Aluminum toxicity in sorghum genotypes as influenced by solution acidity

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Abstract
The effect of pH on AI toxicity in twelve sorghum (Sorghum bicolor (L.) Moench) genotypes differing in Al sensitivity has been studied in nutrient solution culture. Most of the genotypes showed negative growth response to both Al at 15 μmol L-1 Al and H ions over the pH range 3.9–4.8 but there was no relationship between the sensitivities of the genotypes against H and AI ions. At lower pH (3.9 and 4.2) Al appeared to be more toxic for the sensitive genotypes than at higher pH (4.5 and 4.8). This was not due to precipitation of Al-at the higher pH as substantial deviation from the original AI concentration has been avoided by daily adjustment. At the lower pH values the genotypical growth response to Al was closely correlated to the Al-induced changes in both the visual score of root damage and the specific root length (root length per g dry root). In addition, Al severely decreased the uptake of Mg but hardly affected that of Ca and P. Aluminum inhibited the uptake of Mg stronger at the lower pH values. It was concluded that both manifestations of Al toxicity (root damage and Mg deficiency) may be attributed to the Al species dominant at low pH (Al3+) while the contribution of H ions to these negative effects was marginal.