Rice

Rice

Oryza sativa
Family- Poaceae

Rice can be grown two or three times an year without exhausting the fertility of the soil for considerable periods, on the same land. The variability of the varieties that exists in this crop is unrivalled. At one end of the scale we have varieties of rice which can grow in 7.5 m of water and on the other end, those which can be raised on dry lands with only 50-62 cm of rainfall along with cotton and red gram. Rice can be raised on any kind of soil, provided there is an adequate amount of water during the growth periods. The plant can tolerate high degree of acidity and to some extent salinity also. It is reckoned that there are about 1,20,000 morphological varieties of rice in the world and in India the collection exceeds about 30,000.

Oryza and its species

Rice, Oryza app. belongs to the tribe Oryzae of the family Poaceae. Th genus most closely related to Oryza is Leersia. The absence of glumes (sterile lemmas) in Leersia distinguishes it from Oryza. There are about 27 species of Oryza distributed all over the world of which only two are cultivated species viz., sativa and glaberrima.

Oryza coarctate Roxb. 

A coarse, stout grass with spinous-serrate leaves; found in the lower Gangetic basin in Sunderbans, in the estuaries of Mahanadi in Orissa and in Godavari Delta in Andhra Pradesh. It is essentially an aquatic plant and the grain resembles wheat and the rice has a soft chalky texture.

Oryza meyeriana Baill. 

A perennial grass, upto 1m high, with a compact and small root stock; found in Bengal, Orissa, Tamil Nadu extending to South Asia. It bears short erect panicles with awnless spikelets and the grains have good flavour; grass is eaten by cattle.

Oryza officinalis. Wall ex Watt:

A tall, annual or perennial grass with sub-woody roots; profusely branched panicles; spikelets elliptic, awns present. It occurs in Assam, Khasi hills, Sikkim and /maharashtra.

Oryza perennis Moench.

A water-loving potentially perennial plant with creeping or bulbous rhizomes and erect or prostrate culms, sometimes floating; leaves broad 10-20 mm wide with long acute ligules; panicles medium sized; spikelets deciduous and slender. It is a polymorphic species and widely distributed in tropical Africa, America and Asia. In India it has been reported from Orissa, Bengal and Assam. The species have several geographic races: the African O. barthii strongly rhizomatous and spreading or floating habit, O. longistamisnata and the American plant with semi-erect habit O. perennis var. cubensis.

Oryza rufipogon Griff.

An annual, growing in water or swampy places in Assam, West Bengal, Chota Nagpur, Madhya Pradesh, Orissa and eastern parts of Andhra Pradesh; with long erect culms; deciduous, long spikelets, awned and rigid. This species is highly polymorphic and is considered to be a close relative of cultivated rice O. sativa, with which it hybridizes freely.

Oryza sativa L.

An annual or perennial grass without a rhizome; leaves long and narrow 30-50 cm into 1.2-2.5 cm, slightly pubescent with spiny hairs on the margin: inflorescence a terminal panicle varying from close and compact in some to loose and spreading in others: spikelets generally single, but in some in clusters of 2-7; number of spikelets varying from 50-60 to 200-300, large numbers being usually associated with smaller size and a densely packed arrangement; lemma and palea enclosing the kernel are variously coloured, golden yellow, red, purple, brown, smoky black, becoming yellow or golden yellow on ripening the grains; grain varying in size from 5 to 14.5 mm long and 1.9 to 3.7 mm broad, the length/breadth ratio defining the size and shape of grain; kernel most commonly white, but also red, purple or brown pigmentation present.

Sub-species of Rice

Kato (1910) has distinguished cultivated rice into two groups broadly as indica and japonica rice based on hybrid sterility. After continued work on hybrid sterility of rice, some workers have shown that the indica group of earlier workers includes another group called javanica (India-japonica) consisting of rice (Bulu) which exhibit intermediate characters between japonica and indica rice.

