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|DESCRIPTION: Cultivated rice is an annual grass. Depending on the degree of photosensitivity its growth duration may range from 60 to more than 200 days. It has round; jointed culm with long narrow leaves. The culm terminates in an inflorescence called a panicle. The height of the culm may vary from 20 cm to over 5 m in some deep-water rice exhibiting excellent internode elongation ability.
Three types of roots are recognized in rice e.g. seminal roots, mesocotyl roots and nodal or adventitious roots. Seminal roots grow out of radicle and are temporary in nature. The radicle emerges after germination. It elongates to about 15 cm and remains functional up to seventh leaf stage. Mesocotyl roots emerge from the axis between the node of the coleoptile and base of radicle. Mesocotyl roots do not grow under normal circumstances. However, under deep seeding conditions emergence of mesocotyl roots are promoted. The rice root system is essentially adventitious, fibrous and arises from the compressed basal nodes. Roots that develop directly from the nodes are called primary roots. Primary roots produce branched secondary roots, which in turn give rise to tertiary roots and so on. The roots, which develop from nodes below the soil surface, are called crown roots and roots that develop from nodes above the soil surface are referred to as nodal roots. Nodal roots are often found in rice cultivars growing at water depth above 80 cm. Most rice varieties reach a maximum depth of 1 m or deeper in soft upland soils. In flooded soils, however rice roots seldom exceed a depth of 40 cm. Aerobic root environments, as in case of upland rice, promote development of root hairs whereas anaerobic condition as in flooded rice, retard the development of root hairs.
The rice stem is known as culm. The culm is made up of a series of nodes and internodes. The node bears a leaf and in its axil between the sheath pulvinus and nodal septum a bud which may grow into a tiller. The region between two nodes is called an internode. Only a few internodes elongate. The number of internodes, which elongate ranges from 3-8. An internode whose length exceeds 5mm is usually considered as elongated. The internodes of a culm vary in length usually increasing from the lower to the upper ones. The lower internodes are compressed into a solid section. The number of nodes varies from 13 to 16 with only the upper 4 or 5 separated by long inter-nodes. Under rapid increases in water level, some deep-water rice varieties the lower inter-node can also elongate in length by over 30 cm each. The plant height is an inherited character. The plant height of the modern high yielding varieties may vary from 90 to 110 cm. with the dwarfing gene as compared to traditional tall varieties where the plant height may vary from 120 to 150 or more. The plant height in deep-water varieties with inter-node elongation ability may reach 5 m. under certain situation.
A branch of the plant bearing the root, culm leaves and with or without a panicle is called a tiller. Tillers develop from leaf axils at each un-elongated node of the main shoot or from tillers during vegetative growth in an alternate order. On the culm the coleoptile and the first leaf do not produce tillers. Tillering starts from the second leaf when the fifth leaf on the main culm emerges. The primary tillers originate from the lower most nodes and give to secondary tillers, which in turn give rise to tertiary tillers. Tillering ability is a varietal character. The tall traditional varieties are usually low tillering and spreading while the modern high yielding varieties (HYV) have high tillering ability and are compact. In high tillering varieties all the tillers are not productive. Numerous environmental factors also affect tillering ability such as spacing, light, nutrient supply and cultural practices.
The leaves arise from the node and are borne at an angle in an alternate phyllotaxy. Varieties differ in leaf number. A typical rice leaf consists of leaf sheath, auricle, ligule, leaf lamina or blade. The leaf sheath is continuous with the leaf blade. It encloses the culm above the node in varying length, form and tightness. The swelling at the base the leaf sheath is called pulvinus. It is above the nodal septum. The expanded portion of the leaf is called the leaf lamina or leaf blade. It has parallel venation. The prominent ridge on the lower surface is called the mid-rib. At the junction of the leaf blade and leaf sheath on the inside there are two triangular and membranous structures called ligules. On either side, there is a pair of hairy sickle shaped auricles. The white band around the junction between the blade and sheath is called the collar. The first rudimentary leaf at the base of the main culm is bladeless and two keeled bract. It is called the prophyllum. The margins of the prophyllum clasp the young tiller. Prophyllum is also present between each secondary and it's tertiary tiller. The upper most leaf below the panicle is called the flag or boot leaf. The traditional tall varieties have long droopy leaves, whereas the semi-dwarf modern HYVs have short, small and erect leaves.
The floral organs of rice plants are modified shoots. The terminal internode bears the determinate inflorescence; called the panicle that bears spikelets. Panicle: The panicle consists of the panicle base, axis, primary and secondary branches, pedicel, rudimentary glumes and spikelets. The panicle axis extends from the panicle base to the apex interspersed with 8-10 nodes at two to 4 cm apart. Panicle branching is racemose. Primary branches develop from the nodes. Secondary branches develop from the primary branches. Pedicels develop from the nodes of primary and secondary branches, which bear the spikelets. The spikelet of rice consists of three flowers, two of which are reduced, in development. The enlarged cup like apex of the pedicle is homologous with a pair of glumes and called rudimentary glumes. The spikelet consists of a minute axis called rachilla, a pair of sterile lemmas glumes, lemma, palea and the flower. The lemma, palea and the included flower are together known as the floret. The sterile lemmas, lemma and palea are also called bracts. Lemma is the five-nerved keel shaped bracts, which partly encloses the palea. The tip of the lemma and palea is known as apiculus. The filiform extension of the keel of the lemma is called awn. Palea is a three-nerved bract, which fits closely to the lemma.
