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Transcript of How to breed organic sweet corn
How to Breed Sweet Corn for Organic Agriculture www.seedalliance.org�
How to Breed Sweet Cornfor Organic Agriculture
Organic Seed AllianceAdvancing the ethical development and stewardship of the genetic resources of agricultural seedPO Box 772, Port Townsend, WA 98368
This publication was made possible through a grant from Organic Farming Research Foundation and Seed Matters
How to Breed Sweet Corn for Organic Agriculture www.seedalliance.org2
Section I: Introduction...................................................................................................................................................3 Section II: Introduction to Breeding........................................................................................................................3Section III: Breeding Goals...........................................................................................................................................3Section IV: Germplasm..................................................................................................................................................4
Section V: Reproductive Biology and Pollination Techniques...........................................................................5
Section VI: Selection.....................................................................................................................................................10 Inbred-hybriddevelopment.............................................................................................................................................10 Massselection.........................................................................................................................................................................10 Familyselection.....................................................................................................................................................................10 Selectiontechniques............................................................................................................................................................11 Breedingtimeline..................................................................................................................................................................13Glossary............................................................................................................................................................................15
References and Resources.........................................................................................................................................17
Table of Contents
How to Breed Sweet Corn for Organic Agriculture www.seedalliance.org3
Sweetcornisatypeofcornormaize(Zea mays)distinguishedbyoneormoremutationsthatincreasesugarorpolysaccharidecontentinthekernels*.Sweetcornhastraitsthatmakeitbotheasyandchallengingtobreed.Forexample,ingen-eralcorniseasytobothcross-andself-pollinate,providingflexibilityinbreeding.Ontheotherhand,certainchallengesarisebecausesweetcornflavor,tenderness,andotherear-relatedtraitsneedtobeevaluatedafterpollinationhasoccurred.Thisman-ualwillhelpyouunderstandthesetraits,amongothers,andteachyouthemosteffectivewaystoimprovesweetcornforyourorganicfarm.
Sweetcornvarietiescanbeclassifiedbytheirin-tendedend-use.Sweetcornisgrownforanumberofend-uses,includingfreshmarket,canned,frozen,dehydrated,andcreamedcorn.Thismanualwillfo-cusonbreedingforfreshmarketuses.Sweetcornvarietiesarealsoclassifiedbasedonthetypeofendospermmutationthatcausesincreasedsweet-ness.InNorthAmerica,themostcommontypesofsweetcornare:sugary,basedonthepresenceofthesu1allele;sugary enhancer,basedonthepres-enceofbothsu1andsealleles;andsupersweet,basedonthepresenceofthesh2allele.Inmoretropicalareas,suchasHawaii,thebrittle1orbt1alleleisalsocommonlyseen.AllofthesegeneswillbediscussedinmoredetailintheSelection tech-niquessection.
Asyoubeginmakingyoursweetcornbreedingplans,weencourageyoutorefertoOSA’sIntro-duction to On-Farm Organic Plant Breedingforanin-depthexplanationanddiscussionofthefunda-mentalconceptsofplantbreeding.Readingthispub-licationwillfurtheraidyouinyourbreedingwork.Inbrief,thestepsinvolvedinbreedinginclude:
DevelopinganddefiningyourbreedinggoalsIdentifyingparentalmaterial,includingcon-ductingtrialstofindthebestparentsMakingcrossesSelectingandrecombiningyourselections
Thefirststepinanybreedingprojectistoidentifythegoalsoftheproject.Belowaresometraitsthatcanbeimportantinsweetcornbreedingprojects.AdiscussionofselectiontechniquesforthesetraitscanbefoundintheSelectionsection.Thelistbe-lowislong.Noonebreedingprojectwillincludeallofthesetraits.Usethislistasabrainstormingtoolandaimtoworkon4to6traitsatmost.
1.2.
3.4.
I. Introduction II. Introduction to Breeding
III. Breeding Goals
Plant/Agronomic Traits:GerminationEarly vigorSilk dateTassel datePlant heightEar heightAnther color
Defensive Traits:Disease resistance (including Smut, Rust, Northern Corn Leaf Blight, Southern Corn Leaf Blight, Maize Dwarf Mosaic Virus, Stewart’s Wilt, Stalk Rot, Ear Rot, Kernel Rot)
*Definitionsfortechnicalwordsthatareitalicizedandboldedcanbefoundintheglossary
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Ingeneral,whenbreedingsweetcorn,youwillworkwithinoneofthemainendospermmutanttypes:su1(sugary),su1andsecombined(sugaryenhancer),orsh2(supersweet).Su1andsh2arerecessivealleles,meaningtheireffectswillonlybenoticeableiftwocopiesoftheallelearepresent(refertoOSA’sIntroduction to On-Farm Organic Plant Breedingformoreonrecessiveanddomi-nantalleles).
