The underground life of plants

The energy a plant devotes to its roots depends on its proximity to other plants, according to a new study.

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The roots of neighbouring pepper plants were stained by injection. Credit: Ciro Cabal, Princeton University

Using a combination of modelling and experiments, a research team from the US, Brazil and Spain showed that in the presence of competitors, plants overproduce roots to snatch up nearby resources but avoid foraging for nutrients near their neighbours.

This provides a new theoretical foundation for understanding the rules that govern competitive root behaviour, the researchers suggest, and has implications for responding to climate change.

As plants remove carbon dioxide from the atmosphere and deposit it in their structures, knowing how carbon deposition changes in different scenarios could help more accurately predict carbon uptake.

“While the aboveground parts of plants have been extensively studied, including how much carbon they can store, we know much less about how belowground parts… store carbon,” says Ciro Cabal from Princeton University, US, lead author of a paper in Science.

“As about a third of the world’s vegetation biomass, hence carbon, is below ground, our model provides a valuable tool to predict root proliferation in global earth-system models.”

Plants make fine roots to absorb water and nutrients from the soil and coarse roots to transport these substances back to their centre.

Their investment in these systems involves the total volume of roots and the way they are distributed. A plant can concentrate all of its roots directly beneath its shoots, or it can spread them horizontally to forage in the adjacent soil, competing with neighbouring plants.

The new model predicted two potential outcomes when plants find themselves sharing soil. In the first, plants cooperate by segregating their root systems to reduce overlap, which leads to producing less roots overall than they would if they were solitary.

In the second, when a plant senses reduced resources on one side due to the presence of a neighbour, it shortens its root system on that side but invests more in roots directly below its stem.

Natural selection predicts this scenario, because each plant acts to increase its own fitness, regardless of the impact on others. If plants are very close together, this increased investment in root volume, despite segregation of those roots, could result in a “tragedy of the commons”, whereby resources (in this case, soil moisture and nutrients) are depleted.

To test this, the researchers grew pepper plants in a greenhouse individually and in pairs. At the end of the experiment, they dyed the roots of the plants different colours so they could easily see which roots belonged to which plant. They then calculated the total biomass of each plant’s root system and the ratio of roots to shoots.

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Pepper plants growing at the Museo Nacional de Ciencias Naturales in Madrid. Credit: Ciro Cabal, Princeton University

They discovered that when planted near others, pepper plants increased investment in roots locally and reduced how far they stretched their roots horizontally, to reduce overlap with neighbours.

There was no evidence of a “tragedy of the commons”, since there was no difference in the total root biomass or relative investment in roots compared to aboveground structures (including the number of seeds produced per plant) for solitary versus co-habiting plants.

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