Sulfate uptake into duckweed (Lemna gibba G1) was studied by means of [(35)S]sulfate influx and measurements of electrical membrane potential. Uptake was strongly regulated by the intracellular content of soluble sulfate. At the onset of sulfate uptake the membrane potential was transiently depolarized. Fusicoccin stimulated uptake up to 165% of the control even at pH 8. It is suggested that sulfate uptake is energized in the whole pH range by a 3H(+)/sulfate cotransport mechanism. Kinetics of sulfate uptake and sulfate-induced membrane depolarization in the concentration range of 5 μM to 1 mM sulfate at pH 5.7 was best described by two Michaelis-Menten terms without any linear component. The second system had a lower affinity for sulfate and was fully active only at sufficiently high proton concentrations.
Lemna gibba L., grown in the presence or absence of Fe, reduced extracellular ferricyanide with a V max of 3.09 μmol · g(-1) fresh weight · h(-1) and a K m of 115 μM. However, Fe(3+)-ethylenediaminetetraacetic acid (EDTA) was reduced only after Fe-starvation. External electron acceptors such as ferricyanide, Fe(3+)-EDTA, 2,6-dichlorophenol indophenol or methylene blue induced a membrane depolarization of up to 100 mV, but electron donors such as ferrocyanide or NADH had no effect. Light or glucose enhanced ferricyanide reduction while the concomitant membrane depolarization was much smaller. Under anaerobic conditions, ferricyanide had no effect on electrical membrane potential difference (Em). Ferricyanide reduction induced H(+) and K(+) release in a ratio of 1.16 H(+)+1 K(+)/2 e(-) (in +Fe plants) and 1.28 H(+)+0.8 K(+)/2 e(-) (in -Fe plants). Anion uptake was inhibited by ferricyanide reduction. It is concluded that the steady-state transfer of electrons and protons proceeds by separate mechanisms, by a redox system and by a H(+)-ATPase.
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