Muchofthebreedingeffortsinsweetcornarerelativelyrecent.Upuntilthe1960s,allcommercialsweetcornwasbasedonthesu1sugarymutation,which,relativetosupersweetandsugaryenhancedcorn,producedrelativelylesssugar.Inaddition,thesugarproducedbysugaryvarietiesisrapidlycon-vertedtostarchafterharvest,causingverylimitedshelflife.Whereasitispossibletofindmanyolder
sugaryopen-pollinatedvarieties,almostallsuper-sweetandsugaryenhancedtypesofsweetcornarehybrid varieties.
Sourcesforgermplasmincludecommercialseedcompanies,publicbreedingprograms,andtheNa-tionalPlantGermplasmSystem.Someoftheprinci-palsweetcornbreedingcompaniesincludeIllinoisFoundationSeed,CrookhamCompany,Sakata,HarrisMoran,MesaMaize,andSeminis.SomeofthekeypublicprogramscanbefoundatUniversityofWisconsin,OregonStateUniversity,Cornell,andUniversityofHawaii.Whenyousourcematerial,besuretothoroughlyinvestigateallintellectualpropertyclaims.
Becausesweetcornisthesamespecies(Zea mays)asothercorntypes,suchasdent,flint,flour(allcommonlyreferredtoasfieldcorn),andpopcorn,crossescanbemadebetweensweetcornandany
Pest resistance (Corn Earworm, European Corn Bore)Drought toleranceLodging resistance
Ear Traits:Silk colorRow numberTip fillRow straightnessEar shapeEar sizeHusk appearanceHusk protectionEar leaf lengthEase of ear detachmentEase of huskingKernel color
Quality:SweetnessFlavorTendernessKernel depth
IV. Germplasm
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ofthesetypes.However,whenmakingcrossesbetweensweetcornandothertypesofcorn,keepinmindthatqualitytraitssuchassweetness,flavor,andtendernessarecomplexandverydifficulttomaintainorrecoverifalloftheparentsarenothigh-quality.TheUniversityofWisconsinsweetcornprogramhasdevelopedsomematerialthatderivesfromcrossesbetweentemperateandtropi-calfieldcornandmaybeusefulasparentalmateri-alwhenbreedingforcertaintraits,includingplantheight,vigor,anddiseaseandpestresistance.SomeofthismaterialcanbeobtainedfromtheUSDA-GRINsystembysearchingfor“sweetcornTracy.”
Cornisamonoeciousplant:The tasselsrepresentthemale(pollen-bearing)inflorescencesandtheearsrepresentthefemale(seed-bearing)inflores-cences.Cornisprimarilycross-pollinatedbywind.Cross-pollinationisencouragedbytwofactors.First,thetasselsarephysicallyseparatedfromtheears.Second,anthesis,alsoknownastasselingorpol-lenshed,generallybeginsbeforethesilksemerge;although,modernvarietiesmayhavesimultaneousanthesisandsilking.Ittakesbetweentwotosevendaystoshedallofthepollen,withsheddingbegin-ningnearthetopandprogressingdownthetassel.Silkstaketwotosixdaystofullyemerge.Eachindi-vidualsilk,whenpollinated,formsasinglekernelontheear.Althoughcross-pollinationisbyfartheprimarymodeofpollinationundernaturalcondi-tions,cornisself-compatibleandself-pollinationsarerelativelyeasytomake.
Becausetheendospermisthebulkofwhatweseeandeatwhenweconsumesweetcorn,sweetcornbreedersmustworkwithanunusualgeneticeffectcalledxenia.Xeniareferstoobservableeffectsthatoccurduetotheinfluenceofpollen.Insweetcorn,thetwomostimportanttraitsaffectedbypol-lenarekernelcolorandsweetness.Forexample,yellowkernelcolorisdominanttowhite,andifawhitevarietyreceivespollenfromayellowvariety,yellowkernelswillappearamongthewhiteones.
Forthepurposesofplantbreeding,cornpollina-tioncanbedoneinthreeways:open,uncontrolled
pollination,controlledpollinationbydetasseling,andcontrolledhandpollination.
Open pollination:Openpollinationisstraightfor-wardanddonebyplantingtheplantsyouwanttocross-pollinateincloseenoughproximitytoreli-ablycross,typicallyseparatedbynomorethan10feet(3meters).Ifyouareworkingwithvarietiesofdifferingrelativematuritiesyoumayneedtostag-gerplantingtimestoensurethatalloftheplantsaresimultaneouslyreleasingandreceptivetopollen.
Detasseling:Controlledpollinationbydetassel-ingisarelativelysimplemethodtomakemanysimultaneouscrosses.Plantthefemaleorintendedpollenrecipientnexttothemalepollendonorordonors.Removetasselsonthefemaleassoonastheybegintoemerge,checkingregularlytomakesurethatallofthetasselsareremoved.
V. Reproductive Biology and Pollination Tecnhiques
To detassel corn, grab the tassel just above the top-most leaf. Bend the tassel slightly and firmly pull up to break the tassel off the plant. Photo credit: Long Island Seed Project
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Hand Pollination: Forhandpollinations,youwillneedspecialbagstocovertheearsandthetassels.BothcanbeorderedfromLawsonbagcompany,withshoot bagsforsweetcornsoldas#217andtassel bagssoldas#402or#404,dependingonthesize.
