Ask any gambler the secret to leaving a casino with the big bucks, and they'll tell you it's all about knowing when to walk away. Although they may not see it in these terms, the gambler's wisdom is actually exemplary of a highly evolved and desirable trait in humans and animals: the ability to assess risk. A gambler assesses poker chips and slot tokens rather than the more elementary resources (e.g., food and water) of the animal, but in both cases the success of the organism depends on its ability to accurately assess risk based on the availability of these resources.
Risk assessment has previously been documented in dozens of animals, including humans, primates, birds, and insects. Recently, a team from Oxford and Israel's Tel-Hai College demonstrated for the first time that plants are also sensitive to the variability of resources in their environment, and are thus making risk assessments despite lacking a central nervous system.
"We do not yet know how the plants' sense variance functions, or even if their physiology is specifically adapted to respond to risk," said Alex Kacelnik, a professor of Zoology at Oxford.
"But the findings lead us to look even at pea plants as dynamic strategists and to model their decision processes just as one would model an intelligent agent."
In order to show that plants are also sensitive to risk, the researchers grew pea plants whose roots were split between two different pots. For each plant, one of the pots contained a consistent level of nutrients, while the nutrients in the other pot were considered variable. In this way, the researchers were presenting the plant with a choice between which pot to prioritize for its growth.
As the researchers discovered, when the average level of nutrients in a plant's pots was high, the plant was more likely to expand its roots in the pot with steady level of nutrients. When the average level of nutrients in the pots was low, the plant would grow more roots in the pot with variable nutrients.
These findings are consistent with risk sensitivity theory (RST), a leading theory on human and animal decision making which essentially amounts to the observation that animals are more likely to make riskier decisions in situations where taking a safer option wouldn't meet their needs.
For the pea plant, it is safest to prioritize the pot with that has a consistent level of nutrients so long as that level of nutrients is enough for the plant to grow. If the nutrient level in the consistent pot is too low to support the plant's growth, it makes more sense to gamble by choosing the pot with the variable level of nutrients and hope for good luck. This is because in the worst case scenario, the plant is no better off than it would've been in the nutrient-deficient consistent pot; but in the best case scenario, the variable pot works in the plant's favor and provides a nutrient-rich environment in which the plant can thrive.
Obviously plants aren't consciously making a decision between pots, but that's exactly what is so cool about the new research. It demonstrates that plants respond to the changes in their environmental resources much like animals do, even though they lack a nervous system to assess these changes and the risk they pose. By understanding the mechanism used by plants to assess risk, the researchers hope to gain a better understanding of the adaptive significance of resource sensitivity in non-human organisms.
"We do not conclude that plants are intelligent in the sense used for humans or other animals," said Kacelnik. "But rather that complex and interesting behaviours can theoretically be predicted as biological adaptations—and executed by organisms—on the basis of processes evolved to exploit natural opportunities efficiently."