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Measuring Competition in Crop Plants: Mechanisms & Outcomes

Competition is ubiquitous among plants and is one of the crucial interactions that influence the health and performance of crops. By measuring various shoot and root parameters throughout the entire crop cycle, scientists can glean crucial insights into the processes of competition and its outcome on crop plants.

Small soybean plants growing in row

Scott Trimble

November 30, 2021 at 6:07 am | Updated March 16, 2022 at 5:28 pm | 8 min read


Plant to Plant Competition

Competition can be intraspecific (between members of a species) or interspecific (between species). In both cases, competition occurs for resources.

Nutrients, water, and light are the common resources for which plants compete in space and time. While plants obtain light and some essential elements like carbon (C) and oxygen (O) from the air, they get most major and minor nutrients from the soil, such as nitrogen, potassium, phosphorus, calcium, iron, magnesium, etc. Even the oxygen used for roots is derived from the ground.

Plants gain an advantage if they have better access to nutrients and water by extending their root systems or growing taller and growing more leaves to monopolize a greater amount of light than their neighbors.

There are two main aspects of competition that interest scientists today:

  • One chiefly examines the mechanisms of resource competition.
  • The second focuses on the reduction in plant fitness that occurs due to limited, shared resources. Moreover, competition can reduce the resources allotted by plants for defense, making them more susceptible to pests and diseases.

Why Should Plant Competiton be Measured?

Both the mechanisms and results of plant competition are crucial for food production.

In agriculture, the aim is to eliminate competition for resources to maintain high crop productivity and quality. As monocultures are the norm, the crops are from the same species and variety in a single field. Crop science is an exercise in balancing the need to increase plant density while providing proper spacing, fertilization, irrigation, and other agricultural operations to create optimum, competition-free growing conditions for plants.

In open spaces, carbon and oxygen are not limited in the air. However, in closed atmospheres like greenhouses, even these elementary nutrients have to be considered and supplied.

Intraspecific Competition

The intraspecific competition effects between plants of the same variety, sown at the same time, can be one or more of the following:

Competition-density effect: As the density of plants increases, there is a corresponding decrease in plant performance, reflected in a reduction in the overall size and weight of plants.

Size-hierarchy development: Competition can affect the relative growth rate of neighboring plants when a few individuals use disproportionate resources. As a result, biomass accumulation among plants can differ. Along with microsite parameters, the number of plants, and the relative time of emergence can determine the position of a plant in the emerging size hierarchy in a field. The size hierarchy effects of competition can assume commercial importance in the case of vegetables that have to meet pre-market size criteria.

Self-thinning: Density-dependent mortality of plants is unusual in agriculture, as agricultural practices rarely result in densities high enough to lead to mortality. This kind of effect is more common in natural plant communities.

There is also a competition among different organs or sinks of a plant for resource allocation, for example, between vegetative parts and fruits.

Interspecific Competition

In agriculture, interspecific competition effects between weeds and crop plants, and between intercrops, must also be considered. Weeds suppress the growth of young crop plants by shading them and competing for nutrients from the soil and space to grow. Weeds can reduce the quantity and quality of yield, so it is necessary to minimize crop-weed competition by controlling the population of weeds; see Figure 1.

However, crop density and populations can also be optimized to suppress weed growth. The density and biodiversity of weeds will determine their ability to influence crop yield loss.

The competitive ability of various species in an environment depends on many life history factors, such as the following:

  • Time of weed emergence
  • Weed seedling density
  • Morphological and physiological traits, such as leaf area, plant height, root system size, relative biomass, seed size, reproductive strategies, etc.
Figure 1: “Effect of weed competition on physical and chemical fruit quality properties (A) fruit weight (n=30), (B) firmness (n=30), (C) soluble solids content (SSC) (n=3), (D) pH (n=3), and (E) fruit skin colour (b* value; ‘+’ = yellow, ‘–’ = blue) (n=30). SWC: Strong Weed Competition, MWC: Moderate Weed Competition, WWC: Weak Weed Competition. Interaction P: P-value of the interaction between weed competition level × year (2011 and 2012),” Atay et al. 2017. (Image credits: https://doi.org/10.15835/nbha45110556)

Parameters to Measure Plant Competition Mechanisms

Competition among crop plants is analyzed based on both the mechanisms and outcomes of the relationship.

The mechanisms used by crop plants and weeds to get a competitive edge are measured by estimating physiological and morphological parameters.

Leaf area

Leaf area evaluations are essential for measuring competition at three levels: interspecific, intraspecific, and within-plant resource allocation.

  • Measuring leaf area is typical for crop-weed competition, as shading by either species depends on the relative total leaf area. When weeds emerge earlier than crops, they can grow faster and depress crop growth by shading them and limiting access to light.
  • Also, leaf area can indicate plant fitness and photosynthetic potential in intraspecific and interspecific competition, since the final biomass yield depends on leaf area.
  • A balance of resource allocation to vegetative parts measured in leaf area and yield is standard in agriculture.

Several portable instruments like the CI-202 Portable Laser Leaf Area Meter and CI-203 Handheld Laser Leaf Area Meter help take non-destructive, rapid estimations of leaves in the field for single or repeat measurements.

