Physiological Characteristics of Cowpea Rhizobia: Evaluation of Symbiotic Efficiency in Vigna unguiculata

One fast-growing and three slow-growing strains of Rhizobium (isolated from cowpeas) were evaluated for symbiotic performance on Vigna unguiculata (L.) Walp. cultivar California no. 5 blackeyes. Plants inoculated with slow-growing strains 176A22, 176A30, and 176A32 developed a maximum acetylene reduction activity of 24.6, 27.0, and 32 μmol of ethylene formed per plant per h, respectively, versus 6.4 μmol per plant per h in plants inoculated with the fast-growing strain 176A28. When inoculated with approximately equal proportions of rhizobia, the fast-growing strain 176A28 produced 95% of the nodules when challenged with the slow-growing strain 176A22, but formed only 6% of the nodules when challenged with the slow-growing strain 176A30. Consequently, there was no relation between the growth rate in vitro and the capability of rhizobia to compete for nodule-forming sites. Plants inoculated with strain 176A28 and subjected to drought during the vegetative growth period recovered to the same level of nitrogen fixation and nodulation as those that received adequate irrigation. On the other hand, plants inoculated with strains 176A22, 176A30, and 176A32 failed to achieve the same levels of nodulation and nitrogen fixation under drought as compared with irrigated conditions.