The dosage-response curve for auxin-induced cell elongation: A reevaluation.

The dosage-response curve for auxin-induced growth of coleoptile sections has been reevaluated, using initial growth rates than rates obtained hours after application of auxin. The curve is sigmoid rather than the "classical" bell-shape; i.e., the growth rate increases as the IAA concentration is raised from 10(-8) M, but then the same optimal rate prevails over the range of 3×10(-7) to 10(-3) M. In the absence of sucrose this sigmoid shape persists, but with sucrose present the shape slowly changes into a modified bell-shaped curve. It is suggested that the classical bell-shaped curve is due, at least in part, to an auxin-sucrose interaction, and that it is not a true reflection of the kinetics of auxin-induced elongation.