Enzymes of Ammonia Assimilation and Ureide Biosynthesis in Soybean Nodules: Effect of Nitrate

Publisher Website
Google Scholar
Abstract
The effect of nitrate on N2 fixation and the assimilation of fixed N2 in legume nodules was investigated by supplying nitrate to well established soybean (Glycine max L. Merr. cv Bragg)-Rhizobium japonicum (strain 3I1b110) symbioses. Three different techniques, acetylene reduction, 15N2 fixation and relative abundance of ureides ([ureides/(ureides + nitrate + .alpha.-amino nitrogen)] .times. 100)in xylem exudate, gave similar results for the effect of nitrate on N2 fixation by nodulated roots. After 2 days of treatment with 10 millimolar nitrate, acetylene reduction by nodulated roots was inhibited by 48% but there was no effect on either acetylene reduction by isolated bacteroids or in vitro activity of nodule cytoplasmic glutamine synthetase, glutamine oxoglutarate aminotransferase, xanthine dehydrogenase, uricase, or allantoinase. After 7 days, acetylene reduction by isolated bacteroids was almost completely inhibited but, except for glutamine oxoglutarate aminotransferase, there was still no effect on the nodule cytoplasmic enzymes. It was concluded that, when nitrate is supplied to an established symbiosis, inhibition of nodulated root N2 fixation precedes the loss of the potential of bacteroids to fix N2. This in turn precedes the loss of the potential of nodules to assimilate fixed N2.