First,covertheearshootsastheyemerge,butbeforethesilksemerge.Shootswillfirstappearasabulgealongthenodesofthestem,andcanappearwithorwithoutshootleaves.
Sweet corn breeding supplies, including a pollinating apron holding Lawson tassel and shoot bags, Bostich stapling pliers, paper clips, pocket knife, spare staples, and a water bottle.
Thegoalwillbetoplaceashootbagonthetop-mostearshoot.Trimtheshootleavessothatthebagwillfitfirmlyovertheshoot,withthelongersideofthebagbetweentheshootandthestem.Thisprotectstheshootsfromunintendedpollina-tion.Checktheshootbagsregularlytomakesuretheyhavenotfallenoff.
Withinafewdaystofewweeksofputtingonthefirstshootbags,itwillbetimeforthenextstepintheprocess:setting uppollinations.Lookforplantsthathavesomeanthersopenandsheddingpollen,havesomeanthersthatarestillnotopen,andhavesilksthathaveemerged.Sometimesyoumaynotbeabletoseesilkemergingfromashoot,butifyoucutitback(seefollowingimages),theremaybesilksthatareclosetoemerging.Onceaplantmatchingtheabovecriteriaisidentified,cutbackthesilkstocreateabrushofsilksstickingup.Thesesilkswillbeusedforpollinationthefollow-ingday.Tocutbacktheshootforpollination,useaknifetocutacrosstheshoot,paralleltotheground,andaninchortwofromthetop.Cutdeepenoughtoexposeapencil-widthareaofsilks.Donotcutsofarthatyounickthedevelopingear.Replacethe
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shootbagaftercuttingtheshootback,foldingthetopofthebagdowntoindicatethatthisplantisreadytopollinatetomorrow.
Afteryoucuttheshoots,thenextstepisprepar-ingthetassels.Findtasselsthataresheddingpollenbutstillhavesomeunopenedanthersandthensliponatasselbag,pullingitdownsnuglyandpastthetopleaf.Smoothoutthebagandfoldacornerofthebottomofthebagupatanangle,sotheopeningatthebottomissealedagainstthestem.Securethetasselbagwitha#1non-skidstandardpaperclip.Tousecornbreedingtermi-nology,youhavenow“setup”atasselbagthatcanbe“taken down”thenextday.
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Takedownthetasselbagsthenextday,oncethebagsaredryandpollenisshedding.Shakefreshpollenintothetasselbagbyholdingthetasselandbagperpendiculartothegroundwithonehandandstrikingthebagfirmlyagainstyourotherhand.Toavoidlosingpollen,holdthebagsideways,removeitfromthetassel,andfolditinhalf.Withthebagstillfolded,openthefront,creatingapouch.Placethispouchovertheshoottobepollinated.Movingquickly,takeofftheshootbag,slidethetasselbagovertheshootandunfoldthebag.Makesurenottotouchthesilkswithyourhandsbecauseyourhandsarelikelycontaminatedwithpollenfromthefield.Shakethetasselbagtodislodgeanypollenremaininginthebag.Youhavenowmadeacon-trolledpollination.Wrapthetasselbagaroundthestemandstaplethebaginplace,leavingenoughroomforaneartogrow.
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How to Breed Sweet Corn for Organic Agriculture www.seedalliance.org�0
Althoughalltypesofcornarenaturallycross-pol-linating,theycaneasilybeself-pollinated.However,keepinmindthatcornishighlysusceptibletoinbreeding depression.Selectionmethodsforsweetcornfollowthebasicstructureoutlinedforcross-pollinatedcropsinOSA’sIntroduction to On-Farm Organic Plant Breeding.Wehighlyrecommendreadingthisintroductoryguidebeforedelvingintothespecificstrategiespresentedhere.Itisavailableforfreedownloadatwww.seedalliance.org.
Mostmodernvarietiesareproducedthroughaninbred-hybrid system.Forinformationalpurposes,thefollowingsectionbrieflyexplainsthisprocess.However,developingandmaintaininghybridsisnotgenerallyappropriatefororganicon-farmplantbreedingforthefollowingtworeasons.First,in-bredscanbeveryweakandhardtoproduceseedofonorganicfarms.Second,hybridvarietiescan-notberapidlyadaptedtonewenvironments.
Inbred-hybrid developmentThisbreedingsystemconsistsoftwoparts:inbreddevelopmentandhybridtesting.Duringinbreddevelopment,crossesaremadebetweenpromisingparents,includinginbreds,hybrids,andpopula-tions.Familiesareproducedbyself-pollinatingears,eachofwhichisthenplantedtoasinglefam-ilyrowandevaluated.Thebestplantsfromthebestfamilieswillagainbeself-pollinatedtopro-ducefamiliesforthefollowingyear.Thisprocesswillcontinueforfiveormorecyclesofself-pollina-tion,producingveryinbredplants.Asthesefami-liesbecomemoreinbred,theywilltendtobecomeweaker,smaller,andproducesmallerears.Thislossofvigorisanormalpartoftheinbreedingprocess.However,theinbreedingprocesswillalso“fix”thedesirabletrait.
