Avoiding lodging in irrigated spring wheat. I. Stem and root structural requirements

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University of Birmingham

Avoiding lodging in irrigated spring wheat. I. Stem

and root structural requirements

Jesson, Michael; Pinera-Chavez, Francisco; Berry, Pete; Foulkes, M.J.; Reynolds, Matthew

DOI:

10.1016/j.fcr.2016.06.009

License:

Creative Commons: Attribution-NonCommercial-NoDerivs (CC BY-NC-ND)

Document Version

Publisher's PDF, also known as Version of record

Citation for published version (Harvard):

Jesson, M, Pinera-Chavez, F, Berry, P, Foulkes, MJ & Reynolds, M 2016, 'Avoiding lodging in irrigated spring

wheat. I. Stem and root structural requirements', Field Crops Research, vol. 196, pp. 325–336.

https://doi.org/10.1016/j.fcr.2016.06.009

Link to publication on Research at Birmingham portal

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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

Avoiding

lodging

in

irrigated

spring

wheat.

I. Stem

and

root

structural

requirements

F.J.

Pi ˜

nera-Chavez

a,b,∗

_,

_P.M.

_Berry

c

_,

_M.J.

_Foulkes

a

_,

_M.A.

_Jesson

d

_,

_M.P.

_Reynolds

b

a_Division_of_Plant_and_Crop_Sciences,_The_University_of_Nottingham,_Sutton_Bonington_Campus,_{Loughborough,}_{Leicestershire}_LE12_5RD,_UK b_CIMMYT,_Int._Apdo._Postal_6-641,₀₆₆₀₀_Mexico,_DF,_Mexico

c_ADAS_High_Mowthorpe,_Malton,_North_Yorkshire_YO17_8BP,_UK

d_School_of_Civil_Engineering,_The_University_of_Birmingham,_Edgbaston,_Birmingham_B15_2TT,_UK

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received2February2016

Receivedinrevisedform4June2016 Accepted16June2016

Availableonline30June2016 Keywords:

Springwheat Lodging-proofideotype Stemstrength Anchoragestrength Rootplatespread Grainyield Stembiomass

a

b

s

t

r

a

c

t

Amodelofthelodgingprocesshasbeensuccessfullyadaptedforuseonspringwheatgrownin North-WestMexico(NWM).Thelodgingmodelwasusedtoestimatethelodging-associatedtraitsrequiredto enablespringwheatgrowninNWMwithatypicalyieldof6tha−1_and_plant_height_of_0.7_m_to_achieve_a

lodgingreturnperiodof25years.Targettraitsincludedarootplatespreadof51mmandstemstrengthof thebottominternodeof268Nmm.Thesetargettraitsincreasedto54.5mmand325Nmm,respectively, foracropyielding10tha−1_._Analysis_of_multiple_genotypes_across_three_growing_seasons_enabled

rela-tionshipsbetweenbothstemstrengthandrootplatespreadwithstructuraldrymattertobequantiﬁed. ANWMlodgingresistantideotypeyielding6tha−1wouldrequire3.93tha−1ofstructuralstembiomass and1.10tha−1_of_root_biomass_in_the_top₁₀_cm_of_soil,_which_would_result_in_a_harvest_index_(HI)_of_0.46

afteraccountingforchaffandleafbiomass.Acropyielding10tha−1wouldachieveaHIof0.54for0.7m tallplantsor0.41formoretypical1.0mtallplants.Thisstudyindicatesthatforplantbreederstoachieve bothhighyieldsandlodging-proofnesstheymusteitherbreedforgreatertotalbiomassordevelophigh yieldinggermplasmfromshortercrops.

1. Introduction

Lodging is deﬁned as the permanent displacement of plant stemsfromtheirverticalpositionas aresultof windacting on theshoot and rainor irrigationweakening thesoil and reduc-inganchorage strength (Berryet al.,2004).Lodging grainyield reductionsofwheatcanbeintherangeof7–80%(Acrecheand

Slafer,2011;BerryandSpink,2012;Eassonetal.,1993;Fischerand

Stapper,1987;Tripathietal.,2005;WeibelandPendleton,1964)

andcommonlyareaccompaniedbyreductionsofbreadmaking quality(Berryetal.,2004).Infact,asPinthus(1974)indicated,these reductionscanbeatleastasgreatasthatresultingfrom crypo-togamicdiseasesandinsectpestsinhighyieldingenvironments. Lodgingaffectsallcerealspeciesandmanyothercrops,suchas oilseedrapeandsunﬂowers,throughouttheworld.Inwheat, lodg-ingcanincreasesusceptibilitytopestsanddiseases(Berryetal.,

∗ Correspondingauthorat:DivisionofPlantandCropSciences,TheUniversity ofNottingham,SuttonBoningtonCampus,Loughborough,LeicestershireLE125RD, UK.

E-mailaddresses:[email protected],[email protected]

(F.J.Pi ˜nera-Chavez).

2004; Pinthus, 1974), inducenegative effects on crop

develop-ment(decreasinggrainperm2_and_average_grain_weight)₍_Acreche

andSlafer,2011;FischerandStapper,1987)andcomplicate

har-vest(Berryetal.,2004;FischerandStapper,1987;Pinthus,1974).

Widespreadlodgingaffectsfrom15to20%oftheUKwheat grow-ingareaonceeverythreeorfouryears(Berry,1998),although,

Grifﬁn(1998)indicatedalodgingincidenceof10%everyyear.For

theYaquiValley,asurveyconductedduring1981–1991(80 farm-ers’ﬁeldseachyear)indicatedoccurrenceoflodgingfrom18to 40%of thegrowingareain severalyears(Tripathietal.,2004). YieldpotentialoftheYaquiValley(NWMexico)(irrigated envi-ronment)hasbeenestimatedat9tha−1and10.4tha−1fortheUK (rainfedenvironment)(FischerandEdmeades,2010).Yield poten-tiallossesduetolodgingcanbeestimatedfrom0.63to7.2tha−1 fortheYaquiValleyand0.73–8.3tha−1intheUKintheaffected area.Peakeetal.(2014)estimatedalodgingyieldpotentiallossof 1.7tha−1ofirrigatedspringwheatinsub-tropicalAustralia(yield potentialof9tha−1).Ineconomictermsithasbeenreportedthat inaseverelodgingyearthecostforthefarmingindustrywould bearoundUS$188millionintheUKalone(Berry,1998).Forthe YaquiValleythiscostwouldbeUS$29million(assuming40%ofarea affectedfrom76000ha(ServiciodeInformacionAgroalimentariay http://dx.doi.org/10.1016/j.fcr.2016.06.009

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Pesquera,2016),50%yieldlossandUS$215wheatpricepertonne

(Lanticanetal.,2016)).Ifweassumed1.0tha−1ofyieldlossdue

tolodging in10%of theworldwheatgrowingarea, whichwas 222millionhain2014(Lanticanetal.,2016),thentherewillbe agrainyieldlossofaround22milliontonneseveryyear (equiv-alenttoUS$4.7billionassumingaglobalwheatpriceofUS$215). Thiswouldaddanextra3%tothe700milliontonnesproducedin averageworldwideeveryyear(FoodandAgricultureOrganization

oftheUnitedNations,2014).Awheatcropthatwouldlodgeonce

in25yearswouldaddanextra72%tothetotalworldwidewheat productionacrossthose25years.Inascenariowheretheprimary objectiveistoincreasegrainyieldtofulﬁllglobalfooddemands

(Reynoldsetal.,2012,2011)andresearchinitiativessuchas

Inter-nationalWheatYieldPartnershipareinvestinginthis,maintaining lodgingresistancewillbeofparamountimportancetoprotectthe increasedproductivity.

