STUDIES ON PLANT GROWTH‐REGULATING SUBSTANCES

Abstract

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 33‐ 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:s‐ 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:s‐ derivatives. Although the 2:4:6‐trichloro‐ and 2:4:6‐tribromo‐phenoxyacetic acids are almost inactive, a: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.