EFFECT OF NITROGEN FORM ON ALUMINUM TOXICITY IN SORGHUM GENOTYPES

The effect of N form (NH4 or NO3) on Al toxicity was examined with 12 sorghum (Sorghum bicolor (L.) Moench) genotypes grown on nutrient solution (pH 4.2). The negative effect of Al on dry matter yield after two weeks of growth was increased by NH4-N relative to NO3-N in some genotypes and decreased in others. In contrast, the negative effect of Al on internal Mg concentration and specific root length (SRL; m per g dry root) was generally lowered by NH4-N. Moreover, the utilization of the Mg taken up seemed to be negatively affected by NH4-N, as deduced from the occurrence of Mg deficiency symptoms. It was suggested that Al toxicity with the NO3-grown plants was mainly expressed as root impairment (characterized by small SRL) and NH4-induced changes in growth response of sorghum plants to Al were due to a shift of growth control towards Mg deficiency. The modification of Al toxicity on dry matter production of individual genotypes, induced by N form, might depend on their relative sensitivities to both Mg deficiency and root impairment.