Plantbreedershavehistoricallyreducedlodgingriskby intro-ducingdwarﬁng genes to produce shorter varieties. Additional plantheightreductionhasbeenpossiblethroughtheuseofplant growthregulatorsorPGRsthathelpedtoreducefurtherlodgingrisk

(Berryetal.,2004;CrookandEnnos,1995;Pinthus,1974;Tripathi

etal.,2004;WebsterandJackson, 1993).Optimizingcrop

man-agementalsohelpedfarmerstoreducelodgingriskandexamples arereducedseedrate,delayedsowing,reducedanddelayed

nitro-gen(Berry etal.,2004;Websterand Jackson,1993)androlling

thesoil(Berryetal.,2004).Lodgingresistancemustbe

continu-allyimprovedtocountertheescalatinglodgingriskarisingfrom continuedyield increases.However, there maynow be limited potentialtocontinueimprovinglodgingresistancethroughfurther decreasingplantheightbecausetheminimumheightthatis com-patiblewithhighyield(0.7–1.0m(Allan1986;Kerteszetal.,1991;

Richards1992;BalyanandSingh1994;MirallesandSlafer1995a;

Flinthametal.,1997;Berryetal.,2014))hasnowbeenreached

inmanyenvironments.MirallesandSlafer(1995b)suggestedthat dwarfinggenesmayhaveadirecteffecttoreducethefinalgrain weight.Dwarfinggeneshavealsobeenassociatedwithareduction ofwatersolublereservesstoragecapacity(CossaniandReynolds, 2012)andleafextensionrate(Keyesetal.,1989)thatmightreduce grainweight.Thus,areductionofthesolublereservesinthestem (particularlyindryenvironments)(Borrelletal.,1993)orreduction

ofﬁnalleafarea(McCaigandMorgan,1993;MirallesandSlafer,

1995a,b)andradiationuseefﬁciencyatpre-anthesis(Mirallesand

Slafer,1997)arepossiblereasonswhyextremedwarﬁsmcould

signiﬁcantlyreducegrainyield.

Itthereforeseemsthatreducingheighttobelow0.7mmightnot bethebestmechanismtoimprovelodgingresistanceinmodern highyieldingwheat.Ifweconsiderthatthetwotypesoflodging areduetothebending/bucklingofthestembase(stemlodging) ortheover-turningoftheanchoragesystem(rootlodging),then greaterlodgingresistanceinwheatcanbeachievedby strengthen-ingthesestructures(Berryetal.,2003b).Inthepast,stemstrength andanchoragestrengthhavebeenproposedaskeypropertiesof cerealcropsforlodgingresistance(CrookandEnnos,1994,1993;

Eassonetal.,1995,1992;Ennos,1991a,b;Graham,1983;Pinthus,

1974).Largegeneticvariationhasbeenidentiﬁedfortheanchorage andstemstrengthofwinterwheatintheUK(Berryetal.,2003a, 2007).However,breedingtoimprovethesetraitsinhigh yield-ingwheatrequiresmoreunderstandingabouthowtheydevelop andpossibletrade-offswithyield-formingprocessesindifferent environments.

Bakeretal.(1998)developedamodeloflodgingthathasbeen

validatedbyBerryetal.(2003b)forwinterwheatintheUK.The modelwasbasedontheinteractionofplant,soilandwind char-acteristicsandcalculatesthestemandrootlodgingriskaccording tothewindspeedrequiredtoover-turntherootanchorage sys-temortobucklethestembaseofaplant.Apreliminaryattempt

toquantifythestemstrengthandanchoragestrengthrequiredby winterwheattowithstand1in25yearwindgustsintheUKhas

beenmadebyBerryetal.(2007)usingthislodgingmodel.This

indicatedthatsubstantialamountsofdrymattermayneedtobe investedinthestemandanchoragesystemtomakeplants lodging-proofforaperiodof25years,whichwereestimatedat7.9tha−1of stembiomassand1.0tha−1surfacerootbiomass(rootsintheﬁrst top10cmofsoil)(Foulkesetal.,2011).Thiswouldmeanthatthe maximumharvestindex(ratioofgraindrymattertototal above-grounddry matter)fora 0.7mtallcropyielding8tha−1 would onlybe 0.42,rising to0.50 for a cropyielding16tha−1, which issigniﬁcantlylessthanthetheoreticalmaximumharvestindex 0.62estimatedbyAustin(1980).Additionallyitispossiblethatthe investmentindrymatterforthestemandanchoragesystem dur-ingstemelongationwhichisthecriticalphasefordetermination ofgrainnumber(Fischer,1985)maycompeteforresourceswith grainyielddetermination.TheimplicationsofBerryetal.(2007)

arethatthedualrequirementsofbreedingforgreateryieldand greaterlodgingresistancewillbechallenging.However,partsof theanalysiswerebasedonlimiteddatasetsforwinterwheatandit wasnotpossibletodistinguishbetweenthestructuraldrymatter andwatersolublecarbohydrateinthestem,whichmaymeanthat theestimateofstemstructuraldrymattertoavoidlodgingwas over-estimated.

Theaimsofthispaperwereto1)investigatetherelationship betweenstemstrengthandanchoragestrengthandthedrymatter requirementsofthesestructuresforspringwheatinNorth-West Mexico(NWM),2)adaptanexistingmodeloflodgingforwinter wheatforspringwheatandcalculatedlodgingrisk,3)estimatethe structuraldrymatterrequirementstoenablespringwheattoavoid lodginginthis particularenvironment,and4)considertowhat extentthedevelopmentofstructuralcharacteristicsmaycompete withyield-formingprocessesandgrainyield.

2. Experimentalmethods

2.1. Experiments

Fourfieldexperimentswereestablishedduringthefieldseasons 2010–2011,2011–2012,2012–2013and2013–2014(referredto hereafteras2011–2014,respectively)intheexperimentalstation ofCENEB(CampoExperimentalNormanE.Borlaug)locatedinthe ValledelYaqui,Sonora,Mexico(27.9◦Nandlongitude109.9◦W). Thesoiltypeattheexperimentalstationisacoarse,sandyclay, mixedmontmorillonitictypiccaliciorthid,slightlyalkaline(pH7.7) innature(Sayreetal.,1997),bulkdensityof1.32gcm−3andorganic matterof0.7%approximately(CIMMYTinternalrecords).Detailed information about experiments and cultivars (CIMMYT Mexico CoreGermplasmPanelorCIMCOGconsistingof58Triticum aes-tivumandtwoTriticumdurumanddescribedinTableS1)isgivenin acompanionpaperbyPi ñera-Chavezetal.(2016).Thewhole CIM-COGpanelwasestablishedduring2011andasubsetof30cultivars wereusedfor2012and2013(asindicatedinTableS1).Experiments weremanagedunderaconventionalagriculturalmanagementbut maintainingyieldpotentialconditions.Theaverageseedratefor allplotsinexperiments2011–2013was10.6gm−2whichgavea range213–292seedsm−2.Fortheexperimentin2014asubsetof fivecultivarswithcontrastingvaluesforstemstrength,anchorage strengthandstemwallmaterialstrength(cultivars7,19,24,57 and60,seeTableS1)wasestablishedusingseedratesof75,125 and175seedsm−2toevaluatetheeffectoflowplantpopulations onlodgingtraits.Theirrigationscheduleincludedfivetosixflood irrigationevents(includingoneatsowing)duringthecycleand thefertilizationwas200kgha−1ofN(25%beforesowingand75% beforefirstirrigationevent)and50kgha−1ofP(beforesowing).

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Table1

CultivarsfromCIMCOGusedfor2014experiment.

