In vivo chlorophyll a fluorescence transients and the circadian rhythm of photosynthesis in gonyaulax polyedra.

The intensity of chlorophyll a fluorescence during the early part of fluorescence induction at 0, initial fluorescence, and P, peak fluorescence, was higher during the day phase of the circadian cycle than during the night phase in continuous light (LL) conditions and was positively correlated with the rate of oxygen evolution. The circadian rhythm in fluorescence in LL persisted in the presence of 10 pcM 3-(3,4-dichlorophenyI)-l,l-dimethylurea (DCMU), which blocks electron flow from photosystem (PS) I1 in photosynthesis. The rhythmic changes in fluorescence intensity are consistent with a lower rate constant for radiationless transitions during the day phase than during the night phase of the circadian rhythmicity. The circadian changes in the intensity of fluorescence were abolished at 77K, which may indicate the importance of structural changes in membranes in circadian oscillations.