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

Abstract

Bean (P. vulgaris L. cv. Dwarf) roots were inoculated with R. phaseoli and colonized by the vesicular-arbuscular mycorrhizal (VAM) fungus G. fasciculatum Gerd. et Trappe or left uncolonized as controls. The symbiotic associations were grown in an inert substrate using 0, 25, 50, 100 or 200 mg hydroxyapatite (HAP) (Ca10[PO4]6]OH]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 mg HAP, where available P at harvest (7 wk after planting) was 5 .mu.g P/g 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 mg. Intersymbiont competiton 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.