564 Cams, Addicott, Baker, and Wilson 



Although our knowledge of the role of gibberellic acid in abscis- 

 sion is still meager, it is consistent with the concept emerging from 

 other fields of research: that gibberellic acid functions through inter- 

 action with auxin (4). The evidence on auxin and abscission accum- 

 ulated since 1935 indicates that auxin is the principal endogenous 

 regulator of abscission (2). Now it is apparent that gibberellic acid 

 sometimes may be similarly involved in the regulation of abscission 

 (although the frequency with which it is a critical or deciding factor 

 under natural conditions is still obscure). Considered together, all 

 the evidence now available suggests a new hypothesis: that three hor- 

 mones — auxin, gibberellic acid, and the abscission-accelerating hor- 

 mone — interact in a common mechanism which regulates the process 

 of abscission. 



Since knowledge of the physiology of endogenous gibberellic 

 acid, as well as of gibberellic acid's interactions with auxin, is still 

 very fragmentary, speculation on the nature of their interaction in 

 abscission is not yet justified. Thus the recent suggestion that gibber- 

 ellic acid and auxin counteract each other in the regulation of young 

 fruit abscission — gibberellic acid preventing and auxin promoting 

 the abscission (10) — appears premature; and further, it is imsatisfac- 

 tory in failing to account for the numerous investigations which show 

 auxin to be involved in the prevention of young fruit abscission (1, 2). 



Further experiments are being directed to the definition of the 

 role of gibberellic acid in abscission and to the understanding of its 

 physiological and biochemical interactions with auxin and the abscis- 

 sion-accelerating hormone. 



SUMMARY 



Gibberellic acid accelerated abscission in excised cotyledonary 

 nodes of cotton when applied in relatively high concentrations either 

 proximal or distal to the abscission zone. Gibberellic acid retarded 

 abscission when applied in relatively low concentrations proximal to 

 the abscission zone. 



In these experiments gibberellic acid was probably directly in- 

 fluencing the process of abscission. In some experiments, reported by 

 others, gibberellic acid effects were probably indirect; e.g., increased 

 fruit-set following gibberellic acid application appears to be an in- 

 direct effect on abscission resulting from stimidation of fruit develop- 

 ment. Failure to obtain abscission responses from gibberellic acid 

 applications appears due in some cases to limitations of the experi- 

 mental materials. The hypothesis is advanced that the three hor- 

 mones—auxin, gibberellic acid, and the abscission-accelerating hor- 

 mone — interact in a common mechanism which regulates the process 

 of abscission. 



