The Fragrance Mystery: Why Flowers Change Their Scents

Flowers are known to change their aroma to attract pollinators or to repel harmful insects. Floral scents are composed of volatile organic compounds (VOCs) or aroma compounds emitted by floral tissue. The scent of a flower may contain as few as seven to ten different oils, as in snapdragons or petunias, or as many as 100 different chemicals, as is the case with orchids. The scent of a flower is important for plant-pollinator interactions and can also be used to ward off herbivores.

To attract pollinators

Flowers change their aroma to attract pollinators, which is essential for their reproduction. The scent of flowers is composed of volatile organic compounds (VOCs) or aroma compounds emitted by floral tissue, such as flower petals. These VOCs are detected by flower visitors like insects and bats, who use them to locate their host species and choose their flowers.

The aroma of flowers plays a crucial role in mediating plant-pollinator interactions, along with visual cues such as colour, shape, and size. Flowers with showy petals and sepals, nectar guides, unique shapes, large sizes, and distinct colours often attract pollinators. For example, the blue petals of the bird's-foot violet attract bees, while red or yellow flowers tend to attract butterflies and hummingbirds.

The scent of flowers can be quite complex, with some plant odours consisting of more than 100 compounds. The creation of these scents is a delicate balance for plants, as they must generate enough smell to entice pollinators while also conserving energy and carbon. Flowers have different strategies to achieve this balance. For instance, snapdragons decrease scent production 36 hours after pollination, while sexually deceptive orchids emit scents after pollination to reduce their attractiveness to pollinators, directing them towards unpollinated flowers.

Flowers can also mimic the scents of other flowers or even animals to attract pollinators. The European centaury, for example, emits an odour that attracts foraging insects but offers no nectar reward. The bee orchid, on the other hand, mimics the scent and appearance of a female bee, tricking male bees into attempting copulation and inadvertently spreading pollen between the deceptive flowers.

To communicate with other plants

Flowers can change their aroma to communicate with other plants. This form of communication is called plant-plant communication. Plants have a wide array of volatile compounds that they can release to signal other plants. By releasing these cues, plants can learn more about their environment and respond accordingly.

One of the most common types of chemical signals released by plants is volatile organic compounds (VOCs). VOCs are small molecules released as gases that can diffuse through the air away from the plant that produces them. Some VOCs are chemically similar to the pheromones used by many animals. For example, ants use pheromones to find food and identify their nestmates. One of the most common plant VOCs is methyl jasmonate (MeJA), which is produced and released by plants that are under attack. When released, MeJA moves through the air to the non-damaged parts of the same plant and to neighbouring plants, where it activates defence mechanisms.

Plants can also release VOCs to attract beneficial fungi or bacteria. For example, fungi can help plant roots absorb soil nutrients more efficiently, and bacteria can convert nitrogen gas from the atmosphere into nutrients that help plants grow faster.

The composition of floral compounds and the rate of their release are the potential factors that control attraction and repulsion. These two elements can be in response to ecological cues like high plant density and temperature. For instance, in sexually deceptive orchids, floral scents emitted after pollination reduce the flower's attractiveness to pollinators. This mechanism acts as a signal to pollinators to visit unpollinated flowers.

Environmental conditions can also affect plant communication and signalling. For example, high temperatures increase the rate of releasing floral compounds, which can increase the amount of signal released and thus its ability to reach more plants. When plant density increases, plant communication increases as well, as plants are physically closer to each other and their signals can reach many neighbouring plants. This can also increase the signal's reliability and lower the chance that the signal will degrade before it can reach other plants.

To lure in prey

The aroma of flowers is composed of volatile organic compounds (VOCs) or aroma compounds emitted by floral tissue. These scents are one of the most important communication channels mediating plant-pollinator interactions.

Flowers use their scents to attract pollinators, but some carnivorous species also use them to lure in prey. For example, the orchid mantis, which resembles a flower, takes on this appearance to attract prey. Orchid mantises are even better at drawing in insects than some actual blossoms. They not only look like flowers, but they also smell like them, making them extremely effective predators.

Some carnivorous plants use a combination of visual and olfactory signals to attract prey. For instance, certain North American pitcher plants (Sarracenia) and some Asian pitcher plants (Nepenthes) have evolved to emit olfactory and visual signals that attract flower-visiting insects into their traps. These plants produce high-contrast UV patterns, similar to the bull's-eye UV patterns of flowers, along with extrafloral nectar and a flower-like odour.

The Venus flytrap is another example of a carnivorous plant that uses olfactory cues to attract prey. While its traps are visually striking, they also emit a scent that may attract insects from a close distance. The main compounds found in the Venus flytrap's scent, methyl salicylate and benzyl alcohol, are commonly found in flowers and are likely used to signal nectar-seeking insects.

In summary, while most flowers use their aroma to attract pollinators, some carnivorous species have evolved to use scent as a lure to attract prey, employing aggressive mimicry to entice unsuspecting insects.

To repel harmful insects

Flowers release scents composed of volatile organic compounds (VOCs) or aroma compounds. One of the primary functions of floral scents is to deter herbivores and insects that feed on leaves.

Flowers can change their aroma to repel harmful insects. For example, the Soaptree yucca emits an aroma that attracts a single species of yucca moth, which acts as a pollinator. However, some flowers have evolved to exploit this symbiosis, mimicking the scents of other flowers but offering no benefit to pollinators. The European centaury, for instance, emits an odour that attracts foraging insects but does not produce nectar.

Flowers can also change their scent to repel insects after successful pollination. In tropical orchids, floral scent emission is terminated immediately following pollination, reducing energy expenditure on fragrance production. Similarly, snapdragons decrease scent production 36 hours after pollination.

In addition to floral scents, essential oils derived from plants can also effectively repel insects. For example, lavender, basil, thyme, peppermint, and citronella are all known to have insect-repellent properties. These oils can be applied topically, diffused, or used in sprays to deter insects.

To signal readiness for pollination

Flowers change their aroma to signal readiness for pollination. Floral scent is one of the most important communication channels mediating plant-pollinator interactions. The primary functions of floral scent are to deter herbivores and attract pollinators. The scent of a flower is composed of volatile organic compounds (VOCs) or aroma compounds emitted by floral tissue.

Flowers use their scents to attract pollinators and communicate with other plants. Floral scent is often composed of hundreds of VOCs in very variable proportions. The method used to analyse floral scent is a trade-off between accurately detecting and quantifying minor compounds and avoiding detector saturation by major compounds.

The creation of scent is a balancing act: plants must generate enough smell to induce insects to fertilise their flowers, but not so much that they waste energy and carbon. In fact, for many species, scent emission is not constant; snapdragons, for example, decrease scent production 36 hours after pollination.

Flowers offer a reward to pollinators in the form of nectar or nest-building materials; fragrances signal to animals that such incentives are available. The sweet smell of honeysuckle, for instance, beckons butterflies with promises of nectar.

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