Inhibition of Symbiotic Nitrogen Fixation by Nitrate

Symbiotic nitrogen fixation is inhibited in the presence of nitrate. Yarious symbiotic processes are affected including nitrogenase activity which decreases just after addition of nitrate to the medium. Experimental results suggest that this inhibition is not due to reduction of nitrate in the shoots and roots but to nitrate ion metabolism in root nodules where bacterial reduction of nitrate does not appear to play a major role. Considerable nitrate metabolism was found to occur in the plant compartment of the nodules. This could cause inhibition by nodular accumulation of nitrite produced by cytosolic nitrate reductase activity and by competition with bacteroids for the reduction energy.