It appears that Roald Dahl was correct all along: if you
harm a plant, it cries.
I guess, sort of. Not like you or I might shout. Instead, when a plant is
agitated, they start to make louder popping or clicking sounds at ultrasonic
frequencies that are not audible to humans. Scientists believe that this may
be one means by which plants express their anguish to those around
them.
As explained by
evolutionary biologist Lilach Hadany
of Tel Aviv University in Israel, "Even in a quiet field, there are actually
sounds that we don't hear, and those sounds carry information. There are
animals that can hear these sounds, so there is the possibility that a lot
of acoustic interaction is occurring."
It would be very suboptimal for plants to not use sound at all since they
frequently engage with insects and other creatures, many of which rely on
sound for communication.
Stressed plants aren't as docile as you might imagine. They go through some
fairly significant changes, with the emission of some pretty potent aromas
being one of the most perceptible (to us humans, at least). They can change
their form and hue as well.
These alterations may alert neighboring plants to peril, which prompts them
to fortify their own defenses; or they may entice animals to attack pests
that may be endangering the plant.
However, it hasn't been thoroughly investigated whether vegetation produce
other signals, like noises. Hadany and her coworkers discovered a few years
back that plants are capable of hearing. The obvious follow-up query was
whether they can also make it.
They observed tomato and tobacco seedlings under various circumstances to
find out. To establish a benchmark, they first documented unstressed plants.
Then they noted plants that had lost moisture and plants whose stalks had
been clipped. These sounds were made in two different settings: first, a
soundproofed acoustic room, and then, a typical greenhouse.
In order to distinguish between the sounds made by unstressed plants, cut
plants, and dehydrated plants, they next taught a machine learning
program.
Within a radius of more than a meter, plants produce sounds similar to
popping or clicking noises that are much too high pitched for people to
hear. (3.3 feet). Plants that are not under duress don't produce any noise
at all; they simply hang out and go about their daily activities.
Stressed plants, on the other hand, make a lot more noise, averaging up to
40 sounds per hour, depending on the variety. Additionally, dry vegetation
have a distinct sound signature. They begin to clack more before the plant
begins to show obvious symptoms of dehydration, increase as the plant
becomes more parched, and then decrease as the plant dries out.
These noises and the type of plant that released them could be
distinguished by the algorithm. And it's not just vegetation for tobacco and
tomatoes. The researchers conducted tests on a range of plants and
discovered that sound generation seems to be a fairly typical plant
activity. We captured the sounds of wheat, maize, grapes, cacti, and
henbit.
But there are a few unanswered questions. For instance, it's unclear how
the noises are made. In earlier studies, it was discovered that dehydrated
plants undergo a process called cavitation, in which air bubbles develop,
grow, and then burst inside the stalk. Human knuckle-cracking causes a
perceptible pop; it's possible that plants experience a comparable
sound.
We are unsure at this time if other distressing situations can also cause
noises. The plants could also begin to disintegrate like bubble wrap as a
result of pathogens, attacks, UV exposure, temperature fluctuations, and
other unfavorable circumstances.
Additionally, it is unclear whether sound generation in vegetation is a
natural occurrence or an adaptive evolution. However, the team demonstrated
that a program can be taught to recognize and differentiate between
different plant noises. Other creatures might have performed the same
function, for sure.
Additionally, these creatures might have acquired the ability to react in
different ways to the sounds made by distressed plants. According to Hadany,
"for instance, a moth planning to lay eggs on a plant or an animal planning
to eat a plant could use the sounds to help guide their decision." The
ramifications for us as humans are fairly obvious; we could listen for
thirsty plants' alarm sounds and water them before a problem arises.
However, it is unclear if other plants are detecting and reacting. It is
definitely conceivable because prior studies have shown that plants can
adapt their drought endurance in reaction to sound. And it is in this
direction that the study team will direct its next phase.
Now that we are aware that plants do produce noises, Hadany asks, "Who
might be listening?" We are presently studying our capacity to recognize and
understand the sounds in entirely natural settings, as well as how other
organisms, including animals and plants, react to these sounds.
The research has been published in
Cell.
