Botany
The growth duration of the rice plant is 3-6 months,
depending on the variety and the environment under which it is grown.
During this time, rice completes two distinct growth phases
Vegetative and
Reproductive.
The vegetative
phase is subdivided into
Germination
Early seedling growth and
Tillering
The reproductive
phase is subdivided into
Stem elongation
Panicle initiation
Panicle development
Flowering
Milk grain
Dough grain and
Mature grain stage.
A
120-day variety, when planted in a tropical environment, spends about 60
days in the vegetative phase, 30 days in the reproductive phase, and 30
days in the ripening phase.
The
vegetative phase is characterized by active tillering, gradual increase in
plant height, and leaf emergence at regular intervals. Tillers that do not bear
panicles are called ineffective tillers. The number of ineffective tillers is a
closely examined trait in plant breeding since it is undesirable in irrigated
varieties, but it is sometimes an advantage in rainfed lowland varieties where
productive tillers or panicles may be lost due to unfavorable conditions.
The stages under
Vegetative phase includes:
1. Seedling
stage: From sowing to transplanting stage, root formation and leaf formation
takes place.
2. Active
vegetative stage: Growth to maximum tiller number.
3. During
this phase tiller number, height and straw weight increases.
4. The
duration of this phase is primarily a function of the amount of nitrogen
available to maintain the nitrogen content in the plant above the critical
load.
5. Tillering
is closely related to nitrogen content in the plant, especially soluble
nitrogen in the culture.
6. The
critical nitrogen content of the culm is said to be about 7 %.
7. Vegetative
lag phase: From maximum tiller number stage to panicle initiation stage.
8. The
tiller number decreases, increase in height and straw weight continues but less
rapidly than before.
9. Varietal
characteristics and climatic conditions, especially day length and temperature,
determine the duration from sowing to panicle initation.
10. If
total growth duration is short the reproduction phase and the active vegetative
phase overload.
Reproductive phase
The reproductive growth
phase is characterized by culm elongation (which increases plant height),
decline in tiller number, emergence of the the last leaf, booting,
heading, and flowering of the spikelets.
Panicle initiation is the
stage about 25 days before heading when the panicle has grown to about 1
mm long and can be recognized visually or under magnification following
stem dissection.
Spikelet anthesis (or
flowering) begins with panicle exertion (heading), or on the following
day.
Consequently, heading is
considered as a synonym for anthesis in rice.
It takes 10-14 days for a
rice crop to complete heading because there is variation in panicle
exertion among tillers of the same plant and among plants in the same
field.
Agronomically, heading is
usually defined as the time when 50% of the panicles have emerged.
Anthesis normally occurs
between 1000 and 1300 h in tropical environments and fertilization is
completed within 6 h.
Only very few spikelets
have anthesis in the afternoon, usually when the temperature is low.
Within the same panicle it
takes 7-10 days for all the spikelets to complete anthesis; the spikelets
themselves complete anthesis with 5 days.
Ripening follows
fertilization, and may be subdivided into milky, dough, yellow-ripe, and
maturity stages.
These terms are primarily
based on the texture and color of the growing grains.
The
length of ripening varies among varieties from about 15 to 40 days.
The grain yield of
rice is determined by the following four components:
Number of panicles/m2
Number of grains / panicle
Percentage of ripened
grains.
1000 gm weight.
Seeds
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 consists mainly 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 coleoptil(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 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 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 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 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 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 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 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 n 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 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.
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 auricles 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 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 then it leads definite drooping.
Tillering plants
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.