THE PHOTOSYNTHETIC RHYTHM OF ACETABULARIA CRENULATA. I. CONTINUOUS MEASUREMENTS OF OXYGEN EXCHANGE IN ALTERNATING LIGHT-DARK REGIMES AND IN CONSTANT LIGHT OF DIFFERENT INTENSITIES

¹ Tyco Laboratories, Inc., Waltham, Massachusetts

1. Oxygen exchange by Acetabularia crenulata in various light regimes was monitored continuously for as long as 10 days by means of a rate-measuring graphite oxygen electrode.

2. The time course for photosynthesis in 8-hour light periods is bimodal, and consists of an initial burst of oxygen production followed by a depression and a subsequent slow rise to a (usually) higher maximum in the latter part of the period. The two maxima show differential sensitivity to the deleterious effects of long-term exposure to the environment of the oxygen electrode.

3. The light curve for photosynthesis departs from linearity at the comparatively low intensity of 200 ft.-c. but does not reach saturation below 1300 ft.-c., indicating an unusually loose coupling of the light and dark reactions.

4. Both maxima of the diurnal time course of oxygen evolution as well as the free-running rhythm were expressed at a moderate (250 ft.-c.) and at a high (1000 ft.-c.) light intensity. A free-running rhythm was also found at 45 ft.-c. The possibility that only the maximum capacity of photosynthesis fluctuates in the expression of the rhythm was ruled out in an experiment that monitored both capacity and activity alternately in the same cycles.

5. The natural period of the rhythm at 28° C. is approximately 25 hours. The phase can be reset by a single 8-hour photoperiod.

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