Ozone is probably the most important, plant-toxic air pollutant in the United States.
Much of the ozone in the lower atmosphere is produced when sunlight reacts with exhaust gases from internal combustion engines. Ozone is a very active form of oxygen that causes a variety of symptoms. Plant growth is often stunted. Flowering and bud formation can be depressed. Affected leaves of certain plants, such as citrus, grape, and tobacco, commonly wither and drop early.