Cultivar Characterofinterest

BACANORAT88a _Lowest_stem_and_anchorage_strength

CMH79A.955/4/AGA/3/4*SN64/CNO67//INIA66/5/NAC/6/RIALTOa _Highest_anchorage_strength

CROC1/AE.SQUARROSA(205)//BORL95/3/PRL/SARA//TSI/VEE#5/4/FRET2a _Highest_material_strength

WBLL1*2/KURUKU*2/5/REH/HARE//2*BCN/3/CROC1/AE.SQUARROSA(213)//PGO/4/HUITESa _Highest_stem_strength

YAV3/SCO//JO69/CRA/3/YAV79/4/AE.SQUARROSA(498)/5/LINE1073/6/KAUZ*2/4/CAR//KAL/BB/3/NAC/5/KAUZ /7/KRONSTAD

F2004/8/KAUZ/PASTOR//PBW343a

Lowestmaterialstrength

Plantgrowthregulatorswerenotappliedinanyoftheexperiments. Plantemergencedates(at50%ofplantsemerged)wererecordedat 15ofDecember2010,16ofDecember2011,02ofDecember2012 and01ofDecember2013forexperiments2011–2014,respectively. 2.2. Measurements

PlantmeasurementsweredoneatGS65+20days(Zadoksetal., 1974)during2011–2013aswasdescribedinBerryetal.(2000)

(detailedkeyinformationofmeasurementsisgiveninTableS2). Additionally,during2013and2014mainshootmeasurementsof thelengthand breakingstrengthfollowingremovalof the leaf-sheathoftheinternodes1–5weredeterminedforfivecultivars withcontrastingperformancesinstemstrength,materialstrength andanchoragestrength(Table1).Internode1inthemainshootwas identified,definedasthefirstinternodeofmorethan10mm, origi-natingatorjustbelowthegroundsurfaceandwithoutcrownroots emergingfromitsuppernode.Subsequentinternodesascending thestemwerenumberedtwo,three,fouretc.,withtheuppermost internodereferredtoasthepeduncle.Also, dryweightandthe followingdeterminationofthewatersolublecarbohydrates con-tent(WSC)wasmadeontheseinternodes.WSCcontentwasalso determinedforthewholemainshootinallcultivarsusedfor exper-iments2011–2013.TheseanalyseswerecarriedoutintheMaize NutritionQualityandPlantTissueAnalysisLaboratoryfrom CIM-MYT(ElBatan,Mexico)usingtheAnthronemethod(Galiciaetal.,

2008).

2.3. Calculations

Avalidatedmodeloflodgingforwinterwheat(Bakeretal.,1998;

Berryetal.,2003b)wasusedtocalculatethestemfailuremoment

(stemstrengthatthepointoffailure),anchoragefailuremoment (anchoragestrengthatthepointoffailure),thewind-inducedbase bendingmoment(leverageforce)oftheshootandplant,andoverall risktostemandrootlodgingonspringwheat(stemandanchorage failurewindspeed).Thismodelincludedstembasebendingmodel estimationusingBaker(1995)methodandasimpliﬁedversionof therootstrengthmodelofCrookandEnnos(1993).

Thestemfailuremoment(Bs)wascalculatedfromthebreaking

strength(Fs)andlength(h)oftheinternode(Eq.(1)).

Bs=

1

4Fsh (1)

Anchoragefailuremoment(BR)wascalculatedfromtheroot

platespread(d),theshearstrengthofthesurroundingsoil(s)and aconstantof0.43(k3)takenfromBakeretal.(1998)(Eq.(2)).The

surroundingsoilwasassumedtobeatﬁeldcapacitywithashear strengthof6kPa(Bakeretal.,1998).

BR=k3sd3 (2)

Theshoot basebendingmoment(B) wasobtainedfromthe density of air (=1.2kgm−3), the projected ear area (A), the shoot’sheightatcentreofgravity(X),thewindgustspeed(Vg),

theshoot’snaturalfrequency(n),theaccelerationduetogravity

(g=9.81ms−2),theshoot’sdampingratio(␰=0.08)andthedrag coefﬁcientoftheear(Cd=1.0).Thebasebendingmomentofthe

wholeplantwascalculated bymultiplying Bbythenumber of shootsperplant(Bakeretal.,1998):

B=1₂ACdXVg2

1+ g (2n)2X

1+e−␰sin

/4

(3) Naturalfrequencyisconsideredaparameterofmajor impor-tanceofthewind-inducedleverage(basebendingmoment)(Baker etal.,1998),although,heightatcentreofgravityandearprojected areahavealsoagreatinﬂuence(Berryetal.,2003b).Assumingthe wind-inducedleveragedecreaseslinearlyforprogressivelyhigher positionsupthestem(Berryetal.,2006),bendingmomentatthe baseofsubsequentinternodes2–5wascalculatedbymultiplying theleverageexertedatthebaseofinternode1withtheratioof thedistancebetweenthebaseofinternodes2,3,4or5andthe mid-pointoftheearwiththetotalstemheightatthemid-pointof

theear(Berryetal.,2007).Theseratiosweremeasuredinspring

wheatat0.84forthebaseofinternode2,0.70forinternode3,0.50 forinternode4and0.19forinternode5.Ratioatthemid-pointof thelengthinternode5orpedunclewasusedtocalculatethe bend-ingmoment(19%)duetonon-uniformgeometricproperties(Berry etal.,2007).Thismeansthattheleverageexertedatthebaseof internodes2,3,4andthepeduncleshouldbe84,70,50and19%, respectively,oftheleverageexertedatthebaseofinternode1.

Thestem failurewindspeed (VgS)and theanchoragefailure

windspeed(VgR)werecalculatedbycombiningandre-arranging

Eqs.(1)and(2),withEq.(3)(Berryetal.,2003b).LetterNinEq.(5)

indicatesthenumberofshootsperplant. VgS= (2Bs)0.5×

ACdX

1+ g (2n)2X

1+K_␰

−0.5 (4) VgR= (2NBR)0.5×

ACdX

1+ g (2n)2X

1+K_␰

−0.5 (5) 2.4. Statisticalanalysis

Simple linear and non-linear regression analysis and simple linearregression analysiswithgroups wereused toinvestigate relationshipsbetweentraits.Analysisofvarianceusingageneral linearmodelwasusedtotestfordifferencesbetweenyears, culti-varsandleafsheathremovaltreatmentstogetherwithtreatment interactions.AlltheanalyseswerecarriedoutbyGENSTAT15th Edition(VSNInternational,2012).

2.5. Windspeedcharacterisation

Dailywindrundatawassourcedfromalocalmeteorological sta-tionwithinthewheatgrowingareaoftheValledelYaqui,Sonora, Mexico(gridreference27.3◦Nand109.1◦W,38masl)spanninga 40-yearperiodfrom1973to2013.Theweatherstationwaslocated within10kmfromtheexperiments.Thedailywindrundatawere convertedtothemaximumhourlymeanwindspeedforeachday

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bymultiplyingbyafactorof1.606(Berryetal.,2003b).Thehourly meanvalueswereeachconvertedtohourlygustvalues,forgustsof duration=0.3s,usingtheempiricalequationdescribedbyBerry

etal.(2003b): Ugust=Um

1+0.42

Um

sin

₃₆₀₀

(6) where/Umistheturbulenceintensity(TI).Avalueof/Um=0.5

wasused,againfollowingtheworkof Berryetal.(2003b)who usedthevalue determinedby Finnigan(1979)for wind over a wheatcrop.Thesevalueswerethencorrectedfordifferencesinthe roughness,z0,attheairportweatherstationsandthecrop

loca-tions,andalsoforthedifferenceinheightaboveground,z,ofthe airportanemometers(10m),themetstationanemometer(1.5m) andthepertinentwindspeedheightforcroplodgingof2m(Baker

etal.,1998;Berryetal.,2003b).Thiscorrectiontakestheform:

Uc=Uw ln

z−d_z 0

c ln

z−d z0

w (7) wheresubscriptscandwrefertothecropandweatherstation locations,respectively. z0 over thecropshasbeenestimatedas

z0=(h−d),where=1/3,h=1mandd=0.75h,givingavalueof

z0=1/12m, withz0=0.01mattheweatherstation(Berryetal.,

2003b).Finally, an altitudecorrection has been appliedto the

gust wind speed usingthe V99 correctionfactor of (1+0.009h)

speciﬁed in Baker et al. (1998). The correction is taken as the ratio(1+0.009hc)/(1+0.009hw)wherehcandhwarethecropand

weather stationaltitudes respectively. As stated in Berry et al.

(2003b),thesemethodsweredevelopedbasedonUK

Meteorolog-icalOfﬁcedataandshouldthereforebeappliedwithcareinother locations.Inparticular,thesemethodsareonlyapplicablewhere synoptic(non-convective)windsareexpected.

