農(nóng)學(xué)本科畢業(yè)外文翻譯

新舊玉米雜交種在高種植密度條件下的生長(zhǎng)表現(xiàn)摘要抗逆性優(yōu)良的玉米雜交種的遺傳改良是通過(guò)增加種植玉米群體數(shù)量來(lái)為增產(chǎn)做出貢獻(xiàn)的。這項(xiàng)研究對(duì)尼日利亞大草原上不同的年代的玉米雜交種在高種植密度下的反應(yīng)進(jìn)行了評(píng)估。分別在2002年和2003年，對(duì)位于北部幾內(nèi)亞熱帶草原地帶的尼日利亞扎里亞薩馬魯?shù)霓r(nóng)業(yè)研究所試驗(yàn)站進(jìn)行實(shí)地調(diào)查。六個(gè)品種，兩個(gè)來(lái)自于20世紀(jì)80年代，兩個(gè)來(lái)自于20世紀(jì)90年代，兩個(gè)來(lái)自于21世紀(jì)：三個(gè)種植密度，采用裂區(qū)設(shè)計(jì)，3次重復(fù)。種植密度（53,333，66,666和79999株/公頃）為主區(qū)，6個(gè)雜交品種為副區(qū)。種植密度超過(guò)53,333株/公頃的雜交種減產(chǎn)，這可能是由于試驗(yàn)選擇的雜交種是在低種植密度下進(jìn)行的，因此對(duì)種植密度壓力沒(méi)有耐性。這也可能是由于試驗(yàn)區(qū)低的增產(chǎn)潛力，它不允許在高種植密度下增產(chǎn)。測(cè)試雜種之間存在顯著性差異。在所有密度條件下，21世紀(jì)的雜交種的產(chǎn)量高于20世紀(jì)80年代和20世紀(jì)90年代的雜交種。為了在高種植密度下提高玉米的產(chǎn)量，我們建議在在高種植密度下選擇雜交種。關(guān)鍵詞：玉米雜交種；種植密度；玉米產(chǎn)量。光合作用是決定作物產(chǎn)量的最重要的因素之一，因?yàn)榇笥?0%的作物干物重直接來(lái)自于光合作用。這意味著，在農(nóng)業(yè)生產(chǎn)中，光合效率直接關(guān)系到最終產(chǎn)量。因此，許多國(guó)家的研究人員已經(jīng)積極地參與到新的育種方法的研究中，目的是改善作物的光合能力［4,5］，并且通過(guò)我們長(zhǎng)期在不同發(fā)育階段采取多種措施以提高作物生產(chǎn)潛力的研究，高光效育種已經(jīng)被視為重要的方法之一。在過(guò)去的二十年里，通過(guò)植物生理學(xué)家和基因?qū)W家的密切科學(xué)合作，中國(guó)研究人員已經(jīng)采取許多不同手段，廣泛地研究了個(gè)別作物增加產(chǎn)量的潛力，并且已經(jīng)取得了明顯進(jìn)展。這篇文章是對(duì)我們以前在玉米高光效育種研究以及其他研究者的相關(guān)工作的簡(jiǎn)短回顧。不同的光合參數(shù)，例如光截獲能力、光能轉(zhuǎn)換效率、碳同化作用和光合特性的遺傳等等，都有待討論。引言玉米的糧食產(chǎn)量受植物種群密度的影響比其它禾本科的植物明顯，這是因?yàn)橛衩椎姆痔Y能力低，且為雌雄同株的植物，存在一個(gè)相對(duì)短的花期（Sangoi等，2002）。理想的植物種群取決于幾個(gè)因素，例如，水份的供應(yīng)，土壤肥力，成熟期和行距（Argenta等人，2001年）。低的種植密度能延遲冠層的郁閉和減少光截獲，導(dǎo)致高糧食生產(chǎn)低糧食單產(chǎn)（安德拉德等人，1999）。另一方面，較高的種植密度，提高同化物、水分和養(yǎng)分（Edmeades等，2000）的競(jìng)爭(zhēng)。高種植密度也促進(jìn)不結(jié)實(shí)和增加開(kāi)花吐絲間隔（ASI）（Sangoi等人，2001），從而降低了玉米主要產(chǎn)量構(gòu)成單位面積的籽粒產(chǎn)量。在20世紀(jì)后半期，根據(jù)Duvick和卡斯曼（1999），普通玉米籽粒產(chǎn)量每單位面積大幅增加。這一產(chǎn)量增加歸因于遺傳改良，氣候變化，改善作物管理措施，現(xiàn)代雜種更大的容忍低土壤水分脅迫（德懷爾等人，1992年）和雜草干擾（Tollenaar等,1997）。增加產(chǎn)量也被歸因于玉米耐高種植密度（Duvick和卡斯曼，1999;Tollenaar和吳1999）。最佳行的寬度和密度在大田玉米生產(chǎn)系統(tǒng)繼續(xù)加大在美國(guó)的玉米遺傳技術(shù)的發(fā)展（Duvick和卡斯曼，1999年）。雜交玉米不同種植密度的影響（Echarte等人，2000年；Maddonni等人，2001）。最近開(kāi)發(fā)的雜交品種比舊的雜交品種可以承受更高種植密度（Tollenaar,1989）。更近的雜交品種被發(fā)現(xiàn)有較高的抗倒伏能力，更好地抵御環(huán)境壓力的能力，從而產(chǎn)生更少的貧瘠的植物（Tollenaar，1991年）。研究利用雜種在尼日利亞開(kāi)始于70年代早期（1978年Fajemesin，1993年Fakorede等人），并于1979年成為國(guó)際熱帶農(nóng)業(yè)研究所（IITA）玉米改良方案的一個(gè)組成部分（基姆，1997）。1984年對(duì)全國(guó)農(nóng)田進(jìn)行了實(shí)驗(yàn)雜種測(cè)試（fakorede等人。，1999）并發(fā)現(xiàn)產(chǎn)量大大高于廣泛種植的自由授粉品種。自那時(shí)以來(lái)，雜交種被許多商業(yè)種子公司開(kāi)發(fā)和銷(xiāo)售。盡管有產(chǎn)量的優(yōu)勢(shì)，但人們普遍相信尼日利亞雜交耐性較小，因此需要比自由授粉品種投入高。