fungicide on varieties with a moderate level of durable disease resistance3.BreedingforSpecificAdaptationThe breeding and trialling procedures used in most countries are designed toselect varieties which perform well over a wide range of environments. Althoughmost systems, such as that used in Britain, include trials grown at contrasting sites andcontinued over a number of years, the varieties chosen for commercial exploitationare normally those which perform well over a wide range of environments and havebeen selected forwideadaptation from an early stage in the breedingprogramme.Furthermore the trial sites used rarely include areas of low fertility or severecontamination by soil borne diseases, largely because such sites are not usuallyavailable on experimental farms, and trials grown on them frequently have largecoefficients ofvariability.It has been suggested, however, that varieties selected for overall goodperformance over a range of sites may yield less well under specified conditions thanvarieties selected specifically for those conditions. A project to test this hypothesis inrelation to winter wheat breeding has recently been started at Cambridge. Thisinvolves comparisons of the performance of selections made following a normalautumn sowing at the Plant Breeding Institute with that of selections from a very latesowing and from sowings on a sandy site at which water deficit is likely to occur. It isplanned to multiply early generation selections made at each of the three sites so thatthey can be compared in yield trials under each environment. There are obviouslogistic limitations to the economic value of varieties bred to meet narrowly definedecological requirements.It is hoped, however, that these experiments will showwhether it is desirable to breed for specific environments, and that the environmentsconsidered are of sufficiently widespread occurrence to warrant the development ofvarieties specificallyadapted to them.4.BreedingforWaterDeficitIt is in particular felt that there is considerable scope in breeding varietieswhich are tolerant of water deficit. Austin and Quorrie considered a range ofexperiments with wheat and barley grown on light soils in Eastern England and found11
11 fungicide on varieties with a moderate level of durable disease resistance. 3. Breeding for Specific Adaptation The breeding and trialling procedures used in most countries are designed to select varieties which perform well over a wide range of environments. Although most systems, such as that used in Britain, include trials grown at contrasting sites and continued over a number of years, the varieties chosen for commercial exploitation are normally those which perform well over a wide range of environments and have been selected for wide adaptation from an early stage in the breeding programme. Furthermore the trial sites used rarely include areas of low fertility or severe contamination by soil borne diseases, largely because such sites are not usually available on experimental farms, and trials grown on them frequently have large coefficients of variability. It has been suggested, however, that varieties selected for overall good performance over a range of sites may yield less well under specified conditions than varieties selected specifically for those conditions. A project to test this hypothesis in relation to winter wheat breeding has recently been started at Cambridge. This involves comparisons of the performance of selections made following a normal autumn sowing at the Plant Breeding Institute with that of selections from a very late sowing and from sowings on a sandy site at which water deficit is likely to occur. It is planned to multiply early generation selections made at each of the three sites so that they can be compared in yield trials under each environment. There are obvious logistic limitations to the economic value of varieties bred to meet narrowly defined ecological requirements. It is hoped, however, that these experiments will show whether it is desirable to breed for specific environments, and that the environments considered are of sufficiently widespread occurrence to warrant the development of varieties specifically adapted to them. 4. Breeding for Water Deficit It is in particular felt that there is considerable scope in breeding varieties which are tolerant of water deficit. Austin and Quorrie considered a range of experiments with wheat and barley grown on light soils in Eastern England and found
