Pea Plants Don’t Always Play it Safe

Plants seem to cope as well as animals do when faced with challenges in access to nutrients – even if they don’t possess a cognitive capacity or a complex nervous system.

A pea plant. Credit: revstan/Flickr, CC BY 2.0

A pea plant. Credit: revstan/Flickr, CC BY 2.0

Would a heroin addict prefer a source that supplied them a constant quantity of the drug or one that was more unpredictable about the quantity they provided? You’d think that they would prefer the safer choice, but a 2004 study published in the journal Addiction showed that this is not always the case. It appeared that when the subject was satiated, she preferred the constant source that guaranteed a fixed amount of the drug. However, if she was in a deprived situation, she would tend to take a gamble with the variable source.

This fluctuation between a risk-averse and a risk-prone behaviour is exactly what is predicted by the risk sensitivity theory (RST). According to RST, agents should switch between risk proneness and risk aversion depending on state and circumstances, especially according to the richness of the least variable option. This behaviour has been observed in animal foraging patterns, where animals were typically seen to forage longer in patches that had more food. But for the first time, researchers tested this theory on plants. They ended up with tantalising evidence that plants too are capable of such levels of decision-making.

“This field of plant behaviour is very much neglected and overlooked since for many years people have tended to under-appreciate plant ability to behave,” said Hagai Shemesh, an environmental scientist from Tel Hai College, Israel, who, along with compatriot Efrat Dener and Alex Kacelnik from Oxford University, authored the paper on their study which was published on June 30 in Current Biology.

In what is called the split-root technique, the three used pea plants whose roots were split into two separate pots containing different levels of nutrition. This is standard procedure in experiments that try to estimate plant preference, explained Shemesh. “The advantage is that you have the same plant in two environments and can, therefore, control for noise that originates from differences between plants.”

The split-root pea plants are now faced with a decision to make. To which pot should they allocate more resources? This was determined by observing the root growth in each pot. As expected, in the first set of experiments, where one pot had more nutrients than the other, the root growth was more in the richer pot. In the second set of experiments, both pots had the same average nutrient supply, but in pot 1 this was supplied constantly, while in pot 2 the nutrient level varied over time.

At the end of these experiments, Shemesh and team found out that the plant’s decision whether to be risk-averse (more growth in constant pot) or risk-prone (more growth in variable pot) depended on the average nutrient level. When the constant pot supplied the plant with sufficient nutrients, roots proliferated more there, but when the constant pot was poor in quality, the plants seemed to prefer to grow in the variable pot, even though this was a risky choice. This fit in with the RST theory because as in the case with the drug addicts the subject here had nothing to lose and a chance to gain by taking the risk, whereas if it chose to be risk-averse, it was assured a constant yet scant supply.

With these results, the team has successfully shown that for an organism to behave optimally, it need not be aware of the problems or to be capable of thinking. No one thought that plants, which do not possess a cognitive capacity or a complex nervous system like animals do, can strategically cope with such challenges, but for now it seems that they cope at least as good as animals do, said Shemesh.

So how are plants able to do this? “They simply respond with their own physiology,” said Kacelnik. “Nutrients have a different effect locally (at each root ending) and systemically (for the plant as a whole). The combination of these two processes must ensure the correct flexible response.” He admitted that the details of how this happens are not yet known.

“I would say that they have an arsenal of simple rules,” guessed Shemesh. “If X happens, do Y. A number of such rules implemented simultaneously can theoretically produce complex behaviour without the need for cognition or a complex nervous system.” He said that only developing more mathematical growth models would be able to shed light on what these rules could be. “More species should be tested under different levels of resources. Working with model plants can help pinpoint genetic mechanism involved,” he added.