Identi?cation of resistance to Yam mosaic virus(YMV),genus Potyvirus in white Guinea yam(Dioscorea rotundata Poir.)
B.O.Odu a,b,d,R.Asiedu a,b,*,J.d’A.Hughes a,b,S.A.Shoyinka c,O.A.Oladiran d
a International Institute of Tropical Agriculture,P.M.B.5320,Ibadan,Oyo State,Nigeria
b c/o https://www.doczj.com/doc/ea10898278.html,mbourn&Co.,Carolyn House,26Dingwall Road,Croydon CR93EE,UK
c Institute of Agricultural Research an
d Training,P.M.B.5029,Ibadan,Oyo State,Nigeria
d Department of Botany and Microbiology,University of Ibadan,Ibadan,Oyo State,Nigeria
Received20December2002;received in revised form20January2004;accepted24January2004
Abstract
Yam mosaic virus(YMV),genus Potyvirus;family Potyviridae is the most important virus infecting Dioscorea spp.in the tropics.It induces a variety of symptoms ranging from mild mosaic on leaves of infected plants to stunted growth.Field and laboratory studies were conducted in Nigeria to identify Dioscorea rotundata genotypes that are resistant to YMV.A total of24 D.rotundata varieties were evaluated in the?eld under natural infection conditions at four locations in Nigeria during1998and 1999planting seasons,and also in an insect-proof screenhouse using mechanical inoculation and vector transmission.The plants in the?eld were individually scored for virus symptom severity,while those in the screenhouse were indexed for YMVat6,8and 10weeks after inoculation by triple antibody sandwich enzyme-linked immunosorbent assay(TAS-ELISA).Analysis of the symptom severity scores showed highly signi?cant differences(P<0:01)among the genotypes.Varieties TDr1621,TDr1640, TDr35,TDr2224and TDr93-48were identi?ed as resistant to?eld infection by yam viruses based on their low symptom severity scores(<2on a1–5scale).In the screenhouse evaluation by mechanical and vector transmission only TDr1621,TDr 1640,and TDr93-48remained free from YMV infection con?rming resistance to YMV in these varieties.
#2004Elsevier B.V.All rights reserved.
Keywords:Yam mosaic virus,genus Potyvirus;Dioscorea rotundata;White yam;TAS-ELISA
1.Introduction
Yam(Dioscorea spp.)production in West and Cen-tral Africa,principally in Nigeria,Togo,Benin Repub-lic,Ghana,Ivory Coast and Cameroon,has changed dramatically in the past15years from annual output of 9.6million metric tonnes in1985to33.9million metric tonnes in1999(FAO,2000).Nigeria produced about75%of the total output in West Africa and about 65%of total world production in1999(FAO,2000). Many pests and diseases that are of economic importance affect yams.Virus-like symptoms includ-ing stunting,mosaic,necrosis,leaf distortion,mottling and banding on leaves of yams,and brown spots in the tubers have been observed on cultivated yams in the tropics(Harrison and Roberts,1973;Brunt et al., 1990).Viruses reported to infect yams include Dios-corea latent virus(DLV),genus Potexvirus;Dioscorea alata virus(DA V),genus Potyvirus;Cucumber mosaic virus(CMV),genus Cucumovirus;Dioscorea
dume-
Field Crops Research89(2004)97–105
*Corresponding author.Tel.:t234-2-2412626;
fax:t234-2-2412221.
E-mail address:r.asiedu@https://www.doczj.com/doc/ea10898278.html,(R.Asiedu).
0378-4290/$–see front matter#2004Elsevier B.V.All rights reserved.
doi:10.1016/j.fcr.2004.01.009
torum virus(DdV),genus Potyvirus;Dioscorea bul-bifera bacilliform virus(DbBV),genus Badnavirus; Dioscorea alata bacilliform virus(DaBV),genus Badnavirus and Yam mosaic virus(YMV),genus Potyvirus(Thouvenel and Fauquet,1979;Brunt et al.,1990;Hughes et al.,1997;Odu et al.,1999). Of all the yam viruses identi?ed,YMV is the most prevalent(Thouvenel and Dumont,1988,1990;Gou-dou-Urbino,1995).Infection with YMV reduces plant vigour and subsequent tuber size(Craig,1964;Thou-venel and Dumont,1990).It is also reported to cause about40%yield reduction in Dioscorea rotundata (IITA,1981),which is the most important food yam in West Africa.
Protection of crops against pathogens may be achieved by means such as application of chemicals, phytosanitation and the use of biological control agents or incorporation of host plant resistance.The use of resistant varieties has been considered to be the most effective and environmental-friendly means of disease control as part of integrated pest management. In order to produce improved yam germplasm with virus resistance,it is necessary to identify sources of resistance to viruses.Earlier efforts have not been particularly successful(Thottappilly,1992). Essentially,a resistant plant shows reduced or no symptom expression and virus multiplication within it is negligible(Walkey,1991).However,resistance to viruses is often evaluated through the expression of symptoms without quantifying virus multiplication (Fraser,1990).Plants are often classi?ed as resistant when no symptoms are seen,or necrotic lesions develop,and are usually classi?ed as susceptible when symptoms are apparent(Scholten et al.,1996).It is necessary to measure or estimate the intensity of the disease as this is a host response which is a continuous gradient from severe infection to immunity(Walkey, 1991).Ef?cient separation of susceptible from resis-tant genotypes requires the use of a reliable scoring system that will accommodate a range of virus symp-toms as well as diagnostics that will detect symptom-less infection(Hamilton et al.,1981).Evaluation of host plant reaction to viruses based on symptom expression has been reported by several researchers (Fokunang,1995;Msabaha,1981;Stevens et al.,1995; IITA,1984;Thottappilly and Rossel,1993;Steyer et al.,1995).Serological assays such as enzyme-linked immunosorbent assay(ELISA)were used in each of the resistance assessments to complement assessment of symptom expression(Thottappilly and Rossel, 1993;Stevens et al.,1995;Steyer et al.,1995).
The purpose of this study was to identify D.rotun-data genotypes that are resistant to YMVand could be used in yam breeding programmes to build increased levels of resistance to YMV in new yam cultivars.
2.Materials and methods
2.1.Establishment of multilocation trial
Twenty-two cultivars of D.rotundata were selected from germplasm available at the International Institute of Tropical Agriculture(IITA)based on their perfor-mances in previous?eld trials.These cultivars were originally collected from different yam-growing areas in Nigeria and Togo(Table1).The cultivars and two known virus susceptible breeder’s lines(TDr87/ 00211and TDr87/00571included as controls)were planted at four locations in Nigeria representing the major yam-growing ecologies in the country.The locations were Ibadan(forest/savanna transition zone),Ubiaja(forest zone),Abuja(Guinea savanna zone),and Jos(mid-altitude zone).The24genotypes were evaluated for two consecutive seasons(1998and 1999)at all the locations.
At each of the locations,planting was at a density of one plant per square meter and each experimental plot (6m?5m)was planted with30setts.The experi-mental design was a randomised complete block (RCBD)with four replications.Agronomically,each location was treated according to its individual requirements as regards weeding(manually as required)after the application of a combination of pre-emergence herbicides(Gramozone and Premes-tral,5ml là1water)before the emergence of yam plants.Individual staking of each of the plant stands was done at emergence to increase area of leaf expo-sure to sunlight and to facilitate scoring of disease symptoms on individual plants.
