Air Pollution — An Ever-Increasing Problem

The problem of air pollution has been with us ever since man discovered how to make a fire.

The damage to vegetation by atmospheric pollutants has been widely publicized, due to the rapid expansion of industry, the sharp rise in population and increased interest in gardening.

We’ve known for a long time that smoke emanating from chimneys of manufacturing plants, and other instruments of combustion, including automobiles, contains ingredients which are harmful to vegetation. For this reason it is far more difficult to grow plants near industrialized areas than in the rural parts of our country.

Smoke from industry frequently contains ingredients that may damage or kill leaves. The three major pollutants are sulfur dioxide, fluorine compounds and the so-called smog typical of the Los Angeles area. The first two are so-called acid-forming gases, whereas smog is a peculiar combination of many different chemicals.

Sulfur dioxide, perhaps the most common ingredient in the smoke from many factories, can injure vegetation in concentrations as low as one or two parts in a million parts of air. Although highly toxic to plants, it is relatively non-toxic to humans at these low concentrations. At concentrations high enough to injure plants, it may irritate the mucous membranes of individuals who are ex-posed to it for long periods of time.

Both sulfur dioxide and fluorine produce markings on leaves that are quite distinctive. When the gas concentration is high, the leaf cells become water-soaked and then collapse and dry out rapidly, leaving blanched or scorched areas. Mild attacks will merely cause the leaves to turn yellow.

When the concentration is too low or the time of exposure to sulfur dioxide is too short to cause any visible lesions or yellowing of the leaves, the growth or yield of the plant will not be affected. In other words, sulfur dioxide is not a systemic poison and its action is not cumulative. At low concentrations of about three tenths of a part per million parts of air it is absorbed by the leaves and changed to the sulfate form.

When relatively high concentrations of one or more parts per million are absorbed for an hour or more, sulfurous acid or sulfite form in the interior cells. These compounds cause visible damage to the leaf cells and prevent them from functioning normally.

Middle-aged leaves are most susceptible to sulfur dioxide, while the young or growing leaves are quite resistant. Brownish-red or yellow areas between the leaf veins are a rather characteristic symptom. The margins or tips of some leaves may also be discolored or blanched. On coniferous trees the tips, bases or centers of the needles turn red and then turn completely brown and drop off, especially when the gas concentration is high or when the gas has been present for a long period.

Sulfur dioxide and fluorine damage often cannot be told apart in the field. Only a chemical analysis of the leaves will reveal which gas was responsible. Unlike sulfur dioxide, fluorine can be detected in plant tissues because it actually accumulates in the leaves and stems of plants.

Plants vary widely in their tolerance to fluorine and flouride. Cotton, for example, can absorb as many as 5000 parts per million parts of air without showing injury, whereas the gladiolus may be injured when it absorbs as little as 20 to 30 parts per million. Peach, prune and conifers also are very sensitive to fluorine.

Smog is a term used to designate a mixture of smoke and fog. In the Los Angeles area, however, this term refers to a complex of liquids, solids and gases, comprising more than fifty chemical elements and compounds.

The harmful ingredients are believed to be products resulting from a combination of unsaturated hydrocarbons and the ozone in the atmosphere. The hydrocarbons come from oil refineries and related industries as well as from automobile and bus exhausts.

Soot, the solid residue of smoke, may also damage plants although it is not as toxic as sulfur dioxide or fluorine. This material slowly settles on the leaves and eventually may coat them completely. Here, the damage does not result from clogging of the breathing pores (stomates), as some believe, but from the screening out of sunlight. The sharp reduction in light reduces the capacity of the leaf to manufacture food, and consequently results in a serious impairment of the general vigor of the plant. Because the leaves of evergreens remain attached throughout the year, and in some species for several years, a considerable deposit may accumulate and thus greatly reduce the chances of survival of these trees. Soot is one factor that makes it difficult to grow evergreens in large cities.

Toxic gases in the atmosphere may harm vegetation indirectly as well as directly. The acidity of a soil may increase as a result of the interaction between acid gases and the lime particles in the soil. Such a change not only reduces the population of certain beneficial soil bacteria, but also decreases the activity of others. Excessive soil acidity, whether from air pollutants or natural leaching by rainfall or irrigation, can be counteracted by the application of lime.

Gardeners should not conclude, however, that everything that is wrong with their lawns, gardens or trees is due to air pollution. They should bear in mind that unfavorable weather, insects, fungi, bacteria and viruses also produce symptoms that might be confused with gas symptoms.

Over the years I have examined literally hundreds of cases of alleged gas injury to vegetation and have found that only a small percentage of the damage was actually caused by gas. And this experience was gained in areas that were highly industrialized. Before blaming poor growth of vegetation on air pollutants, therefore, one would do well to call in a competent diagnostician to help.

By P Pirone

Related Articles Of Interest:

• Process of Photosynthesis
• HEPA Filters and House Plants For Cleaning Indoor Air?
• Night Blooming Cereus Perfect Low Humidity Little Space Houseplant
• Indoor House Plants and Cooler Winter Temperatures