BOTANICAL STRATEGIES: Root analysis helps understand how competitive plants are

Imagine you’re a pepper plant. You need water and nutrients. Luckily, you can grow roots that grab that stuff from the soil and pipe it back to you. So far, so good. There’s just one problem. Your neighbour — also a pepper plant — needs the same things. There’s only so much to go around. What’s your move? For years, researchers have looked into the tangled problem of root competition, coming up with diverse and sometimes conflicting findings about how plants strategically arrange their roots when the dirt gets crowded. A paper published earlier this month in Science details a new model that appears to reconcile this confusion by accounting for the spatial distribution of roots along with their prevalence. In initial tests performed by the paper’s authors, real plants played by the rules the model laid out.

It takes energy and materials to grow and maintain a root. Ideally, a plant will get more resources out of its roots than it spends on their construction and upkeep. Plants can sense the concentration of water and nutrients in particular soil patches, and apportion roots accordingly, to maximise their yield.

For a solitary plant, this is simple enough. But when other plants are around, the calculus changes. Researchers have borrowed tools from game theory — a way to analyse and optimise decision-making, used by everyone from financial analysts to actual gamers — to try to figure out exactly how. One model, published in 2001, predicted that plants growing close together end up in a “tragedy of the commons,” with each individual in a shared space making more roots than a lone plant would, but also getting fewer rewards.

Some real-world experiments matched this model, finding that plants with neighbors did create more root mass than those growing on their own. But other studies have found the opposite: that competing plants invested less in roots. And others still found no appreciable difference. “There was all this controversy,” said Ciro Cabal, a doctoral student in ecology and evolutionary biology at Princeton University and the lead author of the new study.

Cabal wondered whether that model, and others like it, might be missing a component. They treated all roots the same, no matter their distance from the plant’s stem. But in reality, the farther a root grows from the plant, the more costly it is to produce and maintain. So he and his co-authors built a new model to account for that. “We incorporated space,” he said. “And we found this new theory.”

In their model, a plant facing competition will under-produce those more expensive, wide-ranging roots that might otherwise overlap with a neighbour’s. But it will overproduce roots closer to home, effectively consolidating power and preventing any “I-drink-your-milkshake” style plays. Whether plants with neighbors over- or under produce roots compared with solo plants depends on how far apart the two competing plants are, Cabal said. So those findings from previous studies that seemed to contradict each other are “all possible according to our model.”

If this strategy does turn out to be widely used by plants, people could use it to incorporate more accurate estimates of plant biomass into climate models, Cabal said. And if it’s possible, breeding some of the competitiveness out of plants could also improve yields in agriculture. Farmers are often stymied by self-interested crops that put their energy into roots instead of fruits.

Cara Giaimo is a journalist with NYT©2020