IAA breakdown and its effect on auxin‐induced cell wall acidification in maize coleoptile segments

In excised Zea maus L. coleoptiles incubated in aerated media at high fresh weight per volume ratios, indole‐3‐acetic acid induces transient drops of extracellular pH. Based on the quantitative dependency of the response on the initial auxin concentration we developed a novel auxin bioassay, which allows reliable estimation of IAA concentrations between 10−85 and 10−5M. Using the bioassay and complementary concentration measurements by IAA fluorescence we found the transient IAA‐induced pH response paralleled by a decrease of auxin activity and concentration in the medium. This decline is rapid and starts immediately upon auxin addition, and insofar differs from the well known IAA degradation by epiphytic bacteria in long‐term auxin tests. We conclude that the transient character of the auxin pH response is due to rapid IAA metabolism. The effect occurs under those experimental conditions that are necessary for reliable estimations of auxin‐induced shifts of cell wall pH, which considerably complicates the interpretation of the results.