Pollution's Impact: Photosynthesis And Plant Health

Plants are essential to life on Earth. They are a source of fibre, fuel, shelter, and nutrition, and during photosynthesis, they absorb carbon dioxide and release oxygen, which is vital for humans and other animals to breathe. However, plants are vulnerable to pollution, and when exposed to pollutants, their metabolic functions are altered, making them weak and susceptible to disease and pest infestations. One of the most significant impacts of pollution on plants is the effect on their ability to photosynthesise.

Ozone and other air pollutants reduce growth rates and yields

Ozone is a widespread air pollutant that is particularly harmful to plants. While ozone in the upper atmosphere protects all life on Earth by absorbing the sun's ultraviolet rays, ground-level ozone is a harmful pollutant. Ground-level ozone is formed when other pollutants, such as nitrogen oxides and volatile organic compounds, react in the atmosphere in the presence of sunlight.

Ground-level ozone damages vegetation and reduces biodiversity. It is a major pollutant that affects terrestrial plants and agricultural crops. It damages plants by entering leaf openings called stomata and oxidising (burning) plant tissue during respiration. This damages the plant leaves, slows growth, and causes reduced survival.

Ozone also reduces a plant's ability to photosynthesise. When sufficient ozone enters the leaves of a sensitive plant, it can reduce photosynthesis—the process plants use to convert sunlight to energy to live and grow. In 2019, economic losses due to the impacts of ground-level ozone on wheat yields totalled EUR 1,418 million across 35 European countries.

In addition to ozone, other air pollutants can also reduce growth rates and yields. Direct effects of air pollution on plants occur when toxins deposit on them directly from the air and affect their leaf metabolism and uptake of carbon, which they need to build their bodies and get energy to live. The chemicals responsible for direct pollution include ozone and nitrogen oxides.

Indirect effects happen via the soil and start at the roots. Some air pollutants, like heavy metals (lead, cadmium, mercury) from industrial activities, fall on the ground and change soil chemistry and pH. Plants then have problems obtaining enough nutrients to thrive.

Overall, ozone and other air pollutants can have significant negative impacts on plant growth rates and yields, with economic consequences for agricultural industries.

Pollutants block stomata, preventing gas exchange

Plants are vulnerable to all forms of pollution, and pollutants can affect them in many ways. One of the most common ways is by blocking the stomata and preventing gas exchange.

Stomata are the microscopic pores in the epidermal leaf surfaces that facilitate the passage of gases. They are formed of two specialised cells known as guard cells, which can control their pore apertures by changing their volume and turgor pressure. This is the plant's first response to environmental changes, including light intensity, carbon dioxide levels, temperature, humidity, and water availability.

When pollutants like dust particles enter the leaf through the stomata, they can block the pore openings, preventing their proper function. Smaller particles can even enter the stomata and interfere with the plant's metabolism. This can lead to reduced photosynthetic rates and stunted growth.

Additionally, the accumulation of dust on leaves can increase their temperature, kill leaf sections, and damage chloroplasts, further reducing the plant's photosynthetic capacity.

Plants are also affected by haze, which blocks light and reduces their rate of photosynthesis. Haze also minimises stomatal conductance, which is the exchange of carbon dioxide and water vapour in and out of the plant leaves through the stomata.

Overall, pollutants that block the stomata and prevent gas exchange can have significant negative impacts on plant health and survival.

Chlorophyll deficiency caused by leaf damage

Chlorophyll is a green photosynthetic pigment found in plant leaves. It is responsible for absorbing sunlight, which is essential for photosynthesis. Chlorophyll is vulnerable to damage by various environmental factors, including pollution. Leaf damage caused by pollution can lead to chlorophyll deficiency, affecting the plant's ability to perform photosynthesis.

Pollution can cause leaf damage in several ways. One common way is through the accumulation of dust particles on leaves. Dust particles can block the penetration of sunlight, hindering chlorophyll's ability to absorb light energy. Additionally, dust can physically damage leaf tissues, impacting the chlorophyll present within the leaves.

Air pollution, particularly particulate matter, can also cause chemical injuries to leaves. When dust particles react with water on the leaf surface, it can form an alkaline solution that is toxic to plants. This can lead to chlorophyll degradation and a reduction in the plant's photosynthetic capacity.

