Isolation and characterization of native cowpea rhizobia from Wayanad India

In the present study, eight rhizobial strains (RH-1 to RH-8) were isolated from root nodules of cowpea which grows in acidic soils of Wayanad. They were characterized based on morphological, physiological, biochemical and molecular characteristics. The results on plant growth promoting traits showed all isolates positive to IAA production, four positive to ammonia production and five positive for in vitro siderophore production. Three isolates exhibited antagonistic activity and none of them showed volatile cyanogen production. The 16S rRNA gene sequence analysis revealed all the isolates to show similarity to Rhizobium sp. While rhizobia generally have a pH range of 6.5–7.5 for optimum growth, the isolates RH-1. RH-2, RH-4 and RH-8 grew in a liquid yeast extract-mannitol agar medium at pH-4, suggesting acid tolerance in the strain of Rhizobium. The isolates were authenticated for their nodulation and growth promotion of cowpea in a paper cup experiment and four isolates (RH-2, RH-3, RH-4 and RH-5) were evaluated for their growth promotion in cowpea under field condition. It was observed that inoculation of Rhizobium increased nodulation, growth parameters and yield of cowpea compared to uninoculated control. This study showed that these rhizobial isolates with plant growth promoting traits can be used to promote the growth and yield of cow peain acidic soils of Kerala.