农学本科毕业外文翻译

新旧玉米杂交种在高种植密度条件下的生长表现摘要抗逆性优良的玉米杂交种的遗传改良是通过增加种植玉米群体数量来为增产做出贡献的。这项研究对尼日利亚大草原上不同的年代的玉米杂交种在高种植密度下的反应进行了评估。分别在2002年和2003年，对位于北部几内亚热带草原地带的尼日利亚扎里亚萨马鲁的农业研究所试验站进行实地调查。六个品种，两个来自于20世纪80年代，两个来自于20世纪90年代，两个来自于21世纪：三个种植密度，采用裂区设计，3次重复。种植密度（53,333，66,666和79999株/公顷）为主区，6个杂交品种为副区。种植密度超过53,333株/公顷的杂交种减产，这可能是由于试验选择的杂交种是在低种植密度下进行的，因此对种植密度压力没有耐性。这也可能是由于试验区低的增产潜力，它不允许在高种植密度下增产。测试杂种之间存在显著性差异。在所有密度条件下，21世纪的杂交种的产量高于20世纪80年代和20世纪90年代的杂交种。为了在高种植密度下提高玉米的产量，我们建议在在高种植密度下选择杂交种。关键词：玉米杂交种；种植密度；玉米产量。光合作用是决定作物产量的最重要的因素之一，因为大于90%的作物干物重直接来自于光合作用。这意味着，在农业生产中，光合效率直接关系到最终产量。因此，许多国家的研究人员已经积极地参与到新的育种方法的研究中，目的是改善作物的光合能力［4,5］，并且通过我们长期在不同发育阶段采取多种措施以提高作物生产潜力的研究，高光效育种已经被视为重要的方法之一。在过去的二十年里，通过植物生理学家和基因学家的密切科学合作，中国研究人员已经采取许多不同手段，广泛地研究了个别作物增加产量的潜力，并且已经取得了明显进展。这篇文章是对我们以前在玉米高光效育种研究以及其他研究者的相关工作的简短回顾。不同的光合参数，例如光截获能力、光能转换效率、碳同化作用和光合特性的遗传等等，都有待讨论。引言玉米的粮食产量受植物种群密度的影响比其它禾本科的植物明显，这是因为玉米的分蘖能力低，且为雌雄同株的植物，存在一个相对短的花期（Sangoi等，2002）。理想的植物种群取决于几个因素，例如，水份的供应，土壤肥力，成熟期和行距（Argenta等人，2001年）。低的种植密度能延迟冠层的郁闭和减少光截获，导致高粮食生产低粮食单产（安德拉德等人，1999）。另一方面，较高的种植密度，提高同化物、水分和养分（Edmeades等，2000）的竞争。高种植密度也促进不结实和增加开花吐丝间隔（ASI）（Sangoi等人，2001），从而降低了玉米主要产量构成单位面积的籽粒产量。在20世纪后半期，根据Duvick和卡斯曼（1999），普通玉米籽粒产量每单位面积大幅增加。这一产量增加归因于遗传改良，气候变化，改善作物管理措施，现代杂种更大的容忍低土壤水分胁迫（德怀尔等人，1992年）和杂草干扰（Tollenaar等,1997）。增加产量也被归因于玉米耐高种植密度（Duvick和卡斯曼，1999;Tollenaar和吴1999）。最佳行的宽度和密度在大田玉米生产系统继续加大在美国的玉米遗传技术的发展（Duvick和卡斯曼，1999年）。杂交玉米不同种植密度的影响（Echarte等人，2000年；Maddonni等人，2001）。最近开发的杂交品种比旧的杂交品种可以承受更高种植密度（Tollenaar,1989）。更近的杂交品种被发现有较高的抗倒伏能力，更好地抵御环境压力的能力，从而产生更少的贫瘠的植物（Tollenaar，1991年）。研究利用杂种在尼日利亚开始于70年代早期（1978年Fajemesin，1993年Fakorede等人），并于1979年成为国际热带农业研究所（IITA）玉米改良方案的一个组成部分（基姆，1997）。1984年对全国农田进行了实验杂种测试（fakorede等人。，1999）并发现产量大大高于广泛种植的自由授粉品种。自那时以来，杂交种被许多商业种子公司开发和销售。尽管有产量的优势，但人们普遍相信尼日利亚杂交耐性较小，因此需要比自由授粉品种投入高。传统玉米杂交种和开放授粉品种种植密度为53,333株/公顷，行长为0.75米、宽为0.25米。在尼日利亚草原得到高等植物密度的玉米杂交种，关于此没有任何信息的反馈。本研究的目的是评估玉米杂交种研究成果，在尼日利亚草原过去20年中，玉米种植密度高于53333株/公顷。材料与方法研究的地域和文化实践活动Samaru在2002年和2003年对农业研究所试验站进行了实地调研，主要在几内亚热带稀树草原地带北部，扎里亚，尼日利亚（7.38°E，11.11°N，686m左右）。土壤类型为：粉细砂土。在这个网站我们了解到2002年和2003年的总雨量分别为1009.5毫米、1139.0毫米，平均气温分别为25°C、26°C。从1982到2000年在尼日利亚草原这项研究的发展和广泛的测试中使用了六个杂交种；两个杂交从上世纪80年代（8321-21和8425-8），两个杂交种从上世纪90年代（9801-11和9803-2），两个杂交种从2000年（0103-11和0103-15）。种植试验开始于2002年6月30日和2003年7月1日。在这两年中，试验奠定了一个裂区设计3次重复。三种植物种植密度-53333，66666，79999株/公顷-在主要小区，而这6个杂交种是每个主要小区的副处理。每个副处理有四行;每行行长为0.75米、间距为5米。达到种植目标密度的种植的地块，需经过两个星期整理。试验前茬的作物是大豆（大豆（属）美林），其次是2年休耕的地块。在种植时，N、P和K肥料40kg/ha。在种植五周后施用其他形式的氮肥，以尿素为主，60公斤/公顷（WAP）。控制杂草，整地前两个星期使用草甘膦（N-（膦酰基甲基）甘氨酸），七个星期后用百草枯（l:l-dimethly-4，4'-联吡啶氯化）。从播种到50％的花粉棚（开花期）和50％的丝挤出（吐丝期）的日期是由30株决定的，它们位于每个小区中间两行。开花吐丝间隔（ASI）指开花期和吐丝期之间的日期。根倒伏百分数从中央行录得。粮食产量从每个小区的中央行取得，不包含每一行结束植株。在收获时记录每一个小区总的植株数和穗数。穗数/植株数计算为植株总的穗数除以总植株数。从每小区收获的穗先去壳，再使用迪基约翰水分测定仪（型号14998，迪基约翰公司，阿拉巴马州奥本市）进行水分百分率测定。计算粮食产量时，要将去壳的粮食的水分百分率降至12％。