On the Mechanism of Action of Growth Regulators

Abstract

The relative activities of 117 chemical compounds, including derivatives of indoleacetic acid, phenoxyacetic acid, phenylacetic acid and benzoic acid, were detd. on the basis of the molar concn. required to induce a minimal response in elongation compared with the molar concn. of indoleacetic acid inducing the same elongation (0.15 mm.). With such a measurement of relative activity, only 4-chloro-3-indoleacetic acid is more active than indoleacetic acid (IAA). The same degree of activity as IAA is shown only by alpha-naphthaleneacetonitrile. The relative activity of naphthaleneacetic acid and 2,3,5-triiodobenzoic acid is half that of IAA, and 2,4-dichlorophenoxyacetic acid and 2,4,5-trichlorophenoxyacetic acid have one fourth the activity of IAA. Recent hypotheses concerning the correlation of chemical structure with physiological activity of plant growth regulators were examined as explanations for the activity or lack of activity of di-ortho alkyl substituted analogues and the low activity or inactivity of mono-alkyl substituted analogues. It is concluded that the 2-point ortho reaction mechanism affords the best working hypothesis, although a few compounds at present do not seem to fit the hypothesis.