STUDIES ON PLANT GROWTH-REGULATING SUBSTANCES X. THE ACTIVITY OF SOME 2: 6-AND 3: 5-SUBSTITUTED PHENOXYALKANECARBOXYLIC ACIDS

A study has been made of the growth-regulating activity of a number of phenoxy acids possessing halogen or methyl substituents in the 2:6- or 3:5- positions of the nucleus. The pea curvature, Avena cylinder elongation and tomato-leaf epinasty tests have been used for assessing biological activity. The results show that in general 3:5- is more closely associated with inactivity than 2: 6- substitution. The introduction of a further halogen atom into the 4- position does not enhance activity in the 2: 6- compounds but does so in the 3:5- derivatives. Although the 2:4:6-trichloro- and 2:4:6-tribromo-phenoxyacetic acids are almost inactive, 2:4-dichloro-6-fluoro- and 2:4-dibromo-6-fluoro- phenoxyacetic acids are very highly active. When a methyl group is substituted into the side-chain of certain phenoxyacetic acids possessing 2: 6- substituents, growth-promoting activity is increased. Some of these findings conflict with recent theories on mode of action of phenoxy acids which are discussed in relation to the present work.