专业英语阅读材料适合种子科学与工程专业使用二零一八年九月
专业英语 阅读材料 适合种子科学与工程专业使用 二零一八年九月
Unit 1Plant BreedingPart IReading and ComprehensionBreeding Development1. Past SuccessesDuring the past 5, 000 years, plant breeding has resulted in the domestication andspread of many species far outside their original area of domestication. Thesesuccesses were accomplished with little conscious knowledge of pathologybiochemistry, genetics or plant physiology. In the last 100 years, as our understandingof biological processes has expanded dramatically, large sustained increases in yieldshave been achieved in many crops. Developments in statistics, mechanization andmost recently in computerization have also made integral contributions to these latestsuccesses. Despite being labeled conventional, modern plant breeding alreadyintegrates diverse knowledge and technologies and an awareness of the politics andpracticalities of farming.We are confident that future plant breeding will alsogradually incorporate various tissue culture techniques and, eventually, sophisticatedgene transfer technology.2.Current Plant Breeding and Its ConstraintsPropositions involving the application of new technology to plant breeding mustrecognize both the basic nature of plant breeding and its practical constraints. Plantbreeding depends conceptually upon the existence of genetic variation, itsrecombination and selection of improved genotypes.Details of the process vary,according to whether the species is self-or cross-pollinated, or propagated asexually,but the principles are relatively universal.Practically, sources of variation may be cultivars, land races, or related species.Variability within the cultivated germplasm pool has provided most of the genetic2
2 Unit 1 Plant Breeding Part I Reading and Comprehension Breeding Development 1. Past Successes During the past 5, 000 years, plant breeding has resulted in the domestication and spread of many species far outside their original area of domestication. These successes were accomplished with little conscious knowledge of pathology biochemistry, genetics or plant physiology. In the last 100 years, as our understanding of biological processes has expanded dramatically, large sustained increases in yields have been achieved in many crops. Developments in statistics, mechanization and most recently in computerization have also made integral contributions to these latest successes. Despite being labeled conventional, modern plant breeding already integrates diverse knowledge and technologies and an awareness of the politics and practicalities of farming. We are confident that future plant breeding will also gradually incorporate various tissue culture techniques and, eventually, sophisticated gene transfer technology. 2. Current Plant Breeding and Its Constraints Propositions involving the application of new technology to plant breeding must recognize both the basic nature of plant breeding and its practical constraints. Plant breeding depends conceptually upon the existence of genetic variation, its recombination and selection of improved genotypes. Details of the process vary, according to whether the species is self-or cross-pollinated, or propagated asexually, but the principles are relatively universal. Practically, sources of variation may be cultivars, land races, or related species. Variability within the cultivated germplasm pool has provided most of the genetic
base for improvement in crop yield,and still continues to be the major source in somecrops, e. g.maize. However, many modern varieties derive from a restricted geneticbase into which new genes have been incorporated by back-crossing, thus preservingcarefully established blocks of genes, but not broadening the genetic base. Exoticgermplasm extends the range of variation available and has been very successfullyused in some crops such as tomato.However, wider use of exotic germplasm is constrained by its generally pooragronomic suitability, which necessitates lengthy back-crossing to incorporate the fewdesirable characters into highly adapted backgrounds. Because of this, exoticgermplasm has most commonly been used in attempts to introgress single genes,and it may not prove useful in the improvement of characters under complex geneticcontrol. In very wide crosses, lack of recombination with the host genotype may alsoprovide severe practical difficulties. Thus, in most crops, variability alone does notappear to be a serious limitation, rather it is the accompanying undesired genes whichform the obstacle to rapid progress. Mutation is another possible source of geneticvariation which has proven useful in some situations. Mutation does not necessarilyextend the total variability available, but it may provide it in a more suitablebackground.Recombination of the numerous genes which determine crop yield and quality isthe next usual phase of plant breeding. The large number of these genes, most ofwhich are unidentifiable and have complex interrelationships, requires that a largenumber of individuals be maintained in a breeding programme. Lack ofrecombination due to insufficient homology may be a constraint where very diverseparents are used.In asexually propagated species,variability resulting fromrecombination may be severely limited, or even precluded.The selection of improved genotypes resulting from manipulation of geneticvariation is the final phase in plant breeding.For characters like disease-resistance,which are usually under relatively simple genetic control, selection can be effectivelymade in early generations. Yield, however, cannot be reliably selected at this stage,and its interaction with environment requires replicated comparison of numerous3
