Phytochrome-controlled Nyctinasty in Albizzia julibrissin: III. Interactions between an Endogenous Rhythm and Phytochrome in Control of Potassium Flux and Leaflet Movement

Abstract

Prolonged irradiation during appropriate parts of the diurnal cycle promotes the opening of Albizzia julibrissin leaflets. Leaflets also open without illumination, but such opening starts later and is slower and less complete. Opening in the dark is accompanied by lower potassium efflux from dorsal pulvinule motor cells but equal or greater potassium movement into ventral motor cells than occurs during opening in the light. Far red-absorbing phytochrome inhibits opening in the dark, indicating that its action is similar during endogenously controlled opening and nyctinastic closure; i.e., a high far redabsorbing phytochrome level is associated with low potassium content in ventral motor cells, high potassium content in dorsal motor cells, and a small angle between leaflets.When open leaflets are darkened, there is an immediate and large potassium flux into dorsal motor cells. This is initially independent of red and far red preirradiation, but prior red light appears to promote continued potassium movement into dorsal cells during the latter part of a 90-minute dark period. The situation in ventral motor cells is different; here the effect of prior red or far red irradiation on potassium efflux is evident after 10 minutes of darkness. Phytochrome controls the direction of potassium movement in ventral motor cells during the early part of the dark period (to 25 minutes); potassium moves out of ventral motor cells if leaflets are preirradiated with red light and into these same cells if leaflets are preirradiated with far red light. Kinetic data are consistent with the suggestion that potassium leaving ventral cells enters dorsal cells. However, there must be an additional source of potassium entering dorsal cells since this potassium movement precedes potassium efflux from ventral cells.Pulvinules excised from the lamina or rachilla open and close in response to light and darkness and also move during extended periods of constant intensity light or uninterrupted darkness. This shows that the photoreceptor controlling opening and the oscillator controlling endogenously rhythmic leaflet movement are localized in the pulvinule. In addition, all the potassium that enters expanding cells during leaflet movement and the energy for potassium transport must be available from within the pulvinule.If leaflets are darkened late in the photoperiod, they close more rapidly and show lower inhibition by far red preirradiation or anaerobic conditions. A rhythmic increase in potassium efflux from ventral motor cells appears to be the basis for rhythmic promotion of nyctinastic closure. We suggest this is due to a rhythmic increase in the leakiness of ventral motor cells.