jump over navigation bar

Unit 8 Rice - Oryza sativa and Oryza glaberimma

There are numerous excellent on-line resources available on rice. Many have been developed by the International Rice Research Institute specifically for training purposes and contain wonderful graphics of the rice plant and factors affecting rice production. The volume of materials can be overwhelming, so we have included the basic information that you are required to know for this course in this lesson. However, you are encouraged to pursue the links provided for more in-depth explanation of relevant topics.

Origin, Taxonomy, Genetic and Reproductive Systems

Long Grain Rice

Rice is the staple food crop for about one-half of the world's population. In many Asian societies, rice is featured in creation myths and has important cultural significance in addition to its contribution to the economy and food supply.

Cultivated rices are members of the grass family and belong to two species, Oryza sativa and Oryza glaberrima. O. sativa is the widely cultivated species. O. glaberrima is native to West Africa, where it is valued for its tolerance to drought and other local stresses.

Wild rice (Zizania palustris) is not related to Oryza sativa.

Photo from USDA-ARS
Long grain rice

Rice is thought to have originated in Southeast Asia, in the zone of monsoonal rainfall extending from eastern India through Myanmar, Thailand, Laos, northern Vietnam, and into southern China. A diversity of wild relatives can be found in this region, which probably played a role in domestication. Linguistic evidence also supports the idea that rice was domesticated in Southeast Asia. In several regional languages the general terms for rice and food, or for rice and agriculture, are synonymous.

The inflorescence of rice is a loose panicle, producing about 100-150 seeds per panicle. Rice is normally self-pollinating. It is a diploid species with 2n=24 chromosomes. In 2001 Syngenta published the rice genome map, which was the first genome of any food crop to be completely mapped.

Hybrid rice technology exploits the phenomenon of hybrid vigor and involves raising a commercial crop from F1 seeds. This technology helped China to increase its rice production from 140 million tons in 1978 to 188 million tons in 1990. Research at IRRI and in other countries indicates that hybrid rice technology offers opportunities for increasing rice varietal yields by 15-20% beyond those achievable with improved, semidwarf, inbred varieties.

See the IRRI lesson on hybrid rice seed production:

Growth Requirements; Morphological, Physiological, and Adaptive Traits

Although rice is a C3 plant, it is grown in tropical and subtropical regions throughout the world. It is unique among the cereals in that it thrives under flooded conditions. The oxygen produced by photosynthesis in the leaves is transported down to the submerged roots.

Most rice is grown as lowland or paddy rice (under flooded conditions), but in some areas it is grown in fields that are not flooded. This 'upland' rice still requires abundant moisture and generally yields less than paddy rice.

In the developing world, paddy rice is generally planted in flats and transplanted to paddies when the plants are one to six weeks old. This is a labor intensive process, but it has several benefits. Plants can be transplanted when environmental conditions are favorable, and stands may be more uniform than when crops are seeded directly in the field. This practice also reduces the amount of time that rice must be in the field, which makes it possible to plant rice in areas that have a short growing season, or to plant two crops in a year in areas with a longer rainy season.

In mechanized agricultural systems rice is soaked before planting in sodium hypochlorite to control seed-borne diseases and remove germination inhibitors. The soaked seed may be seeded into the paddy by airplane. Alternatively, seed may be planted directly in the soil which is flooded after the seedlings begin to grow. Paddies may be drained intermittently during the growing season to permit fertilizer to be applied and to prevent the water from becoming stagnant. The paddies are drained shortly before harvest which allows combines with specialized tires to enter the field.

Algae growing in the paddies may fix atmospheric nitrogen.

Ideal soils for rice cultivation are heavy and compact to reduce percolation of water. The process of puddling soils was developed in China. Secondary tillage is used to turn the soil into a muddy or watertight paste to prevent water drainage.

Rice morphology and growth stages

See the IRRI lessons on:

and the IRRI/CIAT/WARDA site on Morphology and growth of the rice plant.

Rice growth and development can be divided into the vegetative phase (including germination, seedling, and tillering stages) and the reproductive phase (including panicle initiation and heading) and the ripening phase (grain filling). The duration of the rice crop is three to six months, depending on the variety and growing conditions. For a 120-day variety, plants would remain in the vegetative phase for about 60 days, in the reproductive phase for about 30 days, and in the ripening phase for about 30 days.

