Modelling anion amelioration of aluminium phytotoxicity

There is good evidence for the ameliorating effect of SO₄ ^2- and F^- on the expression of Al phytotoxicity in acidic solutions. The role of OH^-, in both shifting Al speciation towards hydroxy-Al species and decreasing activities of H⁺ with increasing pH, is still controversially discussed. Grauer and Horst (1992) proposed a model based on the assumption that Al phytotoxicity is a function of the Al saturation (AlS) of exchange sites in the root apoplast and analyzed the predictions of the model in the case of cation amelioration, with special emphasis on H⁺. In this study the model is further developed by considering, in addition to Al³⁺, the complexation of Al with the anions OH^-, F^-, SO₄ ^2-, and Cl^- to form potentially toxic Al species. Association constants of these Al complexes with a ligand (L ^-) which is assumed to simulate the cation exchange sites in the root apoplast, were estimated. Affinity factors for binding to L ^- compared to Al³⁺ were derived from these estimated association constants, and values were, in a first approach, 0.79 for AlOH²⁺, 0.02 for Al(OH)₂ ⁺, and 0.13 for Al(OH)₃ ⁰ (or 0.03 choosing another hydrolysis constant). High toxicity of Al₁₃ (AlO₄Al₁₂(OH)₂₄(H₂O)₁₂ ⁷⁺) could be explained by diminished H⁺ amelioration and a high association constant to L ^-. From estimated association constants for Al-Cl complexes, low affinity factors for L ^- for these complexes were derived. Since the formation of these Al-Cl complexes is not favoured, Cl^- is predicted to have very little ameliorating effect. In the case of SO₄ ^2– and F^- complexes with Al, the derived affinity factors never exceeded 0.05 and, since formation of these complexes is favoured by high association constants, are thus in agreement with experimental results on ameliorating effects. The ranking of the anions for ameliorative effectiveness was estimated to be in the order of OH^->F^->SO₄ ^2–>Cl^-. Hydroxy amelioration in this context is restricted to the speciation effect, which is only significant above pH 4.