Photoperiodic Time Measurement and Circadian Leaf Movement in Pharbitis nil Controlled by the same Clock?

Abstract

Summary Secondary leaves of 4 week old Pharbitis nil (L.) Choisy strain «Violet» plants show a circadian leaf movement rhythm, which can be entrained by light-dark cycles of 24 hours duration if the photofraction is longer than 4 hours and does not exceed 21.5 hours. The leaf movement rhythm undergoes phase shifts, if the plants are transferred to darkness at different times in the circadian cycle. Amount and direction of these phase shifts are determined by the time in the circadian cycle at which darkness begins. The amount of flower induction by a single 12 hour dark period depends, likewise, on the time in the circadian cycle at which it begins. Furthermore, the entraining and phase shifting effect of light on both - the leaf movement rhythm as well as the rhythm of the photoperiodic responsiveness of flower induction towards light breaks - have been studied and comparisons between the two are made. The phase angle difference between both rhythms is not constant, neither under entraining light conditions nor after phase shifting with a 6 hour red light break. This result possibly indicates the involvement of more than one circadian oscillator.