Action of Plant Growth Regulators. IV. Adsorption of Unsubstituted and 2,6-Dichloro-aromatic Acids to Oat Monolayers

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Abstract
The adsorption of chloro-aromatlc acids to monomolecular layers of oat squashes Is reported In earlier papers but it was not possible by the technique used, to measure unambiguously me adsorption of unsubstituted and 2,6-dichloroaromatic acids. This has now been achieved by a modification of the earlier method and involves assessments of competitive adsorption between the unknown acid and a standard acid, using measurements of surface potential. Benzole and phenoxyacetlc acids were not adsorbed but phenylacetlc acid was weakly adsorbed. The 2nd ring in naphthalene and naphthoxyacetlc acids greatly increased adsorption. Substitution of the 2 add 6 positions in the phenyl and phenoxyacetic acids resulted In low adsorption but 2,6-dlsubstltuted phenoxybutyrlc and benzoic acids were more highly adsorbed. The adsorption values from earlier work are combined and discussed in relation to the growth-regulating activity of the acids. It is concluded that there is no direct relation embracing all acids between adsorption and activity, notable exceptions being those substituted by chlorine in the 3-position of the aromatic ring. However, for a number of acids it is suggested that activity is limited not only by their ability to interact at enzyme sites but also by the amount of acid Immobilized by adsorption when moving to these sites. It is also concluded that the hydrophlltc/lipo-philic balance of a growth regulator sometimes used as a guide to Its activity, is an unreliable indication of interfacial behavior.