Gibberellins are not essential for photoperiodic flower induction of Pharbitis nil

The influence on photoperiodic flowering of (2-chloroethyl)trimethylammonium chloride (CCC), an inhibitor of gibberellin (GA) biosynthesis, was studied in the short-day plant Pharbitis nil cv. Violet. The cotyledons contained high levels of endogenous bioactive gibberellins, whereas in the plumules and first leaves the levels were low or undetectable. The first leaf responded to a single dark treatment by inducing flowering when it was 10 mm or wider. Similar seedlings, but without cotyledons, were used as the assay plants to study the effect of CCC on photoperiodic flowering. Treatment with CCC had no effect on flowering of seedlings without cotyledons, although stem elongation was inhibited. By contrast, CCC inhibited flowering of the intact seedlings with cotyledons. Gibberellic acid applied to the shoot apex or to the first leaf promoted flowering in the CCC-treated seedlings without cotyledons. The results indicate that gibberellins are not essential for the flower induction process in leaves, but that they promote flower initiation and/or later processes in the shoot apices.