Rapid calcium exchange for protons and potassium in cell walls of Chara

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Abstract
Net fluxes of Ca2+, H+ and K+ were measured from intact Chara australis cells and from isolated cell walls, using ion‐selective microelectrodes. In both systems, a stimulation in Ca2+ efflux (up to 100 nmol m−2 s−1, from an influx of ∼40 nmol m−2 s−1) was detected as the H+ or K+ concentration was progressively increased in the bathing solution (pH 7.0 to 4.6 or K+ 0.2 to 10mol m−3, respectively). A Ca2+ influx of similar size occurred following the reverse changes. These fluxes decayed exponentially with a time constant of about 10 min. The threshold pH for Ca2+ efflux (pH 5.2) is similar to a reported pH threshold for acid‐induced wall extensibility in a closely related characean species. Application of NH4+ to intact cells caused prolonged H+ efflux and also transient Ca2+ efflux. We attribute all these net Ca2+ fluxes to exchange in the wall with H+ or K+. A theoretical treatment of the cell wall ion exchanges, using the ‘weak acid Donnan Manning’ (WADM) model, is given and it agrees well with the data. The role of Ca2+ in the cell wall and the effect of Ca2+ exchanges on the measured fluxes of other ions, including bathing medium acidification by H+ efflux, are discussed.