C-N BALANCE, NITRATE AND NITRITE REDUCTASES IN BRADYRHIZOBIUM SP (LUPINUS) NODULES EXPOSED TO NITRATE, AND THE RELATIONSHIPS TO NITROGENASE ACTIVITIES

Two Bradyrhizobium sp. (Lupinus) strains (750 and IM 43B) were inoculated on Lupinus luteus and L. mutabilis seedlings planted in Leonard jars. Nitrate at 7 mM N was added and removed from the solutions during the course of plant growth arranged in 4 different test treatments. All of the nitrate treatments significantly increased the C/N in the nodules cytosol; contrarily, most of the treatments depleted C/N in bacteroids, mainly after blooming started. The supply of nitrate augmented total N and NO3- contents in the shoots. Nitrogenase activities (EC. 1.18.6.1) completely recovered after combined N disappeared from the media, and its early presence did not affect the rates of N2 fixation; otherwise, nitrogenase relative efficiencies improved in nitate-treated nodules at the late control. Nitrate and nitrite reductase activities (EC. 1.6.6.1 and EC. 1.7.7.1) were sharply lowered along the course of plant growth, the plant species significantly influenced both activities in the bacteroids, and the rhizobia strain affected them in the cytosol; therefore, some kind of communication seems to be present between both symbionts to stimulate reduction of combined N in the nodules.