Differential symbiotic efficiency by shading of soybean nodulated by B. japonicum and B. elkanii strains

Symbiotic efficiency of soybean Glycine max L. cv. Doko nodulated by BR-33 (CB 1809) strain of Bradyrhizobium japonicum or BR-29 (29W) strain of B. elkanii was investigated under two light intensities: full sunlight or 30% of full sunlight. Plants were harvested during pod-filling and at maturity. Soybeans cultivated under full sunlight and inoculated with strain BR-33 showed higher shoot dry mass and nitrogen accumulation, nitrogenase activity, ureide-N in the xylem sap and grain yield than plants inoculated with strain BR-29, confirming the superior symbiotic effectiveness of strain BR-33. In contrast, soybean submitted to shading treatment showed no significant differences regardless of the strain used as inoculant. When submitted to shading, plants inoculated with BR-29 strain showed an increase in most of the variables observed while plants inoculated with strain BR-33 were not significantly affected by the level of shading. The results suggest that strain efficiency used as an inoculant for soybean can be different in plants cultivated either in the greenhouse or in the field. This can help to explain why the potential of hydrogenase activity to improve symbiotic effectiveness is still a matter of controversy when comparing results from different experiments.