Startingafterthefourthorfifthcycleofself-pol-lination,crossesaremadebetweeninbredfamilies.Thegoalofthesecrossesistofindcombinationsofinbredsthat,whenpaired,producehybridoffspringwithgoodhybridvigoraswellastheotherdesiredcharacteristics.Whengoodhybridcombinationsarefound,theywillbetestedformultipleyearsatmultiplelocations.Iftheyperform
wellovermultipleyearsandlocations,theymaybecomecommerciallyavailablevarieties.Thesevarietiesarethenmaintainedbyperiodicallypro-ducingseedandplantsofbothinbredparentsandremakingthehybridcross.
Mass selectionMass selectioninsweetcornbreedingcanbedonefollowingthebasicprinciplesoutlinedinIntroduc-tion to On-farm Organic Plant Breeding,includinggrowinginauniformfieldandusingagridsystemtoselectevenly.Oneofthebiggestchallengesinbreedingsweetcorn,alongwithmanyotherfruit-ingcross-pollinatedcrops,isthattheearscanonlybeevaluatedafterpollinationhasoccurred.Thismeansthatpollenfrompotentiallyundesirableplantshasalreadyspreadthroughoutthepopula-tionbeforeevaluations.Toaddressthischallenge,practiceasmuchearlyselectionaspossible,re-movingplantsbeforepollinationbasedontraitssuchasgerminationtime,earlyvigor,andearlyplantheight(removetheplantsassoonastasselsemergeandbeforethey’vebeguntoshedpollen).Whenpracticingmassselection,maintainaslargeapopulationaspossible,savingseedfromatleast200plantseachyear.
Family selectionFamily selectionisusedtomakemorerapidgainsrelativetomassselectioninsweetcornbreeding.RefertotheIntroduction to On-Farm Organic Plant Breedingforthebasicconceptsoffamilyselectionincross-pollinatedcrops.Therearethreecommon-lyusedtypesoffamiliesinsweetcornbreeding:half-sibfamilies,full-sibfamilies,andself-pollinat-ed(S1)families.Ahalf-sib familycanbeproducedintwoways.Thefirstisbysavingindividualearsthathavebeenopen-pollinated.Thekernelsoneachearareallrelatedthroughtheirmaternalgeneticsbuthavepaternalgeneticsasdiverseasalloftheplantsinthepopulationsheddingpollen.Thesecondwaytoproducehalf-sibfamiliesisbycombiningthepollenfromseveralplantsintoonetasselbagbeforemakingacontrolledhandpolli-nation.Full-sibfamiliesareproducedbymakingacontrolledpollinationfromthetasselofoneplanttotheearofanother.S1 familiesareproducedbyself-pollinatingasingleplant.
VI. Selection
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Eachofthesefamilytypeshavebenefitsanddraw-backsdependingonyourgoals.Half-sibfamiliesproducedbyopen-pollinationaretheeasiesttocreateandtheyretainthemaximumamountofgeneticdiversity.Ontheotherhand,thatdiver-sitymakesthefamilylessuniformandcanmakeithardertofixthedesiredtraitsandpickthebestfamilies.Half-sibsproducedbycontrolledpollina-tionallowyoutobemoreselectiveinchoosingwhichplantswillcrosswithotherplants.Full-sib familiesrepresentamiddleapproach.Full-sibfamiliesarerelativelyuniform,buttheprocessofcross-pollinationbetweenplantsreducesthepotentialforinbreedingdepressionwhilealsocreatingmorenewgeneticcombinationsrelativetoself-pollination.S1orself-pollinatedfamiliesareusefulforafewreasons.First,theycreatethemostuniformfamiliesofthethreetypeslistedhere,whichmakesiteasiertoseparatethegood,ormostdesirable,familiesfromthebad,orleastdesirable,ones.Second,self-pollinationallowsforthegreat-estexpressionofrecessivetraitsthatcanthenbeeliminatedbyselection.(SeeIntroduction to On-farm Organic Plant Breedingformoreinformationonrecessivetraits.)Third,self-pollinationallowsforselectionofeartraitsafterpollinationbecauseitexcludespollenfromundesirableplants.Sincetheonlypollenaself-pollinatedplantreceivesisfromitself,iftheearlooksandtastesgood,youcanfeelrelativelyconfidentthattheseedwillproduceplantswithsimilarearquality.Thedrawbacktogeneratingfamiliesthroughself-pollinationisthatitisastrongformofinbreeding,andcanquicklyleadtoinbreedingdepressionsincenocrossingisoccurringbetweenplants.Inyourbreedingplan,youmayfinditbeneficialtouseacombinationofthesefamilytypesdependingonthephaseandgoalsofyourwork.
Selection techniquesThissectionincludesbriefdescriptionsofhowtoselectforsomeoftheimportantsweetcorntraitsinthefield.
