Ground-level Ozone the silent plant killer
Ground-level Ozone: the silent plant killer
Ozone — an invisible, odourless gas O3 — is essential for life in the upper atmosphere because it shields the earth from the sun’s harmful ultraviolet rays. The 1987 Montreal Protocol phasing out industrial chemicals that were destroying the ozone layer is considered one of the most successful international environmental actions ever taken.
Tropospheric Ozone, at ground level, it's a whole different ballgame, here the highly oxidative ozone is toxic to plants and even human life. It is categorised as a greenhouse gas — the third worst after carbon dioxide and methane.
Ozone pollution is a growing albeit hidden problem. since it cannot commonly be seen or felt, people ignore it. Scientists have coined the phrase “Ozone: Good up high, bad nearby”.
Various harmful effects of ozone on plants
Tropospheric, or ground level, ozone damages trees and other plants by affecting a host of biological processes at the cellular level negatively impacting plant growth, vitality, photosynthesis, water balance, the flowering process, and the abilities of plants to defend themselves.
- Leaves may have tiny light-tan irregular spots less than 1mm in diameter (flecking), small darkly pigmented areas approximately 2-4 mm in diameter (stippling), bronzing, and reddening.
- Reduced photosynthesis, the process that plants use to convert sunlight to energy to live and grow.
- Slower plant growth.
- Increased risk of disease, damage from insects, effects of other pollutants and harm from severe weather
However, Ozone is not an equal opportunity offender, it seems to affect leguminous plants more than other garden vegetables, probably because it negates a lot of the nitrogen, a critical element in the development and growth of legumes.
Symptoms of Ozone damage to plants
Ozone formation requires sunlight, so it should come as no surprise that periods of high ozone concentration coincide with the peak growing season.
Ozone enters the plant’s leaves through its gas exchange pores (stomata), just as other atmospheric gases do in normal inspiration/expiration.
Ozone dissolves in the water within the plant and reacts with other chemicals, oxidizing (burning) plant tissue and causing a host of problems.
A closer look at how exactly Ozone negatively impacts plant growth
“It paralyzes the plants’ stomata,” said Howard Neufeld, a plant ecologist at Appalachian State University, “and so they release more water than they take in.” Stomata are the microscopic openings on the surface of leaves where trees exchange gasses with the atmosphere. Ozone damages them and interferes with a variety of processes, including photosynthesis.
Ozone also damages leaves and accelerates their aging. “As leaves are injured, photosynthesis goes down; a plant makes less sugars and it has fewer resources,” says Neufeld. “It also affects the movement of sugars to roots, which reduces root growth, making them more susceptible to drought and nutrient deficiencies and disease.”
Ozone’s impacts on soil also affect the rhizosphere (the root system and its associated microbes, fungi, and other organisms). Ozone damage can also alter the timing of leaf fall and shrink leaf size, reducing the amount of litter and affecting the microbial communities that thrive in decomposing leaves. Microbes in the litter and soil are critical to taking up nutrients, helping trees resist disease and use water efficiently.
“When the plants respond to ozone, they consume energy,” said Evgenios Agathokleous, a professor of environmental resources at the Institute of Ecology at Nanjing University of Information Science & Technology in China and one of the top researchers in the field. “When they use so much energy, there is less to provide to organisms in the soil and the chemical composition can be affected.” Less nutritious leaves can also affect the life cycle of animals that feed on them.
Sources of ground-level Ozone
Ground-level ozone is anthropogenic (i.e. created mostly by humans) in the form of airborne pollutants — carbon monoxide (CO), non-methane volatile organic compounds (NMVOCs) and methane (CH4), in the presence of nitrogen oxides (NOx, made up of nitric oxide NO and nitrogen dioxide NO2), which lead to the formation of O3 through photochemical reactions.
When these airborne pollutants meet direct sunlight and warm temperatures, they go through a chemical reaction and become ozone.
Studies show that those knock-on effects can include making plants less nutritious; diminishing the scent trails pollinators follow to find their target; changing the timing of leaf fall, affecting the forest floor and the microbial communities that inhabit it; impacting the root systems of plants and trees and the microbes, fungi, and other organisms that live there; and even reducing harvests of staple food crops such as corn and wheat.
References
- Yale School of the Environment, & Robbins, J. (2021, October 7). Ozone pollution: an insidious and growing threat to biodiversity. Yale Environment 360. Retrieved September 21, 2024, from https://e360.yale.edu/features/ozone-pollution-an-insidious-and-growing-threat-to-biodiversity
- Ozone effects on plants. (n.d.). U.S. National Park Service. Retrieved September 21, 2024, from https://www.nps.gov/subjects/air/nature-ozone.htm
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