Ozone Damage 



A CONSIDERATION IN SELECTING CROP AND LANDSCAPE PLANTS 

 Crekfien Smith, Florence Peterson and Tom Luther 



The hot, hazy days of this past summer were few, 

 but even this year, people may have become un- 

 comfortably aware of the pollutants in the air we 

 breathe. On bad days, our eyes burn, we may have 

 breathing problems, and visibility is greatly reduced. 

 Not surprisingly, plants are also affected by toxic sub- 

 stances in the air. Sensitive plant species show visible 

 symptoms of damage, reduction in growth may occur, 

 and stress from poor air quality may weaken them, mak- 

 ing them more susceptible to invading insect or dis- 

 ease. Many plant species are often injured by air pollu- 

 tion levels lower than those required to produce symp- 

 toms in humans. In this way, plants can provide a warn- 

 ing that pollution levels are approaching dangerous con- 

 centrations. 



The most important plant-damaging air pollutant in 

 our region is ozone (O). Ozone occurs naturally in the 

 upper stratosphere where it provides a protective layer 

 around the earth that screens out harmful high energy 

 radiation from the sun and outer space. But on the sur- 

 face of the earth, tropospheric ozone is a gaseous air 

 pollutant produced when sunlight acts on the products 

 of combustion from cars and other industrial sources. 

 Most ozone damage occurs during periods of air stagna- 

 tion which permit the build-up of ozone concentrations 

 at ground level. The most populated areas produce a 

 variety of man-made pollutants, many of which will react 

 with, and therefore, transform the volatile ozone mol- 

 ecule. Outside the city, these reactions do not occur, 

 thus favoring ozone formation and transport to other- 

 wise pristine areas. Highest levels of ground-level ozone 

 occur in relatively remote areas downwind from urban 

 centers. 



A high level of ozone alone does not cause injury to 

 plants. It must get inside the plant tissue through the 

 open stomates during the normal process of gas ex- 

 change between a leaf and its environment. High mois- 

 ture and good nutrient levels contribute to damage be 

 cause the plant is functioning well and therefore ac- 

 tively engaging in gas exchange. Other environmental 



conditions, such as stomates closing in drought to con- 

 serve water, may slow down gas exchange. So high 

 ozone levels during a drought may cause less damage 

 than in wetter weather. 



Once inside, the ozone attacks the cell membranes, 

 disrupting many important physiological processes. If the 

 cells collapse and die, then visible symptoms will occur 

 on the leaf surfaces. In most broadleaf plants, in jury 

 generally appears as pin-prick red-to-black stipples on 

 the upper leaf surface and between the veins. Dam age 

 is cumulative, so it may appear later in the summer and 

 generally affect older leaves. As uptake continues and 

 injury worsens, leaves may turn yellow and fall. 



Ozone-induced plant injury may be classified as ei- 

 ther "acute" or "chronic". Acute injury involves exposure 

 to peak concentrations of ozone causing cell death and 

 obvious visible injury to leaf surfaces. Chronic injury re- 

 sults from long term exposure to relatively low ozone 

 concentrations which can cause reductions in growth, 

 yield, and reproductive potential, either with or without 

 visual symptoms. Both acute and chronic symptoms may 

 be confused by the effects of pathogens and pests. Of- 

 ten, plants weakened by chronic ozone exposure are 

 more susceptible to other environmental stresses, both 

 natural and man-made. 



One way to assess the impact of ozone on our forests 

 and gardens is to document visible injury on sensitive 

 plant species, known as bioindicators. Bioindicators are 

 plants that exhibit a well-defined and consistent re- 

 sponse to elevated ozone concentrations in the air. Use- 

 ful bioindicator species for our region include black 

 berry, milkweed, and black cherry. Injury on these three 

 manifests as a classic upper-leaf surface stipple. Other 

 species which have proven useful in localized areas in- 

 clude white ash, yellow poplar, sweet gum, bigleaf aster, 

 and spreading dogbane. 



The National Forest Health Monitoring Program (spon- 

 sored by the USDA Forest Service, Environmental Pro- 

 tection Agency, and state governments) is using 

 bioindicators to assess ozone impacts on our nation's 



OCTOBER'f NOVEMBER 1996 



IS 



