Dynamics of floret development determining differences in spike fertility in an elite population of wheat

ContentslistsavailableatScienceDirect

Field

Crops

Research

j o ur na l h o me p a g e :w w w . e l s e v i e r . c o m / l o c a t e / f c r

Dynamics

of

floret

development

determining

differences

in

spike

fertility

in

an

elite

population

of

wheat

Oscar

E.

González-Navarro

_,

_Simon

_Griffiths

_,

_Gemma

_Molero

_,

Matthew

P.

Reynolds

_,

_Gustavo

_A.

_Slafer

a_{CIMMYT(InternationalMaizeandWheatImprovementCenter),ApdoPostal6-641,06600Mexico,D.F.,Mexico} b_{CropGeneticsDepartment,JohnInnesCentre,NorwichResearchPark,Norwich,NorfolkNR47UH,UK}

c_{ICREA(CatalonianInstitutionforResearchandAdvancedStudies),AGROTECNIO(CenterforResearchinAgrotechnology),andDepartmentofCropand}

ForestSciences,UniversityofLleida,Av.RoviraRoure191,25198Lleida,Spain

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received30May2014

Receivedinrevisedform3November2014 Accepted1December2014

Availableonline26December2014

Keywords: Fertilefloret Floretdynamics Floretsurvival TriticumaestivumL.

a

b

s

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c

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Furtherincreasesinwheatyieldpotentialcouldbeachievedthroughabetterunderstandingofthe

dynamicsoffloretprimordiageneration/degeneration,aprocesswhichhasreceivedlittleattention.We

quantifiedgenotypicvariationamongelitegenotypesoftheCIMCOGpanelassembledbyCIMMYTforits

usefulnessforwheatbreeding.Tengenotypes,representingtherangeofvariationforyieldandits

com-ponentsofthewholepanel,weregrownunderhigh-yieldingconditionsinNWMexicofortwogrowing

seasons.Thestageofdevelopmentoffloretprimordiawasdetermined2–3timesweeklyduringstem

elongationforapical,centralandbasalspikeletswithinthespike.Thedynamicsoffloretinitiation/death,

andtheresultingnumberoffertileflorets,weredeterminedforeachspikeletposition.Wefoundthatthe

variationinnumberoffertilefloretswithinthiselitegermplasmwasmuchmorerelatedtothesurvival

offloretprimordiathantothemaximumnumberoffloretsinitiated.Asthetwofloretprimordiamost

proximaltotherachiswerealmostalwaysfertileandmostdistalflorets(florets6–8)werenever

fer-tile,thedifferencesinnumberoffertilefloretswereclearlyattributedtothedifferentialdevelopmental

patternsofintermediateflorets(floretprimordia3,4and5,countedfromtherachis,dependingonthe

spikeletposition).Wefoundsignificantdifferencesamongelitegermplasmindynamicsoffloret

devel-opment.Differencesinfloretsurvivalseemedpositivelyrelatedtothoseinthelengthoftheperiodof

floretdevelopment:thelongerthedurationoffloretdevelopmentthehigherthelikelihoodofthatfloret

becomingfertile.Itisproposedthatthistypeofstudymaybeinstrumentalforidentifyingprospective

parentsforfurtherraisingyieldpotentialwheatbreedingprogrammes.

©2014TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense

(http://creativecommons.org/licenses/by/3.0/).

1. Introduction

Duetotheincreasingglobalpopulationtogetherwithagrowing demandformeatanddairyproducts(implyingagrowingamount of grainsshouldbe usedtoproduce animal foodat alow rate ofconversion),a substantialincreaseofgrainproductioninthe nextdecadesiscritical.Thisisparticularlychallengingasthebasic manageableresourcesforcropgrowthandyield(water,nutrients) willnotincrease(ConnorandMínguez,2012)andtheland avail-ableforcropproductionislikelytodecline(Albajes etal.,2013

and referencesquoted therein).These challengestogether with theneedofmakingfutureproductionofcropsmoresustainable

∗Correspondingauthorat:CropGeneticsDepartment,JohnInnesCentre,Norwich ResearchPark,Norwich,NorfolkNR47UH,UK.Tel.:+4401603450000x2585.

