Characterization of absorption sites for aluminum in the roots

Several experiments on the absorption sites for Al in the roots and the changes in the chemical fonns of Al absorbed with time were carried out. Among plant species the Al content in the roots was positively correlated with the CEC of the dry root powder. Extraction of pectic substances, nucleic acids, cold 10% trichloroacetic acid soluble phosphates (namely inorganic phosphates, ATP. sugar phosphates, etc.) significantly decreased the subsequent absorption of Al. Al content of the dry root powder was considerably higher than that of the excised roots which were treated with Al. The time course of Al uptake by the dry root powder was different from that by the excised roots: after approximately 1 hr of Al treatment the Al uptake by the dry root powder increased remarkably and proportionally to the time of Al treatment, while that by the excised roots increased only slightly. These results indicate that most of Al is bound to the pectic substances in the cell walls but a part of Al enters the protoplast and combines with nucleic acids and acid soluble phosphates. It is thus suggested that the destruction of roots and the high concentration of Al in the medium increase the passive movement of Al into the protoplast. Ca in the medium containing Al may be able to suppress Al toxicity only when the concentration of Ca is considerably higher than that of Al. Only a part of the absorbed Al may be transformed into more basic forms with time during approximately I day after the Al treatment, but thereafter Al absorbed may hardly show any change of form. In the intact plant since Al saturates most of the absorption sites in the roots within a relatively short period namely 30 hr, it is suggested that investigations on the changes in the metabolic reaction of the absorption sites for Al during this period are essential for the elucidation of the mechanisms of Al toxicity.