Principles of Plant Genetics and Breeding

390 CHAPTER 21
performance under, drought stress. Major plant traits
that play a role in plant drought-resistance response
include phenology, development and size, root, plant
surface, non-senescence, stem reserve utilization, photosynthetic
systems, and water use efficiency.
Phenology
Phenology is the study of the times of recurring natural
phenomena, especially as it relates to climate. Botanical
phenology is the study of timing of vegetative activities,
flowering, fruiting, and their relationship to environmental
factors. For the plant breeder, developing a
cultivar with short growth duration (i.e., early flowering)
may help the cultivar escape drought that may
occur in the late season of crop production. However, it
is known that a longer growth duration is associated
with high crop yield. Hence, using earliness for drought
escape may reduce crop yield. Optimizing phenology
is easier when there is a predictable environment. If
the environment is highly unpredictable, using an early
flowering indeterminate cultivar for crop production may
be disadvantageous because stress would be unavoidable.
Under such conditions, cultivars that are either
determinate or indeterminate but have longer growth
duration (late flowering) may have a chance to rebound
after a drought episode to resume growth. However,
late maturing cultivars may face late season stress from
disease and frost.
Research indicates a genetic linkage between growth
duration and leaf number, and often leaf size. Consequently,
early maturing genotypes have a small leaf
area index (LAI) and reduced evapotranspiration over
most of the growth stages. Corn breeders make use of
a phenological trait called anthesis-to-silk interval
(ASI). A short ASI is desirable, whereas a longer ASI
results in poor pollination. Crop plants differ in their
phenological response to drought stress, some (e.g.,
wheat) advancing flowering, whereas others (e.g., rice)
delay flowering.
Plant development and size
Water use by plants is significantly influenced by single
plant leaf area or LAI. Genotypes with small size and
reduced leaf area are generally conducive to low productivity,
while they limit water use. These genotypes may
resist drought but their growth rate and biomass accumulation
are severely slowed. Modern plant breeders
tend to select for genotypes with moderate productivity
and moderate size for use in dryland production.
Plant root characteristics
Root characteristics are critical to ideotype development
for combating drought. The most important control
of plant water status is at the root. The primary root
characteristics of importance are root depth and root
length density, of which root depth is more important
to breeding drought resistance. In cereals where tillering
occurs, deeper root extension occurs when fewer
tillers are produced. Unfortunately, many droughtprone
regions of the world have shallow soils. Under
such conditions, breeders may focus on other factors
such as shoot developmental characteristics and osmotic
adjustment. Also, development of lateral roots at very
shallow soil depth may be advantageous in capturing the
small amounts of rainfall that occur.
Plant surface
Plants interact with the environment through surface
structures. The form and composition of such structures
determine the nature of the interactions. Dehydration
avoidance is positively correlated with yield under stress.
The reflective properties of leaves and resistance to
transpiration depend on plant surface structures. The
stomatal activity primarily determines the resistance of
plant leaves to transpiration, but cuticular properties
(e.g., wax load) also play a role. In sorghum, genotypes
with lower epicuticular wax load (bm) had greater total
leaf transpiration than genotypes with higher wax load
(Bm). Another leaf surface feature with implication in
drought resistance is pubescence. In soybean, high leaf
pubescence genotypes had higher water use efficiency
stemming from lower net radiation and transpiration
with sustained photosynthesis. In wheat and barley, the
role of leaf color in drought resistance has been noted.
Yellow-leaf cultivars (with about one-third less leaf
chlorophyll) tend to perform better than cultivars with
normal green color, under drought stress.
Non-senescence
Environmental factors such as drought, heat, and nitrogen
deficiency are known to accelerate plant senescence.
Certain genotypes, called “non-senescence”, “delayed
senescence”, or “stay-green”, have been identified to
have the capacity to delay or slow senescence. These
genotypes have high chlorophyll content and leaf
reflectance. The expression of stay-green properties
varies among species. In sorghum, the condition is
better expressed when plants have been exposed to