Possible reasons for relative salt stress tolerance in nodules of white lupin cv. Multolupa

The effects of different NaCl concentrations on the growth andnitrogen fixation activity of white lupin (Lupinus albus [L.])was studied over a 6 d period. Plant growth parameters, photosynthesisand shoot respiration were unaffected by NaCl concentrationsup to 150 mol m^–3. However, nitrogenase activity decreasedwith increased NaCl concentration up to 100 mol m_–3, whilstthe O₂ diffusion resistance increased with 100 mol m^–3NaCl, but showed no further change when 150 mol m^–3 NaClwas applied for 6 d. Increases in NaCl concentration decreasednodular starch content while increasing sucrose content, suggestingan osmotic regulation. These changes were associated with a77% decrease in sucrose synthase activity. The effect on theO₂ diffusion resistance was paralleled by changes in glycoproteincontent of the nodules, as determined by immunogold localizationand ELISA. X-ray microanalysis studies of nodules showed that,following a 6 d exposure to 150 mol m^–3 NaCl, Na⁺ ionswere largely excluded from the infected zone, whilst only lowlevels of Cl^- ions penetrated into this region. Na⁺ entry intoroots and leaves was also at a low level. Leghaemoglobin contentdecreased with saline stress, as did superoxide dismutase; whichdecreased by 36% following exposure to 100 mol m^–3 saltfor 6 d. These results are discussed in relation to the relativesalt tolerance of the Multolupa/ISLU-16 symbiosis. Key words: Salt stress, nodules, nitrogen fixation, oxygen diffusion, carbohydrates, Lupinus albus