Effects of Ions and Membrane Potential on the Elongation of the Unicellular Green Alga Closterium

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Cells of the unicellular green alga Closterium ehrenbergii elongated exclusively at septa and for 4-5 hours after cell division. Cell elongation was strongly inhibited by a decrease in either the external concentration of Ca^<2+> or pH, and was also inhibited by several competitive Ca^<2+> channel blockers. Changes in concentrations of other external ions had no effect on the elongation. The average concentrations of ions in the intracellular fluid of the interphase cell before cell division was as follows (in mM): K^+=56.5, Na^+=4.8, Ca^<2+>=2.4, Mg^<2+>= 1.3, Cl^-=59.5; the pH was 7.4. The levels of K^+, Na^+ and Cl^- ions decreased significantly with cell elongation, suggesting that this process, which proceeds with water uptake, surpasses ion absorption. The plasma membrane potential (Vm) in both the interphase cells and in the elongating cells was in the range of -90 to -105 mV (interior negative). The Vm was entirely determined by the simple diffusion of K^+. A decrease in the external concentration of Ca^<2+> caused depolarization, probably by an indirect effect of low Ca^<2+>. Changes in the extracellular level of H^+ and other cautions barely affected Vm. Thus, external Ca^<2+> and H^+ are concluded to affect cell elongation but not via a change in the Vm across the plasma membrane.
日本植物生理学会の論文