Helminthosporol, helminthosporic acid and dihydrohelminthosporic acid stimulated hypocotyl elongation of light-grown cucumber seedlings. The relative activities of the three compounds were in the order H2-H-acid>H-acid>H-ol. The higher the dosage, the longer the promotion of the hypocotyl elongation lasted. The response of the hypocotyl to H-acid depended on the presence of cotyledons. As the amount of the cotyledon attached to the hypocotyl was reduced, the response decreased.When IAA-oxidase was estimated as IAA metabolized per dry weight of the hypocotyl, there was an inverse relationship between IAAoxidase activity and H-ol-induced elongation of the hypocotyl. However, there was no difference between control and H-ol treated materials when IAA metabolized per plant was compared. It is unlikely that the mechanism of H-ol action is closely related to IAA-oxidase activity of the plant.
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