Photosynthesis in crop weed Competition is a crucial factor that influences their growth and ability to outcompete desirable plants. Photosynthesis is the phenomenon of converting light energy to chemical energy in plants through chlorophyll in the presence of sunlight. Weeds are plants that affect the crop and increase the input cost of farmers for managing. Although weeds are harmful to plants yet have some benefits; making soil porous for proper plant growth, providing some nutrients to soil and holding the soil (preventing soil erosion).
Competition between weeds and crops of the same characteristics is severe but if they are of different growth patterns then the impact is bearable. Most weeds are C4 and thus have higher use efficiency in all the inputs/resources. The use of selective herbicides is beneficial for weed-crop competition.
Introduction:-
As we know photosynthesis is a process by which plants prepare their own food (carbohydrates) and releases oxygen by the assimilation of carbon dioxide, water in the presence of sunlight and green pigment chlorophyll mostly in the daytime. Photosynthesis has a necessary role in the survival mechanism of every plant on Earth so far. In order to describe weed competitiveness, firstly competition means fighting for something but in terms of 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 the 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 (property of secreting such chemicals that inhibit the growth of other plants or crops), fast growth, efficiency in nutrient uptake and deep root system. 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).
Significance of Photosynthesis in weed competitiveness
Photosynthesis plays a crucial role in the competitiveness of weeds. Weeds are capable of overcoming successfully with 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, that enable them to adapt to different environmental conditions. Most weeds are C4 plants and crops are C3. C4 plants are more efficient at performing photosynthesis under high temperatures and light intensity, giving them an advantage in hot and sunny environments to perform photosynthesis. 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 mechanisms of photo-respiration at high light and temperature that use oxygen and release carbon dioxide into the atmosphere. The beneficial aspects of carbon dioxide enrichment on water use efficiency and plant water status will enhance the growth of C4 and C3 plants as well as, because of leaf expansion, an important factor in plant growth. It is observed that C3 plants are more efficient than C4 weeds. When carbon dioxide concentration increases from 350 to 675 ppm, the competitiveness of the C3 plant soya bean is higher than the C4 weed Johnson grass.
Moreover, a concentration of carbon dioxide enrichment up to 675 ppm increases the water use efficiency of C4 weeds thus at this concentration, weeds produce more dry matter and leaves. Inadequate 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 crop weed competition, mutual shading of leaves causes a reduction of Photosynthetic Photon Flux Density(PPFD), ultimately causing a reduction in the rate of photosynthesis. The reduction in leaf area by the weed affects yield production greatly 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, FR/R ratio is increased in lower leaves while the FR/R ratio decreases in normal light. In such a situation, a shaded plant produces chemicals that support it to grow longer to reach normal red light. In this condition, plants grow longer than their original mechanism(stem elongation, apical dominance, reduced branches, thinner leaves and leaf area distribution).
If the density of weeds is too strong then it is needed to suppress them through different types of herbicides which do not harm the main crops but the weed population. Allelopathy affects the crops by secreting such chemicals that inhibit the growth of several plants and maybe some weeds. In this way, crops or plants are suppressed and wait until some favourable conditions to support growth and germination. This may increase the longevity of the seed and is thus also 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 the soil thus improving the soil health. Weeds also add nutrients to the soil by secreting some 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 the soil.
Waterlogged soil can be managed by growing weeds that are highly efficient in absorbing water. Weeds are also beneficial in terms of protecting the crop from severe environments such as high temperatures or chilling temperatures by covering the main crops.
Management of crop weed competition for efficient photosynthesis
- Relay cropping; at the end of the first crop, the 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 weed germination.
- Flooding; is a method in which land is watered with excess water and weeds die from suffocation at the root area. This new crop has less exposure to weeds.
- Removal after emergence; after the emergence of the crop, carefully pick out the weed 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 the crop remains unaffected and weeds are removed.
Competitive Structures in Crop Weed Competition:-
- Within species 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 specific areas.
- Between species competition among plants of the different weed species.
- Within-species competition between plants of the same weed species.
Conclusion:-
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, sometimes they favour 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 a level that supports photosynthesis.
Hashir Mahmood
University of Agriculture, Faisalabad
