Chemical structure and biological activity 



Apparently there is no consistent effect of such magnitude that it can be 

 related to the observed differences in gross activity. 



The situation might be influenced, however, by the degree of lipophily 

 of the undissociated molecules, which, together with the molecular size, is 

 a very important factor determining the permeation rate (see Collander, 

 1954). For several aliphatic substances, Collander (1949) found about a 

 five-fold rise in the partition coefficient C'ethei/Qater upon the introduction 

 of a chlorine atom, and the same may be true for aromatic substances. Thus, 

 the water solubility of iV-phenylmaleimide is about twenty times higher 

 than that of the 4-chloro- derivative, and this has been suggested as explaining, 

 in part at least, the higher physiological activity of the chlorinated compound 

 (Overbeek et al., 1955). As pointed out by these authors, however, such a 

 mechanism cannot fully explain the specific effect ois. para chlorine atom, and 

 such specific effects are clearly apparent with the chloro-phenoxyacetic acids, 

 the chloro-phenoxyuobutyric acids and the chloro-irawi-cinnamic acids. It 

 seems, therefore, that we must turn from the less specific solubility effects to 

 the much higher specificity possible in a multipoint adsorption system in 

 order to get a plausible hypothesis as to the nature of the para chlorine effect. 

 As a reminder against over-estimating the lipoid solubility factor, the parti- 

 tion coefficients Qther/^water fo^" phcnylacctic acid and indoleacetic acid may 

 be cited: PA 37, lAA 20 (Collander, 1949). 



Additional evidence for the probable absence of conspicuous penetration 

 factors related to undissociated molecules in a root growth test comes from 

 Burstrom's (1951a; 1952) studies on the effects of PO/B and 4-ClPOeB on 

 wheat roots at different pH values. An increase in the pH value from 4 to 7 

 has, in the presence of a normal calcium supply, no significant effects upon 

 the growth stimulation caused by these substances, in spite of the strongly 

 reduced amount of undissociated molecules present at higher pH values. 

 The non-auxinic inhibitions, on the other hand, are markedly shifted to 

 higher concentrations with increasing pH. 



Though /?ara-chlorination may certainly influence the permeation rate of 

 a regulator substance, and though this rate may be of importance in condi- 

 tioning the magnitude of the gross activity in some types of tests, nevertheless 

 we may conclude that such a factor is likely to be of little significance in 

 root tests. This may perhaps be connected with the localization of the growth 

 centres at the cell surfaces and with the rapid penetration of the regulators on 

 to these surfaces. Further experimental studies are needed, however, for 

 clarification of the situation, but provisionally the permeation factor may 

 be neglected when considering the results of such tests and the emphasis laid 

 upon the probably more important effects of variations in affinity and in 

 intrinsic activity. 



The effects of/xzra-chlorination may then be summarized as follows: 



(?) It generally increases the auxin activity of weak auxins. In the phen- 

 oxyacetic acid series this increase may amount to one hundred times or more, 

 and it appears probable that it is mainly conditioned by an increase in the 

 affinity to the growth centres and in the intrinsic activity of the molecules. 



(n) In the typical anti-auxins, the increase in the growth-stimulating 

 activity upon wheat roots or in the growth-restoring activity upon 2:4-D- 

 inhibited flax roots usually is of the order of about five to ten times or more. 



112 



