Chemical structure and biological activity 



With phenylacetic acid, jfeara-chlorination also increases auxin activity 

 considerably, though less than o?//io-chlorination (Melnikov et al., 1953). 

 The reported decrease in activity from 2-chloro- to 2:4-dichloro-phenylacetic 

 acid makes a more detailed comparison between these substances highly 

 desirable. 



SOME a-PHENOXYPROPIONIC ACIDS 



The racemic forms of a-phenoxypropionic acid (POP) and its para-ch\oro- 

 derivative (4-ClPOP) have been compared in several tests by Fawcett et al. 

 (1953), and their effects upon wheat roots have been studied by Burstrom 

 (1951b) and Hansen (1954). While decidedly more active in the pea test, 

 for example, racemic 4-ClPOP is only about twice as active as POP in 

 inhibiting the growth of wheat roots. The cell length of wheat roots growing 

 in a solution of 4-ClPOP may even be slightly greater than those in roots 

 srowinsf in an equallv concentrated solution of POP. 



After resolution of both substances into their optically active forms 

 (Fredga and Matell, 1952; Matell, 1954), the flax root test indicates a 

 five-fold increase in the activity of D-( + )-POP upon/?ara-chloro-subsdtution, 

 while the physiologically inactive l-( — )-POP is changed into a weak auxin 

 antagonist. Detailed data from these experiments will be published in a 

 later communication. The increased activity after /jara-chlorination both in 

 the auxinic D-series and the anti-auxinic L-series might well be caused by a 

 corresponding increase in the affinity of the chlorinated molecules to the 

 growth centres. 



For the racemic a- (2-methylphenoxy) propionic acid, Synerholm and 

 Zimmerman (1945) found an approximately tenfold increase in cell- 

 elongation activity (tomato shoot) upon/jara-chlorination. With a-(3-methyl- 

 phenoxy) propionic acid, however, the introduction of a para chlorine atom 

 resulted in a decrease in activity. The reproducibility of these results is not 

 easily judged, but irrespective of this, the latter pair of substances should not 

 be cited as a case of absent or reversed para chlorine effect. We must 

 remember that in this instance we are dealing with a mixture of two optically 

 active forms which are probably of opposite physiological character. In spite 

 of a possible increase in the activity of both forms when tested separately, the 

 effect produced by the racemic mixture may be an unchanged or even 

 decreased activity. 



In the present connection, the conspicuous rise in root-growth-inhibidng 

 activity from a-anilinopropionic acid to its 2:4-dichloro-derivative (Aberg, 

 1953b) is also of some interest as it may be caused to a considerable extent by 

 the para chlorine atom. 



SOME ACIDS WITH A STRONG ANTI-AUXIN COMPONENT IN THEIR ACTIVITY 



The anti-auxin effects of a-(4-chlorophenoxy)wobutyric acid (4-ClPO«B) 

 on wheat roots were first described by Burstrom (1950, 1951a), who also found 

 that they occurred at about ten times lower concentrations than the corre- 

 sponding efTects of the unchlorinated a-phenoxywobutyric acid (PO?B). 

 Quite similar results have been obtained by Hansen (1954) and the present 

 author {Figure 3). This pair of substances have also been studied in the flax 

 root test. The action curves of the pure substances show the expected course 



102 



