Oscillations in H+ and Ca2+ ion fluxes around the elongation region of corn roots and effects of external pH

Abstract

Net fluxes of H+ and Ca2+ around the elongation region of low-salt corn (Zea mays L.) roots were measured using the microelectrode ion flux estimation (MIFE) technique. At pH 5.2 two oscillatory components were found. Fast, 7-min oscillations in H+ flux were superimposed on slow oscillations of about 1.5 h. Fast oscillations in Ca2+ flux showed a strong dependence on the H+ oscillations and were normally leading in phase by about 1 to 1.5 min. Both oscillatory components were strongly affected by external pH values. Preincubation for 20 h in buffered pH 4.0 solution suppressed the slow oscillatory component and caused huge H+ influxes in the elongation region. The fast oscillations were 8 times larger in amplitude and were slightly lengthened. Preincubation at pH 6.0 did not suppress the rhythmic character of the ion fluxes but it shifted the average H+ flux to greater efflux. The fast and slow oscillatory components of H+ flux seem to relate to biophysical and biochemical mechanisms of intracellular pH homeostasis, respectively. The origin of the Ca2+ flux oscillations is discussed in terms of the 'weak acid Donnan Manning' model of cell wall ion exchanges.