E-mailaddress:[email protected](O.E.González-Navarro).

amounttoa‘perfect storm’(Godfrayetal.,2010;Fischeretal., 2014).Amongthemajorcrops,wheatisoneofthemostcritical for warrantinghumannourishment:it isthemost widelycrop growngloballyandistheprimarysourceofproteinfortheworld population,representingc.20%ofthedailyintakefordeveloping countries(Braunetal.,2010).Inordertomaintainbalancebetween demandandsupplyalternativewaysandmeanstofurtherraise wheatyieldmustbefound(Chand,2009).Amajorwayto navi-gatethis‘perfectstorm’,facingtherestrictionsmentionedabove, isthroughre-gaininghighratesofgeneticgainsinyield.However, thismaynotbeeasilyachievedasthereismountingevidencethat geneticgainsinyieldhaverecentlybeenmuchlowerthanwhatit wouldberequired(Reynoldsetal.,2012;Fischeretal.,2014).The likelihoodofacceleratingbreedingprogresswouldincreasewith knowledgeofgeneticvariationavailablefortraitsputatively deter-miningyield(Slafer,2003;ReynoldsandBorlaug,2006;Reynolds

etal.,2009).

http://dx.doi.org/10.1016/j.fcr.2014.12.001

Yieldinwheatisgenerallymorerelatedtograinnumberthanto theaverageweightofthegrains(Fischer,2008,2011)asthenumber ofgrainsisfarmoreplasticthanthesizeofthegrains(Sadrasand

Slafer,2012).Consequently,geneticgainsinwheatyieldhavebeen

morerelatedtoimprovementsinthenumberthaninthesizeofthe grains(e.g.Canevaraetal.,1994;Calderinietal.,1995;Sayreetal.,

1997;Shearmanetal.,2005;Acrecheetal.,2008).Asevenin

mod-erncultivarsgraingrowthseemsnotstronglylimitedbythesource

(Borrásetal.,2004;Pedroetal.,2011),itseemslikelythatfurther

increasesinyieldpotentialmayrequireadditionalimprovements ingrainnumber(Reynoldsetal.,2001,2005;AcrecheandSlafer,

2009;Gonzálezetal.,2014).Theidentificationofpotentialtraitsto

increasegrainnumberisofgreatinteresttoensurethatincreased photosyntheticpotentialisfullyutilizedbymatchingitwith ade-quatesinkdemand(Reynoldsetal.,2012;Slaferetal.,2014).To achievethisaim,itwouldbeusefultounderstandthedegreeof variationofphysiologicaldriversofgrainnumberwithinelitelines. Grainnumberislargelydeterminedduringthestemelongation(SE) phase(Fischer,1985;SlaferandRawson,1994).Therefore improve-mentsoftraitsdeterminedduringSEwouldberequiredtofurther increasegrainnumber(Slaferetal.,2005).

Beyond increasing crop growth rate and further improving biomasspartitioningbeforeanthesis, itmayalsoberelevantto optimizethedevelopmentalattributestomaximizespikefertility

(Foulkesetal.,2011;Reynoldsetal.,2012).Thisinvolvestwo

differ-entaspectsofdevelopment:[i]thepatternofpartitioningoftime toanthesisintodifferentphases(Slaferetal.,2001),as lengthen-ingthedurationoftheSEphasemayincreaseyield(Slafer,2003;

MirallesandSlafer,2007);and[ii]thedynamicsoffloret

devel-opment(Kirby,1988),asgrainnumberistheconsequenceofthe developmentalprocessoffloretgeneration/degenerationresulting inacertainnumberoffertileflorets(Gonzálezetal.,2011).

Lookingforvariationindynamicsoffloretdevelopmentwithin modernelitecultivars,couldcontributetotheelucidationofthe mechanismswhich aremost likely to provide opportunitiesto identifysourcesfor apotentialincreasein grainnumber. Floret developmentin wheathas beenlong studied(Stockmanet al.,

1983;SibonyandPinthus,1988;Mirallesetal.,1998;Wangetal.,

2001;Gonzálezetal.,2003a;Bancal,2008;Shitsukawaetal.,2009;

Drecceretal.,2014), especiallyitsresponsetonitrogen

applica-tions(Holmes,1973;LangerandHanif,1973;Ferranteetal.,2010). Itseemsthatduetothedifficultiesinvolvedwiththe developmen-talanalysisofspikemorphogenesisthereisanabsenceofresearch describingvariationforthistraitamongelitewheatcultivars.

Theobjectiveofthepresentstudywastodeterminethedegree ofvariationwithinelitegermplasmofwheatinpatternsoffloret developmentresponsiblefordifferencesinnumberoffertileflorets, andtofurtherunderstandthedifferencesingenerationoffertile floretsamonggenotypesdifferinginyieldcomponents.

