How to Breed Sweet Corn for Organic Agriculture to Breed Sweet Corn for Organic Agriculture How to...

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


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


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


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


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


shootbagaftercuttingtheshootback,foldingthetopofthebagdowntoindicatethatthisplantisreadytopollinatetomorrow.

Afteryoucuttheshoots,thenextstepisprepar-ingthetassels.Findtasselsthataresheddingpollenbutstillhavesomeunopenedanthersandthensliponatasselbag,pullingitdownsnuglyandpastthetopleaf.Smoothoutthebagandfoldacornerofthebottomofthebagupatanangle,sotheopeningatthebottomissealedagainstthestem.Securethetasselbagwitha#1non-skidstandardpaperclip.Tousecornbreedingtermi-nology,youhavenow“setup”atasselbagthatcanbe“taken down”thenextday.


Takedownthetasselbagsthenextday,oncethebagsaredryandpollenisshedding.Shakefreshpollenintothetasselbagbyholdingthetasselandbagperpendiculartothegroundwithonehandandstrikingthebagfirmlyagainstyourotherhand.Toavoidlosingpollen,holdthebagsideways,removeitfromthetassel,andfolditinhalf.Withthebagstillfolded,openthefront,creatingapouch.Placethispouchovertheshoottobepollinated.Movingquickly,takeofftheshootbag,slidethetasselbagovertheshootandunfoldthebag.Makesurenottotouchthesilkswithyourhandsbecauseyourhandsarelikelycontaminatedwithpollenfromthefield.Shakethetasselbagtodislodgeanypollenremaininginthebag.Youhavenowmadeacon-trolledpollination.Wrapthetasselbagaroundthestemandstaplethebaginplace,leavingenoughroomforaneartogrow.

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

How to Breed Sweet Corn for Organic Agriculture www.seedalliance.org��

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-


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-


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


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


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


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.


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.


AuthorsCompleted 20��Jared Zystro, Organic Seed AlliancePictures courtesy of Micaela Colley (OSA) and Jared Zystro (OSA) unless otherwise noted

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