Hybrid rice development and use: innovative approach and challenges

L.P. Yuan

China National Hybrid Rice Research and Development Centre

Hybrid rice has about a 30 percent yield advantage over conventional pure line varieties. From 1976 to 1995 hybrid rice technology had helped China to increase rice production by nearly 300 million tonnes. A well-established package of the technology for hybrid rice seed production would greatly boost the expansion of hybrid rice. The technology has been well developed in China to give an average seed yield of 2.3 tonnes/ha nationwide. The field area ratio between A line multiplication, hybrid seed production and F₁ commercial cultivation has been increased to about 1:50:6 000.

Outside China, 16 countries and two international research institutes are endeavouring to develop hybrid rice technology. The International Rice Research Institute (IRRI) has made great progress in breeding CMS lines. The situation for the development and commercialization of hybrid rice in India and Viet Nam is very encouraging. However, the flow of hybrid rice technology to farmers needs to be accelerated. FAO is playing an important role in supporting national hybrid rice programmes for fighting world hunger.

The exploitation of photoperiod-sensitive and thermosensitive genic male sterile (PGMS and TGMS) lines to develop two-line system rice hybrids has great advantages over the classical three-line or CMS system. China is again a pioneer in this innovative approach of hybrid rice technology. The area under the two-line system hybrid rice is rapidly expanding in China. Recently, a super hybrid rice breeding programme has been set up in the China National Hybrid Rice Research and Development Centre. The bottleneck caused by maintaining a higher harvest index without lodging while huge photosynthates are produced has been overcome by Chinese scientists. The goal of breeding super hybrids yielding 100 kg/ha per day is expected to be realized by the year 2000.

CHINA'S ACHIEVEMENTS IN HYBRID RICE R&D

China is the first country to produce hybrid rice commercially. Hybrid rice research was initiated in 1964 (Yuan, 1966) and the genetic tools essential for breeding hybrid rice varieties, such as the male sterile line (A line), the maintainer line (B line) and restorer line (R line), were developed by 1973 (Yuan and Virmani, 1988). Several hybrid combinations with good heterosis and higher yield potential were identified in 1974. Hybrid seed production techniques were basically established in 1975. The first batch of rice hybrid varieties was released commercially in 1976. Since then the area under hybrid rice has been increasing year after year. It has been proved on a large scale for 20 years that hybrid rice has about a 30 percent yield advantage over conventional pure line varieties (Table 1). From 1976 to 1995, hybrid rice had helped China to increase its production by nearly 300 million tonnes. In recent years the yield of hybrid rice has been around 6.7 tonnes/ha while conventional rice varieties yield 5.2 tonnes/ha on average. In 1994 the area under hybrid rice was 15.7 million ha, which covered 50 percent of the total rice area, and the production of hybrid rice was 57 percent of the total rice output in China. The largest hybrid rice-growing province is Sichuan, which has 95 percent of its rice area (3 million ha) under hybrids and has had an average yield of 7.5 tonnes/ha for several years. Hunan is the second largest hybrid rice-growing province, where the average yield from 2 million ha of second crop is 6.8 tonnes/ha. The highest yield record from a single crop on a large scale (1 000 ha) is 11.2 and 16.8 tonnes/ha on a small plot (0.1 ha). The highest yield record from double cropping hybrid rice is 23.3 tonnes/ha so far. In Hunan Province more than 0.2 million ha of double cropping hybrid rice produces 15 tonnes/ha for years.

In the meantime, hybrid seed production technology has been well developed, giving an average seed yield of 2.3 tonnes/ha nationwide (Table 2). The field area ratio between A line multiplication, hybrid seed production and F₁ commercial cultivation was 1:30:1000 in the late 1970s and has increased to about 1:50:6 000 recently. The highest yield record is 7.4 tonnes/ha created by the Zixing Seed Company of Hunan Province on a small plot (0.2 ha) in 1993.

HYBRID RICE DEVELOPMENT OUTSIDE CHINA

Following China's success in the commercialization of hybrid rice technology in the late 1970s, IRRI revived its hybrid rice research in 1980, and much progress has been made by programmes undertaken in many other countries - Japan, the United States, India, Viet Nam, the Philippines, the Republic of Korea, Thailand, Indonesia, Myanmar, Brazil, Egypt, Colombia, Malaysia and the Islamic Republic of Iran - with support from FAO (Trinh, 1992; 1993; 1994; McWilliam, Ikehashi and Sinha, 1995) as well as from public agencies or the private sector. Technical support in hybrid technology has been provided by IRRI and China: with financial support from FAO, the China National Hybrid Rice Research and Development Centre (CNHRRDC - the former Hunan Hybrid Rice Research Centre) has held six international courses on hybrid rice production and trained more than 70 rice scientists from India, Viet Nam and Colombia.