数据使用SAS方差分析程序进行了方差分析（SAS学院，1990）。分析了两年的数据除了根倒伏的百分率，其中仅在2003年收集的数据split-split-plot模型作为第一要素，种植密度为第二个因素，杂交种作为第三个因素。2003年，使用分割的方法对根倒伏百分率的数据进行了分析，结果是“分区处理间进行的LSD在PW0.05”。原文出处:CommunicationsinBiometryandCropScienceVol.1,No.1,2006,pp.41-48http://agrobiol.sggw.waw・pl/cbcsInternationalJournaloftheFacultyofAgricultureandBiology,WarsawAgriculturalUniversity,PolandREGULARARTICLEPerformanceofoldandnewmaizehybridsgrownathighplantdensitiesinthetropicalGuineasavannaAlphaY.Kamara*1,AbebeMenkir1,IbrahimKureh2,LuckyO.Omoigui1,FridayEkeleme3InternationalInstituteofTropicalAgriculture(IITA)ZIbadan,Nigeria,c/oL.WLamboum&Co.,CarolynHouse,26DingwallRoad,CroydonCR93EEEngland.InstituteforAgriculturalResearch(IAR),AhmaduBelloUniversity(ABU),P.M.B.1044,Zana,Nigeria.MichaelOkparaUniversityofAgriculture,Umudike,P.M.B7267,Umuahia,Nigeria.*Correspondingauthor:A.Y.Kamara,E-mail:A.Kamara@Citation:KamarazA.Y.,Menkir,A.,Kureh,I.zOmoigui,L.O,Ekeleme,F.(2006).Performanceofoldandnewmaizehybridsgrownathighplantdens让iesinthetropicalGuineasavanna.Commun.BiometryCropSci.1(1),41-48.Received:27February2006,Accepted:1June2006zPublishedonline:5August2006©CBCS2006AbstractGeneticimprovementofmaizehybridsforsuperiorstresstolerancehascontributedtoincreasedyieldbyallowinghybridstobeplantedathigherplantpopulations.ThisstudywasconductedtoevaluatetheresponseofmaizehybridsdevelopedintheNigerianSavannafromdifferenterastohighplantdensities.Fieldresearchwasconductedin2002and2003attheexperimentstationoftheInstituteofAgriculturalResearch,SamaruinthenorthernGuineasavannazone,Zaria,Nigeria.Sixhybrids—twofrom1980s,twofrom1990sandtwofromthe2000eras—wereevaluatedatthreeplantdens让iesusingasplit-plotdesignw让hthreereplications.Plantdensities(53,333,66,666,and79,999plantsha1)constitutedthemainplotsandthesixhybridswereassignedtosubplots.Plantdensitiesabove53,333plantsha1reducedgrainyieldofhybrids,whichmightbeduetothefactthatthehybridsevaluatedwereselectedatlowplantdensitiesandwerethereforenottoleranttoplant-densitystress・Itmightalsobeduetothelowyieldpotentialintheexperimentalarea,whichdidnotallowyieldincreasesathighplantdensities.Thereweresignificantdifferencesamongthetestedhybrids.Thehybridsreleasedin2000out-yieldedthehybridsreleasedin1980and1990satallplantdens让ies.Toimprovemaizegrainyieldathighplantdensities,werecommendthatthehybridsbeselectedathighplantdensHies.KeyWords:maizehybrids;plantdensities;maizegrainyield.IntroductionGrainyieldofmaize(ZeamaysL.)ismoreaffectedbyvariationsinplantpopulationdens让ythanofothermembersofthegrassfamilybecauseoflowtilleringability,monoeciousfloralorganization,andthepresenceofarelativelyshortfloweringperiod(Sangoietal.,2002).Theidealplantpopulationdependsonseveralfactors,e.g.,wateravailability,soilfertility,hybridmaturity,androwspacing(Argentaetal.,2001).Theuseoflowerplantdensitiesdelayscanopyclosureanddecreaseslightinterception,leadingtohighgrainproductionperplantbutlowgrainproductionperunitarea(Andradeetal.,1999).Ontheotherhand,higherplantdensitiesenhanceinterplantcompetitionforassimilates,waterandnutrients(Edmeadesetal.,2000).Highplantdensitiesalsostimulatebarrennessandincreasetheanthesis-silkinginterval(ASI)(Sangoietal.,2001)ztherebyreducingkernelnumberperunitarea-themainyieldcomponentofmaize・AccordingtoDuvickandCassman(1999),averagemaizegrainyieldperurdtareaincreaseddramaticallyduringthesecondhalfofthe20thcentury.Thisyieldgainwasattributedtogeneticimprovement,climatechange,improvementincropmanagementpractices,andgreatertoleranceofmodernhybridstolowsoil-moisturestress(Dwyeretal.z1992)andweedinterference(Tollenaaretal.z1997).Yieldgainhasalsobeenattributedtotoleranceofmaizetohighplantdensities(DuvickandCassman,1999;TollenaarandWu,1999).Theoptimalrowwidthandplantdensityinfieldmaize-productionsystemscontinuetointensifyintheUSAasmaizegenetictechnologiesevolve(DuvickandCassman,1999).Maizehybridsdifferintheirresponsetoplantdensity(Echarteetal.,2000;Maddonnietal.z2001).Hybridsdevelopedrecentlycouldw让hstandhigherplantdensitylevelsthantheolderhybrids(Tollenaar,1989).Themorerecenthybridswerefoundtohavedecreasedlodgingathigherplantpopulations,andalsotheywerebetterabletow让hstandenvironmentalstress,resultinginproductionoffewerbarrenplants(Tollenaar,1991)・ResearchontheuseofhybridsinNigeriastartedintheearly1970s(Fajemesin,1978;Fakoredeetal.,1993)andbecameanintegralpartoftheMaizeImprovementProgramattheInternationalInstituteofTropicalAgriculture(IITA)in1979(Kim,1997).Experimentalhybridsweretestedonfarmers'fieldslocatedallacrossthecountryin1984(Fakoredeetal.z1999)andwerefoundtoyieldconsiderablyhigherthanthewidelygrownopen-pollinatedvarieties.Sincethen,manycommercialhybridshavebeendevelopedandmarketedbyseedcompanies.Desp让巳theyieldadvantage,thereiswidespreadbeliefinNigeriathathybridsarelessstresstolerantandthereforerequirehigherinputsthanopen-pollinatedvarieties.Traditionallybothhybridsandopen-pollinatedvarietiesareselectedat53,333plantsha4w让harowwidthof0.75mandw让hinarowdistanceof0.25m.ThereisnoinformationontheresponseofmaizehybridsdevelopedintheNigeriansavannastohigherplantdensities.Theobjectiveofthisstudywastoevaluatetheperformanceofmaizehybrids,developedinNigerianSavannasduringthepast20years,atplantdensitieshigherthan53,333plantsha1.MaterialandmethodsStudysiteandculturalpracticesFieldresearchwasconductedin2002and2003ontheexperimentstationoftheInstituteofAgriculturalResearch,SamaruinthenorthernGuineasavannazone,Zaria,Nigeria(7.38°E,11.11°N,686