Plants' Resilience: Adapting To Air Pollution

Plants have been adapting to natural air pollutants from volcanic eruptions, forest fires, and dust storms for geological eras. However, man-made air pollutants are a recent phenomenon in the evolutionary experience of plants. The specific nature of these adaptations is not generally well-known, but studies suggest that plants respond to increasing concentrations of pollutants through genetic changes. For example, recent studies in poplar and spinach strongly suggest that increased production of the enzyme superoxide dismutase may be a key factor in adapting to sulphur dioxide.

Plants' genetic changes to adapt to air pollution

Plants have responded to increasing concentrations of man-made air pollutants, which are a recent phenomenon in their evolutionary experience. Natural air pollutants from volcanic eruptions, forest fires, and dust storms have been around for much longer. Plants have adapted to man-made pollutants such as sulphur dioxide, fluorides, photochemical oxidants, and acid rain at the community, species, population, and individual levels. For example, some populations of grasses and herbs have evolved tolerances to sulphur dioxide, and some lichens that can tolerate this pollutant have invaded inner-city areas.

Biologists have found that some plant species have used genetic changes to adapt to the pollution they encounter in cities. Genes are segments of DNA that influence how an organism looks and functions, and its development and growth. For example, the gene for an enzyme called mercuric ion reductase, which turns mercury into a less reactive and dangerous form, can be introduced into a transgenic plant. Some plants also host microbes that assist with bioremediation. These plants release chemicals that boost the microbes' ability to break down toxic organic compounds in herbicides.

In addition to genetic changes, plants may adapt to air pollution through morphological and anatomical changes. For instance, studies in poplar and spinach suggest that increased production of the enzyme superoxide dismutase may be a key factor in tolerating sulphur dioxide.

While it may seem beneficial for plants to adapt to city life, there can be hidden costs to these evolutionary changes that biologists are only beginning to understand. For example, a species may become less sensitive to a pollutant, but this could lead to reduced reproduction rates or life expectancy, or increased sensitivity to other stressors.

Plants' ability to absorb carbon dioxide

Research has shown that plants worldwide are absorbing around 31% more carbon dioxide than previously thought. This is due to an increase in photosynthesis, which results in more plant growth. In response to elevated CO2 levels, plants have shown an average increase in above-ground growth of 21% and a 28% increase in below-ground growth.

However, the impact of climate change may hinder plants' ability to absorb carbon dioxide. Scientists have found that 86% of land ecosystems are becoming less efficient at absorbing CO2 from the atmosphere as its levels increase. This is due to limitations in other necessary components of photosynthesis, such as water, nutrients, nitrogen, or sunlight. For example, higher temperatures can cause plants to use more water, and a lack of nitrogen can dilute the amount in leaves, impacting plant productivity.

While plants have shown a remarkable ability to adapt to increasing carbon dioxide levels, the long-term effects of climate change may reduce their capacity to absorb this greenhouse gas.

Air pollution's impact on plant growth

Plants are a source of fibre, fuel, shelter, and nutrition. They play a crucial role in maintaining the balance of the ecosystem and providing oxygen through photosynthesis. However, they are not immune to the detrimental effects of air pollution, which can impact their growth and survival.

Air pollution can have both direct and indirect effects on plants. Direct effects occur when pollutants are absorbed by the plant through its leaves or stems. Indirect effects happen via the soil, impacting the roots. Heavy metals, such as lead, cadmium, and mercury, released from industrial activities, can contaminate the soil, altering its chemistry and pH levels. This, in turn, affects the plant's ability to obtain the necessary nutrients for growth and development.

The impact of air pollution on plants can be observed through various signs of damage. Leaf damage, including yellowing, falling leaves, or injuries, is a common indicator. Other visible signs include necrotic lesions on leaves, changes in leaf colour (chlorosis), reddening, bronzing, and mottling. In addition to aesthetic damage, air pollution can also stunt plant growth, weaken the plant's defence mechanisms, and impair its ability to photosynthesize effectively. This results in diminished productivity and a decline in overall plant health.

