Have you heard the one about putting the banana in the paper bag with the unripe avocado? Leave the bag on the counter for a couple of days and the avocado ripens up. Those are fruits communicating. They’re smelling each other.
Fruits that ripen after being picked, called climacteric fruits,* become softer and sweeter thanks to a plant hormone called ethylene. The gas, produced by the fruits themselves and microorganisms on their skin, causes the release of pectinase, hydrolase and amylase. These enzymes ripen fruits and make them more appealing to eat. A plant can detect the volatile gas and convert its signal into a physiological response. Danny Chamovitz writes in What a Plant Knows that a receptor for ethylene has been identified in plants, and it closely resembles receptors in the neural pathway we have for olfaction or smell.
The gas was discovered in 1901 by a 17-year-old Russian scientist named Dimitry Neljubow of the Botanical Institute of St. Petersburg. I like to imagine Neljubow at his window, gazing at trees twisted and abnormally thickened by their proximity to street lights—why did lights do that?
Neljubow appears to have come to his revelation about ethylene through the careful study of germinating pea plants inside his lab. He planted peas in a pair of pitch-black boxes. Into one, he pumped air from the outside; the other he fed air from his laboratory. Those peas fed the laboratory air grew sideways and swelled up. He then isolated ethylene found in the “illuminating gases” burned by lamps in his lab and on the streets at night
In the 1930s, Florida orange growers noticed something similar. When they kept fruits warm with kerosene heaters, the heat itself did not ripen up the oranges, and yet the fruits ripened (and sometimes rotted). The fruits smelled the ethylene in kerosene, much like you or I would get a whiff wafting over from a neighborhood barbecue. And that’s something we know because of a chance discovery hastened by some leaky pipes in Neljubow’s lab.
Photo of peas grown in increasing concentrations of ethylene by J.D. Goeschle/Discoveries in Plant Biology, 1998. Thanks to Robert Krulwich for inspiration on this one.
* Climacteric fruits include apples, avocados, bananas, cantaloupes, peaches and tomatoes. Others, such as cherries, grapes, oranges and strawberries, do not ripen after being picked.