A manual of rice seed health testing - IRRI books - International Rice ...

two-celled, besifixed, deeply
sagittate, laterally dehiscent, with
long, slender filaments.
—The gynoecium (female reproductive
organ, Fig. 2.5) is
monocarpellary, superior, ovoid,
unilocular with one sessile ovule,
and marginal placentation. There are
two short styles. Stigmata are bifid,
light yellow to amaranthus purple,
and feathery (plumose).
—The fruit is the caryopsis
(pericarp fused with testa). Most are
indehiscent.
—The ripened seed (Fig. 2.6) has
90% starch and 10% protein in the
endosperm. Radicle and plumule are
well-developed in the embryo. The
seed coat varies in color from light
brown to red, to purple. Usually the
seed (mature fruit plus lemma,
palea, rachilla, sterile lemmas, and
awn) is used to grow a crop. Brown
rice (grain) can be used as seed, but
it is more susceptible to microbial
attack due to the absence of the protective
hull (lemma, palea, etc.)
Classification of seeds
Rice seed is usually categorized as
follows:
Nucleus seed —genetically pure
seed available from the original
plant breeder and used for breeder's
seed production
Breeder's seed —seed directly produced
or controlled by the original
plant breeder
Foundation seed —seed directly
produced from the breeder's seed by
technically qualified staff
Certified seed —progeny of foundation
or certified seeds.
Rice ecosystems
Rice-growing environments vary
significantly in different localities
within and between countries. Consequently,
controversies have arisen
regarding acceptable definitions and
classification systems. Scientists and
specialists from different disciplines
and parts of the rice-growing world
2.6 The seed.
formed an international committee
to determine terminology for ricegrowing
ecosystems. Khush (1984)
described the results. Garrity (1984)
reviewed existing taxonomic treatments.
The ecosystem terminology
briefly described below is used in
subsequent chapters.
Terminologies and classifications
of the rice ecosystems are based on
water regime, drainage, temperature,
soil type, and geomorphology.
Rice-growing environments are
grouped in to five main categories.
The irrigated environment has
sufficient water available during the
entire growing season. Rainfall supplements
the water need. The irrigated
environment contributes almost
three-fourths of the total world
rice production. It has been divided
into three subcategories.
—The irrigated environment with
favorable temperature has an
18-38 °C air temperature during the
rice-growing season. This
subcategory constitutes the greater
portion of the irrigated ecosystem.
—The irrigated, low-temperuture,
tropical zone environment has air
temperatures below 18 °C during the
seedling stage. This affects plant
growth. Cold-tolerant and blast-resistant
varieties are best suited to
this ecosystem.
—The irrigated, low-temperature,
temperate zone environment has air
temperatures below 18 °C during the
seedling and flowering stages. Coldtolerant
japonica varieties are best
suited to this environment.
m The rainfed lowland environment
is the most variable rice ecosystem. It
comprises one-fourth of the world
rice area which is dependent on
amount and duration of rainfall,
depth and period of standing water,
frequency and time of flooding, and
soil type and topography. It is divided
into five subcategories.
—The rainfed, shallow, favorable
environment receives adequate water
from rainfall and experiences no
serious drought or flooding.
—The rainfed, shallow, droughtprone
environment may experience
drought or moisture stress at any of
the rice plant’s growth stages.
—The rainfed, shallow, droughtand
submergence-prone environment
experiences both submergence and
prolonged periods of drought.
—The rainfed, shallow, submergence-prone
environment experiences
prolonged rainfall, flooding,
and submergence for as many as
10 d/yr.
8 Rice seed health testing manual