Leaf Area Index

Leaf Area Index (LAI) can also indicate plant health and performance, evaluating light competition in intraspecific and interspecific competition. The amount of light that reaches a plant determines a plant’s ability to conduct photosynthesis and, ultimately, crop yield.

A reduction in light interception by plants due to crowding from neighbors and the importance of this reduction can differ based on the crop stage and crop species. LAI is a better indicator than leaf area for light interception as it is the ratio of one-sided leaf area per ground area.

The CI-110 Plant Canopy Imager can measure light interception without an above-canopy reference by measuring Photosynthetically Active Radiation (PAR) levels and calculating LAI through the Gap Fraction Method.

Photosynthetic Efficiency

Figure 2: “Diurnal variation of net photosynthetic rate (NPR) for the intercropping systems and its control (A. apple–soybean and B. apple–peanut). F0.5, F1.5 and F2.5 were used to represent the sampling points which had different distances (0.5 m, 1.5 m and 2.5 m) from the tree row. Error bars indicate standard deviation,” Gao et al. (2013). (Image credits: https://doi.org/10.1371/journal.pone.0070739)

Since reducing photosynthesis is one of the mechanisms by which competition affects plant productivity, this physiological process is also directly measured in situ. The photosynthetic rate can evaluate the intraspecific and interspecific competition. Figure 2 shows that, in two intercropping systems of apple with soybeans and peanuts, photosynthesis decreases in the crops close to apple trees due to light competition.

Photosynthesis is measured in the fields by portable Infrared Gas Analyzers (IRGAs) like the CI-340 Handheld Photosynthesis System, manufactured by CID Bio-Science Inc.

Root development

Root parameters, such as structure and density, total length, density at different soil depths, and effective membrane transporters can provide information on water use efficiency and nutrient acquisition in intraspecific and interspecific competition.  

The allocation to root systems can change under competition to improve water and nutrient uptake. The longterm underground studies needed to track these changes are now possible with minirhizotrons, such as the CI-600 In-Situ Root Imager and CI-602 Narrow Gauge Root Imager, which take non-destructive, rapid, and accurate scans of root systems through preinstalled transparent root tubes.

Measuring Result of Competition

The result of intraspecific and interspecific competition on crops can be measured in three broad ways.

Nutrient content in plants

The uptake of nitrogen, phosphorus, and potassium (NPK), the major limiting nutrients for any plant, can be affected due to competition. The content of these elements can be measured directly in different plant parts through biochemical assays, or they can be measured indirectly. For example, nitrogen levels in leaves can be measured via photosynthetic rate.

Yield

Figure 3: “Relationship between the spike number (a), grains per spike (b), thousand-grain weight (c) and net yield (d) with mean daily shade intensity of wheat intercropped with fruit trees in 2011 and 2012,” Qiao et al. (2019). (Image credits: https://doi.org/10.1371/journal.pone.0203238.g005)

The amount of biomass produced by a crop is an intuitive means of understanding how a crop plant or population has been affected by competition. The quantification can be based on plant dry matter or economic yield.

Besides yield, morphological parameters like number of fruits, spikes, kernels per spike, 1000-seed weight, plant height, number of tillers, stem thickness, etc. are also used. See Figure 3.

These indicators of yield are used in various competition indices to evaluate the results of the competition.

Quality

Competition for resources will impact yield quality besides the quantity and significantly reduce returns from a farm. These effects are measured by well-established quality parameters like dry matter, soluble solids content, firmness, color, and nutraceutical contents.

Commercially available portable near-infrared spectroscopy tools can precisely measure these parameters in seconds.

Competition Indices

Competition indices are a means to quantify the results of competition. However, these indices do not evaluate the process and mechanisms involved, nor do they track competition progress over time.

Figure 4: “Responses of individual grain yield, dry matter accumulation, and harvest index to plant density. (pl. plants),” Zahi et al. (2018). (Image credits: https://doi.org/10.1016/s2095-3119(18)61917-3)

Moreover, these competition indices focus on interspecific competition between crops and weeds. There is less information available on intraspecific competition.

Competition indices generally fall into three broad categories:

  • Those that seek to quantify the intensity of competition, for example, CI and ASC
  • Those focused on determining the effect of competition, for instance, RY
  • Those that examine the outcome of competition, for example, RReff

Some of the following interspecific competition indices have been revised to evaluate crop intraspecific competition:

  • Relative yield (RY): Compares crop yield in populations with varying plant densities.
  • Competitive intensity (CI): Measures plant size in terms of biomass between plants grown at different densities or spacing increments.
  • Absolute severity of competition (ASC): This index is similar to relative yield, except that one set of plants is grown at no competition and the other is grown at specific densities to compare their yeild.
  • Relative reproductive efficiency (RReff): Plants grown under no/low competition are compared to those at higher densities/competition in seed numbers.

Competition

Though tackling the effects of intraspecific competition is central to industrial agriculture, there is surprisingly little work done to quantify the interaction between members of the same cultivar. Most of the studies are focused on interspecific competition of crop and weeds and between intercrops, to a lesser extent. Hence, scientists recommend more work on the process of competition to understand it and its effects better.

Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture

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