In addition to communication skills, plants must comprehensively perceive every detail of their environment. For example, a tomato plant needs to know when caterpillars are attacking her, a beech tree when and how firmly the wind is shaking her, and an apple tree when she should begin to flower. Plants perceive about twenty environmental signals, more than humans. Like birds, plants use electric fields to orient themselves according to magnetic fields. And just like humans, they smell, taste, see, touch, and maybe even hear. Dieter Volkmann, a cellular biologist and professor emeritus of the University of Bonn (see chapter 8), recently told me that he discussed this information in an interview twenty years ago. He provoked a huge uproar then, but much has been confirmed now.
Smell and taste
Plants smell fragrances and use them for lively conversation. They taste the saliva of predatory herbivores eating their leaves and are capable of identifying the culprit (see chapter 1).
Plants distinctly see light and can measure the amplitude and light quality. Thus, they can regulate many important metabolic processes such as germination, growth direction, and leaf development. But their seeing is not the same as that of humans. We use the retina composed of five different photoreceptors – four rods for different colours of light and one for light-dark differentiation. These photoreceptors send electrical signals to the brain, which translates the millions of signals into an image.
Because plants have eleven different photoreceptors, they perceive light in even greater detail than humans. “Plants constantly monitor their environment. They see when you get near them and know when you lean over them. They even know whether you are wearing a blue or a red shirt,” wrote the renowned biologist Daniel Chamovitz.9 Of course they cannot see images or recognize people because the light signals are translated into growth stimuli, not images. A plant might avoid a shadow, open her blossom, or grow towards the light. She can see if too many harmful UV rays are hitting her, and in response, produce pigments as a kind of internal sunscreen, to protect against UV rays.
Plants sense touch. We all know the mimosa plant, whose leaves fold at the slightest touch. Many climbing plants have tendrils even more sensitive to touch than our skin. The tendrils of bryony (Bryonia dioica) for example, are capable of responding to a thread weighing 0.00025 grams. Human skin is incapable of detecting such subtle contact.
At hearing the sounds, the Arabidopsis began producing defensive chemicals. Upon hearing the sounds a second time, the plant produced toxins faster and in higher concentrations, even though no caterpillars were eating her leaves. Did the plant simply perceive a general vibration and fight back prophylactically, or did she specifically recognize the source as caterpillars based on the chewing sounds?
To answer this question, researchers exposed the plant to the sound of wind (a completely different frequency) and then locust songs (a similar frequency, but different rhythm than the caterpillar’s chewing sounds). The plant did not react to either sound. Apparently, she identified the chewing noise of the herbivores, correctly interpreted the sound as a threat, and responded specifically by producing defensive enzymes and activating genes.10 Many questions remain unanswered, and more research is needed.
Example: Dodder smells tomato plants
The dodder (Cuscuta pentagona) is a predator, does not produce her own food or form roots. She is parasitic of other plants and chooses her victims deliberately. When discovering a plant, she entwines her with a thin stem, penetrates her with suction cups, and sucks her sugar-rich juices. But how does the dodder find the best, juiciest host plant? She recognizes victims by smell. Immediately after germination, her stringy, whitish stem moves back and forth through the air with snake-like movements, searching for a host. The dodder can smell if a less attractive wheat plant or a coveted tomato plant is nearby. She is choosy and seeks only strong, healthy plants even among tomato plants. She avoids injured or diseased plants by decrypting their fragrances. Once she gets the whiff of a luscious victim, she grows towards her quickly and with amazing precision.
Example: Plant odours against stem borer: push and pull
In eastern and southern Africa, maize is the main staple food. Pests such as the stem borer often cause huge damage and destroy up to 80 per cent of crops.
To combat the stem borer, farmers use a highly efficient biological method. First they plant the bean plant Desmodium between the rows of corn plants, completely covering the ground. Her odour repels stem borers and drives them from the field. At the same time, the Desmodium binds a valuable fertilizer (nitrogen) into the soil and protects against erosion. Next, the farmers plant three rows of Napier grass around the cornfields. The scent of this grass attracts and lures stem borers from the cornfields. The Napier grass also produces a sticky substance that traps the larvae of the stem borer. Using this push-pull approach, scents rid the cornfields of the stem borer and increase maize yields by up to 300 per cent without agrochemicals and genetic engineering. Desmodium and Napier grass also provide valuable fodder for animals. Some 90, 000 farmers have already adopted this push-pull method.
These are excerpt from the book “Plant Whispers | A journey through new realms of science.” (Chapter 4) by Florianne Koechlin, translation to English by Thomas Rippel.