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Effect of Salinity on the Functional Components of the Soybean-Rhizobium japonicum Symbiosis¹

A split-root solution culture system was used to partition the effects of salinity on the functional components of the soybean [Glycine max (L.) Merr] - Rhizobium japonicum symbiosis. Nodules were confined to half of the root system. After nodules were well developed (40 days from planting), 120 mM NaCl was applied to the half-root systems in the following combinations: 1)unstressed, no salt to either half-root system; 2) partially stressed shoot and unstressed nodules, salt to only the nonnodulated halfroot system; 3) partially stressed shoot and stressed nodules, salt to only the nodulated half-root system; and 4) stressed shoot and nodules, salt to both half-root systems. Leaf osmotic potentials (s) for the four treatments were: 1) –1.02; 2) –1.12; 3) –1.23; and 4) –1.83 MPa. Nodule s for the treatments were: 1) –0.70; 2) –0.72; 3) –1.12; and 4) –1.21 MPa. While total and specific nitrogenase activity as well as shoot N content were suppressed in treatment 3, the greatest decrease in activity and shoot N was observed when the plant was stressed by having both half-root systems in salt (treatment 4). The rate of leaf expansion in treatment 4 was half the rate observed in the other treatments. We conclude that reduced N₂ fixation by nodulated soybeans growing in saline environments was more a result of the effect of salt on leaf s and expansion, than the direct action of salt on the functional processes of the nitrogenase system.

Key Words: Nitrogenase activity • Split-root • Osmotic potential • Glycine max

² Graduate research assistant, Dep. of Agronomy and Soil Science and professor, Microbiology, Univ. of Hawaii, Honolulu, HI 96822. Current address of senior author: NifTAL Project, P. O. Box "O", Paia, HI 96779.

Received for publication December 13, 1982.