H. R. CARNSi 

 F. T. ADDICOTT- 

 K. C. BAKER 

 R. K. WILSON 



University of California, Los Angeles 



Acceleration and Retardation of Abscission 



by Gibber ellic Acid 



Although there is a voluminous literature on the growth-promot- 

 ing effects of gibberellic acid, there are very few publications on its 

 abscission effects. The first two investigations found no effect of gib- 

 berellic acid on excised leaf abscission zones of Coleus (5) and none 

 with corollas of Papaver and Origanum (8). However, gibberellic 

 acid when applied to flowers or young fruit increased fruit-set in to- 

 matoes (9, 12), and increased fruit-set in cotton. Gibberellic acid 

 sprayed on branches of deciduous trees in late summer had little or 

 no effect on some species, but retarded leaf abscission in other species 

 and accelerated branchlet abscission in Taxodium (6). 



In view of the now considerable knowledge of the effects of auxins 

 on abscission (1) and the recent discovery of an abscission accelerating 

 hormone (7), and in view of the differences among the results of the 

 abscission experiments with gibberellic acid, further investigation 

 of the role of gibberellic acid in abscission and of its interrelations 

 with the above hormones appears well justified. This paper reports 

 an investigation in which both accelerated and retarded abscission 

 resulted from applications of gibberellic acid to cotton explants (ex- 

 cised abscission zones) and discusses the seeming contradictions in 

 the literature. 



MATERIALS AND METHODS 



Seedlings of cotton (Gossyphim hirsutum, 'Acala 4-42' and 

 'M8948') were grown in the greenhouse or under fluorescent lamps. 

 No differences in response were noted between the two cultivars. The 

 planting medium was Sponge Rok, a white volcanic ash. The seedlings 

 were watered with tap water fortified with ferric salts to prevent chlo- 



* Subsequently: USDA, Beltsville, Maryland. 



= Subsequently: Department of Agronomy, University of California, Davis, 

 California. 



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