CIRCADIAN RHYTHM OF Robinia pseudoacacia LEAFLET MOVEMENTS: ROLE OF CALCIUM AND PHYTOCHROME

Abstract— The effect of external calcium level, calcium ionophore A23187 and red light on the circadian rhythm of Robinia pseudoacacia leaflet movements has been studied. Fifteen minute red light pulses shifted the phase of leaflet rhythmic movement with a phase-response curve type 0. Maximum advances and delays (about 10 h and 8 h, respectively) were obtained between circadian time (CT) 10 and CT 12 at the end of a subjective day. An almost null effect was obtained at the end of a subjective night. Phytochrome is the photoreceptor involved in phase shifting since this effect of red light is reversed by 5 min of far red light. Two hour pulses of external calcium, applied as CaCl₂ (10 m M ), and 2 h pulses of calcium ionophore A23187 (10–50 μM) also shifted the phase of leaflet circadian movement and caused the same type of phase-response curve, with maximum advances and delays at the same time as those produced by red light. Two hour pulses of an external calcium chelator, EGTA (5 m M ), and a calcium channel blocker, LaCl₃ (10–50 m M ), damped the circadian rhythm or did not change the phase when they were applied at lower concentration. These results indicate that phytochrome could control the circadian oscillator, which drives Robinia leaflet movements by increasing the intracellular calcium concentration.