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 just below the soil surface. After the coleoptile
emerges it splits and the primary leaf develops.
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 referred
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
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 than vertically, the plant therefore
draws its nutrients from nearby surface of the soil.
Root development is influenced by soil texture, cultivation, water
and air in the soil, the amount of available food supply and by the
system of transplanting.
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 vary 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 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
n the main axis, the number of leaves on the tiller is correspondingly
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 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 its maximum length. The
sheath splits 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 sh21eath, 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.
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.
The articles are situated at the junction of the
Sheath and blade and are sickle shapped. Long slender
teeth are normally present on the convex face of each ligule colour
if present, is always assoicated with colour in the pulvinus. If the
auricles are coloured, so also is the sheath, but the converse is
not true. Some strains of paddy are devoid of auricles.
Rice plants have both auricles and ligules which make a distinguishing
character of rice to differentiate from Echinochloa spp., (a most common
weed in rice fields).
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 then it leads definite
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
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.
Ikisan - Morhoplogy of Rice - Panicles and spikelets
Panicle and spikelets
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 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.