Abstract
The auxin transport inhibitors 2, 3, 5-triiodobenzoic acid (TIBA) and naphthylphthalamic acid (NPA) inhibited adventitious root formation (ARF) induced by indol-3-butyric acid (IBA) on cuttings from etiolated mung-bean seedlings floated on solutions of the growth regulators. The concentrations of TIBA and NPA required for a 25 per cent reduction in ARF with 10 μM IBA were estimated by linear interpolation to be 11.3 μm and 0.42 μM respectively. NPA is a particularly potent inhibitor of IBA-induced ARF. The inhibitory effect of either compound was reversible by higher concentrations of IBA. NPA had no effect when applied after the auxin treatment. The inhibitory effects of TIBA or NPA could not be explained by effects on the uptake or metabolism of [2-14C]IAA. Considering this and other evidence, it is suggested that NPA and possibly TIBA are acting as specific antagonists of auxin in the induction of ARF.
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