The plant growth-regulating activity of dextro and laevo α -(2-naphthoxy)propionic acid

Abstract

$(\pm)$-$\alpha $-$(2$-$\text{naphthoxy)}$propionic acid, an active plant growth-regulating substance, has been resolved into its dextro and laevo rotatory enantiomorphs by fractional crystallization of its (+)-cinchonine salts. The pure stereoisomers have been examined for their effect in inducing a range of growth responses, depending upon cell division, cell enlargement, or both. For these purposes, the Went pea test and the Avena cylinder test have been used for critical assessment in the laboratory. In addition, the capacity to induce an epinastic response in the tomato plant, to induce rooting in cuttings and to stimulate parthenocarpic development of the unfertilized ovaries of the tomato and pear have been examined. An effect on the flowering laterals of the hop is also described. In all cases the (+)-isomer was found to possess high activity, whereas very little, if any, was shown by the (-)-isomer. Although the relative activities of dextro and laevo rotatory forms of an optically active plant growth substance have been previously investigated, the present results demonstrate for the first time that one optically active form of a growth substance can induce pronounced and varied growth responses when applied to plant tissue or intact plants, whereas its enantiomorph can possess negligible activity. The results are discussed in the light of a theory which presupposes that in the primary stages of growth-regulating activity, three essential groupings of the growth substance molecule become associated with specific receptor groups in a surface boundary at the site of action. On this basis only one isomer can possess appreciable activity and the other will be inactive. These considerations are not inconsistent with results obtained in studies on the growth-regulating activity of aryloxy-acetic and -isobutyric acids.