India

India was one of the first countries to start academic studies on hybrid rice. The India Council of Agricultural Research established a goal-oriented network project on hybrid rice in 1989, and this has received further support from the United Nations Development Programme and FAO since 1991. The project is now being operated as a well-organized national research network with 12 centres across the country and the Directorate of Rice Research in Hyderabad acting as coordinator (Paroda and Siddiq, 1996).

The situation for the development and commercialization of hybrid rice in India is very encouraging. Between 1990 and 1996, more than 700 experimental hybrids were developed and evaluated, and the yields of over 100 combinations exceeded that of the best traditional variety by more than 1 tonne/ha. Several hybrid varieties have been released for commercial cultivation. The yield advantage of these hybrids over their check varieties ranged from 16.2 to 44.2 percent (Table 3). Another two hybrids proposed for release are CRH-1 (Coordinated Rice Hybrid-1) from the Directorate of Rice Research and PBH 71 from the Pioneer Overseas Corporation. Besides these, six to eight rice hybrids are being marketed by private seed companies.

After years of extensive trials, a generalized optimum package of hybrid seed production technology has been established in India, with a hybrid seed yield of about 1.5 to 2 tonnes/ha.

In 1995 India planted 10 000 ha of hybrid rice which outyielded inbred varieties by approximately 1 tonne/ha. The area under hybrid rice was 60 000 ha in 1996, and India aims to increase this to 2 million ha by the start of next century (Paroda and Siddiq, 1996) - there are indications, in fact, that this target may be reached earlier than expected. Thus India has emerged as the second largest hybrid rice-growing country in the world.

Viet Nam

Viet Nam started its hybrid rice research in 1983 by introducing breeding lines from China and IRRI to Haugiang in the Mekong River Delta and Hanoi in the Red River Delta. Several experimental rice hybrids from IRRI have shown a 18 to 45 percent yield advantage over Viet Nam's best local inbred varieties at Cuu Long Delta Rice Research Institute (Table 4). Some rice hybrids, such as Shanyou 63, Shanyou Gui 99, Shanyou Quang 12 and Boyou 64, were introduced from China to northern Viet Nam where they have yielded 6.5 to 8.5 tonnes/ha, 18 to 21 percent more than the conventional inbred varieties (Nguyen, Nguyen and Virmani, 1995).

In 1992, Vietnamese Government officials called on FAO to help them intensify development of their hybrid rice technology. With FAO's assistance provided through the project TCP/VIE/2251(A), considerable progress has been made since then in hybrid rice cultivation and seed production. The area under hybrid rice increased rapidly from 20 ha in 1990 to 102 800 ha in 1996 (Table 5), with a yield advantage of 20 to 30 percent over the improved semi-dwarf varieties. The Chinese hybrids are highly adaptable in the northern mountainous area of Viet Nam, where growing conditions are similar to those of southern China, and farmers harvest up to 10 tonnes/ha in Dien Chau (Nghe An Province) and Phu Xuyen (Ha Tay Province). Some Chinese hybrids yield up to 14 tonnes/ha in Dien Bien (Lai Chau Province), 12 tonnes/ha in Hoa An (Cao Bang Province) and 12.6 tonnes/ha in Van Quan (Lang Son Province). The widespread success of hybrid rice in Viet Nam has depended on large amounts of imported seed that has been developed and produced by the Chinese. However the rice hybrids introduced from China are not suited to the tropical conditions of the Mekong River Delta in the south, where IRRI-bred rice hybrids and parental lines instead grow very well.

The yield of hybrid seed production was only 200-680 kg/ha in 1992, but the technology has been improved by years of research in the country. In 1996, F₁ seed yield of 2.1 tonnes/ha was obtained on a large area (Table 6). The yield of F₁ seed production with Boyou 64 was as high as 3 tonnes/ha. However, there is still a big gap between the total seed production and the need for rapidly expanding cultivation area. Viet Nam's dependence on seed imports from China and the resulting scarcity of seed adapted to its southern regions are still major constraints to increased use of rice hybrids in this country.

