Flowering of Pharbitis nil is controlled by an endogenous semidian rhythm

Abstract Flowering of Pharbitis nil after an inductive dark period is greatly influenced by far-red (FR) irradiation during the preceding light period. The response to FR is rhythmic in otherwise constant conditions, and the period of the oscillation is approximately 12 h (i.e. semidian). The rhythm also appears to operate under daily light-dark cycles. The expression of this novel rhythm depends on the time from the beginning of FR pretreatment to the onset of the inductive dark period. The cotyledons are the site of response to both the pretreatment and inductive darkness, and both these conditions must be perceived by the same cotyledon.