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
plateausn.平稳时期(单数为plateau)即将发生impendV.指数indexn.生物量biomassn.体细胞的somaticadj.胞质杂种cybridn.clonaladj.无性系的interspecificadj种间的相互的reciprocaladj.n.染色体组，基因组genome纯合性homozygosity隐性基因recessive gene二对等位基因的diallelicadj同源多倍体autopolyploidn.筛选vscreen突变型，突变体mutantn.抗代谢物antimetaboliten.Exercises1.Fill in the blanks with the information fromthe passage1.has led to the plant domestication during the past 5, o00years.2. In the futuretechnology will be used in plant breedingits3.In theory,plant breeding depends on the existence ofrecombination and selection of improved genotypes.4. In fact, cultivars,maybesourcesofvariation5.In plant breeding, some severe difficulties may arise because of lack ofrecombination with6.The final phase in plant breeding is the selection of improved genotypes6
6 plateaus n. 平稳时期(单数为 plateau) impend v. 即将发生 index n. 指数 biomass n. 生物量 somatic adj. 体细胞的 cybrid n. 胞质杂种 clonal adj. 无性系的 interspecific adj. 种间的 reciprocal adj. 相互的 genome n. 染色体组，基因组 homozygosity 纯合性 recessive gene 隐性基因 diallelic adj. 二对等位基因的 autopolyploid n. 同源多倍体 screen v 筛选 mutant n. 突变型，突变体 antimetabolite n. 抗代谢物 Exercises । .Fill in the blanks with the information from the passage. 1. has led to the plant domestication during the past 5, 000 years. 2. In the future, technology will be used in plant breeding. 3. In theory, plant breeding depends on the existence of , its recombination and selection of improved genotypes. 4. In fact, cultivars, may be sources of variation. 5. In plant breeding, some severe difficulties may arise because of lack of recombination with . 6. The final phase in plant breeding is the selection of improved genotypes
resultingfrom7.It is believed that about 50% of yield increase has resultedfrom8. The next major goal in plant breeding may be9. Some aspects ofare already used in plant breeding10.also provides a mechanism to broaden the germplasm baseI. Find in the text where the following ideas are expressed.1. The latest successes in plant breeding come from developments instatistics,mechanizationand computerization2.Future plant breeding will gradually incorporate various tissue culturetechniques.3. Most of the genetic base for improvement in crop yield has been providedby variability within the cultivated germplasm pool.4. Exotic germplasm has been successfully used in some crops5.Mutation isapossiblesourceof geneticvariation6. The numerous genes determine crop yield and quality7. Variability which results from recombination may be severely limited oreven disappear.8.Tissue culture in plant breeding helps overcome current restrictions in plantbreeding.9.Geneticimprovementcontributestoyieldincrease10. Tissue culture is also used in agronomic traitsAnswer the following question in detail.what methods have been developed in plant breeding by tissue culture according tothe passage?
7 resulting from 7. It is believed that about 50% of yield increase has resulted from . 8. The next major goal in plant breeding may be . 9. Some aspects of are already used in plant breeding. 10. also provides a mechanism to broaden the germplasm base. ॥. Find in the text where the following ideas are expressed. 1. The latest successes in plant breeding come from developments in statistics, mechanization and computerization 2. Future plant breeding will gradually incorporate various tissue culture techniques. 3. Most of the genetic base for improvement in crop yield has been provided by variability within the cultivated germplasm pool. 4. Exotic germplasm has been successfully used in some crops. 5. Mutation is a possible source of genetic variation. 6. The numerous genes determine crop yield and quality. 7. Variability which results from recombination may be severely limited or even disappear. 8. Tissue culture in plant breeding helps overcome current restrictions in plant breeding. 9. Genetic improvement contributes to yield increase. 10. Tissue culture is also used in agronomic traits. Answer the following question in detail. what methods have been developed in plant breeding by tissue culture according to the passage?
