Concomitant inhibition of nitrogen fixation and N-Phenyl-1-Naphthylamine uptake activity of bacteroids of a Bradyrhizohium japonicum strain by nitrate

Abstract

Effect of the treatment of soybean nodules with nitrate on the permeability of the outer membrane of bacteroids of Bradyrhizobium japonicurn strain 138NR was examined using the hydrophobic fluorescent probe N-phenyl-1-naphthylamine (NPN). The incorporation of NPN into the outer and inner membranes of the bacteroid cells isolated from nodules of soybean (Glycine max L. Merro cv. Tamahomare) was followed by the measurement of fluorescence at 30°C. The NPN-uptake activity of the bacteroids was expressed by two parameters, the initial rate of NPN-uptake, k, and the final fluorescence intensity, F _max. Acetylene reduction activity (ARA) of nodules, k, and F _max decreased exponentially during three weeks following the 20 d period after planting (DAP), resulting in high correlations between logarithmic transforms of k-values or F _max per bacteroid and those of ARA per bacteroid. Application of 20 mM KNO₃ to the nodulated roots at 24 DAP inhibited ARA of the roots by 85% of the control after 4 d of treatment. There were concomitant decreases in k and F _max by 75% and 65% of the control, respectively. These results suggest that treatment of soybean nodules with nitrate results in some changes in the permeability of the outer membrane of bacteroids with a concomitant decline of N₂-fixing activity of bacteroids.