THE XANTHOPHYLL CYCLE IN INTERMITTENT LIGHT-GROWN PEA-PLANTS - POSSIBLE FUNCTIONS OF CHLOROPHYLL A/B BINDING-PROTEINS

The xanthophyll cycle in pea (Pisom sativum L. cv Kleine Rheinlanderin) plants has been investigated in vivo. Control plants were compared with those grown under intermittent light (IML plants). IML plants are particularly characterized by the absence of nearly all chlorophyll a/b-binding proteins. The rates of de-epoxidation during 30 min of illumination and their dependence on the incident photon flux density (PFD) have been determined. They were very similar in both types of plants, with the exception that IML plants contained, at any PFD, much higher zeaxanthin concentrations in the steady state (reached after about 15 min of illumination) than control plants. This indicates that the amount of convertible violaxanthin under illumination is dependent on the presence of chlorophyll a/b-binding proteins. The epoxidation rate (examined at a PFD of 15 mu E m(-2) s(-1), after 15 min of preillumination with different PFDs) showed significant differences between the two types of plants. It was about 5-fold slower in IML plants. On the other hand, in both types of plants, the epoxidation rate decreased with increasing PFD during preillumination. Prolonged preillumination at high PFDs resulted in a decrease of the epoxidation rate without a further increase of the zeaxanthin concentration in both continuous light and IML plants. This result argues against a permanent turnover of the xanthophylls under illumination, at least at high PFDs.