Chemical structure and biological activity 



significant stimulation in the range 10"^ to SxlQ-^M (G = 105-8^ 1-4, 

 iV = 10, and G = 104-9^ 1-6, iV = 11, respectively). Apparently we may 

 assume that even in this fairly strong auxin the intrinsic activity is somewhat 

 lower than that of the native auxin of wheat roots, and therefore a weak 

 anti-auxin effect may be demonstrated with this material. Consistent with 

 such an assumption is the lower maximum stimulation obtained in the Avena 

 cylinder test with 4-ClPOA than with lAA or 2:4-D (Muir, Hansch, and 

 Gallup, 1949; Wain and Wightman, 1953). 



In 4-bromophenoxyacetic acid (4-BrPOA) the intrinsic auxin activity is 

 probably further lowered, resulting in a lower root growth-inhibiting activity 

 in the flax root test and stronger stimulations in the wheat root test {Figure 6). 

 In the Avena cylinder test, 4-BrPOA shows about 30 per cent of the gross 

 activity of 4-ClPOA (Muir and Hansch, 1953). 



Figure 6. The effects of the para- 

 halogenated phenoxyacetic acids on the 

 growth of flax and wheat roots. 

 F = 4-fluorophenoxyacetic acid (4- 

 FPOA),andso on. Growth values (G) 

 in per cent of control, presented as in 

 Figure I. 



Molar con en. — " 



In 4-iodophenoxyacetic acid (4-IPOA) the intrinsic auxin activity seems to 

 be very weak or absent; Muir and Hansch (1953) report that it is inactive 

 in the Avena cylinder test. Its affinity to the active sites must be quite consider- 

 able, however, as judged from the strong stimulations obtained with this 

 substance in the wheat root test [Figure 6). In the flax root test it strongly 

 reduces the inhibition caused by 10"'^ M 2:4-D when added in a concentra- 

 tion of 10"" M. 



It is interesting to compare the halogenated phenoxyacetic acids with the 

 corresponding a-phenoxyzj-obutyric acids, which have been studied by Hansen 

 (1954). The anti-auxin activity of the latter substances increases with the 

 size of the halogen atom. If intrinsic auxin activity is wholly absent from the 

 4-ClPOzB molecules, this would suggest that affinity progressively increases 

 from 4-ClPO/B to 4-BrPO/B and 4-IPO/B. Alternadvely, it could mean that 

 the weak residual intrinsic auxin activity is more completely eliminated in the 

 last two substances than in 4-ClPO/'B. 



For 4-methyl- and 4-ethyl-phenoxyacetic acids (4-MePOA and 

 4-EtPOA), it has earlier (Aberg, 1954) been shown that they are able to 

 counteract the inhibiting effects of 2:4-D upon flax root growth, and also 

 that they produce root inhibitions which may be alleviated by addition of 

 an anti-auxin like 1-NMSP. The anti-auxin component of their actixity is 



108 



