Increasing yield potential in rice by exploitation of heterosis

The degree of heterosis in different hybrid rice varieties has the following general trend: indica/japonicaindicajavanicajaponica/javanicaindica/indicajaponica/japonica. The yield potential of the best existing indica/indica hybrids developed by the CMS system, in terms of per day per unit area, is around 75 kg/ha (from seed to seed), about 15 percent higher than pureline varieties. The best two-line system indica/indica hybrids can outyield CMS indica/indica hybrids by 5-10 percent. Heterosis of japonica/javanica hybrids could be used to increase japonica hybrid yields. Their grain quality retains japonica characteristics. Indica/japonica hybrids possess the highest yield potential in both sink and source. Theoretically, they may have a 30 percent yield advantage over the best existing indica/indica hybrids. However, there are some problems in such intersubspecific hybrids - semisterility, plants too tall, very long growth duration, and many poorly filled grains. So far, most problems have been overcome through two-line breeding methods, except for the poor filling of a number of fertilized grains. The strategy of developing very high-yielding indica/japonica hybrids for commercial production is discussed