The Effect of Salinity on Rhizobium Survival, Nodule Function and Nodule Fomation in the Soybean-Rhizobiurn Japonicum
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1982
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Symbiotic nitrogen fixation may be adversely affected by saline environments. This dissertation describes experiments that assess the salt sensitivity of: 1) Rhizobium as free living organisms; 2) soybean nodule function; and 3) soybean nodule formation. In addition, a split-root plant growth system is described which can be used to separate the effects of salinity stress on host yield potential from the effects of salinity on nodule processes.
The growth rate of Rhizobium in culture media is slowed by the addition of NaCl. Some strains were incapable of growth at the highest level of salt used (120 mM NaCl). However, all withstood substantial osmotic shock and most survived for extended periods in saline solutions equivalent to the concentration of sea water. The results show that the effects of increasing moisture tension and salinity on Rhizobium survival in soil are additive.
By independently subjecting nodules and shoots to salinity stress it was possible to show that the soybean-Rhizobium japonicum nodule system was not greatly affected by exposure to 120 mM NaCl. The main reduction in nitrogen fixation was the indirect effect of salinity on leaf expansion, shoot yield potential and the sink for nitrogen.
The early processes of nodule formation were extremely sensitive to NaCl in the rooting medium. When only 26.3 mM NaCl was added to the nutrient solution two hours prior to inoculation, nodule number and mass were reduced by 50% and 79.9 mM NaCl reduced nodule number, mass and nitrogen fixation to less than 10% of the controls. Rhlzobium japonicum, reisolated from nodules from the high salt treatment did not form more nodules under saline conditions than isolates from controls.
By independently subjecting the various processes of the symbiosis to salinity stress it was determined that the early steps in nodule formation are the most sensitive to salinity. This sensitivity indicates that high quality irrigation water must be used during the establishment of symbiotic legumes.
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