Pflra-substitution in regulators with phenyl nuclei 



receptor-regulator complex (corrected for variations in intrinsic activity) and 

 the growth result may be restricted to a rather limited range. Time and 

 penetration factors have to be considered (see Housley et al., 1954), effects 

 upon the metabolic systems regulating the synthesis and destruction of native 

 auxin are likely to occur and may be especially troublesome in systems 

 involving externally applied indoleacetic acid (see Aberg, 1953a; Aberg and 

 Jonsson, 1955), binding and competition at 'inactive sites' may be of import- 

 ance, many substances certainly exert a non-specific toxicity at higher 

 concentrations, and so on. In view of these difficulties it is natural that the 

 attempts at quantitative precision must remain tentative. On the other hand, 

 even limited success in such attempts is valuable as a starting point for 

 further comprehensive quantitative treatment. 



PHENOXYACETIC ACID AND SOME OF ITS MAIN AUXINIC DERIVATIVES 



Phenoxyacetic acid (POA) has often been characterized as wholly inactive 

 as a growth regulator. Some slight positive auxin effects have, however, been 

 reported: e.g. negative stem curvatures induced by highly concentrated 

 lanolin solutions (Zimmerman and Hitchcock, 1942) and positive effects in 

 the Avena cylinder test at high concentrations (Muir et a/., 1949). On the 

 other hand, the conspicuous restorative effects of POA on flax or cress roots 

 inhibited by 2:4-D (Aberg, 1952; Audus and Shipton, 1952) and the 

 positive effects upon wheat root growth and cell length (Hansen, 1954) 

 might indicate a prevailing anti-auxin activity. 



This anti-auxin activity is also clearly shown by the inhibitive action of 

 POA on Avena coleoptile growth when using cylinders with a fairly high 

 residual growth (20 per cent) in absence of added auxin (Ingestad, 1953). 

 In our experiments the residual growth has been even higher (28 per cent) 

 and the inhibition by POA appears at about ten times higher concentrations 

 than in Ingestad's experiments. This may be due to the higher pH used (5-9 

 and 4-5 respectively). It is highly interesting that at strongly increased concen- 

 tions there is a significant decrease in the inhibition, and that positive effects 

 occur in the range above 10~^ M. For example 



G(10-^) = 98-l±4-2; G(10-4) = 83-0±4-4; G(10-3) = 103-9±4-3; 



G(3xl0-3) = 117-0±14-0; A[G(10-3)_G(10-4)] = 20-9±4-9 {N = 1). 



These results, together with the positive effects observed by Muir et al. (1949) 

 at lower concentrations (residual control growth 8 per cent and pH probably 

 lower than in the present tests), suggest that the physiological activity of the 

 POA-molecule is of the intermediate type, fairly low affinity for the growth 

 centres being combined with very low intrinsic auxin activity. 



As regards the conspicuous auxin activity of 4-chlorophenoxyacedc acid 

 (4-ClPOA) there can be little doubt. In the pea test and the Avena cylinder 

 test, its activity is about 5-10 times less than that of 2:4-D (Fawcett et al., 

 1953; Wain and Wightman, 1953; Muir and Hansch, 1953) which 

 correlates well with its inhibiting effects on wheat (Hansen, 1954) and flax 

 roots {Figure 2). 



When comparing the POA and 4-ClPOA curves of Figure 2 we must 

 remember that the inhibiting effects may be of different types. At the 



99 



