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Arbuscular mycorrhiza maintains nodule function during external NH 4+ supply in Phaseolus vulgaris (L.)

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The synergistic benefits of the dual inoculation of legumes with nodule bacteria and arbuscular mycorrhizae (AM) are well established, but the effect of an external NH 4 + supply on this tripartite relationship is less clear. This effect of NH 4 + supply was investigated with regards to the growth and function of the legume host and both symbionts. Nodulated Phaseolus vulgaris seedlings with and without AM, were grown in a sand medium with either 0 N, 1 mM or 3 mM NH 4 + . Plants were harvested at 30 days after emergence and measurements were taken for biomass, N2 fixation, photosynthesis, asparagine concentration, construction costs and N nutrition. The addition of NH 4 + led to a decline in the percentage AM colonization and nodule dry weights, although AM colonization was affected to a lesser extent. NH 4 + supply also resulted in a decrease in the reliance on biological nitrogen fixation (BNF); however, the AM roots maintained higher levels of NH 4 + uptake than their non-AM counterparts. Furthermore, the non-AM plants had a higher production of asparagine than the AM plants. The inhibitory effects of NH 4 + on nodule function can be reduced by the presence of AM at moderate levels of NH 4 + (1 mM), via improving nodule growth or relieving the asparagine-induced inhibition of BNF.

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