If someone asked us to examine a photo of the African savannah – with giraffes, antelopes and zebras everywhere – and write down what we have seen in it on a piece of paper, it is most likely that in our description there is no trace of the plants despite that the image would be full of trees, bushes and grasses . This bias towards animals is so common that there is a term for it: plant blindness. Studied in the 1990s by botanists James Wandersee and Elizabeth Schussler, it consists of an “inability to see or notice the plants that live in our environment, which implies that we do not recognize their importance in the biosphere and in our lives.” It is a paradigmatic phenomenon, since “if tomorrow the plants of the planet disappeared, in a month all life would be extinct “, defends Stefano Mancuso, professor at the University of Florence.
Without them, the ecosystem would collapse, “we would have no food, no oxygen, not even fuel.” For this reason, it is alarming that a fifth of plant species are on the way to extinction . With more than a decade of research behind him, Mancuso is one of the few scientists dedicated to a surprising and controversial field of research: plant neurobiology. The study of the intelligence of plants is, for many, a kind of pseudoscience. However, those dedicated to it say that it will bring a new agricultural revolution, novel environmental management techniques and a different vision of our place in the world.
And this question is not a overnight flower: Charles Darwin himself became interested in researching plants, and his son, Francis Darwin, was one of the first experts in plant physiology. This, in a speech delivered in 1908 at the Congress of the British Association for the Advancement of Science, went so far as to affirm that they are intelligent beings, to the scandal of most botanists.
Today, the vocabulary used in this branch of research continues to be surrounded by controversy. Shortly after its formation, in 2006, the Society for Plant Neurobiology was forced to change its name to the Society for Plant Signaling and Behavior. And the simple use of the word intelligence when talking about plants gives rise to harsh criticism.
Plants can solve problems
Mancuso, who refuses to change the name of the International Plant Neurobiology Laboratory (LINV), which he leads at the aforementioned Italian university, is also not shy about asking the question of whether plants are intelligent. Your answer? “It all depends on how we define intelligence. I see it as the ability to solve problems. And it is known that plants are capable of doing it. If not, they would not survive. So yes, without a doubt they are intelligent, ” he argues. In his latest book, Sensitivity and Intelligence in the Plant World (2015), he offers endless examples that seek to give readers the ability to judge plants for themselves.
“The findings of the last fifty years have shed light on its incredible capabilities,” he defends. However, they have been overshadowed by the purely terminological controversy, which has nothing to do with the experimental results. For him, ” plants are so different from animals that it is almost like being in contact with an alien culture.”
An opinion that perhaps Charles Darwin would have shared. Convinced that the plant world was extraordinary, he sought to understand how plants interact with the environment and what influence external stimuli have on their growth. With the collaboration of his son, he published his results in a book: The power of movement in plants (1880). But in his day vegetables were seen as practically inanimate objects, and his efforts were ridiculed.
Today, we know unimaginable plant secrets then. Although, be careful! There is also a lot of misinformation. Contrary to popular belief, ” there is no use talking to plants, ” says Mancuso. “They only perceive vibrations”, and they react at different frequencies.
According to an article published in 2012 in the journal Trends in Plant Science , the roots go for the lowest frequencies, between 100 and 400 hertz, and grow in the direction of the sound sources. “Around 300 hertz, it is similar to that produced by flowing water,” speculates Mancuso, who participated in this study. And he adds: “ The roots grow in the direction of pipes through which water circulates, even when their outer surface is dry . They may be able to hear it, and associate it with this frequency. “
The aerial part of the plant is not interested in speech or music, but, thanks to the same hearing capacity, it detects if someone is eating its leaves and, most curiously, it knows how to defend itself. “The vibrations produced by a caterpillar when feeding induce changes in the metabolism of the plant”, explains Heidi Appel, discoverer of this phenomenon. “They are a signal for cells to synthesize defensive chemicals to repel the attack.” The research group led by this expert at the University of Missouri (USA) recorded the sounds produced by a caterpillar while gnawing the leaves of an arabidopsis. The funny thing is that the reproduction of these sounds near a specimen free of the attack also induced the production of mustard oil, the defensive compound used by this cruciferous.
For now, it is unknown if there are more signals of this type, but a 2012 investigation revealed that plants are also capable of producing sounds . As they grow, the roots click, “the result of the breakdown of cell walls – made of cellulose and therefore quite rigid – during cell growth,” explains Mancuso. Its function, if it exists, is unknown. However, the fact that vibrations capable of influencing plant behavior have been identified opens the door to the possibility that clicking , the name with which this phenomenon has been baptized, may be something more than it appears.
The defensive strategy of arabidopsis is also used by other species. The plant world is full of experienced chemists, capable of synthesizing compounds so effective that some are used in medicine. Still, they hold secrets that we are only now beginning to unravel. In the case of flowers, the emission of fragrances is related to reproduction. And recent studies indicate, for example, that the volatile compounds produced by the leaves are calls for help , as is the case with the unmistakable aroma of freshly cut grass.
Always aware of your neighbors
At the University of California at Davis (USA), ecologist Richard Karban studies these communications in the small shrub Artemisia tridentata . By carrying out controlled pruning, which emulates the action of insects, he showed that plants pruned in early spring generate a greater amount of volatile substances, which protect them against pests. Interestingly, as with the arabidopsis, a nearby plant that has not been pruned also acquires that protection. Are they talking to each other? Not quite. “As the signaling between different branches of the same bush is limited, the use of volatile compounds guarantees that, in the event of an attack, all the leaves of the same plant activate their defenses”, argues the scientist. “Always attentive to what their neighbors do – he continues – the others detect the presence of these chemicals in the air and, in order not to fall victim to the same aggressor, they implement a preventive defense strategy , especially effective when there is some type of family relationship”.
