Endogenous Circadian Rhythms and the Response of Lemna perpusilla to Skeleton Photoperiods

Abstract

Representing the photoperiodic schedule 11 hours light-13 hours darkness as 11(13), the schedule 1/3(10-1/2)1/4(13) may be regarded as its "skeleton." The effects of certain skeleton photoperiodic schedules on the flowering of Lemna perpusilla 6746 have been studied. When cultures are taken from continuous light and exposed to 4 to 7 repetitions of the schedule 1/4(10-1/2)1/4(13), or similar schedules, the effects depend strongly on which dark period, the shorter or the longer, is presented first. More generally, the relationship between the length of an initial dark period and its effect preceding a given skeleton schedule indicates the presence of an endogenous circadian rhythm which can persist for at least 48 hours in total darkness. Thus these observations agree with the general hypothesis of Bunning and others that such rhythms are fundamental to photoperiodic timing. No evidence either favoring or opposing a significant role of phytochrome in these effects of skeleton photoperiodic schedules is yet at hand.