ADVERSE-EFFECTS OF NITRATE ON STEM NODULES OF SESBANIA-ROSTRATA BREM

When the stems of Sesbania rostrata, growing vigorously in nutrient so lution, are inoculated with Azorhizobium caulinodans ORS 571 and depri ved of nitrate, hundreds of nodules with active N-2 fixation (acetylen e 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 inhibit ed and all aspects of acetylene reduction activity (ARA) are progressi vely curtailed as the nitrate concentration is increased from zero. In 6 mM nitrate, effects are seen at the earliest possible time of measu rement (6 d after inoculation) with maximum inhibition of growth and A RA (about 80%) at 10 d. In plants with stem nodules that have develope d in the absence of nitrate since inoculation, the addition of 9 mM ni trate leads to inhibition of further nodule growth and to prompt decre ase 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 consider ed as resistant to fixed N.