Rice requires a temperature of 10 to 40 °C for germination. Tillering usually begins when a rice plant has about five leaves. Each tiller bears a terminal inflorescence (panicle). Some tillers, however, fail to produce a head - these are called ineffective tillers. Normal tillers remain attached to the plant, but later they develop their own roots and become independent. Rice is grown as an annual crop, although the plant can persist longer because tillers will reroot to produce a ratoon crop.

Rice groups

Source: http://www.hort.purdue.edu/newcrop/Crops/Rice.html

Japonica Group: In general, varieties in this group have short kernels. Stems are stiff, short and upright. Leaves are short, dark green and the second leaf forms a narrow angle with the stem. Plants are pubescent and form many tillers. Panicles are numerous, short, dense and heavy. Spikelets are awnless. This group is generally grown in more northern climates such as Japan, Korea, Northern China, Europe, and California in the USA.

Indica Group: This group is more tropical in adaptation than japonica varieties and includes the kinds grown in Southern Asia, the Philippines, and the South-Central States in the USA. These varieties are characterized by long kernels, long, light green leaves, and tall, somewhat spreading stems that are not as stiff as stems of japonica varieties. Panicles are numerous, long, light in weight, and medium in density. Spikelets are awnless.

Bulu Group: This group is of minor importance compared with the Japonica and Indica groups. Varieties classed here are grown mainly on the islands off Southeast Asia. They are somewhat intermediate in characteristics. The kernels are large, and stems are tall, stiff and upright. Panicles are few in number, of medium length and density, but heavy. Awns are numerous.

Major Production Areas and World Production Statistics

Production of Paddy Rice by Region in 2002
Paddy Rice Production
(1,000 Mt)
South America
North & Central America

Asia produces about 91% of the world's rice. Less than 5% of world rice production is traded internationally. Major rice exporters are Thailand, the United States, Vietnam, Pakistan, and India.

Major Paddy Rice Producing Countries in 2002
Paddy Rice Production
(1,000 Mt)
Viet Nam
United States of America
Korea, Republic of
Sri Lanka
Iran, Islamic Rep of
Korea, Dem People's Rep

For detailed information about rice production in particular countries and regions of the world see the Rice Knowledge Bank from IRRI: http://www.knowledgebank.irri.org.

Agroecological zones for rice

For more information: http://www.knowledgebank.irri.org/gis/gisMaps.htm

The Agroecological zones described in this section are based on the classification system developed by FAO.

Definitions of terms used in AEZ classifications
tropics regions with monthly mean temperature, corrected to sea level, of >18 °C for all months
subtropics regions with monthly mean temperature, corrected to sea level, of <18 °C for one or more months
temperate regions with monthly mean temperature, corrected to sea level, of <5 °C for one or more months
warm daily mean temperature during the growing period of >20 °C
cool daily mean temperature during the growing period in the range of 5 to 20 °C (including the moderately cool range of 15 to 20 °C)
warm/cool daily mean temperature during part of the growing period <20 °C
arid Length of growing period (LGP)<75 d
semiarid LGP 75-180 d
subhumid LGP 180-270 d
humid LGP >270 d

Distribution of agroecological zones in Asia

In Asia, rice is the dominant food crop in the humid subtropics (76% of the area under food grains), humid tropics (75%), and subhumid tropics (51%). It is an important crop in the subhumid subtropics (36%) and semiarid tropics (19%), but is insignificant in the semiarid and cool subtropics.

IRRI considers four broad ecosystems for rice production:

% of rice area worldwide
irrigated rice
rainfed lowland rice
upland rice
flood-prone rice


Types of Rice, Processing, and End-use Quality

Comparison of rice varieties

Rice is grown primarily as food for people. Unlike other cereals, the whole grain is consumed. Consequently, the appearance of the grain, milling, and cooking properties are very important in determining quality. Rice is also used in preparation of commercial breakfast foods such as rice crispies and puffed rice. Sake (or Saki) is a Japanese beer made from unmalted rice.



Photo from USDA-ARS
Types of rice varieties

Types of rice

In the US, grain types are classified according to the length, width, and weight of the grain.

