If you walk through a park, you may enjoy the scents(气味）of flowers, water and soil. If you are especially lucky, you will get to smell fresh-cut grass. It's a beloved scent that transports many people back to their childhood. But there's a dark side to that smell. Indeed, we are in love with the scent of fear.
Plants have many different chemical defenses. That smell is one of them, as the grass responds to an attack, signaling(向......示意）to the surrounding grass that danger is coming. The fresh, "green” scent of a just-mowed lawn is the lawn trying to save itself, says a story at science website Real-Clear Science.
The smell is produced by a mix of chemicals called green leaf volatiles(GLVs, 绿叶挥发物）. When the leafy plants are injured by animals eating them, people cutting them, or any other rough treatment-they give off GLVs into the air. These GLVs are a warning to neighbouring plants that their flowers might be removed, so they can move resources like sugar toward their roots and away from their flowers. This reduces a plant's potential losses and can help it grow back later.
The rush of GLVs does a few other things, too. One chemical helps to close the wound on the damaged plant. Others act as antibiotics(抗生素）and prevent bacterial infections.
Some GLVs may also react with animals that eat plants. Silke All-mann and Ian T.Baldwin, researchers from the University of Amsterdam, Netherlands, found that some caterpillars(毛虫）are changed by the GLV compounds(化合物）when they eat a tobacco plant. Tobacco makes the caterpillars more attractive to predators(捕食者）. For the tobacco plants, this is like having an older brother come to beat up your bully(欺凌者）.
Thankfully, nothing is coming to eat you when you mow the lawn. Instead, we human get treated to some great-smelling GLVs. One is a compound known as"cis-3-hexenal". This is the same chemical that gives strawberries their sweet scent. Similar compounds are also found in apples and olives.
＂Just about all fresh vegetables have some GLV scents to them," Baldwin told Live Science, and fruits may release the compounds as they soften." Throughout evolutionary history, we've used that information to know when something is ripe," Baldwin said. Now we can use it to know when grass is frightened.
