Principles of Plant Genetics and Breeding

490 CHAPTER 28
for 1–3 hours. High temperatures of about 35°C may
kill the pollen grain. Further, once dispersed into the
atmosphere, the pollen grain may lose viability due to
rapid desiccation within a few minutes.
Pistillate flower
The pistillate flowers are borne on a cob that arises
from a husk formed at the sixth or seventh node on the
stem below the tassel. The female spikelet occurs in pairs
of one fertile ovary and one sterile ovary. This is the reason
for the even number of rows of kernels on a corn ear.
Occasionally, both ovaries in a pair become fertilized,
producing kernels, resulting in crowding and irregular
rows of kernels on the ear. A silk grows from the tip of
each ovary until it emerges at the tip of the ear husk.
Silks at the basal part of the ear usually emerge first. A
silk is structurally a stigma and style and hence is receptive
along its entire length. Temperature, soil moisture,
and soil fertility, affect the rate of silk emergence.
Adverse weather such as severe drought may delay or
cause complete cessation of silk emergence.
Receptivity of the stigma
Corn is generally a protandrous plant (the male spikelets
usually mature before the female spikelets). For the
same plant, pollen shed usually precedes silk emergence
by about 1–3 days. Silks are receptive soon after emergence
and remain receptive for up to about 10 days. For
optimum results, the emerged silk should be pollinated
within 3–5 days after first silk emergence. Fertilization
usually occurs within 12–24 hours of pollination. High
temperatures or low humidities adversely affect stigma
receptivity.
Genetic consequences of reproductive biology
1 Heterozygosity. Being predominantly open-pollinated,
natural populations of corn are highly heterozygous
and genetically variable. In theory, each kernel on the
ear could be produced from the fertilization of an
ovule by a different pollen parent. A field of naturally
pollinated corn in effect comprises a population of
hybrids.
2 Xenia. As previously discussed, xenia occurs in corn.
Common breeding methods
Three general approaches are used over the world
for corn improvement for cultivar development –
germplasm introduction, population improvement, and
hybrid breeding. These approaches are used to develop:
(i) open-pollinated cultivars; (ii) population improvement
cultivars; and (iii) hybrid cultivars. Both intrapopulation
and interpopulation improvements are used
in maize breeding.
Mass selection has been used by plant breeders to
modify corn characteristics including height, maturity,
ear characteristics, and grain yield. The method was
used to develop cultivars for new production areas in the
Corn Belt. Modified mass selection was used to achieve
a 2.9% per cycle average yield gain. Mass selection in
corn breeding in the US is now practically non-existent.
However, the method continues to be used in developing
countries by farmers and plant breeders to produce
open-pollinated cultivars.
Other intrapopulation breeding methods used in corn
breeding include ear-to-row (half-sib family), modified
ear-to-row, half-sib family, full-sib family, modified fullsib
family, and selfed family, with varying degrees of
success. Genetic gains averaging 3.8% per cycle were
recorded using modified ear-to-row, while a selfed family
selection average of 6.4% per cycle for grain yield was
recorded. Various interpopulation methods (recurrent
selection, both half and full sib) have been used.
Development of inbred lines in hybrid corn
production
Sources of inbreds for corn hybrid breeding have
changed over the years. Before 1930, corn inbreds were
isolated from open-pollinated varieties. Later, breeders
used inbreds derived from single crosses, modified single
crosses, and three-way crosses. Sometimes, backcrossing
is used to enhance any inbred line in a specific way by
introducing a gene to correct a deficiency. However,
these inbreds were less effective for improving quantitative
traits. To overcome this weakness, breeders resorted
to developing inbred lines from recurrent selection populations
that have been improved for specific quantitative
traits (e.g., grain yield, stalk quality, disease resistance).
Superior inbred lines are also produced by crossing
other inbred lines with superior complementary traits
and then selecting from the progeny.
Inbreds are developed by artificially controlled pollination.
The F1 plants of a cross are hand-pollinated, followed
by pedigree selection through 5–7 generations,
or until a desirable level of uniformity in appearance
and performance is achieved. The inbred lines are evaluated
for combining ability. Single-cross hybrid corn
is most commonly used in commercial production.