Range of grain sizes and shape among typical U.S. long, medium, and short grain rice
Source: B.D. Webb. 1985. In Rice Chemistry and Technology
Length (mm)
Width (mm)
Weight/1000 (g)
7.0 - 7.5
2.0 – 2.1
16 - 20
5.9 – 6.1
2.5 – 2.8
18 - 22
5.4 – 5.5
2.8 – 3.0
22 - 24

Milling and processing

When rice is threshed, the hull (lemma and palea) remains intact - this is known as 'rough rice'. The hulls are left on if the rice is to be used for seed.

  1. In the first stage of milling, wind is blown through the rice to remove dust and trash
  2. The hull is removed (~20% of the kernel weight) to produce brown rice
  3. Further milling removes the bran (the seed coat, embryo, and some endosperm) to produce white or polished rice. The weight of the grain is reduced another 10%.
    A polished rice grain contains approximately 94% starch and 5% protein.


  • Rough rice is soaked and steamed, sometimes under pressure, until the endosperm starch is partly gelatinized.
  • The grain is dried and milled.
Effects of parboiling
  • The endosperm is tougher so the kernels are broken less in milling, increasing the yield of milled rice
  • The rice keeps better due to partial sterilization
  • More of the water soluble vitamins are retained in the kernel. Minerals and B vitamins in the bran and hulls are partially absorbed by the endosperm from the water used in soaking the rice, improving nutritional value of the rice

Quality standards

Components of rice quality in the United States

  1. Hull and bran color - The hull color of U.S. varieties is classified as either light (straw-colored) or dark (gold). Color from the hull can be absorbed into the endosperm during parboiling, which may lead to unacceptable end-products.
  2. General appearance - grain size and shape, uniformity, translucency, chalkiness, color, damaged and imperfect grains. This is a subjective evaluation, but is important in determining the grade of grain.
  3. Grain characteristics - standards for length, width, and weight determine whether grain is short, medium or long-grained
  4. Endosperm translucency and chalkiness - Chalkiness is undesirable in all segments of the rice industry. Breeders select intensively for clear, vitreous kernels. Environmental factors such as harvesting at high moisture content can also affect chalkiness. Chalky kernels will break easily, reducing milling yields.
  5. Milling quality - having a low percentage of broken kernels is an important component of milling quality, because it affects milling yields.
  6. Cooking and processing indices - varieties that are high in amylose (starch made of unbranched chains of glucose molecules) and lower in amylopectin (starch with branched glucose chains) tend to have drier (not so sticky) grains when cooked.

  7. Range of chemical and physical characteristics among U.S. long, medium, and short grain rice types
    Source: B.D. Webb. 1985. In Rice Chemistry and Technology
    Long grain
    Medium grain
    Short grain
    Amylose (%)
    Alkali spreading value
    Gelatinization temp. (°C)
    Gelatinization class
    Water uptake (ml/100g)
    Protein (%)
    Parboiling stability, solid loss (%)
    Amylographic paste viscosity
    Cooked 10 min at 95° C
    Cooled at 50° C
    Brewing Cookability, sec

  8. Eating quality - determined by taste panelists. Protein, amylose, and moisture are the primary determinates of taste in white rice. Brown rice taste scores are a function of protein, amylose, moisture, and fatty acid content.
  9. Moisture content
  10. Test weight
  11. Color
  12. Dockage
  13. Damaged grains
  14. Odors
  15. Red rice

Quality criteria vary from country to country, depending on how rice is consumed and local taste preferences. In the US, it is possible to define quality specifications for various end-uses such as:

  • Direct consumption as white or brown rice
  • Breakfast cereals
  • Baby foods
  • Brewing industry

Nutritional value of rice

The protein in rice is of good quality in comparison to other cereals, but people who subsist largely on polished rice may suffer from beri-beri, due to a deficiency in vitamin B1 (thiamin). Brown rice will contain more thiamin than white rice, but brown rice is also higher in oil content which may cause rancidity over time, reducing the storage-life of brown rice.

Golden Rice

Golden rice has been genetically engineered to produce provitamin A (beta-carotene), in order to alleviate vitamin A (retinol) deficiencies in the diets of poor people in developing countries. Rice plants were developed containing two daffodil genes and one bacterial gene that carry out the steps required for the production of beta-carotene in rice endosperm. According to the World Health Organization (WHO), vitamin A deficiency is the leading cause of preventable blindness in children.

