EFFECT OF NITRATE ON CARBON METABOLISM AND NITROGEN-FIXATION IN ROOT-NODULES OF LUPINUS-ALBUS

Supply of increasing concentrations of nitrate in long-term experiments to lupin plants (Lupinus albus L. cv Multolupa) inoculated with Bradyrhizobium sp. (Lupinus) stimulated specific acetylene reduction activity (ARA). The inhibitory effect of nitrate was not apparent with concentrations of less than 10.5 mM, showing a great tolerance of lupin nodules to nitrate. A positive correlation between ARA and the specific activities of nodule cytosol phosphoenolpyruvate carboxylase (EC 4.1.1.31, PEPC), aspartate aminotransferase (EC 2.6.1.1, AST) and alanine aminotransferase (EC 2.6.1.2, ALT) was found. Supply of nitrate (20 mM) in short-term experiments inhibited the ARA after 1 day of treatment. Following nitrate treatment, sucrose concentration decreased in the nodules, whilst organic acids oxaloacetate and alpha-ketoglutarate increased in bacteroids and malate accumulated in the nodule cytosol. The specific activities of cytosol enzymes were not affected until nodule senescence had set in. However, the specific activities of bacteroid malate dehydrogenase (EC 1.1.1.37, MDH) and AST rose transiently after nitrate treatment. The results provide evidence for anaerobic conditions in the nodules under high nitrate supply.