that correction of water deficit had led on average to an 18% increase in crop yield. Itis, of course, unlikely that a yield advance of this size could be obtained by breeding,but there is considerable genetic variability available in characters associated withwatereconomyLupton et al.demonstrated varietal differences in root development in a rangeof wheat varieties. They showed that short strawed varieties had more roots at depthsof 5Ocm or more below the surface than taller ones, and that this was associated withamoreeffectiveuptakeofphosphorus.Butamoreextensiverootsystemmaynotbeassociated with better drought tolerance. Indeed, an extensive root system in the earlystages of crop development may lead to an early exhaustion of soil water, and thus toa water deficit during the critical period of grain formation.Varietal difference in drought tolerance may therefore be better studied byconsideration of the water relations of the aerial parts. Quarrie studied therelationship of abscisic acid and proline accumulation in droughted plants with thedevelopment of water stress and withchanges in stomatal conductance.Hecompared eight spring wheat cultivars grown under severely droughted conditions inpots and found the expected close correlations of abscisic acid and proline contentswith changing leaf water potential.But he also found varietal differences in the rateof abscisic acid accumulation as leaf water potential decreased, with a suggestion thatthe more drought tolerant varieties accumulated abscisic acid more slowly.Assessmentoftherateof abscisicacidaccumulationmaythereforeprovideamethodof assaying drought tolerance, but it is unlikely that such a relationship has a generalapplication, and it is possiblethat simple morphological characters such as stomataldistribution or even leaf waxiness may be equally useful in assessing varietaldifferences in drought tolerance(1650 words)FromPhysiological Aspects of Crop Productivity byF.GH.LuptonWords and expressionsChickpea鹰嘴豆12
12 that correction of water deficit had led on average to an 18% increase in crop yield. It is, of course, unlikely that a yield advance of this size could be obtained by breeding, but there is considerable genetic variability available in characters associated with water economy. Lupton et al. demonstrated varietal differences in root development in a range of wheat varieties. They showed that short strawed varieties had more roots at depths of 50cm or more below the surface than taller ones, and that this was associated with a more effective uptake of phosphorus. But a more extensive root system may not be associated with better drought tolerance. Indeed, an extensive root system in the early stages of crop development may lead to an early exhaustion of soil water, and thus to a water deficit during the critical period of grain formation. Varietal difference in drought tolerance may therefore be better studied by consideration of the water relations of the aerial parts. Quarrie studied the relationship of abscisic acid and proline accumulation in droughted plants with the development of water stress and with changes in stomatal conductance. He compared eight spring wheat cultivars grown under severely droughted conditions in pots and found the expected close correlations of abscisic acid and proline contents with changing leaf water potential. But he also found varietal differences in the rate of abscisic acid accumulation as leaf water potential decreased, with a suggestion that the more drought tolerant varieties accumulated abscisic acid more slowly. Assessment of the rate of abscisic acid accumulation may therefore provide a method of assaying drought tolerance, but it is unlikely that such a relationship has a general application, and it is possible that simple morphological characters such as stomatal distribution or even leaf waxiness may be equally useful in assessing varietal differences in drought tolerance. (1650 words) From Physiological Aspects of Crop Productivity by F. G. H. Lupton Words and expressions Chickpea 鹰嘴豆
赖氨酸Lysinen.Maltingadj.麦芽的Nitrogenousfertilizer氮肥制度，状况Regimes退化，回归Regression污染Contamination逻辑的Logisticlight soils轻质土proline脯氨酸试验assaying蜡质waxinesspassage IIcrop types and breeding methodsCrops can be classified according to their system of propagation and the relevantmethods of breeding into:(1) Vegetatively propagated crops: potato, cassava, sugar cane etc(2) Autogamous crops: wheat, rice, barley, soya bean, tobacco etc.(3)Allogamous crops:maize, rye, sugar beet, sunflower, oil palm, manygrasses etc.Between groups 2 and 3 there is an intermediate class of crops which are partlyself pollinated and partly cross pollinated, such as sorghum. The breeding methodsappropriate to this class are basically no different from those used for groups 2 and 3and will therefore not be treated separately. Group 3 can be subdivided into twoclasses of importance for breeding: crops in which selection can take place beforeflowering, e. g. sugar beet and cabbage; crops in which selection cannot take placeuntil afterflowering,e.g.maizeand rye.Any attempt to modify the principal methods used in breeding or to developalternative methods must take into account the following properties of plants:13
