Chemical strvicture and biological activity 



fairly strong antagonistic activity against externally applied 2:4-D and lAA 

 in the Avena cyHnder test (McRae and Bonner, 1952) and in the flax root test 

 (Aberg, 1954), and is therefore assumed to be a substance of mainly anti- 

 auxin character, it is clear that at high concentrations it may also exert some 

 auxin activity. This was first shown with the pea test by Thimann (1952) and 

 has been corroborated by other workers (Wain and Wightman, 1 953 ; Osborne 

 et al., 1954). In the Avena cylinder test it has been found inactive, or at 

 fairly high concentrations, inhibiting (McRae and Bonner, 1952; McRae, 

 personal communication ; Muir and Hansch, 1953; Wain and Wightman, 



zzo 



Molar concn. 



Figure 5. The effects of thymoxyacetic acid ( Th), 2 : 6-dichloro-pheno.xyacetic acid (2 : 6-D). and their 

 A-chloro-derivatives {ClTh and 2 : 4: 6-T, respectively) upon the growth of wheat roots and 2 : 4-D-inhibited 

 flax roots {broken curves). Values presented as in Figure 3. 



1953). When the concentration is fvu'ther increased, the growth may, how- 

 ever, be significantly restored, though it usually does not exceed the control 

 growth. The action curve obtained with the present method almost coincides 

 with that of 4-MePO A given in Figure 1. When using shorter test periods stimu- 

 lations over the control growth may even be obtained (Osborne et al., 1954). 

 The existence of a synergistic component in the activity of 2: 6-D has also been 

 suggested (Thimann, 1952). The absence of conspicuous stimulations in the 

 wheat root test {Figure 5) may under the circumstances be interpreted as the 

 complex result of several different actions; thus the effect of compedtion 

 with the native auxin is compensated by synergistic effects, the slight intrinsic 

 auxin effects, and the toxic effects of 2: 6-D molecules. 



For 2:4:6-trichlorophenoxyacetic acid (2:4:6-T), no corresponding 

 positive auxin effects on coleoptile sections at high concentrations have been 

 reported in the literature, nor have indications of such effects been found 

 during the present investigation. A very slight activity is indicated in the 



106 



