Principles of Plant Genetics and Breeding

chambers may be used for experimental purposes, but
they are not amenable to practical breeding of corn.
Breeders in cooler climates may use winter nurseries for
additional breeding work.
Special environment
No special environmental conditions are needed if
the breeding nursery is established in an area adapted
for corn production. Temperature, soil moisture, and
humidity are critical for proper flowering and pollination
of corn. Temperatures above 30°C and low humidity
adversely impact pollen viability. Similarly, drought
causes delay in silk emergence relative to tassel development.
It is important to have a source of supplemental
irrigation to provide moisture at critical times to avoid
tassel firing and male flower abortion.
Artificial pollination
Materials and equipment
Crossing corn requires the use of simple equipment,
including a knife and stapler. Materials include tassel
bags, ear shoot bags, paper clips, a pencil, tags, and a
carrying case or apron with pockets. The bags should be
water repellent.
Emasculation
Corn is a monoecious plant. The female flower should
be covered and protected from unwanted pollen. The
ear shoot should be covered with an ear bag before the
silks emerge from the husk tips. Once covered, artificial
pollination should be undertaken within about 3 days,
otherwise the silks should be trimmed to prevent them
from growing out of the bag and becoming contaminated.
Further, trimming the silks to within 2 cm of the
husk tip is usually done the day before pollination.
Pollination
Where controlled pollination is desired, the tassel is also
covered with a tassel bag and held securely in place
by stapling or using paper clips at least 1 day before pollination.
This prevents contamination from unwanted
sources, and also saves desirable pollen that would
otherwise have been lost to the wind. Copious amounts
of pollen are available on the second or third day following
pollen dehiscence.
BREEDING CORN 495
When selfing, the pollen may be taken directly from
the tassel and deposited on the silk. In cross-pollination,
the pollen may be collected in a bag by shaking the tassel
and dusting the pollen on the ears. A pollen gun may
also be used for multiple pollinations.
The optimum time for pollination depends on temperature
and humidity, but is usually about 3 hours after
sunrise. The ear bag should be replaced immediately after
pollination to avoid contamination. However, under
some conditions, covering the ear shoot for a long time
may cause ear tip rot. Pollination information, including
parents involved in the cross and date of pollination, is
written on the envelopes.
Natural pollination
Controlled artificial pollination (and sometimes natural
pollination) is used to develop commercial hybrid cultivars.
Once developed, hybrid seed (or synthetic seed)
production is conducted in the field using the wind as
the agent of pollination. To maintain the genetic purity
of the new cultivar, and to meet certification standards
for corn, seed production must be conducted in isolation
from other corn cultivars to keep contamination
below the mandated 1%. It is also mandated that an
isolation block for seed increase be located no less than
200 m from corn of a different color or texture.
Common breeding objectives
The breeding objectives presented below are not in
order of importance.
1 Grain yield. Grain yield of corn is a major breeding
objective for this crop, which is among the major
crops that feed the world. It is estimated that corn
yield in the US has increased about 340% between
1940 and 1990. This increase is attributed to the
development of cultivars with high yield potential.
Over the period indicated, producers switched from
open-pollinated cultivars to higher yielding cultivars.
Yield is a complex trait and depends on the genes that
are associated with basic physiological processes, plant
structure, and morphology. Yield components of
corn include number of ears, kernel rows and kernels
per row, kernel test weight, and shelling percentage.
Days to maturity, standability, and resistance to
environmental stress (biotic and abiotic) also affect
grain yield of corn. Features of modern corn hybrid
cultivars compared to older cultivars were summarized