Inordertoallowtheanalysistoconcentrateontheperiodwhen lodgingriskispossible,thehourlygustvaluesweresplitbymonth (i.e.,12groupsofdatawereformedfromthe40yearsofdata,each correspondingtoaparticularmonth).Theprobabilityofthegust speedexceedingacertainvaluewascalculatedforeachmonthon aperdaybasis.Ateachscale,probabilitiesofthegustspeedbeing withinacertainrangewerecalculatedbysortingthevaluesinto 0.5ms−1wide“bins”,withtheprobabilityofgustswithintherange coveredbyeachbincalculatedsimplyfromthenumberofvalues inthatbindividedbythetotalnumberofvalues.Probabilitiesof exceedingacertainvaluewerecalculatedasthesumofthe prob-abilitiesforthebinswhoserangesexceededtherequiredvalue. Theprobabilityofexperiencinganyparticularwind gustduring thelodgingriskperiodwasthencalculatedusingdailygustspeed probabilitiesforthemonthsduringwhichlodgingispossibleand assumingastemlodgingriskperiodof50daysinMarchandApril (assumingonewheatcycleperyear)(windowbetweencultivars withearliestﬂoweringandthelatestmaturitywas52days).Root lodgingriskincreaseswhenthesoilsurfaceiswet(Eassonetal.,

1995;Berryetal.,2003a),typicallywhentheﬁrst50mmofsoilis

atfieldcapacity(Bakeretal.,1998).Thiscanbeattributedtothe movementoftheplantcrowninasaturatedsoilsurfaceafterflood irrigationinirrigatedenvironments (FischerandStapper, 1987) orafterprecipitationinrainfedenvironments(CrookandEnnos, 1994),Moreover,Sterlingetal.(2003),usingaportablewindtunnel inthefield,foundthatrootlodgingoccurredonlywhenthesoilwas saturated.Additionally,itiswellknownthatwatersupplyinmost springwheatworldwideisgivenbyfloodirrigationinflatbeds. Itthereforeseemssensibletoconsidertherootlodgingriskwhen thesoilsurfaceismoist.Forourpurposes,itwasestimatedtheroot lodgingriskperiodfortheNWMenvironmenttobe10days,based onthenumberofdayswhenthefirst60mmofsoildepth (maxi-mumrootplatedepthexceeded50mminCIMCOGpanel)wasat

0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.40 0.45 0.50 0.55 0.60 0.65 0.70

Predicted height at centre of gravity (m)

Me as ur ed h ei gh t a t ce nt re of gr av it y (m )

Fig.1.PredictedandmeasuredheightatcentreofgravityatGS65+7daysand GS65+20,respectively,forplotmeansof2012(closedsquares)and2013(open squares).(—)1:1line.Bestﬁtline,y=0.70x+0.15;R2₌_0.64_(P_<_0.001).

50%ofplantavailablewaterduringthe50daylodgingriskperiod. Atlowersoilmoisturecontent,thesoilisusuallytoostrongto per-mitrootlodging.Springwheatinthisregiontypicallyreceivesthree floodirrigationeventsduringthe50daylodgingriskperiod,each deliveringapproximately73mmwater(0–120cmsoilcoredepth). Windyconditionsoftenoccurpost-anthesiswhichmaycoincide withirrigations,indeedinthatperiodin2014and2016windstorms affectedthearea.Farmerspayspecialattentionwhethertoapply thelastirrigationornot(normallyatmidgrainfilling)becauseof theintensityofthewindyseason.Thisisadifficultdecisiontomake becauseavoidingthelastirrigationsometimesendsingrainyield losses.Usinginternalrecordsoffieldcapacityandpermanent wilt-ingpointfromCENEBandevapotranspirationdatafromnearest weatherstationithasbeenestimatedthatthetop150mmofsoil driesto50%ofplantavailablewaterafter8daysofirrigation.The soiltendstodryfromthetopdownwardswhichindicatesthatthe top60mmofsoilwillbedriedto50%plantavailablewaterwithin about3.2days,givingabout10dayswhenthetop60mmofsoil maybemoistandweakenoughtopermitrootlodging.Areviewof rainfalldataoverthepast40yearsshowedthatthechanceofmore than10mmofrain(enoughtobringthetop60mmofsoiltofield capacity)fallinginonedayduringMarchorAprilwasverysmall.

2.6. Lodgingmodeldevelopment

Thissectiondescribeshowforspringwheatthelodgingmodel wasfurtherdevelopedtoestimatethekeyplantcharacteristics thatdeterminebasebendingmoment(shootheightatcentreof gravity,shootnaturalfrequencyandeararea)fromplant charac-teristicsthataremorecommonlymeasuredbycropphysiologists (grainyield,grainharvestindex,plantheightandshootsm−2).This processwascarriedoutforwinterwheatbyBerryetal.(2004), however springwheat hasfundamental differenceswhich may affecthow theplantcharacteristicsdescribedabovearerelated. Onekeydifferenceisthepresenceofawnsonthespringwheat varieties.Theoretically,shootheightatcentreofgravity(X)canbe calculatedfromstemlength(SL),stemandleaffreshweight(SW),

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0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 16

Earfresh weight(g)

Ea r a re a (c m 2)

b

0.5 1.0 1.5 2.0 2.5 0.40 0.45 0.50 0.55 0.60 0.65 0.70 Height at centre of gravity (m)

N at u ra l f requ enc y (H z)

a

Fig.2. (a)HeightatcentreofgravityatGS65+20plottedagainstnaturalfrequencyforplotmeansof2012(closedsquares)and2013(opensquares).Regressionline: y=0.67x−1.4_(R2₌_0.38;_P_<_0.001)._(b)_Ear_fresh_weight_plotted_against_ear_area_at_GS65₊_20,_for_plot_means_of₂₀₁₂_(closed_squares)_and₂₀₁₃_(open_squares)._Best_ﬁt_line:

y=1.02x+9.95(R2₌_0.44;_P_<_0.001).

earfreshweight(EW)andearlength(EL)followingEq.(8)which

assumesuniformweightofshootandear(Berryetal.,2004). X₌(SLSw+2SLEW+ELEW)

2 (SW+EW)

(8) ThecomponentsofEq.(8)canbecalculatedfromphysiological croptraitsmeasuredcommonly:grainyield(Y,gm−2),thenumber ofearspermetresquare(En),theratioofchaffdryweighttototal

eardryweight(␣),theharvestindex(HI)andthecropheighttothe tipoftheear(h,m)(Eqs.(9)–(11)).

EW= Y

_⁄

₍₁₋_˛) En (9) SW= EW(1−˛) HI −EW (10) SL=h−EL (11)

Eq. (8) was tested using measurements of SW and EW at

GS65+7dinarandomsampleof20plantsperplot,andX,SL,Enand

ELatGS65+20din10plantsperplotinalltheplotsduring2012

and2013experiments.Fig.1showsthatEq.(8)accountedfora sub-stantialproportionofthedifferencesinheightatcentreofgravity, butover-predictedthemeasurementbyabout6%onaverage.The mostlikelyexplanationfortheoverestimateisnon-uniform dis-tributionofthedrymatteralongtheshoot,withmoreatthebase

thanthetop(Berryetal.,2004).

Regressionanalysisperformedwithdatafromallplots mea-suredin2012and2013showedaninversecorrelationbetween thenaturalfrequency(nn)ofthemainshootwiththemeasured

heightatcentreofgravity(Fig.2a)resultinginEq.(12).Theeararea (includingawns)wasshowntocorrelatewiththefreshweightof theearresultinginEq.(13)(Fig.2b).

nn=0.7x−1.4+0.3 (12)

A=9.95+1.02EW (13)

Hence,thefurtherdevelopedEqs.(8)–(13)forspringwheathave demonstratedhowplantheight,harvestindex,shootspersquare metreand yieldcanbeusedtocalculatelodgingmodelinputs; heightatcentreofgravity,natural frequencyandeararea. Suc-cessively,theseparameterscanbeusedinEq.(3)toestimatethe effectofchangestoplantheight,harvestindexandyieldonthebase

0.0 0.2 0.4 0.6 0.8 1.0 0 5 10 15 20 25 30 A nn ua l pr o b ab ili ty

Wind gust speed (m s-1₎

Fig.3.North-WestMexicoseasonalmaximumwindgustprobabilitiesfor50days ofhighstemlodgingrisk()andfor10daysofhighrootlodgingrisk(whensurface soilhorizonismoist)(×).

bendingmomentofasingleshootandthewholeplantonspring wheatgenotypes.

