Tolerance of wheat genotypes to Al toxicity in relation to their rhizosphere pH change, NH4+ and NO3- uptake, and nitrate reduction under Al stress

Two wheat genotypes differing in Al tolerance were employed to study the relationship between changes of rhizosphere pH and NO3- and NH + 4 uptake kinetics under Al stress. Tolerant genotype, Jian86-4, was able to maintain higher rhizosphere pH than the sensitive one, Yangmai-5. The lowest rhizosphere pH of Jian86-4 was higher than that of Yangmai-5 by 0.23 unit. The maximum uptake rate (Vm) and affinity indicated by Km for NO3- and NH4+ were strongly reduced by Al in both genotypes, with greater reduction in the Al-sensitive one. Jian86-4 had larger maximum uptake rate and affinity for NO3- and lower for NH + 4 than Yangmai-5 with and without Al supply. Nitrate reductase activities in leaves and roots were severely decreased by Al, and a greater decrease was noted in Yangmai-5. Al increased the content of free proline in leaves and roots of both two genotypes. The increases were greater in Jian-86-4 than Yangmai-5. It suggested that tolerance of Jian-86-4 to Al was partially achieved by increasing the rhizosphere pH to reduce the solubility, hence the reduction of toxicity of Al, which resulted from the high NO - 3 and low NH4+ absorption, and higher nitrate reductase activity.