Hybrid rice . It is the rice obtained from the cross between two or more genetically different parents. Its achievement is considered an important result in crop breeding, given the increase between 15 to 20% of its yields compared to commercial or self-pollinated varieties.
Summary;What Is Hybrid Rice;10 Facts
- 1 Antecedents
- 2 Development of hybrid rice
- 3 Advantages of hybrid rice over commercial rice varieties
- 1 Superiority of hybrids in morphological characteristics
- 2 Superiority of hybrids in physiological behavior
- 3 Superiority of hybrids in grain yield
- 4 Rice Genetic-Cytoplasmic Male-Sterility System
- 1 Three-line system for obtaining hybrid seed
- 1.1 Line A
- 1.2 Line B
- 1.3 Line R
- 2 Interrelationships between the three lines
- 1 Three-line system for obtaining hybrid seed
- 5 Two-line system for producing hybrid seed
- 1 Advantages of the two-line rice hybrid breeding system
- 2 Characteristics of environmentally induced genetic male sterility
- 6 Hybrid seed production
- 7 internal links
- 8 external links
- 9 Fountains
The genetic improvement of plants , an alternative of great significance, is undoubtedly the exploitation of the phenomenon of heterosis or hybrid vigor in increasing yield potential. This phenomenon has been widely used initially in cross-pollinated crops, such as corn , sorghum , cotton and others; but currently its use is expanding to various autogamous or self-fertilizing crops such as rice .
Professor Yuan Lonping , internationally regarded as “Father of Hybrid Rice “, for his contributions to commercial hybridization in riceIn the past, the adoption of hybrid rice technology had been considered impractical due to the strictly self-pollinating nature of the plant and skepticism about the feasibility of commercial-scale seed production. Fortunately, rice breeders, led by Chinese professor Yuan Lonping, have managed to overcome this obstacle through the development of a cytoplasmic genetic male sterility system, and efficient and inexpensive commercial-scale seed production packages have been established.
Currently, more than 50% of China ‘s rice area is cultivated with hybrid rice , and several countries outside of China are already developing this technology , to the point of exploiting it commercially.Open florets on the mature panicle of a fertile rice plant , showing yellowish anthers.Open florets on the mature panicle of a male – sterile rice plant, showing off-white anthers
Development of hybrid rice
Rice is a strictly self-pollinated crop ; therefore, to develop hybrid rices , it is essential to have a male sterility system. Male sterility by genetic means means that the pollen is not viable, or what is the same, it cannot fertilize the ovule and thus give way to the normal formation of the seed . In this way, the spikelets or florets of the inflorescence ( panicle ) of the rice are not capable of producing seeds by self-fertilization and depend on external pollen to achieve fertilization.fertilization .
Hybrid rice field in a state of maturation, where large and numerous panicles can be seen , which makes it have a better yieldHermaphrodite plants , whose male reproductive organ (stamen) is unable to fertilize the female reproductive organ (pistil), are recognized as male-sterile and only these can be used as female parents (mothers) of hybrid rices . Growing sterile plants alternately with fertile plants in plots properly insulated from foreign pollen can produce a mass quantity of seed due to cross-pollination with an adjacent pollinator.
The result of the fertilization of the florets of sterile pollen plants , also known as ” sterile male ” or “andro- sterile “, gives rise to hybrid seed , which is used by producers in commercial hybrid crops .
Advantages of hybrid rice over commercial rice varieties
A large body of experimental and commercial production evidence has confirmed that, in many respects, hybrid rice exhibits significant “superiority”, which is exhibited summarily in its morphological characteristics, physiological behavior and grain yield .
Superiority of hybrids in morphological characteristics
- Vigorous root system. Hybrid ricehas strong rooting ability, as the quantity and quality of hybrid rice roots are markedly superior to conventional rice varieties.
- Great tillering capacity. Rapid growth in the vegetative state and maintenance of this advantage until panicle( inflorescence ) appearance appears to be characteristic of rice hybrids . Hybrids have greater tillering capacity and vegetative vigor compared to many of the conventional varieties (self-pollinated or self-pollinated).
