Applications of kinetics to auxin-induced growth 



consistent results. There are, however, other experimental facts which do not 

 appear to us to be capable of such treatment. A simple example is the fact 

 that the apparent A'^ of auxin in the coleoptile system is not constant as 

 between one lot of seeds and another. We have found variations of two- to 

 four-fold of the values of Kg in successive yearly harvests oi^ Avena seeds of the 

 variety Siegeshafer. A second example concerns interaction between lAA 

 at low concentrations and some second auxin as for example 2:4-D. The 

 latter auxin inhibits the growth-promoting effect of lAA as shown in Figure 8. 

 This effect is not, as has already been noted (McRae et al., 1953), interpretable 

 with simple kinetic theory without further assumptions. Still another 

 category of effects which appear to require further understanding for their 

 interpretation are the synergisms between chemically different auxins and 



^■^/m-g/l. 



OOf Ta3/l 

 //// a/one 



50 

 ■m.3/1. 



ijs concentration 

 Competitive inhibition of lAA-indiiced Avena coleoptile section growth by 2 : 4-D. 



Figure 8. The growth-promoting activity of indoleacetic acid in the Avena coleoptile section is inhibited 

 by low concentrations oflA-D. This effect, which is apparently a competitive one, needs an explanation. 

 Data after McRae, Foster, and Bormer (1953). 



related substances. Such synergism has been frequently described in the 

 literature. We will consider one specific case which illustrates the problems 

 involved. The lAA derivative 2 methyl-5:7-dibromo-IAA is inactive as an 

 auxin in the coleoptile system. It behaves as a carboxyl group-combining 

 anti-auxin in the presence of 2: 4-D. The same compound, however, 

 enhances the growth-promoting effect of lAA. This effect is upon the 

 apparent K^ of I AA which is decreased by a factor of 4. Maximum growth 

 rate of coleoptile sections in the presence of lAA is unaffected by the addition 

 of 2-methyl-5:7-dibromo-IAA. How can a substance decrease the apparent 

 Kg of an auxin in our system ? How can one and the same substance enhance 

 the growth-promoting effect of lAA but inhibit that of 2:4-D^ Discussion 

 of these questions is not possible in terms of our simple kinetic model and 

 appears to require additions to it. Answers to these questions will doubtless 

 greatly amplify and deepen our understanding of auxin matters. 



We have seen in the initial section of this discussion that to treat the auxin- 

 coleoptile system in kinetic terms requires no assumptions as to what goes on 

 inside the coleoptile and tells us nothing about the nature of the auxin- 

 coleoptile interaction except that this process is growth-rate limiting. The 



306 



