The effects of osmotic stress on nitrogen fixation and respiration of soybean root nodules were investigated. Non-electrolytes, such as mannitol, depressed acetylene reduction when given to detached nodules in sufficient concentration to withdraw water from them. When given to the root systems of whole plants, hypotonic concentrations of mannitol had a depressing effect after 1–2 hours, the exact time depending on the size of the nodules. Equivalent concentrations of salts had a much more rapid effect on whole root systems and on detached nodules (less than 5 minutes); 100 m-equiv./l almost stopped acetylene reduction by small nodules. These effects could not be transmitted via the roots and needed contact between salt solution and nodules. They were accompanied by a decrease in respiratory activity and were fully reversible if the root systems were flushed with water within a few hours of treatment. High salt concentrations, as in sea water, had a rapid (1–2 minutes) effect, which after 20 minutes became irreversible.
The effects of salt stress are interpreted as resulting from alterations in the metabolism of the cortical cells of the nodules.