[Studies on the backward-reactions in the xanthophyll-cycle of Chlorella, Spinacia and Taxus].

The epoxidation of zeaxanthin to the di-epoxide violaxanthin via the mono-epoxide antheraxanthin (called the backward-reaction), is examined with several plant objects and under various conditions.In Chlorella and in the needles of Taxus baccata a backward-conversion can be observed immediately after the termination of strong illumination. The reaction can be accelerated somewhat by exposure of the plant material to pure O2 or dim light.One cannot observe such an epoxidation in leaf disks of Spinacia oleracea under normal conditions (dark, air). It begins only under the influence of dim light or when pure oxygen is supplied. The absence of the backward-reaction under the given experimental conditions is a consequence of a closure of the stomata, which begins during the strong illumination and continues in the succeeding dark period; it is therefore a consequence of anaerobiosis in the plastid-containing cells. Yet a backward-reaction starts if the O2-tension in the cells is increased either by pure O2 given from outside or by the intracellular evolution of photosynthetic O2 (associated with a partial opening of the stomata).The concentration of the intermediate antheraxanthin increases strongly more at the beginning of the O2- or dim-light-promoted backward-reaction than the concentration of the endproduct violaxanthin. Hence it follows that during epoxidation the two O-atoms are added to the 5,6- and the 5',6'-position of the zeaxanthin not simultaneously but one after the other.In isolated chloroplasts or cell fragments no backward-reaction could be observed under various conditions tested. An apparent backward-reaction in lyophilized cells or chloroplasts, which is triggered by light or O2-atmosphere, is a result of the different velocity of the photooxidative destruction of carotenoids.The in vivo epoxidation of zeaxanthin, which probably is catalysed by a Cu-containing enzyme, only proceeds in the presence of molecular O2.