AIR POLLUTION AND PLANTS

Although most people are aware that air pollution can affect their health, it is also true that these same pollutants can adversely affect the growth and reproduction of the plants and trees which make our environment so beautiful. Unlike people, plants are unable to escape from areas of high pollution and must try to grow and adapt to these polluted environments. Air pollutants impact a wide variety of vegetation, including crop plants, landscape plants, forest and native vegetation.

SOURCES OF POLLUTION

Air pollution is a global problem and can affect the physical environment, human health and vegetation. There are many types and sources of pollutants. They can be broken down into general categories such as mobile and fixed sources and natural sources. Examples of mobile sources include cars, trucks, buses, trains and aircraft. Examples of fixed sources would be large industrial operations, refineries, numerous small industries which produce pollutant precursors, agricultural burning, dairy and cattle industries, and fertilizer production processes.

Natural sources can also play a significant role in pollutant production. For example, volcanic activity can release significant amounts of pollutants into the atmosphere which can be distributed globally. Fugitive dust from nonvegetated areas or from agricultural operations are also important environmental concerns.

Finally, emission of organic compounds from trees and plants themselves are termed "biogenic emissions" and can contribute to formation of pollution in certain areas. Although there are naturally occurring atmospheric pollutants, human activity accounts for a signigicant proportion of these pollutants.

Pollutants are formed in the atmosphere when chemical components are fueled by sunlight. These 'photochemical' reactions result in the production of various compounds including oxides of nitrogen (NOx), peroxyacylnitrates (PAN), sulfur oxides and aerosols, polyaromatic hydrocarbons (PAH), ozone and multitudes of other chemical species. Certain urban areas in the U.S. and the world are particularly vunerable to pollutant problems due to both local topography (which may limit air movement within a region, for example Los Angeles), and climate.

During hot summer months a temperature inversion layer may form in the atmosphere which traps pollutants near the ground. Of all atmospheric, ozone is the most important with respect to plant injury. Both in the U.S., and worldwide, ozone accounts for roughly 90% of the plant injury and losses to air pollutants. Other important pollutants include sulfur dioxide, nitrogen dioxide, fluorides, methane, ammonia, PAN and ethylene.

HOW PLANTS TAKE UP POLLUTANTS

Plants exchange atmospheric gases much like humans and animals breathe. However, plants take up carbon dioxide and give off oxygen which is opposite to ourselves. This production of oxygen is partially responsible for maintaining the oxygen atmosphere on the planet. Plants take up and release gases through small pores on the leaf surface called stomata. The uptake of carbon dioxide and conversion to carbohydrates and energy is called photosynthesis. During this process of uptake of carbon dioxide plants can also take up pollutants. Once the pollutants enter the leaf, the pollutants may injure leaf tissue and upset the normal metabolic functioning of the plant. Depending on the pollutant, the upper or lower leaf surface of the plant will exhibit symptoms. Because the most common pollutants are gaseous (ozone, sulfur dioxide, PAN) and are broken down quickly within the plant, it is not possible to quantitatively assay the plant to determine the intensity of pollutant exposure. For these pollutants, reliable air monitoring is necessary to associate plant injury with a pollutant episode. However, a few pollutants, eg. fluoride, are absorbed through the leaf surface and remain within the plant and can be assayed for high concentrations but the occurrence of these pollutants is usually quite low or limited in distribution.

Plants can vary dramatically in sensitivity to pollutants. Many species of crops and landscape ornamentals are quite resistant while others are extremely sensitive. Additionally, within a specific crop sensitivity can vary widely among cultivars. Presumably, subtle genetic differences are responsible for changes in sensitivity although the basis for these differences are not always well understood.

OZONE

Ozone is produced in the atmosphere as a photochemical pollutant. The photochemical cycle involved in the production of ozone utilizes hydrocarbons and nitrogen dioxide which are released into the atmosphere by both mobile and stationary sources (eg. automobiles and industrial processes). In the presence of sunlight which provides energy for the reaction, an oxygen atom is stripped from the nitrogen dioxide and combined with an oxygen molecule to produce ozone (O3). Ozone is a powerful oxidizing chemical which affects many cellular components and biochemical processes.

As mentioned previously, ozone is the most important pollutant with respect to plant injury. Ozone injury on plant leaves is usually characterized by upper leaf surface chlorosis (yellowing), stippled or mottled markings or by a 'bronzed' appearance. At very high concentrations, the leaf tissue will show small dead areas (lesions). Symptoms will vary among plants, and even among varieties within a single type of plant. Crop, landscape, trees and native vegetation can all be susceptible to ozone impact.

