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The effects of nitrate on biological nitrozen fixation in Phaseolus vulgaris Tsui, Samuel


The inhibition by nitrate of nitrogen fixation in nodules of Phaseolus vulgaris cv. 'lca Pijao' was investigated. Acetylene reduction activity dropped 83% within six hours of nitrate exposure and was completely eliminated after seven days. This decrease was correlated to an accumulation of nitrite within the nodule cytosol. Nitrite was not detected within the bacteroids indicating that nitrite acted via the nodule cytosol for disruption of nitrogenase activity. The effect of nitrate and nitrite on leghemoglobin (Lb) was assayed in vitro using the measurement of oxygen uptake by bacteroids. Lb added to a bacteroid suspension at low oxygen concentrations increased the oxygen uptake rate. Pre-incubation of Lb with 10 mM nitrate or nitrite showed that within 30 minutes, the ability of Lb to function as an oxygen carrier declined significantly. Pre-incubation of Lb with nitrite showed a greater decrease in the Lb-enhanced bacteroid respiration than nitrate pre-incubation. It is suggested that nitrate inhibition of nitrogen fixation is mediated by nitrite inactivation of Lb, disrupting oxygen transport to bacteroids and decreasing ATP supply to nitrogenase.

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