QBARS - v31n4 Effects of Air Pollution on Rhododendrons

by Fred R. Davis, Kent, Ohio
Reprinted from The Rosebay, Massachusetts Chap

Fred R. Davis is a chemist with a Ph. D. in physical chemistry from Kent State University. His avocation is raising azaleas and rhododendrons with an emphasis on nutrition. He is also interested in breeding for disease resistance. Dr. Davis is a member of the American Rhododendron Society and the American Horticultural Society.

Man's activities in producing energy, manufacturing goods and disposing of waste materials result in releasing pollutants into the atmosphere which may alter plant metabolism and induce disease. Air pollution damage to plants has been known for several decades. Its extent and importance, however, increased with the world's increasing population, industrialization and urbanization.
The air pollutants which cause plant injury are primarily gases, but some particulate matter or dusts do affect vegetation. The gases which cause widespread damage are sulfur dioxide, nitrogen dioxide, ozone, hydrogen fluoride and peroxyacyl nitrates. High concentrations of or long exposure to these materials causes visible and sometimes characteristic symptoms (e.g., necrosis) on the affected plants. Dosages of these pollutants (less than those that cause acute damage) may suppress the plant's growth and productivity due to the interference with plant metabolism. The purpose of this article is to describe pollutant sources and their effects on plants.

Sulfur Dioxide:
The sulfur oxides are common atmospheric pollutants which arise mainly from the combustion of fuels. Solid and liquid fossil fuels contain sulfur, usually in the form of inorganic sulfides or sulfur-containing organic compounds. Combustion of the fuel forms about 25 to 30 parts of sulfur dioxide to one part sulfur trioxide
Sulfur dioxide may cause acute or chronic leaf injury to plants. Acute injury produced by high concentrations for relatively short periods, usually results in injured tissue drying to an ivory color; it sometimes results in a darkening of the tissue to a reddish-brown. Chronic injury, which results from lower concentrations over a number of days or weeks, leads to pigmentation of the leaf tissue, or gradual yellowing, or chlorosis in which the chlorophyll producing mechanism is impeded. Both acute and chronic injury may be accompanied by the suppression of growth and yield.
Acute injury apparently affects the plant's ability to transform absorbed sulfur dioxide into sulfuric acid and then into sulfates. At high rates of absorption, sulfite is thought to accumulate, resulting in the formation of sulfurous acid which attacks the cells. The amount of acute injury depends on the absorption rate which is a function of the concentration. Different plants vary widely in their susceptibility to acute sulfur dioxide injury. Some species of trees and shrubs have shown injury at exposures of .5 ppm for seven hours, while injury has been produced in other species at three hour sulfur dioxide exposures of .54 ppm and, still others, at eight hour exposures of .3 ppm. From these studies, it appears that acute symptoms will not occur if the eight hour average concentration does not exceed .3ppm. However, sulfur dioxide concentrations from .05 to .25 ppm may react synergistically with either ozone or nitrogen dioxide to produce moderate to severe injury to certain sensitive plants. Rhododendron and kalmia show damage at exposures of .5 ppm for eight hours. The damage consists of bleaching of the interveinal tissue and some chlorosis of the leaves.
Chronic plant injury results from the gradual accumulation of excessive amounts of sulfate in leaf tissue. Sulfate formed in the leaf is additive to sulfate absorbed through the roots and when sufficiently high levels accumulate, chronic symptoms, accompanied by leaf drop, occur.
It has been suggested that sulfur dioxide might suppress growth and yield without causing visible injury. One investigator reported that yields of rye grass grown in unfiltered air were significantly lower than similar yields of plants grown in filtered air. No visible symptoms of injury were observed.
Sulfuric acid mist which may occur in polluted fogs and mists also damages leaves. The acid droplets may cause spotted injury to wet leaves at concentrations of .1 mg/ meter.
We must all be aware of this molecule and its adverse effect on vegetation when its concentration reaches .05 to .2 ppm (annual mean). At these concentrations, one generally observes chronic plant injury and excessive leaf drop.

Nitrogen Dioxide
Nitrogen dioxide (N02) is produced from oxygen and nitrogen in the air by hot combustion sources such as open fires, furnaces, and automobile combustion chambers. Nitrogen dioxide in concentrations of 2 - 3 ppm causes bleaching of plant foliage similar to that caused by sulfur dioxide. There is evidence that at even smaller concentrations it suppresses the growth of some vegetable plants.
This substance is not as severe a pollutant as some other man-made molecules, since during rainfall it dissolves in the water to produce nitric acid which gives rise to soluble nitrates that are beneficial to most plants.