2. Materialsandmethods

2.1. Generalconditions

TwofieldexperimentswereconductedintheMexican Phen-otypingPlatform(MEXPLAT)establishedat theresearchstation “CentroExperimentalNormanE.Borlaug”(CENEB),nearCiudad Obregón,Sonora,Mexico(27◦33N,109◦09W,38masl),with con-ditionsthatrepresentthehigh-yieldingenvironments ofwheat worldwide(Braunetal., 2010).Thesoilis a Chromic Haplotor-rert(VertisolCalcaricChromic),lowinorganicmatter(<1%),and slightlyalkaline(pH=7.7).

2.2. Treatmentsandexperimentaldesign

Experimentsweresownon06December2010and09December 2011,withintheoptimalsowingperiodforthewinter–springcycle

Table1

SubsetselectedfromtheCIMCOGpanel.Foreachentry,thenameofthecultivaror crossisindicated,aswellasthemaintraitforwhichthegenotypewasselectedto bepartoftheCIMCOG.

Entry Name Trait

1 BACANORAT88 Highgrains/m2

2 BCN/RIALTO Late

development

3 BRBT1*2/KIRITATI Largegrains

4 CROC1/AE.SQUARROSA

(205)//BORL95/3/PRL/SARA//TSI/VEE#5/4/FRET2

Highfloret number

5 ATTILA/PASTOR Highfloret

number;late development 6 PFAU/SERI.1B//AMAD/3/WAXWING Early development 7 SERIM82 Wide adaptation

8 SIETECERROST66 Benchmark

9 TRAP#1/BOW/3/VEE/PJN//2*TUI/4/BAV92/ RAYON/5/KAUZ//ALTAR 84/AOS/3/MILAN/KAUZ/4/HUITES Wide adaptation 10 WHEAR/SOKOLL Wide adaptation

ofcerealsintheregion.Sowingdensitywas101.5and108.8kgha−1

respectively, and 200units of N fertilizer (urea) were applied. Weeds wereremoved byhand throughout thegrowing season and diseases and insects prevented by applying recommended fungicidesandinsecticidesatthedosessuggestedbytheir man-ufacturers.

Thetreatmentsconsistedofthetenwheatgenotypes(Table1), allelitematerialbelongingtotheCIMMYTMexicoCoreGermplam Panel(CIMCOG)withgoodagronomicadaptation.Thefullsetof 60genotypesoftheCIMCOGpanelarepotentiallyusefulin practi-calbreedingprogrammesaimingtofurtherraisingyieldpotential andforthatreasonisthemaingermplasmstudiedsofarbythe WheatYieldConsortium(Reynoldsetal.,2011).Forthisparticular study,thenumberofgenotypeshadtoberestrictedtotenbecause ofthedetailedmeasurementsrequired,particularlyregarding flo-retdevelopment(seebelow).However,itisworthnotingthatthe selectedgenotypes do represent fairlywell thewholeCIMCOG panelintermsofyieldanditsmajordeterminantsbothconsidering averagevaluesaswellasrangeofvariation(Table2).

Theexperimentwasdesignedinrandomizedcompleteblocks withtworeplicates,whereplotswereassignedtogenotypes.In season2010–2011plotswere5mlongand3.2mwide,consisting offourraisedbeds0.80mwide,withtworowsperbed(0.24m apart),andinseason2011–2012plotswere8.5mlongand1.84m wide,consistingoftworaisedbeds0.80mwide,withtworowsper bed(0.24mapart)(Fig.1,leftpanel).

2.3. Measurementsandanalyses

Plotswereinspectedperiodicallyandoneplantperplot reg-ularly sampledand dissected under binocularmicroscope (Carl Zeiss,Germany)todetectthetimingofinitiationoftheterminal spikelet in each case. From then on until a weekafter anthe-sis, one plant per plot was randomly sampledtwice or thrice weekly.Thesamplesweretakentothelabandtheapexofthe main shoot dissected under binocular microscope. On the dis-sectedjuvenilespikesthetotal numberof floretprimordiawas countedineachoftheanalysedspikelets.Inadditionthestageof developmentofeachofthefloretswithinparticularspikeletswas determined.Togetherthesemeasurementsrepresentthe variabil-ityexpectedinthespikes,indevelopmentalterms(seebelow).To determinethestageofdevelopmentofthefloretprimordia,we fol-lowedthescaleofWaddingtonetal.(1983).Thisscaleisbasedon

Table2

ComparisonofyieldanditsdeterminantsbetweentheCIMCOGpanelandthesubsetoftengenotypes.Dataaretheadjustedmeansfromacombinedanalysisofthewheat genotypesgrownduringthe2010–2011and2012atCENEB,nearCiudadObregon,Mexico.

