Effects of lanthanum on rhythmic and nyctinastic leaflet movements in Albizzia lophantha

The involvement of extracellular calcium in rhythmic and nyctinasticmovement oi Albizzia lophantha Benth. leaflets has been studiedby testing the effect of LaCl₃ and its interaction with thephytochrome control of these movements. A 2h pulse of LaCl₃(10–50 mM) promotes a loss of rhythmicity, leaving leafletsin an open position, and also overrides the phase shift causedby phytochrome. A 2 h pulse of LaCl₃ (1 mM) decreases the amplitudeof rhythmic oscillations but does not promote arhythmicity normodify the phase shift caused by red light. The red light pulseabolishes the damping effect of 1 mM La³⁺. LaCl₃ inhibits nyctinasticclosure and decreases the phytochrome control of nyctinasticclosure. A subsequent supply of CaCl₂ (10 to 100 mM) does notreverse La³⁺ (10 mM) inhibition of closure. Light-induced openingis independent of LaCl, but rhythmic opening in darkness showsdifferent responses to La³⁺ depending on the time at which La³⁺is applied. Data suggest that extracellular calcium is requiredfor the closure mechanism and for the expression of rhythmicmovement. It could also be involved in the phytochrome transductionpathway and/or in the linking steps between phytochrome andthe circadian clock. Key words: Albizzia lophantha, calcium, circadian rhythm, lanthanum, phytochrome