Improvingwinegrowingwithsapflowdrivenirrigation–a10-yearreviewT.Scholasch1,a1FruitionSciences,Montpellier,France.Abstract

TheFullertonSapflowmeetingpresentedlotsofscientificevidenceshowinghowusefulsapflowmeasurementscouldbeforapplicationinhorticulture.Forthepast10years, our company, Fruition Sciences has been using real time sap flow data toimprove irrigation scheduling across many vineyard regions worldwide. However,despiteitssolidscientificbasis,convertingatheoreticalconceptintoafullyactionabledecisiontoolhasbeenachallenge.Thegoalofthisarticleistoexaminehowandwhyviticulture has adopted sap flow monitoring to improve irrigation strategy forcommercial applications. By dissecting historical steps leading to the successfulimplementationofsapflowinviticulture;itisillustratedhowitispossibletoovercomethechallengeofintroducinganewtechnologyinatraditionalindustry.Inthecontextofprecision farming, adopting sap flowmetricsmeansdisrupting traditionalhabitsandpre-existing statusquoon irrigationandvineyardmanagement. It is discussedhowunderstandingthewinemarketguidedstrategicdecisionsinordertoconvertascientific concept into a business opportunity. Through a few case studies it isdescribedhowsapflowmonitoringhasgainedpopularityinthewineindustryandwhyitcontributestoimprovingvineyardmanagementbeyondirrigationstrategies.Keyword:sapflow,viticulture,irrigation,berryripening,nitrogendeficitINTRODUCTION

Alotofresultshavebeenpublishedonsapflowmeasurementsperformedovermanydifferentcrops.Numerousstudieshavevalidatedtheuseofsapflowmeasurementtoimprovevinewaterusemonitoring.Inthiscontext,Iwouldliketodiscusswhatspecificchallengeshadtobeovercometodisseminatetheuseofsapflowinthewineindustry.Becauseadoptingsapflowmetrics disrupts traditional irrigation practices, it requires an additional effort fromvineyardmanagersandwinemakers.Assuch,asitistypicallythecasewhenintroducinganinnovation,aselectedgroupofearlyinnovatorswastargetedatfirst.

Unless a new legal framework challenges traditional habits, adopting changes invineyardmanagementpracticesisaslowprocess.Forinstance,sinceirrigationhasbecomelegalin2006insouthernFrance,manyvineyardshaveinvestedinirrigationtechnologies.Adrasticchangehasbeenadopted,eveniftherewasnopreviousexperienceonhowirrigationcouldmodulate vineyard historical performances. Thus, when legal incentives exist, newtechnologiescanberapidlyadoptedwhiletheireffectintheshortorinthelongtermmaybeunknown.

Bycontrast,thereisnolegalincentivetopromotetheadoptionofsapflowtechnology.In this context, a creative strategy had to be developed to promote the use of sap flowmonitoring.Whatwerethosesteps?Howandwhywasitpossibletodisseminatetheadoptionofsapflowinthewineindustry?

aE-mail: thibaut@fruitionsciences.com

Whywinemakersareinterestedinsapflowtechnology1.Winescoresandvintageeffect.

Some features intrinsic to the wine industry provide winegrowers with specificincentives to adopt sap flow technology. First, amongwine consumers andwinemakers ahistorical conception that seasonal vine water status variations relate to wine sensorialpropertiesandthuswinepricingalreadyexists.Theeffectofone-seasonclimaticconditionson wine composition is called the “vintage effect” by winemakers. For instance, drierwinegrowingconditionsinBordeauxhavetypicallybeencorrelatedwithhigherwineprices.Thus,underoceanicclimate,dryseasonsareoftenexpectedtohaveapositive“dryvintage”effectonwine.Second,numerousstudieshavereportedhowvinewaterdeficitvariationscanaffectpositivelyfruitandwinecompositionsunderdifferentclimates.Third,followingeachharvest,winecriticsappreciatethe“vintageeffect”byattributingwinescores.Seasoneffectonvineperformance,andparticularlyvinewateruse,isexpectedtoinfluencewinesensorialpropertiesandwinescores.

For those reasons, winemakers are naturally interested by innovative methodsrevealing the footprintofaseasononvinewaterusevariations.Thewineconsumersandcriticsarealsoreceptivetosapflowasawaytoreflectthevintageeffect.2.Highlightingvineyardoriginality.

Wine pricing is very dependent upon the originality of the production site. Winecomposition, suchas concentrations inanthocyaninor tannins isdirectlyaffectedby fruitcomposition, which in turn is affected by viticulture practices. Many scientific articlesdescribehowdifferentirrigationregimesmodulatefruitanthocyaninaccumulationorwinecolorationdensity.Thus,knowinghowtooptimizeseasonalvinewateruseprofilethroughaprecisemonitoringtechniquecanbeastrategicadvantageforthewinemakersinceitdirectlyeffectswinecomposition.Winegrowercanalsousesapflowvariationsoveraseasontorevealaunique“plantsignature”attachedtoauniqueplace.Thetimeprofileofvinewaterusecanbeinterpretedasasite-specifictraithighlightinganoriginalvineyardcontextandframingfruitripeningconditions.Byrecordingcontinuousvinewaterusevariations,winemakerscanalso track the effects of contrasted farming strategiesonvineperformance.Afterharvest,winemaker can “taste” the effects of contrasted vine water deficit profiles on winecomposition.

