Definition
Ethylene is a gaseous plant hormone that carries signals when a plant grows, when fruit ripens, and when leaves fall. It is also one of the most important basic chemicals used to make the plastic bags and plastic products we use every day.
Why a banana ripens faster next to an apple
Seal a green banana or an astringent persimmon in a plastic bag with a ripe apple, and in a few days it's ready to eat. As the apple "breathes," it releases an invisible signal into the air that tells the fruit around it to ripen.
That signal is ethylene, a gaseous plant hormone. Most plant hormones travel through the tissues inside a plant. Ethylene is a gas, so it drifts through the air. That makes it an unusual hormone that passes its signal on to nearby tissues and even to neighboring plants.
Once fruit picks up the ethylene signal, a lot changes inside. The firm cell walls loosen and soften. Starch turns into sweet sugars. Sourness and astringency fade. The green chlorophyll in the skin breaks down, letting the fruit's bright colors show through.
The food industry puts this to good use. Bananas and tomatoes are picked firm and green where they grow, so they survive the trip. Then, just before they go on the shelf, they are given ethylene gas on purpose so they ripen just right.
A survival signal that drops leaves and guards the plant
Ethylene does more than ripen fruit. It helps plants survive in many ways. When the seasons turn and cold winter approaches, a tree has to drop leaves it no longer needs in order to hold on to its water. Ethylene is at work here too.
When a leaf reaches the end of its life, ethylene urges the base of the leaf stalk to form an abscission layer that cuts the leaf loose. An abscission layer is a special band of tissue that dissolves the cell walls where leaf meets branch and creates a dividing line. If a tree can't drop its leaves in time in the fall, winter winds can strip away all its moisture and the tree may dry out.
Ethylene is also released quickly when a plant is damaged, such as when strong wind bends it or insects attack it. When the injured tissue sends out a danger signal, the surrounding parts reinforce their cell walls and make defensive substances that keep germs out.
In this way, ethylene acts as a shield that helps plants respond flexibly to a changing environment, making up for the fact that they can't move.
A bit more precisely: the "rice" of petrochemicals that gives us plastic
Looking at it from a chemist's point of view, ethylene is made of 2 carbon atoms and 4 hydrogen atoms (C₂H₄). It is the simplest hydrocarbon with a double bond. The double bond between the two carbon atoms makes it easy for ethylene to react with other molecules.
That reactivity makes it possible to link ethylene molecules one after another into a long chain. Join tens of thousands of them together, and you get polyethylene (PE), a classic plastic used for grocery bags and all kinds of packaging.
Today, ethylene matters so much to the chemical industry that a country's output of it is used to judge how strong its chemical industry is. That's why industry people nickname ethylene the "rice of petrochemicals," meaning a staple the whole industry feeds on.
It is a light gas that carries messages between plants, and it is also an industrial raw material that forms the backbone of countless everyday goods. In that sense, ethylene is a rare substance that links nature and industry.
Common misconceptions
Ethylene gas is a harmful substance that makes fruit rot.
Ethylene doesn't cause rot. It is an essential plant hormone that helps fruit ripen into something tasty and helps plants protect themselves. That said, if fruit keeps being exposed to ethylene after it is already ripe, it can age too fast and turn mushy.
Where you meet it
Put an astringent persimmon in a box with a ripe apple, and within a few days it turns into a sweet, soft, fully ripe persimmon.
Florists use preservatives that block the action of ethylene to keep flowers from wilting or dropping petals too early.
In one sentence
Ethylene is a gaseous plant hormone that ripens fruit sweet and makes leaves fall, and it is also a key chemical raw material for making plastic.