Plants compete for sunlight by stretching upward and outward to block each other’s access to the sun. Another type of competition is happening underground.
An international team of researchers led by Ciro Cabal, a graduate student in the department of ecology and evolutionary biology at Princeton University, recently shed light on the underground life of plants. The researchers used a model to predict two potential outcomes for root investment when plants find themselves sharing soil.
In the first outcome neighboring plants “cooperate” by segregating their root systems to reduce overlap. That leads to producing fewer roots overall than they would if they were solitary.
In the second outcome, when a plant senses reduced resources on one side due to the presence of a neighbor, it shortens its root system on that side. But it invests more in roots directly below its stem. Natural selection predicts the second scenario. Each plant acts to increase its own fitness regardless of how those actions impact other individuals. If plants are close together the increased investment in root volume, despite segregation of the roots, could result in soil moisture and nutrients being depleted.
The researchers grew pepper plants in a greenhouse both individually and in pairs. At the end of the experiment they dyed roots of the plants different colors so they could see which roots belonged to which plant. Then they calculated the biomass of each plant’s root system and the ratio of roots to shoots. That enabled them to see how much energy and carbon the plants deposited into belowground and aboveground structures when planted alongside neighbors. They also counted the seeds produced by each plant as a measure of relative fitness.
The researchers found that the outcome depends on how close a pair of plants are to each other. If planted very close together, plants will be more likely to heavily invest in their root systems to outcompete each other for finite underground resources. If they’re planted further apart, they’ll likely invest less in their root systems than a solitary plant would.
When planted near others, the pepper plants increased investment in roots locally and reduced how far they stretched their roots horizontally. There was no difference in the total root biomass or relative investment in roots compared to aboveground structures for solitary versus co-habiting plants.
Plants remove carbon dioxide from the atmosphere and deposit it in their structures. A third of the vegetative carbon is stored in roots. Understanding how carbon deposition changes in different scenarios could help farmers more accurately predict carbon uptake, which in turn could help design strategies to mitigate climate change. The research also could help optimize food production; to maximize crop yield, it’s helpful to understand how to optimally use belowground and aboveground resources. The study recently was published in “Science.” Visit science.sciencemag.org and search for "Ciro Cabal" for more information.
Liana Wait is a graduate student in the ecology and evolutionary biology department at Princeton University.