Analysisofvinesapflowvariationsprovidesanopportunitytocreatealinkbetweenwineconsumersandsite-specificconditionsleadingtowineproduction.Thisinformationisusefultodistinguishwhyawineisdifferentfromothersinacompetitivemarket.Aniconicwinemaker,auniquevineyard topographical featureoranoriginalhistorycan imprintan“inimitable”charactertoawinebottle.Similarly,sapflowcontributestohighlightwhatmakesawineuniquebasedonthevineyarditissourcedfrom.3.Aglobalandcompetitiveindustry.

Onaglobalscale,anincreasingnumberofwinegrowingregionsaredevelopingundersemiaridandwarmerconditions.Theemergenceofnewwinegrowingregions(likeChina)increasesglobalcompetition.Withincreasedpressureonvineyardgrowingconditions,duetowaterscarcity,warmertemperaturesandincreasedwinemarketcompetition,agreaterneedtooptimizeviticulturepracticesarise.

As new vineyards are being developed and managed under higher temperatures,optimizingvinewaterusevariationsisbecomingastrategicpracticetomaintainvineyardfinancial sustainability. For instance, warmer and drier winegrowing conditions directlyimpactwinemakerdecisions: timingof harvest canbecomeearlier in response to greaterwaterdeficit;whendehydrated fruit isharvested,cellaroperationsmustadjust forhighersugarconcentrationleadingtohigherwinealcoholcontent.

Inconclusion:winemakersarenaturallysensitivetothebenefitsattachedtosapflowmonitoring. It isanopportunity to improve fruitandwinecompositionvia improvedvine

water statusmanagement. Sap flowprofileshighlight site andvintage specific effects andprovidesanopportunitytoadjustfarmingstrategiesinacontextofclimaticchanges.Assuch,winemakers welcome sap flow technology to improve wine production. Such features,specifictothewineindustryjustifywhyamongstmanyotherhorticulturalcrops,grapehasbeenprompttorevisitthestatusquoonirrigationpractices,evenwithoutlegalincentives.However,acompellingcasefromawinemakerstandpointisnotnecessarilyacompellingcasefromavineyardmanagerstandpoint.Howdidvineyardmanagersbecomeinterestedinsapflowtechnology?

In viticulture, theproduction cycle is slow. It takes2 to3 years afterplanting for avineyardtoproduceandthereisundermostwinegrowingclimatesonlyonefruitproductionperyear.Inthiscontext,therewardforoptimizingirrigationisfuzzy.

First,theopportunitytoimproveirrigationpracticesonlyappliesduringafractionoftheyear.

Second,imposingamoderatelevelofvinewateruseatkeystagesoftheseasonisnottheonlyfactorlikelytoimprovevineyardperformances.However,theriskfornotapplyingenoughwaterisanimmediatepenaltyforthevineyardmanager:fruitsdehydrate;yieldandmoneycanbelost.

Third,asavineyardisaperennialstructure,performanceimprovementresultingfromchanginganirrigationstrategymayneedmultipleseasonsbeforefruitquantityandqualityarepositively impacted.Authorshave reported that amountof fruitproducedoneyear isdeterminedbywaterstressfromthepreviousyear(Guilpartetal.,2014).Longtermstudiesofdifferentirrigationstrategiesandtheirconsequencesonperennialstructuressuchastrunkdiameterorrootmassdistributionwithsoildepthhaveshownthatvineyardperformancesare impactedby theeffectofoneseason to thenext. (EdwardsandClingeleffer,2013). Inpractice,vineyardshistoricallyirrigatedweeklyorbiweekly,areoftenshowingarootprofilepreferentiallydevelopedintopsoilhorizons.Roottipsaretypicallymoreconcentratedwithinthesoilsectionunderneaththedripper.Bygraduallyapplyingwaterlessfrequently,therootstructurewilleventuallybemodified,howeverthismaytakemorethanoneseason.Thus,duetoitsperennialnatureandthecarryovereffectofpastseasonsonyieldandonvineyardrootstructure,productionimprovementinresponsetosapflowdrivenirrigationmaybeslowtoobserveandspanoverseveralyears.Fromthatpointofview,adoptingsapflowtechnologymaylooklikeaslowreturnoninvestment.

Fourth, challenging the status quo on irrigation will impose deep modifications inlogisticsandoverallvineyardmanagement.Forinstance,sapflowdrivenirrigationusuallyleads to the application of larger volumes of water less frequently. This shift in waterallocationprogramsacrossmultiple vineyardswillmodifywaterpumping regimes,waterdistributionschedulingandvineyardworkerstaskhours.Sapflowdrivenirrigationwillalsochallenge vineyard managers’ own expertise particularly when irrigation is traditionallydecidedbasedonvisualobservations.Itmaybedisconcertingatfirsttoacceptthatvinevisualsymptoms can be misleading. When higher sap flow rates are measured under drieratmospheric conditions (i.e., high vapor pressure deficit) it indicates no soil moisturerestrictionevenifleavesarefoldingawayfromthesun.Consequently,triggeringirrigationbased on leaf visual cues under dry air conditions is not justified when sap flow rate issimultaneouslyhigh.

