Beschreibung:
<jats:title>Abstract</jats:title><jats:p>Senescent petioles of <jats:italic>Coleus rehneltianus</jats:italic> Berger. <jats:italic>Phaseolus vulgaris</jats:italic> L. cv. Saxa. <jats:italic>Acer pseudoplatanus</jats:italic> L., and senescent fruit stalks of <jats:italic>Malus domestica</jats:italic> Borkh. cv. Golden Delicious contain at least three abscission accelerating substances, which were isolated by extraction with methanol or with water and by diffusion into agar. They were purified by thin‐layer chromatography and bioassayed in a special abscission test using <jats:italic>Coleus</jats:italic> explants. Two of these abscission accelerators could be conclusively identified by thin‐layer chromatography and by gas chromatography as abscisic acid and xanthoxin. The third substance, which has acidic properties and is less polar than abscisic acid, could not be identified. The concentration and the absolute amount of abscisic acid <jats:italic>in Coleus</jats:italic> petioles were found to decrease during their development, young petioles having the highest concentration. No evidence was found that the three abscission accelerators or synthetic abscisic acid and xanthoxin affect the production of ethylene in <jats:italic>Caleus</jats:italic> explatns. The results obtained do not support the hypothesis that senescent petioles contain a specific “senescence factor”, which stimulates abscission via ethylene production.</jats:p>