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Ozone’s Effects on Plants
Severely affected plants do show symptoms of ozone stress. Leaves may have tiny light-tan irregular spots less than 1mm in diameter (flecking), small darkly pigmented areas approximately 2-4 mm diameter (stippling), bronzing, and reddening. An increasing number of reports have appeared during the past 25 years regarding ozone-induced injury to plant leaves in many countries. (Krupa et al. 2001)
Although research shows that ozone pollution harms forests and that prolonged exposure has serious consequences, the precise extent of ozone damage to mature forests has proven a difficult issue to resolve. Natural ecosystems are highly variable and complex, and laboratory studies can never fully simulate them. Variability extends to individual plant species, subspecies, and varieties; some react to ozone more strongly than others.
Among crop plants, tobacco is a "canary in the mine" (or early warning) for detecting harmful levels of ozone. Plants such as soybean, cotton, peanut, clover, quaking aspen, and yellow poplar (dicotyledons) tend to be more sensitive to ozone than plants such as sorghum, field corn, and winter wheat (monocotyledons). Agricultural researchers study ozone’s effects on major crops that include tobacco, soybeans, cotton, wheat, and corn because they’re important to our agricultural economy.
Studies of ozone’s influence on crop yields differ in their results.
Studies of soybean yield at the University of Maryland found a 10 percent loss
of soybean crop due to current levels of ozone in that state, which are commonly
40-80 ppb during the growing season, with particular episodes much higher. The
same study showed that ozone exposure causes the loss of 6-8 percent of winter
wheat and 5 percent of the corn crop yields to Maryland farmers. (Mulchi 2001)
The National Crop Loss Assessment Network in Raleigh, North Carolina, found a
2-5 percent loss for winter wheat at current levels of ozone (which usually
average between 50 and 55 ppb). (Heagle 2001)
The Ozone We Breathe
Chemical changes in the atmosphere spread throughout other parts of the Earth system, including land, water, and living organisms. Effects of crops’ exposure to ozone appear in the soil as well as in the plants themselves. In soybeans, overexposure to ozone results in the plant metabolizing less carbon dioxide. This reduces carbon flow from the atmosphere to the roots. Reduced carbon flow suppresses nitrogen fixation, and the plant then "mines" the soil for some of the nitrogen it needs to grow. Under conditions of high ozone exposure, soybean farmers who want to maximize their soybean crop production must add more nitrogen to the soil than it normally requires. In Maryland and nearby states, an overabundance of nitrogen runoff from the land causes serious and expensive problems for natural ecosystems and fisheries in Chesapeake Bay. While the exact extent of this nitrogen runoff due to ozone exposure remains to be established, adding more nitrogen to the watershed presents an unattractive solution to the ozone pollution problem.
High ozone concentrations can affect not only plant growth, but soil fertility. Plants exposed to low ozone concentrations normally metabolize a certain amount of carbon dioxide. They send carbon to their roots, and then to the surrounding soil. Microbes in the soil make use of this carbon. Plants that are exposed to high ozone concentrations metabolize less carbon dioxide, so less carbon is available in the soil, and fewer soil microbes grow and thrive. Microbial activities that result in soil enrichment and carbon processing decrease, with the result that soil fertility diminishes.
Ozone’s harmfulness at ground level extends to non-living things. In the earliest days of ozone research, cracks in rubber served as the indicators used by scientists to determine atmospheric concentrations of ozone. Ozone accelerates fading in dyes and speeds deterioration of some paints and other coatings. It also damages cotton, acetate, nylon, polyester, and other textiles. Photographic paper companies caution users about ozone exposure.