Thoseobservationshighlightwhyvineyardmanagerscanbemoresensitivetotheriskandcomplicationsassociatedwithirrigationchangesinresponsetosapflowmonitoringthanitsrewardsonfruitandwinecomposition.Inthiscontext,10yearsago,theadoptionofsapflowdrivenirrigationmetalotofresistanceinviticulture,despiteitssoundscientificbasis.Consequently,sapflowmonitoringwasimplementedatfirstbyearlyadoptersinsituationswherewinemakershadastronginfluenceonvineyarddecisions.

Fromascientificconcepttoabusinessopportunity1.Buildingascientificcaseforasapflowapproachinvineyards

Toconvinceearlyadopterstoimplementasapflowdrivenirrigationstrategy,aseriesoftrialsweredeployedoncommercialvineyards.Thegoalofthosetrialswastodemonstratethepotentialforimprovingirrigationbyevaluatingdifferentplantwaterstatusmonitoringtechniques,excludingsapflowatfirst.In2001bycollaboratingwithamajorCaliforniawinecompanyinNapatwosetsofpreliminaryexperimentswereperformed.

One experiment took place within an irrigated vineyard located in Napa Valley incollaborationwith Dr. ToddDawson (University of California Berkeley).Within the samevineyardat3distinctlocationsthroughthevineyard,watersampleswerecollectedfrom3sources(9samples total):a) fromthedripperduringan irrigationevent,b) fromthesoil,obtained at various depths along the root zone but outside thewet bulb created by dripirrigation, and c) leaf petioles detached one and two days after irrigation. After analysis,isotopic composition of water absorbed by the vine and found in the petioles was notmatchingisotopiccompositionofirrigationwater.However,isotopiccompositionofpetiolewater was closely matching (80%) isotopic composition of soil water coming from soilsectionsunaffectedbyirrigationwater.Theseresultsindicatedthatirrigationwaterwasnoteffective under real field conditions. Results also suggested that industry standards forirrigationbestpracticescouldbeimproved.

Asecondsetofexperimentswasdesignedattwovineyards,covering20acresinNapaValley.Thegoalwastoevaluatestemwaterpotentialandleafstomataconductancevariationsinresponsetoirrigationandheatwaves.Fortwoconsecutiveseasons,contrastedirrigationtreatmentswereappliedwithinthetwovineyardsandvinedatawerecollectedbiweeklyovermultiple locations. The differences between irrigation scheduling consisted of replacingeitherthetotalamountofvinewaterlostbetweentwoirrigation,oronlyafractionthereofusing an estimate of vineyard crop coefficient to compute crop evapotranspiration. Under well-watered irrigation treatment, vines were irrigated in excess of crop evapotranspiration (ETcrop computed using a single crop coefficient as described in Poblete-Echeverría andOrtega-Farias (2013)). Under regulated deficit irrigation treatment, the amount of waterappliedwascomputedsoastomaintainvinestemwaterpotentialwithinprescribedlimitsofdeficit.Simultaneoustotreatmentsapplication,ananalyticalframework(VSIMmodel,LarsPierce,CSUMontereyBay)wasusedtocomparemeasuredvinewaterstatusvariationswithsimulated output in response to climatic demand and irrigation. Themodeling approachconsistedofcombiningacanopycropcoefficientmodelwithasoilwaterbalancemodeltoestimatevinetranspiration,soilmoisturedrainageandwaterpotential.Afterthefirstseason,despite the monitoring of vine water status through modeling and stem water potentialmeasurements, a major crop volume loss (>50%) was observed following a heat waveoccurringinSeptember,afewdaysbeforeharvestandacrossallirrigationtreatments.

Aftertwoconsecutiveseasons,thecombinationoffieldandsimulateddatahighlightedtwomajordifficultiestoimprovethemonitoringofvinewaterstatusvariationsunderfieldconditions:a)theneedtoimprovevinebasalcropcoefficientestimatesbasedonvineactualtranspiration;b)theneedtounderstandbettervinetranspirationresponsetoheatwaveandirrigation.

Fromthepracticalstandpointofawinegrowerwillingtoimproveirrigationstrategies,thosepreliminaryresultshighlightedtheneedtoanalyzeindepthrelationshipsbetweenvinebasal crop coefficient, vapor pressure deficit, soil moisture content and vine water usevariations.The contextwas set to justify the implementationof vine sap flow tooptimizeirrigation.2.Creatingaproofofconcept

Followingpreliminaryresults,thedeploymentofalarge-scaleprojectwasdecided.Thegoalwastoevaluatehowsapflowvariationsmeasuredunderrealfieldconditionscouldbeusedasatooltorefinevineyardirrigation.Startingin2006andforthreeconsecutiveyears,fiveNapapremiumwineriesdeployedsapflowsensorsinCabernetsauvignonvineyards.The

fruitharvestedfromthesectionsundermonitoringwasusedtomakewineseparately.Thisprojectinvolvedthecollaborationwithseveralpartners.

Dr. Dennis Baldocchi and Dr. Laurent Misson (University of California Berkeley)providedaconceptualframework(howtorelatesapflowmeasurementstoeddycovarianceand large-scale models) and measurement tools (Licor 6400). By performing additionalmeasurementsonvinesequippedwithsapflowsensorsduringspecificevents(before,duringandafteranirrigation;beforeandafterveraison;underhighandlowvaporpressuredeficitconditions)strategicdatawereacquiredtorefineavinewaterbalancemodel.

