Making scents: The aromatic world of flowers

The miniscule molecules in plant fragrances have powerful effects

BEN GUARINO • JANUARY 3, 2013

From putrid corpse lilies to perfumed petunias, flowers expel a diverse range of scent molecules into the air. But flowering plants don’t release these odors for the benefit of human noses. Flowers use their scents to attract pollinators, communicate with other plants and for some carnivorous species, lure in food.

More than a thousand years ago Persian physician Ibn Sina discovered how to distill roses into aromatic oils. He placed the flowers in a heated chamber to create a vapor of scented oil and water; after the steam condensed Ibn Sina collected the oil that floated to the top of the watery mixture. Until the past few decades, however, the biochemical process of scent production remained a mystery. In 1953, chemists knew of 20 chemicals in a rose’s fragrant bouquet; by 2006, they had discovered roughly 400. To date, scientists have catalogued 1,700 different scent compounds produced by flowers. Some plant odors are brews of more than 100 compounds, says Natalia Dudareva, a biochemist at Purdue University.

An ounce of rose oil requires the distillation of more than 600 pounds of rose flowers [Image credit: flickr]/一盎司的玫瑰精油需要蒸馏600多磅的玫瑰花

“Scent is a complex mixture of low molecular weight compounds,” says Dudareva. Each scent compound is an organic molecule known as a volatile, a chemical that vaporizes into a gas when released by a plant. These chemicals are miniscule – so small, in fact, that if the average American had the same mass as the moon, the heaviest odor molecule would have the mass of a marshmallow. The tiny size of the molecules allows the compounds to evaporate into the air, where the olfactory organs of insects or bats can sense the odor.

植物释放香味的部分取决于品种。对于许多开花植物来说，气味的产生并不局限于一个区域，而是分布在花瓣的外层和花的其他部分。但有些花，比如兰花，有专门的气味腺体，叫做渗透细胞，它会渗出带有气味的液体，这种气味一接触空气就会蒸发。

Unsurprisingly, many flowers emit scents to aid reproduction. Some flowering plants are generalists and use their odors to entice a host of insects and birds to fertilize their flowers. Others specialize, releasing scents that only appeal to a particular insect. The Soaptree yucca, for example, emits an aroma that attracts a single, aptly named species of yucca moth. As pollinators travel from flower to flower, they collect and deposit pollen, fertilizing the plants.

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

气味的产生是一种平衡行为:植物必须产生足够的气味来吸引昆虫给它们的花施肥，但又不能太多，否则会浪费能量和碳。事实上，对于许多物种来说，气味的释放并不是恒定的；金鱼龙在授粉后36小时会减少气味的产生。

Scientists observe cyclic scent production when studying snapdragons (above) [Image credit: Natalia Dudareva]/科学家在研究金鱼兰时观察到循环气味的产生(上图)

Many 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 beckons butterflies with promises of nectar. Like the creation of scent, nectar production costs the plant resources and energy. Some flowers have evolved to exploit this symbiosis, mimicking the scents of other flowers but giving no benefit to pollinators. The European centaury, for example, emits an odor that attracts foraging insects, but offers no food.

Not only can plants mimic other plants, but some flowers mimic animals. The flower of the bee orchid smells and looks, to the male bee, just like a female bee. The duped male bee attempts to copulate with the orchid’s petals, and the insect spreads pollen between the deceptive flowers. And flowers don’t just imitate bees. A few orchid species smell like female flies, and others replicate the aroma and texture of scarab beetles.

A bee orchid, which mimics the coloration and scent of a female bee [Image credit: Ian Capper via geograph.org]/蜜蜂兰花，模仿雌蜂的颜色和气味

Researchers such as Purdue’s Dudareva study petunias as models for odor-producing flowers. Current biochemical studies are uncovering the processes by which these plants create amino acids, chemicals used in the production of volatile scents. Aromatic amino acids aren’t just precursors to plant fragrance. Some, such as tryptophan, are essential for protein production in animal cells. By studying the ways plants craft odor, scientists learn about the creation of molecules that sustain all forms of life.

普渡大学的Dudareva等研究人员将矮牵牛花作为产生气味的花朵的模型。目前的生物化学研究正在揭示这些植物产生氨基酸的过程，这种化学物质用于产生挥发性气味。芳香氨基酸不仅仅是植物香味的前体。其中一些，如色氨酸，是动物细胞中产生蛋白质所必需的。通过研究植物制造气味的方式，科学家们了解了维持所有生命形式的分子的产生。

Petunias growing in a laboratory greenhouse [Image credit: Natalia Dudareva]/实验室温室里的矮牵牛花

To this day, the flowers that Ibn Sina distilled a millennium ago – damask roses – are the single most important plant species used in perfumery. A few perfumers still use Ibn Sina’s steam extraction technique, albeit modernized, to obtain scents. But the most common method employed today requires submerging flowers in chemicals such as benzene, which disintegrates plant matter but leaves behind the aromatic oils. Once extracted, rose oil can supply a perfume’s entire fragrance, or the oil can be one ingredient in a mix of hundreds.

Flower volatiles can do more for humans than just cover up stink. Caregivers who practice aromatherapy apply flower oils to stop everything from headaches to the lasting effects of torture. But a few doctors, such as psychiatrist Stephen Barrett, dismiss this field as a pseudoscience, citing aromatherapy advocates who make unverifiable claims. In 2008, a team of Australian clinical neuroscientists reviewed the scientific literature regarding aromatherapy. The researchers found that support for aromatherapy was limited, but certain studies showed promising effects, particularly in reducing stress.

In other words, stopping to smell the roses just might put a mind at ease.

换句话说，停下来闻闻玫瑰的花香可能会让你的心灵放松下来。