ABSTRACT
Synchronous cell division in Euglena gracilis (strain Z) was obtained in 24-hour light cycles consisting of 10 hours of light and 14 hours of darkness; cell division was restricted to the dark period. Photosynthetic capacity was found to vary in a cyclic manner during the cell cycle, reaching a peak 2 hours before the onset of darkness. Light reactions were investigated during the cell cycle to determine what role they played in the control of the observed rhythmic changes in capacity. Light-saturation curves showed no major change in the light-limited region. No fluctuations were found in Hill reaction activity or photoreduction of methyl red during the cell cycle. These results imply that the reactions comprising photosystems I and II do not generate the capacity rhythm.Some of the photosynthetic dark reactions were also followed during the cell cycle in an attempt to determine their possible role in the control of the rhythm of photosynthetic capacity. The activity of ribulose-1, 5-diphosphate carboxylase showed no correlation with the rhythm. On the other hand, the activity of glyceraldehyde-3-phosphate dehydrogenase was found to parallel the change in photosynthetic rate under various growth conditions. The rhythm in photosynthetic capacity could be effectively divorced from the cell cycle itself by placing cultures in high frequency light cycles (LD: 2,4) or in stationary growth-phase conditions. If synchronously dividing cultures previously grown in LD: 10, 14 were released into continuous dim illumination and constant temperature, the rhythm of capacity persisted for only one full cycle.
