Effect of oxygen availability on nitrogen fixation by two Lotus species under flooded conditions

The pasture legumes Lotus uliginosus (Schk.) and Lotus corniculatus (L.), known to differ in their tolerance to flooding, were inoculated with Rhizobium loti and flooded for 60 d while subjected to two levels of dissolved pO(2): 0.241 and 0.094 mu mol ml(-1). L. uliginosus showed significantly greater growth (shoot and root) and N-2 fixation under both pO(2)s, compared tot. corniculatus, although growth and N-2 fixation by L. corniculatus was not affected by the low pO(2). Surprisingly, in L. uliginosus., growth, nodulation and N-2 fixation were all increased by low pO(2) while nodulation of L. corniculatus was unaffected by low pO(2). The highest rates of endogenous ethylene production were found with L. corniculatus where low pO(2) plants showed a significant increase over that of the higher pO(2) plants while L. uliginosus plants showed a decline. Root porosity of L. uliginosus doubled in the low pO(2) treatment from a mean of 14.5% in high pO(2) roots to 28.5%, whereas that of L. corniculatus was relatively unaffected by pO(2), being 7% and 9% for high and low pO(2) plants, respectively, The structure of nodules differed little between species and treatments, although nodules/nodulated roots from the L. uliginosus plants had particularly profuse lenticels and aerenchyma. However, L. corniculatus nodules, especially those grown in the lower pO, showed signs of early senescence with vacuolation of infected cells and green coloration when cut open, Leghaemoglobin (Lb) concentrations in nodules from both species were unaffected by low pO(2), although that of L. corniculatus nodules, regardless of pO(2), was significantly greater than L. uliginosus, Concentrations of the intercellular glycoprotein recognized by the monoclonal antibody MAC265 were significantly reduced in nodules from the low pO(2) treatment in both species. Immunogold labelling showed that the MAC265 antigen was localized primarily within intercellular spaces within nodule cortices from both Lotus species, A marked decrease in deposition of the MAC265 antigen within the cortices of L. uliginosus nodules grown in the lower pO(2). is discussed in terms of the relative abilities of the two Lotus spp. to maintain an O-2 supply to the N-2-fixing bacteroids within submerged nodules.