Air Pollution's Impact: Stunting Plant Growth And Health

Air pollution has a detrimental impact on plant growth, primarily by interfering with resource accumulation. When plants are exposed to harmful pollutants, their leaves often exhibit signs of damage, such as discolouration, lesions, or chlorosis (leaf yellowing). Additionally, air pollution can affect the metabolic function of leaves, disrupting photosynthesis and resource allocation within the plant. The effects of air pollution on plants vary depending on factors such as the type of pollutant, the concentration, and the plant species. Some common air pollutants that affect plant growth include ozone, sulphur dioxide, nitrogen dioxide, and particulate matter. These pollutants can cause oxidative damage to plant cell membranes, disrupt photosynthesis, and hinder the plant's ability to absorb nutrients and water from the soil.

Ozone exposure

Ozone enters the leaves of plants through microscopic pores called stomata, which are necessary for the plant's respiration process. Once inside the leaf, ozone damages the plant tissue by oxidizing and burning it, which slows down photosynthesis and results in reduced plant growth. The compounds resulting from oxidation by ozone interfere with the cell's energy production, leading to a decrease in the number of flowers and fruits produced by the plant. Additionally, plants weakened by ozone exposure become more susceptible to pests, diseases, and droughts.

The effects of ozone exposure on plants can vary depending on the plant species. For example, tobacco, soybean, cotton, peanut, clover, quaking aspen, and yellow poplar are more sensitive to ozone than plants like sorghum, field corn, and winter wheat. Studies have shown that current levels of ozone in Maryland, USA, have led to a 10% loss of soybean crops, a 6-8% loss of winter wheat, and a 5% loss of corn crops.

The damage caused by ozone exposure to plants may not always be visible. However, in some cases, leaves may exhibit signs of ozone stress, including tiny light-tan irregular spots, small darkly pigmented areas, bronzing, and reddening.

Nitrogen dioxide

NO2 is a precursor to harmful secondary air pollutants such as ozone and particulate matter. When present in high concentrations, it can lead to excessive accumulation of nitrite and cell acidification, resulting in negative responses such as the generation of reactive oxygen species (ROS) and inhibition of nitrogen assimilation and plant growth. This can cause acute damage to leaves, whole-plant chlorosis, or even death. However, the effects of NO2 exposure vary among different plant species, with some exhibiting resistance or low incorporation of NO2 into their total nitrogen content.

Studies have shown that NO2 exposure can affect various physiological responses in plants, including changes in antioxidant enzyme activity, nitrogen metabolic enzyme activity, and the composition and distribution of nitrogenous metabolic products in plant tissues. Additionally, high levels of NO2 can increase the acidity of rain, lowering the pH of water and soil, which can have harmful effects on biological systems.

On the other hand, at lower concentrations, NO2 can act as an airborne fertilizer. It can be metabolized and incorporated into the nitrate assimilation pathway, forming organic nitrogenous compounds in plants without causing leaf injury. NO2 has been found to positively regulate the vegetative and reproductive growth of plants, influencing processes such as nutrient uptake, photosynthesis, and nutrient metabolism. It triggers plant growth and development by controlling cell proliferation and enlargement.

In summary, while high concentrations of NO2 can be detrimental to plants, lower concentrations can have positive effects on their growth and development. The balance of nitrogen in the environment is crucial for maintaining healthy and productive ecosystems.

Sulphur dioxide

Plants absorb SO2 through their stomata, which can lead to excessive water loss. SO2 inhibits photosynthesis by disrupting the photosynthetic mechanism and reducing the quality and quantity of plant yield. It can also have an indirect effect by contributing to acid rain, which leaches nutrients from the plant canopy and soil.

At high concentrations, SO2 can damage foliage and decrease growth. The impact is more severe when combined with other pollutants such as nitrogen oxides, fluorides, and ozone. Plants vary in their tolerance to SO2, with lichens and bryophytes being among the most sensitive.

While low levels of SO2 can stimulate plant growth, especially in sulphur-deficient soil, excessive exposure turns toxic and damages plants by interfering with physiological and metabolic processes. Sulphur is a structural component of protein disulfide bonds, amino acids, vitamins, and cofactors. It is also involved in the formation of chlorophyll, coenzyme A, and S-adenosyl-methionine.

Overall, SO2 has a detrimental effect on plant growth and development, and its release into the atmosphere through human activities can have significant ecological consequences.

Particulate matter

PM can be divided into coarse particles (PM10) and fine particles (PM2.5). Coarse particles have a diameter of 10 μm or less and can reach the upper part of the respiratory system, while fine particles, with a diameter smaller than 2.5 μm, are inhaled into the lungs. PM2.5 is, therefore, more hazardous to humans because of their greater surface area relative to their volume.

PM affects plants in several ways and can cause both acute and chronic injury. Acute injury results from exposure to a high concentration of gas for a short period, while chronic injury is caused by prolonged exposure to lower gas concentrations.

PM affects plants' growth and development, depending on their physical and chemical nature. It can alter the pH of leaf extracts, which may influence the stomata sensitivity. PM can also block the opening of stomata, preventing their proper function and inhibiting photosynthesis. In addition, PM can change soil pH, making it difficult for plants to obtain the necessary nutrients.

Plants can act as biofilters and remove PM through various mechanisms, including excretion, conjugation, and compartmentalization. The efficiency of PM capture by plants depends on factors such as leaf area index, morphology, anatomy, and stomatal density. Trees, with their large leaf area index, are the most effective aerosol depositors.

To increase the PM removal by plants, species selection, biodiversity increase, and the use of PAH-degrading phyllospheric endophytes and transgenic plants are some of the methods that can be employed.

Heavy metals

The toxicity of heavy metals on plants depends on several factors, including the type of plant, its growth stage and growth conditions, the nature of toxicity of the specific elements involved, the soil's physical and chemical properties, the occurrence and bioavailability of heavy metal ions in the soil solution, and the soil rhizosphere chemistry.

The following are some of the key mechanisms by which heavy metals directly interfere with the physiological, biochemical, and molecular processes in plants:

• Generation of oxidative stress
• Inhibition of photosynthetic phosphorylation
• Enzyme/protein inactivation
• Genetic modifications
• Hormonal deregulation

The following are some of the visual symptoms of highly toxic non-essential heavy metal elements in plants:

• Chlorosis
• Inhibition of seed germination
• Stunting of root and shoot growth
• Reduction of biomass accumulation and yield
• Occasional death of plants

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