Effect of mycorrhizal colonization and light limitation on growth and reproduction of lima bean (Phaseolus lunatus L.).
Background Plants respond with a 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. Methods 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). We quantified both vegetative (height, number of leaves, above and belowground biomass, shoot:root ratio) and generative plant traits (flower, pod, and seed production, seed viability). Results As expected, shaded plants were smaller, had less leaves and total biomass, fewer flowers, pods, and seeds than non-shaded plants. However, individual seeds were significantly heavier in shaded plants, required less time for germination, and had higher germination success than in plants grown under full light. Contrary to our expectations, mycorrhizal fungi neither enhanced performance of plants nor resulted in measurable costs in either light condition. Mycorrhizal inoculation had no effect on plant height, number of leaves, aboveground and total biomass, but significantly reduced belowground biomass and increased shoot:root ratio under both light conditions. Under full light, mycorrhizal plants produced fewer flowers and pods than controls, but there was no significant difference in number of buds, seeds, and total seed weight. However, mycorrhizal plants had a slightly increased average seed weight when grown under full light but, there was no significant difference in the percentage of seeds that germinated and in the time required for germination as compared to controls. Conclusion Our study suggests that mycorrhizal colonization neither provided benefits to lima bean plants grown under full light, or created costs when photosynthesis was limited. Prior to inoculating agricultural systems with commercial mycorrhizal strains, detailed experiments to test for effects on crop yield should be conducted.