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Metabolism and Enzymology

Effect of Oxygen and Malate on NO₃^– Inhibition of Nitrogenase in Soybean Nodules

Laboratoire de Recherche sur les Symbiotes des Racines, Institut National de la Recherche Agronomique, 9 place Viala, 34060 Montpellier cedex, France, Station de Physiologie Végétale, Centre de Recherches Agronomiques de Bordeaux, Domaine de la Grande Ferrade, 33140 Pont-de-la-Maye, France

Soybean (Glycine max cv Hodgson) nitrogenase activity (C₂H₂ reduction) in the presence or absence of nitrate was studied at various external O₂ tensions. Nitrogenase activity increased with oxygen partial pressure up to 30 kilopascals, which appeared to be the optimum. A parallel increase in ATP/ADP ratios indicated a limitation of respiration rate by low O₂ tensions in the nodule, and the values found for adenine nucleotide ratios suggested that the nitrogenase activity was limited by the rate of ATP regeneration. In the presence of nitrate, the nitrogenase activity was low and less stimulated by increased pO₂, although the nitrite content per gram of nodules decreased from 0.05 to 0.02 micromole when pO₂ increased from 10 to 30 kilopascals. Therefore, the accumulation of nitrite inside the nodule was probably not the major cause of the inhibition. Instead, inhibition by nitrate could be due to competition for reducing power between nitrate reduction and bacteroid or mitochondrial respiration inside the nodule. This is supported by the observation of decrease in ATP/ADP ratios from 1.65, in absence of nitrate, to 0.93 in the presence of this anion at 30 kilopascals O₂. Furthermore, the inhibition was suppressed by the addition, to the plant nutrient solution, of 15 millimolar L-malate, a carbon substrate that is considered to be the major source of reductant for the bacteroids in the symbiosis.