Plants defend their territory. When several seeds germinate in the same pot, the normal thing is that each one develops a very high number of roots to guarantee that it has more resources than its competitors. However, according to a study published in 2007 in the journal Nature , if the seeds are daughters of the same plant, they recognize their kinship and develop a greater aerial part, to the detriment of the roots. In tune with these data, Karban observed that the closer the genetic relationship between the plants, the greater the likelihood that they would respond to a nearby signal. “The composition of these chemicals appears to be inherited, as with human blood types,” he explains. After all, improving family members’ chances of survival is an effective strategy to ensure that shared genes reach the next generation.
But the biochemical dialogue takes place in other contexts as well. At the Ben-Gurion University of Israel, a group of biologists specialized in studying how plants adapt to the desert have discovered that the common pea, Pisum sativum , detects if a neighboring plant suffers from drought symptoms and reacts by closing the stomata, small pores present on the surface of the leaves, which are the main route of moisture loss . The only condition is that both plants share soil, since the sign of drought is a compound released by the roots. Furthermore, according to the article published in the journal PLOS ONE , “the results suggest the existence of a chain communication”. Peas not only respond to the misfortune of their neighbors, but also go on to emit stress signals, detected by individuals increasingly distant from the primary source.
For centuries, we have ignored the existence of these information exchanges. However, the animals have been more insightful. For many insects, such as wasps, bed bugs, and even small worms, the scent released from a plant in distress amounts to a cry of “Food is served!” – convenient and hard to ignore.
In 2010, researchers from the Department of Molecular Ecology at the Max Planck Institute elucidated the complexities of one of these relationships. According to the journal Science , when the caterpillars of the Manduca sexta species attack the Nicotiana attenuata , this wild tobacco cousin defends itself by releasing compounds that attract Geocoris , a voracious appetite bug whose favorite bites are caterpillars.
We are facing a complex and quite subtle language, because, although the plant synthesizes the relevant volatile molecule, the compound that attracts Geocoris is only generated when that molecule comes into contact with the caterpillars’ saliva and undergoes a chemical alteration that transforms it into a powerful bed bug magnet. Thus, the caterpillar condemns itself, and the presence of the predator gives the plant the time it needs to quickly manufacture its chemical defense arsenal.
Selective abortions
Taking into account the energy expenditure involved in the synthesis of these compounds, Mancuso argues that behaviors such as these make up “an authentic expression of intelligence, denoting a calculation of risks and a forecast of benefits.” A controversial opinion, very much in line with almost everything that the Italian scientist defends. Decision-making is such a complex cognitive ability that few dare to claim that it exists in the plant world. However, more and more studies suggest that plants are capable of deciding depending on the circumstances and of adopting strategies that allow, for example, to maximize the probability of having offspring.
The best example of this is the case of the Berberis vulgaris shrub, discovered by scientists at the University of Göttingen, in Germany. The barberry, the common name of this species, is frequently attacked by a tiny fly that incubates its eggs inside the fruits. The larvae feed on the seed, but, as each fruit usually contains a pair of seeds, when this happens, the plant aborts the infected one and kills the parasite. However, selective abortion only occurs when the attacked fruit has a second seed capable of germinating.
If there is only one, “the plant seems to speculate that there is a possibility that the larva will die naturally. A slight possibility of reproduction is better than nothing,” says biologist Hans-Hermann Thulke, who studied this relationship. According to the data collected, only 5% of the fruits of a single seed are aborted if they suffer a parasitic infection, while, when there are two per fruit, 75% of those infected are discarded. This decision-making process is one of the most complex recorded to date in the plant world. ” Plants see without eyes, hear without ears, and taste and smell their environment ,” Mancuso explained in a talk in Madrid in 2015. They do not have structures comparable to our specialized organs, although they do not need them either. For the Italian, organs are overrated, brain included.
Your ‘brains’ may be at the root
But then how do they integrate the information they collect from their habitat? Darwin expressed his controversial opinion on the matter in the last paragraph of the book that he published with his son: ” It is not an exaggeration to say that the tip of the root, endowed with sensitivity and being able to direct the movement of the adjacent regions, acts as the brain of a lower animal “ . This argument is known as the root-brain hypothesis.
This idea was rescued just a decade ago by scientists from the Plant Signaling and Behavior Society. One of the most active in his study is František Baluška, a biologist at the University of Bonn in Germany. Leader of a group dedicated to investigating the ability of plants to perceive the environment, he studies what happens when, after removing the tip of a root, it is subjected to different stimuli. In tune with Darwin, he defends that there is a specific region in the roots where decisions of great importance are made: it is a nerve center, as Baluška calls it.
Serotonin or melatonin, whose presence has been detected in plant cells, act in the human brain in functions as important as sleep and moods. To discover what role they play in plants, Susan Murch of the University of British Columbia in Canada altered their concentrations with drugs such as the antidepressant fluoxetine and methylphenidate, used to treat attention deficit and hyperactivity disorder. His studies have shown that the changes cause malformations in the roots. Are we hasty in suggesting that the plant brain is at the root? Perhaps, but if the hypothesis were confirmed, we would be facing living beings that do not have one, but hundreds of millions of command centers, as many as root points.
“Let’s imagine that they function as a colony of insects,” explains Mancuso. “The ants – he continues – are not physically connected to each other, but they act in a coordinated way thanks to chemical signals; the same happens in plants. They have developed a distributed intelligence, typical of swarms .” Still, most botanists refuse to say that plants are intelligent; and others simply concede that they show intelligent behaviors. It is not the same, and these philosophical definitions have important consequences in how we relate to the vegetable orb.
Images: Jon Sullivan / Stefano Zucchinali / Yathin S. Krishnappa / CC