Trait Average CIMCOG Subset

CIMCOG Subset Range LSD0.05 Range LSD0.05

Yield(Mgha−1_{) 6.42 6.40 4.99–7.63 0.7 6.13–6.61 0.7}

Biomass(Mgha−1_{) 14.12 13.97 11.73–15.76 1.5 13.23–14.72 1.5}

Harvestindex 0.46 0.46 0.41–0.52 0.02 0.43–0.49 0.03

Numberofgrains(m2_{) 15,072 16,554 11,626–21,769 1848 13,752–21,950 2639}

Numberofgrains(spike−1_{) 50 50 41–63 8.3 45–56 9.1}

Grainweight(mggrain−1_{) 43 39 30–52 3.1 30–45 4.4}

Daystoanthesis 87 87 78–95 2.5 80–95 1.2

Fig.1.The60CIMCOGlinesweregrownunderraisedbeds(leftpanel);andschematicdiagramillustratingspikeletpositionswithinthespikeaswellasthepositionofflorets withinthespikeletthatwereusedinthisstudytocharacterizefloretdevelopmentinCIMCOG(rightpanel).

gynoeciumdevelopmentfromfloretprimordiapresent(W3.5),to stylescurvedoutwardsandstigmaticbranchesspreadwidewith pollengrainsonwell-developedstigmatichairs(W10),whichare consideredfertile florets(fordetailsseeFig.1inFerranteetal.,

2013a).

Theanalysedspikeletswerethoseontheapical(fourthspikelet fromthetopofthespike),central(middlespikeletofthespike),and basal(fourthspikeletfromthebottomofthespike)positionsofthe spike(Fig.1,rightpanel).Namingoffloretswithinthespikelets followedthesamesystemdescribedbyGonzálezetal.(2003a); thatis, fromF1tothelastdeveloped floretdependingontheir positionwithrespecttotherachis(F1wasthefloretmost prox-imaltotherachisandthemostdistalfloretprimordiawasF6–F8, dependingonthespecificspikeletandgenotypeanalysed;Fig.1, rightpanel).

Toanalysethedynamicsofdevelopmentweplottedthe devel-opmentalscoreoftheparticularfloretsagainstthermaltime(◦Cd), whichwascalculateddailyassuming, asitis standard,thatthe meantemperaturewastheaverageofthemaximumandminimum valuesandthebasetemperaturewas0◦Cforallgenotypesand stagesofdevelopment.Then,foreachsamplingdatewecalculated thenumberoffloretprimordiawhichwerealiveanddeveloping normally;thetimingwhenfloretprimordiawereconsiderednot developingnormallyanylongerwasthatwhenthemaximumstage of developmentof a particularfloret primordium wasreached. Thenthenumberoffloretprimordiawasplottedagainstthermal timearoundanthesisforeachparticulargenotypeandexperiment. Forthisanalysisweconsidereda primordiatobeafloretwhen itreachedatleastthestage3.5inthescaleofWaddingtonetal.

(1983).

Thedataweresubjectedtoanalysisofvariance(ANOVA),and therelationshipsbetweenvariablesweredeterminedby regres-sionanalysis(SASstatisticsprogram,2002).Theadjustedmeans acrossthe2yearswereobtainedbyusingPROCMIXEDprocedure oftheSASstatisticalpackage(SASstatisticsprogram,2002).Allthe effects,years,replications withinyears, blockswithinyears and replications,andgenotypebyyearinteraction(G×E)were consid-eredasrandomeffectsandonlythegenotypeswereconsideredas fixedeffects.

3. Results

Thereweresignificantdifferencesinnumberoffertilefloretsper spikeletineachofthetwoexperiments,andinadditionthese differ-enceswerereasonablyconsistentbetweenyears,withtheunique exceptionofline2(Fig.2).Line8waswithinthelinesexhibiting thehighestlevelsofspikefertilityinbothexperiments,andline9 waswithinthoseexhibitingthelowestvalues(Fig.2).

Therewassignificantvariationinbothcomponentsofthe num-berof fertile florets:themaximum number offloret primordia initiatedandtheproportionofprimordiasurvivingtobecome fer-tilefloretsatanthesis.However,thenumberoffertilefloretswas muchmorestronglyrelatedtothesurvivaloffloretprimordiathan tothemaximumnumberfloretsinitiated(Fig.3).

Tofurtherunderstandtheprocessesinvolvedinthegenotypic differences withintheCIMCOG panelwe studiedthedynamics of generation and survival of floret primordia in apical, cen-tralandbasalspikelets.Thegeneraldynamicswassimilarinall cases(genotypes×spikeletpositions):duringstemelongationthe number of floretprimordiafirstlyincreased rapidly,reachinga

Fig.2. FertilefloretsperspikeletinbothexperimentsforthesubsetoftheCIMCOG panel.Barsoneachdata-pointshowthestandarderrorofthemean.Genotypeswere labelledasinTable1.Genotype2wastheexception,notbehavingconsistently betweenthe2years,andgenotypes8and9werethosehavingrespectivelythe highestandthelowestnumberoffertilefloretsperspikeletofthelinesanalysed consistentlybetweenyears.Datapointsofsomegenotypesareoverlapped.