DynamaxInc.graciouslysuppliedsapflowsensorsanddataloggers.Dynamaxsapflowsensor design consists of a heating sleeve wrapped around the vine. Heat is provideduniformlyandradiallyacrossthestemsection.Thestemheatbalancemethodforsapflowcomputationiseasytocomputeautomaticallyanditsprincipleeasytoexplaintofirsttimeusers.Thesensordoesnotrequiretheintrusionofaneedleintothevine.Byeliminatingtheneedforneedleintrusionintothestem,winegrowerconcernsregardingtheriskofinfectingthe vine and subsequently measuring sap flow from a non-representative vine waseliminated.Theprotocolofinstallationandmaintenanceofnon-intrusivesapflowsensorissimple to implement. The sleeve is flexible andmaintains a snug fit during stem diurnalcontractions.Thisturnsouttobecritical,assapflowsensorsmusttightlyfitoddshapedstemsections. Thanks to their design, sensors can be applied over stems slightly bent or evenpartiallynecroticas it issometimesobserved inresponsetopruning injuries.Becausetheentire stem section is heated, the heat balance method can be applied even if sap flowtrajectorythroughthestemistortuous.Forthosereasonstheselectionofnon-intrusivesapflowsensors,insteadofaneedle,waskeytofacilitatethedisseminationofsapflowdrivenirrigationstrategies.

To further investigate theeffectsofenvironmentalvariationsonvinesap flow, fieldmeasurement trialswerecompletedwithgreenhouse trials in thecontextofaPhDstudy(LEPSE-SUPAGRO,Montpellier,France).LysimetermeasurementswereperformedonpottedCabernetsauvignonvinessubjectedtocontrastedlevelsofsoilmoistureandvaporpressuredeficits. Soilmoisture levelswereexpressedas fractionof transpirable soilwater rangingfrom10%to100%asdescribedbyLebonetal.(2003).Vaporpressuredeficitlevelsrangedfrom1kPa to7kPa.Those targets imposedunder controlled conditionswere selected toreproducethesameenvironmentalconditionsobservednaturallyunderfieldconditionsinNapa,California.

3.Buildingabusinesscase:“Transformingsapflowintoactionabledata”

Bydiscussing the advances of the projectwithwinemakers and vineyardmanagersinvolvedinthestudy,theconditionsunderwhichabusinessmodelcouldbedevelopedwererefined.Tosolidifytheneedforsapflowtechnology,thepracticalbenefitsofsapflowdatabothforthewinemakingindustryandfortheviticultureindustryhadtobehighlighted.Fromawinemakerstandpoint,measuringvinewaterusevariationstobetterunderstandseasonspecificeffectonfruitripeningdynamicswasacompellingargument.Toconvincevineyardmanagers,alistoffivepracticalapplicationsduringtheseasonwasformulatedasfollows:#1:Assesswhetherthereisarealneedforirrigationunderheatwave

With sap flow monitoring, vineyard managers can revisit irrigation practices,particularlyunderheatwaves.Sapflowmonitoringwillchallengewaterpotentialthresholdvalue,asaplant-basedindextoassessvinewaterneeds.Duringaheatwave,increasedsapflowratecanberecordedwhilexylemwaterpotential(midday,stemorpredawn)decreases.Fromanirrigationstandpoint,thosetworesultsseemedtocontradicteachother.Inonehand,sapflowdatasuggestsnoneedforirrigationsinceanincreaseinsapflowrateindicatesnosoilmoisture restriction. In theotherhandadecrease inwaterpotential, canbewronglyinterpretedasaneedforirrigationwhenitisinfactcausedbyhighvaporpressuredeficit.

#2:OptimizetheamountofwatertoapplyVineyardmanagerscantrackthedurationofsapflowincreaseinresponsetodifferent

irrigationvolumes.Inturn,thisknowledgecanbeusedtorefinetheamountofwatertoapplyforthenextirrigation.Becausevineyardrootstructureisperennialandchangesoverseveralseasons, historical sap flow response to irrigation can be used to guide future irrigationstrategies.Whendifferentirrigationvolumesareappliedtoasamefullydevelopedvine,itispossibletoidentifywhichwatervolumemaximizesthedurationofirrigationeffect.Soilwaterholding capacities and root architecture impose anupper limit to themaximal amount ofwaterthatcanbeheldasplantavailableafterirrigation.Theanalysisofsapflowprofileinresponsetocontrastedwatervolumes indicates theupper limitabovewhichaddingmorewaterdoesnotprolongvineresponse to irrigation.Applyingwaterabove thisupper limitwouldbeawasteofwater.#3:Optimizethefrequencyofirrigation

Bytrackingsimultaneouslyclimaticdemandandvinewateruse,vineyardmanagerscandefineandreproducetheirpreferredirrigationstrategiesacrossmultipleseasons.Auserdefinedvinewaterdeficit indexthresholdcanbeusedasa triggertodetermine irrigationfrequencyobjectively.#4:Improveriskmanagementwithacontinuousplant-basedmeasurement

Unlikewaterpotential,whichasadestructivemethodcanonlyprovidediscontinuousmeasurements,sapflowprovidesacontinuousmeasurementinrealtime.Thisinstantplant–basedfeedbackcanbeuseda“safetynet”tomakeirrigationmanagementmoresecure.#5:Useanintegrativemeasurementreflecting“terroir”effect

Regardless of vineyard specificities (vine age, scion and rootstock material, trellissystem, soil properties, topography), sap flow remains fundamentally dependent onatmospheric conditions reflective of the “vintage effect”. But, beyond the effects ofatmosphericconditions,sapflowdynamicsalsointegratecomplexinteractionsbetweenvine,practicesandsoilproperties.Assuch,theso-called“terroir”effect,whichdescribescomplexinteractions between human and environmental factors impacting vine physiologicalresponse, is also characterized. Thus, outside from atmospheric conditions, sap flow alsoreflectstheimpactofpresentandpastpracticesaswellasvinedevelopmentalhistoryonvinewateruseregulation,whichisparticularlyrelevantwithaperennialcrop.

