Botany Made Easy
Plant Communication
Yes, it's true! Plants actually do communicate!
We all know one or two of those somewhat nutty people who talk to their plants... OK, so maybe some of us actually ARE those nutty people who talk to their plants... However, none of us really think our plants talk back to us. Well, sure, they wilt or drop leaves to tell us they feel bad. They perk their leaves up and glisten to tell us they feel good. But none of us crazy plant people actually expect plants to be able to really communicate much more than this. We're right, aren't we? Well, yes and no....
It's true that plants don't communicate to us humans in more complex ways, but to each other - that's a completely different story. In the last 10 years, scientists have discovered that plants absolutely do communicate with each other, with "friends," with "enemies," with everyone they encounter in their little piece of the world. They have a full, rich life of constant communication. So how do they do it?
Plants communicate through their roots by secreting tiny amounts of special chemicals into the soil all through the plant's root zone - what scientists call the rhizosphere. These chemicals, called root exudates, send signals to every other living thing in the root zone. Just like humans with our giant vocabularies, plants are capable of producing more than 100,000 different chemical signals to communicate or accomplish a huge variety of things! Scientists are just beginning to learn about the different types of signals, how they work, and what they do, so we aren't even close to understanding them all. But some of what we do know is fascinating.
Plants Communicate Who They Are
One interesting thing plants communicate to each other is who they are. No one is yet sure how this is done, but we do know that plants are able to recognize plants that are siblings (podmates), plants that are their same species but not close relatives, and plants that are total strangers. They then react accordingly. They compete vigorously when a plant is a stranger, growing long, invasive roots that stretch out farther and farther, trying to fill up the root space and drive the other plant out. We've all seen weeds do exactly this in our gardens, but we didn't realize it was intentional! By contrast, when a plant is a sibling, both plants grow much more shallow roots, allowing the sibling plant to use equal amounts of space, and to support each other, they grow extra long, intertwined branches and leaves. In our greenhouse we see this when we plant podmate seeds in a single pot. The plants become completely intertwined, and tend to stay exactly the same size for years. With unrelated seeds, this doesn't happen. One plant tends to dominate over time, and eventually even kill off the other plants, if we aren't very careful.
Plants Communicate Friendship
Some signals are friendly, and encourage other living things to cozy up to the roots and cooperate with the plant so both will be healthier. One of the best known examples of this is mycorrhizal fungi. These are different types of plant-friendly fungi that attach to the roots of the plants, and basically extend the plant's root system with the fungi's large web of filaments. The fungi are better at absorbing nutrients from the soil because of the huge network of filaments, and they share these nutrients with the plant. But the plant is much better at creating sugars, because they can photosynthesize with their above-ground leaves, something fungi can't do. So the plant feeds sugar to the fungi, and absorbs nutrients from the fungi, making both plants healthier together than they would be apart. There are many of these types of symbiotic relationships in the plant world, and the partners find each other with their chemical communications.
Plants Communicate Hostility
On the other hand, if a noxious, predatory living thing attacks the plant, plants send out signals that are hostile, and drive predators away, kill them, or make the soil unbearable so the predators slowly dwindle away. For many plants, the defensive communication system works all over the plant, not just in the roots. Amazingly, sometimes when only one plant in a group is being attacked by a pest, that plant is able to communicate what is going on to the other plants in the group, and all the plants then put up their defenses against the predator. In the case of aphids, for example, some plants secrete a chemical that attracts bugs that eat aphids. Roots can also send chemicals to leaves to make them taste bad to predators, to thicken their waxy coat to make it harder for predators to cut into the leaves, and they often produce toxins that kill or disable predators in the soil.
Plants Read the Soil and Alter It
Roots also read information from the soil itself. If they don't like what they read, they will exude chemicals to try to change it. If the soil is too hard, they will send out chemicals to try to break it up. If there aren't enough nutrients or minerals, they will try to break it up to release the nutrients they need, suck the nutrients up, then send out chemicals to gel the soil back together again - or to repair it, so to speak. They exude chemicals to make the soil perfect for other living things that are helpful to them, or to make it uninhabitable to living things they consider dangerous.
Many Plant Exudates are Common Substances
The lists of plant exudates sound quite familiar when we dig into them. Common drugs and supplements, antioxidants and toxins, sugars and carbohydrates, amino acids and proteins, many substances we are very familiar with make up the chemicals that plants use to communicate and build or impede relationships. Caffeine in coffee plants is a very familiar example. It kills or drives away predatory bugs trying to eat any parts of the plant. The coffee plant caffeinates the soil around it, and the caffeine in the soil kills or cripples other non-coffee plants that try to grow in that area. But the nectar of the coffee plant has very low doses of caffeine in it, unlike the rest of the plant, which is attractive to bees and other pollinators, helping coffee plants get pollinated. So caffeine both helps friendly organisms and kills non-friendly ones in the same plant.
Hibiscus Pigments are also Exudates
In hibiscus, some of the chemicals that make color pigments in flowers also function as communication exudates in other parts of the plant. Pigments we know well, like flavanoids and polyphenols are root exudates, and anthocyanins function as sticky anti-predator exudates in leaves. No one knows which function came first, but somewhere along the way, hibiscus evolved to use these chemicals in multiple ways, as did all plants with their exudates. We know very little about the specific exudates of hibiscus - we are barely learning about exudates in plants in general. But it's a topic that isn't going away, so hopefully we'll know more in the future.