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
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
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.
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
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.
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.
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:
Types of Rice, Processing, and End-use Quality
Types of rice
In the US, grain types are classified according to the length, width, and weight of the grain.
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.
Effects of parboiling
Components of rice quality in the United States
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:
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 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:
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, & WeedsSee the IRRI lesson on Integrated Pest Management in Rice for information about
Viral diseases and their vectors
Bacterial diseases and their causal agents
Fungal diseases and their causal agents
Important diseases in the US
Brown leaf spot
Important insect pests in the USA
Rice stink bug
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 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.
Review the controversy surrounding 'golden rice':
Take the quiz on this Unit on the Blackboard.
Become familiar with the websites that are available on rice. Review at least one of the training units from the Rice Knowlege Bank.
California Rice Commission. 2006. http://www.calrice.org/
Cooperative Research Centre for Sustainable Rice Production. 2003. RiceScience
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
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
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.