The response of growth and circadian rhythm to cycle length and photoperiod

Abstract

The growth of bean plants (Phaseolus vulgaris L., cv Red Kidney) is inhibited by cycle lengths of 36 and 48 hours. Maximal inhibition occurs when the length of the light period is equal to or shorter than 3/6 of the cycle length. The inhibition does not occur when the photofraction is 5/6 or longer. The rhythmic leaf movement in beans can be entrained to a 30-hour cycle with a photofraction of 3/6 or less. No entrainment occurs to 36-or 48-hour cycles, but such cycles with photofraction of 3/6 or less cause an irregular course of the rhythm. When the photofraction is 5/6 or greater, the leaf movement proceeds as in continuous light, independent of cycle length. In continuous light the rhythm persists for at least 12 days. The parallel response of growth and circadian rhythm to cycle length and photoperiod suggests that a circadian oscillation is involved in the growth process. It further indicates that the response of these phenomena to cycle length and photoperiod involves the same basic timing mechanism.