MODELING CATION AMELIORATION OF ALUMINUM PHYTOTOXICITY

Many studies have demonstrated the role of cations in ameliorating Al phytotoxicity. Until recently, the role of pH on the expression of Al toxicity has always been discussed in the light of pH-dependent change in Al speciation but not in the light of H+ as an ameliorative cation for Al toxicity. Several theoretical approaches have been proposed to characterize the ameliorating effects of specific cations. Based on the assumption that Al phytotoxicity is a function of the Al saturation of exchange sites in the root apoplast (AIS), a model is proposed with the general form: [GRAPHICS] where Am(j) = ameliorating cations, Al(i) = Al species, and K(Amj), K(Ami) = association constants of the respective ions with a ligand, L-, that simulates the apoplastic binding sites, and braces denote molar activities. Association constants for Ca, Mg, and Al ions were estimated from cation-exchange experiments with fresh roots of rye and yellow lupine, and ion speciation was calculated with the GEOCHEM-PC program. Making a sensitivity analysis, we found that the Al saturation (AIS(o)) maximum is strongly dependent on both the choice of association constants, and the ion concentrations in the rooting medium. In contrast, the pH of maximum AIS(o) is rather stable, and is predicted by the model to occur at pH 4.4 +/- 0.4, a value that agrees with several reports of maximal Al toxicity at pH around 4.5. Cations were ranked for their competitiveness with Al3+ in the order H+ > > Ca2+ > Mg2+ > > K+. Limitations of the model, as well as prospects for improved understanding of genotypical differences in Al tolerance, are discussed.