ARS | Publication request: The surface charge density of plant cell membranes: an attempt to resolve conflicting values for intrinsic s.

Submitted to: Journal of Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 18, 2009
Publication Date: April 16, 2010
Citation: Kinraide, T.B., Wang, P. 2010. The surface charge density of plant cell membranes: an attempt to resolve conflicting values for intrinsic s. Journal of Experimental Botany. 61:2507-2518.

Interpretive Summary: Plants in agricultural or natural ecosystems respond critically to nutrients and toxicants. Most of these nutrients and toxicants are electrically charged, that is, they are ions. Ions respond to the electrical potentials at cell surfaces, especially in the roots. The AFSRC has taken the lead in developing electrostatic models for the prediction of and the interpretation of plant responses to ionic nutrients and toxicants. One essential component of these electrostatic models is the membrane surface charge density (SCD). Essentially, this is the number of negative charges per unit area of the membrane surface. The literature, and our own measurements, present conflicting values for SCD. The principal method of measurement (zeta potential measurement) indicates a value six-fold smaller than do other methods. We have assessed these measurements, and conclude that the preponderance of evidence indicates that the smaller value is wrong. We also reveal, for the first time, a discrepancy between theory and zeta potential measurement, and we provide an explanation for the discrepancy. Finally, the study presents an electrostatic model with all required parameters (including SCD) for use by plant physiologists and soil chemists who are increasingly turning to electrostatic models to interpret plant-ion interactions.

Technical Abstract: The electrical potentials at membrane surfaces (Psi) may be computed with electrostatic models incorporating the intrinsic surface charge density of the membrane (Sigma), the ion composition of the bathing medium, and ion binding to the membrane. Ion activities at membrane surfaces may be computed from Psi, and physiological responses to ions are better interpreted with surface activities than with bulk-phase activities. Some of the parameter values needed for the models are well established. The equilibrium constants for ion binding were confirmed for several ions using multiple approaches, and a method is proposed for the computation of other binding constants. Sigma has been estimated by several methods, including computation from the near-surface electrical potentials measured by electrophoreses (zeta potentials). Computation from Zeta potentials yields values in the range –2 to –8 mC m–2, but other methods yield values in the range –15 to –40 mC m–2. The preponderance of evidence supports the suitability of Sigma = –30 mC m–2. A systematic discrepancy between measured and computed Zeta potentials was noted and discussed. A proposed, fully paramatized Gouy-Chapman-Stern model appears to be suitable for the interpretation of many plant responses to the ionic environment.