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Plant Physiology 64:99-103 (1979)
© 1979 American Society of Plant Biologists

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Articles

Regulation of the Photosynthesis Rhythm in Euglena gracilis

II. Involvement of Electron Flow through Both Photosystems 1

Thomas A. Lonergan2 and Malcolm L. Sargent

a Department of Genetics and Development, University of Illinois, Urbana, Illinois 61801

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 O2 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 (H2O 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 O2 evolution. The individual activities of photosystem I (reduced from 2,6-dichlorophenol-indophenol to methyl viologen) and photosystem II (H2O 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.

2 Present address: Department of Biological Sciences, University of New Orleans, New Orleans, Louisiana 70122.

1 This work was supported by Grant GM 16541 from the National Institute of General Medical Sciences to M. L. S. and by a National Institutes of Health Traineeship to T. A. L.






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ASPB Publications PLANT PHYSIOLOGY® THE PLANT CELL
Copyright © 1979 by the American Society of Plant Biologists