FURTHER EVIDENCE FOR A CHEMICAL REACTION BETWEEN PLANT GROWTH-REGULATORS AND A PLANT SUBSTRATE

Abstract

The substitution of atoms or groups in both ortho positions of phenoxyacetic, phenylacetic, phenylbutyric and indoleacetic acids causes the compounds to be ineffective in promoting the elongation of sections of Avena coleoptiles (9,15). The obvious interpretation is that these atoms or groups in the ortho position block a chemical reaction taking place at that point. Considerable evidence exists that electronegative atoms or groups attached to the benzene ring in phenoxyacetic and phenylacetic acids enhance the activity of these compounds, and this feature suggests that the substrate reacting at the ortho position is nucleophilic in nature (15). Bentley (2), however, has reported that 2,3,6-trichlorobenzoic acid is very effective in promoting the elongation of coleoptile sections, although both ortho positions are occupied by chlorine atoms. Investigations of the activities of a number of benzoic acid derivatives have shown that the active compounds have one or both ortho positions occupied by electronegative atoms or groups (9, 26), and it has been suggested that with these compounds the reaction takes place by a displacement of the electronegative atom or group by the nucleophilic substrate (9). One of the active derivatives of benzoic acid is 2,6-dichlorobenzoic acid. If such a substance does react at an ortho position by nucleophilic displacement of a chloride ion, then chemical analysis of the tissue and the solution in which growth has taken place should detect an increase in the amount of chloride ion. Such analyses wrere made and an increase in Cl~ was detected.