Possible influence of Rhizobium on VA mycorrhiza metabolic activity in double symbiosis of alfalfa plants (Medicago sativa L.) grown in a pot experiment

Summary Alfalfa (Medicago sativa, L. cv Aragón) plants were grown under greenhouse conditions in pots of inert sand and vermiculite. The plants were inoculated with Rhizobium meliloti strain 102F28, with Glomus fasciculatus or with a mixture of both microorganisms. Plants inoculated with both Rhizobium and Glomus had the highest shoot dry weight and the lowest root-to-shoot ratio. Roots from dually inoculated plants also had a higher oxygen uptake and nodule nitrogenase activity than those from plants inoculated with either of the two microsymbionts alone. However, the dry weight of the roots from only VAM-infected plants was higher than those from Rhizobium or from Rhizobium plus Glomus-inoculated ones. These differences did not correlate with succinate dehydrogenase activity, which was similar between treatments. Nutrient element concentrations were increased in dually infected plants in comparison with those of plants inoculated with only Rhizobium or Glomus. These data suggest that Rhizobium may affect fungal metabolism and that the effect is not achieved via the tricarboxylic acid pathway.