3 base for improvement in crop yield, and still continues to be the major source in some crops, e. g.maize. However, many modern varieties derive from a restricted genetic base into which new genes have been incorporated by back-crossing, thus preserving carefully established blocks of genes, but not broadening the genetic base. Exotic germplasm extends the range of variation available and has been very successfully used in some crops such as tomato. However, wider use of exotic germplasm is constrained by its generally poor agronomic suitability, which necessitates lengthy back-crossing to incorporate the few desirable characters into highly adapted backgrounds. Because of this, exotic germplasm has most commonly been used in attempts to introgress single genes, and it may not prove useful in the improvement of characters under complex genetic control. In very wide crosses, lack of recombination with the host genotype may also provide severe practical difficulties. Thus, in most crops, variability alone does not appear to be a serious limitation, rather it is the accompanying undesired genes which form the obstacle to rapid progress. Mutation is another possible source of genetic variation which has proven useful in some situations. Mutation does not necessarily extend the total variability available, but it may provide it in a more suitable background. Recombination of the numerous genes which determine crop yield and quality is the next usual phase of plant breeding. The large number of these genes, most of which are unidentifiable and have complex interrelationships, requires that a large number of individuals be maintained in a breeding programme. Lack of recombination due to insufficient homology may be a constraint where very diverse parents are used. In asexually propagated species, variability resulting from recombination may be severely limited, or even precluded. The selection of improved genotypes resulting from manipulation of genetic variation is the final phase in plant breeding. For characters like disease-resistance, which are usually under relatively simple genetic control, selection can be effectively made in early generations. Yield, however, cannot be reliably selected at this stage, and its interaction with environment requires replicated comparison of numerous
advanced lines over many sites and several seasons.Official registration proceduresmay require additional time, and a total span of between seven and fourteen years isusual for the production of a new cultivar.The various applications of tissue culture have the potential to help overcome allthese current restrictions: they may extend the genetic base, allow morerecombination in wide crosses and, for some characters, provide faster and moreefficientmethods ofselection.3.FutureRoleof PlantBreedingThe magnitude of yield increases in many crops during the last 50 years hasfrequently stimulated the question of the limits to genetic improvement. Comparisonof current and past yield levels may be confounded by different agronomic andbreeding inputs and sociopolitical conditions. However, recent analysis of yieldincreases during the last 50 years suggests that about 50% has been attributable togenetic improvement. Yield plateaus have apparently not been reached in most cropsand there is no convincing evidence to suggest an impending end to continued geneticimprovement.However,therateof improvement sometimes seems slow,and sourcesof new major improvements in yield should be sought. Increases in harvest index haveled to large yield increases in the past, but as harvest indices approach about 0. 65these increases cannot be expected to be sustained.Increasing biomass at a highharvest index may therefore become the next major goal in plant breeding.Meanwhile, specific achievements, such as resistance to disease and tolerance toenvironmental stresses,will continue to provide small sustained increases in yield4.Scopeof Tissue Culture inPlant BreedingSome aspects of tissue culture are already employed in plant breeding. Specificdevelopments include micropropagation, embryo culture, haploidy,in vitro selectionand production of somatic hybrids and cybrids by protoplast fusion. Micropropagationisemployed in theclonal propagationof specific cropgenotypes fortheproductionofhybrid seed and for rapid multiplication of specific varieties in the horticulture andforestry industries. The use of embryo culture can overcome post-pollinationincompatibility to enable rescue of interspecific hybrids, and allows the genetic base4
