Seselin, a natural coumarin derivative isolated from citrus roots, inhibited radicle growth in seedlings of cucumber (Cucumis sativa), lettuce (Lactuca sativum), radish (Raphanus sativus), and wheat (Triticum aestivum) grown in the dark. Coumarin similarly inhibited radicle growth of cucumber seedlings. Growth retardation of the cucumber radicles was accompanied by an increased activity of peroxidase and indole-3-acetic acid oxidase. Both compounds antagonized indole-3-acetic acid-induced growth of wheat coleoptiles, whereas coumarin was much less effective than seselin in antagonizing gibberellic acid-induced release of reducing sugars from barley endosperm. It is suggested that seselin plays an important role in the regulation of root growth, and that it is the indole-3-acetic acid oxidase cofactor previously detected in citrus roots.Seselin (I), a pyrano coumarin derivative, was recently isolated from citrus roots (38) and identified in other plants of Rutaceae (5). Coumarin (II) and its derivatives often occur in the plant kingdom; its various biological activities were reviewed by Mayer and Poljakoff-Mayber (27), and its functions as an inhibitor of germination and root growth have been established (20,21,27,28). Coumarin was reported to inhibit the growth of sunflower (18,19) and cucumber hypocotyl sections (22), but Neumann (31) claims that it enhances the growth of excised sections of sunflower hypocotyl.The growth-retarding characteristic of coumarin moved several authors to investigate its effect on IAA-oxidase activity in the retarded tissues, which showed either an increase (13,22) methoxy-7-hydroxy coumarin), a coumarin derivative, inhibited the activity of IAA-oxidase in tobacco (36), and both scopoletin and scopilin (7-glucoside of scopoletin) inhibited the initial IAA oxidation by horseradish peroxidase (35).An unidentified, dialyzable, probably phenolic, cofactor of IAA-oxidase was previously detected in citrus roots (7), and the recent identification of seselin (38) suggested that this compound might be the unidentified cofactor mentioned above. In the following, we present some physiological characteristics of seselin indicating that this compound might be functional in regulating root growth. MATERIALS AND METHODSThe desired concentrations of seselin and coumarin were dissolved in 2 ml of ether, and then applied to two layers of Whatman No. 3 filter paper discs in 9-cm Petri dishes, and evaporated to dryness. Neither seselin nor coumarin were applied to control samples, which were treated with 2 ml of ether only. Three milliliters of glass-distilled water were then added to wet the filter papers on which samples of 10 seeds were allowed to germinate in the dark at 25 C. The plant species involved were local cultivars of cucumber (Cucumis sativa L. cv. Bet. Alpha), lettuce (Lactuca sativum L. cv. Bon Gardinier), radish (Raphanus sativus L. cv. Red radish), wheat (Triticum aestivum L. cv. Mexican 852), citrus (Citrus aurantium L. cv. sour orange), and tomato (Lycopersicum esculentum L....

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