Bylistingthebenefitsattachedtovinesapflowanalysis,itwaspossibletoarticulatewhy a sap flow driven irrigation strategy can provide a competitive advantage forwinegrowing.Atthisstage,FruitionSciencesprojecthadsparkedwinemakersandvineyardmanagers’willingnesstoinvestintosapflowmonitoringmethod.However,acriticalpiecewasmissing inorder tomakesap flowmonitoringactionable in commercial settings.Themeasurementhadtobeimmediatelyandcontinuouslyavailable.Astrategicexpertisehadtobe incorporated into theproject toprovide sap flowmonitoring in real timeand createasustainablebusiness.4.Buildingatechnologicalframeworktoanalyzesapflowdataanduserfeedback

Tomakesapflowdataavailableinrealtime,ourcompanytestedthedeploymentofameshnetworkinthevineyardtocollectsapflowdata.Ourcompanyalsoconceivedaweb-application to analyze and display sap flow variations along fruit and climatic data. Thisframework became the central platform to discuss irrigation decisions and theirconsequenceswithwinegrowers.

Priortoanysapflowsensorinstallation,apreliminarydiscussionwiththewinegrowermustoccurtoselectthebestsamplinglocation(s)withinthevineyard.Vineyardtopography,historical practices and soil variations create an important spatial variability in vineyardwaterstatus.Consequently,asamplingscheme,accountingforvineyardspatialstructurehastobedefinedfirst.Bycombiningdifferentvineyard-mappingtools,usertestimonialsanda

spatialmodeltoextrapolatevinewaterstatus,thelocationofsapflowsamplingsiteswithinthe vineyard isdetermined.To simplify the analysis of sap flowvariations in response toenvironmentalconditions,anindexwascreated:the“WaterDeficitIndex”(WDI).

WDI=T/Kcb*ETref (1)Tisactualvinetranspiration;ETref iscomputedfromanearbyweatherstationusing

PenmanMonteith.Kcbisthebasalcropcoefficientaccordingtothenomenclatureofthedualcoefficientapproach(Allenetal.,1998).Thebasalcropcoefficientiscomputedas

Kcb=Tm/ETref (2)

where Tm is maximal transpiration measured under cultural conditions. Modelssimulatingseasonalchangesinbasalcropcoefficientareusedandadjustedaccordingtosite-specificdata.DynamicsofWDIareusedtointerpretsapflowvariationsanddiscusstheneedforirrigationaccordingly.

Inpractice,winegrowersaretoldthatwhenWDIisat100%,vineistranspiringtoitsmaximal level,whereaswhenWDI is at 0%, the vine is no longer transpiring.A few casestudiesarereportedbelowtoillustratehowsapflowandWDIarebeingusedinpractice.Casestudies1.Effectofincreasedwatervolumesonvineresponse

Researchers have shown that changes in irrigation strategy can change root sizedistribution across the root profile (Edwards and Clingeleffer, 2013). Thus, changes inirrigationvolumesandfrequencyhavetobe imposedgraduallyso thatwaterapplicationsmatchmodificationsinrootwateruptakeprofile.Inthisvineyard(NapaValley,California,cv.Cabernet sauvignon), the goal of thewinegrowing practices is to stimulate a higher rootdensity at greater soil depths. In this context, the purpose of the treatment is to changeirrigationstrategyfromfrequent(every3to7days)tolessfrequentirrigations.Toensurethatchangesinirrigationfrequencydonotincreasethelevelofwaterdeficitbetweentwoirrigations, sap flowandwaterdeficit indexvariationsweremonitored in real timealongthree seasons. Time intervals between two irrigations were increased by adding largeramountofwaterandtrackingsapflowresponse.Theamountofwaterappliedwasgraduallyincreasedfrom5mmforeachirrigation(year1),to8mm(year2),to8to12mm(year3).Totalamountofwaterappliedwas75mmforyear1,72mmforyear2,76mmyear3.Sapflowdatawascollectedoverthesamevinesforthethreeseasons.

Figure1showsseasonaldynamicsobservedinWDIyear1andyear2.WeobservethatWDIreacheshighervaluesfollowinglargerirrigationsappliedinyear2.UsingthesameWDIthresholdpriortoeachirrigation(around40%)andapplyinglargeramountofwaterwitheachirrigation,intervalsbetweenirrigationbecomelarger.Duringyear1irrigationisappliedevery3to7daysandduringyear2,every10to15days.Figure1showsthatirrigationeffectincreasesitslengthintimewhenlargerwateramountsareapplied.Furthermore,overtheperiod June to September,WDI seasonal average increases fromyear1 (<50%) to year2(around55%).Weobservethatapplyingthesameamountofwaterlessfrequentlyreducestheseverityofseasonalwaterdeficit.

