Para-substitution in regulators with phenyl nuclei 



/)flrfl-chlorophenyl group in the systemic herbicide 3-(4-chIorophenyI)- 

 1 : 1 -dimethylurea (CMU) is of some interest too. Its action is certainly 

 different from that of the auxins (Christoph and Fisk, 1954; Muzik et al., 

 1954), but nevertheless the general affinity of the /7«ra-chlorophenyl group to 

 the plasma proteins may be of importance in conditioning it. 



Upon closer inspection deviations are likely to occur within the series of 

 phenomena now compared. Though perhaps similar to a certain extent, the 

 receptor sites for such widely different processes may certainly be expected 

 to differ in other respects. For the mainly auxinic substances, the magnitude 

 of the para chlorine effect upon affinity may be difficult to separate quantita- 

 tively from simultaneous effects on intrinsic activity, and other difficulties are 

 encountered if regulator substances of clearly intermediate character are 

 included in the comparisons. For an anti-auxin with conspicuous residual 

 auxin activity, the inhibiting effect on root growth exerted by fairly high 

 concentrations may thus be of auxin character, or it may be a mixed effect 

 comprising both an auxin component and a component of 'nonspecific 

 toxicity'. For weak auxins the root inhibitions at high concentrations may 

 likewise be mixed. 



CONCLUDING REMARKS 



There still remains to be discussed the possible effects of varying penetration 

 rates of the different regulators to the active sites. The exact location of these 

 sites offers immediate difficulties. In the case of surface-controlled reactions 

 of growing cells, the penetration factor may probably be regarded as rela- 

 tively unimportant when working with such objects as roots. At least in the 

 outer tissues, transport through the cell walls will hardly differ very much for 

 the various substances; when of a purely diffusive character it will naturally 

 be somewhat slower for substances of higher molecular weight. 



If, however, it is assumed that a main part of the regulating action takes 

 place after penetration of the plasma membrane, the factors determining the 

 rate of this process may become important, provided that the rate is too slow 

 to permit the equilibrium concentration to be reached under the dynamic 

 conditions of growth. How far this situation is realized cannot be judged at 

 present, and certainly it must differ for different types of material and even 

 for different rates of growth. 



Nevertheless, it may be of value to consider briefly two possible sources of 

 variations in the uptake of different regulators. It is fairly certain that most 

 of these substances penetrate into the cell mainly in the form of undissociated, 

 lipophilic molecules, though some uptake in the form of anions may also 

 occur (cf. Simon and Beevers, 1952). A difference in the dissociation constant 

 [K) of two regulators tested at the same pH might thus possibly cause a 

 difference in their uptake rates. The effect of j&ara-chlorination on the 

 dissociation of some of the substances used in the present study is shown by 

 the following /v-values : 



