Pflra-substitution in regulators with phenyl nuclei 



with much smaller stimulations than in the case of wheat roots (for PO/B see 

 Aberg, 1952; 3 X lO"" M 4-ClPO/B gives a G value of 1 1 l-3±l-7;. The 

 G'-curves from the combination experiments {Figure 3) show about a six-fold 

 increase in the antagonistic activity of 4-C1P02B as compared to POzB. As 

 both substances seem to be completely or almost completely devoid of 

 intrinsic auxin activity, and assuming in the first instance a negligible influence of 

 the possible difference in their penetration rates (pp. 101-112), this increase 

 may apparently be related to a corresponding increase in their receptor 

 affinity. 



4-ClPOfB is known to counteract the effect of externally applied lAA or 

 2:4-D on the growth ofAvena coleoptile cylinders (McRae and Bonner, 1953) ; 

 in the concentration range used (about 5> 10~' to 5 <10~^M) the effect 



200 

 180 

 160 



1 



^100 

 I 80 



60 



fO 



20 



"I 1 1 r 



yPOiB 



POiB 



_L 



_L 



"1 r 



f-ZlTr 



WO 



300 



200 ■s. 



I 



100 



% 





w'' w 



10~ 



10'"^ 10'^ 10''^ 

 Molar concn. 



10'^ 10'^ W^ 



Figure 3. The effects ofphenoxy'isobutyric acid {POiB) , iranscimamic acid { Tr) . and their A-chloro- 

 derivatives {4-ClPOiB and 4-ClTr, respectively) upon the growth of wheat roots and 2-A-D-inhibited 

 flax roots. The left scale {G) refers to the growth of wheat roots in per cent of control without added 

 regulator (continuous lines), the right one [G') to the growth of the 2 : -i-D-inhibitedflax roots in per cent of 

 control with 10"' M 2:4-Z) only (broken lines). The growth values are plotted against a logarithmic 

 concentration scale. X is the number of independent experiments represented by the points. 



upon control sections was negligible. When tested by the present method 

 (p. 94),4-ClPOiB gives a conspicuous inhibition of coleoptile cyhnder growth 

 at 10-^ M and higher concentrations. Up to 5 X 10-^ M, 4-ClPO/B is a 

 stronger inhibitor than FOiB, but at higher concentrations the curves tend 

 to cross each other. This phenomenon will be further studied as it could 

 possibly indicate a very faint residual intrinsic auxin activity of 4-ClPO?B 

 molecules. 



As in the case of wobutyric acids, there are good reasons to assume that 

 in transc'mna.mic acids the side chain is of a type normally excluding any 



103 



