Horticulture Principles and Practices

concentration in the atmosphere.A class of compounds called chlorofluorocarbons (CFCs)
release chlorine upon breaking down. This chlorine rises and, upon reaching the upper
atmosphere, reacts with ozone, reducing it to oxygen gas. The ozone layer thus becomes
depleted, allowing more harmful ultraviolet radiation to reach the earth. An example of
CFC is freon, a refrigerant and propellant in aerosol cans. Aerosols used in homes
(coolants in refrigerators and some cosmetic sprays) also can deplete the ozone layer.
Pesticides are sources of environmental pollution. Arsenic-based insecticides, for example,
cause chlorosis and necrosis, resulting in poor growth and quality of plant products.
Acid Rain Acid rain is a consequence of air pollution, because the pollutants in the
atmosphere are brought down in the various kinds of precipitation (rain, ice, and snow).
When the pH of rain is below 5.6, it is described as acid precipitation. Acid rain is
produced when sulfur oxide and nitrogen oxide react with water in the atmosphere to
form sulfuric acid and nitric acid, respectively. Normal (unpolluted) rain has a pH of
about 6.0. Acid rain with a pH of less than 3 has been recorded in heavily industrialized
and polluted parts of Scotland, Norway, and Ireland. The eastern United States and
southeastern Canada record an average pH of 4 to 4.5 in rain water. Pollutants can remain
airborne for long distances. Thus, nonindustrial areas can experience acid rain because
of a drift effect from polluting sources in neighboring countries.
Mychorrhizal fungi are known to be affected adversely by acid rain. Acidified
lakes have shown dramatically reduced fish populations. This decline is attributed in
part to aluminum toxicity. Aluminum is a trace element in plant nutrition but comprises
about 5 percent of the earth’s crust. It is not soluble under alkaline conditions. Acid
rain provides the low pH required to dissolve soil aluminum and other heavy metals
such as lead, mercury, and cadmium. Acid rain is also suspected in chemical damage
to certain forests.
Water
Role Water is required for germination, the first step in plant growth. Water from the
aboveground environment comes from precipitation (including rain and snow) and evaporation.
In terms of plant growth needs, the distribution of precipitation throughout the
crop growth period is as important as the total amount. Water is needed for photosynthesis,
the process by which plants manufacture food. Water is the medium by which
minerals and photosynthates are transported through the plant. Plants are cooled through
the process of transpiration.
Moisture Stress The effect of moisture stress on horticultural plants is discussed
more fully later in this chapter. Lack of moisture in the aboveground environment
makes the air less humid, thereby increasing its drying power. The rates of plant
processes such as transpiration, diffusion, and evaporation are affected directly by the
vapor pressure of the air (the part of the total air pressure attributable to the water molecules
present in the air). As previously indicated, if air temperature is increased but
the amount of water vapor in the air stays the same, the relative humidity of the air
decreases. Excessive moisture in the microclimate of plants predisposes them to disease.
Horticultural plants grown indoors are sometimes given a misty spray of water to increase
the humidity of the microclimate, especially in winter when the heaters are turned on
to warm the building.
Acid Precipitation
Precipitation (e.g., rain,
snow, sleet, and hail) with
a pH of less than 5.6.
Moisture Stress
Occurs when a plant is
unable to absorb adequate
water to replace that lost
by transpiration.
Temperature
Temperature, the intensity factor of heat energy, is important in all plant biological,
chemical, and physiological processes. It regulates the rate of chemical reactions and
consequently regulates the rate of plant growth. As a contributing factor to climate, it
plays a major role in plant adaptation and the length of the growing season. The kind
of horticultural plants that can be grown in an area therefore depends on temperature
(in conjunction with rainfall, light, and air movements). As previously indicated,
4.2 Aboveground Environment 97