This technology has both strong advocates and opponents. Some of the issues that are being debated are:

  1. Will the golden rice varieties supply enough provitamin A to be of practical value for individuals deficient in Vitamin A?
  2. The license to market golden rice is owned by Syngenta. Will the technology be able to reach the poor?
  3. How will the trait be incorporated into adapted varieties that will satisfy local preferences for end-use quality?
  4. Are there risks that golden rice will cause allergies?
  5. Will yellow rice be accepted by consumers who value their traditional white varieties?

Recently, an improved golden rice has been developed that has more than 20 times the levels of betacarotene that were present in the original transgenic varieties, utilizing genes from corn rather than from daffodil.

Additional information about the technology from the perspective of the inventors can be found at http://www.goldenrice.org.

For a balanced discussion of these issues and links to further information about the golden rice controversy, see:


Major Diseases, Insect Pests, & Weeds

See the IRRI lesson on Integrated Pest Management in Rice for information about


Viral diseases and their vectors

  • Rice tungro Nephotettix virescens (Distant)
  • N. nigropictus (Stål)
  • Ragged stunt Nilaparvata lugens (Stål)
  • L. acuta (Thunberg)

Bacterial diseases and their causal agents

  • Bacterial blight Xanthomonas oryzae pv. oryzae (Uyeda et Ishiyama 1922) Swings et al 1990

Fungal diseases and their causal agents

  • Blast Pyricularia oryzae Cav.
  • Sheath blight Rhizoctonia solani (Thanatephorus cucumeris) [Frank] Donk)

Important diseases in the US

Seedling blights
  • Caused by sclerotinium and other fungi
  • Attacks young seedlings in warm weather
  • Severe infection kills rice
  • Rice sown early to avoid seedling blight
  • Immediate submergence will check the disease
  • Some fungicides and seed treatments provide good protection
Brown leaf spot
  • One of most serious diseases in Louisiana, Texas, and Arizona
  • Attacks seedlings, leaves, hulls, and kernels
  • Resistant varieties may be the only practical and economic control available

Insect pests

Stem borers

  • Yellow stem borer Scirpophaga incertulas (Walker)
  • White stem borer Scirpophaga innotata (Walker)
  • Striped stem borer Chilo suppressalis (Walker)
  • Dark-headed rice borer Chilo polychrysus (Meyrick)


  • Rice leaffolders Cnaphalocrocis medinalis (Guenée) and others
  • Rice caseworm Nymphula depunctalis (Guenée)


  • Green leaf hopper Nephotettix virescens (Distant)
  • N. nigropictus (Stål)
  • N. parvus Ishihara et Kawase
  • N. cincticeps (Uhler)


  • Brown planthopper Nilaparvata lugens (Stål)
  • Whitebacked planthopper Sogatella furcifera Horvath

Rice bugs

  • Malayan black rice bug Scotinophara coarctata (Fabricius)
  • Rice grain bug Leptocorisa oratorius (Fabricius)

Important insect pests in the USA

Rice stink bug
  • Sucks out contents of kernel during the milk stage leaving an empty seed coat
  • Control measures include early mowing and winter burning of the coarse grasses in which the insect hibernates.
Stalk borers
  • Two are important:
    • Sugarcane borer
    • Rice stalk borer
  • The larvae tunnel inside the rice culms, eat the inner parts and cause panicles to turn white and produce no grain (called white head)
  • The borers prefer large stems
  • They over winter in rice stubble
  • Borers can be killed by heavy grazing of stubble, mashing down or flooding stubble fields (done by holding water from winter rains).


Although flooding in rice paddies serves as a means of weed control, many weeds have adapted to paddy conditions, including some aquatic plants and algae. In the US, herbicides are generally used in combination with flooding to control weeds. In the tropics, control of weeds in rice fields is a major challenge, requiring more labor than any other operation during the crop cycle. In Asia, weeds typically reduce yields by about 20% in farmers' fields. Losses are much greater when the weeds are not controlled.