Botany of the plant

Seedling characters

The grains of rice that lack dormancy germinate immediately upon ripening. First the coleorhiza enveloping the radicle comes out on germination, then the radicle breaks through the coleorhiza and is followed by two or more secondary seminal roots all of which develop lateral roots. Seminal roots are later replaced by secondary system of adventitious roots. The length of the axis between coleoptile and the point of union of the root and culm is called mesocotyle. The coleoptile later ruptures at the apex and the first seedling (primary) leaf emerges. The second leaf that follows the first leaf is differentiated into sheath, blade, ligule and auricles.

Vegetative characters

The roots are fibrous having rootlets and root hair. The secondary adventitious roots are produced from the underground nodes of the young culms and are freely branched. Culm: The jointed stems rice, called culm, is made up of a series of nodes and internodes. The node bears a leaf and a bud.

Tillers arise from the main culm in an alternate pattern. The primary tillers originate from the lowermost nodes and give rise to secondary tillers and then to tertiary tillers.

Leaf

The leaves are in two ranked arrangement borne on the culm and are present one at each node. At the node various structures appear characteristic of different varieties in their structure, colour etc. The leaf sheath is continuous with the blade. There may be a swelling at the base called pulvinus. The blades are generally flat and sessile. The uppermost leaf below the panicle is called the flag leaf. There are two ear-like appendages borne on either side of the base of the leaf called auricles. At the junction of the blade and sheath on the inner side is a membranous or glabrous structure called ligule. The junction of sheath and blade is called the collar or junctura, which may be purple.  

 

Floral characters

The terminal shoot of the rice plant is a determinate inflorescence called the panicle. The spikelet is the unit of the inflorescence. The spikelets are borne on the branched panicle.

Spikelet

The spikelet consists of a minute axis (rachilla) on which a single floret is borne in the axils of 2-ranked bracts. The bracts of the lower pair of the rachilla are called as sterile glumes or empty glumes. The upper bracts or the flowering or fertile glumes  are known as lemma and palea. The lemma is larger, hardened 5-nerved bract enveloping the 3-nerved palea. The extended tips of the lemma and palea are the apiculi. Just at the base of lemma and palea are a pair of scale-like, colourless, transparent lodicules which aid in the opening of the spikelet. At anthesis, the lodicules become turgid and thrust the lemma and palea apart, exposing the fertile stamens.

                        There are six fertile stamens in two rows of three each and anthers are versatile. Ovary is hypogynous, monocarpellary, unilocular with a single basal ovule and a pair of feathery stigmas at the top. The rice fruit is a caryopsis in which the single seed is fused with the wall of the ripened ovary (pericarp) forming a seed like grain. The embryo lies on the ventral side of the spikelet next to the lemma. The remaining part of the caryopsis is the endosperm which provides nourishment to the growing embryo.

 

Grain

 

The embryo contains the embryonic leaves (plumule) and the embryonic primary root (radicle). The plumule is enclosed by the coleoptile and the radicle by coleorhiza. These form the embryonic axis which is bounded by scutellum (cotyledon) lying next to the endosperm. The endosperm is enclosed by an aleurone layer lying beneath the tegmen. The white starchy endosperm consists of starch granules embedded in a proteinaceous matrix.

                    In the glutinous (waxy) varieties the starch contains only amylopectin and stains reddish brown with Iodine test. In the non-glutinous (non-waxy) varieties the starch contains amylose and amylopectin and stains dark blue with Iodine test.

Methods of Cultivation

 

Mainly there are two types of rice cultivation:

Upland Rice: (Dry paddy cultivation) 

Forms only ten percent of the whole rice growing countries of the world. It is grown on hill tops, hill sides or other areas where neither irrigation nor any device to hold rain water are available. This is known as terrace cultivation in parts of Himalayas and China. Seeds are sown either broadcast or either by dribbling or dibbling in plough furrows.

Floating Rice: (Wet paddy cultivation)

In the low-lying areas of West Bengal, Assam, Orissa, Tamil Nadu and Kerala, during the later part of the crop season water rises to a level of 5-6 feet. The seed is sown as in the upland rice and as the crop grows when the monsoon strengthens, flooding takes please and the rice crop also grows rapidly, keeping pace with the rise of water level. When the water level becomes stationary, the growth ceases and is followed by tillering and rooting at higher nodes. Harvesting of crop has to be done in standing water by boats.

