Chemical structure and biological activity 



relatively high concentrations where POA begins to inhibit root growth, 

 non-specific toxic effects are certainly possible, and even if the inhibition 

 should be caused by the residual auxin effect of the POA molecules, the 

 affinity of these molecules for the growth centres must be higher than indicated 

 by the CgQ-value (p. 94). A better measure for this affinity may perhaps be 

 obtained from combined experiments with 2:4-D (Aberg, 1952). We will 

 use the concentration of the antagonist which, combined with 10~'^ M 2:4-D, 

 gives a doubled growth rate as compared to that in 2:4-D alone (C = 200), 

 and this concentration will be denoted C^ (negative logarithm = pC^). 

 A calculation from a simple competitive model system shows that the C j- 

 values of pure antagonists may be roughly comparable to the CjQ-values of 



Mo/ar concn. — ► 



Figure 2. The effects of phenoxyacetic acid {POA) and some of its derivatives upon the root growth of 

 flax seedlings. The derivatives are indicated by means of the substituents : 2-Cl = 2-chloro-phenoxy- 

 acetic acid {2-ClPOA), and so on. Values presented as in Figure 1. 



typical auxins as affinity measures, but because of the complexity of the physio- 

 logical situation and the complications caused by the variations in intrinsic 

 activity we prefer to use them in a purely provisional and exploratory manner 

 in order to see if both sets of values may fit the same general picture. 



Even if we use the pC .^-value for POA (4-0) instead of the pCjo-value (3-4), 

 the comparison gives an increase in affinity of 4-ClPOA (pCgQ = 6-8) of 

 about six hundred times. 



For 2-chlorophenoxyacetic acid (2-ClPOA) there are clear indications 

 of a weak auxin activity in the Avena cylinder test and in the pea test (see, for 

 example, Muir et al., 1949; Wain and Wightman, 1953). Also the experi- 

 ments with wheat roots (Hansen, 1954) and flax roots {Figure 2) indicate weak 

 auxin activity. The growth inhibition of flax roots, however, is strongly 

 counteracted by the anti-auxin 1-NMSP. In 2:4-dichlorophenoxyacetic 

 acid (2:4-D) this weak auxin activity is strongly augmented. As indicated by 

 the Avena cylinder test, the increase may be as high as four hundred times 

 (Muir and Hansch, 1953) though the data of Wain and Wightman (1953) 

 give an increase of only about fifty times. From experiments with flax roots 



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