Botanical Description of Rice

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

1. Germination
2. Early seedling growth and
3. Tillering

The reproductive phase is subdivided into

1. Stem elongation
2. Panicle initiation
3. Panicle development
4. Flowering
5. Milk grain
6. Dough grain and
7. Mature grain stage.
8. 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.

Vegetative 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

1. 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.
2. 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.
3. Spikelet anthesis (or flowering) begins with panicle exertion (heading), or on the following day.
4. Consequently, heading is considered as a synonym for anthesis in rice.
5. 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.
6. Agronomically, heading is usually defined as the time when 50% of the panicles have emerged.
7. Anthesis normally occurs between 1000 and 1300 h in tropical environments and fertilization is completed within 6 h.
8. Only very few spikelets have anthesis in the afternoon, usually when the temperature is low.
9. Within the same panicle it takes 7-10 days for all the spikelets to complete anthesis; the spikelets themselves complete anthesis with 5 days.
10. Ripening follows fertilization, and may be subdivided into milky, dough, yellow-ripe, and maturity stages.
11. These terms are primarily based on the texture and color of the growing grains.
12. 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:

1. Number of panicles/m2
2. Number of grains / panicle
3. Percentage of ripened grains.
4. 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.

 Seedlings

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.

 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 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.