3. Results

3.1. Windgustspeedprobabilities

Theprobabilitiesforexperiencingwindgustspeedsattheheight ofthecropduringstemandrootlodgingriskperiodsatNWMare summarisedinFig.3.

AnnualwindgustspeedreturnperiodsfortheNWM

environ-ment(Table2)showthattheplantmustwithstandawindgustof

22ms−1tohaveastemlodgingreturnperiodof25years,andit mustwithstandawindspeedof18ms−1tohavearootlodging returnperiodof25years.Forareturnperiodof10yearsthecritical windspeedsthatmustbewithstoodfallto19ms−1and16ms−1 forstemandrootlodging,respectively.Consideringlodgingreturn

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268 0 50 100 150 200 250 300 350 400 450 0 5 10 15 20 25 30

Lodging return period

S ho ot le ver age ( N mm )

a

448 0 100 200 300 400 500 600 700 0 5 10 15 20 25 30

P la n t le v era g e (N mm )

b

Fig.4.Shootleverage(a)andplantleverage(b)foracropyielding6tha−1_with_a_crop_height_of_1.0_m₍₎_and_0.5_m_(䊉),_and_crop_yielding₁₆_t_ha−1_(♦)_and₄_t_ha-1₍₎

withacropheightof0.7,fordifferentlodgingreturnperiodsintheNWMenvironment.DottedlineindicatesaNWMwheatcropwithcurrentaverageyieldof6tha−1and putativeminimumcropheightcompatiblewiththisyieldof0.7m.

Table2

SeasonalwindgustspeedreturnperiodforObregon.

Windgustreturnperiod(years) Windgustspeed(ms−1)

stemlodgingrisk rootlodgingrisk

5 18 14

10 19 16

15 21 17

25 22 18

periodastheperiodoftimebetweenlodgingeventsortheperiod oftimebetweentwowindgustpeaksof22ms−1(stemlodging) and18ms−1(rootlodging)ifweusealodgingreturnperiodof25 years(comparablewiththeUKlodgingresistantideotype).Berry

etal.(2004)deﬁnedlodgingproofnessas:“thestructurethatcan

withstandthestrongestwindlikelytooccuroveracerealcroponce everygeneration”.

3.2. Calculatingthelodging-proofideotype

The maximum wind induced shoot and plant leverages for crops with a range of crop height and yield have been calcu-latedforlodgingreturnperiodsof1yearin5–1yearin25(Fig.4) usingEqs.(3)and(6)–(11).Thewind-inducedshootbase bend-ingmoment(leverage)(Nmm)calculatedforthemaximumwind speedexpectedduringtheentire50daystemlodgingriskperiod representtheminimumfailuremoment(stemstrength)ofthestem base(Nmm)for supportingtheshoot.The wind inducedplant leveragecalculatedforthewindspeedsexpectedduringthe10day rootlodgingperiodrepresenttheminimumstrengthofthe anchor-ageforsupportingalltheshootsof asingleplant.Tocarry out thesecalculationsitwasassumedthatthecropshad500shoots perplantand200plantsm−2whicharetypicalfortheYaqui Val-leynearObregon.Theideotypecropwiththecurrentaverageon farmyieldof6tha−1(at12%moisture)fortheNWMenvironment

(FischerandEdmeades,2010),andminimumcropheightof0.7m

thathasbeenobservedtobecompatiblewithhighyield,musthave astemstrengthequivalenttothe268Nmmofshootleverageand anchoragestrengthequivalentto448Nmmofplantleverage.If cropyieldandplantheightareincreasedto10tha−1and1.0m, respectively,thenthestemstrengthrequiredmustbeequivalent

to480Nmmofshootleverageandanchoragestrengthequivalent to803Nmmofplantleverage.

Thesizeoftherootplaterequiredtoavoidlodgingforarangeof croptypesandlodgingreturnperiodshavebeencalculatedusing Eq.(2).Stemmaterialstrength(␴)wascalculatedusingEq.(14)

wherestemwallwidth(t)wasconstant(0.65mm)andthestem radius(ɑ)andstemstrength(Bs)weredeﬁnedbythemaximum windgustofeachlodgingreturnperiod(Table2).Aminimumstem wallwidthof0.65mmwasassumedbecauseitisunderstoodthata thinwalled,butwide,cylinderisthebestwayofachievingstrength fortheminimuminvestmentofdrymatter(Berryetal.,2007)and 0.65mmwasthethinnestwallwidthobservedinthespringwheat experiments. Bs= a₄3

1−

a−_at

4

(14) Thetargetrootplatespreadrangedfrom43.2mmfora0.7m tallcropyielding6tha−1withalodgingreturnperiodof5years,to 62.1mmfora1.0mtallcropyielding10tha−1withalodgingreturn periodof25years.Thesecalculationsassumedthesoilwasrolled aftersowingtoconsolidateit.A0.7mtallcropyielding6tha−1 withalodgingreturnperiodof5yearswouldrequireastem diam-eterof4.04mmwithamaterialstrengthof35MPaoralternatively therequiredstrengthcouldbeachievedwithastemdiameterof 3.51mmwithamaterialstrengthof50MPa.A1.0mtallcrop yield-ing10tha−1withalodgingreturnperiodof25yearswouldrequire astemdiameterof6.09mmwithamaterialstrengthof35MPaor 5.24mmwithamaterialstrengthof50MPa(Table3).

3.3. Biomassandfailuremomentofstemandanchoragesystem Apositiveregression(R2₌_0.63;_P_<_0.001)_was_found_between

thestructuralstemdrymatterperunitlengthandinternode fail-uremomentforinternodes1–2(27 cultivars,2011–2013and5 cultivars,2014)andinternodes3–4(5cultivars,2013and2014). Accordingtothisregressionmodelwhere theresponsevariable wastheinternodefailuremoment(stemstrength),aﬁttedvalue of100Nmmin this parametercouldbeachievedwitha struc-turalstem dryweightperunitlengthof1.13mgmm−1 or with 1.53mgmm−1 of structural plusWSC stemdry weight (Fig.5).

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Table3

NWMideotypetraittargetsfordifferentlodgingreturnperiods.

Character Lodgingreturnperiod(years)

5 10 15 25

0.7mtalland6tha−1

Rootplatespread(mm) 43.2 47.3 49.2 51.1

Internodediameter(mm)a _4.04 _4.23 _4.58 _4.76

Internodediameter(mm)b _3.51 _3.67 _3.97 _4.12

0.7mtalland10tha−1

Rootplatespread(mm) 46.1 50.4 52.5 54.5 Internodediameter(mm)a _4.38 _4.58 _4.97 _5.16

1.0mtalland6tha−1

1.0mtalland10tha−1

*Allcropsassumedtohaveastemwallwidthof0.65mm.

a_Material_strength_of₃₅_Mpa. b_Material_strength_of₅₀_Mpa.

50 100 150 200 250 300 1 2 3 4 In te rno de f ail ur e m o m en t (N mm )

Internodedry weight per length(mgmm-1₎

Fig.5.Dryweightperunitlengthplottedagainstinternodefailuremomentof internode1(diamonds),internode2(circles)for27genotypes(2011–2013 exper-iments)andofinternodes1,internode2,internode3(triangles)andinternode 4(squares)forfivegenotypes(2013and2014experiments).Openfigures indi-catestructuraldryweight(y=103x−16.8;R2₌_0.63;_P_<_0.001)_and_closed_figures

indicateoveralldryweight(y=76.2x−16.3;R2₌_0.64;_P_<_0.001).

