Adverse effects of nitrate on stem nodules of Sesbania rostrata Brem

When the stems of Sesbania rostrata, growing vigorously in nutrient solution, are inoculated with Azorhizobium caulinodans ORS 571 and deprived of nitrate, hundreds of nodules with active N₂ fixation (acetylene reduction) develop on each stem within 10 d. The numbers of nodules that develop are not affected by increasing the nitrate concentration in the medium from 0 to 9 mM. However, as the nitrate concentration is raised above about 2 mM the growth of the nodules is strongly inhibited and all aspects of acetylene reduction activity (ARA) are progressively curtailed as the nitrate concentration is increased from zero. In 6 mM nitrate, effects are seen at the earliest possible time of measurement (6 d after inoculation) with maximum inhibition of growth and ARA (about 80%) at 10 d. In plants with stem nodules that have developed in the absence of nitrate since inoculation, the addition of 9 mM nitrate leads to inhibition of further nodule growth and to prompt decrease in specific ARA. The 'growth' and 'activity' responses of the stem nodules of Sesbania are similar in degree and sensitivity to those of root nodules of other legumes and thus they can no longer be considered as resistant to fixed N.