2.2.Disease evaluation(?eld)
All?elds were exposed to natural infection by the pathogen.The severity of symptom expression was rated2,3,4,and5months after planting on a1–5scale
98 B.O.Odu et al./Field Crops Research89(2004)97–105
according to IITA(1998)and Mignouna et al.(2001), where1:no symptoms,2:moderate or mild symptoms, 3:severe symptoms,4:very severe symptoms,and5: distortion,malformation of leaf or stem.Symptom severity scores were analysed using generalised linear model(GLM)procedure of SAS(1999).The analysis of variance(ANOV A)was done separately for each location and then as a combined analysis.Mean symp-tom severity scores for genotypes at each location were derived from the GLM analysis.Genotypes with a mean symptom severity of2were classi?ed as resis-tant,while others were considered susceptible.
2.3.Serological detection of YMV and
other viruses infecting yams
Leaf samples were collected from plots in the multilocational trials and these were subjected to enzyme-linked immunosorbent assay and indexed for the presence or absence of YMV,CMV,DaBV,DbBV,DA V,DLV and DdV.This was done to detect the occurrence of the viruses at the locations. Triple antibody sandwich enzyme-linked immuno-sorbent assay(TAS-ELISA)as described by Thottap-pilly et al.(1998)and Njukeng et al.(2002)was used in the detection of YMV in the leaf samples.The monoclonal antibody used was YMV-M24,produced against YMV at the IITA,which was found to detect YMV isolates from other countries including Camer-oon,Nigeria,Benin,Togo,Ivory Coast,Burkina Faso and Guinea(Njukeng,1998).YMV-M24recognises a common epitope and therefore forms part of an effec-tive and sensitive TAS-ELISA protocol.
Protein A sandwich enzyme-linked immunosorbent assay(PAS-ELISA),as described by Hughes and Tho-mas(1988),was used to detect DA V,DaBV,DLV,DdV, DbBV and CMV the in leaf samples.All polyclonal antibodies to the viruses were obtained from Dr.A.A. Brunt,Horticultural Research International(HRI),UK except that of DA V which was produced at IITA.
Table1
D.rotundata genotypes planted in the multilocation trial and in the screenhouse for virus resistance evaluation,and their local names and geographical sources
IITA accession numbers Local/source names Original source Sex Country of collection TDr93-1Akwoki Abuja Female Nigeria
TDr93-23Obiaoturugo Obinagu Female Nigeria
TDr93-27Akuru Uturu,Okigwe–Nigeria
TDr93-29Gbango Zaki-biam Male Nigeria
TDr93-31Danacha Zaki-biam Male Nigeria
TDr93-32Amula Zaki-biam Female Nigeria
TDr93-46Sabara Agyaragu,Plat.tNigeria
TDr93-48Asuikwu Ika North LGA,DeltatNigeria
TDr93-49Eikale Ika North LGA,DeltatNigeria
TDr95-107Maria Ndayako,Mokwa Female Nigeria
TDr95-115Nyagode Abuja Male Nigeria
TDr95-127Dandiyo Keffi,Nassarawa Male Nigeria
TDr95-128Gwari Keffi,Nassarawa Female Nigeria
TDr35Atoja Ibadan Female Nigeria
TDr179–Eastern NigeriatNigeria
TDr205–UmudiketNigeria
TDr608Nwopoko Eastern Nigeria Male Nigeria
TDr747––tNigeria
TDr1621Lololo–Female Togo
TDr1640Lassora SOT61–tTogo
TDr2224HT86/00258IITA,Ibadan Male Nigeria
TDr2269––Female–
TDr87/00211Breeder’s line IITA,Ibadan Male Nigeria
TDr87/00571Breeder’s line IITA,Ibadan Female Nigeria
No information is available(–);non-?owering genotypes at the time of this experiment(t);TDr:tropical D.rotundata;IITA:International Institute of Tropical Agriculture.
B.O.Odu et al./Field Crops Research89(2004)97–10599
2.4.Planting in screenhouse
Small setts(30–50g sizes,termed minisetts)from all24D.rotundata genotypes planted in the multi-location experiment were pre-sprouted in plastic bas-kets containing heat sterilised shredded coir(coco-peat).The coco-peat was used to maintain soil fria-bility and to retain water for a longer period of time. The coco-peat was sprayed with water until soaked. This was repeated periodically as required after plant-ing the minisetts.Before transplanting,regular checks were carried out on the planted minisetts to assess sprout and root emergence.Sprouted minisetts were transplanted into6in.Stewart planting pots already ?lled with a mixture of heat-sterilised topsoil and coco-peat(3:1).
Three leaf samples from each plant of each geno-type were collected from the screenhouse and sub-jected to TAS-ELISA to detect YMV infection prior to virus transmission studies.
2.5.Mechanical transmission of YMV
and indexing
In order to produce inoculum for mechanical inoculation,virus-tested in vitro plantlets of D. rotundata breeder’s line TDr87/00211were trans-planted and inoculated with a standard isolate of YMV(Mignouna et al.,2001)and kept in an insect-proof screenhouse.These plants,showing typical symptoms(mosaic and green vein-banding)and con?rmed by TAS-ELISA to be infected with YMV,were used as the source of inoculum.YMV-infected leaves were ground using a sterile pestle and mortar and the extract was diluted10-fold with inoculation buffer(0.03M sodium phosphate buffer pH8.3containing0.2%sodium diethyl dithiocarba-mate(DIECA)).
The sap extract was used to inoculate leaves of D. rotundata plants in the screenhouse after dusting with carborundum(600mesh).After inoculation,the leaves were rinsed with water to remove excess inocu-lum.A total of20plants per genotype(5plants replicated four times)were used in the transmission experiments,while four plants per genotype were employed as negative control checks.Virus infection was diagnosed by symptom expression and indexing by TAS-ELISA.2.6.Vector transmission and indexing
A YMV-infected D.rotundata breeder’s line TDr 87/00211was used as the source of inoculum and Aphis craccivora,previously reported to transmit YMV in a non-persistent manner(Thouvenel and Fauquet,1979),was used as vector.The insects were previously maintained on virus-free cowpea plants.A total of20plants per genotype(5plants replicated4 times)were used in the transmission experiments, while four plants per genotypes were employed as negative control checks.The transmission procedure was essentially as described by Odu et al.(1999). Symptom development and expression were moni-tored,and the plants were indexed by TAS-ELISA to detect YMV.
3.Results
A total of768leaf samples were collected from all the locations of the experiments during1998and1999. Serological indexing of the samples showed the pre-sence of only YMV and DA V(Table2).YMV inci-dence was highest in Ibadan(66.2%)and lowest in Abuja(58.2%),while DA V incidence was highest in Jos(12.3%)and the lowest in Ibadan(0.05%).The symptoms observed in the?elds across locations were found to be associated with infection by YMV.