How to evaluate traits on a relative scale
Toscoreatraitonarelativescale,assignavaluetoaplantorfamilybasedonaratingsystem.Forexample,youmightscoreleafsizeonascalefrom1to9,where1representsatinyleafand9representsalargeone.Whenscoringtraits,beginbywalkingtheentirefieldtoassesstherangeofexpressionwithinthepopulationforthetraitofinterest.Theplantsorfamilieswiththeworstorleastdesirableexpressionofthetraitwillbethe1onyourscaleandtheplantsorfamilieswiththemostdesirableexpressionwillbeyour9.Thenproceedtoscoreallplantsorfamiliesbasedonthatrange,attemptingtousethewholerangeofscoresfrom1to9.MoreinformationonhowtoconducttheseevaluationscanbefoundinOSA’sOn-farm Variety Trials: A Guide for Organic Vegetable, Herb, and Flower Producers.Boththisguideanddatasheetsforevaluat-ingandselectingplantsareavailableforfreedownloadatwww.seedalliance.org.
Germinationisreallyonlymeasurablewhenselect-ingbasedonwholefamiliesorpopulations.Itcaneitherbemeasuredinacontrolledsettingorinthefield.Inside,youcanusestandardgerminationtestingstandards,oryoucancreatestressfulsitu-ations,suchasgerminatingtheseedundercolderthannormalconditions.Trytomakesurethatyouareprovidingasuniformofconditionsaspossible.Inthefield,germination(andemergence)ismea-suredbyplantingaknownnumberofseedfromeachfamilyorpopulationandrecordingthenum-berofplantsthatemergeafterasetperiodoftime.Thiscanbehighlysusceptibletothedepthofplant-ing,compaction,andotherfactors,andisreallyonlyusefulifextracareistakentoplanteverythingasuniformlyaspossible.
Early vigorcanbeevaluatedbyratingthesizeoftheplantsatacertaintimeafterplanting.Forexample,atfourweeks,plantsshouldbelargeenoughandhaveputonsufficientgrowthsothatdifferencesinvigorareapparentandeasilyevaluated.Aswithger-
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mination,thistraitisverysusceptibletotheeffectsofseedingdepthandcompaction.
Silk dateisrecordedwhensilksbegintoemergefromtheplant.Foralldates,inadditiontotheac-tualdate,youcanrecorddatesasdaysafterplant-ingoryoucanusegrowing degree days (GDDs).Bothsilkdatesandtasseldatesareusedtoevalu-atematurity.
Tassel datesarerecordedwhenhalfofthetasselshaveshedtheirpollen.
Plant heightisrecordedatmultiplepointsduringtheseasonandcanbedirectlymeasuredorevalu-atedonascale.
Ear heightismeasuredtothebaseoftheears.Anthercolorisrecordedasanote.Cornanthersaretypicallyeitheryelloworpurple.
Disease resistance(includingsmut,rust,northerncornleafblight,southerncornleafblight,maizedwarfmosaicvirus,Stewart’swilt,stalkrot,earrot,andkernelrot)canbemeasuredeitherbyscoringonarelativescaleorbyestimatingthepercentofsurface(leaf,stalk,ear,orotherplantparts)infected.Withalldiseaseratings,keepinmindthatdiseasepressuremayvarywidelybothwithinafieldandfromyear-to-year.Beforeem-phasizingselectionfordiseaseresistance,makesureyoucanfindexamplesofseverelyinfectedplantsthroughoutthefield.Theseexampleswillindicatethatthediseaseispresentinsufficientquantity/pressuretocauseinfectionandthatthereisvariationintheresistanceamongyourplants/varieties.Ifspecificdiseaseresistancesareimportanttraitsinyourproject,includeparentsorvarietiesthatareknowntohavesomelevelofresistanceaswellasthosethatareknowntobesusceptible.Includingtheseplantsorvarietieswillprovideabaselineforcomparison.
Pest resistance(includingcornearwormandEu-ropeancornborer)isachallengingtraittoselectfor.CornearwormandEuropeancornborercanbeverydevastatinginareaswheretheyarecom-mon,andresistancetothemislimited.Therearesomeexistingvarietiesandpopulationsthathavevaryinglevelsofresistanceeitherbecauseofthick,
tight,longhusks,orbecauseofnaturallyoccurringcompoundspresentinthekernelsandsilks.
Drought tolerancecanonlybeselectedforwhendroughtpressureispresent.Droughttoleranceisselectedforbymeasuringtraitssuchastipfill,plantheight,andthedifferencebetweenthesilkandtasseldatesunderdroughtstress.Itisveryimportantthatthereisequalstressthroughoutthebreedingfield.Availablewatercanbemeasuredwithawaterprobe.
Lodging resistance,likedroughtresistance,canonlybeevaluatedwhencircumstancesallow.Lodgingiswhenplantsfallover,eitherfromtherootsorfromthestalk.Conditionsthatpromotelodgingarewindandwetsoils,oracombinationofthetwo.Lodgingcanberatedonarelativescaleormeasuredasapercentofplantsthatfellover.
