Auxin Activity of Ring-Alkylated Derivatives of Indole-3-Acetic Acid

The growth-promoting activities of ring-alkylated (alkyl = 2-, 4-, 5-, 6-, or 7-methyl ; 4- or 5-ethyl ; 5-propyl or 5-butyl) derivatives of indole-3-acetic acid (IAA) in the Avena coleoptile straight-growth assay were determined and correlated with structural parameters. Auxin activity was expressed as the net elongation of coleoptile sections (in per cent of the response in control experiments with non-substituted IAA) at the optimal concentrations (about 4 -mol/l) of the compounds examined. The elongations observed were dependent on the structures of the substituents and on their position at the indole nucleus. A methyl group in any but the 2-position increased biological activity up to 150% (5-methyl IAA). The elongation response decreased notably as the length of the alkyl chain increased. Thus, for example, 5-ethyl, 5-propyl, and 5-butyl IAA had 74%, 55%, and 46% of the growth-promoting activity of unsubstituted IAA.