Concomitant Inhibition of Nitrogen Fixation and N-Phenyl-1-Naphthylamine Uptake Activity of Bacteroids of a Bradyrhizobium japonicum Strain by Nitrate

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Effect of the treatment of soybean nodules with nitrate on the permeability of the outer membrane of bacteroids of Bradyrhizobium japonicum 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.Merr.cv.Tamahomare) was followed by the measurement of fluorescence at 30℃. 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 KNO_3 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 N_2-fixing activity of bacteroids.
社団法人日本土壌肥料学会の論文