Japan

Japan has been researching hybrid rice since the 1950s. Sinjyo developed the A, B and R lines of japonica rice (Taichong 65) towards the end of that decade but it has not been used commercially (IRRI, 1980). The Ministry of Agriculture, Forestry and Fisheries started the natio-nal hybrid rice breeding programme in 1983, and the first three-line rice hybrid, Hokuriku-ko 1, was developed in 1985, outyielding the inbred check by about 20 percent (Uehara et al., 1997). The National Federation of Agricultural Cooperative Associations (Zen-Noh) and several private companies, such as RAMM Hybrid International Cooperation, Kirin Brewer Co. Ltd and Sumitomo Chemical Co., are also involved in developing and testing rice hybrids using different approaches (Kato, Maruyama and Uchiyamada, 1994).

Zen-Noh has registered three intervarietal japonica hybrids which yielded 5 to 15 percent more than the in-bred check variety, Todorokiwase. The Ministry has also bred several rice hybrids such as Kanto Cross 1, Ouu Cross 1 and Shu Cross 4781 which have a yield advantage of 16 to 19 percent over the inbred check variety, Akihikari. However, because of their poorer grain quality, not all hybrids are suitable for the Japanese market. Another constraint in the development and use of hybrid rice technology in Japan has been the high cost of seed production.

IRRI

China's successful experience with hybrid rice technology encouraged IRRI in 1979 to explore the prospects and problems of using hybrid rice to increase yields. By 1989, two commercially usable CMS lines, IR58025A and IR62829A with a "WA" cytoplasm, were bred at IRRI and shared with national programmes worldwide (Virmani et al., 1996). IR58025A is stable in sterility in tropical countries, whereas IR62829A has good combining ability but its sterility is not stable enough for hybrid seed production under higher temperatures. In recent years, IRRI's capacity to breed genetically diverse CMS lines has increased and 10 to 20 new CMS lines are now bred annually.

Hybrid seed technology for the tropics has been developed at IRRI in collaboration with national programmes, and the institute's technology packages can now result in a hybrid seed yield of up to 2 tonnes/ha in the tropics.

FAO considers hybrid rice technology a key approach for increasing global rice production to help meet the world's growing food requirements. The Organization is therefore organizing a task force on hybrid rice development in Latin American and Caribbean countries (e.g. Columbia and Brazil) and providing support through Technical Cooperation Programme (TCP) activities in some Southeast Asian countries such as Bangladesh, India, Myanmar, the Philippines and Viet Nam. It is hoped that hybrid rice may play an important role in fighting world hunger in the near future.

CONSTRAINTS AND CHALLENGES

Although tremendous progress has been made in research on the commercial utilization of heterosis in rice over the last 20 years in China, from a strategic point of view, the technology is still in its early stages and the high yield potential of hybrids has not yet been fully tapped. A possible strategy for the development of hybrid rice breeding could follow two phases.

i) Breeding methodology, involving three approaches:

• three-line method or CMS system;
• two-line method or PGMS and TGMS system;
• one-line method or apomixis system.

ii) Increasing the degree of heterosis in rice to increase yield potential using:

• intervarietal hybrids;
• intersubspecific hybrids;
• distant hybrids (interspecific or intergeneric hybrids).
• If successful, each phase would signify a new breakthrough in rice breeding and would result in considerable yield increases.
• The existing rice hybrid varieties used in commercial production belong to the category of intervarietal hybrids and use the CMS, or three-line, system. Although research over the years has proved it to be an effective method - and it will continue to play an important role in the development of hybrid varieties this century - the CMS system does meet with certain constraints:

- The yield of existing rice hybrids, including newly developed varieties, has stagnated for some years now (Fig. 1), meaning a plateau has been reached and, unless new methods and novel materials are found, it will be very difficult to increase yield potential further.

- There are only limited sources of male sterility-inducing cytoplasm (used for developing better CMS lines). Currently, about 85 percent of the A lines used in commercial production still belong to the "wild aborted" (WA) type. The dominant cyto-sterility situation of the WA type has a latent weakness that, over time, could make the hybrid rice vulnerable to a destructive pest.

