Effects of Plasmid Content in Rhizobium leguminosarum on Pea Nodule Activity and Plant Growth

Abstract

Summary: Pea plants that were nodulated by Rhizobium leguminosarum strains 248, 300 or 3622 and grown in the absence of combined nitrogen differed significantly in dry weight, leaf area, nitrogen content, nodule mass and nodule number. Transfer of plasmids from these field isolates to strain 16015, a non-nodulating derivative of strain 300, resulted in new strains capable of both nodulation and N2 fixation. In plants nodulated by these new strains the numbers of root nodules were significantly different, but there were no significant differences in N2 fixation, leaf area or dry weight. In several cases the introduction of additional plasmids into strain 300 or strain 16015 impaired symbiotic performance relative to strain 300 itself. Of all plant traits measured in symbiotic associations, leaf area was most highly correlated with the total Kjeldahl nitrogen content of plants after 25 d growth in the absence of combined nitrogen.