Nodulation competitiveness between contrasting phage phenotypes of pigeonpea rhizobial strains.

Competitiveness between (I) lysogenic vs. phage-indicator strains, (II) phage-resistant vs phage-sensitive strains, and (III) large plaque vs. small plaque developing strains was examined under laboratory and field conditions in order to study the involvement of these crucial phage sensitivity patterns in the competition for nodule occupancy of pigeonpea rhizobia. The phage-indicator strain (A039) exhibited higher competitiveness over the lysogenic strain (A025 Sm(r)); the phage sensitive strain (IHP-195) over the phage resistant strain (IHP 195 Sm(r)V(r)); and the large plaque developing strain (A059) over the small plaque developing strain (IHP195 Sm(r)) in association with pigeonpea cv. bahar both under laboratory and field conditions. Dual inoculation of A025 Sm(r) + A039 and A059 + IHP195 Sm(r) (mixed in equal proportion just before treatment) improved the nodule occupancy by inoculant strains against native rhizobia and resulted into higher plant dry weight and yield as compared to their application as single inoculum. The phage-resistant mutant IHP195 Sm(r)V(r) showed reduced competitiveness against native rhizobia, compared to its parental strain. The dual inoculation of parental strain and phage-resistant mutant gave the same result as the inoculation of parental strain alone.

Diverse effects of formate on the assimilatory metabolism of nitrate and nitrite in Rhizobium.

Two strains οf Rhizobium, cowpea Rhizobium 32H1 and Rhizobium japonicum CB 1809, showed a marked stimulation in growth on addition of formate to the minimal medium containing nitrate as the sole source of nitrogen. The amount of accumulated nitrite and specific nitrate reductase activity was much higher in cultures supplemented with formate than in the control medium. In contrast, growth, consumption of nitrite and specific nitrite reductase activity in minimal medium + nitrite was greatly reduced by the addition of formate. A chlorate resistant mutant (Chl-16) was isolated spontaneously which contained a nitrite reductase which was not inhibited by formate. The results suggest that formate serves as an electron donor for nitrate reductase and inhibits nitrite assimilation in Rhizobium.