4 advanced lines over many sites and several seasons. Official registration procedures may require additional time, and a total span of between seven and fourteen years is usual for the production of a new cultivar. The various applications of tissue culture have the potential to help overcome all these current restrictions: they may extend the genetic base, allow more recombination in wide crosses and, for some characters, provide faster and more efficient methods of selection. 3. Future Role of Plant Breeding The magnitude of yield increases in many crops during the last 50 years has frequently stimulated the question of the limits to genetic improvement. Comparison of current and past yield levels may be confounded by different agronomic and breeding inputs and sociopolitical conditions. However, recent analysis of yield increases during the last 50 years suggests that about 50% has been attributable to genetic improvement. Yield plateaus have apparently not been reached in most crops, and there is no convincing evidence to suggest an impending end to continued genetic improvement. However, the rate of improvement sometimes seems slow, and sources of new major improvements in yield should be sought. Increases in harvest index have led to large yield increases in the past, but as harvest indices approach about 0. 65 these increases cannot be expected to be sustained. Increasing biomass at a high harvest index may therefore become the next major goal in plant breeding. Meanwhile, specific achievements, such as resistance to disease and tolerance to environmental stresses, will continue to provide small sustained increases in yield. 4. Scope of Tissue Culture in Plant Breeding Some aspects of tissue culture are already employed in plant breeding. Specific developments include micropropagation, embryo culture, haploidy, in vitro selection and production of somatic hybrids and cybrids by protoplast fusion. Micropropagation is employed in the clonal propagation of specific crop genotypes for the production of hybrid seed and for rapid multiplication of specific varieties in the horticulture and forestry industries. The use of embryo culture can overcome post-pollination incompatibility to enable rescue of interspecific hybrids, and allows the genetic base
of crop species to be significantly broadened. Somatic hybridization by protoplastfusion also provides a mechanism to broaden the germplasm base. Protoplast fusionenables the reciprocal exchange of cytoplasmic organelles and possible geneticrecombination between genetically dissimilar mitochondria or chloroplast genomesAnther culture has enhanced the capacity to generate large numbers of haploidplants.For crop improvement, haploid enables the achievement of rapidhomozygosity, enhanced selection efficiency for recessive genes, and breeding at adiallelic state for autopolyploid species.In vitro screening in tissue culture providesthe capacity to isolate biochemical mutants, particularly those which confer resistanceto antimetabolites. Tissue culture selection is being extended to include agronomictraits for which there is a demonstrated, or presumed, correlation with a definite cellculture response.(1011 words)FromPlant Cell CultureTechnology byM,M.YeomanWordsandexpressionsPathologyn.病理学Sophisticated a.尖端的，深奥的Propositionn.建议Constraintn.局限性Blocks of genes基因区组Exotic adj.外来的农艺的Agronomicadj.V.使成为必要necessitate基因渗入introgress4.阻止,妨碍precludeV重复,复制replicateV.系linen.n.重要性magnitude使混淆confoundV
5 of crop species to be significantly broadened. Somatic hybridization by protoplast fusion also provides a mechanism to broaden the germplasm base. Protoplast fusion enables the reciprocal exchange of cytoplasmic organelles and possible genetic recombination between genetically dissimilar mitochondria or chloroplast genomes. Anther culture has enhanced the capacity to generate large numbers of haploid plants. For crop improvement, haploid enables the achievement of rapid homozygosity, enhanced selection efficiency for recessive genes, and breeding at a diallelic state for autopolyploid species. In vitro screening in tissue culture provides the capacity to isolate biochemical mutants, particularly those which confer resistance to antimetabolites. Tissue culture selection is being extended to include agronomic traits for which there is a demonstrated, or presumed, correlation with a definite cell culture response. (1011 words) From Plant Cell Culture Technology by M. M. Yeoman Words and expressions Pathology n. 病理学 Sophisticated a. 尖端的，深奥的 Proposition n. 建议 Constraint n. 局限性 Blocks of genes 基因区组 Exotic adj. 外来的 Agronomic adj. 农艺的 necessitate v. 使成为必要 introgress v. 基因渗入 preclude v. 阻止,妨碍 replicate v. 重复,复制 line n. 系 magnitude n. 重要性 confound v. 使混淆