Plants frequently encounter stresses,
external conditions that adversely affect growth,
development, or productivity. Stresses can be
biotic, imposed by other organisms (Chapter
21), or abiotic, arising from an excess
or deficit in the physical or chemical environment.
Among the environmental conditions that cause
damage are water-logging, drought, high or low
temperatures, excessive soil salinity, inadequate
mineral nutrients in the soil, and too much or
too little light. Phytotoxic compounds such as
ozone also can damage plant tissues. Resistance
or sensitivity to the stress depends on the species,
the genotype, and the developmental age of the
a wide range of plant responses, from altered
gene expression and cellular metabolism to changes
in growth rates and crop yields. The duration,
severity, and rate at which a stress is imposed
all influence how a plant responds. Several adverse
conditions in combination may elicit a response
different from that for a single type of stress.
Features of the plant, including organ or tissue
identity, developmental age, and genotype, also
influence plant responses to stress (Fig. 22.1).
A response may be triggered directly by a stress,
such as drought, or may result from a stress-induced
injury, such as loss of membrane integrity. Some
responses clearly enable a plant to acclimate
to stress, whereas the functional role of others
is not apparent. Therefore, identifying which
responses promote or maintain plant growth and
development during stress is important for understanding
the stress response process.