The Invisible Threat: The Impact of Ozone on Plant Health

What’s Up, Forsyth? In our next two blog posts, we will discuss how ozone (O₃) affects plants and how studying certain “ozone sensitive” plants can inform us of local air quality issues. In this first blog, we will delve into the physiological effects of O₃ exposure on plants and the subsequent ecosystem and economic effects. In our following blog, we will explain how “ozone gardens”—areas planted with species that physically show ozone damage on their leaves—can help raise air quality awareness in our communities. We will also highlight a recent partnership between Triad Air Awareness and the Paul J. Ciener Botanical Garden to plant Kernersville’s first public ozone garden. Let’s begin by defining ground-level ozone.

What is Ground-Level Ozone?

Stratospheric ozone, commonly called the ozone layer, is naturally occurring in the Earth’s upper atmosphere and protects us from the Sun’s harmful ultraviolet (UV) radiation. However, this same gas is an air pollutant when it forms at surface-level. This is why we often say that ozone is “good up high, bad nearby.” Ground-level ozone, also known as tropospheric ozone, is created when photochemical chemical reactions caused by sunlight and heat occur between precursor air pollutants—nitrogen oxides (NOx) and volatile organic compounds (VOCs).

As a secondary pollutant, ozone does not come directly from sources like tailpipes or smokestacks. That being said, in North Carolina, on-road mobile sources (cars and trucks) are still considered the main cause of ozone pollution as they are responsible for approximately half of our statewide NOx emissions. Although ozone itself is invisible, its effects on plants are not.

Figure 1. Air Awareness Coordinator Sarah Coffey presents on the primary sources of nitrogen oxides in North Carolina at “Ozone Gardening 101” at the Paul J. Ciener Botanical Garden on August 12, 2025.

How Does Ozone Damage Plant Tissues?

We typically focus on the human health effects of breathing ozone, but it can also cause severe damage to plants. In fact, for some plants that are extremely O₃ sensitive, they may experience injury at much lower levels than what would be considered unhealthy for human respiratory systems.  By damaging plant tissues, ozone harms both native plants and cultivated crops and trees. This can have far-reaching effects on entire ecosystems and our economy.

Ozone enters through the plant’s stomata (singular: stoma), which are usually located on the underside of the leaf. A stoma is like a mouth, opening to allow for gas exchange and closing when the plant needs to block substances from getting in or to prevent water loss through transpiration. When stomata are open, anything in the air (good or bad) can come in.

Plants take in carbon dioxide (CO₂) through their stomata to make sugars through photosynthesis—a byproduct of which is the oxygen (O₂) upon which we all depend. When O₃ is present in our ambient air, it enters along with CO₂ and then reacts with the cell membranes and the enzymes responsible for making sugars through photosynthesis (namely, RuBisCo and ATP synthase). These reactions can lead to localized cell death, reduce the rates of photosynthesis, and slow plant growth overall. Figure 2 below, borrowed from NSF NCAR & UCAR Science Education, illustrates how ozone injury occurs.

 

Figure 2. “How Ozone Injury Occurs” from NSF NCAR UCAR Science Education.

In addition to the sensitivity of a plant species or cultivar to ozone, the extent of the foliar (leaf) injury is influenced by other environmental factors, such as soil moisture and the presence of other air pollutants, insects, or plant pathogens. If the soil is dry, plants will close their stomata more to prevent water loss, which reduces the amount of ozone entering the leaf. At the same time, if a plant is drought stressed, weakening its natural defenses, it may be more susceptible to other diseases. In summary, high ozone concentrations affect not just individual plants but survival rates within that population, and these effects are amplified when other sources of disease are present.

The appearance of tissue death begins as small dots or stipples on the top side of the leaf. Stipples, which can be black, red, purple, or tan, are typically uniform in size. Overtime, they can merge and form larger areas of yellowing (chlorosis) and brown/black tissue death (necrosis). It is important to note that O₃ injury only occurs between leaf veins and not on the veins themselves. In fact, the veins typically appear bright green in contrast to the rest of the leaf. Sometimes when there is severe ozone damage leading to necrosis, it can be visible on the underside of the leaf, but other things besides air pollution (like bacterial and fungal pathogens) can also be culprits of foliar injury and tissue death. Because the effects of ozone exposure are cumulative, older leaves show more damage than newer leaves. 

Figure 3. Ozone damage on a cut-leaf coneflower (Rudbeckia laciniata) at the Paul J. Ciener Botanical Garden in Kernersville, NC. Notice the uniform, brown stipples and chlorosis and how the veins do not show damage. The patches of necrosis may be due to other co-occurring plant diseases.

How Does Ozone Hurt Our Forests?

