Effect of mycorrhizal colonization and light limitation on growth and reproduction of lima bean (Phaseolus lunatus L.)

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 an 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 plant performance in terms of total biomass and seed production nor resulted in measurable costs in either light condition. Our study suggests that mycorrhizal colonization neither provided benefits to lima bean plants grown under full light, nor created costs when photosynthesis was limited.