Plants And Pollution: Harmful Or Helpful?

Plants are vulnerable to the effects of pollution, which can enter the environment from diffuse sources. The causes can be outright, such as emissions from a coal-burning power plant, or hard to identify, such as nonpoint source pollution, where several contributors contaminate surface water.

Plants are a source of fibre, fuel, shelter, and nutrition. They absorb carbon dioxide from the air and release oxygen during photosynthesis, which humans and other animals need to breathe. Most plants are capable of removing pollutants from the environment through bioaccumulation and incorporating them into their tissues.

Plants are sensitive to all forms of pollution. The degree to which each plant is affected depends on several factors, such as soil type, pollutant concentration, plant age, temperature, and season.

Plants are affected by both water and air pollution. Water pollution occurs when harmful chemicals, microorganisms, waste, or excess sediment contaminates freshwater and groundwater sources. Air pollution comes from natural and anthropic sources and can have different effects on a global level or on individual plants and animals.

The major atmospheric pollutants are represented by gases, particles in suspension, different ionising radiation, and noise. Gaseous pollutants include sulphur dioxide, nitrogen oxides, fluorides, chlorine, ammonia, ethylene, and other organic substances. Particulate pollutants include soot, dust, fine particles of cement, and various other substances.

How does water pollution affect plants?

Water pollution can have a range of effects on plants, from encouraging growth to causing harm and even death. The impact depends on the type of pollutants involved and the specific plants affected.

Encouraging Growth

Water pollution sometimes provides necessary nutrients and food for plants, leading to an explosion of new growth. For example, agricultural runoff containing nitrogen and phosphorus-rich fertilisers can cause blooms of growth in nutrient-enriched waters.

Harming and Killing Plants

However, water pollution can also harm or kill plants by changing their growing conditions, such as by altering the acidity (pH level) of the water or blocking sunlight.

• Altering pH Levels: Water pollution that changes the pH level of a plant's surrounding environment, such as acid rain, can be harmful or even fatal. Acid rain is formed by the interaction of atmospheric sulfur dioxide and nitrogen dioxide—emitted from natural and human-made sources like volcanic activity and burning fossil fuels—with common atmospheric chemicals like hydrogen and oxygen. When acid rain reaches the ground, it flows into waterways, lowering water pH levels and making it uninhabitable for certain plants.
• Blocking Sunlight: Marine debris on or near the water's surface can block sunlight from reaching plants below. Plants rely on sunlight for the photosynthesis process to create glucose food molecules. By obstructing sunlight, marine debris stunts plant growth.
• Chemical Poisoning: When chemical pollutants build up in aquatic or terrestrial environments, plants can absorb these toxins through their roots, leading to phytotoxicity. Signs of phytotoxicity include poor growth, dying seedlings, and dead spots on leaves. For example, mercury poisoning, often associated with fish, can also affect aquatic plants, as mercury compounds accumulate in their roots and bodies. This initiates a chain of bioaccumulation as animals feed on the contaminated plants, leading to increasing levels of mercury in the food chain.

Water pollution can have far-reaching consequences, affecting not just plants but also animals and entire ecosystems.

How does air pollution affect plants?

Plants are affected by air pollution in several ways. Firstly, ground-level pollutants like ozone physically damage plant leaves, causing chlorosis (abnormal yellowing of leaves) and a deficiency in chlorophyll, which is essential for photosynthesis. High concentrations of ozone can even cause plant leaves to die due to excessive exposure.

Secondly, air pollution can delay flowering in plants as they divert all their resources to fighting and surviving the threat. For example, plants exposed to vehicle exhausts often delay flowering as they combat the emissions.

Thirdly, air pollution can create acidic soil conditions, either from soil pollution or acid rain, which makes it difficult for plants to obtain the necessary nutrients and ions for growth. Acidic soils accumulate aluminium ions, which destroy root systems.

Moreover, air pollution reduces the size of stomata (tiny pores on leaves that facilitate gas exchange) and compromises photosynthesis, further impeding plant growth.

In addition, air pollution can obstruct a plant's growth by creating holes in the upper atmosphere, allowing ultraviolet light to pass through and cause damage. Ozone destruction in the lower atmosphere restricts respiration, obstructs stomata, and prevents photosynthesis, stunting plant growth.

Air pollution can also reduce crop yields and contribute to global warming. Plants absorb pollutants from the air, and when there is a build-up of toxins, they may struggle to grow and perform photosynthesis efficiently, leading to reduced yields. This can have knock-on effects on food availability and prices, impacting people, animals, and the economy.

Lastly, certain air pollutants can have indirect effects on plants via soil contamination. Heavy metals like lead, cadmium, and mercury from industrial activities fall to the ground, altering soil chemistry and pH. This makes it challenging for plants to obtain the necessary nutrients from the soil.

Overall, air pollution has detrimental effects on plants, hindering their growth, reproduction, and survival.

What are the effects of noise pollution on plants?

