Studies on plant growth‐regulating substances XXV. The plant growth‐regulating activity of cinnamic acids

SUMMARY Effects of ring substitution on the plant growth‐regulating activities of trans‐ and cis‐cinnamic acids have been investigated in the wheat cylinder, pea segment and pea curvature tests. Most of the cis‐ acids were shown to be active. Substitution of fluorine, chlorine or bromine into the ring of cis‐cinnamic acid in most cases increased the activity. The results are discussed in relation to mode of action and chemical structure/biological activity relationships: 4‐chlorobenzoic acid is shown to act as a competitive antagonist towards 4‐chloro‐cis‐cinnamic acid.