Q&A: Understanding how plants attract pollinators

Departments - The Growing Edge

Professor Dr. Robert A. Raguso led a study that explores the relationship between plant color, fragrance and pollination.

October 25, 2017

Photo courtesy of Cornell University

A recent study conducted in Greece by Cornell University featuring 41 insect-pollinated plant species explored the relationship between plant color, fragrance and the bees that pollinate plants. Below, Dr. Robert A. Raguso, professor of neurobiology and behavior, explains what was learned from the study, why it’s important to understand bees in this context, and more.

Greenhouse Management: Why was it surprising to learn that it was possible to predict the fragrance of a flower by looking at its color?

Dr. Robert A. Raguso: Reading the most cited literature in pollination biology (e.g. on plant-pollinator networks, their properties and consequences for community structure, stability, ecological services, etc.), most studies focus on abundance and timing rather than traits. And when they do include traits, they tend to be things we can see (density of display, color). That literature neglects to even consider chemical aspects of floral display even though we know that they matter (e.g. the way bees learn to show constancy for rewarding flowers through their smell, color or combination thereof). So, my straw man [argument] is that if you were simply to follow this literature, scent would not even be mentioned. For me, though, who works on scent and has shown that it matters across the spectrum of specialized to generalized plant-pollinator interactions, I was confident that scent chemistry would not be random across this community. What I was not sure of was whether there would be correlations with color (or pollinators), and if so, would they only apply to specialized cases (e.g. the sexually deceptive orchids). Only two of the biosynthetic pathways that make scent also make pigment, so the correlations that we ended up finding were not easily explained as “same pathway” phenomena.

GM: You have also noted that there could be a more “holistic” understanding of pollination. What exactly does that mean and how can that inform the growing community?

RR: I use ‘holistic’ not in a new age sense but in a non-reductionist sense. There has been a tendency to focus on specific aspects of plants — their genomes, their color — on certain traits for which tools have been developed without reference to other traits that, understand them or not, may be important to pollinator perception and foraging decisions. As a sensory ecologist, I’d like to see the day when we have a fuller set of tools to query pollinators on how they perceive and choose flowers, when those choices change, and on what basis they make those choices. My other research projects involve flower shape, respiratory properties (heat, CO2), texture and even relative humidity (as a transient indicator of the presence of nectar). My colleague at Cornell, Charlie Walcott, worked with homing pigeons for much of his career, and the birds made it clear that they were capable of using many different kinds of environmental information (the sun, magnetic fields, odors) to navigate, as we know is true, for bees and butterflies. So, by holistic, I indicate that we’d like to reach a broader understanding of how pollinators work. Growers could then be more intelligent or [utilize] effective ways to arrange both insect-pollinated crop plants, border crops, biological control plantings and pollinator gardens. For urban settings, that might provide us with more insights on gardens that combine human aesthetics with greater rewards for pollinators.

Photo courtesy of Cornell University

GM: Why is it so important to understand bees in this way?

RR: Bees have flexible behavior — they can learn a lot of different traits, can adjust their behavior if those traits lead to unprofitable choices, can even learn faces or to work in the dark if necessary. But it is important to differentiate what they can do, which is interesting from a sensory point of view, versus what they do, which is important from the standpoint of optimizing their performance as pollinators, etc. If we think that they like purple, which has been shown experimentally, but ignore or omit that purple flowers tend to smell a certain way, and maybe that color-odor combination enhances learning accuracy perhaps at the expense of speed, as has been shown for learning of multiple traits, then we are missing half of the story. Imagine your own dining choices withholding all but visual aspects of the buffet. You would be missing a lot of important information. Scent tends to be complex, so you need to parse aspects of pollinator attraction from defense against enemies, so flower scents are not always easy to interpret. But it is worth the effort, in my opinion.