傳統(tǒng)玉米雜交種和開(kāi)放授粉品種種植密度為53,333株/公頃，行長(zhǎng)為0.75米、寬為0.25米。在尼日利亞草原得到高等植物密度的玉米雜交種，關(guān)于此沒(méi)有任何信息的反饋。本研究的目的是評(píng)估玉米雜交種研究成果，在尼日利亞草原過(guò)去20年中，玉米種植密度高于53333株/公頃。材料與方法研究的地域和文化實(shí)踐活動(dòng)Samaru在2002年和2003年對(duì)農(nóng)業(yè)研究所試驗(yàn)站進(jìn)行了實(shí)地調(diào)研，主要在幾內(nèi)亞熱帶稀樹(shù)草原地帶北部，扎里亞，尼日利亞（7.38°E，11.11°N，686m左右）。土壤類(lèi)型為：粉細(xì)砂土。在這個(gè)網(wǎng)站我們了解到2002年和2003年的總雨量分別為1009.5毫米、1139.0毫米，平均氣溫分別為25°C、26°C。從1982到2000年在尼日利亞草原這項(xiàng)研究的發(fā)展和廣泛的測(cè)試中使用了六個(gè)雜交種；兩個(gè)雜交從上世紀(jì)80年代（8321-21和8425-8），兩個(gè)雜交種從上世紀(jì)90年代（9801-11和9803-2），兩個(gè)雜交種從2000年（0103-11和0103-15）。種植試驗(yàn)開(kāi)始于2002年6月30日和2003年7月1日。在這兩年中，試驗(yàn)奠定了一個(gè)裂區(qū)設(shè)計(jì)3次重復(fù)。三種植物種植密度-53333，66666，79999株/公頃-在主要小區(qū)，而這6個(gè)雜交種是每個(gè)主要小區(qū)的副處理。每個(gè)副處理有四行;每行行長(zhǎng)為0.75米、間距為5米。達(dá)到種植目標(biāo)密度的種植的地塊，需經(jīng)過(guò)兩個(gè)星期整理。試驗(yàn)前茬的作物是大豆（大豆（屬）美林），其次是2年休耕的地塊。在種植時(shí)，N、P和K肥料40kg/ha。在種植五周后施用其他形式的氮肥，以尿素為主，60公斤/公頃（WAP）?？刂齐s草，整地前兩個(gè)星期使用草甘膦（N-（膦?；谆└拾彼幔邆€(gè)星期后用百草枯（l:l-dimethly-4，4'-聯(lián)吡啶氯化）。從播種到50％的花粉棚（開(kāi)花期）和50％的絲擠出（吐絲期）的日期是由30株決定的，它們位于每個(gè)小區(qū)中間兩行。開(kāi)花吐絲間隔（ASI）指開(kāi)花期和吐絲期之間的日期。根倒伏百分?jǐn)?shù)從中央行錄得。糧食產(chǎn)量從每個(gè)小區(qū)的中央行取得，不包含每一行結(jié)束植株。在收獲時(shí)記錄每一個(gè)小區(qū)總的植株數(shù)和穗數(shù)。穗數(shù)/植株數(shù)計(jì)算為植株總的穗數(shù)除以總植株數(shù)。從每小區(qū)收獲的穗先去殼，再使用迪基約翰水分測(cè)定儀（型號(hào)14998，迪基約翰公司，阿拉巴馬州奧本市）進(jìn)行水分百分率測(cè)定。計(jì)算糧食產(chǎn)量時(shí)，要將去殼的糧食的水分百分率降至12％。數(shù)據(jù)使用SAS方差分析程序進(jìn)行了方差分析（SAS學(xué)院，1990）。分析了兩年的數(shù)據(jù)除了根倒伏的百分率，其中僅在2003年收集的數(shù)據(jù)split-split-plot模型作為第一要素，種植密度為第二個(gè)因素，雜交種作為第三個(gè)因素。2003年，使用分割的方法對(duì)根倒伏百分率的數(shù)據(jù)進(jìn)行了分析，結(jié)果是“分區(qū)處理間進(jìn)行的LSD在PW0.05”。原文出處: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讓iesinthetropicalGuineasavanna.Commun.BiometryCropSci.1(1),41-48.Received:27February2006,Accepted:1June2006zPublishedonline:5August2006?CBCS2006AbstractGeneticimprovementofmaizehybridsforsuperiorstresstolerancehascontributedtoincreasedyieldbyallowinghybridstobeplantedathigherplantpopulations.ThisstudywasconductedtoevaluatetheresponseofmaizehybridsdevelopedintheNigerianSavannafromdifferenterastohighplantdensities.Fieldresearchwasconductedin2002and2003attheexperimentstationoftheInstituteofAgriculturalResearch,SamaruinthenorthernGuineasavannazone,Zaria,Nigeria.Sixhybrids—twofrom1980s,twofrom1990sandtwofromthe2000eras—wereevaluatedatthreeplantdens讓iesusingasplit-plotdesignw讓hthreereplications.Plantdensities(53,333,66,666,and79,999plantsha1)constitutedthemainplotsandthesixhybridswereassignedtosubplots.Plantdensitiesabove53,333plantsha1reducedgrainyieldofhybrids,whichmightbeduetothefactthatthehybridsevaluatedwereselectedatlowplantdensitiesandwerethereforenottoleranttoplant-densitystress?Itmightalsobeduetothelowyieldpotentialintheexperimentalarea,whichdidnotallowyieldincreasesathighplantdensities.Thereweresignificantdifferencesamongthetestedhybrids.Thehybridsreleasedin2000out-yieldedthehybridsreleasedin1980and1990satallplantdens讓ies.Toimprovemaizegrainyieldathighplantdensities,werecommendthatthehybridsbeselectedathighplantdensHies.KeyWords:maizehybrids;plantdensities;maizegrainyield.IntroductionGrainyieldofmaize(ZeamaysL.)ismoreaffectedbyvariationsinplantpopulationdens讓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讓hstandhigherplantdensitylevelsthantheolderhybrids(Tollenaar,1989).Themorerecenthybridswerefoundtohavedecreasedlodgingathigherplantpopulations,andalsotheywerebetterabletow讓hstandenvironmentalstress,resultinginproductionoffewerbarrenplants(Tollenaar,1991)?ResearchontheuseofhybridsinNigeriastartedintheearly1970s(Fajemesin,1978;Fakoredeetal.,1993)andbecameanintegralpartoftheMaizeImprovementProgramattheInternationalInstituteofTropicalAgriculture(IITA)in1979(Kim,1997).Experimentalhybridsweretestedonfarmers'fieldslocatedallacrossthecountryin1984(Fakoredeetal.z1999)andwerefoundtoyieldconsiderablyhigherthanthewidelygrownopen-pollinatedvarieties.Sincethen,manycommercialhybridshavebeendevelopedandmarketedbyseedcompanies.Desp讓巳theyieldadvantage,thereiswidespreadbeliefinNigeriathathybridsarelessstresstolerantandthereforerequirehigherinputsthanopen-pollinatedvarieties.Traditionallybothhybridsandopen-pollinatedvarietiesareselectedat53,333plantsha4w讓harowwidthof0.75mandw讓hinarowdistanceof0.25m.ThereisnoinformationontheresponseofmaizehybridsdevelopedintheNigeriansavannastohigherplantdensities.Theobjectiveofthisstudywastoevaluatetheperformanceofmaizehybrids,developedinNigerianSavannasduringthepast20years,atplantdensitieshigherthan53,333plantsha1.MaterialandmethodsStudysiteandculturalpracticesFieldresearchwasconductedin2002and2003ontheexperimentstationoftheInstituteofAgriculturalResearch,SamaruinthenorthernGuineasavannazone,Zaria,Nigeria(7.38°E,11.11°N,686masl).Thesoiltypewasafine-loamy,IsohyperthermicPlinthustalf.Thetotalrainfallreceivedatthiss讓ewas1009.5mmin2002and1139mmin2003,andtheaveragetemperaturewas25°Cin2002and26°Cin2003.SixhybridsdevelopedandwidelytestedintheNigeriansavannafrom1982to