Studies on plant growth-regulating substances: XXXIII. The influence of ring substituents on the plant growth-regulating activity of phenylacetic acid

A comprehensive range of phenylacetic acids substituted with nitro, halogen, methyl, amino, hydroxyl and N-acetylamino groups have been synthesized and their growth-regulating activities assessed in the wheat cylinder, pea curvature and pea segment tests. The influence of substituents on molecular shape is shown to be more important in determining activity than their effects on electron distribution. Studies with 2,6-disubstituted phenylacetic acids have indicated that the most active compounds can attain a certain spatial configuration in which one surface of the molecule, including the plane of the ring system, is flat and the carboxyl group is above with its axis of rotation perpendicular to this surface. Positional requirements for growth-regulating activity in phenylacetic acids are shown to be less important than in the phenoxyacetic and benzoic acids.