ALUMINUM TOXICITY IN SORGHUM GENOTYPES AS INFLUENCED BY SOLUTION ACIDITY

The effect of pH on Al toxicity in twelve sorghum (Sorghum bicolor (L. ) Moench) genotypes differing in Al sensitivity has been studied in nu trient solution culture. Most of the genotypes showed negative growth response to both Al at 15 mumol L-1 Al and H ions over the PH range 3. 9-4.8 but there was no relationship between the sensitivities of the g enotypes against H and Al 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 an d 4.8). This was not due to precipitation of Al-at the higher pH as su bstantial deviation from the original Al concentration has been avoide d by daily adjustment. At the lower pH values the genotypical growth r esponse to Al was closely correlated to the Al-induced changes in both the visual score of root damage and the specific root length (root le ngth per g dry root). In addition, Al severely decreased the uptake of Mg but hardly affected that of Ca and P. Aluminum inhibited the uptak e of Mg stronger at the lower pH values. It was concluded that both ma nifestations of Al toxicity (root damage and Mg deficiency) may be att ributed to the Al species dominant at low pH (Al3+) while the contribu tion of H ions to these negative effects was marginal.