Para-substitution in regulators with phenyl nuclei 



most pronounced with 4-EtPOA, and correspondingly we find stronger 

 stimulations in the wheat root test with this substance than with 4-MePOA 

 {Figure 7). The intermediate character of 4-MePOA is also apparent from 

 the type of action curve obtained in the Avena cylinder test {Figure 1). 



In the flax root test with externally applied 2:4-D the antagonistic effect of 

 the 4-alkyl-phenoxyacetic acids culminates with 4-?.fopropyl-phenoxyacetic 

 acid (4-/P-POA), which also gives a significant stimulation of flax root 

 growth in the range 10~'^ to 10~^M. With 4-?^ri.-butylphenoxyacetic acid 

 (4-/B-POA) both this stimulation and the restorative effect upon 2:4-D- 

 inhibited flax roots are slightly weakened (Aberg, 1954). 



For wheat roots, on the other hand, growth stimulations have already 



Figure 7. The effects of some pzn-a- 

 alkyl-phenoxyacetic acids on the growth 

 of flax and wheat roots. Me = 4- 

 methjlphenoxyacetic acid {4-MePOA), 

 and so on. Et = ethyl, iP = isopropyl, 

 tB = tert.-butyl, Growth values (G) 

 in per cent of control, presented as in 

 Figure 1. The iP and XB effects on 

 wheat roots do not differ significantly, 

 and the values for both substances have 

 therefore been jointly plotted. 



10 ' 10" 10 ^ 10 



10 ^ 10'^ 10 "' 



Molar concn. — - 



reached their maximum with 4-EtPOA {Figure 7). This is probably partly 

 related to the lower auxin sensitivity of this material as compared with flax 

 roots. To some extent it may perhaps also be caused by a decreased 'uptake' 

 of the substances with 'bulky' alkyl-substituents (cf. p. 95). 



Summarizing, it may be said that the highest intrinsic auxin activity is possibly 

 attained in the 4-FPOA molecules which, however, show a low affinity to the 

 active sites. In 4-ClPOA this affinity is much increased, and the intrinsic 

 activity is still high, but not high enough to exclude a very faint anti-auxin 

 component of the activity. With further increase in the size of the para- 

 substituent we get substances of clearly intermediate character (4-BrPOA, 

 4-MePOA, 4-EtPOA, and probably 4-1 POA), while the introduction of 

 still larger substituents results in regulators of an almost pure anti-auxin type 

 (4-?P-POA, 4-/B-POA). The lowering of intrinsic auxin activity seems to 

 parallel fairly closely the increase in the van der Waals radius of the para- 

 substituent (Pauling, 1948): e.g. 



/^ara-substituent : F CI Br CHg I 



van der Waals radius : 1-35 1-80 1-95 2-0 2-15 A 



109 