The effect of ozone on agricultural crops has been intensively studied for the past 25 years. Several comprehensive studies have attempted to determine the dollar loss to growers due to the impact of pollutants. The National Crop Loss Assessment Network (NCLAN) was a comprehensive study which investigated the impacts of ozone on major agricultural crops in the United States, including corn, soybean, and cotton. The Network consisted of large field chambers which were set down over the crop in an agricultural field and in which the chamber atmosphere could be controlled. The air quality in the chambers ranged from charcoal filtered air to twice the ambient ozone level.

There were eight identical sites across the U.S., testing the response of these crops to ozone under identical conditions. The data from this multi-year experiment was utilized by agricultural economists in which the cost to both producers (farmers) and consumers (public) was calculated under differing air quality scenarios. The total calculated economic loss (producers + consumers ) at current air quality levels compared to an improvement of approximately 25% was nearly $1.6 billion per year. Similar estimates for California, the nation's leading agricultural state, were between $100 and $500 million in lost revenues each year just to ozone. Clearly, the impact of ozone on agricultural systems is significant and the benefits of controlling ozone levels are obvious.

The aesthetic losses from ozone to forests, native vegetation and landscaping can not be easily quantified but is important in determing the quality of life and our environment.

SULFUR DIOXIDE, NITROGEN DIOXIDE AND OTHER POLLUTANTS

Plant injury can occur due to a variety of other pollutants, including sulfur dioxide, nitrogen dioxide, Pan, ethylene, fluoride, chloride and ammonia. Sulfur dioxide is released as a by product of combustion processes and through a variety of industrial activity. Sulfur compounds in the atmosphere may impact plants directly, or may combine with water to form sulfuric acid particles. The symptoms produced by sulfur dioxide include a bleached appearance, typically white to tan in color, with symptoms appearing on the leaf edges and primarily between the leaf veins. Nitrogen oxides (NOx) are generated both by natural and man-made processes.

In urban areas, automobiles account for a large percent of NOx production. Similar to sulfur compounds, NOx compounds can affect vegetation directly but also combine with atmospheric moisture to form nitric acid. There is no reliable symptom for NO2 injury as it manifests itself distinctly on differing plant types. A primary symptom would be chlorosis (yellowing) of leaves, or occasional bleaching. However, these symptoms are also characteristic of a variety of other pollutants so it is important to be aware of the local conditions and monitoring data to be able to reliably identify this as the source of the problem. Generally, the effect of NO2 on plants is not economically important.

Exposure to the photochemical oxidant Pan results in very distinct symptoms. The injury is usually seen on the lower leaf surface. The leaf underside typically appears silvery or bronze in color. No other pollutant causes injury symptoms similar to this. However, occurrences of Pan are infrequent and are usually restricted to areas near urban centers.

The remaining pollutants, fluoride, ammonia, choride, ethylene result in a variety of distinct symptoms but the occurrence of the compounds is infrequent or in concentrations too low to consistently be considered a problem. Industrial technology has provided solutions which limit the release and impact of these compounds. Diagnosing pollutant injury may be quite difficult at times. Many other problems can mimic pollution injury, including nutrient deficiencies, insect feeding, physiological stress (moisture, heat, cold), viral diseases and herbicides. It is important to know the history of the area, including environmental factors and pesticide control measures, to accurately diagnose pollution injury.

SOLUTIONS

There are very few measures that are available to protect vegetation. Although specific chemicals have been developed to minimize the affect of certain pollutants, particularly ozone, these chemicals are extremely expensive and not effective enough to warrant widespread use. The most common approach to protect vegetation is in the genetic breeding of new varieties which are tolerant to current pollutant levels. This approach is time consuming, however, and is linked to breeding for other desirable characteristics. In the long run, the best approach for limiting pollutant injury to plants is to simply reduce the amount of pollutants released into the atmosphere.

Education is possibly one of the best approaches for reducing air pollution. As we introduce concepts of environmental protection in our school curriculum and incorporate tools such as the Eco Badge^TM and Cybermarch, students will become more educated and aware about pollution problems. As they mature into adulthood, these learned concepts may provide impetus for a more proactive stance on protecting our environmental resources, particularly the air.

Dr. Patrick M. McCool, Environmental Analyst
Environmental Resource Group
326 Broadway
Suttons Bay, MI 49682

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