EVALUATING THE CONTRIBUTION OF MAGNESIUM-DEFICIENCY IN THE ALUMINUM TOXICITY SYNDROME IN 12 SORGHUM GENOTYPES

The contribution of Mg deficiency to Al stress in twelve different sor ghum (Sorghum bicolor (L.) Moench) genotypes was investigated in nutri ent solution culture under conditions of low Mg supply (between 50 and 1000 muM) at two pH values. At pH 4.2, 30 muM Al strongly inhibited M g uptake When dry matter yield was plotted as a function of the plant Mg concentration, similar response curves were obtained in the absence and the presence of Al with three genotypes. With many other genotype s dry matter yields of the control (without Al treatment) and Al-stres sed plants were remarkably different at similar internal Mg concentrat ions, suggesting that growth had been suppressed not by Mg deficiency but by another factor, i.e. Al-induced root damage. At pH 4.8, 30 muM Al hardly induced root damage but reduced Mg uptake and Al-induced Mg deficiency could almost completely account for the growth reaction of all genotypes. Therefore, at this pH the efficiency of uptake or use o f Mg in different genotypes was the basis of their respective suscepti bility to Al toxicity. When specific root length surpassed a certain c ritical range below 80-100 m per g dry root, growth control by Al-indu ced Mg deficiency was nearly abolished. The pH and Al concentration wh ere this range was reached depended on the Al sensitivity of the genot ypes.