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

Net fluxes of Ca 2+ , 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 Ca 2+ 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 Ca 2+ 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 Ca 2+ efflux (pH 5.2) is similar to a reported pH threshold for acid‐induced wall extensibility in a closely related characean species. Application of NH 4 + to intact cells caused prolonged H + efflux and also transient Ca 2+ efflux. We attribute all these net Ca 2+ 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 Ca 2+ in the cell wall and the effect of Ca 2+ exchanges on the measured fluxes of other ions, including bathing medium acidification by H + efflux, are discussed.