Air Pollution: A Lethal Threat To Plants

Air pollution has a detrimental impact on plants, affecting their growth, health, and the wider ecosystem. Plants absorb air pollutants, which can alter their physiological processes, causing damage to their photosynthetic systems, leaf longevity, and carbon allocation. The impact of air pollution on plants can be observed through visible foliage injury, reduced growth, and yield loss. Ozone, a major component of air pollution, slows the growth of many tree and crop species, while nitrogen pollution changes competition between understory plants and soil bacteria and fungi, affecting soil fertility. Other common pollutants such as sulfur dioxide, fluorides, ammonia, and particulate matter also cause injury to vegetation. The negative consequences of air pollution on plants extend beyond individual plants, degrading environments and reducing biodiversity.

Ozone reduces growth and yield and increases plant vulnerability to pests and disease

Ozone (O3) is a major air pollutant and a key component of smog. It is produced through a complex reaction involving nitrogen oxides and reactive hydrocarbons, which are found in automobile exhausts and fossil fuel combustion. Ozone enters plants through the leaves, leading to the overgeneration of reactive oxygen species (ROS) in the mesophyll and guard cell walls. This damages the ultrastructure of chloroplasts and blocks photosynthetic electron transport, reducing the plant's ability to photosynthesise and thereby hindering its growth.

Ozone-induced leaf chlorosis (leaf discolouration) is common, and the effects of ozone on individual plants can have negative impacts on ecosystems, including changes to the specific assortment of plants present in a forest. For example, studies have shown that wheat production is compromised by ozone pollution, and that episodes of high tropospheric ozone reduce seed production and quality in soybeans.

Long-term exposure to high concentrations of ozone disrupts plant physiological processes, including water and nutrient uptake, respiration, and the translocation of assimilates and metabolites. As a result, plant growth and reproductive performance are negatively affected, leading to reduced crop yield and deteriorated crop quality.

Ozone pollution also increases plant vulnerability to pests and diseases. For example, a study in India found that elevated ozone levels interacting with carbon dioxide increased the severity of wilt disease in chickpeas.

Nitrogen pollution changes competition between understory plants and soil bacteria

Atmospheric nitrogen (N) deposition caused by human activities has increased dramatically over the past century and is expected to continue rising. Nitrogen pollution can alter the physiological processes of plants, affecting their growth patterns. This is especially true for understory plants, which are smaller plants that grow in the shade of larger ones.

Soil microorganisms are key regulators of plant growth and ecosystem health in forest ecosystems. Understory nitrogen addition can influence soil microbial communities directly by altering the availability of nutrients for plant roots and microorganisms. The competition for nutrients between roots and microorganisms is especially strong in the rhizosphere, where soil pH can also play a role in determining the outcome.

Research has shown that canopy nitrogen addition (CNA) increases bacterial diversity in the 10-20 cm soil layer but decreases bacterial abundance in the 20-40 cm layer. Conversely, understory removal after CNA reduces competition for nutrients, increasing soil N availability and reducing fungal diversity. The conventional method of understory N addition may skew the effects of natural atmospheric N deposition on soil bacterial communities, highlighting the importance of considering canopy processes in future studies.

The plant community influences soil bacterial diversity and composition. Canopy N addition may indirectly affect soil bacteria by altering the traits of canopy tree leaves, litter, and roots. Canopy N addition has been shown to increase leaf N content, enhancing photosynthesis and the allocation of photosynthates to belowground, which may have positive effects on soil microbial communities. Fine root biomass and production were also found to be higher with canopy N addition, potentially benefiting soil microbes through increased root exudates.

Air pollution can cause physical damage to leaves, appearing as lesions or yellowing

Air pollution can cause physical damage to leaves, which may appear as lesions or yellowing. This damage is caused by contaminants such as ground-level ozone, sulphur dioxide, fluorides, ammonia, and particulate matter. Ozone is a significant greenhouse gas that is useful when present at higher altitudes. However, at ground level, it becomes harmful to both humans and plants. The leaves of plants are particularly vulnerable to ozone, which causes a build-up of oxidants that can lead to vegetation injury. This injury can manifest as necrotic lesions (dead tissue) or chlorosis, which is the yellowing of leaves due to a lack of chlorophyll. Chlorophyll is crucial for photosynthesis, as it captures energy from sunlight to power the food production process. Without chlorophyll, plants cannot produce food or energy.

Plants exposed to sulphur dioxide through acid rain also experience discolouration and hindrance in their photosynthesis abilities. Additionally, sulphur dioxide can interfere with the opening and closing of stomata, leading to excessive water loss. Fluoride, another pollutant, enters leaves through the stomata and accumulates at the margins, causing tissue injury and a characteristic dark band separating healthy and injured tissues.

