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Parasitic and Mutualistic Associations Between a Mycorrhizal Fungus and Soybean: Development of the Endophyte. G. J. Bethlenfalvay, Plant Physiology and Chemistry, Western Regional Research Center, USDA, Berkeley, CA 94710; R. S. Pacovsky(2), and M. S. Brown(3). (2)(3)Plant Physiology and Chemistry, Western Regional Research Center, USDA, Berkeley, CA 94710. Phytopathology 72:894-897. Accepted for publication 11 December 1981. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1982.. DOI: 10.1094/Phyto-72-894.

The development of Glomus fasciculatus, a vesicular-arbuscular mycorrhizal (VAM) fungus, in soybean (Glycine max) roots was determined at 2- to 3-wk intervals for 18 wk. The symbiotic association was grown in 1.5-L pots containing an inert medium supplemented with 200 mg of hydroxyapatite (Ca10[PO4]6[OH]2) as the source of P. Fungal infection, estimated by visual observation of stained mycorrhizal segments was maximal 9 wk after planting. Endophyte biomass, measured by the determination of fungal chitin, increased linearly until week 15, and declined during the final 3 wk of the experiment. Fungal biomass (measured as percentage of mycorrhizal dry weight) peaked between 6 and 9 wk (20%) and declined thereafter. Maximization of percent fungal biomass coincided in time with a reversal in increasing growth inhibition of mycorrhizae relative to uninfected control plant roots. Concentration of P in the mycorrhizae increased rapidly during the first 9 wk and slowly thereafter. The percent difference in P concentration of plant matter increased linearly with decreasing availability of P in the rooting medium. It was concluded that parasitic development of G. fasciculatus under the growth conditions used was due to the diversion of significant amounts of assimilate from the host to the endophyte and to competition for P by the symbionts during the first ontogenic stages of the association.