By Erin Marissa Russell

What Plant Stress Is Like

You may be surprised to learn just how similar plant behavior in the face of stress can be to our own human backlash against stress, according to visual symptoms and other the reactions we are able to detect. Parts of the stress response in plants seem downright human, though unlike in humans, the fight or flight response is not an option for plants because whereas we can escape, they are quite literally rooted to the spot.

Just like our bodies release hormones when we endure stress, plants also also have a hormonal response to stressful conditions. However, hormonal reactions to stress in plants are specifically crafted to counterbalance the exact form of stress the plant is experiencing. For example, a plant going through drought responds by sending abscisic acid to its root system. The abscisic acid works to close little openings in the root system called stomata that are used during photosynthesis—water is lost through these passages, so closing them during a drought helps the plant retain its precious water stores.

Or in response to a caterpillar munching on its foliage, for instance, a plant generates jasmonic acid, which starts a chain reaction that sets chemicals loose in the air as a distress signal to prepare nearby plants for an onslaught of hungry insects. The jasmonic acid also serves as a beacon to bring in beneficial insects that prey on the offending caterpillars.The smell of freshly cut grass, pleasant as it may smell to us, is one such distress signal that plants emit in response to the mangling and slashing they receive from a lawnmower’s blades.

Another way plants respond to stress is by working to heal the damage resulting from the stressful conditions. For example, some plants use a group of compounds called green leaf volatiles much like we use antiseptics, as these compounds support healthy healing of damaged tissue and prevent infection from bacteria or fungi.

A study by the National Center for Atmospheric Research found another form of botanical first aid against stress damage. They observed plants that were suffering from drought conditions and extreme temperatures that boomeranged between unseasonably chilly nights and scorching hot days. They noted the plants were producing unusually high amounts of methyl salicylate, a chemical form of aspirin. The researchers believe that in plants, this chemical form of aspirin activates systemic acquired resistance, which is kind of like a plant’s version of our immune system, helping the plant heal from the current illness and working to ward off future health problems.

One of the most surprising plant responses to stress (and really one of the coolest) is that, like a frustrated person who shouts into a pillow, a stressed plant will actually let out a scream, too. A study from Tel Aviv University in Israel reported screams from plant suffering through drought conditions, with a frequency of up to 11 sounds per hour from tobacco plants and 35 sounds each hour from tomato plants, or when their stems are cut, after which tobacco plants made 15 sounds an hour and tomato plants made noises 25 times every hour. As a comparison, plants that are not undergoing some type of stress tend to make sounds less often than once every hour. Noises like those the study monitored in tobacco and tomato plants were also reported coming from spiny pincushion cacti and henbit deadnettle weeds.

We don’t hear these auditory signals from plants because the pitch is in the ultrasonic range, too high for our human ears to pick up. But other plants or even animals may be able to hear the screams of stressed plants in their vicinity, as their hearing can detect sounds from the ultrasonic range. Making these sounds requires much less energy than what is required for a plant to emit the chemical distress signals we mentioned earlier, and they can quickly contact plants for meters in every direction.

New technology is making it possible for us to hear a translated version of the noises plants make by converting biodata, or the signals that run through plant cells, into sounds the human ear can perceive. This video from Yale’s School of the Environment features an installation in their forest garden that will let you listen to how a device called the MIDI Sprout, when attached to the leaves, translates the sounds plants make into a form humans can hear.

You can take a deeper dive by comparing the sounds sick and healthy plants make in this video from PlantWave. The sick plant begins at two minutes and 55 seconds, while the healthy plant’s sounds begin around five minutes in. If you’d like to learn more about how the MIDI Sprout converts biodata from plants into audible sounds, you’ll enjoy this video from the device’s creators, titled “How to Make Your Plants Sing with MIDI Sprout.”

It is unclear whether the screams reported in the Tel Aviv study are simply a response to the stress plants are feeling or whether plants scream in an effort to convey information to other life forms—and if so,what that information might be. Experts speculate that one day farmers will be able to use devices like the MIDI Sprout to monitor their fields and identify when plants are, quite literally, crying out for water, which will allow them to lessen the damage drought stress can unleash on their crops.