Fig.3.Numberoffertilefloretsperspikeletrelatedtoeitherthemaximumnumberoffloretprimordiainitiated(leftpanel)orthepercentageoftheseprimordiawhich developednormallysurvivingtoproducefertilefloretsatanthesis(rightpanel).Opencirclesrepresentseason2010–11andclosedcirclesseason2011–12.

peakrepresentingthemaximumnumberoffloretprimordiaand finallydecreasedsharplyuntilacertainnumberoffertileflorets isestablishedasthebalanceofthegenerationanddegeneration process(TableA.1).Cultivarsvariedin thedynamicsof genera-tion/degenerationoffloretprimordiadeterminingthenumberof fertilefloretsperspikeletatdifferentspikeletpositions(Fig.A.1). Toillustratethesegenotypicdifferenceswecomparedthis dynam-icsof floret generation/degeneration forthe apical,central and basalspikeletsofthetwogenotypesexhibitingtheextremecases offloretfertility(Fig.2):lines8and9representinghighandlow spikefertilities,respectively.Bothgenotypeshadasimilar maxi-mumnumberoffloretprimordiainitiatedintheapicalandcentral spikelets,whilstgenotype9hadaslightlylowermaximumnumber offloretsinitiatedinthebasalspikeletsthangenotype8(Fig.4).On theotherhand,inallspikeletsthedecreaseinnumberoffloret pri-mordia(floretmortality)wasmorenoticeableingenotype9thanin 8(Fig.4).Interestinglyitseemedthatinallspikeletpositions geno-type9reachedthemaximumnumberoffloretprimordiacloserto anthesisthangenotype8,implyingthatthetimeforfloretsurvival wasconsistentlyshorterinthegenotypewithlowestfinalnumber offertilefloretsatanthesis(Fig.4).

Whenanalysingthedevelopmentoftheindividualfloretsitwas clearthatflorets1and2developednormallyandalwaysreached thestageoffertileflorets:inallspikeletsandallgenotypes(Fig.A.2). Thus,noneofthedifferencesbetweengenotypesinspikefertility wererelatedtothefateofthetwomostproximalflorets.Similarly, noneof thegenotypic differencesinspike fertilitywererelated tothefateof florets6,7 and8;asnoneoftheseflorets devel-opednormallytoreachthestageoffertilefloretsever(Fig.A.3). Therefore,genotypicdifferencesinthedevelopmentalpatternsof

intermediateflorets(3,4and5)werecriticalforestablishingthe genotypicvariationinspikefertility.Focusingontheseparticular floretsitbecameclearthat:

(i)floret3developednormally,achievingthestageoffertile flo-rets,inthetwogenotypesandinallthespikelets:evenwhen thedifferenceinspikefertilitywasnotduetothefateof flo-ret3,adifferenceindevelopmentalrateswasnoticeable:it seemedthatfloret3ingenotype9developedwithsomedelay comparedtothatingenotype8(Fig.5,leftpanels).

(ii)floret 4 in the central spikelets did also develop normally achievingthestageoffertilefloretsinbothgenotypes,though againitseemedthatthisfloretstarteditsdevelopmentin geno-type9withsomedelayrespecttothetimingofdevelopment initiationingenotype8(Fig.5,centralpanel).

(iii)floret4inthebasalandapicalspikeletsdevelopednormallyto becomefertileonlyingenotype8(intheapicalspikeletsonly insomeoftheplantsanalysed)butwasneverfertileinapical andbasalspikeletsofgenotype9(Fig.5,topandbottomofthe centralpanels).

(iv)floret5wasneverfertileintheapicalspikeletsofanyofthe twogenotypes(Fig.5,top-rightpanel),whileinthecentraland basalspikeletsitwasfertileinsomeoftheplantsofgenotype 8andinnoneoftheplantsofgenotype9(Fig.5,central-and bottom-rightpanels).

Even in the case of the floret×spikelet positions in which primordiadidnotcontinue developing normallytoachievethe stageoffertile florets,therewasacleartrend,thoughwithfew

Fig.4. Dynamicsofthenumberoflivingfloretprimordia(thosedevelopingnormallyatthetimeofmeasurement)fromtheonsetofstemelongationonwards,plottedagainst thermaltimefromanthesisingenotypes8and9,whichconsistentlyhadhighandlowspikefertility,respectively,withinthesubsetanalysedfromtheCIMCOGpanelinthe apical(leftpanel),central(middlepanel)andbasalspikelets(rightpanel).