2000wereusedinthisstudy;twohybridswerefromthe1980s(8321-21and8425-8),twofromthe1990s(9801-11and9803-2),andtwofrom2000s(0103-11and0103-15).Trialswereplantedon30June2002and1July2003?Inbothyears,thetrialwaslaidoutinasplit-plotdesignw讓hthreereplications.Threeplantdensities—53,333,66,666,and79,999plantsha1—werethemainplots,whereasthesixhybridswerethesubplotswithineachmainplot.Therewerefourrowsineachsub-plot;therowswere5minlengthandspaced0.75mapart?Plotswereover-plantedandhand-thinnedtoachievethedesiredtargetdens讓yattwoweeksafterplanting.Thepreviouscropatthetests讓ewassoybean(Glycinemax(L.)Merrill),followedbytwoyearsoffallow.Atplanting,fertilizerwasappliedattherateof40kg/haeachofN,P,andK.Add讓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讓hyearsasthefirstfactor,andplantdensityasthesecondfactor,andhybridsasthethirdfactor.Dataon%rootlodgingfor2003werealsoanalyzedusingasplitplotapproach.Means'separationamongtreatmentswasconductedusingtheLSDatP<0.05.ResultsanddiscussionHybridsandplantdensitiesinfluencedsignificantlydaysto50%silking,ASI,%rootlodging,earsperplant,andgrainyield.Hybridxplantdensityinteractionwassignificantforalltraitsmeasuredexcept%rootlodging(Table1).ReproductiveresponsetoplantdensityDaystomidsilkingincreasedw讓hincreasesinplantpopulationforallhybrids.Averagedacrossgenotypes,silkingwasdelayedbythreedaysasplantpopulationincreasedfrom53,333to66,666.Inrelationtoplantpopulationof53,333,daystosilkingwasdelayedbyfivedaysat79,999plantsha4(Table2).Silkdelayduetohighplantpopulationvariedw讓hhybridsandwasnotconsistentbetweentheoldandnewhybrids.HighplantpopulationincreasedASIforallhybridsevaluated.Therewerenosignificantdifferencesamonghybridsatplantpopulationsof53,333and66,666,exceptthattheolderhybridoftheyear1980,8321-21zandtheyear1990hybrid,9801-11,recordedsignificantlyhigherASIthantheotherhybridsat66,666plantsha".Atplantpopulationof79,999plantsha4,theyear2000hybridshadsign讓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讓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讓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讓ies,werecommendthatthehybridsbeselectedathighplantdensities.AcknowledgementsWethankIITA,UnitedNationsDevelopmentProgrammeandtheInternationalFundforAgriculturalDevelopmentforfinancingthisstudyandthestaffofthemaizeresearchgroupatIITAforassistingw計(jì)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讓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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