Nitrate Effect on Nodulation of Soybean by Bradyrhizobium japonicum1

Abstract

It is not known whether NO−3 inhibition of nodulation is primarily due to effects on the host plant or on the bacterials train. This study was conducted to evaluate the effect of NO−3 on nodule appearance and development of nitrogenase (acetylene reduction) activity of various Glycine max (L.) Merr. cultivar ✕ Bradyrhizobiumja ponicum (Kirchner) Buchanan strain combinations. Plants were grown hydroponically in controlled environment cabinets. NO−3 was either resupplied daily (“maintained”) during the growth period or allowed to deplete with plant growth (“rundown”). Nodule development was delayed all NO−3 treatment conditions (both maintained and rundown), the delay being more severe at higher NO−3 concentrations. The external concentration of NO−3 rather than the rate of NO−3 uptake appeared to have a major effect on the initial stages of nodulation. Treatment conditions providing similar rates of NO−3 up take from different solution NO−3 concentrations( 4.0 mM rundown vs. 0.5. mM maintained) resulted in a more marked inhibition of nodule appearance at the higher level of solution NO−3. Maintaining the solution concentration of NO−3 at 1.0 mM following the appearance of nodules greatly retarded, or prevented, the development of nitrogenase activity (C2H2 reduction). Minor differences in the tolerance of nodulation to NO−3 were observed among eight B. j. uponicum strains (preselected from 46 strains). More variation existed among 12 soybean cultivars (preselected from 23 cultivars) with respect to their ability to nodulate in the presence of NO−3. The cvs. Elf and Avoyelles were notable in having good nodulation tolerance to NO−3 when infected by USDA110. This indicated that, with selection and breeding, enhanced NO−3 tolerance of nodulation between soybean and B. japonicum should be possible.