Better understanding how plants compete with each other under high-density cropping conditions may help boost yields
Plants in a field are thought of as all the same throughout their sprouting, growing and maturing stages. But in a field where plants grow in high density and vie for light, an individual plant’s social behaviour can be competitive.
According to researchers at the Leibniz Institute of Plant Genetics and Crop Plant Research in Germany, a single plant’s behaviour may nourish its fitness based on its own selfish traits but potentially to the detriment of the performance of the plant community as a whole.
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“We were intrigued by the differences in morphology and productivity we see in the field between plants in the middle of the plot (surrounded by neighbours) and the plants at the plot borders,” said professor Thorsten Schnurbusch, head of the research group Plant Architecture.
“We wanted to understand the underlying factors of these differences. We speculated that light may be a decisive factor here. We were also inspired by previous works of agronomists and ecologists who suggested using evolutionary and ecological principles to increase crop production.”
Throughout evolution, a plant’s competitive edge for resources such as nutrients, water and light has been the driving force of natural selection. But profound changes occurred when desirable food plants such as wheat that thrived in a wild biodiverse landscape before domestication became managed in a homogeneous monoculture environment. That competitive self-focused approach by individual wheat plants still plays out in a crop field.
“Agriculture relies on community performance,” said Schnurbusch. “But the environment in which crops are grown, their agroecology, is little explored and less understood. It is surprising how little we know about the interactions among plants grown in a dense, real-world community.”
The researchers wrote in the report that ever since plant domestication and the start of agriculture 10,500 years ago, crops have had to adapt to different environments, agronomy practices and deliberate breeding to improve yield, quality and ease of farming. Yet individual crop plants did not lose their fitness or their competitive edge.
“Fitness is a measure of reproductive success that is affected by the selection pressures dominating the environment,” said Schnurbusch. “In nature, competition among plants largely structures plant populations, and the most competitive individuals in the population that can acquire a larger proportion of the resources are likely to be naturally selected. So, fitness in nature centres solely around survival traits.
“Since our crops inherited the genomes of their wild progenitors, shaped by millions of years of natural selection, they likely also carry ‘selfish’ genes and alleles that served their ancestors in nature but may not be appropriate anymore because agriculture evaluates collective performance or surface area productivity, which may conflict with individual fitness.”
As an example, he said in nature, growing tall may be an advantage during inter-plant competition for light. But in today’s agriculture, where all plants are genetically identical with the same height, growing tall may be a waste of resources because resources allocated to the stem could have been allocated to reproduction and seed growth to promote yield.
“Wheat plants are usually cultivated in monocultures of a single cultivar so we can say that the plants in the field are like identical twins in their genetic profile,” he said.
“Sustainable modern agriculture should use more resource-efficient plants that produce most effectively from the available arable land. Being more light- and allocation-efficient are aspects of it.”
The field tests conducted by the researchers involved a population of durum introgression wheat lines where each line mostly (95 percent) carried the genes of a modern durum cultivar with a few genomic regions coming from wild emmer wheat, the direct progenitor of modern wheat.
According to the report, they focused on light as a primary factor affecting plant-plant interactions and assessed morphological and biomass phenotypes of single plants grown in mixtures under sunlight and a simulated canopy shade, and the relevance of these phenotypes for the monoculture community in the field. They found that the responses to canopy shade resembled responses to high density, which contributed to both the individual plant and the plant community.
“We tested wild emmer alleles in the background of a modern cultivar to better contrast genomic regions carrying selfish versus non-selfish alleles,” said Schnurbusch. “All plants were grown and evaluated as single plants in a mixture (neighbouring plants that are of different genotypes) in soil in two glass houses under natural sunlight and simulated canopy shade, reducing the blue and red light and the overall light intensity under the green filter. Then these lines were grown in the field as monocultures. In our analysis, we examined how the individual plant’s characteristics affect the plants’ performance as a group.”
Light was a critical factor in the research. Shading does not harm plants since they are used to having short-term, daily fluctuations of light, longer periods of low light or no light at all during night hours. However, he said that, in a high-density wheat crop, shade limits grain production largely due to the effects of the light spectra in which plants sense they are shaded with lowered intensity of the amount of light reaching them. Consequently, spike fertility and grain set are reduced.
“By simulating canopy shade, we may get closer to the conditions plants are experiencing in high-density stands in the field, which may be helpful for studying and selecting plants for higher grain yield,” said Guy Golan of Plant Architecture at Leibniz Institute and first author of the study. “Co-operative behaviours and highly fertile inflorescences in a light-limited, shaded environment are most important for a thriving grain crop community.”
Schnurbusch said that it is reasonable to expect the level of density of plants in a field to affect the performance output of each individual plant.
“As density in the field increases, the output from each individual plant decreases due to the increasing competition for resources. Modern high-yielding varieties are likely to be more tolerant to the inter-plant competition at high density because they were selected for it. So far, however, we have no clue about such tolerance alleles to high density, unfortunately.”
Currently, they have been collaborating with breeders to test whether simulated canopy shade can help select individual plants during early generations of breeding.
“Having much deeper insights into these interactions, and specifically understanding their molecular and genetic components, is very important to developing more resilient and resource-efficient crop plants in the future. Embracing an agroecological genetics approach may optimize communal yield by better matching crops to their environment as either a monoculture or a mixture.”
The study was recently published in the journal Plant, Cell & Environment.
