It's easy to overlook plants. We appreciate the food and oxygen they provide, yet we tend to see them as passive scenery, not actors like us and other animals. They barely move and don't have nervous systems, let alone brains. How bright can they be?
They may lack animal intelligence, but land plants date back half a billion years, and nothing stupid survives that long. They're also distantly related to animals, and despite all the obvious ways we've drifted apart, scientists periodically discover something that reveals how eerily relatable plants can be.
We know plants communicate, for example, and can learn from experience. And now, in a major new sign of vegetal savvy, scientists have found evidence that plants can do something almost unthinkable for organisms with no brains: They "gamble," assessing their surroundings to make surprisingly good decisions.
"Like most people, including even experienced farmers and gardeners, I used to look at plants as passive receivers of circumstances," says first author Efrat Dener, now a graduate student at Israel's Ben Gurion University. "This line of experiments illustrates how wrong that view is: living organisms are designed by natural selection to exploit their opportunities, and this often implies a great deal of flexibility."
Give peas a chance
The specific plant in question is Pisum sativum, commonly known as the garden pea. For the new study, published in the journal Current Biology, researchers ran a series of experiments to see how a pea plant responds to risk.
First, they grew the plants in a greenhouse with their roots split between two pots of soil. One pot had higher levels of nutrients, and, as expected, the plants grew more roots there than in the other pot. That's an adaptive response, the researchers explain, "similar to animals allocating greater foraging effort to richer food patches."
In the next phase, plants again had roots in two pots, albeit with a tougher choice: Both pots for each plant had the same average nutrient level, but one was constant and the other variable. The average level also differed from plant to plant. This let researchers see what inspired plants to prefer certainty — i.e., constant nutrient levels — and what made them decide to gamble their lives on changing conditions.
Rooting out risk
After letting the peas grow for 12 weeks, researchers measured the root mass in each pot. Many plants had "gambled" by concentrating on their variable pot, but rather than being reckless, they had apparently made perfectly reasonable decisions.
Some plants had been given one pot with steadily high nutrients, plus a second pot with nutrients that wavered high to low, yet averaged the same high level as the first pot. These plants were risk-averse, growing most of their roots in the steady pot.
Other plants were given one pot with steadily low nutrients and another where levels varied, yet averaged as low as the first pot. These plants were risk-prone, preferring to grow roots in the variable pot instead of the constant one.
Both of these are good decisions. The plants had little to gain by gambling in the first situation, since the constant pot offered plenty of nutrients and the variable pot, despite its high average, was prone to streaks of dangerously low nutrients. On the other hand, when average nutrient levels were too low for a plant to thrive, the variable pot at least offered the chance to gamble on a streak of good luck.
Here's a human analogy: If someone offers you a guaranteed $800, or a coin flip that yields $1,000 for heads and nothing for tails, most people realize the first option has a higher average payout. But if you're stranded without money and need $900 to get home, flipping the coin for a chance at $1,000 could be more logical.
"To our knowledge, this is the first demonstration of an adaptive response to risk in an organism without a nervous system," says co-author Alex Kacelnik, a professor of behavioral ecology at the University of Oxford. Economists and zoologists have developed complex models for how humans and other animals make decisions, and now we know those models can also predict the behavior of plants facing similar choices. That's "fascinating," adds co-author and Tel-Hai College plant ecologist Hagai Shemesh, "and points to many interdisciplinary research opportunities."
This doesn't mean plants are intelligent in the same sense used for humans and other animals, the researchers point out, but it does force us to look at brainless vegetation in a different light. And even if they aren't really using logic, it certainly makes all those plants in the background seem a lot brighter. As Kacelnik puts it, "the findings lead us to look even at pea plants as dynamic strategists."