Physicochemical aspects of ion relations and pH regulation in plants -: a quantitative approach

A quantitative physicochemical approach to ion relations of biological solutions is presented, which applies fundamental laws of physical chemistry to these systems and allows analysis of dependent variables ([H+], [OH-] and the dissociation state of partially dissociated ('weak') ions including carbonate species) in relation to independent variables (concentrations of strong and weak ions, dissociation constants and CO2 partial pressure). Within this concept the influence of strong (fully dissociated) ions is confined to their net unbalanced positive charge which is referred to as SID (strong ion difference). The SID concept is then applied to membrane transport processes and ion relations of xylem and phloem sap: simple transmembrane transport of protons between compartments cannot affect pH on either side of the membrane, because rather small deviations from electrical neutrality results in substantial changes of the membrane potential under natural con; conditions. Thus the membrane ATPases as electrogenic pumps cannot control the pH of adjacent compartments but they energize secondary active transmembrane ion transport that results in pH changes. The SID approach is shown to be valid by matching pH values calculated from analysis of xylem and phloem saps with actual measured values. Sensitivity analysis based on the SID approach allows (1) to detect inconsistency in determination of composition in the analysed solutions and (2) quantitatively to analyse the influence of ion export or import and variations of pCO(2) on pH and dissociation state of weak acids of complex biological solutions. The SID concept thus allows the evaluation of the contribution of a proposed pH-regulating or pH-affecting mechanism on a quantitative physicochemical basis.