Since its domestication about 10,000 years ago, rice has undergone tremendous modifications so much as a result of human selection for improved traits that domesticated rice varieties can no longer survive in the wild state. The simple acts of reaping and sowing are selective. Our ancestors may not have known it, but they started the first rice-breeding programs when they began to grow rice plants for their use. Most farmers have a keen eye and a sensitive feeling for plants. Millions of farmers have applied this keen insight and sensitivity for thousands of years to select diverse varieties. Selection was first practiced on the variable and heterogeneous wild and semiwild populations, which must have narrowed the genetic variability. However, several mechanisms in primitive agriculture, such as the introduction of varieties from one region to another and occasional natural crosses, enhanced variability for further selection. Natural crosses between domesticated crop and the weed complexes were another source of variability. The third source of variability was varietal mixtures that primitive agriculturists grew as a protection against disease epidemics. Occasional intercrosses between component varieties generated variability. This conscious and unconscious selection by humans led to the development of over 120,000 rice varieties grown around the world.
Thus farmers themselves were responsible for most rice improvement from the time of its domestication to about 1900. The best known examples are the "rono" varieties such as "Shinriki" that Japanese farmers selected in the 1890s. The rono varieties are shorter and therefore responded to nutrient inputs with higher yields. Rice-breeding stations were established in China, India, and Japan in the early 20th century. Rice breeders' initial activities were the purification of existing varieties (landraces) through pure line selection. This resulted in pure line varieties. Up to the 1960s rice farmers in tropical and subtropical Asia grew thousands of landraces or pure line varieties, and few had been touched by modern agricultural science. These varieties were tall and weak stemmed and late maturing. When nitrogenous fertilizer was applied at rates exceeding 40 kg/ha, traditional varieties tillered profusely, grew excessively tall, lodged early, and yieldedlessthantheywould have with lower fertilizer inputs.
The International Rice Research Institute (IRRI) was established in 1960 in the Philippines to address the problems of stagnant yields. A major breakthrough in raising the yield potential of tropical rice came with the development of IR8 at IRRI in 1966, which resulted in a doubling of the yield potential of rice. IR8 has a short stature and a combination of several other agronomic traits such as sturdy stems for lodging resistance, dark green and erect leaves, and high tillering capacity. Because of lodging resistance it is highly responsive to fertilizer. Since the development of IR8 a series of improved rice varieties have been developed at IRRI and by the National Agricultural Research Systems (NARS). These varieties have been improved in many other traits such as grain quality, disease and insect resistance, growth duration, and tolerance to abiotic stresses. More than 300 varieties have been selected from the breeding materials developed at IRRI (Khush and Virk 2002). These and others developed by NARS are now planted on 80% of the world's rice land. Because of widescale adoption of these varieties and associated technology, world rice production increased 135% in a 35-year period from 257 million tons in 1966 to 600 million tons in 2000, and, during the same period, average rice yield increased from 2.1 t/ha to 3.9 t/ha. Most of the major rice-growing countries achieved self-sufficiency in rice.
During this intensive breeding effort rice varieties have been developed that have genes from various eco-types of rice. Even the genes from wild species have been introduced into modern varieties. Thus the eco-typic differentiation present in the landraces of rices no longer exists in the improved varieties. Genes from numerous landraces have been incorporated into new varieties. For example, widely grown IR64 has 20 landraces in its ancestry (Khush 1987).
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