- Larger panicles and heavier grains . In general, commercially used rice hybridshave about 150 spikelets per panicle – with a maximum of more than 200 – and a population density of7 to 3 million panicles per hectare . The weight of 1,000 kernels is around 28-35 g.
Superiority of hybrids in physiological behavior
- Increased root activity. Root activity of hybrid rice, as measured by exudate pressure, has been found to be consistently higher, even than that of the parents, during the period from tillering to panicle , and especially in the period of its formation.
- larger photosynthetic area. The leaf area of hybrid riceincreases most rapidly in the early stages of growth and reaches a maximum at the time of completion of the paniculation phase. In general, the green leaf area of hybrid rice is larger than that of the parent lines or plants , and dry matter production is generally considered to be the main reason for the larger effective area .
- Lower intensity of breathing. The photorespiration intensity of hybrid rice is lower than those of its parental lines or plants at the time of initiation of panicle or panicle differentiation . The intensity of photorespiration of the hybrids is even lower than that of conventional high-yielding varieties, which generally show relatively low photorespiration.
- Photosynthetic efficiency compared to conventional varieties. Hybrid rice hasa higher or lower photosynthetic efficiency depending on the combinations used, the growth stage and the cultivation conditions . In general, the photosynthetic efficiency of hybrid rice at the tillering or tillering stage is significantly higher than that of the parental lines or plants . Such efficiency is accompanied by a rapid increase in green leaf area , leading to the early establishment of a high-yielding population .
- Distribution of the assimilated. Hybrid rice isnot only superior in the accumulation of assimilates but also in its translocation from the leaf and sheaths to the panicle . At full bloom the total dry weight of the top three leaves of the hybrid is generally 50-70% higher than that of a conventional variety. In addition, in regard to the dry weight per unit of leaf area , it is higher in the hybrid than in the conventional varieties at the time of flowering initiation but becomes lower at the stage of maturity, implying that in thehybrid , more assimilates are translocated from the leaves to the panicle .
Superiority of hybrids in grain yield
It has been shown practically and on a large scale for many years that hybrid rice has a 15 to 30% yield advantage over conventional inbred varieties. All of which is the result of superiority in the aspects analyzed above.
Rice Genetic-Cytoplasmic Male-Sterility System
Rice is a strictly self-fertile species ; therefore, hybrid development must involve an effective male-sterility system to produce hybrid seeds in mass quantities. Cytoplasmic-genetic male-sterility (GCMS) has been identified as a practical way to develop commercial rice hybrids . Knowledge of GCEM and fertility restoration systems help breeders make more efficient use of genetic tools for the development of F1 hybrids.
Three-line system for obtaining hybrid seed
The so-called three lines are: the cytoplasmic-genetic male-sterile line (MEGC or line A), the maintainer line (line B), and the restorer line (line R).
This line refers to a special line for crosses whose anthers are abnormal. There is no pollen or this pollen aborts in the MEGC anthers, therefore, in this line, no seed is formed by self-pollination. However, the pistils of the MEGC line are normal and can produce seeds when pollinated by normal rice . A desirable MEGC line should not only have good agronomic characteristics but also the following characteristics:
- Male- sterilestability : male-sterility should be inherited from generation to generation without any change in pollen sterility and should not be influenced by environmental changes, especially temperature fluctuations .
- Easy restoration: this refers to two aspects; firstly, the proposed male- sterilesystem should have a broad restoration spectrum so that the probability of selecting superior hybrid combinations is greater; second, seed setting of restored hybrids should be stable and less influenced by adverse environments .
- Good flower structure and flower habits: the MEGC line should flower normally and the daily flowering timeshould be synchronized with the male parent; its stigma should be well developed and exserted during and after flowering ; the opening of the glume should last longer and have a greater angle.
Hybrid rice seed production field , where the fringes of Line A (wider) and Line R (narrower) are observed
This line is a specific pollinator used to pollinate the corresponding A line and therefore produces progenies that are still male- sterile . If there were no B line or maintainers, the A line or MEGC could not be maintained and multiplied from generation to generation. The main characteristics of an A line are determined by its corresponding B line; in fact, line A and its maintainer (line B) can be considered “twins”. They are similar to each other in appearance and differ only in some features.
