Messina (Melilotus siculus, Fabaceae) is a waterlogging-tolerant annual forage legume, but data were lacking for the effects of waterlogging on nodulated plants reliant on N2 fixation. Konnerup et al. find that plants inoculated with the appropriate rhizobia, Ensifer (syn. Sinorhizobium) medicae, formed nodules.
Nodulated plants grew similarly well as plants fed NO3–, both in drained and waterlogged conditions. In waterlogged conditions the relatively high respiration rates of nodules relied on O2 movement via the secondary aerenchyma (phellem) in hypocotyl, roots and the outer tissue layers of nodules, which we demonstrated using microelectrodes.