Rice is one of the most important crops in the world, and feeds about 40% of the world population. The yield of rice needs to be increased, since a world food crisis has been predicted for the early 21st century. World annual rice production must increase from its current value of 520 to 810 million tons by 2025 (Hossain 1996). Since rice shows heterosis to give a higher yield, hybrid rice may offer a solution to this predicted food crisis. The yield of hybrid rice is about 15% higher than that of conventional pure line varieties (Yuan 1994). In 1991, hybrid rice covered 17.6 million ha, 55% of the total area of rice in China (Yuan 1994).