Ozone decreases the production of wood for timber trees and fruits/nuts for edible trees. It also reduces the amount of carbon stored in plant tissues (aka carbon sequestration potential). These losses change ecosystem structure, reduce biodiversity, and negatively impact water and nutrient cycling.

There are several O₃ sensitive tree species that are native to North Carolina, including tulip poplar, eastern redbud, and black cherry. In a 2024 Environmental Protection Agency (EPA) study on the effects of ozone on seedlings of quaking aspen, black cherry, and ponderosa pine, researchers found that as ozone increased, tree growth decreased. The losses observed due to O₃ in a controlled setting were then used to model total biomass loss in these species across the United States. For black cherry, which is extremely O₃ sensitive, they predicted more than 30% biomass loss in the Charlotte-Mecklenburg and Triad metro areas of North Carolina.

High ozone exposure also increases the risk of damage to trees from disease, damage from insects, effects from other pollutants, and harm from severe weather. Essentially, O₃ weakens trees, making them more vulnerable to damage from other causes, similar to how inhaling ozone can make our lungs more susceptible to infections.

Can Ozone Affect Crops, too?

Yes—many important crops are ozone sensitive species: soybeans, potatoes, sweet potatoes, peanuts, maize (corn), rice, wheat, cotton, tobacco, tomatoes, grapes. Almost of these are significant to NC agriculture.  A 2024 study in Environmental Pollution found that ozone pollution reduces corn and soybean yields in the US by an average of 3.5% and 6.1%, respectively, resulting in an estimated annual deficit of roughly $2.6 billion for these two crops alone.

The effects felt on agriculture reach far beyond direct yield losses. Higher ozone concentrations can lead to decreased worker productivity. This harms worker health and the ability to carry out tasks efficiently, leading to labor expenditure losses. Ozone also compromises soil health by damaging microbial communities. It disrupts bacteria-fungi interactions, reduces fungal diversity, accelerates soil organic matter decomposition, and depletes soil carbon, further decreasing the productivity of agroecosystems.

Is there Anything Plants Can do to Protect Themselves?

There are some plant characteristics and behaviors that can protect plants from ozone damage. Plants that are naturally higher in antioxidants, like vitamin C, are less susceptible to ozone damage. Other plants that have smaller and fewer stomata may tolerate higher O₃ concentrations since less of the pollutant can enter the leaf.

There are also variations in how photosynthesis is carried out (through different photosynthetic pathways), including C4 and Crassulacean Acid Metabolism (CAM) photosynthesis. These photosynthetic adaptations may increase these plants’ tolerance to ozone exposure. CAM plants open their stomata at night for carbon capture (unlike most plants which open during the day when there is sunlight). Nighttime is also when O₃ levels are lowest, which mitigates the effects of O₃ on CAM plants. C4 plants, as opposed to C3 plants, isolate RuBisCo in specialized cells that aren’t exposed to ambient air, and due to their increased efficiency of water use, they do not open and close their stomata as often. As a result, they are not as affected by O₃ as C3 plants.

Most will close their stomata to reduce the amount of ozone entering the leaf, since the plant recognizes this molecule as being harmful. While this helps in the short term, it poses two major problems if high ozone concentrations occur over extended periods: 1) When plants close their stomata, they cannot take in the carbon dioxide needed for photosynthesis, slowing the overall growth of the plant, and 2) frequent O₃ exposure can lead to overuse of the stomata. Overtime, the stomata can become less responsive to environmental cues and stay open more than they should—a phenomenon known as “stomatal sluggishness”.

What Can We Do to Keep Plants Healthy?

Luckily, there is a lot that we can do to reduce ground-level ozone concentrations to help our local plants and our human neighbors who are sensitive to ozone. (Sensitive groups to ozone include people with lung diseases like asthma, children and teens, older adults, and those who are active outdoors). By making good personal driving choices, we can reduce vehicle-related emissions that lead to ozone formation. These “vehicle emissions reduction activities” (VERA) include driving less whenever possible, avoiding unnecessary engine idling, keeping our cars well-maintained, driving sensibly/being efficient in the way that we drive, and choosing the cleanest car that meets our needs.

In addition to reducing nitrogen oxides, carbon monoxide, carbon dioxide, particulate matter, hydrocarbons, and other pollutants that exit the tailpipes of internal combustion vehicles, practicing VERA makes us better, safer drivers. These practices also save money by reducing fuel consumption. Learn more by watching this video from It’s Our Air.

Along with making good driving choices, environmental education plays a vital role in changing individual perspectives that can lead to widespread behavioral shifts. Ozone gardens are an excellent education tool for raising much-needed awareness about air pollution issues in our communities. Stay tuned for more in our next blog post, “Growing Awareness: How Ozone Gardens Help Us See the Air We Breathe.”