Associative diazotrophs of pearl millet (Pennisetum glaucum) from semi arid region--isolation and characterization.

Diversity of the native diazotrophs associated with the rhizosphere of pearl millet (P. glaucumn), grown in nutritionally poor soils of semi-arid regions was studied with a view to isolate effective nitrogen fixing and plant growth stimulating bacteria with root associative characteristics. The native population varied from 10(3)-10(4) g(-1) of rhizosphere soil after 40 d growth and belonged to genera Azospirillum, Azotobacter and Klebsiella. Another non-diazotrophic root associative group was Pseudomonas sp., which also produced IAA and enhanced plant growth. Some of these rhizobacteria showed high in vitro acetylene reduction activity along with production of indole acetic acid. Out of 11 selected diazotrophs used as seed inoculants, M10B (Azospirillum sp.), M11E (Azotobacter sp.) and M12D4 (Klebsiella sp.) resulted in significant increase in total root and shoot nitrogen at 45 and 60 days of plant growth under pot culture conditions.

Symbiotic effectiveness of bacteriocin producing and non-producing strains of Rhizobium in green gram (Vigna radiata).

Rhizobium strains nodulating green gram [Vigna radiata (L.) Wilczek] were found to produce bacteriocin on modified Bergersen's medium and inhibited the growth of homologous Rhizobium strains. Four bacteriocin producing and four bacteriocin non-producing strains were compared for their effect on nodulation, in planta nitrogenase activity and plant dry weight of green gram. The bacteriocin producers formed more nodules in comparison to non-bacteriocin producers. However, the symbiotic effectiveness of bacteriocin producers was less in terms of plant dry weight in comparison to non-bacteriocin producers.

Rhizobium strains expressing uptake hydrogenase in different host species of cowpea miscellany.

Uptake hydrogenase activity in nodules of green gram (Vigna radiata (L.) (Wilczek)), black gram (Vigna mungo (L.) (Hepper)), cowpea (Vigna unguiculata (L.) and cluster bean (Cyamopsis tetragonoloba (L.) (Taub.)), formed with two Hup+ (S24 and CT2014) and one Hup– (M11) Rhizobium strains, was determined at different levels of external H2 in air atmosphere. Nodules of all the 4 host species formed by inoculation with strains S24 and CT2014, showed H2 uptake but not those formed with strain M11. H2 uptake rates were higher in 1 and 2% H2 in air atmosphere (v/v) than at 5 or 10% levels in all the host species. Variations in the relative rates of H2 uptake were observed both, due to host species as well as due to Rhizobium strains. However, no host dependent complete repression of the expression of H2 uptake activity was observed in nodules of any of the host species formed with Hup+ strains.

Nitrification and simultaneous denitrification by Azospirillum brasilense 12S.

Azospirillum brasilense, an associative diazotrophs from sorghum roots grows autotrophically on NH + 4 and CaCO3. NH + 4 a is also oxidized to NO – 2 and then denitrified. Addition of malate to the autotrophic medium enhances both NH + 4 oxidation as well as NO – 2 dissimilation. The incomplete nitrification linked denitrification results in a rapid loss of nitrogen from the growth medium. The bacterium also shows assimilatory NO3− and NO 2 reductases and fixes nitrogen at < 50 μg N/ml of NH + 4 , NO−3 or NO-2.

In vitro and in vivo nitrogenase activity of Rhizobium mutants and their symbiotic effectivity.

A slow growing nitrogen-fixing strain of Vigna radiata var. aureus (mung bean) Rhizobium which expressed nitrogenase activity in a synthetic medium was isolated from its native population. Mutants with decreased and increased nitrogenase activity were derived from this strain by treatment with acridine orange and ethidium bromide. These mutants were tested for symbiotic effectivity in vivo. The effectivity of mutants with decreased nitrogenase activity in the culture medium was lower than the parent strain; however, the effectivity of mutants with higher nitrogenase activity did not increase above that of the parent. This suggests that the plant is perhaps a limiting factor in the full expression of rhizobial nitrogenase in the nodules.