Screening of Rhizobium Strains Isolated from the Root Nodules of Vigna mungo Cultivated in Rice Fallows for Their Phosphate Solubilizing Ability and Enzymatic Activities

Aims: The present work is aimed to find out the enzymatic activities and phosphate solubilizing efficiency of indigenous rhizobia confined to rice fallows.

Study Design: In this experiment we maintained random block design (RBD).

Place and Duration of Study: This work was carried out in the Department of Botany and Microbiology, Acharya Nagarjuna University between October 2012 and December 2013.

Methodology: In this study, we have isolated 19 Rhizobium strains collected from the healthy root nodules of Vigna mungo cultivated in rice fallows on yeast extract mannitol agar (YEMA) medium. The strains were confirmed as Rhizobia by using Gram staining, growth on YEMA with congo red, growth in Hofer’s alkaline broth, growth on glucose peptone agar, acid production, ketolactose test and nodulating ability was tested on homologous hosts by plant infection tests. Phosphate solubilization ability of the isolated Rhizobium strains were carried out Pikovskaya’s agar medium.

Results: Eight out of 19 strains tested for phosphate solubilizing ability on Pikovskaya’s agar medium containing tri calcium phosphate (TCP) as insoluble phosphate source showed zone of TCP solubilization. The strain VM-2 exhibited maximum solubilization after 48h of incubation, while least activity was found with VM-11. Effect of different carbon and nitrogen sources on phosphate solubilizing ability of Rhizobial strains was tested and maximum phosphate solubilization (799µg/ml) by VM-2 was observed when glucose and ammonium sulphate were used as carbon and nitrogen sources.

Conclusion: In this study it is concluded that along with symbiotic nitrogen fixtation, some Rhizobium species were found to be involved in phosphate solubilization and this ability of phosphate solubilization by the Rhizobium strains can be exploited as PGPR.