BINDING AND ELECTROSTATIC ATTRACTION OF LANTHANUM (LA3-MEMBRANES() AND ALUMINUM (AL3+) TO WHEAT ROOT PLASMA)

A general model for the sorption of trivalent cations to wheat-root (T riticum aestivum L cv, Scout 66) plasma membranes (PM) has been develo ped and includes the first published coefficients for La3+ and Al3+ bi nding to a biological membrane. Both ions are rhizotoxic, and the latt er ion is the principal contributor to the toxicity of acidic soils ar ound the world. The model takes into account both the electrostatic at traction and the binding of cations to the negatively charged PM surfa ce. Ion binding is modeled as the reaction P- + I-Z reversible arrow P IZ-1 in which P- represents a negatively charged PM ligand, located in an estimated area of 540 Angstrom(2), and I-Z represents an ion of ch arge 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) a nd H+ (21,500 M-1). Al sorption is complicated by Al3+ hydrolysis that yields hydroxoaluminum species that are also sorbed. Binding constant s of 30 and 1 M-1 were assigned for AlOH2+ and Al(OH)(2)(+), respectiv ely, then a constant for Al3+ (20,000 M-1) was evaluated experimentall y using the previously obtained values for K+, Ca2+ and H+ binding, El ectrostatic attraction was modeled according to Gouy-Chapman theory. E valuation of parameters was based upon the sorption of ions to PM vesi cles suspended in solutions containing variable concentrations of H+, Ca2+ and La3+ or Al3+. Use of small volumes, and improved assay techni ques, allowed the measurement of concentration depletions caused by so rption to vesicles. Some independent confirmation of our model is prov ided by substantial agreement between our computations and two publish ed 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.