Dose responses of auxins

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Abstract
Published data for the most widely used bioassays for auxins as. well as other auxin‐induced responses, have been analyzed to determine whether the response is ultrasensitive, hyperbolic or subsensitive, i.e. whether the change from 10 to 90% of maximal response requires less than, equal to or more than an 81‐fold increase in external auxin concentration. Auxin‐dependent callus growth is most often ultrasensitive. Other responses, including the curvature of Avena coleoptiles, may show simple Mi‐chaelis‐Menten kinetics. In the majority of cases, however, the response is sub‐sensitive, often very markedly so. The finding of subsensitive responses is consistent with the proposal by A. J. Trewavas that plant development is regulated by changes in the sensitivity to plant growth substances.Detailed data for subsensitive dose responses, particularly auxin‐induced inhibition of root growth, can often be precisely represented by bi‐ or multiphasic isotherms, with the phases separated by sharp transitions. The relationship to auxin uptake and the relationship, if any, between carriers and receptors for auxins remain to be elucidated.

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