Soil microbes help plants to attract bees

According to a UC Berkeley study, mycorrhizal fungi impact flower characteristics and, consequently, the number and duration of visits by bees

by Matteo Cavallito

 

Certain soil microbes may help plants produce larger flowers, thereby attracting more bees. This is suggested by a study published in the journal New Phytologist that focuses on the relationships between microorganisms and roots with the goal of illustrating how variations in floral traits affect interactions between plants and pollinators. The research, involving UC Berkeley scientists, focused on arbuscular mycorrhizal fungi (AMF), which enter into symbiosis with plants, providing them with nutrients and water in exchange for carbon.

Fungal presence affects plant growth

“Floral traits, including floral display and nutritional rewards from pollen and nectar, drive pollinator visitation,” the study explains. “Even within a single plant species, environmental factors can influence the quality and quantity of floral resources. Yet, the ecological interactions driving this variation in floral resources, especially those belowground, remain unknown.”

To explain these processes, the researchers analyzed how the presence of arbuscular mycorrhizal fungi in the soil and the availability of nutrients such as phosphorus influence plant growth and the characteristics of the fungi themselves and flowers. And how these, in turn, influence bee visitation.

The authors examined the effect of different treatment combinations using “two nutrient levels (low vs high P supply) and four different synthetic AMF mixtures (competitor, stress-tolerant, and ruderal species plus a mixture of all four species) and a control, in a factorial design for a total of 10 treatment combinations with five replicates each.” The experiment was conducted in a greenhouse and experimental field at the University of California, Berkeley between June and August 2019.

Attendances increase by 28%, visit time by 47%.

The results were clear. “We found that increased AMF richness of functional diversity enhanced floral display (flower size and number) and rewards (nectar volume and pollen protein) and increased bee visitation,” the study explains. This is a result of increased flower size.

However, the authors explain, there are also some critical issues: “flower size correlates negatively with root colonization but positively with hyphal length, suggesting that AMF traits drive the effects of AMF on flower growth.”

On balance, they say, “Plants grown with AMF inoculum received 28% more bee visits and 47% more bee visitation time“. Plants inoculated with six AMF species (i.e. representing all functional groups) received the highest bee visitation time was highest. P supply did not affect bee visitation or number of bee visits.”

Dynamics at the subsoil level provide important insights

In general, the research concludes, the effect of microbes on floral traits and bee visitation has not been homogeneous. The different types of fungi involved, in fact, result in different consequences on flowers and bee foraging dynamics. Analysis of the interactions occurring in the understory, therefore, may offer new insights into the factors that can increase bee visitation.

“Our findings reveal that the hidden roles of AMF can emerge,” explained Aidee Guzman, researcher and co-author of the study at the University of California Berkeley and now a professor at Stanford University, in a statement. “Floral traits and bee activity varied between compositionally distinct AMF communities, emphasizing that not all AMF communities are the same.”