Principles of Plant Genetics and Breeding

while awnless is conditioned by an 1 an 2 an 3 . Photoperiod
sensitivity is controlled by one or two genes (Se 1 ,
Se 2 ), while one or two dominant genes control shattering.
Genes for host resistance to many diseases have
been identified, including bacterial blight (Xa-1, Xa-2,
etc.), blast (Pl 1 , Pl 2 , and others), yellow dwarf virus
(Ydv), and brown spot (He or he 1 ).
One of the important mutations in rice breeding is
the sd, the recessive gene that conditions semidwarf
stature. It was discovered in a Taiwanese cultivar “Deegeo-woo-gen”.
Hybrid rice using CMS entails crossing
a CMS line (rffr-S) with a restorer line (RfRf-F) to
obtain an F 1 (Rfrf-S). The “wild abortive” or the
“Chinsurah Bone II” are the preferred sources of CMS
for rice hybrids.
General botany
An annual grass, rice has erect culms that may reach
1.8 m in some varieties. It produces about five tillers.
Rice inflorescence is a loose terminal panicle consisting
of spikelets that are self-pollinated. The rice grain is
enclosed by the lemma and palea (constitutes the hull)
that may be straw yellow, red, brown, or black in color.
Depending on the variety, the lemmas may be fully
awned, partly awned, tip-awned, or awnless. Hulled kernels
vary in length from 3.5 to 8 mm, and are 1.7–3 mm
in breadth and 1.3–2.3 mm thick. Furthermore, the
kernels may be hard, semihard, or soft-textured. The
color of unmilled rice kernel is variable and may be
white, brown, amber, red, or purple; lighter colors
(white, light brown) tend to be preferred in the US.
Reproductive biology
Rice has a loose terminal panicle with branches that arise
singly or in whorls. It is predominantly self-pollinated
with less than 1% outcrossing. A panicle may contain
75–150 spikelets, or even several hundred in some varieties.
Rice has perfect flowers that are borne in singleflowered
spikelets. The flower consists of two lodicules,
six stamens (instead of three as in most cereals), and two
plumose stigmas on two styles, surrounded by floral
bracts. The floral bracts (lemma and palea) may be
straw, yellowish-gold, red, brown, or purplish in color.
Further, the lemma may be fully awned, partly awned,
tip-awned, or awnless.
Rice is predominantly self-pollinated. Time of anthesis
is significantly influenced by environmental factors,
BREEDING RICE 501
especially temperature, and to a lesser extent by genotype.
The entire panicle completes blooming in 4–7
days. Peak anther dehiscence occurs between mid to
late morning in tropical regions and at about noon in
temperate regions.
Common breeding methods
Being a self-pollinated species, rice can be improved by
any of the methods of breeding for self-pollinated
species. Field tests can be conducted to evaluate introductions
from various parts of the world to identify
adapted varieties for commercial production. International
breeding programs and non-profit organizations
may engage in breeding genotypes for use in developing
countries with few resources for embarking on elaborate
breeding programs.
Hybridization is the principal procedure used to generate
a segregating population for a breeding program
in rice. Depending on the breeding objective and the
underlying genetics of the trait of interest, breeders may
use any of the breeding methods for self-pollinated
species in rice breeding.
Commercial hybrid rice seed production was started
in the People’s Republic of China in the 1970s, where
hybrid varieties occupy more than 50% of the 75 million
acres of rice cultivated annually. The key factor in the
success of the commercial hybrid program was the discovery
of a source of CMS, called the “wild abortive”, in
a wild rice plant (O. sativa f. spontanea). Fertility restoring
genes were discovered in indica cultivars. Hybrid
rice production is practical in China largely because
of the low cost of labor. To enhance flowering, crosspollination,
and seed set, seed producers implement
practices such as cutting back the flag leaves of female
plants to enhance pollination.
Establishing a breeding nursery
Field nursery
Rice is a warm season species and is successful under
a mean temperature of about 20°C or higher. High
humidity encourages diseases and hence is undesirable.
Heavy-textured soil with impervious subsoil for holding
moisture is desirable for rice. The tolerable pH ranges
between 4.5 and 7.5. Rice is also a short-day plant.
Rice seeding is accomplished by using ground equipment
(broadcast seeders or grain drills). Rice is drilled to