Principles of Plant Genetics and Breeding

Designated ms 1 , ms 2 ,...ms n , over 20 of these genes
are known. Sterility controlled by nuclear genes is also
known in corn, even though it is not of practical application
in hybrid corn production. In corn, two main
sources of male sterility, controlled by the cytoplasm
known as the T (Texas) and S systems, are known.
Though deemed superior to the S, the T system, once
most widely used, fell out of favor when overuse predisposed
maize production in the US to maize leaf blight
in 1920, leading to devastation of the industry. CMScontaining
stock is used as seed parents to eliminate the
cost of detasseling in commercial hybrid seed production.
In addition to sterility genes, fertility-restoration
(RF n ) genes are used in corn. Some seed corn companies
use mechanical detasselling for hybrid seed
production.
Plant height
Studies have been conducted on the brachytic 2 (br2)
gene. This gene drastically reduces plant height.
However, its practical exploitation in breeding maize
has been limited because of the undesirable effects associated
with its use (e.g., reduced broad leaves, delayed
maturity, thick stem).
Restriction fragment length polymorphism (RFLP)
probes capable of detecting more than 500 polymorphic
loci have been developed in maize. A maize RFLP map
has been generated.
General botany
Corn is a monoecious annual and one of the largest of
the cereals, capable of reaching 4.5 m in height. The
male flowers (staminate) occur in the terminal panicle or
tassel at the top of the stalk, while the female inflorescence
(pistillate) is borne in the axils of leaves as clusters,
called a cob, at a joint of the stalk. Long silks (long
styles) hang from the husk of each cob. These pollen
tubes are the longest known in the plant kingdom. As
pollen receptors, each silk must be individually pollinated
in order to produce a fruit or kernel. A fertilized
cob (also called an ear) may contain eight or more rows
of kernels. Furthermore, a stalk may bear 1–3 cobs.
Corn has a variety of morphological features. Some
early maturing types maturing in 50 days may attain a
height of 0.6 m and produce 8–9 leaves, while tall late
maturing types (330 days) may attain a culm or stalk
height of 6 m and bear 42–44 leaves. The hybrid corn
varieties grow in the northern USA attain a height of
BREEDING CORN 489
0.9–2.5 m, bear 9–18 leaves, and mature within 90–120
days. The central Corn Belt hybrids varieties range
between 2.5 and 3 m in height, bear 18–21 leaves, and
mature in 130–150 days. The varieties used on the
Gulf coast and South Atlantic regions are much taller
(3–3.6 m), produce more leaves (22–27), and tiller profusely,
maturing late (170–190 days).
Corn has both seminal and adventitious roots. The
seminal roots may number 3–5 and grow downward at
the time of seed germination. The crown or coronal
roots arise from the nodes of the stem, about 25–50
mm below the soil surface, and may number between 15
and 20 times as many as the seminal roots. The aerial
roots (buttress, prop, or brace roots) arise at nodes on
the stem above ground.
The number of rows of grain is variable among
varieties, ranging between eight and 28. Each row may
contain between 20 and 70 kernels. Most of the corn
varieties grown in the US contain 14, 10, 12, or 18 rows
of kernels, and average about 500 kernels per ear.
Reproductive biology
Floral biology
Each spikelet consists of two staminate flowers. Each
flower has three anthers that are pushed out of the
spikelet as the filament elongates at anthesis. The exsertion
of the anthers is followed by opening of these
structures to shed the pollen. Complete pollen shed
may occur in just a few minutes, or over a longer period.
A tassel may shed all of its pollen in 1 day or even over
a period of about 1 week. The shed pollen pattern is
dependent on the genotype and environmental factors
such as temperature, humidity, and air movement. Corn
produces pollen grains profusely. A normal plant tassel
produces an estimated 25,000 pollen grains per kernel
on each ear of corn.
Pollination
Pollen
Corn pollen is primarily dispersed by wind. Consequently,
corn is about 95% cross-pollinated, most of
the effective pollination of an ear originating from
sources in the immediate vicinity of the ear. Pollen dispersal
is favored by warm temperature and low humidity.
Under such conditions, pollen dispersal may start at
about 3 hours after sunrise in the US Corn Belt and last