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Environmental and Stress Physiology

Assessing the Rhizotoxicity of the Aluminate Ion, Al(OH)₄^–

Appalachian Soil and Water Conservation Research Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Beckley, West Virginia 25802-0867

Dissolved aluminum (III) in acidic soils or culture media is often rhizotoxic (inhibitory to root elongation). Alkaline solutions of Al are also sometimes rhizotoxic, and for that reason toxicity has been attributed to the aluminate ion, Al(OH)₄^–. In the present study, seedlings of wheat (Triticum aestivum L. cv Tyler) and red clover (Trifolium pratense L. cv Kenland) were cultured in aerated aluminate solutions at pH 8.0 to 8.9. The bulk phases of these solutions were free of reactive polynuclear hydroxy-Al (including the extremely toxic species AlO₄Al₁₂[OH]₂₄[H₂O]⁷⁺₁₂ [Al₁₃]) according to the ferron (8-hydroxy-7-iodo-5-quinolinesulfonic acid) assay. At an aluminate concentration of 25 micromolar (23 micromolar activity) and a pH of 8, root elongation was less than 40% of Al-free controls, but at pH 8.9 elongation was 100% of controls. The hypothesis is offered that aluminate is nontoxic and that the inhibition at lower pH values is attributable to Al₁₃ postulated to have formed in the acidic free space of the roots where the ratio /{Al³⁺/}//{H⁺/}³ may rise above 10¹⁰. At this value hydroxy-Al in over-saturated, alkaline solutions begins to undergo rapid conversion to polynuclear species.