We now know that do “talk” to each other;
UNDERGROUND CHAT | Studies show that plants do “talk” to each other, through their roots, leaves and tiny electrical pulses they emit. Image Credit: Stock photo

Highlights

  • Plants’ roots and leaves emit chemicals and electrical signals.
  • They generate voltage-based signalling reminiscent of the animal nervous system.
  • Research going back to the 1970s, now validated by more recent experiments, have uncovered the mystery surrounding signalling in plants; this carries a huge ecological significance.

Do plants talk to each other? Yes. They do. And apparently, they scream too! But not in ways we know. After all, plants belong to another lifeform. 

If you have a yard, try to cut grass. You will notice certain scents. They’re actually a blend of chemicals being emitted. To humans, they may be pleasant. But to plants, it signals danger on the way.

Scientific understanding of how plants communicate — via their roots, leaves and the electrical pulses they emit didn’t emerge in one go. Evidence evolved, or piled up, over a few decades.

They show one of life's fascinating realities: that plants have their own way of relating to each other, mostly for defence purposes. Read on:

What's the evidence that plants do “talk” to each other?

In October 1990, Ted Farmer, a researcher in the Washington State University, worked with local sagebrush which produce methyl jasmonate, an airborne organic chemical.

Botanists believe this chemical wards off insect herbivores. Lab experiments show that when damaged sagebrush leaves were put into airtight jars with potted tomato plants, the tomatoes began producing proteinase inhibitors — compounds that harm insects by disrupting their digestion.

In his paper published in the journal PNAS, Farmer concluded: “interplant communication is real”. That research was carefully conducted, properly replicated and was quite convincing.

Do plants count?
Neuroscientist Greg Gage, who uses sophisticated equipment to study the brain, said that while plants have no brains and blood the way animals have, they do use electrical signals to communicate. In a TED talk, he hooked up with the Mimosa pudica, a plant whose leaves close when touched, which encodes digital information as it moves.

Another plant, known as the Venus flytrap (a carnivorous plant), shows how it uses electrical signals to convey information, trigger movement — and even count (determine if a fly is inside the trap before it takes action, by trapping a fly).
Mimosa pudica,
Mimosa pudica, a plant whose leaves close when touched, encodes digital information as it moves.

How do plants communicate?

There are a number of ways in which plants talk to each other. Expert botanists have discovered these mechanisms over the years.

In a nutshell (no pun intended), it’s through their roots and leaves — as well as the chemicals they release. At least 40 studies going back to the 1970s show this to be the case.

Bio-chat: How do roots talk?

One way plants have a “bio-chat” is by secreting tiny amounts of special chemicals into the soil through the plant's root zone. Botanitsts call this the “rhizosphere”. These chemicals, called “root exudates”, send signals to every other living thing in the “root zone”.

PLANTS ARE MASTERS OF BIOCHEMISTRY
Plants are masters of synthetic biochemistry. In the Western world today, about 40% of the drugs behind the pharmacist's counter are derived from plants that people have used for centuries.

40%

Share of plants in medicine found in today's pharmacies.

Wait…so how was this phenomenon discovered?

One of the earliest research on anti-hervibore defences in plants first came out in 1972. In 1982, experiments made by David Rhoades, Lynn Erckmann and Gordon Orians (University of Washington) found that un-infested willow trees changed their leaves' chemistry in the same way as nearby willows that were being attacked by tent caterpillars.

On July 15, 1983, biologist Jack C. Schultz, and chemist Ian T. Baldwin, at Dartmouth College in New Hampshire, found similar defensive actions among sugar maple and poplar seedlings, as reported in the journal Science.

What they found shook botany. The chemicals emitted by plants “float” like viruses through the air, mostly unnoticed. But these chemicals carry a slight odour that neighbouring plants — and scientist equipped with special instruments — can detect.

What are these chemicals?

The chemicals emanate from the injured parts of the plant, that signify what botanists call a “silent scream” or a sort of “alarm”.

These chemicals include:

  • Alcohols
  • Aldehydes
  • Ketones
  • Esters

Collectively, they are known as volatile organic compounds (VOCs)

Volatile organic compounds (VOCs)
The evidence that plants release VOCs when damaged by herbivores is well established in science.

VOCs are emitted as gases and are airborne. Concentrations of many VOCs are consistently higher indoors (up to 10 times higher) than outdoors.

There is evidence that plants can somehow perceive this volatiles and respond with a defence response.

How do roots talk?

In 2011, a team led by Zdenka Babikova, of the Institute of Biological and Environmental Sciences, University of Aberdeen, in the UK, published their plants-talk-through-their-roots thesis in the journal Ecology Letters.

The roots of most land plants are colonised by mycorrhizal fungi. Not only they provide mineral nutrients in exchange for carbon: they're the earth's internet.

-

Researchers demonstrated that underground signals carried through common “mycelial” networks warn neighbouring plants of aphid attack.

