From the Ground Up: What is fragrance?

Have you ever wondered why some flowers, such as lilacs and roses have a lovely fragrance and others, especially some of the newer hybrids, have little or none? Generally speaking, the answer is in the part of the flower referred to as nectaries. Because native and 'Heritage” plants retain the original genetic make-up of the flower, the nectaries produce the full fragrance of the plant. In hybrids, different cultivars are combined to bring out certain traits like color or size and the nectaries function is not kept intact, so there is little or no fragrance.

The production of fragrance is not for the benefit of our human nose. Flowers use their scents to attract pollinators and communicate with other plants. Many flowers emit scents to aid reproduction. Nectar-feeding pollinators include many species such as bees, butterflies, moths, flies, hummingbirds, and even mosquitoes. For carnivorous plants such as the Venus Fly trap and Pitcher plant, fragrance lures in insects to be captured and digested within the flower cavity for food. Two-thirds of our food production relies on insect-based pollination - think honeybees - so in turn, we rely on a flower's ability to produce fragrance for our food.

Some flowering plants are non-discriminating, using their fragrances to attract a host of insects and birds to fertilize their flowers. Others specialize in releasing scents that only appeal to a particular insect. Pollination occurs when an insect crawls or lands on the pollen bearing flower parts, takes some pollen with it, and unknowingly deposits that pollen on another flower of the same species, hence pollinating the plant and reproducing it through seed.

Nectaries secrete sugars and amino acids to form both nectar and fragrance molecules. Each fragrance compound is a molecule known as a volatile that vaporizes into a gas when released by the plant. Although flowers may appear identical in color or shape, they won't have the same fragrance because of the large diversity of volatile compounds that make up fragrance. To date, scientists have catalogued 1,700 different fragrance compounds produced by flowers, with some single plants having a mix of more than 100 compounds.

Fragrance is a signal that directs pollinators to a particular flower whose nectar or pollen is the food reward. Species pollinated by bees and flies have sweet fragrances, while those pollinated by beetles have strong musty, spicy, or fruity aromas. Little is known about exactly how insects respond to these compounds within floral scents, but it's clear that they are capable of distinguishing among the complex fragrance mixtures. The science behind fragrance is fascinating - and we just thought flowers smelled wonderful.

' Becki Lynch is a Linn County Master Gardener. For questions, contact the Linn County Master Gardener hortline at (319) 447-0647 from 10 a.m. to noon weekdays.