The Ortho Effect in Plant Growth-regulators.

Introduction In an earlier publication (2) a comparison was made of the growth regulating activites of various compounds as indicated by the elongation of sections of Avena coleoptiles. From this comparison it was seen that the substitution of various halogen atoms in the phenoxyacetic acid (POA) structure increased its effect on elongation except when both ortho positions were occupied. Also, substitution of methyl groups in the 2, 4, and 6 positions destroyed the activity of both the POA and the phenylacetic acid (PAA) structures. It was proposed that these compounds require a free or potentially free ortho position to effect cell elongation. This paper reports additional evidence for this ortho effect.