TherewasnoassociationbetweenWSCcontentandinternode fail-uremomentforinternodes1–2(2011–2014)andinternodes3–4 (2013and2014)(R2₌_0.009).

Regardingtheanchoragesystemtherewasapositive relation-shipbetweenrootdryweightperplantandrootplatespreadamong 27genotypeswhichhadaconsistentslopeacrossyears2012and 2013of0.038mmmg−1,butdifferentyaxisinterceptsof24.7and 35.6mm,respectively,andanR2_of_0.74_(P_<_0.001)_for_the

regres-sionmodel(Fig.6).Regressionanalysisonthisassociationfor2011 showeda ﬁttedlinewithaslope of 0.011mmmg−1 and y-axis interceptof28.8mmandanR2_of_0.18_(P_<_0.05).

Experimentsin2013and2014includedascreeningofﬁve geno-typeswhichwereevaluatedforinternodefailuremomentwithand withouttheleafsheath.Analysisofvarianceshowedthatremoving theleafsheathsigniﬁcantlyreducedtheinternodefailuremoment by8Nmm,23Nmm,32Nmm,31Nmmand47Nmmfor

intern-25 30 35 40 45 50 55 0 100 200 300 400 500 600 700 R o o t pl at e spr ea d (mm )

Rootdry weight per plant(mg)

Fig.6.Surfacerootdryweightperplantplottedagainstrootplatespreadof27 springwheatgenotypes.Parallelmodelfor()2012experiment(y=0.038x+24.7) and()2013experiment(y=0.038x+35.6);R2₌_0.74;_P_<_0.001)._Expreiment_(×)

2011showedaregressionlineofy=0.011x+28.8withanR2_of_0.18_(P_<_0.05).

odes1–5,respectively(4,12,19,19and34%,respectively).This variation was statistically signiﬁcant(P<0.05) for internodes2 (SED=5.55),3(SED=4.97),4(SED=4.81)and5(SED=4.38). Differ-encesbetweencultivarswerefoundforallinternodes(SED7.94, P<0.001)andtherewerenosigniﬁcantinteractionsbetween cul-tivarandleafsheathtreatments.

3.4. Quantifyingstemandrootbiomassrequirementsofa lodging-proofwheatcrop

The amount of structural stem and surface root dry matter requiredtoresistlodgingfora rangeofcroptypesandlodging returnperiodsaredescribedinFig.7.Thestructuralstembiomass requiredtoachievespeciﬁedlodgingreturnperiodswasﬁrst esti-matedforeachindividualinternodeusingtheempiricalequation y=103x₋16.8fromFig.6for2011–2014data,wherethe“y”value wastheleverage exertedatthebaseofeachinternodeand the “x”valuewasthestructuraldryweightperunitlength.The

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struc-0 1 2 3 4 5 6 7 8 9 10 11 0 10 20 30

St em d ry w ei gh t (t h a -1)

a

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 0 10 20 30

R o ot dr y w ei g ht ( t ha -1)

b

Fig.7. Stem(a)androotdryweight(b)foracropyielding6tha−1_with_a_crop_height_of_1.0_m₍₎_and_0.7_m_(),_and_crop_yielding₁₀_t_ha−1_with_a_crop_height_of_1.0_m_(♦)

and0.7m(䊉),fordifferentlodgingreturnperiodsintheNWMenvironment.DottedlineindicatesaNWMwheatcropwithcurrentaverageyieldof6tha−1_and_putative

minimumcropheightcompatiblewiththisyieldof0.7m.

turaldryweightofeachinternodewascalculatedbymultiplying thedryweightperunitlengthbytheinternodelength.The struc-turaldryweightofthewholestemwascalculatedbysummingthe dryweightsofallﬁveindividualinternodes.Thetargetstructural stembiomassrangedfrom2.78tha−1fora0.7mtallcropyielding 6tha−1 withalodgingreturnperiodof5yearsto10.1tha−1 for a1.0mtallcropyielding10tha−1withalodgingreturnperiodof 25years.Rootbiomasswasestimatedbyﬁrstcalculatingtheroot platespreadrequiredtowithstandthewindinducedplant lever-age,thenusingempiricalequationy=0.038x+30.2fromFig.6for 2012and2013data,wherethe“y”valueistherootplatespread andthe“x”valueisthesurfacerootbiomassperplant.Thetarget rootbiomassrangedfrom0.69tha−1fora0.7mtallcropyielding 6tha−1withalodgingreturnperiodof5yearsto1.68tha−1fora 1.0mtallcropyielding10tha−1withalodgingreturnperiodof25 years.

3.5. Applicabilityofthelodgingmodel

Thewinterwheatlodgingmodelhasdemonstratedsigniﬁcant accuracytopredicttiming andamountof lodging(Berryet al.,

2003b).However,itsapplicabilityforspringwheathasnotbeen

tested.Theexperimentof2011experiencedenoughnaturallodging totestthelodgingmodeldevelopedforspringwheatby compar-ingtheseverityofnaturallodgingagainstthepredictedlodging risk calculated by themodel. An index for natural lodging for eachcultivarwascalculatedbysummingthepercentageoflodged area(recordedonceortwicea weekduringthelodgingperiod) betweentheﬁrst occurrenceand harvest. Themodel predicted lodgingsusceptibilitywascalculatedbyinputtingthevaluesofthe lodging-associatedcharactersintothemodelandcalculatingthe meanvalueofthestemandrootfailurewindspeedwhichranged 7.6–11.7ms−1.Lodgingoccurredduringearlytomid-grainﬁlling on35cultivarswhereas28werepredictedbythemodel;lodging wasabsentin25genotypesand12werepredictedbythemodel. Consideringthis,fromthetotalof60cultivarsthemodelcorrectly predicted40genotypesforeitherabsenceorpresenceoflodging givingapercentageofcorrectpredictionsof67%.Fig.8isshowing areasonablecorrelationbetweenobservedandpredictedrankings forcultivarlodgingresistance.

0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35

Experimentalranking

Mo d el r ank ing

Fig.8. Predictedandexperimentalrankingofspringcultivarsforlodging suscepti-bilityunderNWMenvironmentduring2011.Rankingisinascendingorder.(—)1:1 line.Bestﬁtline,y=0.60x+6.52;R2₌_0.35_(P_<_0.001).

4. Discussion

4.1. Comparisonofresultswithpublishedliterature

WindspeedanalysisforNWMenvironmenthasdemonstrated that springwheat growingin theseconditions must withstand 22ms−1and18ms−1toresiststemandrootlodging,respectively. ThisindicatesthatspringwheatwillrequirestrongerstemsthanUK winterwheat(18ms−1forstemandrootlodgingrisk)(Berryetal., 2007)andsimilaranchoragestrengthtosupportplantswiththe sameheightandyieldandhavelodgingonlyoncein25years. How-ever,severaldifferencesbetweenspringandwinterwheattypes mustalsobeconsidered.Ithasbeenfoundthatforaheightat cen-treofgravityof0.5mspringwheathadagreaternaturalfrequency of1.5Hzcomparedwithabout1.0Hzforwinterwheat(Berryetal., 2004).Typicalearareaforspringwheataveragedabout19cm2

comparedwith12cm2_for_winter_wheat_with_slightly_greater_yield

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Table4

Springwheatgenotypicrangeforthelodgingkeytraits(Pi ˜nera-Chavezetal.,2016).

Trait Geneticrange

Diameter(mm) 3.35–4.47

Wallwidth(mm) 0.64–0.92

Internodefailuremoment(stemstrength)(Nmm) 134–252

Materialstrength(MPa) 27.4–59.4

Rootplatespread(mm) 34–42

Height(m) 0.73–1.07

*Lodgingprobabilityof1in25years,200plantsm−2_,₅₀₀_shoots_m−2_and_grain_yield

of6tha−1_.

winterwheatexplainsatleastpartofthisdifference.Overallthese differencesmeanthatspringwheatwillhaveagreaterleverage thanwinterwheatforcropswiththesameheight,yieldandears m−2(springwheat=383Nmm;winterwheat=297Nmm).

