ENHANCED COMPETITIVENESS OF A BRADYRHIZOBIUM-JAPONICUM MUTANT STRAIN IMPROVED FOR NODULATION AND NITROGEN-FIXATION

Bradyrhizobium japonicum strain TA-11NOD(+), with altered indole biosy nthesis, exhibited enhanced nodulation and nitrogen fixation on soybea n in previous greenhouse studies. In this study, field experiments wer e conducted at Upper Marlboro, Maryland, in the summers of 1988 and 19 93. In 1988, the site used was essentially free of soybean-nodulating bacteria and seed yield in plots inoculated with either I-110ARS or TA -11NOD(+) was significantly higher by 12 or 20%, respectively, than th at of the uninoculated controls. The 1993 site had an indigenous soil population (about 10(4) cells g(-1)) of symbiotically ineffective soyb ean-nodulating bacteria. Nevertheless, six-week-old 'Morgan' soybean p lants inoculated with strain TA-11NOD(+) had 44% more nodules and exhi bited 50% more nitrogen fixation by acetylene reduction when compared with plants that received the parental strain I-110ARS. Nodule occupan cy, as determined using genetic markers for rifampicin and streptomyci n resistance, was significantly higher for strain TA-11NOD(+) than for strain I-110ARS. Overall, for the two years and the two soybean genot ypes, the yield obtained with TA-11NOD(+) was 6% higher than that obta ined with I-110ARS. Competition experiments were conducted in the gree nhouse and strain TA-11NOD(+) was significantly more competitive than strain I-110ARS in competition with strains USDA 6 or USDA 438.