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

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Abstract
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.

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