Effects of Plant Growth Promoting Rhizobacteria on the Performance of Greengram under Field Conditions

Despite reports on effects of single Plant Growth Promoting Rhizobacteria (PGPR) inoculation on legumes, response of greengram to combined inoculation with phosphate solubilizing asymbiotic Azotobacter and ACC deaminase positive symbiotic Bradyrhizobium sp. (vigna) under field conditions remains unexplored. The present study aims at identifying ACC deaminase producing and phosphate solubilizing bacterial strains and to assess their impact on greengram plants in order to find efficient and friendly co-cultures for developing effective bioinoculants for increasing sustainable production of legumes. Additionally, plant growth promoting activities of Azotobacter and Bradyrhizobium sp. (vigna) were determined using standard methods. The isolated bacterial cultures were characterized morphologically, culturally and biochemically and were identified as Bradyrhizobium sp. (vigna) and Azotobacter chroococcum. Dry matter accumulation in whole plants, symbiotic attributes, nutrient uptake and grain yields were significantly enhanced following co-inoculation of A. chroococcum and Bradyrhizobium sp. (vigna).The inoculation of Azotobacter with Bradyrhizobium increased seed yield by two fold and produced the highest grain protein. A- 75% and 52% increase in P concentration in root and shoots, respectively was observed for A. chroococcum, while P uptake was highest (0.52 mg/g) in shoots following combined inoculation of A. chroococcum with Bradyrhizobium at harvest. The highest N concentration in roots and shoot at harvest were observed with co-culture of A. chroococcum and Bradyrhizobium sp. (vigna). Gram negative Azotobacter and Bradyrhizobium solubilized insoluble phosphate, synthesized indole acetic acid, ammonia, cyanogenic compounds and exopolysaccharides while only Bradyrhizobium showed ACC deaminase activity. The results suggest that two unrelated bacteria belonging to symbiotic and asymbiotic group and capable of facilitating greengram production under field conditions and expressing multiple plant growth promoting activity can be used to produce composite bioinoculants for enhancing greengram production while saving the use of fertilizers.