Red rice

Red rice is a weedy form of Oryza sativa that is a major pest in US rice production systems. It imparts an undesirable rosey color to rice products. It cannot be controlled with conventional herbicides because chemicals that kill red rice will also kill cultivated rice. Two new technologies have been developed to address this problem.

The Liberty Link system involves a transgenic rice with resistance to the herbicide glufosinate (Liberty). Liberty is a broad-spectrum, nonselective, postemergence herbicide with no soil or residual activity. The EPA approved the release of this technology in September, 2003.

Clearfield rice is another herbicide-tolerant system marketed by BASF that has been very successful in controlling red rice since it was released in 2003. Clearfield rice has a natural mutation (it is not a transgenic technology) and is tolerant of a family of herbicides known as the imidazolinones. Presently, NewPath is the only imidazolinone herbicide registered for use in the Clearfield system. Optimum management is critical with this system, however. Red rice with resistance to imidazolinone could develop quickly through outcrossing if the weed is not thoroughly controlled. Rotations with other crops are highly recommended.

Cultural Significance of Rice & Emerging Issues

Rice and malaria

During the 16th and early 17th centuries, malaria was a major disease in southern Europe, and it was believed to be spread by the bad air (mal-air) of swampy areas. Major drainage projects were undertaken in southern Italy, and wetland rice cultivation was discouraged or even forbidden in some regions. These measures delayed the spread of rice cultivation in Europe.

Methane emission from rice fields

Methane (CH4) is an important greenhouse gas, which contributes to global warming. Flooded ricefields are suspected of putting 115 million t of methane into the atmosphere each year. This is at least equal to the total production from all of the world's natural swamps and wetlands. Practical means for mitigating these emissions are being investigated at IRRI.

Nitrogen management in rice

Most of the nitrogen used by the rice crop worldwide is supplied in the form of inorganic fertilizer. Ways to promote biological nitrogen fixation, to control negative environmental effects from the use of nitrogen fertilizer, and to improve the efficiency of nitrogen utilization in rice farming systems are currently active areas of research. Scientists hope to enhance N2 fixation by blue-green algae (Azolla) and heterotrophic bacteria in the root zone. The potential for developing symbiotic associations between nitrogen-fixing bacteria (Rhizobia) and rice in root nodules using genetic transformation is also under investigation.

Crosses with O. glaberimma

O. glaberimma is native to West Africa and is tolerant to a number of local stresses such as drought and insect pests. Crosses were made between O. sativa and O. glaberimma at WARDA in Côte d'Ivoire to generate a promising array of NERICA rices. They smother weeds, and resist drought, pests and problem soils. They also inherited higher productivity from O. sativa, and can double production with just a few inputs. For this pioneering work, Dr. Monty Jones was named a winner of the 2004 World Food Prize.



Review the controversy surrounding 'golden rice':


(See References for this unit for additional links)


Take the quiz on this Unit on the Blackboard.

Recommended self-study

Become familiar with the websites that are available on rice. Review at least one of the training units from the Rice Knowlege Bank.


BIOTHAI, CEDAC, DRCSC, GRAIN, MASIPAG, PAN-Indonesia and UBINIG. 2001. Grains of delusion: Golden rice seen from the ground.
http://www.grain.org/briefings/?id=18 [pdf version]

California Rice Commission. 2006. http://www.calrice.org/

Cooperative Research Centre for Sustainable Rice Production. 2003. RiceScience Portal.

FAO. 2004. International Year of Rice. http://www.fao.org/rice2004/index_en.htm

IRRI. 2006. Rice Knowledge Bank. http://www.knowledgebank.irri.org

IRRI. 2004. RiceWorld. http://www.riceworld.org
This site is presently under renovation

National Research Council. 1996. Lost Crops of Africa: Volume I, Grains, Washington, D.C.: National Academy Press. http://books.nap.edu/books/0309049903/html/index.html
Good reference for O. glaberimma and other traditional grain crops of Africa

Paine, J. et al., 2005. Improving the nutritional value of golden rice through increased pro-vitamin A content. Nature Biotechnology, March 27, 2005. http://www.botanischergarten.ch/Rice/Paine-et-al-2005-NB-Golden-Rice.pdf

Rice Online. 2006. http://www.riceonline.com/home.shtml

University of California Cooperative Extension Rice Project. 2005.