Thefollowingtraitsinvolveharvestinganeartoassessquality.Toevaluatethesetraitswhilestillsavingseed,youeitherneedtobeworkingwithfamiliesorhavemultipleearsonaplant.
Silk color canberatedonascalefromlightsilkstodarksilks.Consumersgenerallyfinddarksilksunattractiveinfreshmarketcorn.
Row numberisevaluatedbycountingthenumberofrowsofkernelsthatsurroundtheearatitswid-estpoint.Thereisgenerallyanevennumberofrowsonanearofsweetcorn.
Tip fillisameasurementofhowfarupthetipoftheearthekernelsgo.Itcanberatedonascalewherefullkerneldevelopmentextendingallthewaytothetipoftheearispreferred.Handpollinationsoftencauseincompletepollinationandpoortipfillsokeepthisinmindwhilemakingselectionswhenmakingcontrolledpollinations.
Row straightness canbeevaluatedonarelativescalefromstraighttolessstraight.Incompletepollinationscanreducerowstraightness,sothisisanothertraitthatislessreliablewhenevaluatinghand-pollinations.
Ear shapecanberatedonascalewhereuniform,cylindricalearsaregenerallypreferredtotriangu-
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larshapedears.
Ear sizecanbemeasuredbyearlengthandearwidth.Inbreedingcanrapidlyreduceearlength.Huskappearancecanbeevaluatedonarelativescale,withdarkgreenhusksgenerallypreferredtoyellowishhusks.
Husk protectionreferstohowfarthehuskextendspastthetipoftheear.Itcanbedirectlymeasuredorscoredonarelativescalewithlongerhuskscon-sideredmoredesirable.
Ear leaf lengthisameasurementoftheleafthatextendspasttheear.Itcanbemeasureddirectlyorscoredonarelativescalewithlongerflagleavesconsideredmoreattractivetoconsumers.
Ease of ear detachment canbescoredbasedonhowharditistoharvesttheearfromthecornplant.Makesureonlyonepersonisharvestingandevalu-atingthistraittoensureconsistency.
Ease of huskingcanbescoredbasedonhowharditistohusktheear.Aswitheardetachment,makesurethatonlyonepersonisdoingthehuskingandratingtomaintainconsistency.
Kernel color cancapturetwoparameters.Firstisthecolortype.Sweetcornistypicallywhite,yellow,orbicolor,withyellowbeingdominanttowhite.Cornkernelcoloriscontrolledbymanygenesthatcreatepigmentinvarioustissuelayers,creatingblue,red,green,andpurplecolors.However,colorsbesidesyellowandwhiteonlyappearaftersweetcornhaspassedthestagewhereitisharvestableforfresheating.Thesecondparameterofkernelcoloristheintensityofcolor.Ingeneral,sugaryenhancedcorntendstohavepalerkernelsrelativetosugaryandsupersweettypes.
Sweetnessisacomplextrait,andiscontrolledbyanumberofbothmajorandminorgenes.Thesu1geneincreasessugarandwater-soluble polysac-charides (WSP)abovethoseinnon-su1corn.Thesegenemodifiesthe su1gene,andwhenbotharepresent,sugarandWSPareincreased.Thesh2genegreatlyincreasessugarcontentovertheseothertypesbutresultsinloweredWSPinthekernels.Becausecorncarryingthesh2genehas
lessWSPthansu1orsu1 setypes,ittendstobeperceivedaslesscreamy.Sweetnesscanberatedsubjectivelybytastetesting.Brixtestingisusedtoevaluatesweetness,butitisimportanttonotethatBrixtestingusesarefractometertomeasuresolublesolidsratherthanjustsugaralone.ThismeansBrixtestingcanbeanunreliablemeansofdeterminingsweetness.
Flavorcanberatedonascaleorrecordedasde-scriptivenotesreflectinganyexceptionallygoodoroff-flavorsbeyondsweetness.
Tendernesscanbeevaluatedonascalethroughtastetests.Tendernessreferstothethicknessoftheseedcoat(pericarp).Thethickeritis,thelesstenderthekernelsare.However,ingeneral,seedwiththinnerpericarpsaremorepronetoproblemswithgerminationandcoldsoilemergence.
Kernel depthismosteasilyevaluatedonascaleduringtastetests.However,itcanalsobemeasureddirectlyondrykernels.
Breeding timelineThefollowingtimelineisbasedonOSA’sapproachtobreedingcross-pollinatedcrops.
Year 0:Setyourbreedinggoalsandevaluatepo-tentialparentsbasedonhowwelltheymeetyourgoals.Usevarietytrialstohelpyouevaluateanddeterminewhichvarietiesorotherparentalmate-rialtouseinyourbreedingwork.SeeOSA’sOn-farm Variety Trials: A Guide for Organic Vegetable, Herb, and Flower Producersformoreinformationonconductingvarietytrials.
Year 1:Ifnecessary,makecrossesbetweenyourselectedparents.Thecrossescanbemadethroughopenpollination,detasseling,orhandpollinationasappropriate(seeReproductive Biology and Pollination Techniquessectionabove).
