DENITRIFICATION IN LUCERNE NODULES IS NOT INVOLVED IN NITRITE DETOXIFICATION

Dissimilatory reduction of ionic nitrogen oxides to gaseous forms such as nitrous oxide or nitrogen can be carried out by free living or sym biotic forms of some strains of Rhizobium meliloti. In this paper we i nvestigate whether bacteroid denitrification plays a role in the allev iation of the inhibitory effects of nitrate on nitrogen fixation both in bacteroid incubations as in whole nodules. The presence of a consti tutive nitrate reductase (NR) activity in isolated bacteroids caused n itrite accumulation in the incubation medium, and acetylene reduction activity in these bacteroids was progressively inhibited, since nitrit e reductase (NiR) activity was unable to reduce all the nitrite produc ed by NR and denitrification occurred slowly. Even nodules infiltrated with nitrate and nitrite failed to increase gaseous forms of nitrogen substantially, indicating that nitrite availability was not limiting denitrification by bacteroids. In spite of the low rates of bacteroida l denitrification, the effect of nodule denitrification on the inhibit ion of nitrogen fixation by nitrate in whole plants was tested. For th at purpose, lucerne plants (Medicago sativa L. cv. Aragon) were inocul ated with two Rhizobium meliloti strains: 102-F-65 (non denitrifying) and 102-F-51 (a highly denitrifying strain). After a seven days nitrat e treatment, both strains showed the same pattern of inhibition, and i t occurred before any nitrate or nitrite accumulation within the nodul es could be detected. This observation, together with the lack of alle viation of the ARA inhibition in the denitrifying strain, and the limi ted activity of dissimilatory nitrogen reduction present in these bact eroids, indicate a role other than nitrite detoxification for denitrif ication in nodules under natural conditions.