Fig.5.Developmentalprogressoffloretprimordia3,4and5(fromlefttorightpanels)inapical,centralandbasalspikelets(fromtoptobottompanels)fromtheonset ofstemelongationonwards,plottedagainstthermaltimefromanthesisingenotypes8and9ofthesubsetanalysedfromtheCIMCOGpanel.Thefloretsarefertilewhen achievingthestage10inthescaledevelopedbyWaddingtonetal.(1983).

exceptions,forthefloretprimordiaofgenotype8tohavedeveloped morethantheequivalentfloretsofgenotype9(Figs.5andA.3). 4. Discussion

Futurewheatbreedingneedstobeextremelyefficientasthe land allocated to wheat (and most other major food crops) is unlikely to increase significantly, and theuse of inputs cannot increaseatsimilarratesastheyhaveinthelasthalf-century(Chand,

2009;Reynoldset al.,2012;Halland Richards,2013).Although

farmyieldsmaybemuch lowerthanyieldpotential,theyseem toberelated(SlaferandAraus,2007;FischerandEdmeades,2010) andthereforethereisagreementthatgeneticgainsinyield poten-tialwillneedtobeaccelerated(Reynoldsetal.,2009).Toidentify opportunitiesformajorimprovementsincropphotosynthesisis essential(Reynoldsetal.,2000; Parryetal.,2011), butwillnot translate in yield gains without further gains in sink strength, themajordeterminantof whichis grainnumber. Infact, geno-typicdifferencesinyieldaremostfrequentlyassociatedwiththose ingrains perm2 _{(Slafer etal., 2014)and geneticgainsin yield} have been mostly explained by improvements in this

compo-nent(Calderinietal.,1999andreferencesquotedtherein).Further

improvinggrainnumberwouldrequiretheidentificationof varia-tioninitsphysiologicaldeterminantswithinhigh-yielding,well adapted populations for breeding.As wheatis a cleistogamous plant,amajordeterminantofgrainnumberisthenumberoffertile floretsproduced.Unfortunately,studiesonthedynamicsoffloret primordiageneration/degeneration,whichultimatelydetermines

spikefertility,areratherrare,likelybecausetheintrinsicdifficulties ofdeterminingthesedynamics.

Mostoftherelativelyfewstudiesonfloretdevelopment dynam-icswerefocusedontheeffectsofenvironmentalfactorsaffecting grainnumber.Inthesecases,itwasconsistentlyrevealedthat flo-retsurvivalwasmorecriticalthantheinitiationofprimordiafor mostenvironmentalfactorsaffectingthenumberoffertileflorets atanthesis.Examplesofthisincludecasesinwhichspikegrowth duringpre-anthesiswasalteredbyshading(FischerandStockman, 1980), nitrogenavailability(SibonyandPinthus, 1988;Ferrante etal.,2010), photoperiodcondition(Gonzálezetal.,2003b)and combinationsofsomeoftheseenvironmentaltreatments(Langer

andHanif,1973;WhingwiriandStern,1982;Gonzálezetal.,2003b,

2005).Regardinggenotypicvariation,whichiskeyforgenetically improvingatrait,therehavebeenreportsonlybasedonthe intro-gressionofsemi-dwarfinggenes.Miralleset al.(1998)reported thatRht1andRht2allelesincreasedthelikelihoodofrelatively dis-talfloretprimordiatosuccessfullyprogresstotheproductionof fertilefloretsandattributedthistoanimprovedassimilate allo-cationofresourcestothegrowingspikebeforeanthesis(Siddique

etal.,1989;Slafer andAndrade,1993).Asopportunitiesto

fur-therincreasepartitioningtothejuvenilespikeinrespectofmost moderncultivarsarerestricted,variationinfloretdevelopmentand spikefertilitywithinelitegermplasmmustbeidentified.In the presentstudywereportedvariationinthedynamicsoffloret pri-mordiainapanelassembledforitspotentialrelevanceforbreeding tofurtherraiseyieldpotential.Thegenotypicvariationin maxi-mumnumberoffloretsinitiatedwasmarginalwhereasvariation

infloretprimordiasurvivalwasfoundtobethemaindeterminant ofthegenotypicvariationinthenumberoffertilefloretsat anthe-sis.Thefactthatfinalnumberoffertilefloretswasrelatedtofloret primordiasurvivalandratherindependentofthemaximum num-beroffloretsinitiatedisinagreementwithresultsreportedwitha comparisonoffourmoderndurumwheatsbyFerranteetal.(2010,

2013a).Thus,itseemsthatthedifferencesbetweenelitegenotypes

inspikefertilityarebasedonsimilarprocessesresponsiblefor dif-ferencesinspikefertilitywhenplantsaregrownundercontrasting environmentalconditions.

