Effect of light upon membrane potential, conductance, and ion fluxes inRiccia fluitans

Abstract

Summary. The membrane potential Era, slope conductance g,,, and fluxes of S6Rb+/K+ and 36C1-have been measured on cells of the aquatic liverwort Riccia fluitans in artificial pond water of pH 4 to 8 and 0.1 to 10 mM K +, in the dark and 1 mW/cm 2 of white light. In the dark, E m reflects a passive diffusion potential according to the Goldman equation with relative ionic permeabilities, Pn/PK/PNa = 10:1:0.02. In the light, E m of -200 to -240 mV exceeds the most negative ion diffusion potential, i.e. E~, by more than 100 mV, is more sensitive to H + than K +, and is reduced to the dark level by uncouplers of phosphorylation. At 1 mM K +, gm is 33 and 50 gS/cm 2 in the light and dark, respectively, gm is sensitive to Ho + only in the light, and more sensitive to K + in the dark. Current-voltage relationships are given. Light increases the influx at the plasmalemma of S6Rb+/K+ and 36C1 less than would be expected from the increase of E~. It is concluded that the electrogenic pump operates in the dark as a constant current source which is shunted by the diffusive channels, whereas in the light E m approaches the H +-dependent electromotive force of the electrogenic pump.The origin of the common notion that electrogenesis at the cell membrane of green plants utilizes photosynthetic energy may be dated back to the days of the founder of membrane biology, W. Pfeffer, whose coworker Haacke (1892) recorded light-induced voltages from green tissues of higher plants. In current hypotheses the plant cell membrane is represented by an electrical equivalent circuit which contains ion-specific passiv e diffusive elements in parallel with an ATP-powered electrogenic proton pump (see Slayman, 1974;Bentrup, 1975). However, the complex influence of light upon ion transport (see MacRobbie, 1971;Higinbotham, 1973), and furthermore, the intimate linkage of energy metabolism of the chloroplasts to that of the other cellular compartments (see Heber, 1974), renders unlikely that light acts only via ATP-consuming pumps, the passive elements being coupled only electrically to them. The regulation of cytoplasmic pH as outlined by Raven and Smith (1973), requires a more refined mode of control. In fact, Spanswick (1972), and Vredenberg and

Membrane Potentials of Vallisneria Leaf Cells and Their Relation to Photosynthesis

“…Earlier it was concluded that such a treatment increases the ionic permeability of the cell membranes (15). This has now been confirmed by resistance measurements; after such a treatment the resistance of the epidermal cells was less than 1.5 MQ2 compared with 7 MfZ in the control cells (Table III). Electropotentials in Light, LEvo.…”

“…Resistance was 4.1 ± 0.7 MUl in the dark, and 3.7 ± 0.4 MUl in the light. There was no significant difference between the cell membrane resistance in light and dark as has been found in some other cells (7,19,20,22 (Table I) and of CCCP (Table II). Both 2 tLM CCCP and anoxia depolarized DE,O by about 13 mv.…”

“…It is also conceivable that CCCP interferes with the electrogenic mechanism itself presumably acting as a proton carrier as was, e.g. concluded for Riccia (7), since uncoupling of phosphorylation seemed to occur in this plant only at high CCCP concentrations and the depolarization caused by CCCP was accompanied by a decrease of the membrane resistance (7). The (5).…”

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

“…On the other hand, the mean value of EK was only -110 mV under this condition (Table I). This indicates that the transmembrane potential is to a great extent established by an electrogenic pump (Trgbacz et al, 1989), comparable to results in the liverwort R. fluitans (Felle and Bentrup, 1976).…”

“…More recent data (Schroppel-Meier and Kaiser, 1988;Thaler et al, 1992) indicate that the cytoplasmic C1-activity is probably 4 0 mM, at least in glycophytes. the cytoplasmic and vacuolar C1-concentration, respectively (Felle and Bentrup, 1976). The cytoplasmic NOs-activity in Conocephalum was at the lower end of recently published values (Miller and Zhen, 1991;King et al, 1992).…”