Principles of Plant Genetics and Breeding

482 CHAPTER 27
detectable during the first week following pollination.
At normal maturity, the tagged spikes are harvested and
threshed.
Common breeding objectives
The success of modern wheat cultivars is largely due to
high-yield potential, wide agroecological adaptation,
and high responses to agronomic inputs (fertilizers,
irrigation). Yield components of wheat are spikes ×
number of grains/spike × weight of grain (or number
of grains/unit area × weight of grain in that unit).
Breeders should determine what balance of these components
to include in a cultivar for an agroecological
niche. The right balance is determined taking into
account the photoperiod, heat units, and the moisture
and fertility status of the target area. For example, in one
study, a top-yielding wheat cultivar, “Seri 82”, produced
3,778 kg/ha in 98 days (maturity) at 22°12′N
but 8,544 kg/ha in 140 days at 30°53′N. It is not
possible to simultaneously select for all the yield components
because of the presence of negative intercorrelations
among them.
1 Grain yield. Breeding for high yield potential in
wheat can be accomplished by hybridizing highyielding
genotypes and selecting transgressive segregants
from the progeny with desired traits. However,
it is the discovery and use of dwarfing genes that
dramatically increased yield potential in wheat. Shortstatured
cultivars have high tillering capacity and also
increased grain yield per spike. Manipulation of
harvest index by incorporating semidwarf genes has
resulted in high lodging resistance, high biomass, and
high harvest index and consequently a high rate of
partitioning of assimilates into the grain for higher
grain yield.
2 Yield stability. Some breeders have used the concept
of shuttle breeding (selecting F 2 segregating populations
in one location and the F 3 in another, etc.) to
develop cultivars with wide adaptation and high yield
potential (e.g., cultivars such as “Siete Cerros” and
“Pavon 76” developed at CIMMYT). The capacity
to sustain high yield potential over a broad range
of growing environments is desirable in a cultivar.
Breeders conduct G × E evaluations of genotypes in
yield trials to identify those with yield stability.
3 Agromorphological traits. As previously discussed,
short stature and lodging resistance are important
breeding objectives in wheat breeding. Semidwarf
cultivars are lodging resistant. Selection for short
stature often impacts other plant characteristics. For
example, semidwarfs tend to be photoperiod insensitive,
and have reduced seed size and protein content.
4 Adaptation:
(a) Winter hardiness. Winter-hardy wheat cultivars
are needed in places where plants are likely to
be exposed to unseasonable low temperatures.
Regions with high rainfall tend to hold moisture
in the soil for a longer time. Under wind chill
temperatures and alternate freezing and thawing,
wheat plants grown in such soils are prone to
heaving. The red soft winter wheat types are
more resistant to heaving injury than the hard
red wheats. Selecting for winter hardiness should
be done under natural conditions.
(b) Drought resistance. Germplasm of the Crimean
origin has drought resistance and narrow leaves.
Similarly, durum wheats have drought resistance
and are adapted to the drier production
regions of North Africa and the Middle East. At
CIMMYT, breeders have used a shuttle breeding
approach in drought breeding. Generations F 2 ,
F 5 , and F 6 are tested under optimal conditions,
while F 3 and F 4 are evaluated at reduced fertility
and moisture conditions. The assumption is that
input efficiency and input responsiveness can be
incorporated into one genotype. Traits of interest
under optimal conditions are disease resistance,
good tillering capacity, head development, leaf
retention, and grain plumpness. Under low input,
breeders select for delayed leaf senescence, tiller
viability, grain plumpness, reduced spike sterility,
relative high yield, and relative higher pest
resistance.
(c) Aluminum (Al) tolerance. Tolerance to Al is
needed in cultivars grown for production regions
where the soils are acidic. Breeders may select
for Al tolerance under artificial conditions in the
lab. In fact, breeding efforts in Brazil produced
genotypes with high Al tolerance. However,
yield of those genotypes were poor. Breeders at
CIMMYT have improved upon the yield potential
of the Al-tolerant genotypes.
5 Disease resistance. The strategy of using resistance
genes singly in wheat breeding is not effective and
hence not widely practiced any longer. Rather, a
combination of multiple hypersensitive resistance
genes is preferred. Rust-resistance genes (Lr 9 , Lr 19 ,
Lr 24 ) and stem-resistance genes (Sr 24 , Sr 26 , Sr 31 ) once
deployed singly are now used in combination to
promote stability to stem rust in North America and
Australia. However, this strategy is effective only when
breeding is centrally coordinated in the country and