Figure 1: National average yields of hybrid and conventional rice planted in China, 1976-1995

- The heterosis level of japonica hybrids is not as good as that of indica hybrids. Furthermore, the sterility of japonica CMS lines (BT type) now being used is not stable enough to produce very pure F₁ seeds. Therefore, the cultivation area of japonica hybrids has been limited to around 0.1 million ha for many years while, what is worse, the area of japonica hybrids is declining.

- There is a lack of very early-maturing combinations with strong heterosis suitable for the first crop in double rice-cropping regions along Yangtse Valley, the main rice production region in China. This is another major reason why the area under hybrid rice cannot be increased further.

PROGRESS IN TWO-LINE HYBRID RICE IN CHINA

The discovery in 1973 of Nongken 58 S, a PGMS/TGMS japonica rice line (Shi, 1981; 1985), provided the first genetic source for the development of two-line system hybrid rice. The major feature of such PGMS/TGMS lines is that, under longer day length and higher temperatures they show complete pollen sterility, in which case they can be used for hybrid seed production, while under shorter day length and moderate temperatures they show almost normal fertility and thus can multiply themselves by selfing. In 1987, China initiated a collaborative research project involving the exploitation of PGMS/TGMS lines to develop two-line system rice hybrids. This method has the following advantages over the classical CMS or three-line system:

There are no constraints caused by the restoration-maintenance relationship, since the male sterility of PGMS and TGMS is controlled by only one or two pairs of recessive genes. Nor is there any need for special R genes for fertility restoration, so the choice of parents in developing heterotic hybrids is greatly broadened. Experience has shown that more than 95 percent of existing rice cultivars can restore fertility of the PGMS/TGMS lines to a normal degree, while in the CMS system only about 5 percent of the existing lines can be used as restorers.

• The cost of hybrid seed will be cut down because of the simplified production procedure. With the PGMS/TGMS system, no maintainer line is needed in seed multiplication for male sterile lines.
• Negative effects from the sterile cytoplasm will be avoided, and the vulnerability to destructive diseases or insects that is usually induced by a unitary cytoplasmic resource may be eliminated.
• After more than ten years of nationwide collaborative studies, important progress has been made in two-line system hybrid rice, both in theoretical research and practical application, and a yield advantage of 10 percent over three-line system hybrids has been attained. With techniques advancing each day, two-line system hybrid rice is becoming more popular in large-scale application. The area planted to two-line system hybrid rice has reached about 64 000 ha.
• CNHRRDC has successfully bred the first practical and widely used PGMS/TGMS line, Pei'ai 64S, and the first two-line system hybrid combination, Pei'ai 64S/Teqing, certified for commercial use. Pei'ai 64S/Teqing has been expanded to cover more than 2 000 ha (in 1997) and has been ranked first five times in the history of rice production in Hunan as well as the whole country (Table 7).

These new records indicate that the two-line system hybrid rice has broken through the old yield plateau established long ago by the three-line system hybrid rice and that it has shown great potential for increasing yields even though it is still in its initial stages.

Up to now, ten hybrid combinations and more than ten commercially used PGMS/TGMS lines have been certified and registered in China. It should be noted, however, that the key selection criterion for PGMS/TGMS lines is that the critical sterility-inducing temperature (CST) must be relatively low and should be based on historical meteorological data from the target areas. For example, in the rice regions in central China, the CST of commercial PGMS/TGMS lines should be limited to about 23.5°C.

Another important consideration is the drift up of CST when seed multiplication of PGMS/TGMS lines is normally conducted without any artificial selection. A "core seed" concept has been proposed (Yuan, 1994b) and an effective production procedure for core and foundation seeds has been established so as to ensure that the PGMS/TGMS lines are reliable on a commercial scale (Yuan, 1994b; Deng, Fu and Yuan, 1997).

BREEDING FOR SUPER HYBRID RICE

The target of super high-yielding rice

Several research programmes have been dedicated to super high-yielding rice since 1980s but there is apparently no consensus on what the yield level of these varieties should be. Based on the present situation of hybrid rice production and breeding, and especially considering the breakthrough achieved recently in China, CNHRRDC has set up a super hybrid rice breeding programme with the aim of achieving a super hybrid yield of 100 kg/ha per day by the year 2000 (Yuan, 1997).