Table2
The proportion of D.rotundata leaf samples infected by viruses collected from four locations in Nigeria(Ibadan,Abuja,Jos, Ubiaja)during the growing seasons in1998and1999as detected by enzyme-linked immunosorbent assay a
Location YMV b(%)DA V c(%)Others d(%) Ibadan66.20.050.0
Abuja58.210.20.0
Jos59.412.30.0
Ubiaja62.17.40.0
a Leaf samples were collected from24D.rotundata genotypes at research stations in the respective locations.
b Yam mosai
c virus(YMV),genus Potyvirus.
c Dioscorea alata virus(DA V),genus Potyvirus.
d Others tested for wer
e Dioscorea alata bacilliform virus (DaBV),genus Badnavirus;Dioscorea bulbifera bacilliform virus (DbBV),genus Badnavirus;cucumber mosaic virus(CMV),genus Cucumovirus;Dioscorea latent virus(DLV),genus Potexirus; Dioscorea dumetorum virus(DdV),genus Potyvirus.
100 B.O.Odu et al./Field Crops Research89(2004)97–105
Fig.1shows the percent disease incidence (based on visual symptom)of four resistant genotypes and the two breeders ’lines used as susceptible checks (TDr 87/00211and TDr 87/00571)at Ibadan in 1999plant-ing season.This same trend was observed at all the other locations where the ?eld trial was conducted for the two planting seasons.The two susceptible checks had 100%incidence from the ?rst month of scoring,and this was maintained throughout the period of disease rating.
Results from combined analysis of variance of virus symptom severity score of 24D.rotundata genotypes evaluated at four locations in 2years are presented in Table 3.Location,location ?year,location ?year ?genotype mean squares were all highly signi ?cant (P ?0:01).‘‘Genotype ’’was the largest (55.2%)
source of variation.The genotypes differed signi ?-cantly in virus symptom severity at each of the four locations (Table 4).
Table 5shows the means of virus symptom severity scores from the four locations for the 2years.Five genotypes (TDr 93-48,TDr 35,TDr 1640,TDr 2224and TDr 1621)had mean symptom severity scores less than 2.00,while TDr 87/00571,a susceptible breeder ’s line had the highest score.
Preliminary serological indexing by TAS-ELISA prior to mechanical inoculation showed that some of the genotypes (TDr 747,TDr 87/00211,TDr 87/00571,TDr 93-29,TDr 93-31,TDr 93-32)were already infected with YMV (Table 6).Eleven geno-types expressed YMV associated symptoms and were considered susceptible to the virus.Four weeks after mechanical inoculation,TDr 93-49showed mild mosaic symptoms typical of YMV.However,symp-toms were only seen on the inoculated leaves and
only
Fig.1.Incidence of virus symptoms on plants of representative D.rotundata genotypes planted at Ibadan in 1999(TDr 87/00211and TDr 87/00571are known susceptible breeders ’lines).
Table 3
Combined analysis of variance showing mean squares and their proportion (%)for virus severity scores of 24 D.rotundata genotypes tested at four locations in two seasons (1998and 1999)Source of variation
d.f.Mean square Variation (%)Year 1 2.749** 1.6Location
3 3.727** 6.5Location ?year
30.3006*0.5Replication (location ?year)240.1909** 2.7Genotype
23 4.1644**55.2Year ?genotype 230.4326** 5.8Location ?genotype
690.1752**7.0Location ?year ?genotype 690.0797 3.2Error
552
0.0540
17.3
*Signi ?cant at P <5%level of probability.**
Signi ?cant at P <1%level of probability.
Table 4
Analyses of variance showing mean squares for virus symptom severity scores of 24 D.rotundata genotypes planted at four locations in 2years (1998and 1999)Source of variation d.f.Location Ibadan
Abuja Jos Ubiaja Year
10.2230.336 2.7100.383Replication (year)60.0540.1330.3640.213Genotype
23 1.423**0.939** 1.170** 1.158**Year ?genotype 230.1850.14090.2030.142Error 138
0.0540.0430.6890.035CV (%)
10.91
9.42
11.77
8.36
**
Signi ?cant at P <1%level of probability.
B.O.Odu et al./Field Crops Research 89(2004)97–105101
these leaves tested positive for YMV.Newly formed leaves from the inoculated part of the plant expressed symptoms and also tested positive for YMV when indexed by TAS-ELISA following mechanical inocu-lation with the virus.
Genotypes TDr1621,TDr1640and TDr93-48did not develop YMV associated symptoms and also tested negative for the presence of YMV by TAS-ELISA.TDr2224,TDr35,TDr95-115and TDr95-128did not express any symptoms but tested positive by TAS-ELISA for YMV.
Five genotypes(TDr2224,TDr35,TDr93-46and TDr93-49)tested positive by TAS-ELISA for YMV following mechanical inoculation but negative when subjected to vector transmission(Table6).4.Discussion
The genotypes evaluated in the?eld were found to differ greatly in the symptoms they exhibited as well as the severity of symptom expression under exposure to a range of factors such as possible strains of the viruses,climatic conditions and different inoculum pressures(Matthews,1991).
Mechanical and vector transmission of YMV were very effective methods for screening D.rotundata genotypes.The susceptible control(TDr87/00211) was always infected during the screening process,thus suggesting that all genotypes that remained uninfected have resistance to YMV.Natural transmission in the ?eld can be useful to demonstrate whether YMV
Table5
Mean virus symptom severity scores a of24D.rotundata genotypes planted at four locations in two cropping seasons(1998and1999)in Nigeria
Genotype Ibadan Abuja Jos Ubiaja Mean S.E.
19981999199819991998199919981999
TDr35 1.77 1.54 1.79 1.80 1.78 1.70 1.67 1.65 1.710.05 TDr1621 2.02 1.47 2.00 1.57 1.98 1.61 2.01 1.78 1.810.04 TDr2224 2.01 1.67 2.16 1.52 2.12 1.73 2.04 1.63 1.860.06 TDr1640 1.83 1.65 1.79 1.78 2.39 1.93 1.90 1.67 1.880.06 TDr93-48 1.75 1.50 1.90 2.04 2.58 1.77 2.03 2.15 1.970.07 TDr2269 2.15 1.75 2.04 1.87 2.14 1.96 2.08 2.11 2.010.05 TDr93-29 1.62 2.15 2.09 1.94 2.50 2.26 1.93 1.97 2.060.06 TDr93-49 2.03 1.59 2.11 1.58 2.97 2.29 2.37 1.78 2.090.08 TDr179 2.04 1.76 2.13 1.86 2.68 2.08 2.31 1.87 2.090.07 TDr95-107 2.19 1.81 2.11 2.03 2.33 2.21 2.21 1.93 2.100.06 TDr93-27 1.90 1.93 2.37 2.14 2.47 2.04 2.23 1.98 2.130.07 TDr93-32 2.07 2.28 2.08 1.96 2.55 2.43 2.35 1.90 2.200.05 TDr747 2.42 2.16 2.33 2.27 2.54 1.85 2.27 2.07 2.240.05 TDr608 2.03 2.04 2.04 2.04 2.75 2.76 2.22 2.22 2.260.13 TDr95-128 1.90 1.78 2.29 2.29 2.68 2.74 2.21 2.60 2.310.07 TDr95-115 2.02 2.17 2.34 2.41 2.75 2.46 2.10 2.27 2.310.06 TDr93-46 2.21 2.03 2.46 2.38 2.75 2.72 2.10 2.09 2.340.06 TDr93-1 1.94 2.29 2.38 2.34 2.66 2.63 2.38 2.51 2.390.06 TDr93-23 2.04 2.58 2.27 2.51 2.48 2.37 2.58 2.42 2.410.06 TDr93-31 2.51 2.20 2.40 2.20 3.00 2.32 2.57 2.18 2.420.06 TDr95-127 2.46 2.64 2.64 2.48 2.80 2.59 2.61 2.73 2.620.05 TDr205 2.85 2.86 2.58 2.58 3.15 3.15 2.82 2.82 2.850.06 TDr87/00211 2.83 3.08 2.76 2.94 2.63 3.02 2.71 3.03 2.870.04 TDr87/00571 3.24 3.32 2.80 3.44 3.10 3.43 3.20 3.47 3.250.05 Mean 2.16 2.09 2.25 2.17 2.57 2.34 2.29 2.20
S.E.0.030.040.030.040.050.050.040.04
CV(%)13.17.99.29.710.98.38.412.7
a Means obtained from evaluation of120plants/genotype on a scale of1–5,where1:no symptoms,2:moderate or mild symptoms,3: severe symptoms,4:very severe symptoms,and5:distortion,malformation of leaf or stem,complete plant infection.
