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Inhibitors from Carob (Ceratonia siliqua L.)

II. Effect on Growth Induced by Indoleacetic Acid or Gibberellins A₁, A₄, A₅, and A₇¹

^a Department of Biology, San Fernando Valley State College, Northridge, California 91324

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.

Concentrated whole extract and fractions B₁ and C were tested for reduction of growth caused by gibberellins A₁, A₄, A₅, A₇, and a neutral gibberellin-like substance from beans in the dwarf-5 maize bioassay. Each gibberellin was inhibited and required the same amount of inhibitor for a 50% reduction of the induced growth. The inhibiting effect could be completely overcome by increasing the amount of gibberellin while maintaining the same concentration of inhibitor. Fractions B₁ and C were also tested with gibberellins A₂ and A₄ in the cucumber hypocotyl test. Both inhibitory fractions reduced growth but were more effective against gibberellin A₃ than gibberellin A₄ in the assay. The ability to reduce gibberellin-induced growth and not reduce IAA-induced growth indicates that the inhibitors from carob have a greater specificity of action than that previously reported for any inhibitor.