Ozone Air Pollution Harms Plants
What is Ozone Air Pollution?
- Ground-level ozone (O3) is formed during hot, sunny days from vehicle exhaust and power plant emissions (nitrogen oxides [NO+NO2] and volatile organic compounds [VOC's]).
- Ground-level O3 should not be confused with the ozone layer which naturally occurs high above the earth. This "good" ozone protects us from harmful ultraviolet (UV) radiation. Ground-level O3 does not occur naturally, it is a pollutant from human sources.
- In humans, O3 pollution causes respiratory problems, decreased lung capacity, and aggravated asthma. It is especially harmful to sensitive populations such as people with lung conditions, children, and the elderly.
Ozone Pollution is Harmful to Plants
Black stippling caused by ozone on tall milkweed NPS photo
- Ozone pollution weakens plants and makes them more susceptible to diseases and insects. It also decreases a plant's ability to produce and store its food. It causes less production of flowers and seeds, and can lead to lower yields in many crop species.
- In St. Louis and much of the Midwestern United States, the most O3 pollution is produced from April 1st through October 1st. Thus the growing season and the O3 season are the same.
- Sensitivity to O3 varies between and within plant species. Plants that show visual symptoms of O3 damage, like many of the plants in our gardens, are "bio-indicators"--species that can be used to monitor the health of an environment. Many plants begin to show symptoms at O3 concentrations of 40 parts per billion (ppb) and higher.
- Ozone symptoms on plants often appear as a dark stippling on the top side of leaves in-between the leaf veins. Continuous exposure to elevated O3 levels can cause leaf cell death and early leaf drop.
How Ozone Damages Plants
Image provided by H. Neufeld, from NASA's Ozone-Induced Foliar Field Guide
- Ozone enters plants through pores on leaves called stomata (singular: stoma). Once in the leaf O3 reacts with other chemicals creating reactive molecules (such as hydrogen peroxide H2O2, and hydroxyl radicals OH-). These highly reactive molecules cause a variety of problems in the leaf, including weakening cell walls, interruption of photosynthesis, and the release of anthocyanin pigments which cause stippling.
- The image above represents a cross section of a leaf under a microscope. It shows the progression of O3 entering through the stomata on the bottom side of the leaf, to O3 reacting and forming reactive oxygen species (ROS) molecules, to the defensive production of anthocyanins (pigments that darken cells) on the top side of the leaf, and finally cell death.
- Stomata allow carbon dioxide (CO2) to enter leaves for photosynthesis, and allow water (H2O) and oxygen (O2) to leave the plant. Ozone enters leaves along with CO2 and whatever else is in the air. Stomata regulate this gas exchange and water loss by opening and closing in response to environmental conditions such as daylight, temperature, humidity, and CO2 concentrations.