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Nitrogen is fixed biologically by widespread soil rhizobacteria like Azotobacter, Gluconacetobacter, Azospirillum, etc. These rhizobacteria proliferate in the host
plant's rhizosphere, fix atmospheric nitrogen and supply it to the plants. Resistance to azide
can be used as a potentially useful method to select efficient strains to be used as
biofertilizer for various crops (Yadav et
al., 1999 and 2000).
Sodium azide, a potent inhibitor of
terminal segment of electron transport chain, can be reduced to ammonia and dinitrogen by the
enzyme nitrogenase. The process is ATP-dependent and requires a strong reducing agent
during nitrogen fixation and also some of the toxic compounds like cyanide can also be
converted into harmless compounds. Resistance to azide has also been used as one of the
important methods to isolate spontaneous mutants of Rhizobium with enhanced nitrogen fixing
ability (Ram et al., 1978). Recently, azide resistant mutants of Rhizobium were isolated, which are more competitive than the native rhizobia under natural conditions (Yadav et al., 1992). Azide is also known to affect electron transport chain by inhibiting the cytochrome c oxidase
enzyme. In Azospirillum brasilense, cytochrome c oxidase is required under microaerobic
conditions when high respiration rate is needed (Marchal et al., 1998).
Studies on the rate
of consumption of dissolved oxygen by suspension of bacteroids from soybean root
nodules showed the presence of two terminal oxidase systems (Bergerson and Turner,
1975).
These enzymes are oligomeric, cytochrome containing complexes and are used for energy
requiring reactions such as synthesis of ATP. High affinity pathways produce up to five times
greater ATP concentration in bacteroids than the low affinity pathways. The differential
respiratory activity of azide resistance strains may be influenced by the variable activity of
cytochrome c oxidase and the amount of ATP produced. Keeping the above facts in view, this study
was undertaken to determine the azide resistance and its possible relationship to the rate
of respiration, cytochrome c oxidase activity, ATP concentration and nitrogen fixing ability
among important soil bacteria such as Azotobacter, Gluconacetobacter and Azospirillum.
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