Effect of low pH on the enzyme activities of the ammonium assimilation pathways in the symbiotic association Bradyrhizobium sp peanut (Arachis hypogaea L.)

The activities of glutamine synthetase (GS, E.C. 6.3.1.2) and glutamate synthase (GOGAT, E.C.1.4.1.13) were determined in leaves, stems, roots and nodules of peanut plants inoculated with Bradyrhizobium sp. SEMIA 6144 and grown in soils with neutral reaction (pH 7.0) or acid reaction (pH 5.5). Leaves, stems and nodules showed no significant differences in those activities between the different trial groups. A high increase in the activities of these enzymes was observed in inoculated roots as compared to non-inoculated ones. The maximum increase of the specific activity of GS and GOGAT was found in inoculated roots grown in soils with acid reaction. Since inoculation and low pH of soil were necessary for a maximum in the activity of the ammonium assimilation enzymes, we suggest that Bradyrhizobium sp. SEMIA 6144 strain is the most sensitive part of the symbiotic association to soil acidity.