Part IIReading and PracticePassage IAims of crop breeding1.BreedingforIncreasedProteinYieldWhen selecting for increased grain yield, it is important that the plant breedershould at the same time consider the protein content of his new varieties. This isobviously important indeveloping countries, wherewheat comprises a largepartofthe human diet. But it is essential to view the problem in its proper perspective and inparticular to appreciate that protein content is of secondary importance when the totalenergy value of the diet is limiting.Under these circumstances proteins consumed willbe used as an additional source of energy and will not therefore be available forgrowth or tissue replacement.Nevertheless, programmes to breed wheat and barleyvarieties with in creased protein percentages, and with improved amino acid balanceof these proteins have been started in India and other developing countries. At thesame time, increased areas of wheat cultivation in these countries following theintroduction of highyieldingshort strawed varietieshas led to a reduction intheareadevoted to grain legumes such as chickpea .This has resulted in a reduction in theprotein content of the diet in some areas, and in particular in a fall in the content oflysine and other limiting amino acids.Even in Western Europe, cereals provide an important part of the protein in thehuman diet, and also in the diet of animals, which consume 47% of the wheatand 65% of the barley grown in the United Kingdom. So far as human diet isconcerned, protein content is most important in relation to bread making quality inwheat and tomalting quality in barley.When a range of wheat or barley varieties are grown under the same conditions,there is a strong negative association between protein percentage and grain yield,8
8 Part II Reading and Practice Passage I Aims of crop breeding 1. Breeding for Increased Protein Yield When selecting for increased grain yield, it is important that the plant breeder should at the same time consider the protein content of his new varieties. This is obviously important in developing countries, where wheat comprises a large part of the human diet. But it is essential to view the problem in its proper perspective and in particular to appreciate that protein content is of secondary importance when the total energy value of the diet is limiting. Under these circumstances proteins consumed will be used as an additional source of energy and will not therefore be available for growth or tissue replacement. Nevertheless, programmes to breed wheat and barley varieties with in creased protein percentages, and with improved amino acid balance of these proteins have been started in India and other developing countries. At the same time, increased areas of wheat cultivation in these countries following the introduction of high yielding short strawed varieties has led to a reduction in the area devoted to grain legumes such as chickpea . This has resulted in a reduction in the protein content of the diet in some areas, and in particular in a fall in the content of lysine and other limiting amino acids. Even in Western Europe, cereals provide an important part of the protein in the human diet, and also in the diet of animals, which consume 47% of the wheat and 65% of the barley grown in the United Kingdom. So far as human diet is concerned, protein content is most important in relation to bread making quality in wheat and to malting quality in barley. When a range of wheat or barley varieties are grown under the same conditions, there is a strong negative association between protein percentage and grain yield
though the protein content of any variety may be increased by appropriateapplications of nitrogenous fertilizer. This situation was demonstrated by Pushmanand Eingham who compared the yields of a range of winter wheat varieties grownunder three contrasting regimesThey also showed that despite the negativeregression of protein percentage on yield observed under each regime, certainvarieties consistently had higher protein contents than would have been expected fromthe overall regressions, suggesting that it should be possible for the plant breeder toselect for high yielding varieties with satisfactory protein content. They also showedthat the protein yield per hectare of modern varieties was up to 10% greater than thatof older varieties they had replaced.Despite the encouraging results obtained by Dobereiner, Day and von Bulowthere is no immediate likelihood of developing cereal plants capable of synthesisingproteins from atmospheric nitrogen. Increase in protein percentage can therefore onlybe achieved by selection for varieties which either assimilate soil nitrogen moreeffectively or which achieve a better distribution of protein within the plant. It ishowever important to appreciate that any increase in grain protein, whether byreduction of nitrate, or in the longer term by fixation of atmospheric nitrogen, canonlybe achieved by the expenditure of photosynthetic energy which might otherwisehave been used in the synthesis of carbohydrates.Furthermore, the energy required to synthesize a gramme of protein is aboutdouble that required to synthesize a gramme of carbohydrate.There are thus four ways in which selection for increased grain protein contentmay be attempted:() The breeder may select for maximum dry weight at anthesis. Such selectionmay also be necessary in order to achieve increased yield, though it may be difficultto combine with selection for shorter strawed varieties.(2) He may select for varieties which break the close correlation of total drymatter and protein content. Such varieties have been identified by Vogel, Johnsonand Mattern, and have been widely used in breeding programmes in many countries.(3) He may select for varieties in which a high proportion of total plant nitrogen9
