Plants competing for sunlight—stretching upward and outward to block each other’s exposure to the rays of the sun—is a usual sight. However, there is another competition occurring underground, unobserved.
Similar to how people look for free snacks in the break room when their colleagues are present, plants alter their use of underground resources upon being planted close to other plants.
In a study published recently in the Science journal (and featured on the cover), an international research team headed by Princeton graduate student Ciro Cabal highlights the underground life of plants. In the study, a combination of modeling and a greenhouse experiment was used to determine whether plants invest distinctly in root structures when planted independently versus being planted close to a neighbor.
This study was a lot of fun because it combined several different kinds of mind candy to reconcile seemingly contradictory results in the literature: a clever experiment, a new method for observing root systems in intact soils and simple mathematical theory.
Stephen Pacala, Study Senior Author and Frederick D. Petrie Professor in Ecology and Evolutionary Biology, Princeton University
“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—that is, roots—store carbon,” stated Cabal, a PhD student in Pacala’s lab. “As about a third of the world’s vegetation biomass, hence carbon, is belowground, our model provides a valuable tool to predict root proliferation in global earth-system models.”
Plants synthesize two different kinds of roots: fine roots for absorbing nutrients and water from the soil, and coarse transportation roots to carry such substances back to the center of the plant. The 'investment' of plants in roots includes both the entire volume of roots generated and the method by which such roots are distributed across the soil.
A plant can focus all of its roots immediately below its shoots, or it could spread its roots out horizontally to forage in the adjacent soil—which leads to competition with the roots of neighboring plants.
The model developed by the team expected two possible results for root investment when plants discover themselves sharing the soil. Firstly, the neighboring plants 'cooperate' by separating their root systems to decrease overlap, which results in the synthesis of less roots entirely than they would if they were alone.
Secondly, if a plant senses decreased resources on one side because of the existence of a neighbor, it curtails its root system on that side but invests more in roots immediately below its stem.
Natural selection expects the second case since each plant acts to increase its own fitness, irrespective of how those actions affect other individuals. If plants are planted very close to each other, this higher investment in root volume, regardless of the segregation of such roots, can lead to a disaster of the commons, whereby the resources (in this case, soil moisture and nutrients) are exhausted.
The researchers tested the predictions of the model by growing pepper plants in a greenhouse both separately and in pairs. At the end of the experiment, the roots of the plants were dyed with different colors so that they could easily observe which roots belonged to which plant.
They then estimated the total biomass of every plant’s root system and the ratio of roots to shoots, to verify whether plants altered how much carbon and energy they deposited into aboveground and belowground structures when planted near neighbors, and counted the number of seeds made by every plant as a measure of relative fitness.
The researchers identified that the results are based on how close a pair of plants are present to each other. If plants are planted very close together, they will be more likely to heavily invest in their root systems to try to surpass each other for finite underground resources; if they are planted apart from each other, they will probably invest less in their root systems compared to a solitary plant.
Particularly, they discovered that when the pepper plants are planted next to others, they exhibited an increased investment in roots locally and decreased the extent to which they stretched their roots horizontally, to minimize overlapping with the neighbors.
There was no proof for a 'tragedy of the commons' scenario, as there was no difference in the total root biomass or relative investment in roots than aboveground structures (such as the number of seeds produced for every plant) for single versus co-habiting plants.
Plants capture carbon dioxide from the air and deposit it in their structures—and one-third of this vegetative carbon is stored in roots. Gaining insights into how carbon deposition varies in various scenarios could enable a more accurate prediction of carbon uptake, which could help design methods to prevent climate change.
Also, this study could help maximize food production, knowledge of how to optimally utilize belowground and aboveground resources can optimize crop yield.
The other co-authors of the are Ricardo Martínez-García, a former postdoctoral fellow in EEB who is currently a professor at the South American Institute for Fundamental Research; Aurora de Castro, who worked on the project as part of an undergraduate thesis for the Department of Biogeography and Global Change at the Spanish National Museum of Natural Sciences.
Additional co-authors include Fernando Valladares, an associate professor in the Department of Biology, Geology, Physics and Inorganic Chemistry at Rey Juan Carlos University and a researcher in the Department of Biogeography and Global Change at the Spanish National Museum of Natural Sciences.
Cabal, C., et al. (2020) The exploitative segregation of plant roots. Science. doi.org/10.1126/science.aba9877.