Symbiotic efficiency of Hup+, Huphr and Hup- Bradyrhizobium japonicum and Bradyrhizobium elkanii strains in cowpea cultivars

Abstract

The effectiveness of Bradyrhizobium strains with characteristics Hup⁺ (SR and USDA-110), Hup^- (29W) and Hup^hr (SEMIA-587), was evaluated in cowpea (Vigna unguiculata L.), cultivars IPA-202, BR-3 and VITA-4. Results showed that nodulation in VITA-4 was superior, with a positive interaction when inoculated with strains SEMIA-587 and USDA-110. However, when comparing nodule efficiency, the combination IPA-202 x SEMIA-587 presented the highest nitrogenase activity, with a relative efficiency around 1.0. Nitrogenase activities were similar in plants inoculated with the strains SR and SEMIA-587, and higher, with strains USDA-110 and 29W. These results indicate that strains with Hup⁺ and Hup^hr were able to achieve higher enzymatic activities. Leghemoglobin (Lb) content detected in strains SR and USDA-110 was positively correlated to nitrogenase activity. However, correlations between Lb and nitrogenase activity were highly variable in nodules containing strain 29W-Hup^-, indicating that imbalances in the nitrogenase system in the absence of hydrogenase may affect the flow of Lb to the bacteroids. Significant differences on the plant N content were found among the strains studied, but not among cultivars. Plant inoculated with Hup⁺ strains were more efficient to accumulate N, indicating that N-fixing systems retaining H₂ may accumulate additional N in the host.