Battling Silent Invader: Unmasking Parthenium Weed Menace And Biocontrol Solutions

Photosynthesis is the phenomenon of converting light energy into chemical energy in plants through chlorophyll in the presence of sunlight.

Battling Silent Invader: Unmasking Parthenium Weed Menace And Biocontrol Solutions

Photosynthesis is the phenomenon of converting light energy into chemical energy in plants through chlorophyll in the presence of sunlight. Weeds are plants that affect crops and increase the input cost for farmers to manage them.

Although weeds are harmful to plants, they have some benefits: they make soil porous for proper plant growth, provide some nutrients to the soil, and hold the soil (prevent soil erosion).

Competition between weeds and crops with the same characteristics is severe, but if they have different growth patterns, the impact is bearable. Most weeds are C4, thus having higher use efficiency for all the inputs and resources. The use of selective herbicides is beneficial for weed-crop competition.

As we know, photosynthesis is the process by which plants prepare their own food (carbohydrates) and release oxygen through the assimilation of carbon dioxide, water in the presence of sunlight, and the green pigment chlorophyll, mostly in the daytime.

Photosynthesis has played a necessary role in the survival mechanisms of every plant on earth so far. In order to describe weed competitiveness, firstly one should know about competition, which means fighting for something, but in agriculture, it means fighting for resources when they are not in excess. The term competitiveness depends upon emergence time, density, pattern of growth, rate at which growth occurs, survival in different conditions, and allelopathy (secreting chemicals).

‘Weed competitiveness’ refers to the ability of a weed species to compete with different plants for resources such as water, nutrients, light, carbon dioxide, and space.

Weeds affect plants and desirable crops in a very harmful way. They are very vigorous and have a strong genetic makeup to survive in stressful conditions. They affect the production of crops and their yield.

The competitiveness of weeds is mainly due to allelopathy factors (the property of secreting chemicals that inhibit the growth of other plants or crops), fast growth, efficiency in nutrient uptake, and deep root systems. Allelochemicals are secondary metabolites. Sometimes, the terms competition and interference are confusing. Interference is the combined effect of both competitiveness and allelopathy (secretion of allelochemicals).

Role of photosynthesis in weed competitiveness:

Photosynthesis plays a crucial role in the competitiveness of weeds. Weeds are capable of overcoming crops due to their vigorous nature and ability to perform photosynthesis more efficiently. This behaviour allows them to produce more energy and grow faster.

Weeds have developed different photosynthetic pathways, such as C3, C4, or CAM, which enable them to adapt to different environmental conditions.

Mostly, weeds are C4 plants, and crops are C3. C4 plants are more efficient at performing photosynthesis under high temperatures and high light intensity, which gives them an advantage in hot and sunny environments.

This allows them to produce more energy and grow faster than crops, thus making it more difficult for crops to compete with them for resources. C3 crops start a mechanism of photo-respiration at high temperatures and high light intensity that uses oxygen and releases carbon dioxide into the atmosphere.

The beneficial aspect of carbon dioxide enrichment on plant water use efficiency is that it will enhance the growth of C4 plants, but C3 plants start photorespiration because they are not used to high temperatures and high light intensity. It is observed that C3 plants are more inefficient than C4 weeds. When carbon dioxide concentration increases from 350 to 675 ppm, the competitiveness of C3 plant soya bean is higher than that of C4 weed Johnson grass.

Moreover, concentrations of carbon dioxide enrichment up to 675 ppm increase the water use efficiency of C4 weeds; thus, at this concentration, weeds produce more dry matter and leaves. In an adequate water supply, C4 plants have no effect on CO2 concentration. Overall, photosynthesis plays a crucial role in determining the competitiveness of weeds in the agriculture system, ultimately leading to crop yield loss and reduced agricultural productivity.

In the community of mixed weed-crop competition, mutual shading of leaves causes a reduction in photosynthetic photon flux density (PPFD), ultimately causing a reduction in the rate of photosynthesis. The reduction in leaf area by the weed greatly affects yield production because photosynthates that are transferred for grain filling are produced in leaves.

In relation to photosynthesis, weed crop competitiveness also describes the FR/R ratio. When weeds shade the main plant, far-red light reaches the lower plant; the FR/R ratio is increased in the lower leaves, while the FR/R ratio decreases in normal light. In such a situation, a shaded plant produces chemicals that encourage it to grow longer to reach normal red light.

In this condition, plants grow longer than their original mechanisms (stem elongation, apical dominance, reduced branches, thinner leaves, and leaf area distribution). If the density of weeds is too high, then it is necessary to suppress them through different types of herbicides that do not harm the main crops but the weed population.

Allelopathy affects crops by secreting chemicals that inhibit the growth of several plants and may be weeds. In this way, crops or plants are suppressed and wait until some favourable conditions support their growth and germination. This may increase the longevity of the seed and thus be helpful for plant survival mechanisms.

Weed growth is beneficial too in the way that it provides organic matter to the soil when it decays. They also make soil porous when roots grow in it, thus improving its health. Weeds also add nutrients to the soil by secreting chemicals from the roots that contain essential growth elements.

Weeds are very aggressive in absorbing water and nutrients; thus, they regulate the movement of ions and nutrients in soil. Waterlogged soil can be managed by growing weeds that are highly efficient at absorbing water, and weeds are also beneficial in that they protect the crop from severe environments such as high temperatures or chilling temperatures by covering the main crops.

Management of weed-crop competition for efficient photosynthesis:

Relay cropping:

At the end of the first crop, a second crop is sown before the harvesting of the first crop. In this way, soil is covered after the emergence of a new crop and less space is left over for weeds germination.


It is a method in which land is watered with excess water and weeds die from suffocation at the root area. Thus, newly emergent crops have less exposure to weeds.

Removal after emergence:

After the emergence of the crop, carefully pick out the weeds from the root and perform this activity two times again before the crop reaches 45–60 days after sowing.

Selective herbicides:

Selective herbicides should be used so that crops remain unaffected and weeds are removed.

Competitive Structures in Crop-Weed Competition:

  • Within species, there is competition among plants of the same cultivated species
  • Between species competition among plants of different cultivated species
  • Interspecific competition among plants of cultivated species and weeds grown in a specific area
  • Between species competition among plants of the different weed species.
  • Within-species competition between plants of the same weed species


Photosynthesis has a vital role in the competitiveness of weeds. If the rate of photosynthesis in weeds is higher, then the growth of weeds is also high. In the same way, if the rate of photosynthesis in plants is higher, then the growth of plants is also high.

Sometimes, weeds reduce the growth of plants, but sometimes they encourage the crops to grow. Observing the role of photosynthesis in weed competitiveness is beneficial for developing effective weed management strategies that can mitigate the impact of weeds on crop production.

This could include the development of crops that are better adapted to local environmental conditions or the use of selective herbicides that target specific weed species while leaving crops unharmed. In a sufficient supply of water, carbon dioxide concentration has no effect on the C4 plant’s growth mechanism, but the concentration of carbon dioxide should be up to the level that supports photosynthesis.