masl).Thesoiltypewasafine-loamy,IsohyperthermicPlinthustalf.Thetotalrainfallreceivedatthiss让ewas1009.5mmin2002and1139mmin2003,andtheaveragetemperaturewas25°Cin2002and26°Cin2003.SixhybridsdevelopedandwidelytestedintheNigeriansavannafrom1982to2000wereusedinthisstudy;twohybridswerefromthe1980s(8321-21and8425-8),twofromthe1990s(9801-11and9803-2),andtwofrom2000s(0103-11and0103-15).Trialswereplantedon30June2002and1July2003・Inbothyears,thetrialwaslaidoutinasplit-plotdesignw让hthreereplications.Threeplantdensities—53,333,66,666,and79,999plantsha1—werethemainplots,whereasthesixhybridswerethesubplotswithineachmainplot.Therewerefourrowsineachsub-plot;therowswere5minlengthandspaced0.75mapart・Plotswereover-plantedandhand-thinnedtoachievethedesiredtargetdens让yattwoweeksafterplanting.Thepreviouscropatthetests让ewassoybean(Glycinemax(L.)Merrill),followedbytwoyearsoffallow.Atplanting,fertilizerwasappliedattherateof40kg/haeachofN,P,andK.Add让ionalNfertilizer,intheformofurea,wasappliedattherateof60kgN/hafiveweeksafterplanting(WAP)・Weedswerecontrolledusingglyphosate(N-(phosphonomethyl)glycine)twoweeksbeforelandpreparation,andparaquat(1:1-dimethly-4,4'-bipyridiniumdichloride)atsevenWAP・Daysfromsowingto50%pollenshed(anthesisdate)and50%silkextrusion(silkingdate)weredeterminedusing30plantsinthemiddletworowsofeachplot.Anthesis-silkinginterval(ASI)wascalculatedasthedifferencebetweendaystoanthesisandsilking.Thepercentrootlodgingwasrecordedfromthetwocentralrows・Grainyieldwasrecordedfromthetwocentralrowsofeachplot,excludingtheendplantsineachrow.Thetotalnumberofplantsandearswerecountedineachplotatharvest・Thenumberofears/plantwasthencalculatedasthetotalnumberofearsatharvestdividedbythetotalnumberofplantsharvested.EarsharvestedfromeachplotwereshelledandthepercentgrainmoisturewasdeterminedusingaDickey-Johnmoisturetester(Model14998,Dickey-JohnCorporation,Auburn,Alabama).Grainyield,adjustedto12%moisture,wascomputedfromtheshelledgrain.Dataweresubjectedtoanalysisofvariance(ANOVA)usingtheGLMprocedureinSAS(SASInstitute,1990)・Dataforthetwoyearswiththeexceptionof%rootlodging,whichwascollectedonlyin2003,wereanalyzedaccordingtoasplit-split-plotmodelw让hyearsasthefirstfactor,andplantdensityasthesecondfactor,andhybridsasthethirdfactor.Dataon%rootlodgingfor2003werealsoanalyzedusingasplitplotapproach.Means'separationamongtreatmentswasconductedusingtheLSDatP<0.05.ResultsanddiscussionHybridsandplantdensitiesinfluencedsignificantlydaysto50%silking,ASI,%rootlodging,earsperplant,andgrainyield.Hybridxplantdensityinteractionwassignificantforalltraitsmeasuredexcept%rootlodging(Table1).ReproductiveresponsetoplantdensityDaystomidsilkingincreasedw让hincreasesinplantpopulationforallhybrids.Averagedacrossgenotypes