Page 142 - Fundamentals of Air Pollution

112 8, Effects on Vegetation and Animals

The leaf structure has several important functions, three of which are
photosynthesis, transpiration, and respiration (2). Photosynthesis is accom-
plished by chloroplasts in the leaf, which combine water and CO 2 in the
presence of sunlight to form sugars and release O 2. This process is shown
in Eq. (8-1).

Transpiration is the movement of water from the root system up to the
leaves and its subsequent evaporation to the atmosphere. This process
moves nutrients throughout the plant and cools the plant. Respiration is
a heat-producing process resulting from the oxidation of carbohydrates by
O 2 to form CO 2 and H 2O, as shown in Eq. (8-2).

These three functions involve the movement of O 2, CO 2, and H 2O
through the epidermal layers of the leaf. The analogy to human inhalation
is obvious. With the diffusion of gases into and out of the leaf, pollutant
gases have a direct pathway to the cellular system of the leaf structure.
Direct deposition of particulars matter also occurs on the outer surfaces of
the leaves.
The indirect pathway by which air pollutants interact with plants is
through the root system. Deposition of air pollutants on soils and surface
waters can cause alteration of the nutrient content of the soil in the vicinity
of the plant. This change in soil condition can lead to indirect or secondary
effects of air pollutants on vegetation and plants.
Injury to plants and vegetation is caused by a variety of factors, of which
air pollution is only one. Drought, too much water, heat and cold, hail,
insects, animals, disease, and poor soil conditions are some of the other
causes of plant injury and possible plant damage (3). Estimates suggest
that less than 5% of total crop losses are related to air pollution. Air pollution
has a much greater impact on some geographic areas and crops than others.
Crop failure can be caused by fumigation from a local air pollution source
or by more widespread and more frequent exposure to adverse levels of
pollution.
The subtle interaction of air pollutants with these other stressors to plants
and vegetation is the subject of ongoing research. For some plant systems,
exposure to air pollutants may induce biochemical modifications which
interfere with the water balance in plants, thereby reducing their ability to
tolerate drought conditions.

II. EFFECTS ON VEGETATION AND CROPS

The effects of air pollution on plants range from subtle to catastrophic,
as shown in Fig. 8-1. Historically, these effects have been classified as