Kinetics of Auxin Interaction

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Abstract
It has been found empirically that the promotion of Avena coleop-tile section growth by auxin can be treated quantitatively on the basis of the supposition that the auxin molecule exerts its effect through combination with a suitable receptor site within the plant. The growth-promoting activity of an individual species of auxin molecule can be characterized by two quantities, KS and Vmax each of which may be experimentally determined. KS describes'' the affinity of the auxin for the receptor site within the plant while Vmax is the growth response which obtains when all receptive sites are saturated with the particular auxin. The values of KS and of Vmax for chemically different auxins are in general different. Chemically different auxins such as IAA, 2,4-D and NAA compete with one another for the same receptive sites within the plant, as shown by the following steps: (a) formulation of the quantitative relations between growth rate and auxin concn. which would be expected on the basis of competition of auxins for the same sites, (b) derivation of the parameters KS (combined) and Vmax(combined) for systems containing 2 different auxins in terms of the constants KS and Vmax for each auxin alone, (c) demonstration that the values Ks(combined) and Vmax(combined) measured experimentally for growth rate in the presence of auxin mixtures are identical with those calculated on the basis of (b) above. Auxins of low KS and low Vmax are capable of inhibiting or augmenting the activities of auxins of similar KS and greater Vmax. Whether inhibition or promotion results depends on the concns. of the 2 substances and upon the differential in Vmax for the 2 substances.

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