Horticulture Principles and Practices

Light
Intense Light Strong and direct sunlight may scorch certain plants. Potted plants
placed in south-facing windows receive direct sunlight unless the presence of a tree in
the direction of the sun’s rays filters the light. Intense light also causes the foliage of certain
plants to bleach and look pale and sickly.
Low Light Inadequate sunlight induces etiolated growth (spindly) and yellowing of
leaves. Plant vigor is reduced, and leaves drop prematurely.
Nutrients
Nutrient Deficiency Generally, an inadequate supply of any of the major plant nutrients,
especially nitrogen, causes plants to be stunted in growth and leaves to yellow
(chlorosis). Deciduous plant leaves may prematurely senesce and defoliate. In addition
to yellowing, lack of potassium shows up later as marginal leaf burns of leaves; young
and expanding leaves show purple discoloration when phosphorus is lacking in the soil.
Calcium deficiency in tomato shows up as blossom end rot.
Nutrient Excess Excess acidic soils may cause excess availability of trace elements
(e.g., iron and aluminum), which can lead to toxicity in certain plants.
5.7.2 HUMAN-RELATED ACTIVITIES
Industrial Production
Industrialized and heavily populated areas often experience excessive amounts of chemical
pollutants in the air. These toxic gases damage horticultural plants. Acute amounts
of sulfur dioxide cause chlorosis and browning of leaves and sometimes necrosis (cell
destruction and death). Fluoride injury has been recorded in sensitive plants such as ponderosa
pine as reddish-brown bands that appear between necrotic and green tissue. Ozone
is a major pollutant that is produced primarily from the photochemical action of sunlight
on automobile emission. It can cause chlorosis and necrosis in a wide variety of plants.
Pesticide Application
Improper application of pesticides may cause collateral damage to cultivated plants.
Applying sprays on a windy day may cause the chemicals to drift onto desirable plants,
resulting in deformed leaves, discoloration, and in some cases death of tissue and possibly
the entire plant. Herbicide damage appears suddenly and may last through the cropping
season. Often, the symptom is bleaching; in severe cases, it may be followed by leaf
drop. Unlike the effect of herbicides, collateral damage from insecticides shows up as
browning of the foliage.
Fertilizer Application
Chemical fertilizers are frequently applied to houseplants or outdoor plants in production.
Eagerness for good yield may lead some growers to overfertilize their plants, resulting
in a buildup of excessive fertilizer in the soil. High amounts of salts create sodic
soil conditions. A higher salt-soil concentration than root fluids can cause dehydration
of roots. Instead of the roots absorbing soil moisture, they become depleted of moisture.
Plant growth is inhibited under such conditions, and plants wilt (as they would under
drought conditions) and eventually die.
182 Chapter 5 Plant Physiology
SUMMARY
The variety of activities that have been described to occur at various phases in plant
growth and development are the results of certain growth processes. These processes
provide the raw materials and the energy required for building new tissues and nurturing