The sensitivity of plants to pollutants varies across species. Some plants have demonstrated a higher tolerance to specific pollutants, such as sulphur dioxide. Certain grasses, herbs, and lichens have evolved to adapt to higher concentrations of sulphur dioxide, even invading inner-city areas. However, many other species have seen their distribution severely limited due to the presence of pollutants. The impact of air pollution on plant growth can also be influenced by the interaction of multiple stress factors, such as water stress, creating a complex interplay of processes within the plant.

While plants have shown some capacity to adapt to air pollution, it is important to recognize the potential evolutionary costs of these adaptations. As noted by environmental medicine expert Wirgin, gaining an advantage, such as reduced sensitivity to a pollutant, may come at a price. For example, a plant that evolves to become less sensitive to a particular pollutant may face trade-offs in terms of reproduction, life expectancy, or increased sensitivity to other stressors.

Ozone's adverse effects on plants

Ozone is a dangerous and widespread pollutant, which is formed from gases emitted from sources such as tailpipes, factories, power plants, and motor vehicles. While ozone is beneficial in the upper atmosphere, at ground level, it is harmful to both humans and plants.

Plants have microscopic pores on the undersides of their leaves called stomata, which can be thought of as the 'mouths' of the plant. Plants open and close these stomata to 'breathe'. When the stomata are open, gases in the air surrounding the plant can enter the leaves. This process is necessary for photosynthesis, as plants require carbon dioxide from the air to produce sugars for food. However, when the stomata are open, ozone can also enter the leaves and damage the cells that produce sugars. This can lead to reduced plant growth, decreased production of wood, fruits, and vegetables, and a reduction in the amount of carbon stored in plant tissues.

Plants can protect themselves from ozone damage in several ways. For example, they can temporarily close their stomata to prevent ozone from entering the leaves. Additionally, plants with higher levels of antioxidants, such as vitamin C, are less susceptible to ozone damage.

Ozone damage to plants can have significant ecological and economic impacts. It can affect the growth and productivity of crops, forests, and natural vegetation, leading to reduced yields and ecological imbalances. The impact of ozone on plants can also have indirect effects on other organisms within the ecosystem, including humans, as it can disrupt food sources and the overall health of the environment.

While there have been improvements in air quality due to stricter emissions controls and cleaner technologies, ozone pollution remains a concern. High levels of ozone in the atmosphere can have adverse effects on plants, and even short-term exposure can cause damage. As such, it is important to continue efforts to reduce ozone-forming emissions and protect both human and ecological health.

Plants' role in reducing air pollution

Plants play a crucial role in reducing air pollution and mitigating the effects of climate change. Atmospheric pollutants can have toxic effects on plants, but vegetation also actively contributes to air purification and the reduction of pollutants in the atmosphere.

Trees, for example, help to reduce the intensity of ionizing radiation and noise in urban and industrial areas. They create a microclimate that buffers temperature differentials between day and night, preventing the occurrence of warmer temperatures that stimulate the production of volatile pollutants. Additionally, leaves play a vital role in retaining particulate matter, especially when wet and dry atmospheric deposition increases.

Plants also engage in carbon sequestration, a process where they remove carbon dioxide from the atmosphere and store it in their tissues. By reducing air pollution, plants can enhance their carbon sequestration capabilities, benefiting the environment and human health. This is because aerosol particles in the air can hinder the photosynthetic process in plants, which is essential for their growth and nourishment. Particulate matter in the air can cover leaf blades, reducing light penetration and blocking the opening of stomata, which are essential for gas exchange during photosynthesis. A 10% reduction in photosynthesis can lead to a 5-8% decline in crop yield.

Furthermore, certain plant species have demonstrated remarkable adaptations to air pollution. For instance, some grasses, herbs, and lichens have evolved tolerances to sulphur dioxide, a common air pollutant from the combustion of coal and other industrial activities. These adaptations involve increasing the production of certain enzymes or morphological and anatomical changes.

While plants are adapting to air pollution, it is important to note that the reduction of air pollution is beneficial for plants. Lower levels of air pollution improve plant health and productivity, and their ability to combat climate change. Therefore, efforts to reduce air pollution are crucial for preserving plant ecosystems and the overall ecological balance.

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