Mechanism of Nitrate Tolerance in Tn5 Mutants of Cicer-Rhizobium Strains*

The activities of nitrate reductase (NR) and nitrite reductase (NiR) and production of indole-3-acetic acid (IAA) by symbiotic nitrate tolerant Tn5 mutant AC-10 of Cicer-Rhizobium strain F-75 and mutants BC-35 and BC-46 of strain G36–84 developed earlier, have been studied under ex planta condition. The rhizobial mutants and their parental strains were grown with nitrate (0.0, 0.5, 1, 2 or 4 mM), aeroblcally and microaeroblcally. The overall activities of NR were 70–91% lower in aerobically grown and 78–87% lower in microaerobically grown mutant cells compared to their parental strains. Similarly, the overall activities of NiR were 36–55% and 27–37% lower in aerobically and microaerobically grown mutant cells, respectively, compared to their parental strains. On the contrary, the overall production of IAA In the culture medium by aerobically grown mutant cells was significantly higher compared to their parental strains. Based on these results, it has been suggested that impaired NR activity and a favourable NIR/NR ratio preventing nitrite accumulation in the rhizobial mutants, may be responsible for imparting nitrate tolerance to chickpea - Rhizobium symbiotic system.