PROGRESS OF TW0-LINE SYSTEM HYBRID RICE BREEDING
L.P.Yuan (Hunan Hybrid Rice Research Center, Changsha)
Recently two new kinds of rice genetic tools viz., photoperiod sensitive genic male sterile (PGMS) lines and thermo-sensitive genic male sterile (TGMS) lines have been successfully developed in China. Their male sterility is mainly controlled by a pair of recessive nuclear genes, and has no relation to cytoplasm. Exploitation of these MSlines to develop hybrid rice has the following advantages over the classical three-line or CMS system:1. B line is not needed. Under longer daylength the PGMS lines or under higher temperature the TGMS lines show complete pollen sterility, thus they can be used for hybrid seed production in these conditions. Under shorter daylenghth or temperate conditions they show almost normal fertility, thus can multiply themselves.2. The choice of parents in developing heteroic hybrids is greatly broadened.Studies showed that over 97.6% varities tested (within subspecies) can restore such MS lines. In addition, PGMS and TGMS genescan be easily transferred to any rice cultivar.3. There is no negative effects caused by sterile cytoplasm and the unitary cytoplasm situation will be avoided.Several better inter-varietal hybrid combinations produced by two-line,system were under regional trial and farmer's field trail in 1989.The results were promising, some of them outyielded the best existing hybrids by about 5%. It is estimated that the planting area of two-line system hybrid rice will be extended more than 17,000 ha in 1990.The development of inter-subspecies especially indica/japonica hybrids with very high yield potential through two-line system is still under way.The major problems presented in such F_1 hybrids, namely semi-sterility, too tall plant height and long growth duration, have been overcome. But the plumpness of many fertilized grains is not good in most combinations. Now, efforts are focused on solving this problem.