INVOLVEMENT OF THE XANTHOPHYLL CYCLE IN REGULATION OF CYCLIC ELECTRONFLOW AROUND PHOTOSYSTEM-II

In our previous study (Gruszecki et al., 1995) we have postulated that the mechanism of cyclic electron transport around photosystem II, act ive under overexcitation of the photosynthetic apparatus by light is u nder control of the xanthophyll cycle. The combination of different li ght quality and thylakoids having various levels of xanthophyll cycle pigments were applied to support this hypothesis. In the present work photosynthetic oxygen evolution from isolated tobacco chloroplasts was measured by means of mass spectrometry under conditions of high or lo w levels of violaxanthin, being transformed to zeaxanthin during dark incubation in an ascorbate containing buffer at pH 5.7. Analysis of ox ygen evolution and of light-induced oxygen uptake indicate that the de -epoxidation of violaxanthin to zeaxanthin results in an increased cyc lic electron transport around PS II, thus dimishing the vectorial elec tron flow from water. An effect similar to de-epoxidation was observed after incubation of thylakoid membranes with specific antibodies agai nst violaxanthin.