High Toxicity of Hydroxy-Aluminum Polymer Ions to Plant Roots

Investigations on the differential toxicity among Al ion species were carried out. The results are summarized as follows. 1) In the Al solutions containing similar concentrations of total ionic Al, polymer-Al ions predominated at pH 4.7 whereas monomer-Al ions predominated below pH 4.7. 2) At pH 4.7, the Al content of excised roots was maximum, while the K content was minimum. 3) At extremely low pH below ca. 4.5, the greater loss of K from roots associated with the low pH was prevented when Al was present in the solution. 4) The toxicity of Al ion species to the root growth and to the K retention in roots was in the order of polymer-Al ions > monomer-Al ions>AlSO4 +. 5) The increase of the basicity of Al solutions increased the concentration of the polymer-Al ions in the solution while the retardation of root elongation and the loss of K from roots were enhanced. 6) The modified ferron method was suitable for the identification of Al ion species, but the aluminon method enabled to determine the amount of precipitated Al. In conclusion, polymer-Al ions appear to be preferentially absorbed by roots, they destroy the plasmalemma of root cells, and finally inhibit considerably root elongation.