Ecology

Rice is most suited to tropical countries. It requires high temperature ranging from 20-25 degree C. A warm and bright Sun is necessary when the crop matures. Rice grows in the countries where the annual rainfall ranges from 120-150 cm. Otherwise heavy irrigation is essential.

                                           In India rice is grown under diverse soil conditions over a wide range of pH. The most important groups of soil under which the crop is grown in India are the alluvial soils, red, laterite and lateritic, black, saline and alpaline, and peaty and marshy soils.

                    Two crops are mainly grown for rice in India namely the main season or kharif season between July-December and the off-season or rabi season between January-April. In certain areas with short duration varieties and depending upon soil and water resources a third crop can also be grown. In certain areas of India, three crops have been raised in one year.

Cultural practises

The cultural practises of rice mainly include: sowing, transplanting, irrigation, manuring and harvesting.

Sowing

There are three methods of sowing practises 

1. Broadcasting
2. Dibbling and 
3. Drilling

Dry system

In the broadcasting method when the field is in right condition the seed is broadcast. It is an advantageous practise that if the seeds are soaked for 6-8 hrs previous to sowing, the germination will be hastened.

In the dibbling method of sowing, the seed is dibbled behind furrows in the country plough at regular intervals. In this method the seed rate is lessened by 30% and yields are high.

In the drilling method seed is drilled into the soil with a distance of 18 inches between two rows and the inter culturing operations are done by bullock driven implements.

Wet system

The wet system is practised where there is an adequate and assured supply of water throughout the season. In this method, sprouted seed may be directly sown in a puddled or levelled fielder, the crop may be transplanted with with seedlings raised in a nursery. Of these transplanting is more common and is adopted wherever water facilities are available.

There are three methods of raising seed beds:

1. Dry
2. Semi dry and
3. Wet

All these methods are adopted in different rice tracts. For raising seedlings a new method called DAPOG method has been devised.

 

Modified DAPOG method 

Seed beds:

Separate seed beds are prepared with channels all around. The beds are 1m wide, 8m long and 10cm long. The surface of the bed is packed tightly and kept at uniform level. The beds are covered with polythene sheets. sand or ash upto a thickness of 2.5cm is spread on the beds.

Germinating seeds:

The seeds are soaked in water for 12 hours and kept for germination for one day. The pre-germinated seeds are sown in sand or ash media uniformly at one grain thickness. Seed rate is one kg per sq. m. of nursery area.

Covering: 

The seeds are covered with gunny bags for the first three days and kept moist by sprinkling water thrice a day. The germinated seeds are gently pressed twice a day. The channels around the beds are filled with water upto the rim.

Transplanting:

The seedlings when they are 20-28 days old are ready for transplantation and are planted in the puddled field on the same or next day. At the time of planting the tops of leaves are cut for uniform growth later.

Weeding and Inter-culturing

Weeding is usually done by hand or by mechanical devices like Japanese rotary weeder. Due to weeding, the soil gets stirred round the plants.

The right stage for harvesting is when the ear is nearly ripe and the straw has just turned yellow. The crop is harvested and allowed to dry for 3-4 days and then brought to threshing yard. The Japanese pedal thresher is in modern use by rice cultivators.

Winnowing: After threshing the panicles, the paddy is winnowed to separate the chaff by allowing the grain to fall from a height of about 7.5 m.

Rotation: Rice is rotated normally with leguminous crops for enriching the soil fertility, like red gram (Cajanus cajan), black gram (Vigna mungo), green gram (Vigna aureus) or horse gram (Dolichos biflorus). A mixture of green gram and wild indigo (Tephrosia purpurea) is also grown after rice.

Ratooning: Raising a ration crop of paddy is not a common practise in India but is grown in some areas of Tamil Nadu state. In this method, the main crop is harvested and then the field is irrigated, weeded and manured and the stubble are allowed to shoot up. In general, the yield of a rationed crop is lesser than the main crop.

Cytogenetics of rice

The diploid (2n) chromosome number of rice is 24. Rice has been thoroughly investigated cytogenetically and its relationships with other species have been determined by hybridisation and chromosome pairing at diakinesis and metaphase 1 and genome designations have been given to different species of Oryza.

