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Plants competing for sunlight, stretching up and out to block each other’s exposure to the sun’s rays, is a normal sight. However, there is another underground competition, unnoticed.
Image credit: Ciro Cabal, Princeton University Department of Ecology and Evolutionary Biology.
Similar to how people seek out free snacks in the break room when their colleagues are present, plants alter their use of underground resources when planted next to other plants.
In a study recently published in Science journal (and featured on the cover), an international research group led by Princeton graduate student Ciro Cabal sheds light on the underground life of plants. In the study, a combination of modeling and a greenhouse experiment was used to determine whether plants distinctly invest in root structures when planted independently of being planted near a neighbor.
This study was a lot of fun because it combined different types of mental 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, senior author of the study and Frederick D. Petrie professor in ecology and evolutionary biology, Princeton University
“While the aboveground parts of plants have been extensively studied, including the amount of carbon they can store, we know much less about how underground parts, i.e. roots, store carbon.“Said Cabal, a doctoral student in Pacala’s lab.”Since about a third of the world’s plant biomass, hence carbon, is underground, our model provides a valuable tool for predicting root proliferation in models of the global earth system.. “
Plants synthesize two different types of roots: thin roots to absorb nutrients and water from the soil and coarse transport roots to bring these substances back to the center of the plant. The “investment” of plants in the roots includes both the entire volume of roots generated and the method by which those roots are distributed in the soil.
A plant may concentrate all of its roots immediately below its shoots, or it may spread its roots horizontally to forage in adjacent soil, which leads to competition with the roots of neighboring plants.
The model developed by the team predicted two possible outcomes for root investment when plants discover they share soil. First, neighboring plants “cooperate” by separating their root systems to decrease overlap, which results in synthesizing fewer roots altogether than they would if they were alone.
Second, if a plant perceives a decrease in resources on the one hand due to the existence of a neighbor, it limits its root system on that side but invests more in the roots immediately below its stem.
Natural selection provides the second case as each plant acts to increase its physical fitness, regardless of how these actions affect other individuals. If plants are planted very close to each other, this increased investment in root volume, regardless of the segregation of those roots, can lead to a disaster of the commons, whereby the resources (in this case, moisture soil and nutrients) are depleted.
The researchers tested the model’s predictions by growing pepper plants in a greenhouse either separately or 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 root biomass of each plant and the root-to-shoot ratio, to see if the plants altered the amount of carbon and energy they deposited in above-ground and underground structures when planted near neighbors. , and they counted the number of seeds produced by each plant as a measure of relative fitness.
The researchers identified that the results are based on the proximity of one pair of plants to the other. If the plants are planted very close together, they will be more likely to invest heavily in their root system to try to outdo each other for finite underground resources; if they are planted by each other, they will likely invest less in their root system than a solitary plant.
Specifically, they found that when pepper plants are planted alongside others, they showed greater investment in roots locally and decreased the extent to which they stretched the roots horizontally, to minimize overlap with neighbors.
There was no evidence of a “ tragedy of the commons ” scenario, as there was no difference in total root biomass or relative investment in roots versus aboveground structures (such as the number of seeds produced for each plant) for the single with respect to the cohabitant plants.
Plants capture carbon dioxide from the air and deposit it in their structures, and a third of this vegetative carbon is stored in the roots. Gaining insight into how carbon deposition varies in various scenarios could allow for a more accurate prediction of carbon uptake, which could help design methods to prevent climate change.
Furthermore, this study could help maximize food production, knowing how to optimally use underground and above-ground 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 a dissertation for the Department of Biogeography and Global Change at the Spanish National Museum of Natural Sciences.
Other co-authors include Fernando Valladares, associate professor in the Department of Biology, Geology, Physics and Inorganic Chemistry at the Rey Juan Carlos University and a researcher in the Department of Biogeography and Global Change at the Spanish National Museum of Natural Sciences.
Journal reference:
Cabal, C., et al. (2020) The exploitation segregation of plant roots. Science. doi.org/10.1126/science.aba9877.
Source: https://www.princeton.edu/
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