「植物看得見你」公開課筆記／3.3 Ethylene & Fruit Ripening

In ancient China, the peasants would burn incense in a room full of pears in order to get them to ripen. They were sure that the incense had some magical property, maybe through the Gods, that help the fruit ripen. Similarly, in the early 20th century, citrus farmers in Florida would ripen oranges in sheds that were heated with kerosene heaters.

Maybe you consider that it was heat, but surprisingly that when electric heaters came into use, it was useless in causing the oranges or lemons to ripen. This showed that it wasn't the heat that was causing the fruit to ripen, it was actually the smoke itself.

In 1924, a USDA scientist discovered that within the smoke, there's one particular volatile chemical which is called ethylene. It induces the fruit ripening. It's a very small molecule, two carbons, four hydrogens, and it's a gas. It diffuses in the air spaces within and around the plant, where the plants are so sensitive that they can respond to one molecule within 100 million molecules of air.

Why would fruits respond to this bi-product of kerosene smoke or of incense smoke? What do they care about what we're burning?

If you have an avocado that you've bought in the market, but it's still very hard, how can you get this avocado to ripen? You would put it together in a bag with a very ripe banana.

The reason goes back to a finding from the 1930s by Richard Crane, from Cambridge University. He discovered that ripening apples release ethalyne. Actually all fruits release ethylene as they ripen naturally. Ethylene is a universal plant hormone found in all plants, necessary for fruit ripening.

Ecologically, colored fruit is a great advertisement for animals or humans, who want to come pick the fruit to eat it. From the plant's point of view, the plant wants to have its fruit eaten, that way it can disperse its seeds. If all of the fruit ripen at the same time, there is an added increased display of color, which is more apt to call the herbivores that the fruit-eating animals to come.

Can we actually say that the fruits are talking to each other?

Ethylene also regulates other plant processes, such as leaf senescence and petal senescence. Senescence is when the leaves age and fall off the off the tree or the plant. This was actually also noticed in 19th century Victorian England, because all of the lights were powered by gas -- Kerosene lamps, which give off ethylene. It had the awful side effect of killing most of the house plants. A plant called Aspidistra is naturally resistant to ethylene, which became one of the most popular houseplants in 19th century.

Because of these qualities, ethylene is immensely important in agriculture. Farmers are looking for ways of actually removing ethylene from fruits that are in transit, or from flowers that are in shops, in order to prolong both fruit life and flower life on the shelf.