Stemfailuremomentvaluesforthebottominternoderanged

from134 to252Nmm (Table 4).These values are higher than

therangeofthestemfailuremomentmeasuredincultivartrials carriedout onwinterwheatin theUKwhich rangedfrom122

to230Nmm (Berry et al.,2003b, 2007).Thisstudy hasshown

astrongrelationshipbetweenstemstrengthandstructuralstem biomassandbetweenanchoragestrengthandsurfacerootbiomass. Ifthereislimitedscopetoreducelodgingriskbyfurthershortening cropsthenstemstrengthandanchoragestrengthwillneedtobe increased,andthismayhaveasubstantialbiomasscostthatwill competeagainstgrainyieldformation.Apreviousstudyonwinter wheat(Berryetal.,2007)estimatedthatastemdryweightperunit lengthof1.65mgmm−1wasrequiredtoachieveastemstrength of100Nmm.However,theestimatedbiomassrequiredforstem strengthincludedbothstructuralandwatersolublecarbohydrate (WSC)andmaythereforehaveover-estimatedtheamountof struc-tural stem biomass required for stem strength. In the present study,WSCcontentwasnotrelatedtothestemstrengthwhereas structuralbiomass(composedmostlyoflignin,celluloseand hemi-cellulose)wasstronglyand positivelyassociated withthestem strength.Knappetal.(1987)statedthatlodgingcouldbenotrelated toﬂuctuationsinWSCandstructuralcarbohydratescontent.Onthe otherhand,Ma(2009)foundthewheatgeneTaCM(involvedin ligninbiosynthesis)wasassociatedwithstemstrengthand lodg-ingindexandWiersmaetal.(2011)foundapositiveassociation betweenlodgingresistanceandaciddetergentlignin(ADL)whilst

Wangetal.(2012)proposedthatcelluloseplaysanimportantrole

intheabilityofwheatstemstoresistlodging.Thereistherefore lit-tleevidencetosuggestthatWSCcontributestostemstrength.The presentstudymeasuredthat1.13mgmm−1ofstructuralbiomass isrequiredtoachieveastemstrengthof100Nmm.Ifitisassumed thatspringandwinterwheathavesimilarstembiomass/strength properties,thenthisindicatesthatthestudyofBerryetal.(2007)

mayhaveover-estimatedthestembiomassrequiredtoachieve speciﬁcstrengthtargetsby40–50%.

Rootbiomassperplantin thetop10cmofsoil rangedfrom about200–500mgperplantandarootplatespreadof30–55mm (Fig.6).Thisiswithinasimilarrangetoastudycarriedoutinthe UKwhichobservedasurfacerootbiomassof100–400mgperplant andarootplatespreadof25–45mm.Bothstudieshadsimilarplant populationsofcloseto160–180plantsm−2.Thisstudyhasshown thatbreedingfora widerrootplatewillrequiregreater invest-mentinrootbiomassinthetop10cmofsoil.Inordertoincrease rootplatespreadby10mmanadditional263mgofsurfaceroot biomassperplantwasrequired.Fortheaverageplantpopulation (estimatedat163plantsm−2),thisequatestoanadditionalsurface rootbiomassofapproximately0.43tha−1 toincrease rootplate spreadby10mm.Thiscompareswithawinterwheatstudy car-riedoutintheUKwhich,forasingleﬁeldexperiment,estimated

anadditional0.28tha−1ofsurfacerootdrymattertoincreasethe spreadoftherootplateby10mm(Berryetal.,2007).This compar-isonindicatesthatspringwheatgrowninNWMenvironmentmay requireagreaterinvestmentinadditionalsurfacerootbiomassto widenitsrootplatethanwinterwheatgrownintheUK.Thiscan beaconsequenceofdifferencesinspeciﬁcrootweight(dryweight perunitlength)relatingto‘rootthickness’betweenUKwinterand NWMspringwheat(unfortunatelynotmeasuredforthelatter). Variationinrootbiomasshasbeenfoundtobeaconsequenceof secondarythickeningoftheupperpartsofroots(Berryetal.,2007).

4.2. Implicationsofachievingalodging-proofplant

It hasbeenestimatedthat toachieve a 1in 25yearlodging returnperiodforatypicalspringwheatcropgrownintheNWM environmentyielding6tha−1 (at12%moisture)withaheightof 0.7m will require approximately 3.93tha−1 of structural stem biomass. Unpublished data fromexperiments described in this studyin2011–2013showsthatonaverageanadditional0.80tha−1 ofbiomassisrequiredfortheleaflaminaandsheath,andthechaff tograindryweightratioof0.22(Pi ˜nera-Chavezetal.,2016)givesa chaffdryweightof1.16tha−1.Thisgivesatotalnon-grainbiomass of5.89tha−1.Strawyieldsofupto6tha−1 orjustover6tha−1 have beenobserved in NWM (Pi ˜nera-Chavezet al., 2016). This ideotypewouldrequireasurfacerootbiomassofapproximately 1.10tha−1 which wasnot beenachievedbyanycultivarinthis study.Itthereforeappearsthatforthisideotypeitshouldbe pos-sibletoachievestembiomassrequirementsbutnotrootbiomass requirementswithcurrentgermplasm.TheNWMenvironmentcan supportgreateryieldthantheaverage6tha−1currentlyachieved and couldbeupto9tha−1 (Fischerand Edmeades, 2010).It is estimatedthatcultivarsyielding10tha−1(withaheightof0.7m) will require greater above-ground non-grain biomass equating to4.67tha−1 (assuming nochangeinleafand leafsheath)and 1.28tha−1 of surface root biomass to achieve a lodging return periodof25years.Itshouldfurtherberecognisedthatthe breed-ingprogramatCIMMYThasincreasedtheplantheightofwheat to1.0moraboveintheperiodof1966–2009(Aisawietal.,2015), wheretheraisedbedplantingsystemmayfavortallercrops bet-teratcapturingthelightinthegapsbetweenthebedsearlyin theseason(Fischeretal.,2005).IntheUK,Berryetal.(2014)has shownthatbreedershavenotshortenedvarietiessincethe1990s. Thisindicatesthatachievingahighyieldandashort(0.7mtall) cropmaybechallenging.Ifayieldof10tha−1canonlybeachieved witha1.0mtallcrop,thentheabove-groundnon-grainbiomass requirementincreasesto10.1tha−1andthesurfacerootbiomass to1.68tha−1.Thesebiomassrequirementswillbeverychallenging tomeetandillustratethatbreedersmustbreednotonlyforgreater totalbiomass,butalsothisbiomassmustbeoptimizedcarefullyto maximisestrengthperunitofbiomass.Certainly,itwillbe possi-bletoincreasetotalbiomassasshownintheUK(Shearmanetal.,

2005)andinNWM(Aisawietal.,2015).Recently,severalstudies

haveidentiﬁedQTLsthatcouldbeusedtoincreasebothyieldand strawbiomass(above-groundbiomass)inwheat(Berryetal.,2008;

Lietal.,2014;Xuetal.,2014).Othercerealssuchasricehavealso

shownQTLsrelatedtobothyieldandstrawbiomass(Sujietal., 2012).Optimizinghowtheadditionaldrymatterispartitionedto maximiseitsusefulnesswillbeveryimportant.Targetsfor improv-ingtheefﬁciencywithwhichnon-grainbiomassisusedinclude; maximizingstemstrengthperunitofstembiomass,maximizing grainweightto earweightratio, minimizingtheproduction of infertiletillersandachievinghighyieldswithshortercrops. Breed-ingforwiderstemsseemstobethemostefﬁcientwaytoincrease thestemstrengthwhichtogetherwithareducedleveragegivenby ashorterplantrepresentsastrategicoptiontominimizestructural

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biomassrequirements.Additionally, breedingfor morecompact ears(unawned)inspringwheatcouldfurtherreducethisleverage andconsequentlyreducingmorethebiomassrequirements. How-ever,carefulmustbetakenbecauseawnedearshavebeenrelated todroughtandheatresistance(Blum,1986).

