Do plants talk?
From the call of a hen to warn and protect her chicks from the circling hawks to the barks of the dog at an intruder of a house to placing that 911 call to the police, various organisms including humans adopt several means of communication to warn, protect and defend themselves for survival.
How about plants? They are primary producers harnessing the sun energy through photosynthesis to produce biomass.
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They are at the base of the food chain.
Which means they are food to many organisms including man, which feed (prey) on them to survive in a term known as herbivore.
They have a ‘right to life’ but lack a central nervous system to physically defend themselves.
How do they defend themselves?
Do they ‘talk’ to warn their neigbours or call for help? Can they even choose their circle of ‘friends’ as we do?
The evidence for plant communication is only a few decades old.
Two studies published in 1983 demonstrated that willow trees, poplars and sugar maples can warn one another about insect attacks.
In fact, undamaged trees near ones that are infested with hungry bugs begin to pump out bug-repelling chemicals to ward off attack. They somehow know what their neighbours are experiencing and react to it.
What is even more interesting is that the communication is not only between plants of the same kind but even amongst different plant species.
Communication in plants
Let’s look at communication within one plant; how does one leaf knows it’s being eaten and how does it tell other parts of the plant to start manufacturing defensive chemicals?
To prove that electrical signals are at work, Farmer E.E and Ryan C.A (1990) placed microelectrodes on the leaves and leaf stalks of Arabidopsis thaliana and allowed Egyptian cotton leafworms to feast away.
Within seconds, voltage changed in the tissue radiated out from the site of damage towards the stem and beyond.
As the waves surged outward, the defensive compound jasmonic acid accumulated even far from the site of damage.
The inter-plant communication is even more interesting. Rhoades F (1979), looked at how the Sitka willow altered the nutritional quality of its leaves in response to infestation by caterpillars and webworms.
In the lab, when he fed the insects with leaves from infested trees, the worms grew more slowly.
But their growth was also stunted when he fed them leaves from undamaged willows that lived near the trees being eaten.
The same biochemical change seems to be happening in both groups of trees, and Rhoades’ conclusion was that the untouched willows were getting a message from those under attack.
What about the communication between different plant species? Farmer E.E and Ryan C. A (1990) worked with local sagebrush, which produce copious amounts of methyl jasmonate, an airborne organic chemical that Ryan thought plants were using to ward off insect herbivores.
In their experiment, 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.
Another form of plant communication is using the Wood Wide Web (WWW).
This is done through a network of Mycorrhiza. A mycorrhiza is typically a mutualistic symbiosis between a fungus and a plant root.
The mycelia is not just a medium for the transfer of nutrients, Zeng R, S et al (2010) found that when plants are attacked by harmful fungi, they release chemical signals into the mycelia that warn their neighbours.
This conclusion was reached when pairs of tomato plants were grown in pots.
Some of the plants were allowed to form mycorrhizae.
Once the fungal networks had formed, the leaves of one plant in each pair were sprayed with Alternaria solani, a fungus that causes early blight disease.
Air-tight plastic bags were used to prevent any above-ground chemical signaling between the plants. After 65 hours, Zeng tried to infect the second plant in each pair.
He found they were much less likely to get blight, and had significantly lower levels of damage when they did, if they had mycelia.
So not only do the mycorrhizae allow plants to share food, they help them defend themselves.
But the fungal internet connections are not only for beneficial purposes.
Some plants steal from each other using the internet.
There are plants that do not have chlorophyll so unlike most plants, they cannot produce their own energy through photosynthesis.
Some of these plants such as the phantom orchid get the carbon they need from nearby trees via the mycelia of fungi that both are connected to.
Allelopathy
Plants choose their friends in a phenomenon known as allelopathy. From the release of biochemicals known as allelochemicals, a plant may kill another plant it doesn’t like as a neighbour and encourage the growth of another it likes.
For instance, the chemical responsible for the walnut’s toxicity, Juglone, has a direct specific toxicity effect on the nightshade plants (tomatoes, peppers, eggplants, potatoes), pines and birch trees.
In this case, the walnut can choose other plants except the aforementioned as its neighbours.
However, in some other plants, this allelopathic behaviour is non-specific.
Thus, it affects all other plants within its vicinity. A typical example is the eucalyptus.
Another example is Leucaena leucocephala; it contains a toxic, non-protein amino acid in its leaves that inhibits the growth of other trees but not its own seedlings.
There is a growing distaste for the use of synthetic chemicals in our agricultural system.
Many reasons account for this including their negative effect on the ecological system and the growing resistance of some insects.
By nature, plants have various means of defence against pest and diseases.
Mainly, through structural and the production of various types of chemicals that they deploy via various communication networks for their defence, plants are able to send signals to other nearby plants, fight competition and choose their neighbours.
Continuous research in this area will help develop crop cultivars with more self-resistance and allelopathic potential that may help in better resistance to biotic and abiotic stress thereby limiting the use of synthetic chemicals in agriculture.
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The writer is a PhD candidate of the Life Science College of the Fujian Agriculture and Forestry University, Fuzhou, China.