Page 143 - Fundamentals of Air Pollution 3E

II. Effects on Vegetation and Crops 113

visible symptoms and nonvisible or subtle effects (4). Visible symptoms are
deviations from the normal healthy appearance of the leaves. For broadleaf
plants, a healthy leaf has good color, with a normal cell structure in the
various layers. Deviations from this healthy appearance include tissue col-
lapse and various degrees of loss of color. Extensive tissue collapse or
necrosis results from injury to the spongy or palisade cells in the interior
of the leaf. The leaf is severely discolored and loses structural integrity.
Dead tissue may fall out of the leaf, leaving holes in the structure. Less
dramatic discolorations are caused by a reduction in the number of chloro-
plasts, a symptom referred to as chlorosis. Injury to the outer or epidermal
layer is referred to as glazing or silvering of the leaf surface. When the
pattern is spotty, the terms flecking and stippling are used to describe the
injury,
Other forms of visible injury are related to various physiological alter-
ations. Air pollution injury can cause early senescence or leaf drop. Stems
and leaf structure may be elongated or misshapen. Ornamentals and fruit
trees can also show visible injury to the blooms of the fruit, which can
result in decreased yield.
The nonvisual or subtle effects of air pollutants involve reduced plant
growth and alteration of physiological and biochemical processes, as well
as changes in the reproductive cycle. Reduction in crop yield can occur
without the presence of visible symptoms. This type of injury is often
related to low-level, long-term chronic exposure to air pollution. Studies
have shown that field plantings exposed to filtered and unfiltered ambient
air have produced different yields when no visible symptoms were present
(5). Reduction in total biomass can lead to economic loss for forage crops
or hay.
Physiological or biochemical changes have been observed in plants ex-
posed to air pollutants, including alterations in net photosynthesis, stomate
response, and metabolic activity. Such exposure studies have been con-
ducted under controlled laboratory conditions. An understanding of the
processes involved will help to identify the cause of reduction in yield.
Laboratory studies have also investigated the interaction of air pollutants
and the reproductive cycle of certain plants. Subtle changes in reproduction
in a few susceptible species can render them unable to survive and prosper
in a given ecosystem.
The major air pollutants which are phytotoxic to plants are ozone, sulfur
dioxide, nitrogen dioxide, fluorides, and peroxyacyl nitrate (2). Table 8-1
lists some of the types of plants injured by exposure to these pollutants.
The effects range from slight reduction in yield to extensive visible injury,
depending on the level and duration of exposure. Examples of the distinc-
tion between air pollution injury and damage are also given in Table 8-1.
Visible markings on plants or crops such as lettuce, tobacco, and orchids
caused by air pollution translate into direct economic loss, i.e., damage.
In contrast, visible markings on the leaves of grapes, potatoes, or corn