A plant growth-inhibiting substance was isolated from de-etiolated sunflower (Helianthus annuus) seedlings. Its structure was identified as 8- epixanthatin from IR, 1H NMR and EI mass spectra. 8-Epixanthatin was detected in sunflower and burweed shoots, but not in lettuce, radish, oat and corn. 8-Epixanthatin inhibited auxin-induced growth of sunflower hypocotyl and oat coleoptile sections at concentrations higher than 100 μM and 30 μM, respectively, and the elongation of cress roots at concentrations higher than 30 μM. However, it did not affect gibberellin-induced growth of lettuce hypocotyls and second leaf sheaths of rice at the various concentrations used. The light-grown seedlings were observed to contain a higher level of 8- epixanthatin than the dark control, and the difference in its level was noticeable before the growth inhibition by light appeared. These results suggest that 8-epixanthatin with its auxin-inhibiting activity might contribute to the light-induced growth inhibition of sunflower seedlings.
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