In the absence of exogeneous nitrogen supply, evaluation of a symbiosis effectiveness of Bradyrhizobium japonicum USDA 110 in a supernodulating soybean mutant, SS2-2, its wild type, Sinpaldalkong 2, and control genotype, Jangyeobkong, was conducted in this study. Nodules in SS2-2 were initially white and similar to its wild type, Sinpaldalkong 2. At the late stage, the wild type nodules became dark pinkish by maturation, by contrast, mature nodules in SS2-2 remained light green to pinkish, indicating a lack of leghemoglobin. Tap root length was short in nodulated symbiotic SS2-2 than that of its wild type and the control genotype. Nodulated root length and nodule density on root length were significantly increased by B. japonicum inoculation, but no significant increase was observed on root length and percentage of nodulation to total root length. Regardless of Bradyrhizobium inoculation, SS2-2 showed higher nodule dry weight and higher acetylene reduction activity (ARA) when compared with its wild type and the control genotype. Inoculation of B. japonicum leaded the increase of ARA in 47 days after planting (DAP), in part because of nodule development. Supernodulating mutant, SS2-2, less responded to B. japonicum induction in terms of nitrogen fixation and nodulation characteristics than its wild type. Thus, interaction of supernodulating soybean mutant with Bradyrhizobium had less symbiotically associated response than normal nodulating soybean.

Nodule development was regulated partially by host plant factors originating in the shoots and roots. This study was performed to identify the origin of the factors regulating nodulation in supernodulating soybean (Glycine max [L.] Merr.) mutant 'SS2-2' which was isolated recently from ethyl methanesulfonate (EMS) mutagenesis of 'Sinpaldalkong 2'. Self- and reciprocal-grafts were made among three soybean genotypes which consisted of two supernodulating mutants, SS2-2 and 'nts 382', and a normal nodulating Sinpaldalkong 2. Self-grafted supernodulating mutants were characterized by greater nodule number, nodule dry weight, and C2 H2 reduction activity than self-grafted wild types. They were also characterized by relatively higher nodule to root dry weight. Significant shoot genotypic effects were observed on nodule number, nodule dry weight, and C2~;H2 reduction activity per plant, whereas varying root genotypes had no effects. From this result, it is surmised that supernodulating characters are controlled by a graft-transmissible shoot factor, and mutant SS2-2 may have similar nodulation mechanism to the former supernodulating nts 382. In all grafts, both supernodulating mutants and Sinpaldalkong 2 maintained the similar balance between above ground and below ground parts regardless of significant differences in partitioning of dry matter into root and nodule between supernodulating mutants and Sinpaldalkong 2.