When it comes to plant aromas, we tend to forget that we, as humans, are not the target audience, and these odors were not designed with us in mind—we are really passive spectators to a show that luckily most of us happen to enjoy. This past spring Mother Nature demonstrated just what it means to have a target olfactory audience at Madison’s Olbrich Botanical Gardens. For the first time in about 12 years, one of the four massive titan arum (Amorphophallus titanum) plants that reside at Olbrich bloomed, an event that typically only happens for 24-48 hours at a time and 4-5 times total throughout this plant’s roughly 40 year lifespan. More informally (and aptly) known as the “corpse flower” due to its carcass-adjacent coloring and distinctly foul odor, hundreds of plant enthusiasts and hopeful spectators queued for hours to catch a glimpse and whiff of the pungent plant. Until rare events like this happen, it can be easy to forget just how interesting and complex plants really are. We romanticize and lend meaning to flowers and relish in the sweet fragrance they provide, while often completely overlooking the intricate biological and chemical processes that comprise the science of floral scent.
What’s in a Name Scent?
Plants employ scent as a vital tool predominantly used to accomplish the same basic goals of most lifeforms on earth: to reproduce and not get eaten. Producing scent can help them accomplish both goals by attracting pollinators and acting as a deterrent for pests and predators.
Both floral and fruity scents are volatile organic compounds (VOCs), each a complex mixture of low molecular weight compounds, that plants release which easily evaporate and disperse in the air. Like all organic compounds, VOCs have a carbon chain backbone. The length of the carbon backbone and other atoms connected to the chain, are what influence odor differences in VOCs. For example, sulfur atoms attached to the carbon chains of carrion flowers is what contributes to their signature variations of rotten odors.
In recent years, biochemical research has also been investigating the role amino acids play in plant physiology and metabolism, particularly their role in the production of volatile scents. In addition to their role as precursors to plant fragrance, aromatic amino acids like tryptophan are also fundamental building blocks for protein production in animal cells. In this way, studying the methods and modes of how plants create scents also provides insight into the creation of molecules that are essential to human and animal lives as well.
Attractive is Subjective
What makes a scent attractive? It should come as no surprise that, just like people, different pollinators have different preferences and are drawn to different tastes, colors and of course, scent. Most of their fragrance preferences can be categorized (non-scientifically) into the following groupings: Sweet & Floral, Fresh & Herby, and Foul & Rotten.
Sweet & Floral
When you first think about a flower’s scent, the sweet smell given off by roses, lilies, or peonies might be the type of aroma that comes to mind. These more sugary sweet fragrances, produced in the petals of these flowers, are preferred and sought after by butterflies, bumblebees, hummingbirds and honeybees.
Many sweet-scented plants often have brightly colored flowers as well, serving as dazzling billboard advertisements to further grab the attention of hungry pollinators flying by. Typically, flowers that attract these pollinators release their scents during the day, while flowers that attract nocturnal pollinators like moths and bats, typically release their scents at night. Instead of colorful flowers, plants like dragon fruit, night-blooming cereus, and saguaro cacti actually have large white flowers that open up at night to release their strong perfume-y fragrances. The white coloring of the petals also give these flowers a kind of glowy appearance to help guide their night-vision pollinators.
Fun Fact: Did you know that honeybees are responsible for carrying out more pollination than any other insect? Nearly 80% of all flowering plants in the United States, including 130 types of fruits and vegetables, comes from honeybees alone!
— “The Value of Birds and Bees” —
Fresh & Herby
Most people who enjoy gardening or cooking at home are familiar with the fresh, invigorating scents that cling to our hands after handling members of the Lamiaceae family such as basil, mint, rosemary, and lavender. Many of the species contain essential oils that get secreted by way of tiny glandular hairs on the surface of their leaves. Over time as the oils build up, the hairs swell and will release their scent very easily when disturbed by the wind or when the leaves are touched, which explains the lingering scent on your skin after picking or chopping fresh herbs.
The scents emitted from these can range from minty to bitter, from fresh to sharp, and can help play a role in defending the plant from predators and pests. For examples, the bitter aroma of some herbs is said to repel slugs, aphids, and some larger animals. Sage, rosemary and thyme are believed to deter some species of rabbits, while alliums like garlic are thought to fend off squirrels.
Fun Fact: In addition to their pest control prowess, herbs also attract beneficial pollinators and can sometimes enhance the flavor of their plant neighbors, making many of them excellent companion plants in vegetable gardens!
“Companion Planting: Herbs that Pair Perfectly As Growing Partners”
Foul & Rotten
Just as the old dating adage goes “there’s somebody for everybody”, there seems to truly be a scent for every pollinator as well. Flies and beetles tend to grow and thrive in horribly smelly conditions, and are often thus drawn to plants and flowers with scents that imitate rotting flesh or excrement. That brings us back to our friend titan arum, which imitates a dead animal with dark burgundy interior and putrid odor in order to entice flesh-hungry pollinators towards its female flowers.
Roughly 43 other species join titan arum in the carrion flower genus (scientific name Stapelia), supporting its mission to look like Jurassic-era nightmares and to smell equally horrifying in order to keep attracting pollinators (which—I can only assume—is part of their long-game plot to outlive the human race).
References:
- Brewer, G. (2019) The Giant, Pongy Plant: Titan arum. Kew.org. Accessed May 2022.
- Dudareva, N. (2005) Why do flowers have scents? Scientific American. Accessed May 2022.
- Guarino, B. (2013) Making Scents: The aromatic world of flowers. Scienceline.com. Accessed May 2022.
- Mostafa, S., Wang, Y., Zeng, W. and Jin, B. (2022) Floral Scents and Fruit Aromas: Functions, Compositions, Biosynthesis, and Regulation. Front. Plant Sci. Accessed May 2022.
- Olorunshola, Y. (2021) Why Do Plants Smell? Kew.org. Accessed May 2022.