102 B.O.Odu et al./Field Crops Research89(2004)97–105
isolates exist that can infect the plants and induce symptoms.However,if there are few initial infection foci(Kaniewski and Thomas,1998),the procedure may not be reliable.This study established that mechanical inoculation is a more effective way of screening yam germplasm for resistance to YMV than natural?eld infection.The detection of infection in the six D.rotundata genotypes(TDr747,TDr87/00211, TDr87/00571,TDr93-29,TDr93-31and TDr93-32) prior to vector and mechanical transmission of YMV. This shows the importance of vegetative transmission for YMV survival from one season to another.The genotypes also probably served as sources of infection to other uninfected ones in the?eld.They were excluded from the transmission studies based on their initial health status.
Observation of symptom expression and the use of ELISA for the detection of virus infection in this study have shown that TAS-ELISA is both sensitive and consistent.TAS-ELISA,using a more speci?c mono-clonal antibody,detected symptomless infection in genotypes that may have been identi?ed as resistant if symptom expression had been employed as the sole method of indexing.
There were differences between the results obtained from mechanical inoculation and the vector
Table6
Results of mechanical and vector transmission of yam mosaic virus(YMV),genus Potyvirus to D.rotundata genotypes in a screenhouse environment
Genotype ELISA result before
test inoculation of YMV a ELISA result8weeks after virus mechanical
transmission b
ELISA result8weeks after
vector transmission of YMV Reaction in ELISA Symptoms
TDr747tnt nt nt
TDr87/00211tnt nt nt
TDr87/00571tnt nt nt
TDr93-29tnt nt nt
TDr93-31tnt nt nt
TDr93-32tnt nt nt
TDr2224àtNo symptomsà
TDr35àtNo symptomsà
TDr95-115àtNo symptomst
TDr95-128àtNo symptomst
TDr93-46àtMild mosaicà
TDr93-49àtMild mosaicà
TDr179àttMosaict
TDr205àttMosaict
TDr2269àttMosaict
TDr608àttMosaict
TDr93-1àttMosaict
TDr93-23àttMosaict
TDr93-27àttMosaict
TDr95-107àttMosaicà
TDr95-127àttMosaict
TDr1621ààNo symptomsà
TDr1640ààNo symptomsà
TDr93-48ààNo symptomsà
Healthy controlàà
Diseased controltttt
Tested negative for the presence of infection(à);tested positive for the presence of infection(twice the absorbance of healthy control and less than1.00A405)(t);strong positive(twice the absorbance of healthy control and greater than1.00A405)(tt);nt:not tested as plants were already naturally infected.
a Enzyme-linked immunosorbent assay.
b Tested for both local and systemi
c infection.
B.O.Odu et al./Field Crops Research89(2004)97–105103
transmission tests of some of the D.rotundata gen-otypes.The inability of?ve D.rotundata genotypes namely TDr2224,TDr35,TDr93-46,TDr93-49 and TDr95-107to get infected with YMV by vector (A.craccivora)transmission indicates their resis-tance to the vector used in this study.These geno-types were infected with YMV when transmission was mechanical.There are a number of reported cases of plant resistance to identi?ed vectors of viruses(Lecoq et al.,1981;Berlinger and Daban, 1987;Gunasinghe et al.,1988).
Since vectors are relied upon to transmit virus in the ?eld,the foregoing explains why TDr2224and TDr 35were both classi?ed along with TDr1621,TDr 1640,and TDr93-48as resistant genotypes?eld trial. TDr2224and TDr35were actually infected after mechanical inoculation with the virus in the screen-house as revealed by TAS-ELISA(Table6)but they did not exhibit any known symptoms of YMV-infec-tion,and may be termed as tolerant to the virus. On the disease symptom severity scoring scale of1–5(Mignouna et al.,2001)TDr1621,TDr1640,and TDr35with scores ranging from1to2are shown to be resistant to YMV,while TDr205,TDr87/00211and TDr87/00571with scores of3–5are classed as susceptible.TDr93-1was classi?ed as a resistant parent by Mignouna et al.(2001)based on severity score of less than2.They also detected YMV at low but above background levels in ELISA tests in the genotype.In our study TDr93-1had severity scores ranging from2to3in this?eld trial.It also tested positive for the presence of YMV prior to inoculation in the screenhouse experiment.Higher levels of resis-tance have therefore been identi?ed in TDr1621,TDr 1640,and TDr93-48.
Some D.rotundata genotypes inoculated with YMV expressed symptoms of mosaic and mild https://www.doczj.com/doc/ea10898278.html,d mosaic symptoms may be indicative of relatively small amount of chlorophyll loss,but photosynthesis may be severely reduced due to an inhibition of an important photosynthetic component (Naidu et al.,1984).Some inoculated genotypes, showed no symptoms of infection but virus was detected in them by TAS-ELISA,while no virus was detected in others(Table6).The resistance mechanism in the former resembles tolerance,while those found to be free of virus exhibited immunity. Some of the variations in symptom severity could be associated with differences in virus concentration although some genotypes can modify expressed symp-toms independently of the rate of virus replication (Kuhn et al.,1981).In the case of inoculated yam genotypes that did not show YMV-associated symp-toms but tested positive for YMV,it is concluded that the resistance mechanism allows replication and spread of the virus but suppressed symptom expres-sion.These genotypes,although without symptoms, could be reservoirs of inoculum and source of infec-tion to susceptible genotypes when planted in the?eld. However,they may be less likely to in?uence the development of more severe strains of the virus. Identi?cation of resistance to YMV from the avail-able Dioscorea germplasm is an important contribu-tion to the genetic improvement of yam.Contrary to the common challenge to hybridisation in yams all the resistant genotypes produce?owers with the exception of TDr1640.
