Para-substitution in regulators with phenyl nuclei 



class, though both the initial inhibition and the final stimulation are 

 comparatively slight. 



The very strong difference in the inhibiting activities of 2-naphthoxyacetic 

 acid (2-NOA) upon flax and wheat roots {Figure 1) is paralleled by a 

 similar shift in the anti-auxin direction for several other 2-naphthoxy deriva- 

 tives. It seems probable that here we meet with a case of species difference 

 in the auxin system. For wheat roots and Avena coleoptiles, however, the 

 sensitivity might be of the same type, the action curves indicating at first an 

 'anti-auxin component' of the activity which is, at higher concentrations, 

 superseded by the auxin effects. It is highly interesting to find that 10~^ M 

 2-NOA has a conspicuous restoration effect upon wheat roots inhibited by 

 SxlO^'^M I-naphthylacetic acid (Burstrom, 1955). 



Mofar concn. — •■ 



Figure 1. Theej]ectsof-\-n:ethylphenoxyaceticacid{-\-MePOA) and 2-naphthoxyacetk acid {2-NOA) 

 upon the growth of fax roots (F), cress roots (C), wheat roots (W), and Avena coleoptile cylinders (A). 

 Growth (G) in per cent of control, plotted against a logarithmic concentration scale. N is the number of 

 independent experiments represented by the points. 



Further data, especially on the interaction of 2-NOA with other growth 

 regulators in the growth of coleoptile cylinders, might be necessary for the 

 final elucidation of these 'anti-auxin' effects (action II according to Burstrom, 

 1955). 



The quantitative treatment of the auxin-anti-auxin interactions can only 

 be made in a tentative way at present, but analogies from simple model 

 systems are certainly very useful in making more precise hypotheses as to the 

 mechanisms vmderlying the growth results. It seems reasonable to start from 

 the general laws governing chemical and adsorption equilibria, making as few 

 additional assumptions as possible. This has been done by McRae et al. 

 (1952, 1953) in applying the enzyme kinetics of Michaelis and Menten, and 

 by Hellstrom (1953) in a more general approach. In both cases it must be 

 assumed that a relatively slight proportion of the regulator molecules is 



97 



