Symbiotic efficiency of HUP+, HUPHR and HUP- Bradyrhizobium japonicum and Bradyrhizobium elkanii strains in cowpea cultivars

The effectiveness of Bradyrhizobium strains with characteristics Hup(+) (SR and USDA-110), Hup(-) (29W) and Hup(hr) (SEMIA-587), was evaluated in cowp ea (Vigna unguiculata L.), cultivars IPA-202, BR-3 and VITA-4. Results show ed that nodulation in VITA-4 was superior, with a positive interaction when inoculated with strains SEMIA-587 and USDA-110. However, when comparing no dule efficiency, the combination IPA-202 x SEMIA-587 presented the highest nitrogenase activity, with a relative efficiency around 1.0. Nitrogenase ac tivities were similar in plants inoculated with the strains SR and SEMIA-58 7, and higher, with strains USDA-110 and 29W. These results indicate that s trains with Hup+ and Hup(hr) were able to achieve higher enzymatic activiti es. Leghemoglobin (Lb) content detected in strains SR and USDA-110 was posi tively 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 ab sence of hydrogenase may affect the flow of Lb to the bacteroids. Significa nt differences on the plant N content were found among the strains studied, but not among cultivars. Plant inoculated with Hup(+) strains were more ef ficient to accumulate N, indicating that N-fixing systems retaining H-2 may accumulate additional N in the host.