Growth Regulating Effects of Phenoxy Compounds 229 



auxinic [e.g., (+)3,5-Cl2POP] and an already strong auxin activity 

 can be considerably strengthened [e.g., (+)2,4,5-Cl3POP]. This points 

 to an active role of the d methyl group in the attachment of the 

 regulator to the receptor site and in its function (2). Such a situa- 

 tion is not universal, however, as the d forms of the a-naphthylpro- 

 pionic acids have weaker auxin components in their activity than the 

 corresponding acetic acids (9). 



For all phenoxyacetic acids possessing clear auxin character the 

 introduction of an a-methyl group in the l position results in a pro- 

 nounced change in the antiauxin direction. In other cases the effect is 

 weaker and less clear. 



Unfortunately, only a few of the a-isobutyric acid derivatives 

 have been included in the present study. POiB shows a rather pure 

 antiauxin activity, while slight traces of auxin activity are possibly 

 present in 4-ClPOiB and 3,5-CloPOiB as judged from the type of con- 

 centration-response curves in the oat cylinder test. This may be seen 

 in connection with the occurrence of very slight activity of 4-ClPOiB 

 also in the pea curvature test, and with the rather conspicuous activ- 

 ity of 2,4,5-Cl3POiB both in the wheat cylinder and the pea curva- 

 ture tests (12). This latter case is especially interesting, as the corre- 

 sponding (— )-propionic acid shows a rather pure antiauxin charac- 

 ter. Both a-methyl groups may perhaps influence the binding and 

 position of the side chain, and in 2,4,5-Cl3POiB the influence of the 

 group in the d position would predominate. The presence of an auxin 

 component in the activity of this substance is analogous with its pres- 

 ence in a-(2-naphthoxy)isobutyric acid (10, and unpublished data). 

 Its presence in a-(indole-3)isobutyric acid, on the other hand, repre- 

 sents another situation, as in this case both corresponding a-propionic 

 acids show strong auxin activity (8). 



SUMMARY 



It was found that a rather large number of the studied phenoxy 

 compounds cannot simply be classified as auxins or antiauxins, but 

 show intermediate character. In discussing the relation between chem- 

 ical structure and physiological activity this must be appreciated, and 

 the existence of the intermediate regulators might also be able to 

 shed some light on the difficult question of the mechanism of auxin 

 action. 



LITERATURE CITED 



1. Aberg, B. On the effects of weak auxins and antiauxins upon root growth. 

 Physiol. Plant. 5: 305-319. 1952. 



2. . On optically active plant growth regulators. Lantbrukshogsk. Ann. 



Uppsala. 20: 241-295. 1953. 