The flower consists of six stamens and a pistil. Each stamen is composed of two celled versatile anthers borne on slender filaments. The pistil consists of an ovary with one ovule, short style and bifurcate plumose (hairy) stigma. Two fleshy, transparent structures located at the base of the flower, adnate to palea are called lodicules. Lodicules represent the perianth (calyx and corolla).
The rice fruit is a caryopsis with a single seed fused to the ripened ovary wall, pericarp. Lemma and palea enclose the caryopsis. The rice grain is the ripened ovary with lemma, palea, rachilla sterile lemmas. The rice grain is also known as rough rice. The Lemma, palea, sterile lemmas, rachilla and the awn whenever present constitute the hull or husk. Dehulled rice grain is called brown rice. The caryopsis is enclosed by the pericarp. Next to the pericarp there are two layers of cells representing the remains of the integuments, or seed coat. The bulk of the caryopsis is the starchy endosperm. An aleurone layer encloses the endosperm, which lies below the tegmen. The embryo is located on the ventral side of the caryopsis. The embryo consists of plumule, mesocotyl, radicle, epiblast and scutellum.
The plumule (embryonic leaves) is enclosed by the coleoptile, whereas the radicle (embryonic root) is ensheathed by coleorhiza. The embryonic axis mesocotyl is bounded on the inner side by scutellum adhering to the endosperm and on the outer side by the aleurone layer. The vascular trace fused with the lateral part of the scutellum is called the epiblast.
USES: Raw or parboiled milled rice is cooked and used as food. It forms the main course of the meals for millions of people in Asia and else where. Many processed foods such as popped or puffed rice products are produced from brown rice or parboiled rice for use as breakfast cereals and snack foods. Beer, wine and spirits can be produced from the grain. Rice is mainly grown for food but it is also used in cosmetics, laundering starch, and textiles. An oil is produced and used as cooking and salad oil, for soap manufacture and it is made into a plastic packaging material. Husks are used as fuel, as an addition to concrete, for making hardboard and as an abrasive.
The harvesting of rice produces straw as a by-product. Rice straw has poor palatability, bulkiness, low digestibility, low protein, high lignin and high silica contents. It is the most common feed ingredient for cattle in many developing countries in Asia and Pacific region, particularly in Bangladesh, India, Myanamar, Nepal, Pakistan, Sri Lanka and Thailand. Urea-treated rice straw as cattle feed is popular in Bangladesh. Rice straw is for making and repairing thatched houses in Bangladesh, Nepal and India. It is used for construction of grain storage structures, ropes, packaging material and bags in India. It is also used as fuel in Vietnam. In China, India, Indonesia and Pakistan rice straw is used in the production of paper.
The chief by-products produced during milling are husks, bran and Œbrokens‚. Husks are used as domestic fuel in many countries. In Myanmar rice husks are used as fuel for the operation of rice mills, for parboiling in Bangladesh, India, Myanmar Nepal and Sri Lanka, in China for the production of steam or gas for power generation. Rice husks are also used as packaging and insulation material. Although rice husk exhibits a low protein and high fiber content, they are used as animal feed in Bangladesh, China and India. Rice husks on burning produce 20 % (by weight) of ash containing 90% silica. In India, rice husk ash is fillet in the rubber industry and in the steel industry as an insulating material.
Rice bran is highly nutritious. It contents lipids, protein, minerals and vitamins. Because of its nutritional value, it is used as a feed for poultry and livestock. Refined bran oil is utilized as a salad oil as well as for cooking. Crude bran oil is used in the production of soap. In China oryzanol is extracted from crude bran oil, which has pharmaceutical use. De-oiled rice bran is used to for the commercial extraction of phytin.
Rice brokens have a variety of uses such as animal feed, making rice flour and malt sugar for breweries. It is used in a variety of ways to prepare processed rice based food products.
The seed has nutrient, remineralizing, antidiarrhoeic and emollient properties and administration is as decoction and powder.
KILLING T.: Growth arrested below 10°C; rice exhibits little or no frost tolerance. GROWING PERIOD: Annual grass, wetland types growing 80-150 days, while floating rice may require 180-200 days to mature. COMMON NAMES: Rice, riz, arroz, reis, ris, arishi, arroz, byeo, cau, chawal, dao, dhan. FURTHER INF.: Rice is grown in four ecosystems, which are broadly defined on the basis of water regimes. The ecosystems are irrigated, rain-fed lowland, upland and flood-prone, which cover 55, 25, 13 and 7% of the world's rice area respectively and account for 76, 17, 4 and 3% of the world's current rice production. The cultivation of indica is confined to areas between 0° and 25° latitude, it is seldom grown above 1200 m in elevation and performs best below 600m. They are hardy, resistant to diseases and tolerate unfavourable growing conditions. Rice can be grown very successfully in areas of low humidity and rainfall if there is adequate water available for irrigation but rice prefers medium to high humidity. The genetic diversity of O. sativa is most prevalent in the area extending over Asam, Bangladesh, Myanmar, Thailand, Laos, and Yunnan, China. The genus Oryza belongs to sub-family Oryzoideae of family Poaceae (Gramineae). It has two cultivated species of rice, Oryza sativa L., and Oryza glaberrima Steud. Both the cultivated species of rice are diploid (2n=24 chromosomes). The site of rice (O. sativa L.) domestication, whether one or many is debatable. Hybrid rice yields on an average 6.6 tons/ha compared with 5 tons/ha for conventional rice varieties.
| Grassland Index
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