Effects of Rhizophagus intraradices and Rhizobium trifolii on growth and N assimilation of white clover

The inoculation response of single arbuscular mycorrhiza fungi (AMF) or rhizobia (Rh) in relation to nitrogen (N) acquisition of plants is well established, while the combined effect of both AMF and Rh is poorly known with regard to N assimilation for changes in amino acids of white clover (Trifolium repens). A pot study was carried out to evaluate the effect of single versus dual inoculation of AMF (Rhizophagus intraradices) and rhizobium (Rhizobium trifolii) on plant growth, leaf and root N contents, root amino acid contents, and root N-related enzyme activities in white clover. One hundred days after inoculations, Rh inoculation significantly stimulated the root colonization by R. intraradices. A single inoculation of AMF or Rh improved the plant growth (biomass production, root projected area, and root volume), root N acquisition, and dual inoculation of AMF and Rh further expanded some of these positive effects on root projected area and root N contents than single inoculation. All the inoculations notably increased activities of root asparagines synthase, nitrate reductase, and glutamate synthase, whilst dual inoculation displayed a much stronger effect in asparagine synthase activity than single inoculation. Single Rh treatment increased root glutamate and proline content, single AMF inoculation induced an increase in glutamate, aspartate, arginine, and ornithine content, while dual inoculation stimulated the accumulation of aspartate and proline. These results suggested the cooperation between the AMF and Rh inoculations, which magnified the positive effect on partly N metabolites and N-assimilation relevant enzymes of white clover.