Evaluate the effects of the low partial pressure of O2 on nitrogen fixation in soybean using a positron-emitting tracer imaging system

Nitrogen fixation activity changes under various environmental conditions, of which partial pressure of oxygen is one of the most effective factors in legume plant. The effects of low partial pressure of oxygen in the rhizosphere on the symbiotic nitrogen fixation activity and translocation to various organs were evaluated in a real-time analysis by a PETIS. Soybean nodules were treated with mixed gas containing ¹³N labeled N₂ with various proportions of O₂, and the nitrogen fixation in the nodules was visualized by the PETIS. The results showed that under natural condition (pO₂: 0.20 atm) the nitrogen fixation ability of soybean plant was highest compared to that of lower O₂ conditions (pO₂: 0.00 and 0.10 atm). The nitrogen fixation activity of soybean nodules was strongly depressed with low O₂ concentrations, although it was not inhibited completely even at 0.00 atm pO₂. In contrast to the nitrogen fixation, the export of fixed-N from the nodules was enhanced by lower O₂ concentration.