Detailed pachytene studies have also been done to ascertain genomic relationships among the species of Oryza. Rice has long been considered to be a secondarily balanced polyploid and that the presently cultivated species is diploidized with complete bivalent pairing with 12 bivalents. Rice forms 12 bivalents at diakinesis and at metaphase 1 and there is a regular distribution of chromosomes at anaphase 1 into 12-12 at the poles. There are 12 linkage groups corresponding to the 12 haploid chromosomes. The linkage groups have been determined genetically by hybridisation and segregation of genes and they have been correlated with certain specificity. 

Mutants of rice

Rice is a thoroughly worked out genus as far as the spontaneous and induced mutations are concerned and a great deal of work has been done on the genetics of these mutations. Mutations have been found in the entire plant from the root to the grain which gave clues to the identification of genes and the formulation of linkage groups.  Some mutantions starting from the seedling stage upto the grain are:

1. Chlorophyll mutations
2. Grain mutations
3. Panicle mutations
4. Leaf mutations
5. Culm mutations

Nutritional value of rice

Like all cereals rice is not a complete food. Rice should, therefore be supplemented by other foods which can supply additional amounts of protein, fat, various vitamins and minerals. While rice has nutritional properties approximately similar to those of other cereals in a similar state, it is somewhat superior in certain respects but inferior in others. Rice, like wheat, is  a poor source of fat and milling removes most of the fat from both the cereals. Whole maize is richer in fat than whole rice or wheat.

Effect of washing and cooking

Rice is the only cereal which is always washed in the course of domestic preparation and this process has a serious effect on its nutrient content. due to washing the losses of thiamine, riboflavin and niacin are as follows in husky sand milled rice respectively:

21.1 and 43 percent for thiamine

7.7 and 25.9 percent for riboflavin

13.0 and 23.0 percent for niacin

Rice is nearly always washed before cooking in order to remove dust, insects, husk etc It is also done to remove fine starch particles from the surface of the grains which causes stickiness when cooked.

While cooking may deplete rice of nutrients, cooking losses are in general less serious than losses due to washing. The method of cooking which causes most depletion is that of using excess water and subsequently discarding the surplus water. As much as 30 percent of thiamine remaining in washed rice is removed by cooking in this way.

Parboiled Rice

Parboiling is an ancient method of processing paddy which originated win India. The process of parboiling includes four different stages, soaking, steaming, drying and pounding or milling.

It is first soaked in water for one or two days when the grain absorbs water. After soaking, the paddy is steamed at atmospheric pressure for 30 minutes or less, through the use of steam pipes or heating in a limited amount of water in a vessel. During steaming, thiamine and other water-soluble nutrients originally present in the germ and aleurone layer, diffuse through the grain which is an advantage over milled rice.

After steaming, the paddy must be dried in direct sunlight. Drying should be done neither fast nor slow. Parboiled paddy after being dried is pounded or milled in the same way as milled rice. 

Parboiling toughens the grain and reduces the percentage of 'breakages' during milling in addition to its nutritional value. It is alleged that adult beri-beri is less common in populations consuming parboiled rice than those using raw rice.

Diseases of rice

1. Blast caused by Piricularia oryzae
2. bunt caused by Neovossia horrida
3. False smut caused by Ustilaginoidea virens
4. Foot rot or Bakanae disease caused by Gibberella fujikuroi
5. Helminthosporium blight caused by Cochliobolus meyabeanus
6. Irregular stem rot caused by Helminthosporium sigmoideum var. irregularae
7. Narrow brown leaf spot caused by Cercospora oryzae
8. Bacterial blight caused by Xanthomonas oryzae

Insect pests

1. Paddy stem borer, caused by Schoenobius incertellas elk. Symptoms: Caterpillers bore into stem, central shoot wither and produces dead heart.
2. Swarming caterpillar caused by Spodoptera mauritia Boisd. Symptoms: caterpillars appear in big swarms in seed beds causing damage to seedlings.
3. Paddy bug caused by Leptocorsia varicornis Fabr. Symptoms: Bugs suck, the milky sap of tender grains, sometimes crop destroyed completely.
4. Paddy gall fly, caused by Pachydiplosis oryzae W. Symptoms: Maggots attack base of growing points and produce silvery gall (silver shoot).
5. Rice hips, caused by Hips armigera Ol. Symptoms: Small blue black beetle covered with spines, grubs make long winding tunnels in leaves, more harm fun in seed bed.

