Structural and physiological bases for effectivity of soybean nodules formed by fast-growing and slow-growing bacteria

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Abstract
Five weeks after planting, the total dry weight of the association between the soybean cultivar Maple Arrow and the slow-growing rhizobia, Bradyrhizobium japonicum USDA 16, was 1.6 times that with the fast-growing rhizobia, Rhizobium fredii QB1130. In contrast, with the uncultivated variety, 'Peking', the USDA 16 association possessed only 0.58 of the dry weight of the QB1130 association. These growth characteristics could not be fully explained in terms of number or mass of nodules formed. Rather, the specific nodule activity (rate of H2 evolution in Ar–O2 per unit nodule mass) was lower in the poorly effective symbioses ('Maple Arrow' × QB1130; 'Peking' × USDA 16). Nodules of the less effective symbioses were characterized by large deposits of starch in the cortex and in uninfected cells of the central zone, a thick cortical zone, relatively less infected cell area in the central zone, and fewer bacteria per unit crosssectional area of infected cell. Therefore, there were fewer bacteria per nodule in these symbioses. When total nitrogenase activities were expressed per bacteroid number or per bacteroid volume, no significant differences were observed among the four symbioses. These results indicate that the poor effectivity of the 'Maple Arrow' × QB1130 and 'Peking' × USDA 16 symbioses was due to a limitation of either bacterial release from the infection thread or bacterial proliferation within the infected cells.