China National Hybrid Rice Research and Development Center, Changsha Province, China
STRATEGY OF HYBRID RICE BREEDING
The development of hybrid rice breeding may be divided into the following phases:
In terms of breeding methodology there are three approaches:
- Three-line method or CMS system
- Two-line method or PGMS and TGMS system
- One-line method or apomixis system
For increasing the degree of heterosis, i.e. yield potential, the exploitation of heterosis in rice may be divided into three levels:
- Intervarietal hybrids
- Intersubspecific hybrids (indica/japonica hybrids)
- Distant heterosis (using yield enhancing genes from other species or genera)
Each of the phases marks a new breakthrough in rice breeding and will result in a marked increase in rice yield if it is attained.
The existing rice hybrid varieties used in commercial production belong mainly to the category of intervarietal hybrids using the CMS system. Experience gained over many years proves that CMS can still play an important role in the future, although the system is not free from constraints and problems:
The yield of the existing three-line intervarietal hybrids including newly developed ones has been stagnant for years. Their average yield per hectare nationwide reached 6.6 tonnes in the late 1980s, but has remained at around 6.8 tonnes in recent years. A yield plateau has been reached and yield potential cannot be further increased without the invention and adoption of new methods and novel materials.
The sources of male-sterility-inducing cytoplasm for developing better CMS lines are poor. At present, about 85 percent of the CMS lines used in commercial production still belong to WA type. This may make the hybrid rice susceptible to a sudden outbreak of insect pests.
RECENT PROGRESS AND ACHIEVEMENTS
In the field of rice heterosis breeding, many Chinese rice scientists have attempted to explore new technological approaches for increasing the yield potential of rice. The most successful attempt to date has been the development of intersubspecific (indica/japonica) hybrids using the two-line method plus morphological improvement. It represents a new phase of hybrid rice breeding in China.
Experiments, large-scale demonstration and commercial production have all proved that the two-line intersubspecific hybrid rice (referred to by the media as super hybrid rice) can outyield high-yielding three-line intervarietal hybrid rice by around 20 percent. For example, in 2000, the pioneer combination, P64S/9311, gave an average yield of 10.5 t/ha when it was tested in 20 locations. The size of the test plot was 6.7 ha/location. The average yield of this hybrid in commercial production was 9.6 t/ha (240 000 ha) in 2000 and 9.2 t/ha (1.2 million ha [Mha]) in 2001. Another combination, P64S/E32, yielded a record 17.1 t/ha in an experimental plot (720 m2) in 1999.
There has been a breakthrough in the breeding of early-season hybrid rice. A newly developed short-growth-duration two-line indica/japonica hybrid is being demonstrated: the area under demonstration is 7 ha and it is estimated that the average yield is 9.5 t/ha, which may outyield CK1 (three-line intervarietal hybrid) and CK2 (inbred variety) by more than 20 and 40 percent, respectively.
Efforts are now focused on developing second-phase super hybrid rice capable of yielding 12 t/ha on a large scale (i.e. 15% higher than pioneer super hybrids) and good progress is being made. There were three new two-line indica/japonica hybrid combinations outyielding CK (pioneer super hybrid) by between 8 and 18 percent in replicated trials in 2001. Two promising combinations yielded 13.5 and 15 t/ha in experimental plots (plot size 100 m2) in winter 2001 in Hainan Island. It is expected that the goal of breeding second-phase super hybrid rice can be achieved by 2005.
Crop improvement practices have to date pointed to just two effective ways of increasing the yield potential of crops through plant breeding: morphological improvement and heterosis utilization. However, morphological improvement alone gives only limited potential, while heterosis breeding produces undesirable results if not combined with morphological improvement. Any other breeding approaches and methods which apply high technology (e.g. genetic engineering) must be incorporated into good morphological characters and strong heterosis, otherwise there will be no actual contributions to yield increase. On the other hand, further development of plant breeding for high target depends on the progress of biotechnology.
