Ozone’s Effects on Plants

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Because ozone formation requires sunlight, periods of high ozone concentration coincide with the growing season. Just as in damage to people, ozone damage to plants can occur without any visible signs. Many farmers are unaware that ozone is reducing their yields. Ozone enters the plant’s leaves through its gas exchange pores (stomata), just as other atmospheric gases do in normal gas exchange. It dissolves in the water within the plant and reacts with other chemicals, causing a variety of problems. Plant physiologists are still trying to understand the specific pathways and locations of ozone’s effects within plant cells. Physiologists know that some cell membranes become leaky, possibly because of ozone’s ability to interact with lipid (fatty) components and/or membrane proteins. Photosynthesis slows, resulting in slower plant growth. Compounds resulting from oxidation by ozone interfere with the cell’s energy production in the mitochondria. Such ozone-induced compounds also decrease the numbers of flowers and fruits a plant will produce, and they impair water use efficiency and other functions. Plants weakened by ozone may be more susceptible to pests, disease, and drought.

evidence of exposure to high concentrations of ozone, plant leaves

Browning on potato leaves shows evidence of exposure to high concentrations of ozone. (Photograph courtesy UDA-ARS Air Quality Program, North Carolina State University; photo by Gerald Holmes)

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)

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.
Fluorescence imaging technology captures soybean plant responses, Fluorescence imaging

Fluorescence imaging technology captures soybean plant responses, Fluorescence imaging

Fluorescence imaging technology captures soybean plant responses to elevated levels of ozone. Within each image, the two leaves on the left-hand side grew in control chambers, and the two leaves on the right-hand side grew in chambers with moderately elevated ozone concentrations. Purples and blues in ozone-exposed leaves indicate that the leaves are carrying out photosynthesis less efficiently than leaves in the control chambers, where deeper reds and yellows appear. (From Kim, M.S., McMurtrey, J.E., Mulchi, C.L., Daughtry, C.S.T., Chappelle, E.W., and Chen, Y.R. Steady-state multispectral fluorescence imaging system for plant leaves. Applied Optics, 40:157-166. 2001)

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.

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