Potential for plant growth promotion in groundnut (Arachis hypogaea L.) cv. ALR-2 by co-inoculation of sulfur-oxidizing bacteria and Rhizobium.

The use of Rhizobium inoculant for groundnut is a common practice in India. Also, co-inoculation of Rhizobium with other plant growth-promoting bacteria received considerable attention in legume growth promotion. Hence, in the present study we investigated effects of co-inoculating the sulfur (S)-oxidizing bacterial strains with Rhizobium, a strain that had no S-oxidizing potential in groundnut. Chemolithotrophic S-oxidizing bacterial isolates from different sources by enrichment isolation technique included three autotrophic (LCH, SWA5 and SWA4) and one heterotrophic (SGA6) strains. All the four isolates decreased the pH of the growth medium through oxidation of elemental S to sulfuric acid. Characterization revealed that these isolates tentatively placed into the genus Thiobacillus. Clay-based pellet formulation (2.5 x 10(7) cf ug(-1) pellet) of the Thiobacillus strains were developed and their efficiency to promote plant growth was tested in groundnut under pot culture and field conditions with S-deficit soil. Experiments in pot culture yielded promising results on groundnut increasing the plant biomass, nodule number and dry weight, and pod yield. Co-inoculation of Thiobacillus sp. strain LCH (applied at 60 kg ha(-1)) with Rhizobium under field condition recorded significantly higher nodule number, nodule dry weight and plant biomass 136.9 plant(-1), 740.0mg plant(-1) and 15.0 g plant(-1), respectively, on 80 days after sowing and enhanced the pod yield by 18%. Also inoculation of S-oxidizing bacteria increased the soil available S from 7.4 to 8.43 kg ha(-1). These results suggest that inoculation of S-oxidizing bacteria along with rhizobia results in synergistic interactions promoting the yield and oil content of groundnut, in S-deficit soils.