Plants Vs Pollutants: Understanding The Most Harmful Toxins

Plants are affected by a variety of pollutants, including air, water, and soil pollution. Air pollution, such as ozone and nitrogen oxides, can directly harm plants by depositing on their leaves and interfering with their metabolic functions. It can also have indirect effects, with pollutants like heavy metals falling onto the ground and changing the chemistry of the soil, making it difficult for plants to obtain the necessary nutrients. Water pollution introduces toxins that are harmful to plants and can facilitate the growth of rival plants. Soil pollution poisons the soil, reducing its biodiversity and organic matter, and making it difficult for plants to obtain nutrients. Light pollution can hinder plants from flowering and affect their photosynthesis. Acid rain, caused by the interaction of atmospheric sulfur and nitrogen dioxides with the atmosphere, can harm and kill plants by lowering the pH of water and causing direct damage to leaves. These pollutants have far-reaching impacts on plant health, growth, and reproduction, with potential consequences for the entire food chain and the environment.

Ozone and nitrogen oxides

The harm caused by nitrogen oxides is usually not visible, but it manifests as poorer growth and reduced productivity in plants. Studies have shown that high levels of NOx in glasshouses can inhibit plant growth. Additionally, NO2, when combined with SO2 (sulphur dioxide), can have even more detrimental effects on seedling grasses than the individual gases alone.

Ozone, on the other hand, is formed when volatile organic compounds react with nitrogen oxides in the presence of sunlight. Ground-level ozone is of particular concern as it can harm both human health and plants, unlike the naturally occurring ozone in the upper atmosphere that protects life on Earth by absorbing the sun's harmful ultraviolet rays. Ground-level ozone damages plants by entering leaf openings called stomata and burning plant tissue during respiration, causing leaf damage and reduced survival rates. It also impedes the process of photosynthesis, slowing plant growth and increasing plants' susceptibility to other pollutants and severe weather.

Both ozone and nitrogen oxides can have significant impacts on sensitive vegetation and ecosystems, including forests, parks, and natural habitats. The effects on individual plants can lead to broader ecosystem changes, such as alterations in plant species composition, habitat quality, and water and nutrient cycles.

Particulate matter

PM from different sources impacts the chemical composition of plants and is often used as an indicator of and a tool for monitoring environmental pollution. The size of the particles also plays a role in how they affect plants. Ultrafine PM, for example, poses a higher environmental risk.

PM pollutants can cause acute and chronic injury to plants. Acute injury results from exposure to high concentrations of gas over a short period and manifests as necrotic lesions on foliage. Chronic injury, on the other hand, results from prolonged exposure to lower gas concentrations and takes longer to develop.

PM pollutants can affect plants in the following ways:

• Morphological impact: PM deposition on leaf surfaces can reduce plant growth through its effect on leaf gas exchange, flowering, and reproduction. It can also alter the leaf extract pH of plants, which may influence the stomata sensitivity to air pollutants.
• Physiological impact: PM pollutants can cause physical damage to plants by blocking stomata or altering the optical absorption of leaves. This can lead to impaired growth, photosynthesis, and transpiration.
• Biochemical impact: PM pollutants can bind to potentially phytotoxic compounds, affecting plants through dry or wet deposition. They can also cause oxidative stress in plants, triggering membrane and cell damage.
• Impact on biodiversity and climate change: Deposition of fine PM on forests can act as a source of nutrients, but it can also alter leaf surface properties, increasing the duration of surface wetness and modifying the habitat for epiphytic organisms. This can lead to increased risks from pathogens.
• Genotoxic effects: The build-up of resistant plant species due to PM pollution is also a concern, as it can have genotoxic effects on the ecosystem.

Overall, PM pollutants can have a wide range of detrimental effects on plants, including morphological, physiological, biochemical, and genotoxic impacts.

Photochemical smog

Ozone, the most abundant and phytotoxic component of photochemical smog, interferes with leaf metabolism and inhibits the uptake of carbon, which plants require for energy and growth. It also obstructs stomata, restricting respiration and stunting plant growth. When exposed to ozone, plants exhibit signs of damage on their leaves, such as tiny light and dark spots, followed by bronzing, reddening, and chlorosis.

Peroxyacetyl nitrate, another toxic component of photochemical smog, can be detected by bronzing or silvering of young leaves. It acts as a reservoir for nitrogen oxides, which can be transported long distances and affect ozone chemistry downstream. PAN is particularly harmful to young plants, reducing their chances of survival.

Nitrogen dioxide, the least toxic component of photochemical smog to plants, still causes injuries to the lower surface of leaves. Nitrogen oxides, in general, affect the metabolic function of leaves and interfere with net carbon fixation by the plant canopy.

Acid rain

The impact of acid rain on plants also depends on their age, with younger plants generally being more sensitive. Additionally, plants with hairy leaf surfaces tend to trap more dust and are more susceptible to damage. The composition of the leaf surface, such as the presence of a striated cuticle, can also influence the extent of damage.

To safeguard plants from acid rain, it is recommended to provide cover, such as planting tender specimens under large trees or using gazebos or covered porches. Regular soil testing is also important to monitor pH levels and nutrient content, allowing for the addition of necessary minerals and nutrients.

Water pollution

Plants need water to survive, and 95% of a plant's body is made up of water. Water carries nutrients to different cells in the body and supports the process of photosynthesis. If plants do not have enough water, their leaves will curl up and they will eventually die. However, if plants have too much water, they may also die because their roots will suffer from a lack of oxygen and begin to rot. Therefore, plants need the right amount of water and the right pH level (5.5 to 7.5) to survive.

When chemical pollutants build up in aquatic or terrestrial environments, plants can absorb these chemicals through their roots, causing phytotoxicity. Signs of phytotoxicity include poor growth, dying seedlings, and dead spots on leaves. For example, mercury poisoning can affect aquatic plants as mercury compounds build up in plant roots and bodies.

Waterborne pathogens can also cause disease in plants. These pathogens can come from various sources, including run-offs, irrigation sources, or even the soil itself. Recirculating irrigation water can expose plants to water molds, severely damaging them after just a couple of days of exposure.

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