Figure2comparesWDIseasonaldynamicsrecordedyear1andyear3.Whileasimilaramountofwaterisappliedoverseason1andseason3,seasonalWDIishigherduringseason3 (average WDI >60%). We speculate that applying larger irrigation volumes for 2consecutiveyearshas increased thedepthof root absorption sites.These changes in rootarchitecture probably contributed to the maintaining of higher seasonalWDI value (lesswaterdeficit)andamoreefficientuseofwater.

2.Effectofsapflowprofileonberrysugaraccumulation

Research has demonstrated that relationships exist between berry compositionalchangesduringripeningandseasonalvinewaterdeficitvariations(Delucetal.,2009).The

purpose of this treatment is to highlight the specific relationship between berry sugaraccumulation dynamics and seasonal vine water use. Conclusions from this study aim atimproving winegrower practices, such as irrigation, cover crop selection and canopymanipulation in order tomodulate berry ripening. In this treatment, sap flow andwaterdeficitindexvariationsarecollectedinanon-irrigatedvineyard,locatedinsouthernFrance,over 2 seasons (cv. Syrah). Thermal time (in °C.d) is computed by integrating hourlytemperaturesaboveaminimumtemperaturethresholdsetat10°C.

Figure1. Comparisonofwaterdeficit indexdynamics in response to increasing irrigationvolumebetweenyear1(5mmperirrigation)andyear2(8mmperirrigation).

Figure2. Comparisonofwaterdeficit indexdynamics in response to increasing irrigation

volumebetweenyear1(5mmperirrigation)andyear3(8to12mmperirrigation).

ClimaticcontextandWDIprofileanalysisPriortobudbreak,year1andyear2(November1st-February1st)aresimilarinterms

ofcumulatedrain(130mmforbothyears).BetweenFebruary1standJune1st,cumulativerainis210mmforyear1and330mmforyear2.FromMarch1sttoSeptember1st,1500°C.dwereaccumulated foryear1andyear2.Over theberry ripeningperiod (900-1500 °C.d),vaporpressuredeficitwasgreaterthan3.6kPafor13daysduringyear1;0dayduringyear2.

Asexpectedduringadrieryear,Figure3showsthatWDIreacheslowvalues(<40%)from900°C.donwardinyear1.Relativetotheclimaticdemand,thelevelofplantavailablewater is limited. Thus, seasonalWDI is relatively low,which also reduces photosyntheticactivity.However,duringawetteryear,WDIisconstantlyabove50%,whichfavorsahigherphotosyntheticactivityinyear2.

Figure3.Comparisonofwaterdeficitindexdynamicsinanon-irrigatedvineyard(year1:lessrainandhigherVPD;year2:morerainandlowerVPD).

Berrysugaraccumulationprofileanalysis

Twohundredberriesweresampledateightdifferentdatesoverasameareaof50vines,centeredon sap flowmonitoredvines.Figure4 shows thedynamicsof sugaramountperberry.Duringyear1,asalowseasonalWDIisbeingrecorded,initiationofsugaraccumulationislate(after1000°C.d).Themaximalamountofsugarperberryisreachedlate(after1500°C.d).Incontrast,duringyear2,asahighseasonalWDIisbeingrecorded,initiationofsugaraccumulationstartsearlier (before1000 °C.d).Themaximalamountof sugarperberry isreachedearlier(around1300°C.d)andis20%higherthanyear1.Similarly,maximalberryweightis20%higheryear2(datanotshown).

EffectofWDIonsugaraccumulation

WhenWDIismaintainedbelow50%,sugaraccumulationislow.Mostlikelyduringyear1,alowwaterusereducesphotosyntheticactivity,whichaffectsnegativelyactivesugaraccumulationintheberry.WhenWDIismaintainedabove50%,sugaraccumulationishigh.Mostlikelyahigherphotosyntheticactivitycombinedwithagreaterwaterusepromoteanearlierrateofberrysugaraccumulationandahigheramountofsugarperberry.

Figure4.Comparisonofberrysugaraccumulationrateinanon-irrigatedvineyard.(year1:

lessrainandhigherVPD;year2:morerainandlowerVPD).3.Relationshipbetweennitrogenuptakeandsapflow

In the context of a dry winter, soil nitrogen mineralization rate can be reduced.Consequently,afteradrywinter,evenifsoilmoistureisnolongerlimitedaftertheseason

start,vinenitrogenaccumulationcanbereduced(Mendez-Costabeletal.,2014).However,when soilmoisture supply is limited early season, even after awetwinter, vine nitrogenuptakecanalsobereducedforlackofsufficientwateruptakewhiletheamountofnitrogenmay not be limiting. Thus,whennitrogen deficit is diagnosed, it is difficult to distinguishwhetheralownitrogensupply(followingadrywinter)oralowsoilmoisturesupplyearlyseasonistheculprit.Asbothconditionscanleadtoalackofnitrogenuptake,decidingwhichcorrectiveactionisbestischallenging.

