Chemical Nature of Growth Regulators 165 



the sections in auxin solutions increases the positive curvatures. While 

 such tests are carried out without mixing the two types of growth 

 regulators in the solutions, still they may be presumed to act together 

 in the tissue. Consequently it seems logical to suppose that hemi- 

 auxins are in fact synergists with auxins. Some interesting compounds 

 in this category are y-phenylbutyric acid (XXIV), cyclohexaneacetic 

 acid (VI), and 2-cyclohexene-l-acetic acid (VIII). Some of these com- 

 pounds appear to resemble auxins closely, but some are of an entirely 

 different molecular constitution, e.g. vinylacetate (XXV). 



CHjCH-CH.COOH CH,COOCH=CH, 



XXIV XXV XXVI 



Anti-Auxins 



Figures 72 and 73, describing the effects of synergists with auxins, 

 also indicate that a variety of compounds can inhibit growth in the 

 presence of auxin. Such inhibition effects serve to reduce the effective- 

 ness of the auxin, but it is not clear in the case of most synergists 

 whether they are directly antagonizing the action of auxin or whether 

 they are inhibiting growth through more indirect means. 



Perhaps the first compound asserted to reduce auxin effectiveness 

 more or less directly was y-phenylbutyric acid (XXIV) (Skoog et al, 

 1942). Contemplation of the molecular structure led to the suggestion 

 that this compound had approximately the correct configuration for 

 auxin activity, but its low reactivity hindered its functioning as such. 

 Consequently, it would become attached to the active sites where 

 auxins act but would not readily consummate the auxin performance, 

 and would result in a direct reduction of auxin effectiveness. Enzymol- 

 ogy readily recognizes the inhibition of reactions by compounds of 

 slightly different properties from those of true substrates. 



The search for compounds which would antagonize auxins in 

 plant growth was stimulated by Aberg's report (1950) that two deriva- 

 tives of naphthalene could reduce the inhibition of roots by auxin. 

 There followed a report by van Overbeek et al (1951) that trans- 

 cinnamic acid, which lacks auxin activity because of its spatial con- 

 figuration, inhibits growth, and the inhibition may be reversed by 

 auxin. The ^ran^-cinnamic acid effect is illustrated by figure 74, in 

 which it can be seen that a given amount (15 mg./l.) is inhibitory at 

 low auxin concentrations and the inhibition disappears at higher 



