Effects of light limitation on legume-mycorrhizae interactions.

Plants respond with a sink stimulation of photosynthesis when colonized by fungal mutualists, which compensates for costs of carbohydrate allocation to the microbes. Problems may arise when light is limited and plants cannot increase photosynthesis. We hypothesize that under such conditions the costs for maintaining the mutualism outweigh the benefits, which ultimately turns the beneficial microbes into parasites exploiting resources and reducing host fitness. We study these plant-microbe interactions under different light availabilities using lima bean plants and mycorrhizal fungi. In our study, we apply two levels of light (full light and light intensity reduced by 75%) and two levels of microbial inoculation (sterile soil and mycorrhiza). Fitness-relevant plant parameters are measured including plant vegetative growth as well as flower and seed production. Preliminary results indicate a reduction of flower and seed production in full light mycorrhizal colonized plants, with no difference between shaded treatments. Our study will promote our current understanding of costs and benefits of plant-microbe interactions and will provide insights into effects of these interactions on plant fitness under variable ambient conditions.