Principles of Plant Genetics and Breeding

500 CHAPTER 29
stiff, lodging-resistant stalks, making them more responsive
to heavy fertilization. The longer grained varieties,
called indica types, have taller, weaker stems that
lodge under heavy fertilization. The US produces
mainly indica rice (about 65% of the annual production)
primarily in Arkansas, Mississippi, Louisiana, and Texas,
while California produces mostly medium- and shortgrained
rice.
Rice may also be classified in terms of maturity – early
maturing (about 120–129 days), midseason (about
130–139 days), or late maturing (about 140 days or
more). Rice may be scented (aromatic) or unscented,
the two most common scented types being basmati
and jasmine. Basmati rice has a distinctive odor, doubles
in grain size upon cooking, and is non-sticky (grains
remain separate). It is cultivated mainly in the Punjab
area of central Pakistan and northern India. Jasmine rice
is grown mainly in Thailand and is more preferred by the
Asian community in the US. Cooked jasmine rice is soft,
moist, and sticky. The stickiness derives from the types
of starch in the grain. The endosperm starch of rice may
be glutinous or commonly non-glutinous (non-sticky).
The glutinous property is conferred by amylopectintype
starch.
Germplasm resources
Over 85,000 rice accessions are maintained at IRRI in
the Philippines, representing the largest rice germplasm
collection in the world. The Africa Rice Center (ARC)
in the Ivory Coast maintains large numbers of accessions,
especially O. glaberrima genotypes. The US
Department of Agriculture (USDA) maintains about
16,000 accessions of rice, including a large number of
breeding lines. Genetic testers, developed mostly by
Japanese researchers, are available at the Rice Genetics
Cooperative.
Cytogenetics
Twenty species of the genus Oryza are known, with a
basic chromosome number of 12. The genus has six
genome groups: A, B, C, D, E, and F. The cultivated
species O. sativa (2n = 2x = 24) has the genome formula
AA, whereas O. glaberrima (2n = 2x = 24) is designated
A g A g because it does not pair well with O. sativa. There
are two major species of the cultivated rice – O. sativa
and O. glaberrima – the latter being native to Africa
and cultivated in West Africa and Central Africa. Other
wild species are O. alta (2n = 2x = 48; CCDD), O.
australiensis (2n = 2x = 24; EE), O. longistaminata
(2n = 2x = 48; CCDD), O. brachyantha (2n = 2x = 24;
FF), and O. punctata (2n = 2x = 48; BBCC).
The 22 species are divided into diploids (2n = 2x =
24), e.g., O. australiensis, O. barthii, O. glaberrima, and
O. sativa, and tetraploids (2n = 2x = 48), e.g., O. alta,
O. puncata, and O. grandiglamis. Rice chromosomes
have a tendency to form secondary associations during
meiosis of normal diploids. Asynaptic plants occur in
nature, as well as haploids, triploids, and tetraploids.
They may be induced artificially. Primary trisomics have
also been developed by various researchers for rice. Also,
chromosome maps have been developed by different
workers.
Crossing between plants from different ecogeographic
races, or even within a race, may exhibit some hybrid
sterility, inviability, or weakness. In practice, plant
breeders are able to overcome these sterility problems
through rigorous selection to obtain fertile lines. Wild
germplasm has proved a useful genetic resource for rice
breeding. The wild abortive cytoplasmic male-sterility
(CMS) cytoplasm that is commercially used was obtained
from O. sativa f. spontanea.
Genetics
Rice genetics is quite extensive. Color development in
rice is complex. Anthocynin pigmentation is controlled
by a complementary genetic system, the CAP gene
system. C is the basic gene for chromogen production,
while A controls the conversion of chromogen to
anthocynin. P, which is variable, determines the site of
pigment expression (e.g., Pg, Pm, Ps, Px, etc., according
to the site). The CAP gene system is affected by modifying
genes, and occasionally an inhibitor gene in some
species. Also, multiple alleles and various dominance
levels are associated with C, A, and Pl. Color development
is also affected by light intensity, growth stage,
and fading and leaching. Color of the different layers
of seed coat may be affected by different genes or sets
of genes. A smooth hull is desirable for mechanical
harvesting and processing. Pubescence on the surface of
the blades and hull is controlled by a dominant gene,
glabrous (gl).
Semidwarfism is desirable in rice and is controlled by a
pair of recessive alleles, sd1. However, studies indicate
that semidwarfism is a complex, quantitative trait. One
three-gene unit with cumulative effect controls awning
in rice, fully awned being An 1 An 2 An 3 (or An 1 An 2 an 3 ),