Adaptation of corn roots to exogenously applied auxin

Abstract

The ability of intact primary roots of corn ( Zea mays L. Bear Hybrid WF 9 × 38) to adapt to growth‐inhibitory concentrations of auxin was studied using a highly sensitive position sensor transducer to measure growth. The timing, concentration dependence and temperature dependence of adaptation were studied as well as the time course of loss of adaptation upon removal of auxin. The rate of root elongation is inhibited 80% within 40 min after application of 10 −7 M IAA. Within 90 min growth rate begins to recover. For concentrations of IAA equal to or greater than 10 −7 M , recovery of growth rate (adaptation) is incomplete. Corn roots show a similar pattern of adaptation to the synthetic auxins NAA and 2,4‐D. The Q 10 for adaptation is high (3.2) and comparable to that for root growth (3.3). Upon removal of exogenous IAA, loss of adaptation occurs with full sensitivity to the hormone regained within 20 min. Based on the auxin specificity and the Q 10 for adaptation it is concluded that adaptation occurs neither by a change in the auxin degradation capacity of the root nor by a diffusional redistribution of applied auxin. It is suggested that adaptation involves metabolic processes, perhaps a metabolically dependent alteration of the number or affinity of auxin binding sites.