Increased Soybean Productivity with a Rhizobium japonicum Mutant1

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Abstract
A mutant of Rhizobium japonicum strain USDA 110 with approximately 100% greater C2H2‐reduction capacity in free‐living culture than the wild‐type strain was analyzed on soybean [Glycine max (L.) Merr., ‘Clark’] under controlled environmental and field conditions. The mutant strain, C33, which also was deficient in nitrate reductase activity, increased dry weight and N content approximately 40% (P ≤ 0.05) relative to the wild type 110 after 45 days in plants grown under a photosynthetic photon flux density (PPFD) of 1,500 (µmol/m2 sec with 2mM KNO3. Similar trends, but fewer significant bacterial effects, were measured in plants grown under a PPFD of 700 µmol/m2 sec with 2 mM KNO3. In field trials, strain C33 increased the 2‐year mean of seed yield and seed N content significantly (P ≤ 0.05) compared with the wild‐type strain 110. The promotive effects on seed yields and seed N content were about 12%, but 9 to 10% increases in above‐ground biomass and N content at the R5 growth stage associated with strain C33 were not significant. A greater promotive effect of strain C33 on seed yield and seed N content during the second year in the field was associated with low soil N availability. Estimates of seasonal N2 fixation, based on both the 15N‐dilution assay and a difference in total N relative to a non‐nodulating control, showed that both Rhizobium strains fixed more N2 in the second year. During both years more than 80% of the bacterial isolates from root nodules of plants inoculated with strain C33 were identified as C33 on the basis of bacteriophage sensitivity and resistance to chlorate. The results indicated that, although it was possible to select a symbiotically superior mutant of Rhizobium japonicum in free‐living culture, the beneficial effects produced in the plant under controlled environments were variable, and promotive effects in the field could be measured only under conditions of low soil N availability.