The processes inhibited and promoted by abscisic acid in photoperiodic flowering of Pharbitis nil

A dual role of abscisic acid (ABA) in photoperiodic flowering of Pharbitis nil, cv. Violet was studied. ABA given before an inductive dark treatment lowered the flowering response without influencing the critical night length and the most effective time of a night break. ABA given after a dark treatment accelerated the transport of flowering stimulus and enhanced flower initiation response at shoot apex. Endogenous ABA level in cotyledons was determined by gas chromatography-selected ion monitoring. There was no remarkable difference between ABA levels under short-day and long-day conditions. Fluridone, an inhibitor of ABA biosynthesis, given before and during an inductive dark treatment affected flowering response only slightly. The results indicate that exogenous ABA inhibits the early phase of the flowering without influencing the time-measuring process, and promotes both transport of the flowering stimulus and flower-initiation process, although endogenous ABA plays little role in the regulation of flowering.