The Glycine‐Glomus‐Rhizobium symbiosis

Soybean [Glycine max, (L.) Merr, cv. Lancer] plants were grown in a sterile rooting medium watered daily with a nutrient solution containing 4, 20, 100, or 500 μM, P. Plants were inoculated with Rhizobium japonicum, strain 61A118 and grown in the presence or absence of the endomycorrhizal fungus Glomus fasciculatus, Gerdemann et Trappe. Plants grown at the highest P regime had six times higher shoot dry weight than those grown in the lowest P regime. Nodulation did not occur at 4 μM P. Nodule dry weight increased 200‐fold from the 20 to the 500 μM P treatment. Percentage P in shoots and nodules differed significantly among all treatment levels. Acetylene reduction by nitrogenase increased logarithmically with increasing amounts of P. Hydrogen evolution was not detectable at the 20 μM P level. The relative efficiency of nitrogen fixation increased with increasing P stress. Infection by Glomus fasciculatus, at the 500 μM P level was negligible and did not affect the parameters measured. At all other treatment levels the mycorrhizal plants had significantly higher rates of N2 fixation, plant and nodule mass and P content.