Differential traits of rhizobia associated to root-nodules of gum acacia (Senegalia senegal), shittah tree (Vachellia seyal), pigeon pea (Cajanus cajan L) and cowpea (Vigna unguiculata)

In the present study, rhizobia partners of gum acacia (Senegalia senegal L.), shittah tree (Vachellia seyal), pigeon pea (Cajanus cajan L.) and cowpea (Vigna unguiculata L.) were characterized for their specific traits to each of the three host plants. In total 12 soil samples were collected and used to trap root-nodule rhizobia. Averagely, 24 rhizobia strains were isolated, of which 3, 4, 6 and 11 were respectively from root-nodules of S. senegal, V. seyal, C. cajan and V. unguiculata. Isolates were morphologically rod-shaped and measured 2.5-3.2 mm in diameter. They were wet (22), wet/sticky (01) and wet/creamy (01) in appearance. Isolates (23) were convex, and flat (1) in elevation. They were all Gram-negative, and none form endospores. They indicated positive reactions to ammonia and indole acetic acid tests, but showed various reactions to catalase, phosphate solubilization, starch, triple sugars ion agar and urea tests. Nodulation and nitrogen fixation capacity were confirmed by either pink or brown internal coloration of nodules. Inoculation of hosts with isolates significantly (p < 0.0001) increased both the plant and nodule weights compared to that of control. This study has revealed that all the identified isolates possess are Rhizobium, belonging to the same cross-inoculation group, thus may be useful in increasing the symbiotic nitrogen fixation in the studied legumes. Theses findings provide the basis for further research on the phylogeny of rhizobia strains nodulating legumes, as well as their use as potential inoculants to improve production in semi-arid lands such as those of the northern Cameroon.