In this context, a joint analysis of sap flow and nitrogen accumulation can helpdiscriminatingbetweentwosituationslikelytocausenitrogendeficit.Asanillustration,wecomparedsapflowdynamicsandnitrogendynamicsobtainedinvineyardAandB,locatedinNapa, California (cv. Cabernet Sauvignon) over two consecutive years. Along sap flowvariations; temporal variations in leaf nitrogen concentrationwere analyzedweekly. Leafnitrogenreadingswereperformedusingleaffluorescencemethod.Publishedguidelineswereused tocharacterizenitrogen levelas low,moderateorhigh(datanotshown).SiteAwasdiagnosedwithlownitrogenyear1andhighnitrogenyear2.SiteBwasdiagnosedwithahighnitrogenyear1andlownitrogenyear2.Climaticcontext

Year1wasa“drier”yearastheamountofwinterraincumulatedbetweenNov.1st–March1stwaslessthan380mm.Year2wasa“wetter”yearandcumulatedwinterrainoverthesameperiodwasgreaterthan750mm.InsiteA,maximumKcbwaslowerinyear1(30%)comparedtoyear2(40%)asmorewateravailableearlyseasonduringyear2stimulatedthedevelopmentofalargertranspiringleafarea.InsiteB,maximumKcbwassimilarbothyears(50%).GrowingdegreesdaysarecumulatedsinceMarch1st.

WDIandNitrogenaccumulationprofilesanalysis

WDIprofilestypicalofvineyardsiteexperiencingwell-wateredconditionsearlyseason.

InvineyardA,nitrogendeficitisobservedyear1duringtheperiod400-800°C.d.Figure5 shows thathighwateruptake isobservedduring thatperiod, as indicatedbyhighWDIvalues(>80%).Year2,leafnitrogenconcentrationishigh,inspiteofamoderatereductioninsap flow early season as indicated by lower WDI value. Thus, year 1, low nitrogenaccumulationisnotdirectlyrelatedtolowwatersupplyconditions.Areductionintheinitialpoolofavailablenitrogenearlyseasonissuspectedtocausealowleafnitrogenaccumulation.We speculate that dry soil conditions during the winter have probably limited nitrogenmineralization rate and consequently the amount of nitrogen available early season. Anitrogen fertilization directly via a foliar spray is recommended as the corrective action.Interestingly, year2 receivedmorewinter rain and ahigher level of availablenitrogen isexpectedearlyseason.Thisprobablyleadtoagreaterleafnitrogenconcentration.

WDIprofilestypicalofvineyardsiteexperiencingwaterdeficitearlyseason.

InvineyardB,nitrogendeficitisobservedyear2duringtheperiod400-800°C.d.Figure6shows thata lowwateruptake isobservedduring thatperiod,as indicatedby lowWDIvalues (<60%over 500-650 °C.d). Because a period of lowwater use co-occurswith lownitrogenaccumulation,wespeculatethatearlywaterdeficitconditionshavecontributedtoreducingnitrogenuptake,evenifwinterrainsupplywashigh(i.e.,morefavorabletohighnitrogensupplyearlyseason).Earlyseasonirrigationaloneorcombinedwithafertilizationthroughtheirrigationlineisrecommendedasthecorrectiveaction.Interestingly,duringthe“dry” year 1, vineyard managers applied 3 irrigations early season (400- 600 °C.d) tocompensateforthelowwinterrainsupply.ConsequentlyWDIincreasedsharplyaround500°C.dandremainshigh(>80%)until650°C.d.Highwateruseconditionsaremaintainedduringtheperiodofnitrogenuptake.Nitrogensupplyearlyseasonisnotlimitingandleafnitrogenconcentrationishigh.

Byanalyzing jointlynitrogenuptakeandsapflow,plantnutritionaldiagnosiscanbeimprovedandleadtobetter-targetedpractices.

Figure5.VineyardA :High sap flowand lownitrogen is observedyear1. Foliar sprayof

nitrogenisrecommendedtocorrectnitrogendeficit.

Figure 6. Vineyard B: Low sap flow and low nitrogen is observed year 2. Irrigation and

possiblyfertigationisrecommendedtocorrectnitrogendeficit.CONCLUSIONSANDPERSPECTIVESSapflowisafundamentaltooltodevelopprecisionfarming

Becauseitisafundamentalplant-basedindex,sapflowseasonalvariationsanalysisisa goodentrypoint todevelopaholistic vineyardmonitoring strategy in conjunctionwithotherplant-basedmethods.Apart fromirrigation,manypracticaldecisionsrelatedtovinehealthandperformancecanbeimprovedwheninterpretedwithintheframeworkofsapflowvariationssuchasleafareamanipulationorfertilizationdecisions.Winegrowershavebeenusingsapflowasabaselinetotracktheimpactofdifferentfarmingstrategies.Inacontextofmore elevated temperatures, sap flow response analysis is critical to adjust practices inresponsetoextremeclimaticeventssuchasmorefrequentheatwavesordrierwinters.

Inthecontextofprecisionviticulture,sapflowresponseanalysisisusefultolegitimatenewpracticesortotargetspecificareasaccordingtovineyardspatialvariability.Becauseitisacontinuousmeasurement,sapflowisalsousefulforgroundvalidationofothersensingapproaches,whicharenotnecessarilycontinuous.Forinstance,sapflowmeasurementscanbecombinedwiththermalimagerytoevaluatethetemporalstabilityofspatialindexes.Sapflow measurements can also be combined with fluorescence methods to evaluate the

relationshipbetweenwaterdeficitandotherplantandfruitindexessuchasnitrogenorfruitcoloraccumulation.Compared toothervinewater statusmonitoring techniques, sap flowoffers a plant-basedmeasurement,which does not rely upon indirectmeasurements or acomplex conceptual framework (i.e., aerial pictures, environmental monitoring, modelingapproaches, etc.…). As such, winegrowers can learn directly from observing the vinesequippedwithsapflowsensorsandperformotherplant-basedmeasurementsonoraroundthesamevines.Theknowledgegeneratedfromafewcarefullyselectedlocationscanthenbeappliedoverlargerareaswherelessinformationisavailable.Oneclassicfeedbackfromfirsttimesapflowusersundersemi-aridclimatesisto learnthatplant“look”isdeceiving.Thecombinationofvisualcueswithcontinuousvinewaterusemonitoringteacheshowtomoveawayfromtraditionalirrigationhabitswithconfidence.Aswinegrowersdeveloptrustinsapflowtechnology,othervineyardmonitoringtechnologiesaremoreeasilyadopted.Asmoredataiscollectedfromthesamesamplingareas,vineyardmonitoringbecomesmorepreciseduetothesynergiesbetweendifferentvineyardinformationsources.