Good plant type is the foundation for super high yield. Since the concept of ideotype was proposed by Dr Donald, rice breeders have proposed several models for super high-yielding rice. The most famous is the new plant type proposed by Dr Khush. Its main features are: big panicles (250 spikelets per panicle); fewer tillers (3-4 productive tillers per plant); and short and sturdy culm. Experience will show whether or not this model can realize super high yield.
Studies of a high-yielding combination, P64S/E32 (with a grain yield of 17.1 t/ha) showed that the super high-yielding rice variety has the following morphological features:
Tall erect-leaved canopy
The upper three leaf blades should be long, erect, narrow, V-shaped and thick. Long and erect leaves not only have a larger leaf area but can also receive light on both sides and do not put each other in the shade; light is therefore used more efficiently. Narrow leaves occupy a relatively small space and therefore allow for a higher effective leaf area index. The V-shape makes the leaf blade stiffer and not prone to droopiness. Thick leaves have a higher photosynthetic function and are not easily senescent. These morphological features mean a huge source of assimilates essential to super high yield.
Lower panicle position
The tip of the panicle is only 60 to 70 cm above the ground during the ripening stage. Such architecture enables the plant to be highly resistant to lodging - an essential character for breeding super high-yielding rice varieties.
Larger panicle size
Grain weight per panicle is approximately 5 g and there are about 300 panicles/m2. The theoretical yield potential is 15 t/ha.
Grain yield = Harvest Index (HI) x biomass. Nowadays HI is very high (above 0.5). Further lifting of the rice yield ceiling should rely on increasing biomass because further improvement of HI is quite limited. From a morphological point of view, raising plant height is the effective and feasible way of increasing biomass. However, this approach will cause lodging. To solve this problem, many breeders are trying to make the stem thicker and sturdier, which usually results in HI decrease, however, which means that it is difficult to obtain super high yield. The plant model of a taller leafy canopy can combine the advantages of higher biomass, higher HI and higher resistance to lodging.
Utilization of indica/japonica heterosis
Studies show that the heterosis level in rice has the following general trend: indica/japonica > indica/javanica > japonica/javanica > indica/indica > japonica/japonica. Indica/japonica hybrids possess very large sink and rich source, the yield potential of which is in theory 30 percent higher than intervarietal indica hybrids. Therefore, efforts have focused on using indica/japonica heterosis to develop super hybrid rice. However, there are numerous problems in indica/japonica hybrids (in particular, very low seed set) which must be solved in order to use their heterosis in practice. Thanks to wide compatibility (WC) genes and the use of intermediate type lines as parents instead of typical indica or japonica lines, a number of intersubspecific hybrid varieties with stronger heterosis and normal seed set have been successfully developed.
Utilization of favourable genes from wild rice
This is another promising approach in the development of super hybrid rice. Based on molecular analysis and field experiments, two yield-enhancing quantitative traits loci (QTLs) from wild rice (O. rufipogon L.) were identified. Each of the QTL genes contributed to a yield advantage of 18 percent over the high-yielding CK hybrid Weiyou64 (one of the most elite hybrids). By means of molecular marker-assisted backcross and field selection, an excellent R line (Q611) carrying one of these QTLs is developed. Its hybrid, J23A/Q611, outyielded CK hybrid by 35 percent in a replicated trial for the second cropping in 2001. Its yield potential on a large scale is being evaluated.
The yield standard of second phase super rice (12 t/ha) may be achieved by 2005. Reaching this target means that 2.25 t/ha more rice can be produced (i.e. an increase of 30 million tonnes [Mt] of grains per year) and 75 million more people can be fed once 13 Mha are commercialized.
Rice still has great yield potential which can be further tapped using advanced biotechnology. C4 genes from maize have been successfully cloned and are being transferred into the parental lines of super rice. Using these transgenic lines to develop super hybrid rice, the yield potential of rice could be further increased by a big margin. Relying on this progress, the phase III super hybrid rice breeding programme is proposed with a yield target of 13.5 t/ha by 2010.