Binding and electrostatic attraction of lanthanum (La3+) and aluminum (Al3+) to wheat root plasma membranes

A general model for the sorption of trivalent cations to wheat-root (Triticum aestivum L cv. Scout 66) plasma membranes (PM) has been developed and includes the first published coefficients for La3+ and Al3+ binding to a biological membrane. Both ions are rhizotoxic, and the latter ion is the principal contributor to the toxicity of acidic soils around the world. The model takes into account both the electrostatic attraction and the binding of cations to the negatively charged PM surface. Ion binding is modeled as the reaction P- + IZ <==> 'PIZ-1 in which P- represents a negatively charged PM ligand, located in an estimated area of 540 A2, and IZ represents an ion of charge Z. Binding constants for the reaction were assigned for K+ (1 M-1) and Ca2+ (30 M-1) and evaluated experimentally for La3+ (2200 M-1) and H+ (21,500 M-1). Al sorption is complicated by Al3+ hydrolysis that yields hydroxoaluminum species that are also sorbed. Binding constants of 30 and 1 M-1 were assigned for AlOH2+ and Al(OH)+2, respectively, then a constant for Al3+ (20,000 m-1) was evaluated experimentally using the previously obtained values for K+, Ca2+ and H+ binding. Electrostatic attraction was modeled according to Gouy-Chapman theory. Evaluation of parameters was based upon the sorption of ions to PM vesicles suspended in solutions containing variable concentrations of H+, Ca2+ and La3+ or Al3+. Use of small volumes, and improved assay techniques, allowed the measurement of concentration depletions caused by sorption to vesicles. Some independent confirmation of our model is provided by substantial agreement between our computations and two published reports of La3+ effects upon zeta potentials of plant protoplasts. The single published report concerning the electrostatic effects of Al on cell membranes is in essential agreement with the model.