13 Lysine n. 赖氨酸 Malting adj. 麦芽的 Nitrogenous fertilizer 氮肥 Regimes 制度，状况 Regression 退化，回归 Contamination 污染 Logistic 逻辑的 light soils 轻质土 proline 脯氨酸 assaying 试验 waxiness 蜡质 passage II crop types and breeding methods Crops can be classified according to their system of propagation and the relevant methods of breeding into: (1) Vegetatively propagated crops: potato, cassava, sugar cane etc. (2) Autogamous crops: wheat, rice, barley, soya bean, tobacco etc. (3)Allogamous crops: maize, rye, sugar beet, sunflower, oil palm, many grasses etc. Between groups 2 and 3 there is an intermediate class of crops which are partly self pollinated and partly cross pollinated, such as sorghum. The breeding methods appropriate to this class are basically no different from those used for groups 2 and 3 and will therefore not be treated separately. Group 3 can be subdivided into two classes of importance for breeding: crops in which selection can take place before flowering, e. g. sugar beet and cabbage; crops in which selection cannot take place until after flowering, e. g. maize and rye. Any attempt to modify the principal methods used in breeding or to develop alternative methods must take into account the following properties of plants:
--Dioecy. Inbreeding by self fertilization is not feasible. The sexes may differ intheir value as a crop. Sex manipulation to give pure male or female strains may bepossible. Examples are asparagus, spinach and papaya.--Monoecy. Inbreeding by self fertilization is possible. Hybrid varieties can beproduced. Sex manipulation to give pure female strains may be possible.Examplesare maize, castor oil plant and cucumber.--Hermaphrodite flowers.Emasculation is not readily effected. Male sterilityor self incompatibility is necessary for hybrid production, especially in self-pollinatedcrops. Examples include tomato, rye, wheat and rice.1.Vegetatively Propagated CropsIn these crops, improvement is achieved mainly through careful choice of parents,followed by the production of a large F1 population and selection for favourablecharacteristics. Selection in early years is mainly for characteristics with high H?(broad-sense heritability) the clones from every F1 seedling maintained beingsubjected to increasing selection intensity in successive years.Once the clonescontain a sufficient number of plants, selection for wide adaptability and forcharacters with low H is also carried out.Infection of the plants by viruses constitutes a major hazard during maintenanceand propagation. Measures must also be taken against other pathogens transmitted byvegetatively propagated material.In order to introduce desirable characters, especially various kinds of resistance,much use is made of crosses with related species.This normally necessitates recurrentback crossing.Fertility is not particulary important as a varietal character.Thus manypotato varieties are male sterile or only moderately fertile2.AutogamousCropsImprovement in self-pollinated or, more strictly, autogamous crops began withthe selection of wild progenitors and proceeded to selection from land races.Although land races in Europe were subjected to intensive selection in the last quarterof the 19th and the first quarter of the 20th century, they are now used primarily as asource of special characters. In developing countries land races still comprise a wealth14
14 -Dioecy. Inbreeding by self fertilization is not feasible. The sexes may differ in their value as a crop. Sex manipulation to give pure male or female strains may be possible. Examples are asparagus, spinach and papaya. -Monoecy. Inbreeding by self fertilization is possible. Hybrid varieties can be produced. Sex manipulation to give pure female strains may be possible. Examples are maize, castor oil plant and cucumber. -Hermaphrodite flowers. Emasculation is not readily effected. Male sterility or self incompatibility is necessary for hybrid production, especially in self-pollinated crops. Examples include tomato, rye, wheat and rice. 1. Vegetatively Propagated Crops In these crops, improvement is achieved mainly through careful choice of parents, followed by the production of a large F1 population and selection for favourable characteristics. Selection in early years is mainly for characteristics with high H 2 (broad-sense heritability) the clones from every F1 seedling maintained being subjected to increasing selection intensity in successive years. Once the clones contain a sufficient number of plants, selection for wide adaptability and for characters with low H 2 is also carried out. Infection of the plants by viruses constitutes a major hazard during maintenance and propagation. Measures must also be taken against other pathogens transmitted by vegetatively propagated material. In order to introduce desirable characters, especially various kinds of resistance, much use is made of crosses with related species. This normally necessitates recurrent back crossing. Fertility is not particulary important as a varietal character. Thus many potato varieties are male sterile or only moderately fertile. 2. Autogamous Crops Improvement in self-pollinated or, more strictly, autogamous crops began with the selection of wild progenitors and proceeded to selection from land races. Although land races in Europe were subjected to intensive selection in the last quarter of the 19th and the first quarter of the 20th century, they are now used primarily as a source of special characters. In developing countries land races still comprise a wealth