Acknowledgements
The authors gratefully acknowledge the?nancial support by Gatsby Charitable Foundation(GCF),UK. References
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to与for的用法和区别 一般情况下, to后面常接对象; for后面表示原因与目的为多。 Thank you for helping me. Thanks to all of you. to sb.表示对某人有直接影响比如，食物对某人好或者不好就用to; for表示从意义、价值等间接角度来说，例如对某人而言是重要的，就用for. for和to这两个介词，意义丰富，用法复杂。这里仅就它们主要用法进行比较。 1. 表示各种“目的” 1. What do you study English for? 你为什么要学英语？ 2. She went to france for holiday. 她到法国度假去了。 3. These books are written for pupils. 这些书是为学生些的。 4. hope for the best, prepare for the worst. 作最好的打算，作最坏的准备。 2．对于 1．She has a liking for painting. 她爱好绘画。 2．She had a natural gift for teaching. 她对教学有天赋/ 3．表示赞成同情，用for不用to. 1. Are you for the idea or against it? 你是支持还是反对这个想法？ 2. He expresses sympathy for the common people.. 他表现了对普通老百姓的同情。 3. I felt deeply sorry for my friend who was very ill. 4 for表示因为，由于（常有较活译法） 1 Thank you for coming. 谢谢你来。 2. France is famous for its wines. 法国因酒而出名。 5．当事人对某事的主观看法，对于（某人），对…来说（多和形容词连用）用介词to，不用for.. He said that money was not important to him. 他说钱对他并不重要。 To her it was rather unusual. 对她来说这是相当不寻常的。 They are cruel to animals. 他们对动物很残忍。 6．for和fit, good, bad, useful, suitable 等形容词连用，表示适宜，适合。 Some training will make them fit for the job. 经过一段训练，他们会胜任这项工作的。 Exercises are good for health. 锻炼有益于健康。 Smoking and drinking are bad for health. 抽烟喝酒对健康有害。 You are not suited for the kind of work you are doing. 7. for表示不定式逻辑上的主语，可以用在主语、表语、状语、定语中。 1．It would be best for you to write to him. 2．The simple thing is for him to resign at once. 3．There was nowhere else for me to go. 4．He opened a door and stood aside for her to pass.
of与for的用法以及区别 for 表原因、目的 of 表从属关系 介词of的用法 （1）所有关系 this is a picture of a classroom （2）部分关系 a piece of paper a cup of tea a glass of water a bottle of milk what kind of football，American of soccer？ （3）描写关系 a man of thirty 三十岁的人 a man of shanghai 上海人 （4）承受动作 the exploitation of man by man．人对人的剥削。 （5）同位关系 It was a cold spring morning in the city of London in England． （6）关于，对于 What do you think of Chinese food？ 你觉得中国食品怎么样？ 介词 for 的用法小结 1. 表示“当作、作为”。如: I like some bread and milk for breakfast. 我喜欢把面包和牛奶作为早餐。What will we have for supper? 我们晚餐吃什么?
2. 表示理由或原因,意为“因为、由于”。如: Thank you for helping me with my English. 谢谢你帮我学习英语。 Thank you for your last letter. 谢谢你上次的来信。 Thank you for teaching us so well. 感谢你如此尽心地教我们。 3. 表示动作的对象或接受者,意为“给……”、“对…… (而言)”。如: Let me pick it up for you. 让我为你捡起来。 Watching TV too much is bad for your health. 看电视太多有害于你的健康。 4. 表示时间、距离,意为“计、达”。如: I usually do the running for an hour in the morning. 我早晨通常跑步一小时。We will stay there for two days. 我们将在那里逗留两天。 5. 表示去向、目的,意为“向、往、取、买”等。如: let’s go for a walk. 我们出去散步吧。 I came here for my schoolbag.我来这儿取书包。 I paid twenty yuan for the dictionary. 我花了20元买这本词典。 6. 表示所属关系或用途,意为“为、适于……的”。如: It’s time for school. 到上学的时间了。 Here is a letter for you. 这儿有你的一封信。 7. 表示“支持、赞成”。如: Are you for this plan or against it? 你是支持还是反对这个计划? 8. 用于一些固定搭配中。如: Who are you waiting for? 你在等谁? For example, Mr Green is a kind teacher. 比如,格林先生是一位心地善良的老师。
For的用法 1. 表示“当作、作为”。如: I like some bread and milk for breakfast. 我喜欢把面包和牛奶作为早餐。 What will we have for supper? 我们晚餐吃什么? 2. 表示理由或原因,意为“因为、由于”。如: Thank you for helping me with my English. 谢谢你帮我学习英语。 3. 表示动作的对象或接受者,意为“给……”、“对…… (而言)”。如: Let me pick it up for you. 让我为你捡起来。 Watching TV too much is bad for your health. 看电视太多有害于你的健康。 4. 表示时间、距离,意为“计、达”。如: I usually do the running for an hour in the morning. 我早晨通常跑步一小时。 We will stay there for two days. 我们将在那里逗留两天。 5. 表示去向、目的,意为“向、往、取、买”等。如: Let’s go for a walk. 我们出去散步吧。 I came here for my schoolbag.我来这儿取书包。 I paid twenty yuan for the dictionary. 我花了20元买这本词典。 6. 表示所属关系或用途,意为“为、适于……的”。如: It’s time for school. 到上学的时间了。 Here is a letter for you. 这儿有你的一封信。 7. 表示“支持、赞成”。如: Are you for this plan or against it? 你是支持还是反对这个计划? 8. 用于一些固定搭配中。如: Who are you waiting for? 你在等谁? For example, Mr Green is a kind teacher. 比如,格林先生是一位心地善良的老师。 尽管for 的用法较多，但记住常用的几个就可以了。 to的用法: 一：表示相对，针对 be strange (common, new, familiar, peculiar) to This injection will make you immune to infection. 二：表示对比，比较 1：以-ior结尾的形容词，后接介词to表示比较，如：superior ,inferior,prior,senior,junior 2: 一些本身就含有比较或比拟意思的形容词，如equal，similar，equivalent，analogous A is similar to B in many ways.