Studies have shown that mycorrhizal mycelia can also act as a conduit for signalling between plants, acting as an "early warning system” for herbivore attack.

MYCELIUM
Mycelium refers to tiny “threads” of the greater fungal organism that wrap around or bore into tree roots. Taken together, mycelium composes what's called a “mycorrhizal network,” which connects individual plants together to transfer water, nitrogen, carbon, other minerals and information the plants understand.

How do leaves communicate?

Dozens of studies show how plants communicate through their leaves, too.

A more recent research was published in July 2018 in the journal Plant Methods. In this study, a team led by Leandro do Prado Ribeiro, a researcher in Santa Catarina, Brazil, showed initial evidence of how high-tech (hyper-spectral) imaging shows direct plant responses to stressors — but also to the multiple ways plant communities are able to communicate.

STOCK forest
Studies have shown that mycorrhizal mycelia can also act as a conduit for signaling between plants, acting as an "early warning system” for herbivore attack.

The team claimed they were the first to publish such study showing how phyto-compound data correlate with leaf “reflectance” — one aspect of what they termed “plant–plant communication”.

Within seconds, voltage changes in the tissue radiated out from the site of damage toward the stem and beyond. As the waves surged outward, the defensive compound jasmonic acid accumulated — even far from the site of damage.

-

University of Lausanne's Ted Farmer, a plant signalling pioneer, focuses on how plants activate their defences, usually within seconds, following “wounding” by herbivores. This phenomenon involves electrical signals known as slow-wave potentials (SWPs).

It was a powerful discovery of an almost entirely unrecognised way that plants transmit information — with electrical pulses and a system of voltage-based signalling similar to the animal nervous system.

Do plants really emit electric signals?

Experiments now prove that they do. It turns out that ONE leaf knows it’s being eaten. And that leaf has a way to tell other parts of the plant to start making defensive chemicals against an intruder.

To prove that electrical signals are at work, his team placed microelectrodes on the leaves and leaf stalks of Arabidopsis thaliana (native to Africa and parts of Asia). Then they allowed cotton leafworms to feast away.

Within seconds, voltage changes in the tissue radiated out from the site of damage toward the stem and beyond. As the waves surged outward, the defensive compound jasmonic acid accumulated — even far from the site of damage.

Does it really happen among wild plants?

Another researcher, Ted Karban, repeated Farmer’s experiment in the wild. He clipped sagebrush plants, imitating the injuries caused by the sharp teeth of insects. This induced the plants to produce methyl jasmonate and other airborne chemicals — then the wild tobacco nearby started pumping out the defensive enzyme polyphenol oxidase.

At the end of the season, these tobacco plants had much less leaf damage than others from grasshoppers and cutworms. Another experiment showed: that when wild-growing lima beans are exposed to volatiles from other lima bean plants being eaten by beetles, they grow faster and resist attack.

It’s now known that every green plant that’s been studied releases its own mix of VOCs, and many species register and respond to these plumes.

MOVIE: 'The Secret Life of Plants'
The 1979 film 'The Secret Life of Plants' (after a 1973 book of the same name) had wowed audiences with time-lapse photography that made plants seem to writhe with vitality as they unfurled their leaves and pushed out roots.

The film claimed that science had proven that plants were conscious and could sense “human emotions”.

This made some scientists more cynical.

In 1985, the two “talking tree” papers published in 1982 and 1983 (by Rhoades and Baldwin) were dismissed by eminent ecologist John Lawton (later knighted), and Simon V. Fowler, as “poorly designed”, in his paper “Rapidly Induced Defenses and Talking Trees: The Devil’s Advocate Position”, published in the American Naturalist journal.

Lawton said that Rhoades’ findings may have been accidental. But subsequent studies show plants do talk through their roots and leaves.

This sub-discipline of botany is now getting more attention due to new tools emerging that help researcher both in lab and wild settings.

What are the implications?

It's huge. The possibility that plants routinely share information isn’t just intriguing botany — this knowledge could enhance our understanding that a forest is more than what we see. Moreover, it could revolutionise agriculture, especially in finding ways to naturally improve crop resistance to pests, potentially curbing widespread use of toxic pesticides and herbicides.

In a plantation, a technique may emerge: plants with sensitive or potent defensive responses could be grown alongside commercial field crops. As such, these “guard crops” would act as watchers or early-warning sentinels who are first to detect and react to danger, possibly “alerting” neighbouring crops.

One final mind-bending realisation

Most of us probably thought plants were “brainless”. But we now know better.

The signals they send, receive and interpret can now be measured in electrical and chemical terms. In a way, it's akin to human heart or brain producing electrical pluses that can be read by the latest medical diagnostic tools.

In the case of trees and plants, botanical sensors show plants form a living, pulsating internet that has always existed for eons and millennia.

It's only now that we gaining greater understanding of this phenomenon, which may have to do with the rest of our lives.