Abscisic acid both promotes and inhibits photoperiodic flowering of Pharbitis nil

Abscisic acid (ABA) has been reported to have diverse effects on photoperiodic flowering. Activity of a natural ABA, (+)‐( S )‐abscisic acid (S‐ABA), was recently suggested to be somewhat different from that of racemic ABA, which has been used in previous work. Use of S‐ABA might enable clarification of the role of ABA in flowering. S‐ABA inhibited flowering of the short‐day plant Pharbitis nil (cv. Violet) when given before or 4 h after the start of a 14‐h inductive dark period, and promoted flowering when given 12 h after the start of the dark period or later. The flower‐promoting effect was observed when ABA was applied to the shoot apex. These results indicate that ABA has a dual effect on photoperiodic flowering of P. nil : it may inhibit the time‐measuring process as well as promote some processes that proceed after generation of the flowering stimulus.