,silkingwasdelayedbythreedaysasplantpopulationincreasedfrom53,333to66,666.Inrelationtoplantpopulationof53,333,daystosilkingwasdelayedbyfivedaysat79,999plantsha4(Table2).Silkdelayduetohighplantpopulationvariedw让hhybridsandwasnotconsistentbetweentheoldandnewhybrids.HighplantpopulationincreasedASIforallhybridsevaluated.Therewerenosignificantdifferencesamonghybridsatplantpopulationsof53,333and66,666,exceptthattheolderhybridoftheyear1980,8321-21zandtheyear1990hybrid,9801-11,recordedsignificantlyhigherASIthantheotherhybridsat66,666plantsha".Atplantpopulationof79,999plantsha4,theyear2000hybridshadsign让icantlylowerASIthanthosefromtheoldereras.ASIforyear1980hybridsdidnotsignificantlydifferfromthoseoftheyear1990hybrids.RootLODGING,NUMBERofearsperplantandgrainyieldPlantdensitiesabove53,333increasedrootlodging(Table3).Differencesamonghybridswerethelargestat79,999plantsha-1.Theyear1980and1990hybridsrecordedhigherpercentageofrootlodgingthantheyear2000hybrids.Highplantpopulationsstimulatedbarrennessinallthehybridsevaluated.Olderhybridsrecordedlowernumberofearsperplantthanthenewerones.Nevertheless,onaverage,theyear1990hybridshadlowernumberofearsperplantthanthoseoftheyear1980(Table4).Table1.ANOVAofresponsetoplantdensityoffloweringtime,anthesis,earsperplant,lodging,andgrainyieldofsixmaizehybrids.SourceDfMeanSquareDaystosilkingDaystoanthesisASIEarsperplantGrainyieldReplicate22.9182.2181.1550.015315491029*Years13.2261.1900.43880.00994657935ErrorA23.9550.90821.23690.017831651244Density2306.4**14.74**186.8**2.019**41109981**YearsxDensity21.0301.4820.64730.003808775800ErrorB80.37280.81270.59820.0076162433270Hybrids528.74**6.252**9.844**0.1420**8078008**YearsxHybrids53.5431.3050.53860.006097136050Density*Hybrids104253*2.935*2.177**0.08553**2495884**YearsxDensityxHybrids101.2101.3690.68460.006490163561ErrorC602.0881.3220.63440.017836714003DfLodgingReplicate241.19*Density2171.2**ErrorA436.13Hybrids526.06*DensityxHybrids1015.97ErrorB308.770*Significcintatthe0.05probabilitylevel.**Significantatthe0.01probabilitylevel.Table2.Effectofplantpopulationonagronomicperformanceofmaizehybridsfromdifferenterasofbreeding・DaystosilkingDaystoanthesisASIPlantsha'1TreatmentEra53,33366,66679,999Mean53,33366,66679,999Mean53/33366,66679,999MeanYears200262656865•61626362.013532003626568656262&362.31353Hybrids0103-11200062666865.061636362.51342.60103-15200059636662.959626261.10242.09801-11199063666865.762626262.21463.79803-2199064657066.362626462.71373.68321-21198063666865.962626362.41463.68425-8198062646764.26161&261.60342.6Mean61.462.162.7135Plantdensitiesabove53,333plantsha-1reducedgrainyieldby22%forplantpopulationof66,666andby56%forplantpopulationof79,999.Grainyieldsoftheyear2000hybridswereusuallyhigherthanthoseof1980and1990hybridsatallplantdensities(Table4).Theyear1990hybridsrecordedthehighestgrainyieldreductionathigherplantdensities.Table3.Effectofplantpopulationonlodgingofmaizehybridsfromdifferenterasofbreeding.Plantsha-1HybridsEra53.33366.66679.999Mea312.616.014.30103-15200011.318.618.016.1980141199012.016.621.616.89803-2199012.616.623.017.48321-21198016.617.618.017.48425-8198015.619.623.019.4Mean13.817.019.9LSD(0.05)Density[D]=0.68LSD(0.05)Hybrids[H]=0.96LSD(0.05)HxD=1.55Table4.Effectofplantpopulationonnumberofearsperplantandgrainyieldofmaizehybridsfromdifferenterasofbreeding.EarsplanZ Yield(kgha")Plantha1TreatmentEra5333366,66679,999Mean53#33366,66679,999MeanYears20021.060.820.600.83383834052001308120031.050.810.560.814223329018693127Hybrid070.850.680.8746833620252636100103-1520001.090.960.830.9643265050324742089801-1119901.060.600.5607439881907160124999803-219901.100.850280733506362477326348321-2119801.010.760.5107641992680120026938425-819801.010:900.640.853482320322642983Mean1.060.820.58403133471935LSD(0.05)Years[Y]forEarsperplant=0.05,forYield=325.3LSD(0.05)Density[D]forEarsperplant=0.06,forYield=398.4LSD(0.05)Hybrids[H]forEarsperplant=0.09,forYield=563.4LSD(0.05)YxDforEarsperplant=0.09zforYield=1097.4LSD(0.05)YxHforEarsperplant=0.13,forYield=926.6LSD(0.05)HxDforEarsperplant=0.15,forYield=1169.3Therewasadifferentialresponseofthemaizehybridstohighplantdensities,althoughhighplantdensitiesgenerallyreducedgrainyieldofallthehybridsthatwereevaluatedduringthisstudy.Generally,newerhybridsoftheyear2000weremoretoleranttohighplantpopulationthanthoseoftheyears1980and1990.Athighplantdensities,daystosilkingandASIwerefewerintheyear2000hybridsthanintheyear1980and1990hybrids.ThefewerdaystosilkingandshorterASIamongthe2000-erahybridsdemonstratethatlossofsynchronybetweenmaleandfemaleinflorescencewaslesspronouncedinthemodernhybridsatdensestands.Thissuggestsgreatertoleranceofmodernhybridsthantheolderhybrids.Sangoietal.(2002)reportedsimilarresultsforBrazilianhybrids.TheyfoundthatincreaseinplantpopulationlengthenedtheASImoredrasticallyfortheolderhybridsthanthemodernhybrids.AnincreaseinASIischaracteristicofmaizeunderenvironmentalstress,suchasN-deficiency,droughtandhigherplantdensity(BolanosandEdmeades,1996).IncreaseinASIreducesnumberofkernelsperear(Sangoietal.,2002)・Anasynchronousfloweringcanlimitgrainproductionperearduetolackofpollen,lossofsilkreceptivityorearlykernelabortion(CarcovasandOtegui,2001).Althoughkernelnumberwasnotdeterminedinthisstudy,itisspeculatedthatincreasedASIathighplantdensitiesparticularlyfortheolderhybridsmighthavereducedkernelnumber,leadingtolowergrainyields.Higherplantdensitiesincreasedrootlodging.Thiswasmorepronouncedinolderthannewerhybrids.Thisresultagreesw让hSangoietal.,(2002)whoreportedincreasedstalkandrootlodginginolderhybridswhengrownatsuprahighdensities.PaszkiewiczandButzen(2005)reportedthatnewerhybridsweremoretolerantofhighplantpopulationforrootandstalklodgingthanolderhybridsinareashighlypronetolodgingintheUSA.Earsperplant,whichisameasureofbarrenness,wereverystronglyrelatedtograinyield.Thissuggeststhatreducedbarrennessathighplantdensitiesislinkedtotoleranceofmaizehybridstohighplantdensities.Earsperplantwerereducedby23%at66,666plantsha4andby43%at79,999plantsha1.Threehybrids(2fromtheyear1980hybridsand1fromyear1990)recordedreductionsrangingfrom45%-82%.Sangoietal.(2002)foundthathighplantdensitiesabove50,000plantsha4stimulatedbarrennessinBrazilianmaizehybrids.They,however,foundolderhybridstohavefewerearsperplantthanthemodernhybridsathighplantdensities.Similarly,moderncultivarsoftheyear2000hybridswerefoundinourstudytoproducemoreearsperplantthantheolderhybridsathighplantdensities.TherewassignificantgrainyieldreductionatplantdensitiesabovetheoptimumpopulationintheNigeriansavannas・Otherstudiesconductedelsewherehadreportedacurvilinearresponseofmaizehybridstoplantdensity(TollenaarandWu,1999;Sangoietal.,2002;Echarteetal.,2000).Moststudiesfoundmodernhybridstoyieldhigherthanolderhybridsathighplantdensities.Theresultspresentedhereinshowedthatallhybridsstudiedwerenottolerantofhighplantdensitiesabove53,333plantsha1・Thisiscontrarytotheresultsfromotherstudies.IntheUSAforexample,Milleretal.(1995)foundmaizehybridstoperformbetterathigherdensities・WiddicombeandThelen(2002)alsofoundmaizehybridstogivehigheryieldsat90,000plantsha1inthenorthernU.S.CornBelt.Theauthorsalsosuggestedthat90z000plantsha1wasnottheoptimalplantpopulationforthehybridsevaluated.ThepoorperformanceofmaizehybridsinthisstudymaybeduetothefactthatselectionofmaizeintheNigeriansavannasisgenerallydoneatlowplantdensitiesof53,333plantsha1・Thismayhavecausedhybridsselectedatthispopulationdensitytobeintoleranttohighplantpopulations.Enhancementsinmaizetolerancetointensecompetitionforincidentphotosyntheticphotonfluxdensity,soilnutrientsandsoilwater,havebeenobtainedbyselectingthebestyieldinginbredlinesunderhighplantpopulationdensitiesacrossawidetestingarea(SangoiandSalvador,1998).Consequently,changesinplantdens让ystresstolerancearemostlytheresultofindirectratherthandirectresponsestoselection(TollenaarandWu,1999).Tomakeprogressinselectionfortolerancetoenvironmentalstresssuchashighplantdensityordrought,thereisaneedtodevelop/selectthemaizelinesathighplantdensities.Theenvironmentmayalsoinfluencegrainyieldsofmaizeathigherdensities.ResearchbyPioneerHi-BredInternationalfrom1999-2000inlocationsacrosstheUSAandCanadahasshownthatmaizehybridresponsetoplantpopulationwasaffectedbyyieldpotentialofthegrowingenvironment(PaszkiewiczandButzen,2005).Ifyieldpotentialinanenvironmentislow,higherplantdensitiesmayreducegrainyieldofmaize.ThismaybetruefortheGuineasavannaecologyofnorthernNigeria.Soilstherearegenerallypoorduetolanddegradationarisingfromcroppingintensification(Oikehetal.z2003).ConclusionsHighplantdensitiesabove53,333plantsha1reducedgrainyieldinmaizehybridsintheNigerianSavanna・Thisiscontrarytoresultsobtainedelsewherewherehighgrainyieldswereobtainedatplantpopulationsfarabove53,333・Thismaybeduetothefactthatthemaizehybridsevaluatedwereselectedatlowplantdensitiesandwerethereforenottoleranttoplantdensitystress.Itmayalsobeduetothelowyieldpotentialoftheexperimentalarea,whichdoesnotallowyieldincreasesathighplantdensities.Thereweresignificantdifferencesamongthemaizehybridsevaluated・Hybridsreleasedin2000out-yieldedthehybridsreleasedin1980sand1990satallplantdensities.Toimprovemaizegrainyieldathighplantdens让ies,werecommendthatthehybridsbeselectedathighplantdensities.AcknowledgementsWethankIITA,UnitedNationsDevelopmentProgrammeandtheInternationalFundforAgriculturalDevelopmentforfinancingthisstudyandthestaffofthemaizeresearchgroupatIITAforassistingw计hfieldoperations.ApprovedmanuscriptnumberIITA/05/JA/44.ReferencesAndrade,F.H.,Vega,C.,Uhart,S.,Cirilo,A.Canterro,M.,Valentnuz,O.(1999).Kernelnumberdeterminationinmaize・CropSci.39,453-459.Argenta,G.,Silva,P・R.F.,Sangoi,L・(2001).Maizeplantarrangement:analysisofthestateoftheart.CienciaRural31,1075-1084.Bolanos,J.zEdmeades,G.O.,(1996).Theimportanceoftheanthesis-silkingintervalinbreedingfordroughttoleranceintropicalmaize.FieldCropsRes.31,233-252.Carcova,J.,Otegui,M.E.(2001).Eartemperatureandpollinationtimingeffectsonmaizekernelset.CropSci.49,1816-1822・Duvick,D.N.,Cassman,K.G.(1999).Post-greenrevolutiontrendsinyieldpotentialoftemperatemaizeinthenorth-CentralUn让edStates.CropSci.39,1622-1630.Dwyer,L.M.,Stewart,D.W.,Tollenaar,M.(1992).Analysisofmaizeleafphotosynthesisunderdroughtstress.Can.J.PlantSci.72,477-481.Echarte,L.zLuque,S.zAndrade,F.H.,Sadras,V.O.,Cirilo,A.,Otegui,M.E.,Vega,C.R.C.(2000).ResponseofmaizekernelnumbertoplantdensityinArgentineanhybridsreleasedbetween1965and1993.FieldCropsRes.68,1-8.Edmeades,G.O.,Bolanos,J.,Elings,A.,Ribaut,J.M.,Baenziger,M.(2000).Theroleandregulationoftheanthesis-silkingintervalinmaize.In:Westgate,M.E.,Boote,K.J.(Eds.).Physiologyandmodellingkernelsetinmaize.CSSA.Madison,WI,43-73・Fajemesin,J.M.(1978).AnoverviewofthemaizeimprovementprogramoftheNationalCerealsInstitute.In:Ojomo,O.A.,FajemisinzJ.M.,Reminsin,S.U.(Eds.).ProspectsofhybridmaizeproductioninNigeria.IITA,Ibadan,Nigeria,7-11.Fakorede, 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