Phosphorus deficiency delays the onset of nodule function in soybean

Effects of phosphorus (P) deficiency on nodulation were examined in soybean grown in nutrient solution for 7 weeks. Increasing P supply increased shoot growth of nitrogen (N-2)-fixing plants from week 5 and that of nitrate-fed plant from week 4 after treatment. Nitrogen (N-2)-fixing plants had a greater P requirement for maximum growth at week 5. Increasing P supply from I to 16 mu M increased N concentration in N-2-fixing plants at week 4 but did not affect it from week 5. By contrast, P deficiency increased N concentration in nitrate-fed plants. Increasing P supply improved nodule formation from week 3. Nodule mass was affected more by P supply than nodule number, which, in turn, was affected more than plant growth. However, P supply did not decrease nodule specific N-2 fixation from week 5. The results suggest that P deficiency impaired symbiotic N-2 fixation through delaying onset of nodule function and decreasing nodule development.