Calcium-(organo)aluminum-proton competition for adsorption to tomato root cell walls: experimental data and exchange...

Aluminum interacts with negatively charged surfaces in plant roots, causing inhibition of growth and nutrient uptake in plants growing on acid soils. Pectins in the root cell wall form the major cation adsorption surface, with Ca2+ as the main adsorbing cation. Adsorption of Al3+ and Ca2+ to isolated cell wall material of tomato (Lycopersicon esculentum L.) roots was examined at pH 3.00-4.25 and in the presence of the aluminum chelators citrate and malate. Al3+ displaced Ca2+ from its pectic binding sites in the cell wall to a large extent but apparently also bound to non-Ca binding groups, displacing protons. Aluminum adsorption depended on the pH of the solution, with little Al adsorbing to the cell wall material at very low pH (<3.50). Under very acid conditions Al3+ replacing Ca2+ at pectic cross-links is therefore not expected to play a role in Al toxicity. Equimolar concentrations of citrate decreased Al competition for Ca binding sites almost completely, whereas malate only had an intermediate effect. The competition of (organo) Al3+, Ca2+, and H+ for cell wall binding sites was described adequately using the Gaines-Thomas exchange model.