Furthermore, air pollutants such as sulphur dioxide (SO2) can cause chlorophyll destruction. SO2 reacts with water in the plant to form sulphuric acid, which leads to leaf bleaching. The destruction of chlorophyll pigments results in a decrease in the plant's ability to perform photosynthesis.

Leaf damage caused by pollution can also make plants more susceptible to diseases and pest infestations. This indirect effect can further impact chlorophyll health and, consequently, the plant's photosynthetic efficiency. Overall, pollution-induced leaf damage can have both direct and indirect effects on chlorophyll deficiency, ultimately affecting the plant's survival and productivity.

Soil pollution changes soil chemistry and pH

Soil pollution is defined as the presence of toxic chemicals (pollutants or contaminants) in the soil, in very high concentrations, which pose a risk to human health and the ecosystem. Soil pollution can occur because of natural processes, but it is mostly caused by human activities.

When soluble organic and inorganic acids collect at a faster rate than they can be neutralized, it causes the acidification of the soil. When acidic cations (positively charged ions) like hydrogen and aluminium are abundant in the soil, it becomes acidic. Soil will be alkaline when basic cations like calcium, magnesium, potassium, and sodium are dominant.

Soil pollution can be caused by the overuse of pesticides, industrial waste, poor waste management, and urban activities. The discharge of industrial waste into soils can result in soil pollution. As mining and manufacturing activities increase, soil degradation also increases. The extraction of minerals affects soil fertility, and the by-products are often contaminated and disposed of in an unsafe manner.

The overuse of pesticides and insecticides in agriculture can also cause soil pollution. These chemicals are not produced in nature and cannot be broken down naturally. They seep into the ground and reduce the fertility of the soil.

Air pollution can also lead to soil pollution. Atmospheric acid deposition can cause soils to become more acidic. In areas with moderate acidic deposition, the natural acidifying effect of a fast-growing forest stand is likely to override the effect of atmospheric deposition.

Soil pollution has a major impact on human health. Plants grown on polluted soil absorb most of the pollution and then pass it on to humans. Consuming these plants may cause chronic diseases that are untreatable. Children are especially susceptible to exposure to contaminants as they come into close contact with the soil by playing on the ground.

Water pollution introduces toxins and excess nutrients

Water pollution is a pressing issue that poses a threat to drinking water sources, human health, and ecosystems. It occurs when harmful chemicals, microorganisms, waste, or excess sediment contaminates freshwater and groundwater sources. One of the significant issues associated with water pollution is nutrient pollution, which is caused by the introduction of excess nitrogen and phosphorus into aquatic ecosystems. While nitrogen and phosphorus occur naturally in these environments, human activities such as fertilizer use, wastewater management, fossil fuel burning, and runoff from soaps and detergents, contribute to excess nutrient pollution.

This excess of nutrients acts as a fertilizer, leading to the overgrowth of algae, a process known as eutrophication. Eutrophication has severe ecological consequences. The excessive growth of algae blocks light, hindering the growth of underwater plants, such as seagrasses. As the algae and underwater plants die, they decompose, depleting the oxygen levels in the water. This creates a condition called hypoxia, which can be fatal for aquatic life, including fish, crabs, oysters, and other organisms.

Moreover, nutrient pollution is linked to harmful algal blooms (HABs). These blooms are characterized by the rapid growth of toxic algae that produce harmful toxins. When fish and shellfish consume these toxic algae, the toxins accumulate in their bodies. Consequently, humans who eat contaminated seafood may experience adverse health effects, including skin rashes, liver and kidney damage, neurological issues, and respiratory problems. The distribution, frequency, and intensity of HABs are increasing globally, posing a growing threat to both ecosystems and human health.

Nonpoint source pollution, which originates from various dispersed sources, is a significant contributor to nutrient pollution. Controlling and regulating nonpoint source pollution is challenging due to the difficulty in identifying the specific sources of contamination. Atmospheric nutrient inputs, such as emissions from power plants, large industries, and automobiles, also play a role in nutrient pollution, and their sources can be difficult to pinpoint and control.

The impacts of water pollution, including nutrient pollution, on plants and photosynthesis are significant. Plants rely on water not only for their survival but also for the process of photosynthesis. When water quality is compromised by pollution, it can hinder a plant's ability to carry out photosynthesis effectively, leading to reduced growth and productivity. Water pollution can also affect the availability of nutrients for plants, further impairing their photosynthetic capabilities.

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