Uses of rice

Rice products that are sold in Indian markets are the following:

Parched Rice: 

Parboiled rice is prepared for making murmura. It is prepared by heating to a high temperature in an iron pan containing sand. Little rice is thrown into the heated sand and stirred rapidly, when the rice cracks and swells. After cracking operation, the rice is sieved for separating it from sand. Parched rice is distended, crisp product, with greyish to a brilliant white colour.

Parched Paddy:

It is prepared by drying the paddy in earthen pots and then moistened by adding hot water which is immediately decanted. The jars are kept overnight in an inverted position. Next morning the moistened paddy is dried in hot Sun and then parched in hot sand in the same way as parched rice preparation.

Flaked Rice: 

It is a type of parboiled rice made flat and thin by pressure. It is prepared by soaking paddy in water for 2-3 days to soften the kernel and then boiled in water for a few minutes. After cooking, the water is drained off and the paddy is heated in a shallow earthen vessel or iron pan till the husk bursts open. Then it is pounded by a wooden pestle which flattenes the rice kernel and remove the husk.

Flaked rice is thin and papery and off white in colour.

Sake: 

Sake is an important alcoholic beverage of Japanese for thousands of years. The raw materials used in sake brewing are rice and water. During the fermentation process, addition of koji, for the decomposition of protein contained in rice will be done. The sake mash called moromi, liberates sugars from rice grains and are fermented successively by yeast. This process is called parallel fermentation, involves progressive decomposition of starch and other substances and a slow fermentation at a low temperature. These contribute to a high ethanol production  which can be upto 20 percent in the moromi mash. 

In the beginning rice is polished upto 70 to 75 percent and then steamed. Then the moto mash (sake yeast) and koji are mixed which results in moromi mash. The fermentation of moromi mash takes about 20 to 25 days. The final stage is ageing and fermentation of sake after which it is stored and blended, and adjusted to proper ethanol content by adding water. It is treated with activated carbon followed by filtration, bottled and pasteurized.

                       

Bran:

This is the most important by-product of rice milling industry and is used as cattle feed. Chemically bran contains: moisture, fat, protein, nitrogen free extract, fibre and ash. It also contains in small amounts sucrose, reducing sugars, thiamine and nicotinic acid and its proteins are superior to those of rice kernel. In addition, it also contains several minerals like calcium, phosphorus, potassium, sodium, magnesium, iron, manganese, chlorine and silica. 

Bran-oil:

Bran-oil is extracted immediately after milling the rice, otherwise it deteriorates rapidly. As soon as the bran is removed an extremely high lipase activity begins and causes free fatty acid in the oil to increase rapidly. Bran-oil is extracted by expression in hydraulic press or extraction with solvents. Bran-oil contains about 3-9 percent wax, which has to be removed by filtering or centrifuging.

It is used as an edible oil as refined groundnut or cotton seed oil and has better qualities for storage due to the presence of antioxidants (alpha and gamma-tocopherols). It is used in the hydrogenation of oils for preparing vanaspati, making soaps, oleins and stearins. It is used in the textile industry, leather industry, flexible film industry and enamels.

Bran Wax:

It is obtained as a by-product in bran-oil extraction and contains esters of waxy acid with higher alcohols. It is used in chocolate industry; coating for candy and lozenges; preparation of wax emulsions applied to fruits and vegetables and cosmetic like lipsticks.

Husk:

Paddy husk is used as a fuel mainly in rural areas of India, in brick kilns, in house building, compost making and brick making.

Straw:

Used for feeding cattle. It is used in the manufacture of straw boards. Pulp made from rice straw finds use in paper industry. It is used in thatching, making hats, mats, ropes and baskets.