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Parasitic and Mutualistic Associations Between a Mycorrhizal Fungus and Soybean: Development of the Host Plant. G. J. Bethlenfalvay, Plant Physiology and Chemistry, Western Regional Research Center, USDA, Berkeley, CA 94710; M. S. Brown(2), and R. S. Pacovsky(3). (2)(3)Plant Physiology and Chemistry, Western Regional Research Center, USDA, Berkeley, CA 94710. Phytopathology 72:889-893. 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-72-889.

Soybean plants (Glycine max ‘Kent’) inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus fasciculatus were grown in 1.5-L pots in a perlite/sand rooting medium containing 200 mg of hydroxyapatite (Ca10[PO4]6[OH]2) as the source of P. Plants were harvested at 2- or 3-wk intervals for 18 wk. Fungal biomass as percent total dry weight (of the host and symbiont) was calculated from measurements of fungal chitin and had a maximum value of 7.3% at 6 wk after planting. Total and percent P content of VAM-infected and uninfected control plants were determined, and the decrease of available (NaHCO3-extractable) P in the rooting medium was monitored. As a result of development of the endophyte, growth inhibition of VAM plants relative to the controls increased up to the ninth wk. It was reversed after 9 wk, and, after 15 wk, VAM plants had significantly greater dry weight and P contents than the controls. The data suggest that early growth inhibition was a result of carbohydrate demand on the host by the endophyte, at a time when the shoot-to-root ratio and photosynthetic source capacity of the host were low. Available P in the rooting medium of VAM plants decreased to 10 μg P/g rooting medium during the 3 wk preceding the reversal in growth inhibition, indicating that mycotrophic growth of the host induced by increased P uptake by the endophyte commenced at this P concentration.