介词for用法归纳 用法1：(表目的)为了。如： They went out for a walk. 他们出去散步了。 What did you do that for? 你干吗这样做？ That’s what we’re here for. 这正是我们来的目的。 What’s she gone for this time? 她这次去干什么去了？ He was waiting for the bus. 他在等公共汽车。 【用法说明】在通常情况下，英语不用for doing sth 来表示目的。如： 他去那儿看他叔叔。 误：He went there for seeing his uncle. 正：He went there to see his uncle. 但是，若一个动名词已名词化，则可与for 连用表目的。如： He went there for swimming. 他去那儿游泳。(swimming 已名词化) 注意：若不是表目的，而是表原因、用途等，则其后可接动名词。(见下面的有关用法) 用法2：(表利益)为，为了。如： What can I do for you? 你想要我什么？ We study hard for our motherland. 我们为祖国努力学习。 Would you please carry this for me? 请你替我提这个东西好吗？ Do more exercise for the good of your health. 为了健康你要多运动。 【用法说明】(1) 有些后接双宾语的动词(如buy, choose, cook, fetch, find, get, order, prepare, sing, spare 等)，当双宾语易位时，通常用for 来引出间接宾语，表示间接宾语为受益者。如： She made her daughter a dress. / She made a dress for her daughter. 她为她女儿做了件连衣裙。 He cooked us some potatoes. / He cooked some potatoes for us. 他为我们煮了些土豆。 注意，类似下面这样的句子必须用for： He bought a new chair for the office. 他为办公室买了张新办公椅。 (2) 注意不要按汉语字面意思，在一些及物动词后误加介词for： 他们决定在电视上为他们的新产品打广告。 误：They decided to advertise for their new product on TV. 正：They decided to advertise their new product on TV. 注：advertise 可用作及物或不及物动词，但含义不同：advertise sth＝为卖出某物而打广告；advertise for sth＝为寻找某物而打广告。如：advertise for a job=登广告求职。由于受汉语“为”的影响，而此处误加了介词for。类似地，汉语中的“为人民服务”，说成英语是serve the people，而不是serve for the people，“为某人的死报仇”，说成英语是avenge sb’s death，而不是avenge for sb’s death，等等。用法3：(表用途)用于，用来。如： Knives are used for cutting things. 小刀是用来切东西的。 This knife is for cutting bread. 这把小刀是用于切面包的。 It’s a machine for slicing bread. 这是切面包的机器。 The doctor gave her some medicine for her cold. 医生给了她一些感冒药。 用法4：为得到，为拿到，为取得。如： He went home for his book. 他回家拿书。 He went to his friend for advice. 他去向朋友请教。 She often asked her parents for money. 她经常向父母要钱。
of 1....的,属于 One of the legs of the table is broken. 桌子的一条腿坏了。 Mr.Brown is a friend of mine. 布朗先生是我的朋友。 2.用...做成的;由...制成 The house is of stone. 这房子是石建的。 3.含有...的;装有...的 4....之中的;...的成员 Of all the students in this class,Tom is the best. 在这个班级中,汤姆是最优秀的。 5.(表示同位) He came to New York at the age of ten. 他在十岁时来到纽约。 6.(表示宾格关系) He gave a lecture on the use of solar energy. 他就太阳能的利用作了一场讲演。 7.(表示主格关系) We waited for the arrival of the next bus. 我们等待下一班汽车的到来。
I have the complete works of Shakespeare. 我有莎士比亚全集。 8.来自...的;出自 He was a graduate of the University of Hawaii. 他是夏威夷大学的毕业生。 9.因为 Her son died of hepatitis. 她儿子因患肝炎而死。 10.在...方面 My aunt is hard of hearing. 我姑妈耳朵有点聋。 11.【美】(时间)在...之前 12.(表示具有某种性质) It is a matter of importance. 这是一件重要的事。 For 1.为,为了 They fought for national independence. 他们为民族独立而战。 This letter is for you. 这是你的信。
1.表示各种“目的”，用for （1）What do you study English for 你为什么要学英语？ （2）went to france for holiday. 她到法国度假去了。 （3）These books are written for pupils. 这些书是为学生些的。 （4）hope for the best, prepare for the worst. 作最好的打算，作最坏的准备。 2．“对于”用for （1）She has a liking for painting. 她爱好绘画。 （2）She had a natural gift for teaching. 她对教学有天赋/ 3．表示“赞成、同情”，用for （1）Are you for the idea or against it 你是支持还是反对这个想法？ （2）He expresses sympathy for the common people.. 他表现了对普通老百姓的同情。 （3）I felt deeply sorry for my friend who was very ill. 4. 表示“因为，由于”（常有较活译法），用for （1）Thank you for coming. 谢谢你来。
（2）France is famous for its wines. 法国因酒而出名。 5．当事人对某事的主观看法，“对于（某人），对…来说”，（多和形容词连用），用介词to，不用for. （1）He said that money was not important to him. 他说钱对他并不重要。 （2）To her it was rather unusual. 对她来说这是相当不寻常的。 （3）They are cruel to animals. 他们对动物很残忍。 6．和fit, good, bad, useful, suitable 等形容词连用，表示“适宜，适合”，用for。（1）Some training will make them fit for the job. 经过一段训练，他们会胜任这项工作的。 （2）Exercises are good for health. 锻炼有益于健康。 （3）Smoking and drinking are bad for health. 抽烟喝酒对健康有害。 （4）You are not suited for the kind of work you are doing. 7. 表示不定式逻辑上的主语，可以用在主语、表语、状语、定语中。 （1）It would be best for you to write to him. （2） The simple thing is for him to resign at once.
1.两者都可以引出间接宾语，但要根据不同的动词分别选用介词to 或for：(1) 在give, pass, hand, lend, send, tell, bring, show, pay, read, return, write, offer, teach, throw 等之后接介词to。 如： 请把那本字典递给我。 正：Please hand me that dictionary. 正：Please hand that dictionary to me. 她去年教我们的音乐。 正：She taught us music last year. 正：She taught music to us last year. (2) 在buy, make, get, order, cook, sing, fetch, play, find, paint, choose,prepare, spare 等之后用介词for 。如： 他为我们唱了首英语歌。 正：He sang us an English song. 正：He sang an English song for us. 请帮我把钥匙找到。 正：Please find me the keys. 正：Please find the keys for me. 能耽搁你几分钟吗(即你能为我抽出几分钟吗)? 正：Can you spare me a few minutes? 正：Can you spare a few minutes for me? 注：有的动词由于搭配和含义的不同，用介词to 或for 都是可能的。如：do sb a favour＝do a favour for sb 帮某人的忙 do sb harm＝do harm to sb 对某人有害
1. 两者都可以引出间接宾语，但要根据不同的动词分别选用介词to 或for： (1) 在give, pass, hand, lend, send, tell, bring, show, pay, read, return, write, offer, teach, throw 等之后接介词to。 如： 请把那本字典递给我。 正：Please hand me that dictionary. 正：Please hand that dictionary to me. 她去年教我们的音乐。 正：She taught us music last year. 正：She taught music to us last year. (2) 在buy, make, get, order, cook, sing, fetch, play, find, paint, choose,prepare, spare 等之后用介词for 。如： 他为我们唱了首英语歌。 正：He sang us an English song. 正：He sang an English song for us. 请帮我把钥匙找到。 正：Please find me the keys. 正：Please find the keys for me. 能耽搁你几分钟吗(即你能为我抽出几分钟吗)? 正：Can you spare me a few minutes? 正：Can you spare a few minutes for me? 注：有的动词由于搭配和含义的不同，用介词to 或for 都是可能的。如： do sb a favou r do a favour for sb 帮某人的忙 do sb harnn= do harm to sb 对某人有害
for的用法： 1. 表示“当作、作为”。如: I like some bread and milk for breakfast. 我喜欢把面包和牛奶作为早餐。 What will we have for supper? 我们晚餐吃什么? 2. 表示理由或原因,意为“因为、由于”。如: Thank you for helping me with my English. 谢谢你帮我学习英语。 Thank you for your last letter. 谢谢你上次的来信。 Thank you for teaching us so well. 感谢你如此尽心地教我们。 3. 表示动作的对象或接受者,意为“给……”、“对…… (而言)”。如: Let me pick it up for you. 让我为你捡起来。 Watching TV too much is bad for your health. 看电视太多有害于你的健康。 4. 表示时间、距离,意为“计、达”。如:
I usually do the running for an hour in the morning. 我早晨通常跑步一小时。 We will stay there for two days. 我们将在那里逗留两天。 5. 表示去向、目的,意为“向、往、取、买”等。如: Let’s go for a walk. 我们出去散步吧。 I came here for my schoolbag.我来这儿取书包。 I paid twenty yuan for the dictionary. 我花了20元买这本词典。 6. 表示所属关系或用途,意为“为、适于……的”。如: It’s time for school. 到上学的时间了。 Here is a letter for you. 这儿有你的一封信。 7. 表示“支持、赞成”。如: Are you for this plan or against it? 你是支持还是反对这个计划? 8. 用于一些固定搭配中。如:
加入收藏夹 主题： 介词试题It’s + 形容词 + of sb. to do sth.和It’s + 形容词 + for sb. to do sth.的用法区别。 内容： It's very nice___pictures for me. A.of you to draw B.for you to draw C.for you drawing C.of you drawing 提交人：杨天若时间：1/23/2008 20:5:54 主题：for 与of 的辨别 内容：It's very nice___pictures for me. A.of you to draw B.for you to draw C.for you drawing C.of you drawing 答：选A 解析：该题考查的句型It’s + 形容词+ of sb. to do sth.和It’s +形容词+ for sb. to do sth.的用法区别。 “It’s + 形容词+ to do sth.”中常用of或for引出不定式的行为者，究竟用of sb.还是用for sb.，取决于前面的形容词。 1) 若形容词是描述不定式行为者的性格、品质的，如kind，good，nice，right，wrong，clever，careless，polite，foolish等，用of sb. 例： It’s very kind of you to help me. 你能帮我，真好。 It’s clever of you to work out the maths problem. 你真聪明，解出了这道数学题。 2) 若形容词仅仅是描述事物，不是对不定式行为者的品格进行评价，用for sb.，这类形容词有difficult，easy，hard，important，dangerous，（im）possible等。例： It’s very dangerous for children to cross the busy street. 对孩子们来说，穿过繁忙的街道很危险。 It’s difficult for u s to finish the work. 对我们来说，完成这项工作很困难。 for 与of 的辨别方法： 用介词后面的代词作主语，用介词前边的形容词作表语，造个句子。如果道理上通顺用of，不通则用for. 如： You are nice.(通顺，所以应用of)。 He is hard.(人是困难的，不通，因此应用for.) 由此可知，该题的正确答案应该为A项。 提交人：f7_liyf 时间：1/24/2008 11:18:42
to和for的用法有什么不同（一） 一、引出间接宾语时的区别 两者都可以引出间接宾语，但要根据不同的动词分别选用介词to 或for，具体应注意以下三种情况： 1. 在give, pass, hand, lend, send, tell, bring, show, pay, read, return, write, offer, teach, throw 等之后接介词to。如： 请把那本字典递给我。 正：Please hand me that dictionary. 正：Please hand that dictionary to me. 她去年教我们的音乐。 正：She taught us music last year. 正：She taught music to us last year. 2. 在buy, make, get, order, cook, sing, fetch, play, find, paint, choose, prepare, spare 等之后用介词for 。如： 他为我们唱了首英语歌。 正：He sang us an English song. 正：He sang an English song for us. 请帮我把钥匙找到。 正：Please find me the keys. 正：Please find the keys for me. 能耽搁你几分钟吗(即你能为我抽出几分钟吗)? 正：Can you spare me a few minutes?
正：Can you spare a few minutes for me? 3. 有的动词由于用法和含义不同，用介词to 或for 都是可能的。如： do sb a favor＝do a favor for sb 帮某人的忙 do sb harm＝do harm to sb 对某人有害 在有的情况下，可能既不用for 也不用to，而用其他的介词。如： play sb a trick＝play a trick on sb 作弄某人 请比较： play sb some folk songs＝play some folk songs for sb 给某人演奏民歌 有时同一个动词，由于用法不同，所搭配的介词也可能不同，如leave sbsth 这一结构，若表示一般意义的为某人留下某物，则用介词for 引出间接宾语，即说leave sth for sb；若表示某人死后遗留下某物，则用介词to 引出间接宾语，即说leave sth to sb。如： Would you like to leave him a message? / Would you like to leave a message for him? 你要不要给他留个话？ Her father left her a large fortune. / Her father left a large fortune to her. 她父亲死后给她留下了一大笔财产。 二、表示目标或方向的区别 两者均可表示目标、目的地、方向等，此时也要根据不同动词分别对待。如： 1. 在come, go, walk, move, fly, ride, drive, march, return 等动词之后通常用介词to 表示目标或目的地。如： He has gone to Shanghai. 他到上海去了。 They walked to a river. 他们走到一条河边。
202X中考英语：to和for的区别与用法中考栏目我为考生们整理了“202X中考英语：to和for的区别与用法”，希望能帮到大家，想了解更多考试资讯，本网站的及时更新哦。 202X中考英语：to和for的区别与用法 to和for的区别与用法是什么 一般情况下, to后面常接对象; for后面表示原因与目的为多。 Thank you for helping me. Thanks to all of you. to sb. 表示对某人有直接影响比如，食物对某人好或者不好就用to; for 表示从意义、价值等间接角度来说，例如对某人而言是重要的，就用for. for和to这两个介词，意义丰富，用法复杂。这里仅就它们主要用法进行比较。 1. 表示各种“目的” 1. What do you study English for? 你为什么要学英语? 2. She went to france for holiday. 她到法国度假去了。 3. These books are written for pupils. 这些书是为学生些的。 4. hope for the best, prepare for the worst. 作最好的打算，作最坏的准备。
2.对于 1.She has a liking for painting. 她爱好绘画。 2.She had a natural gift for teaching. 她对教学有天赋。 3.表示赞成同情，用for不用to. 1. Are you for the idea or against it? 你是支持还是反对这个想法? 2. He expresses sympathy for the common people.. 他表现了对普通老百姓的同情。 3. I felt deeply sorry for my friend who was very ill. 4 for表示因为，由于(常有较活译法) 1.Thank you for coming. 谢谢你来。 2. France is famous for its wines. 法国因酒而出名。 5.当事人对某事的主观看法，对于(某人)，对?来说(多和形容词连用)用介词to，不用for.. He said that money was not important to him. 他说钱对他并不重要。 To her it was rather unusual. 对她来说这是相当不寻常的。 They are cruel to animals. 他们对动物很残忍。
keep的用法 1. 用作及物动词 ①意为"保存；保留；保持；保守"。如： Could you keep these letters for me, please? 你能替我保存这些信吗？ ②意为"遵守；维护"。如： Everyone must keep the rules. 人人必须遵守规章制度。 The teacher is keeping order in class.老师正在课堂上维持秩序。 ③意为"使……保持某种(状态、位置或动作等)"。这时要在keep的宾语后接补足语，构 成复合宾语。其中宾语补足语通常由形容词、副词、介词短语、现在分词和过去分词等充当。如： 例：We should keep our classroom clean and tidy.(形容词) 我们应保持教室整洁干净。 You'd better keep the child away from the fire.(副词)你最好让孩子离火远一点。 The bad weather keeps us inside the house.(介词短语)坏天气使我们不能出门。 Don't keep me waiting for long.(现在分词)别让我等太久。 The other students in the class keep their eyes closed.(过去分词) 班上其他同学都闭着眼睛。 2. 用作连系动词 构成系表结构：keep＋表语，意为"保持，继续(处于某种状态)"。其中表语可用形容词、副词、介词短语等充当。如： 例：You must look after yourself and keep healthy.(形容词) 你必须照顾好自己，保持身体健康。 Keep off the grass.(副词)请勿践踏草地。 Traffic in Britain keeps to the left.(介词短语)英国的交通是靠左边行驶的。 注意：一般情况下，keep后接形容词较为多见。再如： She knew she must keep calm.她知道她必须保持镇静。 Please keep silent in class.课堂上请保持安静。 3. ①keep doing sth. 意为"继续干某事"，表示不间断地持续干某事，keep后不 能接不定式或表示静止状态的v-ing形式，而必须接延续性的动词。 例：He kept working all day, because he wanted to finish the work on time. 他整天都在不停地工作，因为他想准时完成工作。 Keep passing the ball to each other, and you'll be OK.坚持互相传球，你们就
to , of 和for的区别 1.to有到的意思，常常和go，come，get连用引出地点。Go to school , go to the shop , go to the cinema. 常见的短语：the way to 去---的路 On one’s way to 在某人去---的路上 以上的用法中，当地点是副词home，here，there等是to 要去掉。如：get home，the way here To后跟动词原形，是不定式的标志 It is +形容词+(for/of +人+)to do sth.(括号内部分可以省略) It is easy for me to learn English. It is very kind of you to lend me your money. 当形容词表示人的行为特征时用of表示to do的性质时用for Want, hope ,decide, plan , try , fail等词后跟to do I want to join the swimming club. Would like to do I’d like to play basketball with them. It is time to have a break. Next to , close to , from ---to--- 2.for 为，表示目的。 Thank you for Buy sth for sb =buy sb sth It is time for bed. Here is a letter for you.
