Vernolepin: a new, reversible plant growth inhibitor.

Vernolepin (5 to 50 micrograms per milliliter), a novel sesquiterpenoid dilactone obtained from Vernonia hymenolepis, inhibits extension growth (from 20 to 80 percent) of wheat coleoptile sections. Inhibited tissues appear normal and their respiration is unaffected. If the inhibited sections are washed and subsequently treated with indole-3-acetic acid, the tissues respond to the auxin, but the degree of elongation is determined by the length of prior treatment with vernolepin. Administered simultaneously, increasing concentrations of auxin will significantly reduce the inhibitory effect of vernolepin, but there is no evidence for a competitive interaction between the two substances.