Para-substitution in regulators with ptienyl nuclei 



Further studies will also be made in order to test the possibility of a 'syner- 

 gistic' component in the activity of PF. When tested for auxin effects upon 

 shoot parts, PF has invariably been found inactive. In the Avena cylinder 

 test 5^- 10""*M causes inhibition, while lower concentrations are without 

 effect (Muir and Hansch, 1951). 



The introduction of a para chlorine atom in PF to give 4-chloro-benzoic 

 acid (4-chloro-phenylformic acid, 4-ClPF) strongly increases the growth- 

 stimulating activity both in respect to wheat roots and to 2:4-D-inhibited 

 flax roots {Figi/re 4). When tested on Avena coleoptile sections with the same 

 methods as used for PF, positive auxin effects are still found to be absent, 

 while the inhibiting effects begin to appear at somewhat lower concentrations 

 (10-^ M, Muir and Hansch, 1951). 



Afo/ar concn. 



Figure 4. The effects of some chlorinated tnLnscinnnmic acids (Tr) on the growth of wheat roots, and 

 the effects of benzoic acid (PF) and its 4-chloro-derivative {4-ClPF) upon the growth of wheat roots and 

 2-A-D-inhibited flax roots. Values presented as in Figure 3. 



Both thymoxyacetic acid (Th) and its/;ara-chloro-derivative (ClTh) have 

 previously been found to exert conspicuous anti-auxin effects in the flax 

 root test when combined with 2:4-D or lAA (Aberg, 1954). Some further 

 data have now been collected, and the G' curves o^ Figure 5 clearly show about 

 a five-fold increase in the anti-auxin activity in respect to 2:4-D from Th to 

 ClTh. Also the positive effects on wheat root growth begin at lower con- 

 centrations for ClTh than for Th. The action curve of the latter substance 

 also indicates some complications in the form of a possible 'synergistic' 

 component in relation to the natural auxin system, which may be absent in 

 respect to 2:4-D. This could possibly cause the depression of the G curves 

 compared to the G' curves. 



For 2:6-dichlorophenoxyacetic acid (2:6-D) both positive auxin effects 

 and anti-auxin effects are well documented. Though this compound exerts a 



V 105 



