What is scent? Why is it important for plants?
Scent is defined as a distinctive smell which conjures up various olfactory qualities, such as floral, fruity, spicy, minty, or musky. Other less pleasant attributes are putrid, foul, smoky, or burned (but we tend to use the term odor for those smells).
Many of us associate flowering plants with pleasurable fragrances, such as the sweet, delicate scent of lavender, which has natural calming effects. However, the angiosperms all around us have much more important reasons for releasing certain aromas. Plants are sessile organisms, which means they do not have the capacity for self-locomotion, as many animals do. Instead, plants rely on their sense of smell to communicate with other organisms and respond to their environment. In other words, plants can detect scents in their surroundings, and they also produce scents for advantageous purposes.
What is the function of floral scent?
In the horticulture world, scent is a chemical language that plays a fundamental role in communication. Each scent is a tiny but complex organic molecule – a volatile organic compound or VOC – that easily vaporizes when released from a plant. Once in the atmosphere, the olfactory organs of some insects, bats, and other pollinators can detect the scent. Surprisingly, no two plant species produce the exact same scent. Furthermore, pollinators can discriminate who’s who among a vast array of plants in their surroundings. It turns out that this interaction is a fascinating story.
The unique scent of any flower serves several purposes. A floral scent may attract potential pollinators to support plant reproduction and signal or guide certain animals to its food sources at just the right time. For example, plants that rely on pollination by bees or butterflies have relatively sweet scents, and their flowers emit the highest scent levels during the daytime, especially from mature flowers ready to offer ample pollen to visiting insects. Conversely, plants that rely on pollination by moths or bats produce the highest scent levels at night, when those animals are actively seeking food sources.
These species-specific volatile organic compounds allow pollinators to distinguish the various scents of angiosperms. Just as flower shape and color entice certain pollinators to desirable flowering plants based on their own body shape and color preferences, scent attracts insects and other animals to distinct flower species. This enhances their capacity for effective pollen transfer while they are foraging for food resources. Thus, scent can optimize successful angiosperm reproduction.
But not all pollinators are lured by sweet-smelling flowers. Most flies and some beetles prefer foul or putrid odors, such as that released by the corpse plant, Amorphophallus titanium, which smells like rotting flesh. This plant lacks showy petals but boasts a large tube-like structure that contains tiny flowers inside. Carrion-feeding flies and beetles are attracted to the widely-dispersed stench of this huge plant. In addition, the corpse plant, or titan arum, uses a biochemical process called thermogenesis to create heat during flowering. This leads to more rapid release of VOCs and heightened attraction for scavenging pollinators who transfer pollen from male to female flowers when they visit.
A flower’s scent may also serve as a deterrent to herbivores browsing for their next meal, providing a type of safety shield to help ensure the plant’s survival. For example, some herbs such as rosemary, basil, and mint contain essential oils that give off a remarkably strong smell, which serves as a defense mechanism against hungry deer, rabbits, and other four-legged creatures. And the sharp, tangy scent of sage, thyme, garlic, and onions deters aphids, slugs, and squirrels from chomping on those plants, protecting them from destructive herbivorous activity.
Of course, some flowering plants are “generalists,” emitting scents that appeal to a wide variety of pollinators to assist with fertilization. Others, such as the Soaptree yucca or Yucca elata, attract just one kind of insect. In that example, only the yucca moth responds to the scent of that plant.
Regardless of the unique smells they produce, most angiosperms time their scent output to coincide with maximum pollen availability in their flowers. Mature flowers release more intense scents than young flowers, but later reduce the quantity and quality of their floral bouquets, thereby directing pollinators to move on to other flowers to increase the efficiency of the reproductive process.
How do plants produce and control their scents?
Plants use different scents to communicate with other organisms, but they also deliver messages internally, telling their own leaves, roots, stems, and flowers when to grow, develop, and make use of resources. Researchers recently discovered that plants have “transporter” proteins to control the emission of the volatile organic compounds that produce odors and fragrances. Located in cell membranes, these transporters determine the timing and quantity of VOCs to be released within and from a plant.
Symbiosis
Scent’s role in flowering plants includes the development of vital symbiotic relationships between angiosperms and pollinators. Floral fragrances are special signals that announce, “If you visit my flowers, you will reap desired rewards.”
To illustrate this partnership, insect and bird pollinators benefit from gathering food resources like nectar and pollen from flowering plants while simultaneously gathering materials they can use for building nests or hives, such as propolis. In turn, the flowering plant benefits from the abundant pollen transfer which ensures angiosperm fertilization and reproduction.
Plant Selection for a Fragrance Garden
If you are intrigued by floral aromas, consider planning a fragrance garden. Your sense of smell is intricately connected to your emotions, and certain scents can evoke pleasurable memories. Selecting plants based on their olfactory appeal, as well as their visual interest (color, texture, foliage, size, shape, bloom time), can elevate the overall impact of your garden design.
Keep in mind that site location and conditions will determine optimal choices for your new garden spot, but if you decide to focus on fragrance, find a convenient place just beyond your doorstep where you can readily enjoy the aromatic scents. For easy reference, here are some excellent sources of information:
Scent-sational plants for a fragrance garden
Fragrant Native Plants | Virginia, USA | Plant NOVA Natives
Fragrant Native Plants for the Mid-Atlantic Region
5 Fragrant Plants to Make Your Garden Smell Amazing
Resources