Applications of kinetics to auxin-induced growth 



We have compared the activities of the auxins ori/zochlorophenoxyacetic 

 acid and /^arflchlorophenoxyacetic acid with that of 2:4-D. Estimates of the 

 two single-point interaction affinities of oriAochlorophenoxyacetic acid were 

 made with orf^ochlorophenetole and 2:6-dichlorophenoxyacetic acid 

 respectively. For determination of the single-point interaction affinities of 

 parachlorophenoxyacetic acid we have used /?arachlorophenetole and 

 2:6-dichlorophenoxyacetic acid. The single-point interaction affinities as 

 estimated from the appropriate Kj values are given in Table 1. The inter- 

 action constants for the carboxyl combining inhibitors do not vary greatly as 



Table 1 



Calculated equilibrium constants for Avena coleoptile section-phenoxyacetic acid complexes. 



{After Foster (1953).) 



Compound 



ortho 



Ki 



carboxyl 



Ks 



^ max 



relative 



o-Cl phenoxyacetic acid 

 p-C\ phenoxyacetic acid 

 2 : 4-dichlorophenoxyacetic acid 



5xlO-«M 

 1 X 10-* M 

 2X10-5M 



4xlO-«M 

 4xlO-«M 

 2xlO-«M 



3x10 

 5x10 

 5 X 10-' 



M 



1 

 3 

 5 



between the differently substituted phenoxy compounds. The interaction 

 constants for the ortho combining group vary, however, by more than an 

 order of magnitude. Thus there is a five-fold increase in ortho attachment 

 affinity as between ortho- and /jflraphenetole and still another five-fold 

 increase in affinity as between parachloro- and 2:4-dichloro-substituted 

 ortho combining inhibitors. These changes are reflected in corresponding 



Table 2 

 Distribution of receptor sites among the several complex species for Avena coleoptile section-phenoxyacetic 



acid interaction. {After Foster (1953).) 

 The distribution is calculated for each compound at its optimal growth-promoting 

 concentration. POA = phenoxyacetic acid. 2:4-D = 2:4-dichIorophenoxyacetic acid. 



changes in the over-all values of A'^^ which increase as the ortho interaction 

 affinities decrease. We may say that as the ortho interaction affinities 

 decrease, the tendency of carboxyl group attached-auxin to complete two- 

 point attachment, for ES^ to go to ES^^^ decreases. Our calculations, 

 summarized in Table 2, indicate that for 2:4-D approximately four times as 

 many receptor sites contain two-point attached as one-point attached auxin 

 molecules at any one instant. For j&arachlorophenoxyacetic acid this ratio 



304 



