Susan Dudley of the McMaster University in Hamilton, Ontario, maintains that plants do not only communicate, smell, taste, see, and perhaps even hear, but that they are social beings. Plants as social partners? Until recently, it was considered self-evident that only humans and certain animals displayed social behaviour, certainly not plants. Susan Dudley disagrees. The evolution biologist says that one of the characteristics that define socially active living beings is awareness of their place in the community. The yellow jewelweed (Impatiens pallida) is quite capable of this feat. She can distinguish whether close relatives or strangers of the same species were growing beside her. Susan Dudley shared her findings in a Skype interview.
Susan Dudley, you say that plants can recognize their siblings.
“We have been able to demonstrate this through experiments with the yellow jewelweed. This plant is frequently found in forests in Canada and Europe. We planted two sets of four seedlings next to each other in separate pots. The pot with the four seedlings that were not related to each other, we called ‘strangers’. The second pot with the four seedlings from the same mother plant we called ‘relatives’. We observed that jewelweed growing next to ‘strangers’ lets her roots grow very quickly. If, however, the jewelweed grows next to ‘relatives’, she invests less energy in root growth and progresses more slowly.
Do plants really have consideration for “relatives” and fight against “strangers”?
“Yes. It was interesting that we could only observe these different effects when the roots touched. But among plants that were close to one another, but in separate pots and not touching, we detected no differences. So some form of communication must exist between the roots. We have known for some time that roots extrude signalling substances to communicate.”
Is the language used by plant roots for underground communication known? Do you know which signal substances they use to recognize each other?
“No, we do not yet know. Plant roots extrude many signalling molecules into their surroundings including phenols, flavonoids, organic acids, and proteins. These substances are like a foreign language to us. We know plants communicate with each other, but we do not understand their language and cannot decipher it. The situation is similar to mine here in Taiwan where I am currently researching for a few months. A wealth of information exists around me, but I cannot decipher it because I hardly know a single word of Chinese. And as with plants, I do not know their signals. At the moment, we only know that the signal substances are water-soluble and liquid.”
Could your observations with the yellow jewelweed be just a coincidence? The composition of the potting soil or other environmental factors may have caused these differences.
"With a second attempt, we excluded precisely such effects and used wild thale cress (Arabidopsis). We cultivated the plant seedlings in containers of sterile solution. We knew that the roots of Arabidopsis would leave signalling chemicals in this solution. After seven days, we took out the seedlings. We then put ‘related’ and ‘stranger’ Arabidopsis in this solution. The only thing the ‘relatives’ and ‘strangers’ could encounter in this solution was the signal chemicals from the predecessor plant. Again, we observed significant differences: Roots of the ‘relatives’ remained unbranched and grew straight down. In contrast, roots of the ‘strangers’ grew faster and developed many lateral roots, as if they wanted to prevail against a competitor. These plants behaved competitively as soon as they recognized alien signals, snatching minerals and water from their foreign neighbours.”
Do all plants recognize their family members?
“We do not know. There are strong indications that different plants can react quite differently. Some invest more into root growth when they share their pot with ‘strangers’, as we have seen. Others react exactly the opposite way and grow more roots when ‘relatives’ are in the pot. Still others exhibit differences in growth above the ground. Although we know very little at this time about the highly complex, differentiated behaviour of plants, the point is that plants clearly know if a relative or stranger is growing beside them.”
To test whether plants behave differently when growing among their own or a different species, Amanda Broz of the Colorado State University in Fort Collins grew a spotted knapweed (Centaurea maculosa) in a greenhouse either next to another knapweed or an Idaho fescue. The researchers then used fragrances to simulate a pest attack and found the knapweed’s response was not consistent. If the neighbour was a knapweed, she produced toxins in her leaves to ward off the pest. If she was in the midst of Idaho fescue, however, she did not produce toxins and instead invested energy into leaf and stem growth. Such behaviour is quite reasonable, Broz explains. In the midst of Idaho fescue, the knapweed lets her neighbours fend off pests while she concentrates on aggressive growth. This strategy probably explains her successful propagation.
Plants know their place in the plant community. They know who is of the same species and even how closely related they are to a plant of the same species. So we can say that plants have a social life.
A lively discussion is taking place among animal psychologists as to if animals have consciousness, and if so, which do. Several criteria must be met to be considered a conscious being. For one, the animal must know his place in the animal community. Plants seem to be capable of this feat. I am not suggesting that plants are conscious – we do not know; but currently, we have no scientific arguments to categorically deny this possibility.
Example: Silver birch with rhododendron smell
Jarmo Holopainen of the University of Eastern Finland, Kuopio, observed an almost unbelievable combination of two very different plant species. During autumn, the silver birch forests growing in the northern Scandinavian mountains are oftentimes visited by swarms of green leaf weevils (Polydrusus flavipes). Holopainen’s group observed that some of these silver birch trees (Betula pendula) are particularly well equipped to fight these insects, specifically when a wild rosemary (Rhododendron tomentosum) is growing in the neighbourhood. The researchers found that these particularly fortified birch trees secrete a sticky chemical mixture of fragrance molecules. The fragrant cocktail repels the insects effectively, but does not originate from the leaves of silver birch trees. The fragrance is a defence mechanism of the rhododendron. The silver birch trees simply absorb the fragrance from their neighbours, then disperse it from their own leaves – a clever strategy. It seems that the fragrance mixture protects the silver birch against the aphid too. Jarmo Holopainen was quite impressed that the silver birch copied the scent of the wild rosemary to ward off enemies. That is, he says, “a good example of the complicated ecological effects that need to be considered at the level of a plant community rather than individual plants”.