Para-substitution in regulators with phenyl nuclei 



{Figure 2) we get an increase of about two hundred times, and from the cell 

 length data of Hansen (1954) about twenty times. 



For 2-methylphenoxyacetic acid (2-MePOA), the weak but conspicuous 

 auxin activity is clear from the results of the Avena cylinder test (Muir and 

 Hansch, 1953; cf. also above p. 96), the wheat root test (Hansen, 1954), 

 and the flax root test {Figure 2; the inhibition is strongly counteracted by 

 1-NMSP). With this type of compound, the introduction oi a. para chlorine 

 atom to give 2-methyl-4-chlorophenoxyacetic acid strongly augments 

 the auxin activity. Both the flax root test and the cell length data from 

 wheat roots indicate an increase of about one hundred times. 



Thus, with the above three pairs of substances, all having at least one 

 ortho position free and with acetic acid as a side chain, we have found a very 

 conspicuous increase in the gross auxin activity accompanying the introduction 

 of a para chlorine atom into the nucleus. In root growth tests the transport 

 factors are to a large extent eliminated, and it also seems probable that the 

 effects of possible diflferences in the penetration rates are not very pronounced 

 (see the Concluding Remarks, p. 112). Preliminarily, we may therefore 

 assume a real difference \n primary activity. Much of this difference is probably 

 due to an increased affinity to the growth centres, but as is evident from the 

 behaviour of the pair POA-4-C1POA, we must also reckon with an influence 

 upon intrinsic activity. That there may be an anti-auxin component also in the 

 effect of 2-MePOA upon some tissues is suggested by its peculiar type of 

 action curve in the Avena cylinder test (p. 96). For 2-ClPOA, an intrinsic 

 activity lower than that of 2:4-D is suggested by the difference in the maximum 

 stimulations obtained in the Avena cylinder test with these two substances 

 {Mxxir et al., 1949; Wain and Wightman, 1953). 



It is thus possible that the true estimate of the affinities of 2-MePOA and 

 2-ClPOA might be somewhat higher than those indicated by the pCjQ 

 values, and that the para chlorine atom influences intrinsic activity as well 

 as affinity. The effect of such a substitution must certainly be expected to vary 

 to some extent with the type of the parent substance, and it would not be 

 suiprising if the affinity of POA is more strongly influenced than that of 

 2-MePOA or 2-ClPOA. 



In order to demonstrate the generality of the para chlorine effect among 

 the auxinic phenoxyacetic acids, the following substances may be cited Irom 

 Wain and Wightman (1953): 3-chloro-, 2:3-dichloro-, and 2:5-dichloro- 

 phenoxyacetic acid. With all of these compounds the introduction o( a. para 

 chlorine atom conspicuously increases the auxin activity as judged from the 

 Avena cylinder and pea tests. 



In the phenylsulphideacetic (or phenylthioglycollic) acid series, the para 

 chlorine effect seems to be of exactly the same type as for the phenoxy- 

 acetic acids. This is true for unsubstituted phenylsulphideacetic acid and for 

 its 2-chloro-, 3-chloro-, 3-methyl-, and 2 : 5-dichloro-derivatives (Sugii and 

 Sugii, 1953; Kato, 1954). 



There is little doubt that /^ara-chlorination will turn out to have a similar 

 effect also in the anihno-acetic acids (CeH^-XH-CHa-COOH). The 4-chloro- 

 and 2:4-dichloro-derivatives show conspicuous activity in the Ave?ia cylinder 

 and pea tests (Muir and Hansch, 1953; Veldstra and Booij, 1949), while no 

 auxin activity has been reported for anilinoacetic acid itself 



101 



