Regulation of the Photosynthesis Rhythm in Euglena gracilis II. Involvement of Electron Flow through Both Photosystems

Rhythmic changes in the light reactions of Euglena gracilis have been found which help to explain the basic reactions effected in the circadian rhythm of O 2 evolution. Diurnal changes in the slope of light intensity plots indicated that the maximal rate of photosynthesis changed throughout the circadian cycle. No evidence was obtained consistent with the premise that changes in chlorophyll content, as measured by total chlorophyll or chlorophyll a/b ratio, or photosynthetic unit size are responsible for this rhythim. The rate of light-induced electron flow through the entire electron chain (H 2 O to methyl viologen) was rhythmic both in whole cells and in isolated chloroplasts, and the highest rate of electron flow coincided with the highest rate of O 2 evolution. The individual activities of photosystem I (reduced from 2,6-dichlorophenol-indophenol to methyl viologen) and photosystem II (H 2 O to 2,6-dichlorophenol-indophenol) did not, however, change significantly with time of day, suggesting that the coordination of the two photosystems may be the site of circadian control. Evidence consistent with this concept was obtained from studies of low temperature emission from systems I and II following preillumination with system I or II light.