IAA BREAKDOWN AND ITS EFFECT ON AUXIN-INDUCED CELL-WALL ACIDIFICATIONIN MAIZE COLEOPTILE SEGMENTS

In excised Zea mays 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 th e response on the initial auxin concentration we developed a novel aux in bioassay, which allows reliable estimation of IAA concentrations be tween 10(-8.5) and 10(-5) M. Using the bioassay and complementary conc entration measurements by IAA fluorescence we found the transient IAA- induced pH response paralleled by a decrease of auxin activity and con centration in the medium. This decline is rapid and starts immediately upon auxin addition, and insofar differs from the well known IAA degr adation by epiphytic bacteria in long-term auxin tests. We conclude th at the transient character of the auxin pH response is due to rapid IA A metabolism. The effect occurs under those experimental conditions th at are necessary for reliable estimations of auxin-induced shifts of c ell wall pH, which considerably complicates the interpretation of the results.