Uptake of Indole-3-acetic Acid and Indole-3-acetonitrile by Avena Coleoptile Sections

Abstract

Avena coleoptile sections floated on solutions of 1-C14-IAA or 1-C14-IAN took up radioactivity rapidly for the 1st half hour and then at a nearly linear rate for 4 hours. The internal concentration in the tissue exceeded the external concentration after 1-2 hours (depending on the absolute concentration), so that there was true accumulation. The uptake of 1AA was about 5 times higher at pH 4.5 than at pH 7.5, while that of IAN was little affected by pH. Addition of unlabeled IAA actually promoted the uptake of labeled IAA (or IAN) by 50% or more. Much of the activity taken up in the 1st 30 minutes was lost again to unlabeled auxin solution or even to water; it is therefore considered to be in solution in the cell walls or other "free space." This is supported by the fact that the initial rapid uptake of both substances was not significantly decreased in purified N, although the subsequent rate was no more than 1/2 of that in air. This initial physical uptake was much larger with IAN than with IAA, and suggests that IAN may be concentrated in some lipophilic part of the tissue. At physiological concentrations, the rate of uptake of IAN was from 2 to almost 10 times as great as that of IAA, which explains most or all of the observed greater growth activity of the IAN in Avena.