Themodelhypotheticallyapplicableisthatwheat(andallother cereals)mayproduceanexcessivenumberoffloretprimordia with-outpenalties asit is energeticallyinexpensive.However, when progressingtolaterdevelopmentalstages,growthofthese primor-diarequiresincreasingamountsofresources,sotheplantadjusts thenumberofprimordiathatbecomefertileflorets(Sadras and

Slafer,2012).Thisadjustmentwouldbequantitativelyrelatedwith

theavailabilityofresourcesforthegrowingjuvenilespikebefore anthesis.Thisisfurtherreinforcedbyevidencethatthetriggers forfloretprimordiadeatharenotpurelydevelopmentalprocesses

(Ferranteetal.,2013b)butlikelyresource-driven(Gonzálezetal.,

2011).Bancal(2009)suggestedthatfloretdeathstartswhenthe

firstfloretofthecentralpositionreachesaWaddingtonscaleof 7–8;whichinthepanelofelitelinesanalysedisnottrueforallthe cultivars(e.g.,theonsetoffloretdeathingenotype7iswhenthe proximalfloretatthecentralpositionscores9.3intheWaddington scale(TableA.1)).

Muchofthedifferencesbetweenthesetofgenotypesanalysed fromtheCIMCOGpanel,intermsofspikefertility,were associ-atedwithdifferencesinfloretsurvivalthatcanbetracedbackto theprocessesoffloretdevelopment.Comparingthetwoextreme genotypesof thisstudy(interms offertilefloretsproducedper spikelet),itseemedclearthatthecultivarmaximizingfloret sur-vivalhasaconsistentlylongerperiodoffloretdevelopment.Thus,it seemedpossibletospeculatethatadvancingdevelopmentprogress oflabilefloretsincreasesthelikelihoodofafloretprimordia becom-ingfertilefloret.Forinstancegrowingaparticulargenotypeunder relativelyshorterphotoperiodsduringtheperiodoffloret devel-opment(andspikegrowth)beforeanthesisnormallybringsabout

significantincreasesinfloretprimordiasurvival(Gonzálezetal.,

2003b;Serragoetal.,2008).Itseemsconsistentwiththisthat

geno-typeshavingslightly longerperiodsof floretdevelopmentmay increasethenumberoffertilefloretsthroughreducingthe pro-portionofprimordiadying,inlinewiththeearlierhypothesisthat lengtheningthestemelongationphasewouldbringaboutincreases inthenumberofgrainsperm2_{(Slaferetal.,2001).}

5. Conclusion

Weconcludedthatwithinelitewheatgermplasm,whichcould be used directly in breeding programmes, there is variation in developmentaldynamics of the floretswhich are ultimately responsiblefordifferencesinspikefertility.Genotypeswithmore fertilespikesexhibitedanimprovedsurvivaloffloretprimordia relatedtoalongerperiodoffloretmortality:thelongertheperiod themoretime(andresources)willbeavailableforallowinglabile primordiatocontinuedevelopingnormallythereforereducing flo-retmortality.Selectinglinesexhibitingthispropertyasprospective parentsmayhelpinfurtherraisingyieldpotentialinwheat. Acknowledgements

ThisworkwassupportedbytheSustainableModernizationof TraditionalAgriculture(MasAgro)initiative fromtheSecretariat ofAgriculture,Livestock,RuralDevelopment,FisheriesandFood (SAGARPA) and by the National Council on Science and Tech-nology(CONACYT) scholarship310626to O.E.G.N.We thankA. Ferrante(Univ.ofLleida,Spain)forthetrainingprovidedon dis-sectionanddeterminationoffloretdevelopmentalstages,andJ.D. Preciado-Díazforhelpingintensivelyondatacollectiononthe sec-ondgrowingseasonofthisstudy.

AppendixA. Appendix

SeeFig.A.1. SeeFig.A.2. SeeFig.A.3.

Fig.A.1.Dynamicsofthenumberoflivingfloretprimordiafromtheonsetofstemelongationonwards,plottedagainstthermaltimefromanthesis,intheapical(leftpanel), central(middlepanel)andbasalspikelets(rightpanel).

Fig.A.2.Developmentalprogressoffloretprimordia1and2inapical,centralandbasalspikelets(fromtoptobottompanels)fromtheonsetofstemelongationonwards, plottedagainstthermaltimefromanthesis.

