Impact of apoplast volume on ionic relations in plant cells.

Ionic relations of plant cells with free-running transmembrane voltage, V, can be modelled by electrocoupling of known, V-relevant and V-gated transporters. For cells in vitro, with constant external substrate concentrations, alternating states of solute uptake and release can be predicted. Here the model is extended to treat cells with limited external substrates, e.g., parenchyma cells in situ with a small external space, the apoplast. This model accounts also for cytoplasmic H+ buffering and apoplastic buffering of H+, K+ and Ca2+ by fixed anions. In general, the model converges to thermodynamic equilibrium for K+ between apoplast and symplast, and to equal steady-state rates of uptake and release for Cl- and H+. Oscillations of this model are rare and very sensitive to the volume portion of the apoplast.