I will study for our country. 3.of表示所属关系意思是：---的 a map of the world a friend of mine
for和of引导的不定式结构的区别 不定式是一种非谓语动词，不能单独作谓语，因此没有语法上的主语。但由于不定式表示的是动作，在意义上可以有它的主体。我们称之为逻辑主语。 提起不定式逻辑主语，人们首先想到的会是“for+名词（宾格代词）+不定式”的复合结构。如：It is important for us to study English well．然而，有时不定式的逻辑主语须要用“of+名词（代词宾格）”才行。例如：It is kind of you to help me．而不能说：It is kind for you to help me．在选择介词“for”还是“of”时，人们往往总是凭感觉而定。有时受习惯影响，多选介词“for”。于是常出现这样的错误：It was careless for him to lose his way．It is cruel for you to do so．由于众多语法书对这种结构中使用“for”与“of”的区别介绍甚少，一些人对其概念认识尚不完全清楚，笔者认为有必要就这一问题作些探讨与介绍。 一、在句中的语法作用不同 a．不定式for结构在句中可以作主、宾、表、定、状、同位语： 1．It is easy for Tom to do this work．（主语）汤姆做此工作是容易的。 2．I'd like for him to come here．（宾语）我喜欢他来这里。 3．His idea is for us to travel in two different groups．（表语）他的想法是：我们分成两组旅行。 4．Have you heard about the plan for you to go abroad．（定语）你听到让你出国的计划吗？ 5．The word is too difficult for him to pronounce well．（状语）这单词太难，他念不准。 6．In the most schools，it is the custom for the headmaster to declare the newterm start．在大部分学校，校长宣布新学期开始是一个习惯。 b．不定式of结构只能在句中作主语。 1．It was careless of him to leave his umbrella in the train．他把伞丢在火车上真是太粗心了。 2．It is awfully good of you to come to see me off at the station．谢谢你来车站送我。 二、逻辑主语的名词有所不同
for 表原因、目的 of 表从属关系 介词of的用法 （1）所有关系 this is a picture of a classroom （2）部分关系 a piece of paper a cup of tea a glass of water a bottle of milk what kind of football，american of soccer？ （3）描写关系 a man of thirty 三十岁的人 a man of shanghai 上海人 （4）承受动作 the exploitation of man by man．人对人的剥削。 （5）同位关系 it was a cold spring morning in the city of london in england． （6）关于，对于 what do you think of chinese food？ 你觉得中国食品怎么样？ 介词for 的用法小结 1. 表示“当作、作为”。如: i like some bread and milk for breakfast. 我喜欢把面包和牛奶作为早餐。what will we have for supper? 我们晚餐吃什么? 2. 表示理由或原因,意为“因为、由于”。如: thank you for helping me with my english. 谢谢你帮我学习英语。 thank you for your last letter. 谢谢你上次的来信。 thank you for teaching us so well. 感谢你如此尽心地教我们。 3. 表示动作的对象或接受者,意为“给……”、“对…… (而言)”。如:
介词for用法完全归纳 ？ 本站特约作者陈根花 ？ 用法1：(表目的)为了。如： They went out for a walk. 他们出去散步了。 What did you do that for 你干吗这样做？ That’s what we’re here for. 这正是我们来的目的。 What’s she gone for this time 她这次去干什么去了？ He was waiting for the bus. 他在等公共汽车。 【用法说明】在通常情况下，英语不用 for doing sth 来表示目的。如： 他去那儿看他叔叔。 误：He went there for seeing his uncle. 正：He went there to see his uncle. 但是，若一个动名词已名词化，则可与 for 连用表目的。如： He went there for swimming. 他去那儿游泳。(swimming 已名词化) 注意：若不是表目的，而是表原因、用途等，则其后可接动名词。(见下面的有关用法) 用法2：(表利益)为，为了。如： What can I do for you 你想要我什么？ We study hard for our motherland. 我们为祖国努力学习。 Would you please carry this for me 请你替我提这个东西好吗？ Do more exercise for the good of your health. 为了健康你要多运动。【用法说明】(1) 有些后接双宾语的动词(如 buy, choose, cook, fetch, find, get, order, prepare, sing, spare 等)，当双宾语易位时，通常用 for 来引出间接宾语，表示间接宾语为受益者。如： She made her daughter a dress. / She made a dress for her daughter. 她为她女儿做了件连衣裙。 He cooked us some potatoes. / He cooked some potatoes for us. 他为我们煮了些土豆。 注意，类似下面这样的句子必须用 for： He bought a new chair for the office. 他为办公室买了张新办公椅。 (2) 注意不要按汉语字面意思，在一些及物动词后误加介词 for： 他们决定在电视上为他们的新产品打广告。 误：They decided toadvertise for their new product on TV. 正：They decided to advertise their new product on TV. 注：advertise 可用作及物或不及物动词，但含义不同：advertise sth＝为卖出某物而打广告；advertise for sth＝为寻找某物而打广告。如：advertise for a job=登广告求职。由于受汉语“为”的影响，而此处误加了介词 for。类似地，汉语中的“为人民服务”，说成英语是 serve the people，而不是 serve for the people，“为某人的死报仇”，说成英语是avenge sb’s death，而不是 avenge for sb’s death，等等。