Effect of Plasma Membrane Ion Currents on Chlorophyll Fluorescence and Excitation Quenching in Chara Chloroplasts

Abstract

Illuminated giant cells of characean algae exhibit membrane excitability as well as the spatial patterns of photosynthesis and transmembrane H+ fluxes. The excitation of plasmalemma under these conditions results in the transient degradation of external alkaline and acid zones and inhibits photosynthesis in the alkaline zones. The generation of action potential in the patterned internodes is followed by cell hyperpolarization that peaks in 1 min and lasts up to 15 min. In order to exclude the influence of drifting resting potential on the chloroplast response to plasma membrane excitation, the voltage clamp mode was applied in this work, and chlorophyll fluorescence changes caused by a short depolarizing pulse were monitored. The depolarizing shift of membrane potential under voltage clamp conditions was found to induce a large depression of $$F_{{\text{m}}}^{{{'}}}$$ chlorophyll fluorescence and photosynthetic activity, provided that inward Ca2+ and Cl– currents were triggered and that a steady-state inward H+ flux (or OH– efflux) persisted before the application of an electric stimulus. The depolarization-induced ion currents measured in the alkaline and acidic cell regions under light and in darkness were found to differ significantly. The results are consistent with the notion that the massive inward H+ flow occurring in the alkaline cell regions under illumination is associated with the acidic shift of cytoplasmic pH. Divergent amplitudes of ionic currents in different cell parts can be partially determined by the presence of numerous plasmalemmal invaginations, charasomes specifically localized in the acid zones, as well as by sharp local changes in external pH in acid zones during the perforation of cell wall with a measuring microelectrode.