The morphological features of super high-yielding hybrid rice

Recently, several two-line intersubspecific hybrids with super high-yielding potential have been identified. For instance, the yield trial of Pei'ai 64S/E32 was conducted on a total area of 0.24 ha at three locations in 1997, where its average yield was as high as 13.26 tonnes/ha and its growth duration 130 days. Pei'ai 64S/E32 has reached the standard of super high-yielding rice, although only in trials on small plots, and it is a good example for breeding super high-yielding rice.

Recent studies and analyses on Pei'ai 64S/E32 have shown that the most important morphological feature of super high-yielding rice lies in the uppermost three leaves, which should be long, erect, narrow, V-shaped and thick. These morphological features allow the very rich source of assimilates necessary for a super yield.

Another prominent feature is that this hybrid has both large and uniform panicles in population. The number of panicles per m² is 260 and the average grain weight per panicle is 5.3 g, while the coefficient of panicle weight variance is only 20 percent. This feature means that the hybrid not only has a very large sink but also a high population photosynthesis efficiency.

The third outstanding characteristic of this hybrid is that it has a taller erect-leaved canopy with drooping panicles. Its canopy height is more than 120 cm while the top of the filled panicles is only 60 to 70 cm above the ground. Such an architecture is more productive in terms of photosynthesis and, at the same time, makes the hybrid highly resistant to lodging, which is also one of the essential characters required for breeding super high-yielding rice (Fig. 2).

Figure 2: Super hybrid rice plant

Strategies for breeding super hybrid rice

i) Utilization of intersubspecific heterosis. It has been proved that the heterosis of intersubspecific hybrids is much stronger than that of intervarietal hybrids; therefore, utilization of intersubspecific hybrids is the most feasible approach for realizing super yields. At present efforts have been focused on using Pei'ai 64S as the major female parent in the selection of super high-yielding combinations. Because Pei'ai 64S is an intermediate type between indica and japonica, it has a very wide compatibility. To exploit the heterosis of intersubspecific hybrids and improve the efficiency of super high-yielding hybrid breeding, the emphasis is on the development of various widely compatible lines, especially those that have a broad spectrum of compatibility, including restorer lines and male sterile lines of indica type, japonica type and the intermediate type with different growth durations. This will ensure abundant parental lines for various super high-yielding hybrids that can be adapted to different ecological environments.

ii) Utilization of favourable genes from wild rice. In 1995, based on molecular analysis and field experiments carried out as part of a cooperative research programme with Cornell University, CNHRRDC identified two favourable QTL genes (yld1 and yld2) from wild rice (O. rufipogon L.). Each of the QTL genes contributed to a yield advantage of 18 percent over the high-yielding hybrid V64 (one of the most élite hybrids in China, with a yield potential of 80 kg/ha per day). By means of molecular marker-facilitated backcrossing and selection, the development of near-isogenic lines carrying these two QTL genes is under way (Xiao et al., 1996).

iii) Utilization of IRRI's new plant types. Khush predicted that: "These new plant types are likely to have 20 percent higher yield potential than the existing high-yielding indicas. The new plant types will be employed in developing indica/japonica hybrids which may have a yield advantage of 20 to 25 percent over the best inbred lines. A combination of the two approaches may raise the yield potential of tropical rice by 50 percent." It seems that IRRI's new plant type varieties could play an important role in hybrid rice breeding for super yields in the future by using them as parental lines.

iv) Utilization of biotechnology. Genetic engineering techniques, such as anther culture, marker-aided selection and gene transformation, offer reliable opportunities for accelerating breeding progress, increasing selection efficiency and transferring genes across species and generic barriers. These will play an important role in breeding for super hybrid rice. For example, if biotechnology can be used to transfer apomixis to rice from grass species, hybrid rice production will be revolutionized and will reach even higher levels.

CONCLUSIONS

The whole package of hybrid rice technology, mainly developed in China, is now available to most rice-growing countries. Generally speaking, the intervarietal hybrids can outyield improved pure line varieties by a margin of around 20 percent and the intersubspecific hybrids have a further 15 to 20 percent advantage.

The classic three-line method can be applied universally while the two-line method, although more advanced, can only be used in certain areas where the climatic conditions are suitable for both hybrid seed production and PGMS/TGMS line multiplication. China's existing rice hybrids may be adaptable to temperate and subtropical regions for commercial production. In tropical areas it will be necessary to develop locally adaptable parental lines as well as hybrid combinations or to introduce IRRI's lines and hybrids for trial and use.