of adapted genotypes. Considerable progress is possible by means of simple breedingmethods such as line selection; this procedure ensures that the strains obtained arewell adapted.Methods using crosses and back crosses as a source of genotypic variability werebeing applied by the end of the 19th century and have since then grown in importance.Back crosses are necessary whenparental material fromthe areas of diversity is usedAlso of growing importance is the use of multiple crosses to combine characteristicsdistributed among more than two parents.Mass selection is now little used inautogamouscrops.Almost all current selection methods are based on line selection and itsmodifications. In a programme of line selection, selection for yield is often notintroduced until the F5. Considering the many factors involved in yield, this entails adistinct possibility that favourable alleles may be lost in early generations, and it istherefore usually desirable to advance selection for yield to an earlier stage. Selectionforperformance in dense populations rather than selection of singleplants in spacedplantings is receiving increasingattention.As a means of obtaining homozygous lines in a short time from manypopulations in a restricted area, without the risk of unwanted spontaneous selectionpressure, use maybe madeof the'single seed descentmethod'.This method involvesharvesting one seed per plant from the F2 to the F6 and sowing them in the greenhouse in order to raise two or three generations per year depending on the crop.However, the method has its disadvantages: if favourable gene combinations are notto be lost in the course of early generations, large numbers of plants are required; also,the procedure provides no possibility of obtaining information on genotype x yearinteractions.Another method of obtaininghomozygous linesis by meansofchromosome doubling inhaploids.Doubled haploidsallowa rapid evaluation of aparticular cross combination, but the method has so far found little application inpractice.Its further use will depend largely on how far the production of haploids canbe simplified.Use is also made of F1 hybrids in autogamous species.In certain cases the15
15 of adapted genotypes. Considerable progress is possible by means of simple breeding methods such as line selection; this procedure ensures that the strains obtained are well adapted. Methods using crosses and back crosses as a source of genotypic variability were being applied by the end of the 19th century and have since then grown in importance. Back crosses are necessary when parental material from the areas of diversity is used. Also of growing importance is the use of multiple crosses to combine characteristics distributed among more than two parents. Mass selection is now little used in autogamous crops. Almost all current selection methods are based on line selection and its modifications. In a programme of line selection, selection for yield is often not introduced until the F5. Considering the many factors involved in yield, this entails a distinct possibility that favourable alleles may be lost in early generations, and it is therefore usually desirable to advance selection for yield to an earlier stage. Selection for performance in dense populations rather than selection of single plants in spaced plantings is receiving increasing attention. As a means of obtaining homozygous lines in a short time from many populations in a restricted area, without the risk of unwanted spontaneous selection pressure, use may be made of the 'single seed descent method'. This method involves harvesting one seed per plant from the F2 to the F6 and sowing them in the green house in order to raise two or three generations per year depending on the crop. However, the method has its disadvantages: if favourable gene combinations are not to be lost in the course of early generations, large numbers of plants are required; also, the procedure provides no possibility of obtaining information on genotype year interactions. Another method of obtaining homozygous lines is by means of chromosome doubling in haploids. Doubled haploids allow a rapid evaluation of a particular cross combination, but the method has so far found little application in practice. Its further use will depend largely on how far the production of haploids can be simplified. Use is also made of F1 hybrids in autogamous species. In certain cases the