THE ACTION OF TRI-IODOBENZOIC ACID ON GROWTH

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Abstract
Tri-iodobenzoicacid (TIBA) increases the growth-promoting action of indoleacetic acid (IA) and other auxins. The effect is large in the curvature of slit pea stems and is exerted by concs. of TIBA from 0.2 to 125 rag. per liter. The TIBA itself does not cause curvature, provided the stems are depleted of auxin, but it can increase the curvature produced by indoleacetic acid by a factor of 3 times. The effect is smaller (25-75%) in the straight growth of Avena coleoptiles and in the standard Avena curvature test. In these two reactions the mol ratios TIBA/IA required for increased growth-promoting action are from 0.12 to 1.5. At higher mol ratios, or in high concs. alone, TIBA inhibits growth. It is suggested that the effects are due to competition between TIBA and the auxin for a common substrate.

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