Effect of Aluminum on the Uptake and Metabolism of Phosphorus by Barley Seedlings

The uptake of P(32) and its incorporation into phosphorylated compounds was examined in the roots of barley seedlings which had been pretreated with aluminum.The rate at which phosphorus increased in Al-roots was greater than in controls, especially during the first 15 minutes of incubation. It was shown that the increased phosphorus in Al-roots was P(i) and that it was almost completely exchangeable. Similar increases over controls were found when root segments were incubated in phosphorus solutions containing 10(-3)m DNP and at low temperature. The increased P(i) in Al-roots did not result in an increase in the total amount of phosphorus incorporated into phosphorylated compounds.Aluminum treatment markedly decreased the incorporation of P(32) into sugar phosphates but increased the pool size of ATP and other nucleotide triphosphates present in the roots. The specific activities of P(32) in ATP in Al-roots and controls were similar indicating that the rates of ATP synthesis were similar in each case.Preliminary investigations showed that aluminum citrate inhibited both purified yeast hexokinase and phosphorylated sugar production by crude mitochondrial extracts from barley roots.The results suggest that there are 2 reactions between aluminum and phosphorus: 1) at the cell surface or in the free space which results in the fixation of phosphate by an adsorption-precipitation reaction; 2) within the cell, possibly within the mitochondria, which results in a marked decrease in the rate of sugar phosphorylation, probably effected by the inhibition of hexokinase. The evidence does not support the view that aluminum enhances phosphorus uptake or that the superficial reaction between aluminum and phosphate interferes with phosphorus transport.