Photoperiodic Entrainment Patterns in the CO(2) Output of Lemna perpusilla 6746 and of Several Other Lemnaceae

The CO(2) output of Lemna perpusilla 6746 in "skeleton photoperiods" consisting of alternating 10(1/2)-hour and 13-hour dark periods separated by (1/4)-hour illuminations was recorded under stable high and low nitrate conditions. The phase relationship finally attained between light schedule and output is the same regardless of which dark period is given first, but entrainment is more rapid (as is flowering) with an initial 13-hour dark period. In all respects other than bistability-the assumption of two different stable phase relationships depending on the initial dark period-both flowering and the course of CO(2) output conform to Pittendrigh's model derived from Drosophila eclosion rhythms, confirming the view that an endogenous circadian rhythm, or biological clock, underlies the photoperiodic control of flowering in this plant. Experiments with rigorous temperature control show that earlier results with long light exposures were in part due to temperature changes; in consequence, it is clear that entrainment patterns with high nitrate differ even more from those in low nitrate than was previously evident, and not simply by the addition of a "nitrate peak." Other Lemnaceae tested with a few simple light-dark schedules in both types of media show a variety of responses, with no obvious correlation to photoperiodic response type.