Alfalfa response to elevated atmospheric CO2 varies with the symbiotic rhizobial strain

Abstract

Increased atmospheric CO2 was shown to affect a variety of physiological processes in plants, including photosynthesis and growth with repercussions on crop yield and nutritive value. Perennial alfalfa (Medicago sativa L.) is a sustainable crop with a deep root system, living in symbiosis with rhizobium for nitrogen (N) fixation. The objective of the project was to determine the combined effects of elevated CO2 and rhizobial strains on photosynthesis, growth, N fixation, and nutritive value of alfalfa, and on soil microflora. Alfalfa inoculated with two different strains of rhizobia (Sinorhizobium meliloti strains A2 and NRG34) was grown 2 months at day/night temperatures of 22/17°C under either 400 (near ambient) or 800 (elevated) µmol mol−1 of CO2. The photosynthetic response of alfalfa to elevated CO2 differed according to the rhizobial strain. At the end of the experiment, elevated CO2 stimulated photosynthetic rates by 50% in plants associated with A2 but there was no significant increase in plants nodulated with NRG34. Nitrogenase activity (+38%) and shoot growth (+60%) were stimulated under 800 µmol mol−1 of CO2 for alfalfa inoculated with both strains. Root dry weight was significantly higher at 800 µmol mol−1 of CO2 only with strain A2. Fibre concentration decreased in response to elevated atmospheric CO2 in alfalfa inoculated with strain A2 resulting in plant material with greater nutritive value when inoculated with A2 compared to NRG34. In the soil, elevated CO2 increased the proportion of fungi in the microbial community while decreasing Gram− bacteria. For alfalfa inoculated with rhizobial strain A2, photosynthetic rates, nitrogenase activity, and growth were all stimulated by increased atmospheric CO2 compared to less consistently positive responses to elevated CO2 when inoculated with NRG34. Our results show that it is possible to identify rhizobial strains to improve plant performance under predicted future CO2 concentrations with no negative effect on nutritive value.