9 though the protein content of any variety may be increased by appropriate applications of nitrogenous fertilizer. This situation was demonstrated by Pushman and Eingham who compared the yields of a range of winter wheat varieties grown under three contrasting regimes . They also showed that despite the negative regression of protein percentage on yield observed under each regime, certain varieties consistently had higher protein contents than would have been expected from the overall regressions, suggesting that it should be possible for the plant breeder to select for high yielding varieties with satisfactory protein content. They also showed that the protein yield per hectare of modern varieties was up to 10% greater than that of older varieties they had replaced. Despite the encouraging results obtained by Dobereiner, Day and von Bulow there is no immediate likelihood of developing cereal plants capable of synthesising proteins from atmospheric nitrogen. Increase in protein percentage can therefore only be achieved by selection for varieties which either assimilate soil nitrogen more effectively or which achieve a better distribution of protein within the plant. It is however important to appreciate that any increase in grain protein, whether by reduction of nitrate, or in the longer term by fixation of atmospheric nitrogen, can only be achieved by the expenditure of photosynthetic energy which might otherwise have been used in the synthesis of carbohydrates. Furthermore, the energy required to synthesize a gramme of protein is about double that required to synthesize a gramme of carbohydrate. There are thus four ways in which selection for increased grain protein content may be attempted: (1) The breeder may select for maximum dry weight at anthesis. Such selection may also be necessary in order to achieve increased yield, though it may be difficult to combine with selection for shorter strawed varieties. (2) He may select for varieties which break the close correlation of total dry matter and protein content. Such varieties have been identified by Vogel, Johnson and Mattern, and have been widely used in breeding programmes in many countries. (3) He may select for varieties in which a high proportion of total plant nitrogen
is translocated to the grain.Austin et al.report significant differences in this ratio,defined as nitrogen harvest index, though high values may be associated with varietiesin which photosynthetic activity of the leaves and ears falls off more rapidly afteranthesis,leaving moretimeforthetranslocation of nitrogen from those organstothe grain.(4) Finally he may select for genotypes in which nitrogen uptake continues afteranthesis, but the metabolic energy required for this process may again be associatedwith genotypes with long continued Photosynthetic activity, and hence with lowernitrogenharvest index.2.BreedingforReliability of PerformanceAny analysis of theprospects of breeding for yield must consider also problemsof breeding for reliability of performance and the need for more widespread use ofimproved husbandry techniques so as to narrow the gap between national averageyields and those obtained by the best farmers. Detailed consideration of problems ofdisease and pest control lie outside the scope of the present review, though losses dueto these organisms are one of the principal causes of year to year variations in yieldMention should however be made of the recent work on breeding for durableresistance to leaf diseases and the use of mixtures and multilines in order to limit therisk of losses due to newly arising virulent races especially of airborne pathogensattackingleavesandstems.It has been suggested that work on breeding for disease resistance is no longernecessary because the diseases can now be readily and cheaply controlled byfungicides. But I consider this a dangerous concept, partly because the cost offungicides and their application is likely to increase, but more particularly because ofthe veryreal dangerthat strains of thepathogens may develop withvirulence againstsprayed crops.Such virulence has already been reported on several occasions, as inthecaseof ethirimol(乙菌定)toleranceofpowderymildewinbarleyIthereforesuggest that although fungicides have a very valuable role to play, their action shouldbe monitored in variety trials with untreated controls.This should lead to anintegrated control system in which losses are restricted by the use of low rates of10
10 is translocated to the grain. Austin et al. report significant differences in this ratio, defined as nitrogen harvest index, though high values may be associated with varieties in which photosynthetic activity of the leaves and ears falls off more rapidly after anthesis, leaving more time for the translocation of nitrogen from those organs to the grain. (4) Finally he may select for genotypes in which nitrogen uptake continues after anthesis, but the metabolic energy required for this process may again be associated with genotypes with long continued Photosynthetic activity, and hence with lower nitrogen harvest index. 2. Breeding for Reliability of Performance Any analysis of the prospects of breeding for yield must consider also problems of breeding for reliability of performance and the need for more widespread use of improved husbandry techniques so as to narrow the gap between national average yields and those obtained by the best farmers. Detailed consideration of problems of disease and pest control lie outside the scope of the present review, though losses due to these organisms are one of the principal causes of year to year variations in yield. Mention should however be made of the recent work on breeding for durable resistance to leaf diseases and the use of mixtures and multilines in order to limit the risk of losses due to newly arising virulent races especially of airborne pathogens attacking leaves and stems. It has been suggested that work on breeding for disease resistance is no longer necessary because the diseases can now be readily and cheaply controlled by fungicides. But I consider this a dangerous concept, partly because the cost of fungicides and their application is likely to increase, but more particularly because of the very real danger that strains of the pathogens may develop with virulence against sprayed crops. Such virulence has already been reported on several occasions, as in the case of ethirimol(乙菌定) tolerance of powdery mildew in barley. I therefore suggest that although fungicides have a very valuable role to play, their action should be monitored in variety trials with untreated controls. This should lead to an integrated control system in which losses are restricted by the use of low rates of