Particulate matter and heavy metals can settle on leaves, causing direct damage to leaf structure and disrupting the plant's defence mechanisms. This damage can also be caused by the indirect effects of air pollution, such as changes in soil pH, which lead to the solubilisation of toxic metal salts. These toxic salts can then be absorbed by the plant, causing further injury.

The impact of air pollution on leaves is not limited to physical damage. Pollutants like ozone and CO2 can modify the physico-chemical characteristics of leaf surfaces by promoting the production of cuticular waxes. These changes can have repercussions for insect recognition and can reduce the nutritional and taste qualities of leaves for insects, disrupting the plant-insect relationship.

Pollutants can alter plant-environment relationships on a regional scale

Air pollutants can alter plant-environment relationships on a regional scale. This can occur through the impact of pollutants on the physiological processes of plants, affecting their growth patterns and overall health. For example, ground-level ozone (O3) enters plant leaves and reduces their ability to photosynthesise, slowing growth and increasing vulnerability to pests and diseases. This can lead to a reduction in plant biodiversity and ecosystem services. In addition, nitrogen oxides (NOx) and ammonia (NH3) in the air are deposited on land and in water bodies, resulting in an excess of nitrogen, which can negatively impact soil fertility and plant competition.

Ozone is a major component of "oxidants", which are produced by a complex reaction involving nitrogen oxides and reactive hydrocarbons from automobile exhausts and fossil fuel combustion. Ozone injury to vegetation has been observed and documented in many areas throughout North America, including Ontario and Los Angeles. The impact of ozone on plants can be seen as necrotic lesions (dead tissue) or the yellowing or chlorosis of leaves. Some plants, such as soybeans, exhibit specific symptoms, including flecking, bronzing, or bleaching of leaf tissues.

The susceptibility of plants to ozone injury is influenced by environmental and growth factors. High relative humidity, optimal soil nitrogen levels, and water availability increase the risk of injury. The stage of leaf maturity also plays a role, with younger leaves being more resistant, while middle-aged and basal leaves are more susceptible.

Other common air pollutants that impact plants include sulfur dioxide, fluorides, ammonia, and particulate matter. Fluorides cause tissue injury by entering the leaf through the stomata and accumulating on the margins. Studies have shown that plants such as apricot, barley, blueberry, and grape are particularly sensitive to fluoride damage. Ammonia injury to vegetation has also been frequently observed in Ontario due to accidents involving the storage, transportation, or application of ammonia-based fertilizers.

The effects of air pollution on plants can vary based on the length of exposure to the pollutant, the plant species, its stage of development, and the environmental factors that contribute to the build-up of pollutants. These factors collectively determine the susceptibility or resistance of plants to injury.

Plants can recover from air pollution damage if provided with proper water and nutrients

Air pollution can have a detrimental impact on plants, affecting their growth, leaf longevity, and patterns of carbon allocation. However, plants are resilient and can recover from air pollution damage if provided with the right care. Proper watering and nutrient supply are crucial for aiding plants in their recovery from the adverse effects of air pollution.

Air pollution can cause oxidative stress in plants, leading to the production of free radicals and reactive oxygen derivatives that damage cell membranes and result in leaf necrosis or tissue injury. The severity of damage depends on the type of pollutant, the concentration, and the duration of exposure. Pollutants such as ozone, sulfur dioxide, nitrogen dioxide, and hydrofluoric acid are particularly harmful to plants.

When plants are exposed to air pollution, their defense mechanisms may become overwhelmed, resulting in irreversible damage such as cell death. However, external factors can influence the plant's response to air pollution, and proper care can aid in recovery. Providing adequate water and nutrients can help strengthen the plant's defense mechanisms and promote healing.

Water plays a vital role in a plant's recovery from air pollution damage. Drought conditions can lead to the closure of stomata, which are small pores on the leaves, protecting the plant from absorbing additional pollutants. Ensuring proper soil moisture can help regulate stomatal opening and closing, allowing the plant to better manage pollutant intake.

In addition to water, providing essential nutrients can aid in a plant's recovery. Nutrients such as nitrogen, when supplied in optimal amounts, can promote healthy growth and enhance the plant's resilience to air pollution. Balanced fertilization can ensure the plant has the necessary nutrients to repair damaged tissues and restore physiological functions.

While air pollution poses a significant threat to plant health, proper horticultural practices can aid in recovery. By ensuring adequate water availability and maintaining optimal nutrient levels, plants can recover from the adverse effects of air pollution and restore their vital ecological functions.

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