The rice grain, commonly called as seed, consists of the true fruit
or brown rice (caryopsis) and the hull, which encloses the brown rice
Brown rice mainly consists of the embryo and endosperm. The surface
contains several thin layers of differentiated tissues that enclose
the embryo and endosperm.
The palea, lemmas, and rachilla constitute the hull of Indica rices,
In Japonica rices, however, the hull usually includes rudimentary glumes
and perhaps a portion of the pedicel.
A single grain weighs about 10-45 mg at 0% moisture content. Grain
length, width, and thickness vary widely among varieties. Hull weight
averages about 20% of total grain weight.
Germination and seedling
development start when seed dormancy has been broken and the seed
absorbs adequate water and is exposed to a temperature ranging from
about 10 to 40oC.
The physiological
definition of germination is usually the time when the radicle or
coleoptile (embryonic shoot) emerges from the ruptured seed coat.
Under aerated conditions the seminal root is the first to emerge
through the coleorhiza from the embryo, and this is followed by the
coleoptile.
Under anaerobic conditions, however, the coleoptile is the first
to emerge, with the roots developing when the coleoptile has reached
the aerated regions of the environment.
If the seed develops
in the dark as and when seeds are sown beneath the soil surface,
a short stem (mesocotyl) develops, which lifts the crown of the
plant to just below the soil surface. After the coleoptile emerges
it splits and the primary leaf develops.
The root
The rice root system
consists of two major types: crown roots (including mat roots)
and nodal roots. In fact both these roots develop from nodes,
but crown roots develop from nodes below the soil surface.
Roots that develop
from nodes above the soil surface usually are referred to as nodal
roots.
Nodal roots are
often found in rice cultivars growing at water depths above 80
cm.
Most rice varieties
reach a maximum depth of 1m or deeper in soft upland soils. In
flooded soils, however, rice roots seldom exceed a depth of 40
cm.
That is largely
a consequence of limited O2 diffusion through the gas spaces of
roots (aerenchyma) to supply the growing root tips.
The rice plant
is an annual grass with round, hallow, jointed culms, rather flat
leaves and a terminal panicle. It has fibrous roots which consists
of rootlets and root hairs.
The seedlings first
have the embryonic roots and later the adventitious roots that
are produced from the underground nodes of the young culms.
On germination
primary root develops from the base of the grain, quickly followed
by two additional roots, all subsequently giving rise to short
lateral roots.
The main rooting
system of the plant, however develops from the nodes of the stem
below ground level. In the "floating rices", whorls of adventitious
roots are formed from the first three very short nodes, giving
rise to whorls of permanent adventitious roots.
Tillers are produced
at the nodes and adventitious roots are produced from lower nodes
of these culms, so that the plant quickly develops a mass of adventitious
roots.
Under normal conditions
the root system is fairly compact, the roots tending to develop
horizontally rather then vertically, the plant therefore, draws
its nutrients from near the surface of the soil.
Root development
in influenced by soil texture, cultivation, water and air in the
soil, the amount of available food supply and by the system of
transplanting.
The Stem
The main axis of
the stem is differentiated from the growing point of the embryo,
enclosed at first by the coleoptile.
The ultimate height
of the stem depends on the number of internodes and environmental
conditions.
Early strains of
short maturation period have lower internodes than those with
a long maturation period.
The number of internodes
may very from about ten to twenty.
The culm is more
or less erect, cylindrical, and hollow except at the nodes, and
varies in thickness from about 6-8 mm.
Nodes are clearly
defined by the presence of a distinct thickening, the pulvinus,
immediately above the node. The pulvinus may be coloured, varying
in intensity from a "touch" of purple to a deep uniform purple.
The colour of the
pulvinus is always associated with colours in the leaf sheath.
The internodes may be green coloured.
The pigment in
the coloured form may be different in the epidermis or in the
parenchyma or confined to the bundle sheaths.
A bud may form
in the axil of each leaf of the main stem, but normally only the
lowermost bud from the crowded nodes at ground level develop into
branches, thus a typical tillered plant develops.
The Leaf
The number of leaves
borne on an axis is equal to the number of nodes since the number
of nodes on the tillers is progressively more than on the main
axis, the number of leaves on the tiller is correspondingly lower.
The first leaf
of the plant is the sheathing leaf or coleoptile. The second leaf
emerging through the lateral sheath of the coleoptile is reduced
in size and has practically no blade.
The remaining leaves
are normal, except the uppermost or "flag" which is slightly modified.
The bud of potential tiller is enclosed in the sheath. The normal
vegetative leaf has sheath, auricles and blade.
The leaves are
born at an angle of every node and they possess two parts viz.,
blade or expanded parts and the leaf sheath which wraps the culms.