Years 2-4: Plantapopulationofatleast250plantsfromthepreviousyear’scrossesandallowthemtofreelycross-pollinate.Removetheleastdesirableplants,discardingapproximately10to20%ofthetotalpopulation.Saveseedfromatleast200plantseachyeartohelpavoidinbreedingdepression.Itisimportantnottoselecttooaggressivelyatthis
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pointbecausethegenesfromtheparentsarestillcombiningtogetherandsettlingintoreliableandpredictablearrangements.
Year 5:Atthispoint,ifyouwouldprefertoonlypracticemassselection,continuetodoso,selectingmoreheavilythanbefore,butstillsavingseedfromaminimumof200plants.
Alternatively,youcanbeginfamilyselection.Youcanapproachthisinoneoftwoways:bycreatinghalf-siborS1families.Tocreatehalf-sibfamilies,firstremovealloftheundesirableplantsbeforepollinationandthenallowtheremainingplantstocross-pollinate.Selectearsfromatleast100to200ofthebestplantsinthefieldandkeepthemseparate.Eachearrepresentsahalf-sibfamily.Giveeachfamilyanumber.TocreateS1families,self-pollinateasmanydesirableplantsasyoucan,preferably500to1000plants.Selectthe100to200bestplantsoutoftheself-pollinatedplants.Savetheseedseparatelyfromeachear,givingeachfamilyanumber.
Year 6:Ifpracticingmassselection,continueasinYear5.
Ifpracticingfamilyselection,plantoneshortrowof10to20plantsperfamily.Ifspaceallows,youcanplantasecondreplicationbyplantingallofthefamilyrowstwice.Savesomeseedfromeachfamilytoserveasremnantseedforlaterplanting.Evaluateallofthefamiliesforthetraitsofinterest.Selectatleast20familiesfromtheoriginal100to200plants.Becausecornpollinatesbeforeyoucanevaluatetheears,werecommendthatyoudonotsavetheseedofthesefamilies,butinsteadusetheremnantseedofthebestfamiliesfornextyear.
Year 7:Ifpracticingmassselection,continueasinYear6.
Ifpracticingfamilyselection,thisyearyouwillrecombine(cross)yourselectedfamilies.Thereareafewwaystorecombineyourfamiliesdepend-ingonyourplansforthefollowingyears.Ifyouaresatisfiedwiththeprogressyoumadeusingfamilyselectionandwishtorecreateapopulationthatyouwillmaintainviamassselection,combineequalamountsofseedfromeachfamily,plantthe
seed,andallowtheplantstoopenlypollinate.Ifyouwouldliketocreatefull-sibfamilies,plantthefamiliesinthefollowingpaired rowschemewithcontrolledhandpollinations:
Inthefirstrow,plantseedfromyourfirstfamilyInthesecondrow,plantseedthathasbeencombinedfromallofthefamiliesexceptthefirstfamilyInthethirdrow,plantseedfromthesecondfamilyInthefourthrow,plantseedthathasbeencombinedfromallofthefamiliesexceptthesecondfamily.Inthefifthrow,plantseedfromthethirdfamilyInthesixthrow,plantseedthathasbeencombinedfromallofthefamiliesexceptthethirdfamily.Continueinthesamefashionfortheremainingrows,plantingarowwithseedfromasinglefamilyfollowedbyseedcombinedfromalloftheotherfamilies.
Hand-pollinateeachplantinthefirstrowwithpol-lenfromaplantinthesecondrowandvice-versa.Thenhand-pollinateplantsfromthethirdrowwith
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Plants from Family 1
Plants from Families 2, 3, 4
Plants from Family 2
Plants from Families 1, 3, 4
...etc
Recombining using paired rows
How to Breed Sweet Corn for Organic Agriculture www.seedalliance.org��
pollenfromthefourthrowandvice-versa,andsoon.Saveseedseparatelyfromeachear,asthesewillbeyourfamiliesfornextyear.
Year 8 (and beyond):Ifpracticingmassselection,continueasinpreviousyears.Ifcontinuinganothercycleoffamilyselection,plantandevaluatefam-ilyrowsasyoudidinyear6.Nextyearrecombinethemasinyear7.Onceyouaresatisfiedwithyourvariety,combinetheseedasdescribedinyear7.
Anthers:male,pollen-bearingstructures.Acorntasselcontainshundredsofanthers.
Anthesis:incorn,theperiodwhenacornplantissheddingpollen.
Detasseling: theactofremovingthetasselfromacornplantorcornplants.
Endosperm: astarchytissuethatconstitutesthemajorityofthematerialinsideeachcornkernel.
Family:agroupofgeneticallyrelatedplants.Oftenthenatureoftherelationshipisspecified.Asexamples,seehalf-sib families,full-sib families,andS1 families.
Family selection:selectingplantsorfamiliesbasedontheoverallperformanceofafamily.
Full-sib family: afamilystructurewheretheplantsinthefamilysharethesamemotherandthesamefather.