Fig.A.3.Developmentalprogressoffloretprimordia6,7and8(fromlefttorightpanels)inapical,centralandbasalspikelets(fromtoptobottompanels)fromtheonsetof stemelongationonwards,plottedagainstthermaltimefromanthesis.

TableA.1

Floretmortalityrateasalinearmodelfromthemaximumnumberoffloretprimordiavs.thenumberoffertilefloretsforthemeanofseason2010–11and2011–12.

Position Entry Maximumnumberoffloretprimordia Numberoffertileflorets Floretmortalityrate

Floret primordia TT(before anthesis) Waddington ScaleofF1 SE Fertile florets SE TT(before anthesis) Primordia ◦_Cd−1_(×100) r2 _P Apical 1 6.5 250 7.6 ±0.31 2.75 ±0.25 0 −1.536±0.1060 0.977 <0.0001*** 2 5.75 394 5.9 ±0.55 3 ±0 145.5 −1.055±0.2146 0.829 0.0044** 3 6.5 391.5 7.2 ±0.25 3.25 ±0.25 0 −0.911±0.0995 0.903 <0.0001*** 4 5.5 88 9.3 ±0.42 3.5 ±0.289 0 −2.318±0.3824 0.974 0.1041ns 5 6.75 159.5 8.3 ±0.37 3.5 ±0.289 0 −1.802±0.4563 0.839 0.029* 6 6.5 104.5 8.8 ±0.42 3.5 ±0.289 0 −2.708±0.4124 0.956 0.0224* 7 6 236.5 7.5 ±0.35 3 ±0.577 0 −1.070±0.1264 0.935 0.0004*** 8 6 290 7.6 ±0.51 3.25 ±0.479 0 −0.652±0.1182 0.813 0.0009*** 9 7 194.5 8.1 ±0.23 3.25 ±0.479 0 −1.182±0.3567 0.687 0.0211* 10 6.5 141.5 8.8 ±0.37 3.5 ±0.25 0 −1.398±0.3334 0.854 0.0247* Central 1 8 250 8.2 ±0.27 4.5 ±0.289 0 −1.403±0.0998 0.975 <0.0001*** 2 7 394 7 ±0.35 4.5 ±0.289 0 −0.614±0.0832 0.872 <0.0001*** 3 7.25 432 7.5 ±0.35 4.25 ±0.25 0 −0.695±0.0914 0.853 <0.0001*** 4 7 123.5 9.3 ±0.27 4.5 ±0.289 0 −1.785±0.7602 0.734 0.1433ns 5 7.5 312 7.7 ±0.43 4 ±0 0 −0.965±0.1405 0.871 0.0002*** 6 7.5 334 8.3 ±0.37 4.5 ±0.289 0 −0.802±0.1218 0.844 0.0002*** 7 7.25 269.5 8 ±0.32 4.5 ±0.25 0 −1.148±0.0730 0.976 <0.0001*** 8 7.75 318.5 7.3 ±0.55 4.5 ±0.289 0 −0.800±0.1220 0.843 0.0002*** 9 7.25 194.5 8.5 ±0.17 4 ±0 0 −1.355±0.2463 0.858 0.0027** 10 7.25 260 8.3 ±0.23 4.25 ±0.25 0 −1.054±0.1124 0.936 <0.0001*** Basal 1 7.25 288 7.6 ±0.31 4 ±0 0 −1.182±0.0847 0.970 <0.0001*** 2 7 235.5 7.5 ±0.20 4 ±0 0 −1.208±0.1999 0.901 0.0038** 3 7.25 315 8.1 ±0.24 4.25 ±0.25 0 −0.918±0.0672 0.964 0.0001*** 4 7 391.5 7 ±0.61 3.75 ±0.25 0 −0.718±0.0587 0.943 <0.0001*** 5 6.75 312 7.2 ±0.43 3.75 ±0.25 0 −0.969±0.1770 0.811 0.0009*** 6 7.5 334 7.7 ±0.25 4.5 ±0.289 0 −0.849±0.1226 0.857 0.0001*** 7 6.5 200.5 8.1 ±0.27 4 ±0.408 0 −1.269±0.1534 0.945 0.0012** 8 7.25 318.5 7.25 ±0.32 4.75 ±0.25 0 −0.762±0.0830 0.913 <0.0001*** 9 6.75 278 7.8 ±0.31 3.25 ±0.479 0 −0.988±0.1540 0.855 0.0004*** 10 7.25 260 8.1 ±0.23 3.5 ±0.289 0 −1.036±0.2817 0.693 0.0103*

SE=standarderrorofthemean. * _<0.05.

** _<0.01. ***_<0.001.

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