OF THE XANTHOPHYLL CYCLE IN HIGHER-PLANT CHLOROPLASTS

Barley leaves were exposed for several min to a white light of photon flux density 1000 mu mol m(-2) s(-1), leading to a massive conversion of the xanthophyll violaxanthin to antheraxanthin and zeaxanthin in th e absence of lipid peroxidation. Using electron spin resonance spectro scopy and different spin-labeled stearate probes, we observed that thi s light treatment noticeably decreased thylakoid membrane Lipid fluidi ty. The light-induced membrane rigidification (i) was proportional to the amount of zeaxanthin present in the membranes, (ii) was blocked by dithiothreitol, a potent inhibitor of the violaxanthin de-epoxidase, (iii) was slowly reversible in the dark, (iv) was not observed in thyl akoids of an Arabidopsis mutant that has no xanthophyll cycle and (v) was accompanied by a substantial increase in the thermostability of th e ionic permeability properties of the thylakoid membranes. The amount of xanthophyll-cycle pigments found in photosystem II was observed to significantly decrease after illumination. Photoacoustic and chloroph yll fluorometric analyses of the illuminated leaves revealed that stro ng illumination decreased the quantum yield of photosynthetic oxygen e volution and the pigment antenna size of photosystem Il in green light (preferentially absorbed by carotenoids) but not in red light (absorb ed by chlorophylls only). Taken together in the light of previous in v itro data on carotenoids incorporated into artificial membranes, our r esults indicate that the xanthophyll cycle could be an 'emergency mech anism' that rapidly provides thylakoid membrane lipids with rigidifyin g carotenoid molecules upon sudden increase in light intensity. The si gnificance of this mechanism for the membrane function and adaptation to stressful light and temperature conditions is discussed. (C) 1997 E lsevier Science B.V.