Effect of Rhizobium japonicum Mutants with Enhanced N2 Fixation Activity on N Transport and Photosynthesis of Soybeans during Vegetative Growth1

Abstract

The effects of two mutant strains of Rhizobium japonicum with enhanced capacities N2 fixation, on ureide transport, photosynthesis,root respiration, and growth in soybeans [Glycine max (L.) Merr. ‘Wells’] were examined during vegetative growth. Three‐week‐old plants inoculated with the mutants were larger and had higher rates of acetylene reduction, photosynthesis, xylary ureide transport, leaf allantoinase activity and higher concentrations of ureides in shoot tissues than did plants inoculated with the wild‐type strain. The relative composition of xylem sap N was unaffected by Rhizobium strain. Ureides comprised over 90% of the sap N in all treatments. After 5 weeks growth, however, there were no statistically significant differences among treatments in rates of the physiological processes and growth parameters examined. Cross contamination of nodules containing the mutant strains with the wild‐type strain was not responsible for the apparent loss of the initial advantages to the host plant (in terms of N2 fixation and growth) brought about by the use of the mutant strains. The use of modifications in Rhizobium genetics for improving the dependency of the host plant on N2 fixation for its N requirements is briefly discussed.