Differential responses of thesunn4andrdn1-1super-nodulation mutants ofMedicago truncatulato elevated atmospheric CO2

Background and Aims Nitrogen fixation in legumes requires tight control of carbon and nitrogen balance. Thus, legumes control nodule numbers via an autoregulation mechanism. ‘Autoregulation of nodulation’ mutants super-nodulate are thought to be carbon-limited due to the high carbon-sink strength of excessive nodules. This study aimed to examine the effect of increasing carbon supply on the performance of super-nodulation mutants. Methods We compared the responses of Medicago truncatula super-nodulation mutants (sunn-4 and rdn1-1) and wild type to five CO2 levels (300–850 μmol mol−1). Nodule formation and nitrogen fixation were assessed in soil-grown plants at 18 and 42 d after sowing. Key Results Shoot and root biomass, nodule number and biomass, nitrogenase activity and fixed nitrogen per plant of all genotypes increased with increasing CO2 concentration and reached a maximum at 700 μmol mol−1. While the sunn-4 mutant showed strong growth retardation compared with wild-type plants, elevated CO2 increased shoot biomass and total nitrogen content of the rdn1-1 mutant up to 2-fold. This was accompanied by a 4-fold increase in nitrogen fixation capacity in the rdn1-1 mutant. Conclusions These results suggest that the super-nodulation phenotype per se did not limit growth. The additional nitrogen fixation capacity of the rdn1-1 mutant may enhance the benefit of elevated CO2 for plant growth and N2 fixation.