Testing for UV reflectance to gain insight into the benefits of nativars compared to native plants

Market demand for native plants in ornamental landscapes has drastically increased since the early 2000s, as consumers, landscapers, landscape designers and landscape architects are increasingly aware of the environmental benefits provided by native plants.

But despite their popularity, native plants remain a relatively small portion of the overall ornamental plant market. Only 13–26% of plants sold in the U.S. are native, and a majority of the plants marketed and sold as native plants are often native cultivars.

Native cultivars are native plants that have been bred or selected for specific ornamental traits (e.g., novel color or compact form) or resistance traits. In ornamental markets, native cultivars might fit better into nursery propagation protocols, or may better appeal to shoppers at the point of purchase, compared to true natives.

Many gardeners are interested in native plants because of the purported benefits to bees and other pollinators. However, it is unclear what impact native plant breeding might have on pollinator perception and use of these plants.

Testing what bees see

In 2019, we started an experimental garden at the Oak Creek Center for Urban Horticulture at Oregon State University, to examine pollinator visitation to Oregon native plants and native cultivars. Our study includes seven native species, and one to three cultivars of each native plant. In 2020 and 2021, Jen Hayes, a Ph.D. student at Oregon State, monitored bee abundance and diversity associated with each plant. She’s also measuring plant traits, to better understand how changes in particular plant traits might increase or decrease pollinator visitation to each plant.

One of the plant traits that we are measuring is UV reflectance from flowers. Humans can’t see into the UV spectrum. Thus, any changes to UV flower patterns would be largely undetectable to us. However, bee vision is optimized in the UV spectrum. In fact, many flowers have UV nectar guides, that better attract bees’ attention. A nectar guide is a region of UV reflectance that helps a bee quickly locate the center of flower, and thus facilitates the rapid collection of nectar. When a plant breeder selects for brighter flowers or unique color patterns, if UV color patterns are also affected, these changes could impact bees.

In 2020, we started working with OSU honors biology student Svea Bruslind, who is minoring in photography. Svea and Jen are working out insect-vision photography protocols to help us see a bee’s eye view of our study flowers. The goal is to document differences that might be invisible to us, but visible to bees, between native plants and native cultivars.

Ultimately, we will compare these differences to our bee visitation data, to help us better understand how different suites of plant traits might increase or decrease bee visitation to particular flowers. For now, let’s take a look at three sets native plants and native cutivars, to see how breeding can alter UV messaging to bees.

Nemophila menziesii (native plant) and ‘Penny Black’ (native cultivar)    

Nemophila menziesii, commonly known as baby blue eyes, appears blue to us in visible light. However, under UV filters, we can see the classic UV “bullseye” in the center of the flower. This bullseye is one type of nectar guide that helps bees to quickly locate pollen the reward center of the flower. You might also notice that under UV light, the pollen-loaded anthers also fluoresce bright blue against the lighter background of the petals.

The ‘Penny Black’ cultivar appears dark purple, with white fringed petals, under visible light. Under UV light, we see a reduction in the classic UV bullseye. We also wee an inversion of the UV markings, which now appear at the edge of each petal. The anthers also appear to be duller, compared to the native plant, under UV light.

Clarkia amoena (native plant) and ‘Dwarf White’ (native cultivar)

Clarkia amoena, commonly known as farewell-to-spring, bears a pink blossom. Under UV filters, the entire flower appears to be a diffuse shade of UV blue. The native cultivar, ‘Dwarf White’ has white petals. Under UV light, the flower appears to bear the same shade of UV blue, as the true native.

Eschscholzia californica (native plant) and ‘Purple Gleam’ (native cultivar)

Eschscholzia californica, commonly known as California poppy, appears bright orange to us under visible light. Under UV light, the petals largely appear orange. But, we can see that the anthers fluoresce bright blue, under UV filters. Unlike the baby blue eyes and farewell-to-spring, California poppy does not provide nectar to bees. When bees visit California poppy, they are only collecting pollen. Thus, it makes sense that this flower lacks a nectar guide, and that the anthers fluoresce UV.

The native cultivar, ‘Purple Gleam’ is a purple blossom with a white center under UV light. Under UV light, this flower appears to have the classic UV bullseye, even though this flower does not offer bees any nectar. How bees perceive this floral marking, in the absence of a nectar reward, remains to be seen.

As you can see, changes to floral color under visible light can result in seemingly big changes under UV light (for baby blue eyes and California poppy). Or, changes in floral color under visible light might result in little to no change under UV light (for farewell-to-spring). What these changes might mean to bees is something we will continue to study in 2022, and beyond.

One of the end goals of our program is to identify suites of plant traits that breeders might cue in on, to best attract bees. Rather than having to test every plant for relative bee attractiveness, we hope that a focus on plant traits will help breeders create beautiful and environmentally friendly plants that are attractive to both consumers and to bees.

Dr. Gail Langellotto is a professor of horticulture at Oregon State University, stationed at the main campus in Corvallis, and the statewide coordinator for the Extension Master Gardener Program. She can be reached at [email protected].