Reduction in toxic aluminum to plants by sulfate complexation

Excess SOin solution has been demonstrated to reduce aluminum (Al) toxicity in several studies, but the mechanism of SO-induced reduction of Al toxicity was not clearly understood. In this study, Size Exclusion Chromatography (SEC; Sephadex G-25, superfine resin) was employed to investigate the interaction between Al and SOin unhydrolyzed Al solutions at pH 3.5. A dual peak elution of Al was observed when Al solution with SO/Al molar ratio ≥ 2.0 was injected into the SEC column. The first Al peak, which was eluted substantially earlier than the bed volume of the column, where the monomeric Al species are expected, was due to co-elution of Al with SOas evident from the position of this peak corresponding to the elution volume for peak elution of SO. The elution fraction at the first Al peak contained SOand Al at a SO/Al molar ratio = 2.7. The co-elution of Al and SOwould appear to be due to outer sphere SOcomplexation which has been referred to as “SO-complexed Al” in this paper. The Al peak for co-elution of Al and SOincreased, while the peak at the bed volume decreased with an increase in SO/Al molar ratio of the injected sample. A nutrient-solution experiment was conducted to evaluate the differential toxicity of SO-complexed Al (eluted by the SEC column), and monomeric Al in the absence of SO[as Al(NO)] in the range of 0 to 40 μM total applied Al concentrations. The SOcomplexed Al was less phytotoxic to soybean (Glycine max L.) root growth than the monomeric Al. The shielding effect of SOin the outer sphere of Al would appear to be responsible for the reduction in Al toxicity