Effect of combined nitrogen on symbiotic nitrogen fixation in pea plants

The nitrogen-fixing activity of the symbiotic system of Pisum sativum with Rhizobium leguminosarum is adversely affected by combined nitrogen. Both ammonium chloride and potassium nitrate, when added to the roots, lower the nitrogenase activity (acetylene-reduction) of intact pea plants. During a 3-day treatment of the plants with combined nitrogen, when the in vivo nitrogenase activity falls to less than 50%, the nitrogenase activity of isolated bacteroids, treated with toluene and supplied with ATP and reductants, does not decrease. Thus, the potential nitrogenase activity of the root nodules is unaffected by short-term combined-nitrogen treatment of the plants. The adverse effect of ammonium chloride on the nitrogenase activity of pea plants is counteracted by the addition of sucrose or of methionine sulfoximine (an inhibitor of ammonia assimilation) to the rooting medium. A higher light intensity also diminishes the effect of ammonium chloride on nitrogenase activity.Ammonium chloride has no specific inhibitory effect on the nitrogenase activity of isolated pea bacteroids, neither in the anaerobic, nor in the aerobic assay. On the other hand, ammonium chloride does inhibit the nitrogenase activity of detached root nodules within a few hours. At a lower oxygen concentration in the assay this inhibition is more pronounced. The effect of ammonium chloride on detached nodules is relieved by simultaneous addition of methionine sulfoximine.Various carbon compounds (glucose and tricarboxylic acid cycle intermediates) stimulate the nitrogenase activity of detached nodules; only dicarboxylic acids of the tricarboxylic acid cycle support the nitrogenase activity of isolated bacteroids. Efficiencies of nitrogen fixation as to consumption of carbon compound are similar in both system , although lower than in the intact system. Ammonium chloride does not affect respiratory activities of detached nodules or of isolated bacteroids.It is concluded from the above-mentioned results that combined nitrogen, added to intact plants or detached nodules, does not affect nitrogenase activity directly. The results support the photosynthate theory, i.e. the photosynthate supply of the nodules is reduced and consequently the nitrogenase activity decreased, owing to the consumption of carbon compounds for the assimilation of the added combined nitrogen.