Impact of arbuscular mycorrhizal fungi (AMF) and atmospheric CO2 concentration on the biomass production and partitioning in the forage legume alfalfa

The influence of mycorrhizal symbiosis, atmospheric CO2 concentration and the interaction between both factors on biomass production and partitioning were assessed in nodulated alfalfa (Medicago sativa L.) associated or not with arbuscular mycorrhizal fungi (AMF) and grown in greenhouse at either ambient (392 μmol mol−1) or elevated (700 μmol mol−1) CO2 air concentrations. Measurements were performed at three stages of the vegetative period of plants. Shoot and root biomass achieved by plants at the end of their vegetative period were highly correlated to the photosynthetic rates reached at earlier stages, and there was a significant relationship between CO2 exchange rates and total nodule biomass per plant. In non-mycorrhizal alfalfa, the production of leaves, stems and nodules biomass significantly increased when plants had been exposed to elevated CO2 concentration in the atmosphere for 4 weeks. Regardless CO2 concentration at which alfalfa were cultivated, mycorrhizal symbiosis improved photosynthetic rates and growth of alfalfa at early stages of the vegetative period and then photosynthesis decreased, which suggests that AMF shortened the vegetative period of the host plants. At final stages of the vegetative period, AMF enhanced both area and biomass of leaves as well as the leaves to stems ratio when alfalfa plants were cultivated at ambient CO2. The interaction of AMF with elevated CO2 improved root biomass and slightly increased the leaves to stems ratio at the end of the vegetative growth. Therefore, AMF may favor both the forage quality of alfalfa when grown at ambient CO2 and its perennity for next cutting regrowth cycle when grown under elevated CO2. Nevertheless, this hypothesis needs to be checked under natural conditions in field.