Just as plants compete for sunlight stretching upward and outward, so their roots move through the underground to source water and nutrients.
A plant’s investment in its root structure is a combination of both the volume of root branches and the way in which they spread. They may grow directly down or spread out horizontally and tangle with neighbours. It all influences how they gather resources to grow.
Scientists at Princeton University have studied the underground life of plants to find out whether they invest differently in their roots when planted alone compared to when they grow alongside neighbours.
“Most people see plants as static organisms, but that is because they move at a much slower pace than we do,” said graduate student Ciro Cabal of the university’s department of Ecology and Evolutionary Biology, who led an international team of researchers.
He said lack of study has led to a generalization of the concept that competing plants overinvest in their root systems and by doing so, they overexploit the soil resources and deplete them.
This “tragedy of the commons” can happen, Cabal said, but it is relative to the spatial configuration of plant populations.
The team’s model predicted two potential outcomes for root investment when plants share soil. In the first outcome, the neighbouring plants co-operate by segregating their root systems to reduce overlap but that results in less root structure overall compared to growing in a solitary space. In the second outcome, when a plant senses reduced resources on one side because of a neighbour’s presence, it will shorten its root system on that side but invest more in roots directly below its stem.
A news release stated that each plant acts to increase its own fitness despite how those actions might impact other plants. But if plants are close together the increased investment in root volume could result in a depletion of essential resources.
“What is essential for plants is to be able to absorb resources from soil such as water or mineral nutrients,” said Cabal. “It is well known that only fine roots can do this, while the coarse lignified roots are transportation organs.”
He said the biology behind root recognition is a fascinating field yet still under development. It has been proven that if a root in a soil patch encounters another root, it can recognize whether the other root is self or non-self, and even whether it is from another plant of the same or different species.
“Several stimuli have been suggested to drive this perception mechanism, from the exudation and detection of biomolecules to the use of electrical signals,” he said. “Plants that cannot use these recognition mechanisms would be expected to behave exploitatively, as is our main result in the paper, because that would be the optimal behaviour derived from natural selection.”
The researchers grew greenhouse pepper plants both individually and in pairs. The plants’ roots were dyed different colours so it was clear which root belonged to which plant. They calculated the biomass of each plant’s root system and the ratio of roots to shoots to see if plants changed the volume of energy and carbon deposited into below-ground and above-ground structures when planted alongside neighbours. They counted the number of seeds each plant produced as a measure of relative fitness. The study provided two model solutions, one based on natural selection and the other on root recognition.
Cabal said that, in natural selection, no self/non-self/non-kin root recognition mechanisms were involved.
“Plants only sense in each patch of soil a given amount of resources entering the system and a rate at which the resources leave or leak away. The plant will adjust its root density to the balance between the two processes of resources entering and leaving the soil patch and what it takes to grow roots to forage in that soil location. If other plants are foraging in the same location, the plant may not be directly aware of it, but will perceive that there is a higher resource leaking rate. If the cost to the plant is low, it will produce more roots as a response to increased leaking, but if the cost of growing roots is high, the plant will produce less roots with increased leakage.”
The second solution required active root recognition between plants and allowed them to make a collective decision to maximize their overall yield. In the study, when the pepper plants were planted near each other they increased investment in roots locally and reduced how far they stretched those roots horizontally to reduce overlap with neighbours. This avoided the “tragedy of the commons” scenario. There was no difference in the total root biomass or investment in roots compared to plant growth aboveground, including seeds produced. However, how this applies to crop production is a work in progress.
“Our take-home message for farmers and agtech scientists is simple: plants in the wild may act selfishly, and this leads to a tragedy of the commons in which a plant’s collective yield is minimal,” said Cabal. “We also produced a theoretical solution for root distribution of plants collectively maximizing their root efficacy in a shared space. This alternative solution is the best possible root foraging behaviour a farmer can dream of, and it simply corresponds to plants growing the same root densities as if they were alone but withdrawing from every soil location where a neighbour is closer than the focal plant.”
Their experiment with peppers showed that the productivity of agricultural varieties of crop plants can be improved. It will require further research, specifically targeting each cultivar of interest. He said that the model solutions can be used to calculate the optimal plant density in crops, help design new crop varieties, or be used to recommend mechanisms to interrupt roots intermingling physically.
Carbon uptake in root structures changes with different scenarios and learning more about carbon will help design strategies to address climate change.
“The design of strategies to mitigate climate change is inevitably tied to our capacity to predict the future,” said Cabal. “We need to develop earth system models that produce climatic predictions and use these models to test how predictions change under different scenarios involving the different actions we can take and different natural changes that may occur. The problem with these models is that they need to rely on very complex earth subsystems which are not well understood. One of these is vegetation as plants are a fundamental driver of carbon movements in the atmosphere.”
The research may help optimize food production since, to maximize crop yield, understanding how to best use belowground resources will be invaluable.
The research was published in the journal Science.