Pflra-substitution in regulators with phenyl nuclei 



As the intrinsic activity of these molecules is thought to be completely or almost 

 completely blocked, this increase may preliminarily be taken as related 

 mainly to the affinity for the growth centres. 



{Hi) The affinity effect must certainly be expected to vary within fairly wide 

 limits for different types of substances. Tentatively, the estimate obtained 

 from anti-auxins of the phenoxy type may, however, be applied to substances 

 like POA, 2-ClPOA, and 2-MePOA, for which the para chlorine effect could 

 then be divided into about a five- to ten-fold increase in receptor affinity and 

 about an equal or stronger increase in intrinsic auxin activity. 



The present hypothesis evidently postulates the existence of a graded series 

 of intermediate growth regulators with intrinsic activities between those of the 

 typical auxins and the very low or absent activities of the typical anti-auxins. 

 The interactions of substances like y-phenylbutyric, 4-methylphenoxyacetic 

 4-ethylphenoxyacetic, and carvacroxyacetic acid (2-Me-5-iP-POA) with 

 2:4-D and 1-NMSP strongly indicate that they occupy such an intermediate 

 position (Aberg, 1952; 1954). The auxin and anti-auxin effects shown by 

 substances like 2:6-dichlorophenoxyacetic acid or 4-bromophenoxyacetic 

 acid in different types of tests, and the peculiar compound action curves of 

 such substances in the Avena cylinder test (p. 96) also point to the same 

 conclusion. That even fairly strong auxins may have an intrinsic activity below 

 the possible maximum is evident from the quantitative treatment of the 

 IAA-2:4-D-interactions in the Avena cylinder test (McRae, Foster, and 

 Bonner, 1953) and from the slight stimulations of wheat roots by low 

 concentrations of 4-chlorophenoxyacetic acid (Figure 6). 



The maximum stimulations obtainable under certain conditions with 

 Avena coleoptile sections may be directly related to intrinsic activity, the 

 maximum stimulations of wheat roots may be inversely related to the same 

 measure, and data for judging the position of a certain substance may be 

 obtained from combination experiments like those made with flax roots. 

 In all these different methods disturbances may be caused in several different 

 ways (uptake and transport factors, non-specific toxicity, synergism, and so 

 on). Therefore, the safest way of approach to a comprehensive quantitative 

 treatment of the primary reaction, or reactions, underlying the diverse growth 

 responses seems to be the parallel use of different test methods and the 

 attempted synthesis of the results into a coherent picture. 



REFERENCES 

 Aberg, B. (1950). On auxin antagonists and synergists in root growth. Physiol. Plant. 



3, 447. 

 Aberg, B. (1951). The interaction of some auxin antagonists and 2:4-D in root 



growth. Physiol. Plant. 4, 627. 

 Aberg, B. (1952). On the effects of weak auxins and antiauxins upon root growth. 



Physiol. Plant. 5, 305. 

 Aberg, B. (1953a). On the interaction of 2 : 3 : 5-triiodobenzoic acid and maleic 



hydrazide with auxins. Physiol. Plant. 6, 277. 

 Aberg, B. (1953b). On optically active plant growth regulators. LantbrHogsk. Ann. 



20,241. 

 Aberg, B. (1954). Studies on plant growth regulators. IX. Para-alkyl-phenoxy- 



acetic and -propionic acids, and some related derivatives of naturally occurring 



phenols. Physiol. Plant. 7, 241. 



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