Those countries whose hybrid rice programmes are still in their initial stages should currently aim at developing intervarietal hybrids by using the three-line method. Some countries such as India and Viet Nam, where the development of hybrid rice technology is under way, could start two-line hybrid and intersubspecific hybrid research programmes in addition to using the three-line method. The one-line method and utilization of distant heterosis are is still at the experimental stage, and only highly advanced biotechnology institutes are likely to undertake research in these areas.

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Développement et utilisation du riz hybride: approche novatrice et défis

Depuis quelques années, le riz hybride occupe 50 pour cent de l'étendue des rizières et son rendement est supérieur de 30 pour cent à celui des variétés de lignée pure conventionnelles en Chine. De 1976 à 1995, les technologies du riz hybride ont aidé ce pays à accroître sa production de près de 300 millions de tonnes. Les technologies de production des semences se sont bien développées et permettent d'obtenir un rendement moyen de semences de 2,3 tonnes à l'hectare pour l'ensemble du pays. Sur le terrain le rapport entre multiplication de lignée A, production de semences hybrides et cultures commerciales de F₁, a été accru à environ 1:50:6 000.

Outre la Chine, 16 pays et deux instituts de recherche internationaux s'efforcent de mettre au point des technologies du riz hybride. L'IRRI a beaucoup progressé dans la sélection des lignées à stérilité mâle cytoplasmique (CMS). Le développement et la commercialisation du riz hybride en Inde et au Viet Nam sont très encourageants. Cependant, le transfert des technologies aux agriculteurs doit être accéléré. La FAO joue un rôle important en soutenant les programmes nationaux de développement du riz hybride pour lutter contre la faim dans le monde.

L'exploitation des lignées P/TGMS pour développer des riz hybrides par hybridation double présente un gros avantage par rapport au système classique d'hybridation triple. Là aussi, la Chine est un pionnier dans ce domaine. La superficie vouée à l'hybridation double s'accroît rapidement dans ce pays. Récemment, un grand programme de sélection de riz hybride a été mis en place au Centre national de développement et de recherche sur le riz hybride. Dans ce programme stratégique, l'hybride rassemblerait toutes les caractéristiques de forte efficacité de photosynthèse, large récepteur, indice de récolte élevé et résistance à la verse. L'objectif de la sélection de super-hybrides ayant un rendement de 100 kg à l'hectare par jour devrait être atteint en l'an 2000.

Fomento y utilización del arroz híbrido: métodos innovadores y retos

El arroz híbrido ha ocupado en los últimos años el 50 por ciento de la superficie destinada al arroz y ha tenido un rendimiento superior en un 30 por ciento al de las variedades convencionales de líneas puras en China. De 1976 a 1995, la tecnología del arroz híbrido contribuyó a aumentar la producción arrocera china en casi 300 millones de toneladas. La tecnología de producción de semillas en China se ha perfeccionado notablemente, obteniéndose rendimientos de 2,3 toneladas/hectárea como promedio en todo el país. La proporción entre multiplicación de líneas A, producción de semillas híbridas y cultivo comercial de F₁ ha aumentado a 1:50:6 000.

Fuera de China, 16 países y dos institutos internacionales de investigación están promoviendo tecnologías para el arroz híbrido. El IRRI ha hecho grandes progresos en el mejoramiento de líneas de ECM. La situación en lo que respecta al fomento y comercialización de arroz híbrido en India y Viet Nam es muy alentadora. Sin embargo, es necesario acelerar la transferencia de tecnología para el arroz híbrido a los agricultores. La FAO está desempeñando una importante función mediante el apoyo a programas nacionales sobre el arroz híbrido para luchar contra el hambre en el mundo.

La explotación de líneas de EGMF/T para obtener híbridos de arroz con un sistema de dos líneas presenta grandes ventajas con respecto al sistema clásico de tres líneas. China está también a la vanguardia en este método innovador de la tecnología del arroz híbrido. La superficie cultivada con arroz híbrido de dos líneas se está ampliando rápidamente en China. Ultimamente, el Centro Nacional de Investigación y Desarrollo del Arroz Híbrido de China ha creado un programa estratégico para obtener superhíbridos que reúnan todas las características de una fotosíntesis altamente eficiente y una gran absorción, un alto índice de producción y resistencia al encamado. Se prevé que el objetivo de obtener superhíbridos con un rendimiento de 100 kg/ha por día se alcanzará para el año 2000.