Drymatterharvestindicesfor theselodgingproofideotypes equateto0.46fora0.7mtallcropyielding6tha−1,0.54fora0.7m tallcropyielding10tha−1and0.41fora1.0mtallcropyielding 10tha−1.Theseﬁguresaresomewaybelowtheestimatedpotential harvestindexforwheatof0.62(Austin,1980).Winterwheatgrown intheUKwithayieldof8tha−1andheightof0.7mwasestimated tohaveaharvestindexof0.42(Berryetal.,2007).However,itis likelythatthisstudyover-estimatedthestembiomassrequirement byincludingwatersolublestemcarbohydrateinthestembiomass measurements.Ifthesamerelationshipbetweenstemfailureand structuralstemweightobservedforthispresentstudyforspring wheatisusedforwinterwheat,andaleafandleafsheathbiomass of1.0tha−1isincluded,thenthisgivesaharvestindexof0.49.The relativelyhighlevelsofnon-grainbiomassandlowharvestindices thatareestimatedtoresultfrombreedingcropswithalodging returnperiodof25yearssuggestthatthehighinvestmentin non-grainbiomassmaycompetewithyieldformationandlimittherate ofbreedingimprovement ingrainyield.Thispotentialtrade-off arisesfromtheoverlappingofthedevelopmentperiodsoflodging traitsandkeyyield-determiningprocessessuchasﬂoret develop-mentandproductionofwatersolublereserves.Infact,Slaferand

Rawson(1994)statedthatalltheprocessesincludedfromGS30to

GS60(Zadoksetal.,1974)areconsideredofmajorimportancefor

yieldconstruction.Crooketal.(1994)describedthedevelopment ofthelodgingcharacters(stemandrootstrength)fromtillering (GS20)untilmaturity(GS87)andconcludedthatthesetraitsceased developsoonafteranthesis(GS65).Theremaybeanetyieldbeneﬁt fromacceptingashorterlodgingreturnperiod,sincetheadvantage oflowernon-grainbiomassinvestmentonyieldpotentialmay out-weighyieldlossesfrommorefrequentlodging.Ifthelodgingreturn periodisreducedfrom25yearsto10yearsthentheharvestindex increasesfrom0.46to0.51foracropyielding6tha−1withheight of0.7m, increasesfrom0.54 to0.58fora 0.7mtallcrop yield-ing10tha−1,andincreasesfrom0.41to0.46fora1.0mtallcrop yielding10tha−1.

Thispaper showeda significanteffect of theleafsheath on thestemstrengthmeasured20daysafterGS65oninternodes2–5 (peduncle)wherethepresenceoftheleafsheathincreasedstem strengthby12%forinternode2toanincreaseof34%onthe pedun-cle.Theeffectoninternode1wasnotsignificantbecausetheleaf sheathwasmostlysenescedornotpresentatGS65+20d.Theleaf sheathhasbeenreportedtohaveanimportantmechanical role ensuringtheplantstandingabilityinotherspeciesincluding; Arun-dinariatecta(Poaceae)(Niklas,1998),Poaararatica,Bromuserectus, Arrhenatherumelatius(Poacea),Luzulanivea(Juncaceae),Carex arc-tata(Cyperaceae) (Kempe etal., 2013)and Triticale (Zebrowski, 1992).Thesefindingsindicatethatleafsheathisamechanical com-ponentofthestemespeciallysoonafterflowering,however,its effectswilldiminishasthecropmaturesastheleafsheathdries andeventually fallsoff. Thisstudyhasestimatedthestructural requirementstoavoidlodgingforaplantatharvestwithout leaf-sheathssurroundingtheinternodes.Thisapproachisappropriate forplantsatharvestandislikelytobeappropriateduringafew weekspriortoharvestforthelowerinternodes,whichmost com-monlybuckle,andwhoseleaf-sheathssenescefirst.However,itwill probablyover-estimatethestemstrengthrequiredtoavoidlodging atearliergrowthstages(e.g.,atflowering)becausethe contribu-tionoftheleafsheathisnotincluded.Furtherworkisrequiredto quantifyhowthecontributionoftheleaf-sheathtothestrengthof eachinternodediminishesastheplantdevelopssothatthe

mini-mumstrengthofthetruestemrequiredforvariouslodgingreturn periodscanbemodelledmoreaccurately.

NWMspringwheatlodgingideotypetraitvaluesforatypical yieldcropwitha25yearreturnperioddifferfromtheequivalent UKwinterwheatlodgingideotypevaluesasfollows;springwheat requiresa10%smallerrootplateanda7%strongerstemstrength. Rainfallispracticallyabsentduringthelodgingriskperiodinthe NWMenvironmentandwatersupplyhastobeprovidedby peri-odicirrigation.Thisconditionreducestherootlodgingriskperiod to10daysofgrainﬁllingperiodwhich,inturn,reducesthe maxi-mumwindgustspeedrequiredtowithstandrootlodging.Drier,but windier,conditionsinNWMcomparedwiththeUKmeanthatboth springandwinterwheatideotypesmustwithstandthesame max-imumwindgustspeed(18ms−1)fora25-yearrootlodgingreturn period;however,yieldof8tha−1 fortheUKideotypecompared with6tha−1forNWMcontributetothegreaterrootplatespread requiredbytheUKideotype.Thegreaterstemstrength require-mentforspringwheatismainlyduetoahighermaximumwind gustspeedontheNWMenvironment(22ms−1)andthegreater earareaofspringwheat.

Thegeneticrangesforthekeylodgingtraitsaredescribedin

Table4andincompanionpaper(Pi ˜nera-Chavezetal.,2016).This

showsthatitshouldbepossibleforplantbreederstoachievesome oftheideotypedimensionsforaspringwheatcropyielding6tha−1 withheightof0.7m.Nevertheless,ifitisassumedthatyieldwill increaseinthefollowingdecadesthenthebiophysicaltargetswill increase.For example,ifyield isincreasedto10tha−1 thenthe stemdiameterwillincreaseby8%,rootplatespreadby6%andstem strengthby18%.Inthiscaseitwouldbeunlikelythatplantbreeders couldachievealodgingproofplantwithalodgingreturnperiodof 25yearswithcurrentgermplasm.Ouranalysisalsoshowedthatthe targetdimensionswillbefurtherincreasedifyieldimprovements mustalsobeaccompaniedbycropheightsofmorethan0.7m.

5. Conclusion

Testingofanadaptedlodgingmodelforelitespringwheatlines showedittobeusefultoolforrankingthesusceptibilityto lodg-ingofcultivarsundercrop,soilandweatherconditionsinNWM. Thishasenabledthecalculationofthetargetlodgingresistance traitsofthelodgingresistantideotypeforthisparticular environ-ment.Apositivestemandrootbiomasscorrelationwiththestem strength(internodefailuremoment)andanchoragestrength(root platespread)wasidentiﬁedwhichenabledthestructuraldry mat-terrequirementstobecalculatedforlodgingproofness.Ithasbeen establishedthatanyimprovementtoachievealodging-proofcrop thatlodgesonly onceina periodof 25years wouldrequirean increaseinthestembiomasswhichinturncouldimplya trade-offwithgrainyieldifimprovementofthelatterdependssolely onincreasingtheHI.Alternatively,decreasingoftheproportionof strawbiomasstothetotalabove-grounddrymatterwouldincrease theriskoflodgingunlessmoretotalbiomassismadeavailableto strawthroughincreasingRUE.Thisstudythereforeindicatesthat forplantbreederstoachievebothhighyieldsandlodging proof-nesstheymusteitherbreedforgreatertotalbiomassordevelop highyieldinggermplasmfromshortercropsof0.7morless.

Acknowledgments

Theauthorsacknowledgethesupportgiven byCIMMYTand SAGARPAthroughtheMasAgroInitiative,CONACYTforproviding themainauthorwithapostgraduatescholarship.Wearealso grate-fultotheWheatPhysiologyGroupfromtheGlobalWheatProgram atCIMMYT.

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AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,athttp://dx.doi.org/10.1016/j.fcr.2016.06.009.

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