SapflowdrivenirrigationreduceswaterandenergyuseIngeneral,monitoringofvinewaterdeficitwithsapflowleadstoareductioninwater

usecomparedtotraditionalpractices.Assuchsapflowtechnologydeploymentprovidesagreatopportunitytoincentivizeasmarteruseofirrigationwater.Becauseitcontributestosavewater,sapflowdrivenirrigationisbeingtestedbywaterandenergyagencies.In2014,theMetropolitanWaterDistrictofLosAngelesinvestigatedtheamountofwatersavingsthatcouldbeobtainedbyadopting sap flowdriven irrigation.Traditional and sap flowdrivenirrigationswereappliedsidetosidewithinthesamevineyardstocomparetheamountofwaterusedaccordingtoeachstrategy.Figure7showsthe3Californiawineregionswheretrialsweredeployedtocomparethetwotreatmentsoversixvineyards.Figure8showsthatthe volumes of water use for irrigation are different for each site. However, despite sitespecific differences in total amount ofwater applied, sap flowdriven irrigation strategiesreduced the total amount ofwater by 60%on average compared to traditional irrigationstrategies.Interestingly,averageyieldswerehigherby15%inthesapflowdrivenirrigation(datanotshown).Intraditionalirrigationtreatmentsyieldlosseswereobservedasaresultofberrydehydrationbeforeharvest.Nosignificantdifferenceswereobservedintotalamountofsugarperberrybetweenthetwostrategies.Assapflowtreatmentsimposelongerperiodsofdrought,wespeculatethatvinewater-useregulationmechanisms(suchasmediatedbyABA)maycontributetodecreaseberrysusceptibilitytodehydrationpreharvest.In2016theCalifornia EnergyCommission funded a 3 years project to assess the benefits of sap flowdriven irrigation on water pumping energy over large scale vineyards (>300 acres). Theprojectiscurrentlyongoingandmaycontributetofurtherdisseminatethebenefitsofsapflowuseinviticultureandbeyond.

Figure7.Locationofsapflowexperimentsinthecontextofthe2014watersavingproject(fundedbytheMetropolitanWaterDistrictofLosAngeles).

Figure8.Comparisonofwaterappliedbetweenvarioustraditionalandsapflowirrigation

treatments(fundedbytheMetropolitanWaterDistrictofLosAngeles).LiteraturecitedAllen,R.G.,Pereira,L.S.,Raes,D.,Smith,M.,(1998).Cropevapotranspiration.Guidelinesforcomputingcropwaterrequirements.FAOIrrigationandDrainage,paperNo.56.(Rome,Italy:FAO).

Deluc,L.G.,Quilici,D.R.,Decendit,A.,Grimplet,J.,Wheatley,M.D.,Schlauch,K.A.,Mérillon,J.-M.,Cushman,J.C.,andCramer,G.R.(2009)WaterdeficitaltersdifferentiallymetabolicpathwaysaffectingimportantflavorandqualitytraitsingrapeberriesofCabernetSauvignonandChardonnay.BMCGenom.10,212.doi:10.1186/1471-2164-10-212.

Edwards,E.J.,andClingeleffer,P.R.(2013).Interseasonaleffectsofregulateddeficitirrigationongrowth,yield,wateruse,berrycompositionandwineattributesofCabernetSauvignongrapevines.Austral.J.GrapeWineRes.19,261-276.doi:10.1111/ajgw.12027

GuilpartN.,MetayA.,andGaryC.(2014).Grapevinebudfertilityandnumberofberriesperbuncharedeterminedbywaterandnitrogenstressaroundfloweringinthepreviousyear.Europ.J.Agron.54,9-20.

Lebon,E.,Dumas,V.,Pieri,P.,andSchultz,H.R.(2003).Modellingtheseasonaldynamicsofthesoilwaterbalanceofvineyards.Funct.PlantBiol.,30,699-710.

Mendez-Costabel,M.P.,Wilkinson,K.L.,Bastian,S.E.P., Jordans,C.,McCarthy,M.,Ford,C.M., andDokoozlian,N.(2014). Effect ofwinter rainfall on yield components and fruit green aromas ofVitis vinifera L. cv.Merlot inCalifornia.Austral.J.GrapeWineRes.20,100-110.doi:10.1111/ajgw.12060

Poblete-Echeverría,C.A.,andOrtega-Farias,S.O.(2013).Evaluationofsingleanddualcropcoefficientsoveradrip-irrigated Merlot vineyard (Vitis vinifera L.) using combined measurements of sap flow sensors and an eddycovariancesystem.Austral.J.GrapeWineRes.,19,249-260.doi:10.1111/ajgw.12019