Diurnal variation in situ of photosynthetic capacity in ulva is caused by a dark reaction.

Ulva lactuca (sea lettuce) undergoes large diurnal oscillations of light-saturated photosynthetic O(2) evolution in situ. Freshly collected samples from Great Harbor, Woods Hole, Massachusetts, had a maximum white light-saturated rate at noon that was 2.5-fold higher than the rate of matched samples collected at midnight. When kept under constant low level illumination, the cycle persisted for at least 36 hours, and after 2 weeks damped out to a constant level that was halfway between the minimum and maximum rates. The cyclic oscillations were apparent whether expressed on a weight or chlorophyll content basis, occurred in both lightly and heavily pigmented samples, and were not attributable to changes in chloroplast shading due to variations in chloroplast orientation within the frond cells. There were no cyclic variations in the initial slopes of the light saturation curves, in photosynthetic unit size, or in relative quantum efficiency. Measurement of the "fast" turnover time of photosynthesis by the delayed dual flash technique revealed no diurnal variations of this parameter. These results indicate that the cyclic variations in photosynthetic activity are modulated by a dark reaction at a step occurring after reduction of plastoquinone by electrons from photosystem II.