It is a pollinating variety used to pollinate the A line and produce hybrids that return to normal fertility and thus can produce seeds by selfing. As a restorative top line you should have:
- ) A strong restorative capacity, eg the seed settingof hybrids should be the same as that of the normal variety.
- ) Better agronomic characteristics and a good combining ability.
- ) A higher plant height , a growthduration similar to or somewhat longer than line A.
- ) Well developed anthers with a large number of pollengrains , good flowering habit and normal dehiscence.
Interrelationships between the three lines
When the three lines have been obtained, it is possible to start the production of hybrid seeds on a large scale and thus be used commercially in rice . Line A (MEGC) is planted in rows alternated with line B (maintainer) in an isolated plot to multiply seeds from line A each year .
In another isolated plot , line A and line R (restorative) are planted in alternate rows to produce hybrid seeds , which are planted and cultivated by farmers as hybrid rice .
Two-line system to produce hybrid seed
Two new genetic tools were recently developed in China for application in rice : genetically photosensitive male- sterile lines (MEGFS) and genetically thermosensitive male- sterile lines (MEGTS). Alterations in the fertility of these lines induced by environmental factors are known, in general, as genetically sensitive environment male-sterility (MEGSA). The MEGFS and MEGTS lines play an important role in the development of the two-line hybrid rice breeding system.
Advantages of the system for obtaining rice hybrids with two lines
Exploitation of environmentally sensitive lines for hybrid rice has the following advantages over the classical three-line method or GCEM system:
- It can simplify the hybrid seed production processand reduce the cost of hybrid seed since B-line is not needed. MEGSF lines under long day conditions and MEGTS lines under high temperatures show complete pollen sterility . Under conditions of short days or low temperatures they show almost normal fertility and can multiply by self- pollination .
- Since male-sterility is controlled by recessive genesin the MEGSF and MEGTS lines, almost all common rice varieties can easily restore fertility to these male- sterile lines . Therefore, the choice of parents to develop superior hybrids is much wider and, as a result, the probability of obtaining better hybrid combinations is greater.
- The MEGSF and MEGTS genes can be easily transferred to any ricecultivar to obtain new male- sterile lines for different breeding programs .
- The male-sterility in the MEGSF and MEGTS lines is unrelated to the cytoplasm.
Characteristics of environmentally induced genetic male sterility
MEGSF lines The characteristics of MEGSF rice are:
- The stage of photoperiodsensitivity for impaired fertility is the period from secondary branch primordium development to pollen mother cell formation .
- The critical daylength to induce impaired fertility is 13.75 to 14 hours.
- The critical intensity of lightto induce sterility is above 50 Lux.
- Temperature hasa certain influence on the alteration of fertility ; beyond certain temperature limits the length of the day will have no effect on the alteration of fertility as indicated in the following model.
This model is very important and useful to guide the development of the MEGSF lines. The high critical temperature affects the multiplication of the male- sterile line under short-day conditions, while the low critical temperature affects the sterility fluctuation of the MEGSF line when it is occasionally exposed to low temperatures under long-day conditions. According to this model there are two important points in the programs for obtaining MEGSF lines. Firstly, the low temperature that induces male sterility must be sensibly low; second, the range of photosensitive temperatures (RTSF) must be very wide.
MEGTS Lines Alteration of fertility of MEGTS lines is primarily temperature induced . Existing MEGTS lines become completely male- sterile under high temperature conditions and regain fertility at low temperatures ; day length has little influence on fertility . The characteristics of the MEGTS lines are as follows:
- The thermosensitive stage for impaired fertility is the period from the formation of the pollenmother cell to the start of the meiotic division.
- The critical temperature to induce male sterility is 23 to 29°C, with variations between the different lines.
- Under critical temperatureconditions for three consecutive days, the MEGTS lines recover male fertility .
The most important practical problem for the use of these lines is that the critical temperature that induces male-sterility must be relatively low, especially in temperate zones. In tropical and sub-tropical areas, MEGTS lines are more useful than MEGSF lines since the day length is relatively short and the temperature is rather high, while MEGSF lines are preferable for high latitudes .
Hybrid seed production
Site selection The selection of a suitable site for hybrid seed production is of paramount importance. In the isolated area , the essential requirements are a field with fertile soil , an adequate irrigation and drainage system , abundant solar radiation and minimal problems with insects and diseases , especially those to which the quarantine regulations refer.
