Effects of arbuscular mycorrhizal fungi and rhizobia on nitrogen assimilation of white clover

Symbiotic microorganisms such as arbuscular mycorrhizal fungi (AMF) and rhizobia (Rh) both promote host plant N acquisition, but whether and how the combination affects host plant nitrogen (N) assimilation remains unclear. To clarify the role of AMF and rhizobia on N assimilation in white clover (Trifolium repens), the effect of single or combined inoculation with an arbuscular mycorrhizal fungus Paraglomus occultum and a rhizobium Rhizobium trifolii on plant growth, photosynthesis, leaf N and amino acid contents, and the activities of N assimilation-related enzymes were analyzed in white clover under potted conditions. The results obtained were as follows: (1) The plant height, stolon length, leaf number, shoot biomass, total biomass, chlorophyll b and total chlorophyll contents, steady-state light quantum efficiency, and leaf N contents of white clover were significantly increased by single inoculation with Paraglomus occultum, single Rhizobium trifolii, and combined inoculations of Paraglomus occultum and Rhizobium trifolii, along with the order of combined inoculations of Paraglomus occultum and Rhizobium trifolii > single Paraglomus occultum > single Rhizobium trifolii > non-inoculation control. (2) Combined inoculations of Paraglomus occultum and Rhizobium trifolii significantly increased the contents of various amino acids including alanine, arginine, asparagine, aspartate, glutamine, glutamic acid and histidine in leaves of white clover, and distinctly improved the activities of nitroreductase, nitrite reductase, glutamine synthetase, glutamate synthetase, glutamate dehydrogenase, asparagine synthetase, and aspartate aminotransferase. In addition, the introduction of Rhizobium trifolii significantly promoted the colonization of Paraglomus occultum to roots of white clover, suggesting the partnership between the two. Therefore, it is concluded that combined inoculations of AMF and rhizobia promote N assimilation, generate more amino acids, and further promote the growth of white clover by activating the activities of N assimilation-related enzymes. It is further demonstrated that AMF and rhizobia have synergistic effects on promoting N assimilation of white clover.