Ecological studies of root-nodule bacteria introduced into field environments. 4. Symbiotic properties of Rhizobium japonicum and competitive success in nodulation of two Glycine max cultivars by effective and ineffective strains

Abstract

Rhizobium japonicum strain evaluation experiments with soybean (Glycine max) confirmed that strain CB1809 was ineffective in nitrogen fixation with the related cultivars Hardee and Geduld but was highly effective with cv. Hampton. There were no major symbiotic differences between the cultivars with other strains. Three strains including CB1809 were used, singly, in pairs, or all together, as inocula for Hardee and Hampton in competition studies in the field. The ratio of strains recovered from nodules was used as an index of competitive success. A few nodules contained more than one strain. There was a marked host x strain interaction in nodulating competitiveness related to symbiotic effectiveness, the order of competitive success being CB1809 > CC709 > CB1795 for Hampton and CC709 > CB1795 > CB1809 for Hardee. With Hardee, strain CB1809, although unsuccessful itself in forming nodules in the presence of the other inocula, appeared to suppress nodule formation by those strains. Nodulation by CB1795 in the presence of CB1809 was suppressed to such an extent that plant growth was retarded and not different from that of uninoculated control plants, although CB1795 alone nodulated Hardee abundantly and effectively. There was no evidence that naturally-occurring ineffective R. japonicum posed any competitive threat to the nodulating ability of effective inoculant strains.