Interactions ofRhizobium japonicum strains in soybean nodules and their contribution to nitrogen fixation

Rhizobium japonicum strain 8-0 StrR applied as inoculum to Clark 63 soybeans formed small ineffective nodules which had very low nitrogenase activity compared to nodules formed by two effective strains, 110 TetR and 138 KanR. Mean numbers of cells per milligram of nodule tissue for plants up to 34 days old were 7.7×106 for 8-0 StrR, 4.1×108 for 110 TetR and 7.6×108 for 138 KanR. Cell counts per unit mass of nodule were independent of plant age for strains 110 TetR and 138 KanR, however, for strain 8-0 StrR, 25 and 34 days old plants had fewer viable cells per nodule mass than 18 day old plants. When a mixture of two effective strains was used, the nodules of individual plants were predominantly caused by either 110 TetR or 138 KanR. In one experiment the predominance was random, but in another, strain 110 TetR clearly dominated. Strain 138 KanR was absent in some nodules on 18 day old plants, and in others, less than 102 cells per nodule were found. When strains 8-0 StrR and 138 KanR were used as mixed inoculum, most of the nodules had strain 8-0 StrR but strain 138 KanR was detected in many nodules and was generally evident in the largest nodules. Nitrogenase activity by many individual nodules was low except for nodules which had cells of 138 KanR. Nitrogenase activity by whole root systems of these plants was relatively high and similar to plants that had only nodules of strain 138 KanR. Similar relationships were observed for a mixed inoculum of 8-0 StrR and 110 TetR. In general, mixed inoculations resulted in nodules with a particular strain being dominant for each individual plant. Double infections within individual nodules were not uncommon and such nodules often had disproportionate numbers of cells of two competingR. japonicum strains.