The majority of flowering plants benefit from both pollination and mycorrhizal mutualisms, yet we know little of the extent to which such above- and belowground interactions might interact to structure outcomes of interactions for plants, pollinators, and fungi. This knowledge gap is critical to agriculture, where >75% of crops benefit from animal pollination, and bee pollinators are threatened by farm management practices, including pesticide application, disease spread by managed bees, and loss of habitat. Mycorrhizal mutualisms are common in crop systems, yet growers rarely manage for such beneficial soil microbes that could increase crop yield.
Using pollinator-dependent highbush blueberry (Vaccinium corymbosum), in this study we ask whether mycorrhizal mutualisms 1) affect aboveground plant traits; 2) alter foraging behavior of bee pollinators; and 3) limit yield loss at farms with low bee abundance. We treated potted, 4-year-old blueberry plants of four cultivars with an inoculum containing two ericoid mycorrhizal fungus species, and maintained other plants as uninoculated controls. In two years we measured leaf, stem, flower, nectar, and pollen traits, as well as duration of flower visits by both managed honey bees and wild bees. In addition, we placed plants at a blueberry farm with low bee abundance, where we crossed mycorrhizal status with a pollen supplementation treatment to assess interaction effects of mycorrhizal and pollination mutualisms on plant reproduction.
Results/Conclusions
Association with mycorrhizae increased leaf area and number while decreasing damage by foliar herbivores. Mycorrhizal plants had larger flowers than controls, and in some cultivars, those flowers produced greater rewards for pollinators. Bees made longer visits to flowers of mycorrhizal plants, suggesting they might transfer more pollen between flowers. Neither mycorrhizal inoculation nor pollen supplementation affected overall fruit production, but both treatments increased fruit mass. Moreover, we discovered a synergy between the two factors, such that largest berries were matured from flowers of mycorrhizal plants that had received supplemental pollen. We interpret this result as evidence that such belowground symbioses can increase plant reproductive success by trait-mediated indirect effects on bee pollinator foraging behavior, but that mycorrhizae may also affect other factors limiting yield, such as limitation of water, nutrients, or other resources. We conclude that plant reproduction may be affected by interactions between multiple classes of organisms with which plants interact, a result that has implications for management of both bee habitat and soil microbial communities on farms.