Longping High-tech International Training center
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隆平高科国际培训学院 Longping High-tech International Training center 1.3 An Overview of the breeding procedures The relation between the various nurseries described above is shown in the above diagram. 2 Principles in Selecting Parents for Better Hybrid Combinations The selection of parents for making hybrid combinations plays a very important role in exploitation of heterosis in rice.The following five major factors should be considered: 2.1 Genetic Diversity Genetic diversity is the base for inducing heterosis. Within a certain range, the greater the genetic diversity between the two parents,the stronger the heterosis.Here,genetic diversity mainly consists of the two parents being from distant geographic areas or of different ecotypes or different subspecies. 2.2 Complementary characteristics Complementation of the good characteristicsof both parents makes the combined characteristics of their hybrids superior to the parents so that the Fl shows obvious heterobeltiosis. For instance, Zhen-Shan 97 A,the female parent of Shan-You 6, is blast-resistant, with short growth duration,weak tillering ability and heavier 1000-grain weight; whereas the male parent IR26 is resistant to bacterial leaf blight, has a longer growth duration, strong tillering ability and lighter 1000-grain weight. However, the F1 hybrid exhibits resistance not only to blast but also to bacterial leaf blight, with an above-average growth duration, good tillering ability, and above-average 1000-grain weight. 2.3 Combining ability As a new CMS line or restorer line is developed, an evaluation of its combining ability is needed so as to avoid blindness in selection of parents for crossing. The general combining ability (GCA) of parents has greater effects on yield potential and certain economic characteristics of hybrid rice. It is necessary to select parents with a high effective value in combining ability with respect to tillering ability,panicle number per plant, number of filled grains per panicle, 1000-grain weight,and grain weight per plant. Some parents possess a specific combining ability (SCA) with respect to certain economic factors. For instance, the 1000-grain weight of F1 hybrids is generally between those of the two parents.While the CMS line Er-Jiu-Nan 1 is crossed with IR24 or IR661, the hybrid shows superiority over its better parent in 1000-grain weight. 2.4 Heritability The parents for rice hybrids should be selected mainly according to the parameter of their generalized heritability. A high heritability characteristic indicates that it is inherited in a major proportion by the parents and less influenced by the environment. It has been practically proven that productive panicles, grain weight per plant and tillering ability are of weak heritability,while growth duration,brown rice percentage and 1000-grain weight are of stronger heritability and number of filled grains per panicle and percentage of empty grains are of intermediate heritability. 42 42 中国杂交水稻技术援外培训基地 China-Aid Hybrid Rice Technology Training Center It is very important to understand the heritability and dominance-recessiveness relationship of the major characteristics of CMS lines and restorer lines prior to their being selected for producing hybrid combinations with good heterosis. 2.5 High yielding potential A hybrid rice variety can produce higher absolute yield only when one or both parents are an HYV.The superior hybrid combinations now used in China, such as the Wei-You system (Wei-You 6, 30,35,49,64,etc.)and the Shan- You system(Shan-You 63,Gui 33,Gui 99, etc.)are all made by using CMS lines and restorer lines which not only have a great genetic diversity but are also developed from elite varieties. 3 Breeding Hybrid Rice for Good Grain Quality In general,the conventional rice varieties with a good grain quality have lower yields. Hybrid rice breeding has the advantage of quickly integrating high yield with good grain quality.However, genetic segregation for some quality characteristics will appear among rice grains produced in the F1 plants since grains harvested from F1 rice hybrids are the F2 seed generation. Research by IRRI and Chinese scientists has shown that genetic segregations for chemical characteristics of F2 grains borne on the commercial F1 crop do not impair its cooking and eating qualities,and hybrids with desired grain quality can be produced with the appropriate selection of parents.A comparison of the milling, physical and chemical qualities of grains harvested from the IRRI-bred hybrid IR64616H and the conventional pure line variety IR57301-195-3-3 indicated that the grain qualityof the hybrid was not inferior to the conventional variety. Comparison of grain quality of the hybrid IR64616H and the pure line variety IR57301-195-3-3 Characteristics IR64616H IR57301-195-3-3 Grain size Long,slender Long,slender Amylose(%) 24.9 22.1 Protein(%) 7.6 7.1 Gel consistency(mm) 84.8(soft) 61.2(soft) Gel temperature.(1-9 sc 4.3 4.1 Milling recovery(%) 64.9 66.4 Head rice(%) 44.5 48.2 Chalkiness(%) 8.1 4.8 Acceptability(%) Raw 87.4 84.6 43 Cooked 78.0 76.6 43 隆平高科国际培训学院 Longping High-tech International Training center Source:Adapted from Proc. of the Rice Varietal Improvement Group of the Philippine Seed Board,Philippine Rice Research Institute,Maligaya,Munoz,Nueva Ecija, 1992. Following are the major determinants of grain quality and their genetic features in hybrid rice: (1)Head rice recovery in hybrid rice can be higher or lower than that of the better parent. Therefore,by carefully selecting the parents,hybrids with higher head rice recovery can be obtained. However,when either of the parents has a high rate of broken grains, the F1 hybrid will normally give a lower head rice recovery than the better parent. (2) Chalkiness and endosperm translucency are important quality characteristics for market acceptability.The genetic feature of chalkiness is similar to that of head rice recovery.For a better grain quality, both parents should be free from or have less chalkiness. Parents with widely different endosperm translucencies should not be involved in a combination because a clear-cut segregation in endosperm translucency of F2 grains would reduce market acceptability. (3) Amylose content of a hybrid is generally in between that of its parents. The amylose content of hybrid varieties should meet the consumer preference in different areas. Generally,an intermediate content is popular. Hybrids between low- and high-amylose parents will give an intermediate amylose content in F2 bulk seed samples and are preferred by most rice consumers. (4) Aroma in rice is controlled by recessive genes. By crossing aromatic A lines with non-aromatic restorers (A/R) or vice versa, only a small portion of F2 grains are aromatic. Such scented rice is just right for consumer preferences. (5)In waxy/non-waxy combinations,there is one fourth of waxy rice in the F2 grains. Such mixed rice is required only in some special cases. 44 中国杂交水稻技术援外培训基地 China-Aid Hybrid Rice Technology Training Center Download 72.07 Kb. Do'stlaringiz bilan baham: |
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