Cytoplasmic Ca2+, K+, Cl-, and NO3- Activities in the Liverwort Conocephalum conicum L. at Rest and during Action Potentials

Abstract

lntracellular Caz+, K+, CI-, and NOs-activities were measured with ion-selective microeledrodes in the liverwort Conocephalum conicum 1. at rest, during dark/light changes, and in the couese of adion potentials triggered by light or electrical stimuli. l h e average free cytosolic Caz+ concentration was 231 f 65 nM. We did not observe any light-dependent changes of the free cytosolic Caz+ concentration as long as no adion potential was triggered. During adion potentials, on average a 2-fold increase of the free cytoplasmic Caz+ concentration was recorded. lntracellular K+ adivity was 76 f 10 mM. It did not depend on K+ concentration changes in the bath solution between 0.1 and 10 mM. The average equilibrium potential for K+ in the standard medium containing 1 mM K+ was -110 mV, which differed significantly from the resting potentia1 of -151 f 2 mV. During adion potentials, either a slight decrease or no changes in intracellular K+ activity were recorded.The average CI-adivity was 7.4 f 0.2 mM in the cytoplasm and 43.5 f 7 mM in the vacuole. l h e adivities of NO,-were 0.63 I : 0.05 mM in the cytoplasm and 3.0 f 0.3 mM in the vacuole. For both anions the vacuolar activity was 5 to 6 times higher than the cytoplasmic activity. After the light was switched off both the CIand the NOs-adivity showed either no change or a slight increase. lllumination caused a gradual return to previous values or no change. During action potentials a slight decrease of intracellular CI-activity was recorded. It was concluded that in Conocephalum, as in characean cells, chloride channels are involved in the depolarization phase of the action potentials. We discuss a model for the ion fluxes during an action potential in Conocephalum.APs in plants play an important role in intracellular signaling and in the regulation of different physiological processes (Davies, 1987; Dziubifiska et al., 1989;Wildon et al., 1992;Fromm and Spanswick, 1993). To understand the function of APs in plant physiology, their ion mechanism has to be known in detail. Data conceming the sequence of ion fluxes during APs in plants were obtained primarily on the highly specialized giant characean cells (Beilby, 1984).According to the present model, APs in these plants are

Electrochemical Potential Gradients of H+, K+, Ca2+, and Cl- across the Tonoplast of the Green Alga Eremosphaera Viridis

“…This may also explain why the exclusion of the pipette tip from the vacuole was a relatively slow process that took 1 to 4 min to be completed. Similar observations exist for other plant cells (Findlay and Hope, 1976;Mertz and Higinbotham, 1976;Trebacz et al, 1994). The transient change of the electrical potential frequently accompanying the exclusion of the electrode tip from the vacuole (Figs.…”

“…Light-dependent changes of the cytosolic K+ activity in the millimolar range were reported by Felle and Bertl (1986b) in the case of the liverwort Riccia fluitans, whereas Trebacz et al (1994) observed no light-dependent changes in the liverwort C. conicum. In E. viridis we did not observe light-dependent changes of the cytosolic K+ activity, but small changes may have been obliterated by the relatively high cytosolic K+ activity.…”

“…A similar situation was recently proposed for the intracellular K+ activity of the liverwort Conocepkalum conicum (Trebacz et al, 1994). Some reports exist about the K+ concentrations in the cytoplasm and the vacuole of different plants (Flowers and Lauchli, 1983;Leigh and Wyn Jones, 1984;Maathuis and Sanders, 1993) according to which the vacuolar K+ concentration is either the same or lower than the cytosolic one, depending on the growth conditions.…”

“…Non-turgor-resistant ion-selective microelectrodes were used. Impalements were made possible by turgor reduction, which, as in preceding studies (Thaler et al, 1992;Trebacz et al, 1994), was achieved by the addition of 400 mM sorbitol to the artificial pond water. Electrode potentials were measured by a high-input impedance amplifier (Frankenberger, Germering, Germany) and registered by a pen recorder (model 314; Kontron, Munich, Germany).…”

“…For C1-measurements only 0.1 mM KCL was used to minirnize C1-leakage. The preparation of ionselective electrodes has been described in detail elsevlrhere (Trebacz et al, 1994). In short, a silanization procedure in a hot oven was used, and the electrodes were filled froin the back side.…”