Discussion on the Aspects of High-Yielding Breeding in Rice

In the last 60 years, great success on rice production was achieved via improved cultivation technique, utilization of chemical fertilizer, variety improvement and so on. In the aspect related to development of high-yielding rice varieties, the successes can be summarized as three points: (1) development of semi-dwarf varieties of indica rice with the gene of sd-1 in late 1950s and early 1960s; (2) development of elect-panicle varieties of japonica rice with the gene of qEP9-1 in 1960s; (3) development of hybrid rice with the gene of CMS of WA, BT, HL, etc. in 1970s. Generally speaking, rice yield is composed of three components: the panicle number of unit area, the filled grain number per panicle and the grain size (usually measured by the weight of 1000 rice grains). In order to develop super-high-yielding-variety in rice, the key point is to explore new resources of the genes related to plant type, tillering ability, panicle size, grain size, insect and disease resistance, grain filling ability, and so on. Such gene resources usually can be obtained from different subspecies, i.e. indica and japonica. Some important genes also can be obtained from other species for which biotechnique approach is essential including gene cloning and development of molecular makers for the genes. In recent years, lots of target genes related to the characteristics mentioned above have been cloned, such as Sd1, Gn1a, Ga20x-2, Gs3, Bt, EPSPS, qPEP9-1. On the other side, new breeding techniques such as transgenic approach and molecular assisted selection (MAS) to pyramid target genes (or gene combinations) need to be accepted and to be joined with conventional breeding techniques. With reviewing the development for high-yielding variety achieved in last century, several points need to be noticed and discussed: (1) The tillering ability of rice plays an important role in regulating population density related to the efficiency of photosynthesis. The panicle number of unit area also relates to the tillering ability of the varieties. For high and the stabe yield, medium or even higher than medium tillering ability is necessary for the elite varieties. (2) Panicle size is usually measured by the number of filled grains per panicle. Extensive variation on panicle size can be found in different rice varieties. The heterosis of hybrid rice mainly shows the enlargement of panicle size. For developing high-yielding varieties, more attention needs to be paid to enlarging the panicle size. (3) Grain size is one of the most important factors influencing yield potential in rice. The range of grain size measured by 1000-grain weight is 23–29 g in commercial varieties and 15–70 g in rice germplasm. In general, the larger the grain size, the poorer the grain quality. From the point of view of balance between grain yield and grain quality, a medium-large grain size is most favorite for high-yielding varieties.