Interactions between Nitrogen Fixation, Mycorrhizal Colonization, and Host-Plant Growth in the Phaseolus-Rhizobium-Glomus Symbiosis

Bean (Phaseolus vulgaris L. cv. Dwarf) roots were inoculated with Rhizobium phaseoli and colonized by the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum Gerd. and Trappe or left uncolonized as controls. The symbiotic associations were grown in an inert substrate using 0, 25, 50, 100, or 200 milligrams hydroxyapatite (HAP) (Ca₁₀PO₄]₆OH]₂) per pot as a P amendment. Plant and nodule dry weights and nodule activity increased for both VAM and control plants with increasing P availability, but values for VAM plants were significantly lower in all parameters than for controls. Inhibition of growth and of N₂ fixation in VAM plants was greatest at the lowest and highest P regimes. It was smallest at 50 milligrams HAP, where available P at harvest (7 weeks after planting) was 5 micrograms P per gram substrate. At this level of P availability, the association apparently benefited from increased P uptake by the fungal endophyte. Percent P values for shoots, roots, and nodules did not differ significantly (p > 0.05) between VAM and control plants. The extent of colonization, fungal biomass, and the fungus/association dry weight ratio increased several fold as HAP was increased from 0 to 200 milligrams. It is concluded that intersymbiont competition for P and photosynthate was the primary cause for the inhibition of growth, nodulation, and nodule activity in VAM plants. Impaired N₂ fixation resulted in N stress which contributed to inhibition of host plant growth at all levels of P availability.