The Fragrance Of Flowers: Nature's Aromatic Mystery

Flowers emit aroma to attract insects and other animals for pollination. The scent is typically a complex mixture of low molecular weight compounds emitted by flowers into the atmosphere. The structure, colour and odour of the scent are critical factors in attracting pollinators. Although flowers can be identical in their colour or shape, there are no two floral scents that are exactly the same because of the large diversity of volatile compounds and their relative abundances and interactions.

To attract pollinators

Flowers emit aroma to attract pollinators, such as insects and hummingbirds. The scent is a signal that directs pollinators to a particular flower whose nectar and/or pollen is the reward. The structure, colour, and odour of the scent are critical factors in attracting pollinators. The scent also functions as both long- and short-distance attractants and plays a prominent role in the localisation and selection of flowers by insects, especially moth-pollinated flowers, which are detected and visited at night.

The colour of the flower also plays a role in attracting pollinators. For example, plants with red or yellow flowers tend to attract butterflies and hummingbirds. Some flowers feature nectar guides that are specific to particular pollinators.

The shape of the flower is another factor that attracts pollinators. For example, the flower of the native azalea is uniquely shaped for a ruby-throated hummingbird to sample the nectar and contact the exserted stamens and stigma.

The size of the flower is also important, as it needs to provide a broad landing area for pollinators. For example, the large flowers of the American lotus are easily seen from afar and provide a broad landing area for beetles.

To repel harmful insects

Flowers emit aromas for a variety of reasons, one of which is to repel harmful insects. Some plants, when infested with herbivores like caterpillars, release a blend of aromatic compounds to attract beneficial insects like parasitic wasps, which then kill the parasites. This is an example of how flowers use their scent to protect themselves from harmful insects.

Flowers can also emit aromas that are unpleasant to certain insects, causing them to stay away. For instance, the strong scent of lavender is known to be disliked by mosquitoes, making it an effective natural bug repellent. Similarly, basil, thyme, mint, and citronella plants are used to repel various insects, including mosquitoes, moths, and flies.

In addition to emitting aromas, some plants have natural defenses that make them unappealing to insects. For example, the true perennial geranium, Russian sage, and lavender are known to deter insects due to their tough, fuzzy, or hairy foliage. The lily of the Nile possesses thick, strappy foliage that easily shrugs off insect invaders.

The use of flowers and plants that emit aromas to repel harmful insects provides a natural and eco-friendly alternative to chemical pesticides. These plants not only help keep bugs away but also add beauty and fragrance to any outdoor space.

To communicate with other plants

Flowers emit aromas to communicate with other plants. They use volatile organic compounds (VOCs) to interact with various hosts and determine locations. For example, tomato plants emit VOCs that induce responses in neighbouring tomato plants. Within seconds, the neighbouring plants experience a rapid depolarisation in cell potential and increase in cytosolic calcium.

VOCs can also help plants defend themselves against herbivores. For instance, the Venus Fly Trap emits VOCs to specifically target and attract starved prey.

Plants also use VOCs to communicate with other species. For example, the Cuscuta pentagona (field dodder) uses VOCs to distinguish between wheat and tomato plants.

To lure in food

Flowers emit aromas for a variety of reasons, one of which is to lure in food. This process is known as luring in pollinators, typically insects and birds, which help the flowers reproduce. The fragrance of the flower signals to the pollinators that the plant is ready to be fertilized.

The scent of a flower is a complex mixture of low molecular weight compounds, with each scent compound being an organic molecule known as a volatile—a chemical that vaporizes into a gas when released by a plant. These chemicals are minuscule, allowing them to evaporate into the air where they can be sensed by the olfactory organs of insects or birds. The structure, colour, and odour of these compounds are critical factors in attracting pollinators.

Flowers that are pollinated by bees and flies tend to have sweet scents, while those pollinated by beetles have strong musty, spicy, or fruity odours. For example, the European centaury emits an odour that attracts foraging insects but offers no food, while the Soaptree yucca emits an aroma that attracts a single species of yucca moth.

The production of scent is a balancing act for plants. They must generate enough smell to induce insects to fertilize their flowers, but not so much that they waste energy and carbon. In addition, plants tend to have their scent output at maximal levels only when the flowers are ready for pollination and when their potential pollinators are most active. For example, plants pollinated by moths and bats tend to produce the most scent in the evening or at night, while plants pollinated by bees and butterflies produce the most scent during the day.

To attract humans

While flowers do not emit aromas for humans, humans have long appreciated the scents of flowers, using them for aromatherapy and perfumes. Aromatherapy using flower oils is used to treat a range of conditions, from headaches to the lasting effects of torture.

The ancient Egyptians were perhaps the first to use perfume as a cultural resource, with perfume playing an integral part in religious ceremonies and the embalming of the dead. The importance of scent continued through history, with the Persian physician Ibn Sina discovering how to distill roses into oils over a thousand years ago. This technique laid the foundation for modern perfume creation.

Today, humans continue to be drawn to the scents of flowers, with many flocking to botanic gardens to experience the rare bloom of the corpse flower, which emits a noxious odour reminiscent of rotting flesh.

While floral aromas are not intended to attract humans, humans have certainly found ways to benefit from and appreciate the scents of flowers.

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