Paraquat is an herbicide that has been widely used for its effects on unwanted weeds and vegetation. It continues to be a popular choice among farmers for its rapid action in killing green plant tissue.
However, the exact mechanism in which it kills plants has also been seen to be the cause for its high toxicity in humans. In fact, the lasting effects of paraquat exposure in humans have prompted victims into filing a paraquat lawsuit – the majority of them farmers and other agricultural workers who had chronic exposure to paraquat in the workplace.
The mode of action of paraquat in plants is believed to have the same effect in human cells, causing them to be destroyed and eventually die.
Read on to learn more about the effect of paraquat on photosynthesis, and how this effect leads to eventual plant death.
How does Paraquat Affect Photosynthesis?
Paraquat affects photosynthesis by inhibiting it. The herbicide diverts electron flow in Photosystem I, a membrane-protein complex located on the outer surface of the thylakoid membrane. Electrons first go through photosystem II, and then photosystem I to complete the process of photosynthesis.
However, the mode of action of paraquat is the inhibition of photosystem I, where it diverts electrons from the normal electron transport necessary for photosynthesis, which, in turn, interferes with the whole process.
This results in the production of highly chemically reactive free radicals, which causes rapid cell membrane damage, spilling their contents into the intercellular spaces. This membrane disruption appears as yellowing and desiccation in plants sprayed with paraquat.
Visible effects of paraquat in treated plants become apparent within hours under bright sunlight, as absorption of the chemical is increased by high light intensity. On the other hand, wilting of plants sprayed with paraquat may take several days during cool or cold conditions.
Effects of Photosynthesis Inhibition by Paraquat on Plants
Photosystem I (PS I) inhibitors such as paraquat and its sister herbicide diquat are also referred to as cell membrane disruptors because of their contact activity with plant tissues. Aside from the paraquat mode of action that takes place inside the plant, there are also more obvious effects that can be seen on the surface of plants sprayed with paraquat. The effects of photosynthesis inhibition by paraquat in plants are listed below.
1. Necrosis of plant foliage
In plants, necrosis means the death of plant cells or tissues. Within four to six hours of paraquat application, necrosis of contacted plant foliage may occur. Leaves of affected plants may turn brown and may have small black dots or holes.
2. Water-soaking of leaves
Plant leaves may develop a water-soaked appearance that occurs within hours of treatment. This happens as a result of the leakage of cell contents into intercellular spaces. What follows is the rapid browning or wilting of affected plants.
3. Spotting and necrotic lesions
Shortly after paraquat application, reddish-brown spotting may appear on the leaf surface. Necrotic lesions may also appear as round spots, brown edges, or blotches of dead tissue between leaf veins.
4. Stem cankers
A stem canker may appear as a dark, sunken area on a stem that ends up killing the remaining stem portion of a plant treated with paraquat. This often happens as a result of applying high doses of the toxic herbicide to the stem of plants. Stem cankers may also occur in woody plants due to the application of large amounts of paraquat.
What Part of Photosynthesis Does Paraquat Inhibit?
The part of photosynthesis that paraquat inhibits is the electron transport, which is a crucial part of exactly how plants convert the sun’s energy into chemical energy that they can use. If this necessary stage in photosynthesis is disrupted, it can lead to damage caused by the production of toxic reactive oxygen species. This can effectively induce cell membrane damage and eventual cell death.
How Does Paraquat Affect Chlorophyll?
Paraquat affects chlorophyll by reducing its levels in plants affected by paraquat. Chlorophyll is the green pigment of plants that plays a special role in the process of photosynthesis. Its job is to capture sunlight, which can then be turned into chemical energy and be used by plants as food.
Chlorophyll gives plants their green color. This is why when a plant is sprayed with paraquat and chlorophyll levels are reduced, yellowing of leaf tissue quickly occurs.
Why Is it Important to Know How Paraquat Affects Photosynthesis?
The ability of paraquat to inhibit photosynthesis is called its “mode of action.” Different classes of herbicides have their own mode or mechanism of action, which describes the way these chemicals control susceptible plants. It also involves the biological processes that different herbicides disrupt in order to kill plants.
The diquat and paraquat mode of action is called the inhibition of photosystem I. These two chemically-related herbicides are also called photosynthesis inhibitors. Knowing and understanding paraquat’s mode of action is important for users to select the proper herbicide for their crop and to design the best management practices in case herbicide-resistant weeds emerge.
Knowing how paraquat affects photosynthesis also helps users and other people be aware of how damaging it can be to plant cell membranes, and the fact that the chemical also works the same way in human cells may also serve as a warning for people to be more careful when working around paraquat. Paraquat safety can be observed by reading the product label and avoiding exposure as much as possible.
How Does Paraquat Affect the Environment?
Paraquat affects the environment through its adverse impacts on terrestrial and aquatic ecosystems. According to a report published by the World Health Organization for the International Programme on Chemical Safety, the mere use of paraquat as a herbicide indicates that it is toxic for aquatic and terrestrial vegetation.
A 2019 study supported this view by stating that paraquat cannot rapidly degrade in the environment and is instead adsorbed in clay lattices, which – according to the study authors – require urgent environmental remediation.
In fact, microorganisms can only degrade less than 1% of paraquat in soil particles, and its half-life can last up to three to 6.6 years. Another study concluded that microorganisms could completely degrade residues of paraquat in the soil in six years. Needless to say, such a prolonged half-life can cause serious adverse effects on humans, animals, and the environment.
Application of high amounts of paraquat results in widespread residues in the soil surface and aquatic environments that can enter and even alter the food chain. What’s worse is that the toxic chemical can also end up causing soil and water pollution.
Reports have also confirmed the presence of paraquat in various aquatic water bodies, where it gets run off, and adversely affects certain aquatic organisms. According to a 2020 study, fish that has been exposed to paraquat for a longer amount of time exhibited significant damage to vital functions, such as respiration, excretion, and metabolic regulation.