Imagine you’re offered a choice between $800 or a coin toss to win $1000. Heads, you end up with the full $1000; tails, you lose everything. For most of us, it’s a no-brainer. We take the $800. But you would likely toss the coin if you were stuck on a desert island with no money, and needed $900 for a flight out. Pea plants, it turns out, make the same decision. When faced with hard times, the species gambles, scientists report. The discovery is the first to show that plants—not just animals—have the ability to switch from being risk-avoiders to risk-takers.
“Like most people, I used to look at plants as passive,” says lead author Efrat Dener, a master’s student in environmental sciences at Ben-Gurion University of the Negev in Beersheba, Israel. His group’s experiments show “how wrong that view is.” Although plants do other things—such as bending toward sunlight and responding to humidity—they haven’t been thought of as “dynamic strategists,” says Dener’s co-author Alex Kacelnik, a behavioral ecologist at the University of Oxford in the United Kingdom. That is, they haven’t been shown to be able to respond when times are tough by changing their behavior and taking a chance.
Humans, primates, birds, and social insects take fewer risks when faced with a steady supply of food. But when the supply is uncertain, they switch strategies and take more risks. For instance, in lab experiments, honey bees turn to gambling when they’re starving, choosing to sip nectar from a tube that may dispense plentiful amounts or nothing. And dark-eyed juncos (small songbirds) that are cold will ignore a seed dispenser that regularly releases three seeds, and choose one that may give out six—or zero.
To find out whether plants do the same, Dener and his colleagues carried out a series of experiments on pea plants (Pisum sativum) raised in a greenhouse. The plants were grown with roots split between two pots. Each pot contained the same concentration and type of nutrients. But the level of nutrients in one pot was constant, whereas it varied in the other. After 12 weeks, the scientists measured the plants’ root mass and their allocation of roots inside each pot.
They found that the plants varied their distribution of roots depending on the nutrient level in each. In some tests, the plants faced a choice between a pot with a steady supply of high nutrients and one with variable levels. These plants, not surprisingly, were risk-averse, and grew most of their roots in the constant pot.
But plants switched strategies when faced with a choice between a dicey pot with variable levels of nutrients and a pot with constant but low amounts of nutrients—so low, they were below what a plant needs to survive. In this case, the plants, like the person on a desert island, gambled. They sent out more roots in the variable pot, basically tossing a coin to see whether they would get lucky and encounter the nutrients they needed to survive, the scientists report today in Current Biology. Thus, normally risk-averse, pea plants become risk-prone when growing in dire conditions.
“To our knowledge, this is the first demonstration” of this kind of risk response in an organism without a nervous system,” Kacelnik says. He adds that this doesn’t mean the “plants are intelligent” in the way that we think of humans or other animals. But they do have some way of sensing or evaluating the different conditions in the pots, although the scientists do not yet know what this is.
That the scientists have shown risk sensitivity in pea plants—“not really anyone’s top candidate for cognitively advanced organism of the year—is a surprise,” says David Stephens, a behavioral ecologist at the University of Minnesota, Twin Cities.
But that may be because most people aren’t aware of plants’ abilities, says James Cahill, a plant ecologist at the University of Alberta, Edmonton, in Canada. “We know plants can process information and have memory.” For example, bittersweet nightshade plants can tell whether flea beetles or tortoise beetles are feeding on their leaves, and mount a different chemical defense against each species. The new study “builds on this [knowledge]. They’re showing that risk matters to individual plants. It’s a great step and novel contribution to the developing field of plant behavior.”
Still, it is a first step, Cahill and Stephens say—but one that is so exciting that both they and the team of researchers are calling for more studies.