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Journal of Applied Botany and Food Quality

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Mycorrhizal fungi, Mycorrhizas in agriculture, Lima bean -- Growth, Lima bean -- Reproduction, Symbiosis


Plants can respond with sink stimulation of photosynthesis when colonized with fungal or bacterial root symbionts, compensating costs of carbohydrate allocation to the microbes. However, constraints may arise under light limitation when plants cannot extensively increase photosynthesis. We hypothesize that under such conditions the costs for maintaining the symbiosis outweigh the benefits, ultimately turning the mutualist microbes into parasites, resulting in reduced plant growth and reproduction.

Using lima bean (Phaseolus lunatus) as experimental plant, we applied two levels of light (full light, 75% shading) and microbial inoculation (sterile soil, mycorrhizal fungi) and quantified both vegetative and generative plant traits.

As expected, shaded plants produced less vegetative biomass and seeds than non-shaded plants. However, individual seeds were significantly heavier in shaded plants and required less time for germination. While under both light conditions mycorrhizal plants showed a significantly reduced belowground biomass, mycorrhizal fungi neither enhanced overall plants performance in terms of total biomass and seed production nor resulted in measurable costs in either light condition. Our study suggest that mycorrhizal colonization neither provided benefits to lima bean plants grown under full light, nor created costs when photosynthesis was limited.


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Originally appeared in the Journal of Applied Botany and Food Quality, volume 86 (2013), published by Julius Kühn-Institut - Bundesforschungsinstitut für Kulturpflanzen (Federal Research Centre for Cultivated Plants).

A version of this work was subsequently published as a Portland State University Honors Thesis in 2014. It may be accessed at



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