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Why do flowers have scents?

Natalia Dudareva, an associate professor in the department of horticulture and landscape architecture at Purdue University, explains.

Flowers of many plant species produce a scent. This scent is typically a complex mixture of low molecular weight compounds emitted by flowers into the atmosphere and its structure, color and odor are critical factors in attracting pollinators. Although flowers can be identical in their color 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. Thus, scent is a signal that directs pollinators to a particular flower whose nectar and/or pollen is the reward. Volatiles emitted from flowers function as both long- and short-distance attractants and play a prominent role in the localization and selection of flowers by insects, especially moth-pollinated flowers, which are detected and visited at night. Species pollinated by bees and flies have sweet scents, whereas those pollinated by beetles have strong musty, spicy, or fruity odors.

To date, little is known about how insects respond to individual components found within floral scents, but it is clear that they are capable of distinguishing among complex scent mixtures. In addition to attracting insects to flowers and guiding them to food resources within the flower, floral volatiles are essential in allowing insects to discriminate among plant species and even among individual flowers of a single species. For example, closely related plant species that rely on different types of insects for pollination produce different odors, reflecting the olfactory sensitivities or preferences of the pollinators. By providing species-specific signals, flower fragrances facilitate an insect's ability to learn particular food sources, thereby increasing its foraging efficiency. At the same time, successful pollen transfer (and thus, sexual reproduction) is ensured, which is beneficial to plants.

Plants tend to have their scent output at maximal levels only when the flowers are ready for pollination and when its potential pollinators are active as well. Plants that maximize their output during the day are primarily pollinated by bees or butterflies, whereas those that release their fragrance mostly at night are pollinated by moth and bats. During flower development, newly opened and young flowers, which are not ready to function as pollen donors, produce fewer odors and are less attractive to pollinators than are older flowers. Once a flower has been sufficiently pollinated, quantitative and/or qualitative changes to the floral bouquets lead to a lower attractiveness of these flowers and help to direct pollinators to unpollinated flowers instead, thereby maximizing the reproductive success of the plant.

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