Principles of Plant Genetics and Breeding

392 CHAPTER 21
be the desert, where irrigation may be used to dispense
water at desired rates.
Selection under managed stress environments
Most drought breeding research is conducted under
managed stress environments. A common facility for
such research is the rainout shelter. This is essentially a
mobile roof that protects selected plots from rain.
Selection based on yield per se
The concept of genotype × environment (G × E) is used
to evaluate genotypes at the end of a breeding program.
The desired genotype is one with minimal G × E interaction,
and stable yield in its target environment and
across other environments. Selection for yield under
stress is generally an inefficient approach. The generally
low heritability of yield becomes even lower under
stress. To get around this problem, some breeders resort
to the tedious and expensive approach of screening very
large populations. Some also use molecular markers to
tag and select certain yield-related quantitative trait loci
(QTLs) to aid in the selection process.
Selection based on developmental traits
Most breeders tend to select for drought resistance on
the basis of certain developmental traits, major ones
including root size and development, and stem reserve
utilization. Root studies are tedious to perform. The
most widely used technologies for detailed studies of
roots are the rhizotrons and lysimeters. These pieces
of equipment are available in various forms and provide
various kinds of information. The minirhizotron
even has a minute video camera included in the rig to
record roots as they appear on the external surface of the
tubes in which the plant is growing. Some researchers
grow plants in soil-filled tubes (polyethylene tubes) and
remove and wash away the soil, usually at flowering
time, to measure root length. To select by using stem
reserve utilization for grain filling, researchers completely
inhibit the photosynthetic source (e.g., spraying
plants with oxidizing chemicals such as potassium
iodide) and measuring grain filling without photosynthesis,
to compare with normal plants.
Selection based on assessment of plant water status
and plant function
Methods used in this category of selection approaches
include assessing stress symptoms (e.g., leaf rolling, leaf
desiccation, leaf tip burning). An infrared thermometer
is also used to measure canopy temperature, whereas
infrared photography is used to measure spectral reflection
from leaves. Selection based on plant function
includes the measurement of cell membrane stability
and chlorophyll fluorescence.
Breeding methods
In breeding for drought resistance, breeders may select
for early maturity for use in avoiding the stress. Also,
genotypes with proven drought resistance may be used
in hybridization programs to transfer the resistance
genes into superior cultivars. The selection methods
discussed and the various breeding methods previously
discussed are used in the selection process.
Cold stress
In the USA, the US Department of Agriculutre (USDA)
has developed a plant adaptation map (plant hardiness
zones) based on temperature requirements, to guide
in the selection of plants for use in various parts of
the country. Tropical and semitropical plants are cold
intolerant.
Overview of cold stress concepts
Plants use a variety of adaptive mechanisms to combat
environmental stress caused by cold temperature. Seed
dormancy is a physiological condition that delays seed
germination until the embryo has gone through an
after-ripening period, during which certain biochemical
and enzymatic processes occur for the seed to attain full
maturity. In many temperate zone trees and shrubs,
buds undergo a dormant stage, starting in late summer
or early fall, ending only when the buds have been
exposed to an extended period of cold or increasing day
length in spring. Most winter annuals and biennials have
a cold temperature requirement (vernalization) before
they will flower. When plants are exposed to gradually
decreasing temperatures below a certain threshold, they
acclimatize (low-temperature acclimation) to the stress,
a process called cold hardening.
In spite of various adaptations to cold, plants may
be injured through exposure to cold temperatures in a
variety of ways, depending on the temperature range.
One type of injury, called chilling injury, occurs at
exposure to temperatures between 20 and 0°C. Some
injuries are irreversible. Common chilling injuries include