Growing degree days (GDD):Ameasurementofaccumulatedheatthroughoutthegrowingsea-son.Incorn,GDDsarecalculatedinthefollowingfashion:foreachdayafterplanting,findtheaver-agedailytemperature.Iftheaveragetemperatureislessthan50°F,recorditas50°F.Iftheaver-agetemperatureisgreaterthan86°F,recorditas86°F.Subtract50°Ffromtheaverage.Thisisthenumberofdegreedaysaccumulatedthatday.Continuethisprocessforeachday,addingeachday’sdegreedaystothetotal.Forexample,iftheaveragedailytemperaturethedayafterplantingwas55°F,then5(55-50)GDDswereaccumulatedthatday.If,onthefollowingday,theaveragedailytemperaturewas75°F,then25(75-50)GDDswereaccumulatedthatday,andatotalof30GDDswereaccumulatedsinceplanting.
Half-sib family:afamilystructurewheretheplantsinthefamilysharethesamemother.
Husk:theleafyoutercoveringofanearofcorn.
Glossary
How to Breed Sweet Corn for Organic Agriculture www.seedalliance.org�6
Hybrid:theproductofacrossbetweengeneticallydistinctparents.
Inbred:avarietyproducedbyinbreedingoveranumberofgenerations.
Inbred-hybrid system:amethodofproducingvarietiesbyfirstdevelopinganumberofinbreds,followedbytestinghybridscreatedfromcrossesoftheseinbreds.
Inbreeding:matingbetweenrelatedindividuals.
Inbreeding depression: thedecreaseinvarietalfitnessduetoinbreeding.
Kernel:aseedofcorn.
Lodging:aphenomenonwherebycornfallsoverduringthegrowingseason.Lodgingtypicallyoc-cursduringwindevents.Itcanoccurasstalklodg-ing,whichoccurswhenthecornstalkbreaks,orasrootlodging,whichoccurswhentheplantspulloutofthegroundattheroots.
Mass selection: aformofselectionwhereindi-vidualplantsareselectedbasedontheirindi-vidualperformance.
Paired rows:aschemeforarrangingplantstobecrossedsothattwoadjacentrowsarecrossedwitheachother.
Pericarp:theoutermostlayerofacornkernel.
S1 family:afamilystructurewheretheplantsinthefamilyallresultedfromthesameself-pollination.
Setting up:preparingthecornshootandtasselforafuturepollination.
Shoot:adevelopinghuskandearofcorn.
Shoot bag: awaterproofedpaperbagdesignedtofitoverashootandprotectitfromunintend-edpollinations.
Silk:thestylesandstigmasofcorn.
Silking:theperiodwhensilkemergesfromthecornshootandisreceptivetopollination.
Sugary: sweetcornhomozygousforthesu1gene,withincreasedsugarandwatersolublepolysaccha-ridecontentovernon-sweetcorn.
Sugary enhancer:sweetcornthatishomozy-gousforboththe su1geneandthe segene.Sugaryenhancercornhasmoresugarthansugarycorn,andhasmorewatersolublepolysaccharidesthansupersweetcorn.
Supersweet:sweetcornthatishomozygousforthe sh2gene.Supersweetcornhasincreasedsug-arycontentrelativetosugarycorn,buthaslesswatersolublepolysaccharides.
Taking down: completingcornpollinations.
Tassel: amaleinflorescenceofacornplant.
Tassel bag:awaterproofedpaperbagusedtocap-turepollenfromtassels.
Water soluble polysaccharide (WSP):aclassofmoleculesmadeupofasmallnumberofsugarsbondedtogether.WSPgivessugaryandsugaryenhancercorntheircreamytexture.
Xenia: effectsthatoccurduetotheinfluenceofpollenandareobservableintheyearthatpollina-tionoccurred.
How to Breed Sweet Corn for Organic Agriculture www.seedalliance.org�7
References and ResourcesColley,M.,andJ.Myers.2007.On-farm Variety Trials: A Guide for Organic Vegetable, Herb and Flower Pro-ducers.OrganicSeedAlliance,PortTownsend,WA.
Kaukis,K.andD.W.Davis.1986.SweetCornBreed-ing.p.477-522.InBassett,M.J.(ed.)Breeding vegetable crops.AVIPub.Co.,Westport,Conn.
Mueller,D.andR.Pope.2009.CornFieldGuide.[Online].Availableat:http://www.agronext.iastate.edu/corn/docs/corn-field-guide.pdf(verified28Mar.2014).IowaStateUniversity,Ames,IA.
Navazio,J.andJ.Zystro.2014.Introduction to On-farm Organic Breeding.OrganicSeedAlliance,PortTownsend,WA.
Tracy,W.F.2000.Sweetcorn.pp.155-199,InA.R.Hallauer(ed.)Specialty CornsSecondEdition.CRC,BocaRaton,FL.
How to Breed Sweet Corn for Organic Agriculture www.seedalliance.org�8
AuthorsCompleted 20��Jared Zystro, Organic Seed AlliancePictures courtesy of Micaela Colley (OSA) and Jared Zystro (OSA) unless otherwise noted
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