Inhibitors from Carob (Ceratonia siliqua L.): II. Effect on Growth Induced by Indoleacetic Acid or Gibberellins A(1), A(4), A(5), and A(7)

Two inhibitory fractions (B₁ and C) from extracts of immature fruit of carob were tested for their ability to inhibit the action of indoleacetic acid (IAA) in three bioassays. There was no reduction of IAA-induced reactions in the Avena curvature test, abscission of debladed coleus petioles, or growth of cucumber hypocotyls. The highest ratio of inhibitor to IAA was 10,000 times greater than the ratio necessary to inhibit by 50% the growth caused by an equivalent amount of gibberellin A₃ in pea seedlings. At the highest concentration used, fraction C alone caused curvature of Avena coleoptiles. The inhibitory fractions appeared to enhance the effect of IAA in the cucumber test.