Nitrogen assimilation in soybean nodules

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Abstract
It has been shown that symbiotic nitrogen fixation in leguminous nodules requires molecular oxygen for ATP production through oxidative phosphorylation; but, on the other hand, O2 inactivates nitrogenase (3). Determination of the internal pO2 level of soybean nodules using oxygen microelectrodes revealed that the pO2 decreased sharply across the nodule cortex, and it was very low in the central tissue which contained bacteroids (13). Recently, nitrogen fixing activity has been induced in a free-living Rhizobium under a low O2 concentration. Bergersen (5) demonstrated that nitrogenase activity appeared when the free oxygen level was between 0.1-0.01 μm in chemostat cultures of the Rhizobium. To supply O2 to bacteroids under conditions of low pO2, leghemoglobin is essential for nitrogen fixation in leguminous nodules. The nitrogenase activity of nodule bacteroid suspensions at a low pO2 concentration was greatly enhanced by the addition of purified oxyleghemoglobin (4).