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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 the response on the initial auxin concentration we developed a novel auxin bioassay, which allows reliable estimation of IAA concentrations between 10−8.5 and 10−5 M. 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 …

The endogenous control of cell wall pH of Zea mays L. coleoptiles has been investigated using abraded as well as non-abraded segments. Regardless of the initial medium pH, coleoptile segments characteristically change the external pH: after about 2 h a pH maximum (neutral peak; NP) around 6 is reached, followed by a slow pH decrease that levels off in a steady state at 4.8 after 7 h (acid equilibrium; AE). This basic behavior is independent of medium composition, but critically requires O2; it is not altered when the number of coleoptiles per volume is increased to over 10 col./mL. Thus, …