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

The effects of different NaCl concentrations on the growth and nitrogen fixation activity of white lupin (Lupinus albus [L.]) was studied over a 6 d period. Plant growth parameters, photosynthesis and shoot respiration were unaffected by NaCl concentrations up to 150 mol m−3. However, nitrogenase activity decreased with increased NaCl concentration up to 100 mol m−3, whilst the O2 diffusion resistance increased with 100 mol m−3 NaCl, but showed no further change when 150 mol m−3 NaCl was applied for 6 d. Increases in NaCl concentration decreased nodular starch content while increasing sucrose content, suggesting an osmotic regulation. These changes were associated with a 77% decrease in sucrose synthase activity. The effect on the O2 diffusion resistance was paralleled by changes in glycoprotein content of the nodules, as determined by immunogold localization and ELISA. X-ray microanalysis studies of nodules showed that, following a 6 d exposure to 150 mol m−3 NaCl, Na+ ions were largely excluded from the infected zone, whilst only low levels of Cl- ions penetrated into this region. Na+ entry into roots and leaves was also at a low level. Leghaemoglobin content decreased with saline stress, as did superoxide dismutase; which decreased by 36% following exposure to 100 mol m−3 salt for 6 d. These results are discussed in relation to the relative salt tolerance of the Multolupa/ISLU-16 symbiosis.