Insulation Pollen grains are very small and light and can travel great distances on the wind ; To ensure the purity of hybrid seed and prevent pollination by other undesirable varieties, hybrid seed production plots should be strictly isolated by the following methods:
- distance isolation. An isolation distance of more than 100 meters is generally satisfactory; Within this distance it is not acceptable to growother varieties of rice in the same season , unless they have the same parental pollen .
- Isolation in time. A period of 20 days is generally required; that is, the emergence of the panicles of undesirable varieties around the production plots should be 20 days earlier or 20 days later than in line A.
- isolation barriers. In some places topographical features – hills or forests– or tall crops – maize , sugar cane , jute , sorghum – covering a distance of more than 30 meters can be used as means of isolation.
Synchronization of flowering The formation of the seed in the female parent -line A- depends on cross- pollination , so it is important to synchronize the start date of paniculation of both parents, especially in the case of those hybrid combinations in which the R and A lines are quite different in the length of the growing period . Also, to extend the pollen supply , the R line is usually sown twice at an interval of five to seven days and alternately every two or three rows.
Row ratio and orientation and planting patterns Row ratio refers to the proportion of male and female rows in hybrid seed production plots. The furrow ratio design depends on:
- the length of the growthperiod of the R line;
- the vigor of the growthof the R line;
- the amount of pollenproduced;
- the plantheight of line R.
For the R line, about 45,000 groups of plants per hectare are generally necessary . Line R is transplanted with one or two seedlings in each hole at a distance of 15 cm between plants and 200 to 250 cm between one row of restorer and another, with the rows of line A between them. In the seed production plots sown with double rows of the R line, these are spaced between 30 and 40 cm. Line A is transplanted with two seedlings per group at a distance of 15 x 15 cm or 15 x 20 cm, for which about 300,000 groups of seedlings per hectare are necessary .
Prediction and adjustment of the date of formation of the panicle Although the sowing intervals between both parents can be determined with some certainty, the synchronization of flowering may not be obtained due to variations in temperature and differences in the management of the crop. crop. Therefore, it is necessary to predict the start date of paniculation and take all appropriate measures as soon as possible to make the necessary adjustments for good timing.
Gibberellin Application Gibberellin ( GA3 ) is an effective plant growth hormone that stimulates cell elongation. The application of GA3 is the key to high seed yields and has several advantages, as it can:
- cause the base of the panicle of line A to be less enclosed in the sheath of the flagleaf or to emerge entirely from this sheath ; that way the entire panicle can grow above the flag leaf;
- increase the exertion of the stigma;
- adjust plant height; _
- cause faster growth of small tillers so that they get closer to the larger tillers.
Supplemental pollination It is recommended to shake the panicles of the R lines by dragging a rope or a stick during anthesis to cause pollen flightand better and wider distribution, thus increasing the rate of outcrossing . This process is most effective on days with no wind or a light breeze. Where seed production plotsare irregular in shape or undulating in topography , and provided sufficient manpower is available, the stick-passing method is more recommended. In other conditions, the method of passing a thick rope is practiced, with which thepanicles of line R, passing against the wind at a speed of 1 to 1.5 m per second and parallel to the furrows. Pollination is usually carried out in the morning hours when line A flowers. If only line R flowers and line A does not flower, supplementary pollination should not be done . In the afternoons, when the R line is still flowering, supplemental pollination should be continued even if the A line has closed its glumes. In general, supplemental pollination is carried out at 30- minute intervals three to five times a day until no more pollen is present.on the R line; it is not necessary to do it when the intensity of the wind is greater than that of a breeze.
Off-type plants The genetic purity of hybrid rice seeds used in commercial production must be greater than 98%; to meet this standard the purity of the R and A lines must be greater than 99%. Therefore, in addition to ensuring strict isolation, it is necessary to remove all off-type plants from seed production plots. This removal is done two or three times: before paniculation initiation, at the initial stage of paniculation initiation, and before harvest . Maintainer plants and semi -sterile plants that appear in row furrows along with other plantsmixed off-types in both the male and female furrows must be completely eliminated.