The Physio1ogical Activity of 2 : 6-Substituted Phenoxyacetic Acids

Abstract

Anumber of 2 : 6-substituted phenoxyacetic acids in which the side chain had been substituted by ailcyl groups to form α-propionic, α-n-butyric and α-n-valeric acids were investigated to assess their ability to act as growth-regulators. Besides employing standard tedmiques of bioassay, further experiments were under taken to determine the effects of these compounds on the vegetative growth and development of intact plants of Helianthus annuus. It has been established that all the compounds tested induced curvature in the Went pea curvature test and that without exception the activity of the parent phenoxyacetic acid (2:6-dichloro-, 2:4:6-trichloro-,2:6-dimethyl-, and 2:4-dichloro-6-methyl-) was increased by side chain substitution. In the Avena straight growth assay, the a 2:6-dichloro- and 2:6-dimethyl- phenoxypropionic and butyric acids brought about statistically significant increases in length of the coleoptile sections, when measurements were made at the end of 24 hours. Their activity was of the same order as that exhibited by a 2:4-dichlorophenoxyacetic acid. Investigations were carried out on the uptake of water by sections of pea internode and the extension growth of Avena coleoptile aectiona when both the concentration and the length of treatment were varied. For some compounds it was found that over a certain range of concentration water uptake and extension growth were accelerated in the initial 6–8 hours. Subsequent to this, inhibition occurred so that no significant increases above control values were apparent at the end of 24 hours, and in some cases an actual loss of water or shrinkage in length took place. It was thus possible to demonstrate that 2:6-dimethylphenoxyacetic acid acts as a growth regulator in pea extension growth and that 2:6-dichiorophenoxyacetic acid is active in the Avena test. The changes in the leaf area of individual pairs of leaves, together with the lengths of the internodes and shoot weight were followed after application of measured amounts of each compound to the first pair of leaves of H. annuus. A number of the 2:6-compounds were capable of modifying growth in ways similar to 2:4-dichlorophenoxyacetlc acid. However, much higher concentration had to be applied in order to produce effects comparable to those caused by the 2:4-dichloro- compound. Measurements of the percentage of applied compound which penetrated into the leaf showed that there were no marked differences in this respect between the different phenoxy acids. On the other hand, experiments in which the treated leaves were left on the plant for varying periods of time led to the conclusion that the a : 6-substituted compounds are less readily translocated than 2:4-dichlorophenoxyacetic acid.