Nitrate reduction and nitrogen fixation in symbiotic association Rhizobium - legumes

The inhibitory effect of nitrate on nitrogenase activity in root nodules of legume plants has been known for a long time. The major factor inducing changes in nitrogenase activity is the concentration of free oxygen inside nodules. Oxygen avail­ability in the infected zone of nodule is limited, among others, by the gas diffusion re­sistance in nodule cortex. The presence of nitrate may cause changes in the resistance to O2 diffusion. The aim of this paper is to review literature data concerning the effect of nitrate on the symbiotic association between rhizobia and legume plants, with special emphasis on nitrogenase activity. Recent advances indicate that symbiotic associations of Rhizobium strains characterized by a high nitrate reductase activity are less suscepti­ble to inhibition by nitrate. A thesis may be put forward that dissimilatory nitrate re­duction, catalyzed by bacteroid nitrate reductase, significantly facilitates the symbi­otic function of bacteroids.