Monday, March 12, 2007

About Rice (Oryza sativa L.)

K K Vinod

[Following is the text of my short presentation made to my class at UNIVERSITY OF NEBRAKSA-LINCLON]

Rice is a semiaquatic annual grass belonging to the genus Oryza. The genus oryza includes 24 species, of which 22 are wild and two namely Oryza sativa and Oryza glaberrima are cultivated. O. sativa is grown all over the world while Oryza glaberrima has been cultivated in West Africa for about 3500 years. There are more than 120,000 varieties of cultivated rice (IRRI, 2001). It is believed that rice domestication occurred independently in China, India and Indonesia, thereby giving rise to three races of rice: sinica (also known as japonica), indica and javanica (also known as bulu in Indonesia).

Cultivated rice is diploid (2n=24) and belongs to AA genome. The sativa rice varieties of the world are commonly grouped into three subspecies namely indica, japonica and javanica. Rice grown in India belongs to the indica subspecies. They are characterised by having leaves slightly pubescent and pale green in colour. Indicas are awnless or possess short and smooth awns. The rice grown in Japan belongs to japonica subspecies. Japonicas are adapted for cultivation in the subtropical and warm temperate regions. Japonica varieties mostly have oval and round grains. They may be awned or awnless. Leaves are narrow and dark green in colour. Subspecies javanica is characterised by a stiff straw, long panicle with awned grains, sparse tillering habit, long duration and low sensitivity to difference in day length. These are found mainly in Indonesia.

Rice was domesticated more than 10,000 years ago is possibly one of the oldest domesticated species. Huke and Huke (1990) observes that the domestication of rice ranks as one of the most important developments in history, for this grain has fed more people over a longer period of time than has any other crop. Rice is the staple cereal for more than 50% people (~3.25 billion) around the world, cultivated in about 9% of the earth's arable land, which is the largest single use of land for producing food. Rice provides 25 to 85 percent of the calories in the daily diet and 15% of per capita protein (IRRI, 2002). In Asia, where rice is the major energy providing food, it accounts for 50-80% of daily caloric intake, especially among the poor (IRRI, 2001). Unlike other major cultivated grains like wheat and corn which are also used for feeding livestock, rice is exclusively used for human consumption.

With China, India and Indonesia producing the most of the world’s rice, Asia accounts for over 90% of the world's production of rice. Only 6-7% of the world's rice crop is traded in the world market. Production of rice in The United States accounts to 1.5% of the world's production, with Arkansas, California and Louisiana producing 80% of the U.S. rice. Thailand, Vietnam, China and the United States are the world's largest exporters (IRRI, 2002).

Rice is the only cereal that can be grown for long period in standing water. Even though predominantly semi-aquatic, rice is grown under many different conditions and production systems, including upland and dry conditions. (FAO, 2004a). 57% of the world’s rice is grown on irrigated land, 25% on rainfed lowland, 10% on the uplands, 6% in deepwater, and 2% in tidal wetlands (Chopra and Prakash, 2002). The flooded rice paddy sustains rich aquatic biodiversity, providing a home for fish, plants, amphibians, reptiles, mollusks, and crustaceans (FAO, 2004b).

Rice has many characteristics, making it useful in various ways to be included in cereals, snack foods, brewed beverages, flour, oil, syrup, flakes and religious ceremonies. Rice grains can be short, medium and long or waxy (sticky) or non-waxy. Some are aromatic (Alford and Duguid, 1998; Chaudhary et al., 2001), some are colored including brown, red, purple and black (FAO, 2004c) and some are of medicinal value. The variation in characteristics makes one variety more popular in one region of the world than another.

Rice breeding

The primary breeding objective in rice growing countries has been high yield potential. Plant breeders have greatly contributed to the development of high-yielding crop varieties and have changed the morphology and physiology of crop plants, and incorporated desirable traits and resistant gene(s) into traditional varieties while stabilizing or increasing crop production. Dramatic advancement in productivity has achieved by incorporation of the semi-dwarf gene from Dee-Gee-Woo-Gen into traditional tall, leafy rice. The semi-dwarf rice varieties are now planted in 60% of the world's rice land.

High-yielding varieties have made a great contribution to the world's food supply, but they also have several major problems. The high yields of these varieties can only be attained with a high level of inputs, in particular heavy applications of fertilizer. This has led to problems associated with pest outbreaks in certain areas, while increased rice production has resulted in lower rice prices.

In rice breeding, the ideal plant type sought by breeders have been high yield potential; resistance to major diseases and insects; and improved grain and eating quality. However, there are few conflicting objectives like, high grain quality tends to result in unstable yields and also, too much emphasis on disease and insect resistance and stable yields leads to poor grain quality. Hence, breeding efforts should be fashioned in a way to sustain the yield under unfavorable conditions, and to maximize yields when conditions are favorable. 

The following breeding approaches should be emphasized in producing varieties for sustainable rice production.

·         - High-yield potential under low inputs.
·         - Heterotic F1 hybrid
·         - New plant type
·         - Premium grain and eating quality to meet consumer demand, and to provide grain suitable for processing.
·         - More genetic diversity.
·         - Durable host resistance to major diseases and insects.
·         - Wider range of growth duration for various purposes.
·         - Proper levels of tolerance to environmental and climatic stresses in specific areas.

Common breeding method used in rice is pedigree breeding method.  Other than the introduction of semi-dwarf gene (sd1), popularization of male sterile systems in early 1980’s, hybrid rice production has met dramatic increase in rice yields in China. Three line breeding of hybrid rice carrying wild-abortive cytoplasmic male sterility has been utilized in commercial scale (Kim and Rutger, 1988). The advent of environmentally sensitive male sterility systems (TGMS and PGMS) paved way for the development of two-line hybrid breeding in rice.  Transfer of cytoplasm from wild species to cultivated backgrounds used backcross procedures widely.

Host resistance to various biotic stresses is a very important aspect of high yields, and can be expected to play a significant role in sustainable rice production. There are now numerous varieties resistant to rice blast, bacterial blight, various virus diseases, and plant hoppers and some possess multiple resistance to diseases and insects. Varieties with the Xa4 gene resistant to bacterial wilt have been grown in the Philippines for the last 15 years, and continue to be resistant. It is extremely difficult to identify polygenic resistance and incorporate it into improved germplasm (Khush and Virmani, 1985). Current studies on host resistance to crops emphasize the durability of resistance (Ikehashi and Kiyosawa, 1981; Ahn, 1982; Lee et al., 1989). Polygenic traits rather than absolute resistance would be preferable in sustainable agricultural production (Hauptli et al., 1990).

Improvements in rice quality are very important in meeting the demands of consumers for healthy, high-quality food. Many traditional varieties in both the tropics and the temperate zone have excellent cooking and eating quality, but a low grain yield (Khush and Juliano, 1985). For many years, breeders have focused their attention on quality improvement, but there seems to be some unknown genetic barrier to incorporating this trait into high-yielding varieties.

Biotechnological Advances

Modern day crop breeding in rice is supplemented with biotechnological tools.  Success stories are fast emerging with the development of golden rice (Ye et al., 2000), and many efforts are on to develop transgenic rice with various incorporated traits, including resistance to pests, herbicides etc.  Successfully the Xa21 gene conferring resistance to Bacterial leaf blight has been cloned. The deciphering the entire rice genome has been completed. Marker assisted frameworks of quantitative trait loci are being developed intensively which will help in developing strong target trait directed marker assisted selection programs.


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Chopra, V.L. and S. Prakash, 2002. Evolution and Adaptation of Cereal Crops. Science Publishers Inc, NH, USA. 
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