Effect of pH on Ionic Species of Aluminum in Medium and on Aluminum Toxicity Under Solution Culture

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Abstract
In a medium with a lower pH, upward translocation of Al to shoots increased, whereas Al absorption by roots decreased. Al content of excised roots considerably oversaturated root CEC at pH 4.5–5.0. Monomer-Al ions predominated in the medium with a lower pH while at pH 4.7 the hydroxy-Al polymer ions predominated and at a pH of above ca. 4.7 the precipitated highly basic Al polymers predominated. Al tolerance at ca. pH 4.5 was in the order of buckwheat> garland chrysanthemum, soybean, Japanese radish> cucumber> tomato, barley>maize. At ca. pH 4.5, Al tolerance was positively correlated with Al contents of roots and tops. In another experiment, Al contents in roots of barley, wheat and oats were similar in the solution where the monomer-Al ions (pH 4.1) predominated, and increased in the solution where hydroxy-Al polymer ions (pH 4.5) predominated. The degree of increase in Al content of the roots was parallel to the Al tolerance. K content of roots considerably decreased in Al-injured roots. In conclusion, the following mechanisms are proposed. Hydroxy-Al polymer ions may be more toxic to plant roots than monomer-Al ions. As the Al-excluding power of the plasmalemma of root cells of the Al tolerant plant species is stronger than that of the Al sensitive ones, the Al tolerant plants may exclude more Al outside the plasmalemma of root cells than the sensitive ones and a larger amount of hydroxy-Al polymer ions may finally polymerize and precipitate there. On the other hand, monomer-Al ions though excluded may not polymerize and therefore the Al content of roots may not oversaturate root CEC at lower pH.