Ozone (03) is one of the most widely occurring air pollutants and one of the most destructive to plants. Ozone originates from the ozone-rich stratosphere by vertical winds. It may be a by-product of photochemical reactions between nitrogen oxides and plant produced terpenes, particularly in conifer forests. Ozone is also produced from the activities of man and his civilization.
Exhausts of automobiles and other internal combustion engines are probably the most important sources of ozone and other phytotoxic pollutants. Tons of incompletely burned hydrocarbons and NO2 are released into the atmosphere daily by automobile exhausts. In the presence of ultra-violet light from the sun, the nitrogen dioxide reacts with oxygen and forms ozone and nitric oxide. However, in the presence of unburned hydrocarbon radicals, the nitric oxide reacts with these instead of ozone, and consequently, the ozone concentration increases. Ozone, too, can react with certain unsaturated hydrocarbons, but these products, organic peroxides, are also toxic to plants. The noxious fumes produced by automobiles and other engines are swept up by warm air currents from the earth's surface rising into cooler air above where the fumes are dissipated. However, during periods of calm, stagnant weather, an inversion layer of warm air is formed above the cooler air and this impedes the upward dispersion of atmospheric pollutants. The pollutants then are trapped near the ground where after sufficient build-up they may seriously damage living organisms. Ozone causes mottling acid chlorosis of leaves that is confined primarily to the upper leaf surface. The spots may be small or quite large and may vary in color from bleached white to tan, brown or black, depending on the plant and on the severity of injury. In some plants, such as citrus, grapes, pine, yews and broadleaf evergreens, ozone injury causes premature defoliation and stunting.
Ozone enters leaves through stomata. Once in the leaf, it concentrates in the palisade layer where it causes collapse and bleaching or discoloration of the palisade cells. Ozone primarily affects expanding leaves, but not very young or mature leaves. Several mechanisms by which ozone can damage plants have been suggested, including inhibition of mitochondria) activity, destruction of the permeability of the cell membrane, inactivation of auxin, and inhibition of protein synthesis and photosynthesis. Although each of these effects has been observed in at least some of the hosts affected, it is not clear how ozone brings these about.
Plants can be protected from ozone damage in several ways. For example, plants escaped damage from ozone when they were watered with ascorbic acid (presumably an inhibitor of oxidation in cells); also, certain antioxidants and antiozonants such as dithiocarbamates provide some protection. In addition, some plant varieties are considerably more resistant to ozone injury than others.

Hydrogen Fluorides and Fluorides
Fluorides are emitted from stacks of factories and are spread by diffusion or carried by air currents. Hydrogen fluoride (HF) is very toxic to plants, e.g., corn, peaches and tulips, where injury is caused by concentrations as low as 0.1 to 0.2 parts per billion (ppb.). Fluoride accumulation in foliage generally injures the leaf margins of dicotyledon plants and tips of leaves of monocotyledon plants. Injured areas turn tan to dark brown, die and may fall from the leaf. Plants differ in their sensitivity to fluoride. The more tolerant ones are able to accumulate much more fluoride (up to 200 ppm) without showing necrosis. Instead, they develop a slight chlorosis, followed sometimes by premature defoliation. Actively growing plants, particularly when their leaves are wet, are generally more susceptible to fluoride damage. Fluoride seems to be absorbed by the leaves through the cuticle and translocated to the leaf margins and tips. When a toxic concentration is reached the cells from the epidermis collapse and die. Fluorides may also escape from the plant through volatilization and washing, and the plants may recover from chronic fluoride symptoms within a couple of months. The author has witnessed very severe fluoride damage to evergreens in the neighborhood of an industrial plant which manufactured fluorides for catalysts. This type of damage is irreversible.

Peroxyacyl Nitrates
Peroxyacyl nitrate is produced from the exhaust pipes of cars, with ozone, nitrogen dioxide and probably other oxidizing substances in the presence of sunlight.
Peroxyacyl nitrates cause a plant disorder known as "silver leaf" which produces spots on the lower leaf surfaces of the plants of many herbaceous crops. The color of the spots may range from bleached white to bronze. The silvering or glazing of the lower leaf surfaces injured by peroxyacyl nitrates results from air filling the space created by dehydration and shrinking of the mesophyll cells. Meanwhile, the guard cells become congested and the epidermis cells collapse.
Peroxyacyl nitrate injury has been observed primarily around metropolitan areas where large amounts of hydrocarbons are released into the air from automobiles. The problem is particularly serious in areas such as Los Angeles and New Jersey where the atmospheric conditions are conducive to inversion layer formation. Many different kinds of plants, vegetable and ornamental, are affected by peroxyacyl nitrates.

Carbon Monoxide
Carbon monoxide (CO) is a gas produced by incomplete combustion of carbonaceous material. All internal combustion engines produce this gas. The amount produced is a function of the temperature and oxygen supply, with well tuned engines producing less than improperly tuned engines.
Carbon monoxide injury involves foliage damage and poor plant vigor. Some pines, particularly white pine, will lose their needles when carbon monoxide injury occurs.

Man has to learn to control air pollutants so that there can be an improvement in the quality of life. Epidemiological studies clearly indicate an association between air pollution and health effects of varying severity. Air pollutants have produced both acute and chronic injury to many species of plants accompanied by suppression of growth and yield.
Some of the material in this article was taken from reports issued by the U.S. Dept. of Health, Education & Welfare (Public Health Service), "Air Quality Criteria for Sulfur Oxides", (February, 1969).