Plants are affected by noise pollution in a variety of ways, and this can have a detrimental impact on their growth and survival.

Noise pollution can disrupt the behaviour of birds and animals that plants rely on for seed dispersal and pollination. For example, a study by ecologist Clinton Francis found that Western scrub jays, which are important dispersers of piñon pine seeds, avoided noisy areas. This resulted in a 75% decrease in piñon pine seedlings in those areas. The absence of these birds can have long-term effects on the plant community, even after the noise has stopped.

Noise pollution can also impact the types of plants that grow in an area. For instance, black-chinned hummingbirds, which are pollinators, are drawn to noisy sites near natural gas well compressors, possibly to avoid Western scrub jays that prey on their eggs and chicks. This can lead to an increase in flowering plants in these areas.

Additionally, noise pollution can have indirect effects on plants by impacting the animals and insects that depend on them for food. For example, leaf-eating insects may be attracted to or repelled by certain types of noise, which can affect the health of the plants they feed on.

Overall, noise pollution can have far-reaching consequences for plant communities and ecosystems, and it is important to consider the potential long-term impacts on plant diversity and survival.

What are the effects of soil pollution on plants?

Soil pollution has adverse effects on plant growth and ecosystem functioning, and it poses risks to the environment and human health. Soil pollution is a major global environmental issue, with the US Environmental Protection Agency tracking nearly 9 million hectares of possibly contaminated land, and in China, 19% of agricultural soils show contamination exceeding the levels of environmental quality standards. In Europe, 2.5 million sites are potentially contaminated, and 350,000 of them require remediation. Metal(loid)s and polycyclic aromatic hydrocarbons (PAH) contribute considerably to this contamination.

Plants exposed to soil pollution exhibit thicker roots, a reduction of root elongation, and a thickened epidermis, cortex, or endoderm. Consequently, the root absorptive surface and soil resource uptake decrease.

Plants growing in soils polluted with heavy metals may show a reduction in growth, performance, and yield. Evidence also suggests that metal pollution significantly reduces the diversity of bacterial communities by up to 90%.

The distribution of heavy metals in agricultural soils has recently been documented, with copper and zinc exceeding safe thresholds in 23% and 18% of the EU-27's arable lands, respectively. This is mainly due to the use of copper as a fungicide and the application of animal manure.

The impact of soil pollution on plant growth and development is complex and depends on various factors, including the type of contaminant, soil matrix, and plant species and/or genotype. Soil pollution can have direct toxic effects on plant metabolism and indirect effects on the development of plant parts responsible for resource capture, such as light, water, and nutrients.

Plants' ability to capture resources, such as water, nitrogen, and light, can be impacted by soil pollution. For example, leaf structure can be affected, and photo-chemical energy production may decrease. Root association with fungi and bacteria, which are pivotal in plant nutrition and water acquisition, can also be disrupted.

Overall, soil pollution has adverse effects on plant growth, development, and ecosystem functioning. It reduces plant growth rates, biomass, and resource acquisition, and can lead to a decrease in plant species diversity.

How does pollution affect plant-insect relationships?

Plants are at the forefront of air pollution, and their relationship with insects is greatly affected by it. The three key parameters that govern plant-insect interactions – recognition, nutritional qualities, and defence systems – are all impacted by pollution to varying degrees.

Changes in Insect Recognition of Plants

Air pollution often leads to a change in plant colour, which strongly influences the colour of associated insects. For example, in England during the 19th century, heavy air pollution led to the disappearance of lichens and the blackening of birch tree trunks. This resulted in a mutation of the birch's insect, the birch moth, to a darker form to better camouflage itself from predators.

Air pollution also disrupts the chemical communication between plants and insects. Pollutants such as ozone interfere with the chemical mediators that plants use to recognise insects, such as their oviposition sites.

Changes in Plant Nutritional Resources for Insects

Air pollution induces the presence of external elements in or on plants, which can have serious consequences on insects that feed on them. The accumulation of toxic air pollutants in plant organs, such as heavy metals, arsenic, and certain plant protection products, can cause poisoning and mortality in insects.

Additionally, air pollution can cause an excess of nutrients in water, leading to excessive plant growth. This can be harmful as it results in weak plants that are more vulnerable to disease and weather.

Changes in Plant Defence Systems

Phenolic compounds and the foliar cuticle, which constitute a plant's chemical defence system and physical defence barrier, respectively, can be modified by pollutants. Ozone and carbon dioxide, for instance, increase the concentration of phenolic compounds in plants, reducing the nutritional and taste qualities of leaves for insects.

Furthermore, ozone and carbon dioxide improve the characteristics of the physical barrier represented by the foliar cuticle by promoting the production of cuticular waxes. This modification can affect how insects recognise plants.

Overall Impact on Plant-Insect Relationships

While the specific effects of pollution on plant-insect relationships vary, the net effect of global changes on these relationships is complex and challenging to predict. It requires a comprehensive understanding of the mechanisms that modulate the interaction strength between plants and insects.

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