The Sheath
The sheath is always
present as a whole or part of the internode from the pulvinus
upwards. As the base of the sheath tend to exceed the length of
the internode and consequently enwrap the base of the succeeding
sheath to a variable extent, from the tenth leaf upwards.However,
the internodes are longer and the sheaths are relatively and progressively
shorter than the internodal length.
Growth of the sheath
is mainly from the base and may continue after the blade has attained
about its maximum length. The sheath is split at the base, is
finely ribbed, and is more or less glabrous.
Colour, if any
may be confirmed to the base, or may be distributed through out
the sheath, and may be visible either on the outer surface or
on the inner surface, or both.
The pigment occurs
in the epidermal cells, in tissues surrounding the bundles, or
distributed throughout the ligule.
Ligule
The ligule is present
in all most all varieties of paddy and its membranous, and tends
to split as it develops. The ligule may be colour less or coloured
, a faint pink or purple. A coloured ligule is always associated
with colour in the sheath.
Auricles
The articles are
situated at the junction of the hseath and blade and are sickle
shapped. Long slender teeth are normally present on the convex
face of each ligule colour if present, is allways assoicated with
colour in the pulvinus. If the auricles are coloured, so also
is the sheath, but the converse is not true. Some straines & paddy
are devoid auricles.
Rice plants have
both auricles and ligules which make a distinguishing character
of rice to differentiate from Echinichloa
spp. , (a most common weed in rice fields).
The Blade
The leaves are
long and narrow, usually pubescent or hispid, with a distinct
midrib, but varying considerably in length. The leaves of many
varieties are coloured, the colour being usually concentrated
in the midrib region and on the margins, though occasionally the
whole leaf is coloured.
The uppermost leaf
or "flag" of the axis posseses a blade always shorter and broader
than the lower leaves. As the panicle emerges from the sheath,
its blade is nearly parallel to the panicle axis. After the panicle
has emerged the blade falls.Ultimately the panicle is either at
an acute angle to the axis, more or less horizontal there to or
definitely drooping.
Each stem of rice is
made up of a series of nodes and internodes. The internodes vary in
length depending on variety and environmental conditions, but generally
increase from the lower to upper part of the stem. Each upper node
bears a leaf and a bud, which can grow into a tiller.
The number of nodes
varies from 13 to 16 with only the upper 4 or 5 separated by long
internodes. Under rapid increase in water level some deepwater rice
varieties can also increase the lower internode lengths by over 30
cm each.
The leaf blade is attached
at the node by the leaf sheath, which encircles the stem. Where the
leaf blade and the leaf sheath meet is a pair of claw like appendages,
called the auricle, which encircle the stem. Coarse hairs cover the
surface for the auricle.
Immediately above the
auricle is a thin, upright membrane called the ligule. The tillering
stage starts as soon as the seedling is self supporting and generally
finishes at panicle initiation.
Tillering usually begins
with the emergence of the first tiller when seedlings have five leaves.
This first tiller develops between the main stem and second leaf from
the base of the plant.
Subsequently when the
6th leaf emerges the second tiller develops between the main stem
and the 3rd leaf from the base.
Tillers growing from
the main stem are called primary tillers. These may generate secondary
tillers, which may in turn generate tertiary tillers.
These are produced
in asynchronous manner. Although the tillers remain attached to the
plant, at later stages they are independent because they produce their
own roots. Varieties and including spacing, light, nutrient supply,
and cultural practices.
The major structures
of the panicle are the base, axis, primary and secondary branches,
pedicel, rudimentary glumes, and the spikelets.
The panicle axis extends
from the panicle base to the apex; it has 8-10 nodes at 8 to 4 cm
interval from which primary branches develop.
Secondary branches
develop from the primary branches. Pedicels develop from the nodes
of the primary and secondary branches; the spikelets are positioned
above them. Since rice has only one fully developed floret (flower)
per spikelet, these terms are often used interchangeably.
The inflorescence is
a spikelet borne on a long peduncle. The rachis bears branches, either
single or in pairs, from which arise the spikelets. The number of
spikelets on panicle varies considerably with the variety from a few
to some hundreds.
The spikelet, or flower
is borne on a short stalk, the pedicel. There are two short, rudimentary,
bristle like outer glumes , and the flower is enclosed in two inner
glumes of boat shaped.Flowering glumes or lemma are provided with
fine nerves and the palea is similar in size and texture to the lemma
but is three nerved. Both the lemma and palea may be awned or awnless.
The flower consists
of two small, oval, thick, and fleshing bodies, the lodicules situated
at the base of the axis. Unlike other cereals, paddy has six well
developed and functioning stamens provided with short filaments and
a pistil with a plumose stigma.
The stigma is some
what longer than broad, smooth and bears two styles and sometimes
a short, rudimentary third. The three some times growing together
at the base.
Morphology
