Effective rhizobia enhance legume growth during subsequent drought despite water costs associated with nitrogen fixation

Purpose Drought induces mortality in plants; however, how symbioses mediate the plant drought response remains understudied and may be crucial to understanding how shifts in symbioses under climate change scenarios may impact plant drought induced mortality. Methods Drought was experimentally imposed in parallel on two legume hosts, Acmispon strigosus and Lotus japonicus , whose roots are nodulated by nitrogen fixing rhizobia in different genera. Within each host taxon, greenhouse experiments factorially combined legume genotypes varying in root nodule investment with rhizobia varying in nitrogen fixation capacity, to investigate how costs and benefits of symbiosis modulate plant performance under drought. Results Net benefits of symbiosis with nitrogen fixing symbionts, namely increased number of leaves and shoot mass, were maintained under drought irrespective of nodulation level or host species, mitigating the carbon strain during drought. Net benefits of symbiosis occurred despite increased water cost associated with the maintenance of nitrogen fixation, costs that were evident even in pre drought baseline conditions. Conclusion Our results demonstrate that benefits of root nodule symbiosis can be robust to environmental perturbations, and potentially mediate carbon starvation during drought induced mortality events. The investment in symbiosis and its impacts on biomass suggest that in long term droughts legumes may be more limited by carbon than water. Ongoing stability of these associations is predicted, even under extended drought conditions.