Review article. Physicochemical aspects of ion relations and pH regulation in plants - a quantitative approach

A quantitative physicochemical approach to ion relations of biologicalsolutions is presented, which applied fundamental laws of physicalchemistry to these systems and allows analysis of dependent variables([H⁺], [OH^-] and thedissociation state of partially dissociated ('weak') ions includingcarbonate species) in relation to independent variables (concentrations ofstrong and weak ions, dissociation constants and CO2 partial pressure).Within this concept the influence of strong (fully dissociated) ions isconfined to their net unbalanced positive charge which is referred to asSID (strong ion difference). The SID concept is then applied to membranetransport processes and ion relations of xylem and phloem sap: simpletransmembrane transport of protons between compartments cannot affect pH oneither side of the membrane, because rather small deviations fromelectrical neutrality results in substantial changes of the membranepotential under natural conditions. Thus the membrane ATPases aselectrogenic pumps cannot control the pH of adjacent compartments, but theyenergize secondary active transmembrane ion transport that results in pHchanges. The SID approach is shown to be valid by matching pH valuescalculated from analysis of xylem and phloem saps with actual measuredvalues. Sensitivity analysis based on the SID approach allows (1) to detectinconsistency in determination of composition in the analysed solutions and(2) quantitatively to analyse the influence of ion export or import andvariations of pCO2 on pH and dissociation state ofweak acids of complex biological solutions. The SID concept thus allows theevaluation of the contribution of a proposed pH-regulating or pH-affectingmechanism on a quantitative physicochemical basis.Keywords: Electrical neutrality, membrane potential, pH regulation,phloem sap, SID, xylem sap.