Effect of plant density on biomass yield of Cynara cardunculus M.Sanz,P.V.Mauri,M.D.Curt,J.Sanchez,M.Garcıa-Mu ller,A.PlazaandJ.FernandezAbstract

Cynara cardunculusL.isaperennialherbthathasbeenproposedasanenergycrop in Mediterranean environments because of its adaptation to dry and hotsummers.Plantdensityisanimportantfactorforbiomassproductionbutitisrelatedto water availability; therefore, in water-constrained conditions it should beoptimized.InthisworktheeffectofplantdensityontheproductionofC. cardunculusisstudiedatfieldlevelintherain-fedconditionsofcentralSpain.TheexperimentwasconductedunderMediterranean-continentalconditions,430mmannualrainfalland3-monthdryperiod,onaverage.Sowingwascarriedoutbeginningautumnbymeansofaseeddrillerwith0.80cmbetweenrows.Afterone-yearestablishment,twoplantdensitieswerecomparedin1-haplots:thestandardplantdensityandasparseplantdensity,designedasadouble-rowsystem; itwasachievedby lifting twooutof fourcrop rows. Treatments were evaluated for plant density, stalk density, number ofheads, plant height, biomass production and dry matter content in summertime.Cynaraplantstendedtobehigher(215vs.205cm)andmoreproductive(0.83vs.0.69kgdmpl-1) indouble-rows,despite intra-variability.However, this apparenthighergrowthdidnotcompensatethereducedplantdensityandfinally,theyieldinbiomasswaslowerthaninthesingle-rowtreatment(8vs.13tdmha-1harvestablebiomass).Moreover, a higher proportion of cauline leaveswas recorded,which affected thequalityof thebiomassasa solidbiofuel sinceN content isusuallyhigher in leavesthaninstems.Therefore,evidencewasprovidedthatthedouble-rowdesigndoesnotresultinyieldimprovement.

Keywords:cardoon,energycrop,biomassyield,cropmanagementINTRODUCTION

Cynaracardunculus L., commonly known as cardoon or cynara, is a perennial herbwith an annual growth cycle that extends from autumn to summer, season in which theaboveground plant parts dry up and seeds are ripe. Contrary to artichoke, for energypurposesitisgrownaccordingitslifeform.Thecropisharvestedeveryyearinsummertimeand re-grows in early autumn. Seeds can be separately harvested for oil and thelignocellulosic biomass for solid biofuel (Fernandez et al., 2006). C. cardunculus, as aMediterraneannativespecies(Wiklund,1992),isadaptedtoMediterraneanclimates,whicharecharacterizedbylowannualprecipitation,irregularrainfalldistributionandhotanddrysummers.Theseconditionsrepresenta limitation formostenergycrops ifno irrigation issupplied. The adaptive advantage of this species alongwith other characteristics, like thehigh biomass potential and the double application (solid and liquid biofuels) of the cropproduce, make Cynara an interesting energy crop for dry farming in Mediterraneanenvironments.Research on Cynara as an energy crop has been carried out in Spain and otherEuropeancountriesintheframeworkofanumberofR+D+iProgrammes.Progresshasbeenmade in many different areas, i.e., management techniques, yields, biomass properties,applications,densification,oil characterisation,biodieselandothers.Asothercrops indry

farming,yieldsaredependentonrainfall.Anotherimportantfactorforbiomassproductionisplantdensity;asaruleofthumb,thehighertheplantpopulation,thehigherthebiomassproductionha-1 is.Byusingdenseplantationsothereffectsmayarise likechangesinplantgrowth habit (branching, height) and competence for water. Thus, in water-constrainedconditions it is extremely important to optimize plantdensity. The effect of this factor onbiomass yield of Cynara has been little investigated. A previous work was the study byRaccuiaetal.(2011),whichwasconductedin20-m-2plotsinSicily.InthisworktheeffectoftwoplantationdesignsoftheCynaraenergycrop,leadingtodifferentplantdensities,wasstudiedinalarge-fieldexperimentconductedindryfarmingincentralSpain.Planttraits,biomassproductionandbiomasspartitioningweredeterminedatharvesttimeforthedesignsofsinglerowsanddouble-rows.MATERIALSANDMETHODS

LocationandplantmaterialThe experiment was carried out in the property ‘El Encın’ of IMIDRA (InstitutoMadrileno de Investigacion y Desarrollo Rural, Agrario y Alimentario) at latitude40°31’12”N; longitude3°18’13”W,altitude603ma.s.l.This isanexperimental farmof theAutonomousCommunityofMadrid (Spain), sited in themunicipalityofAlcaladeHenares(Figure1).

Alcala de Henares

Figure1.LocationanddigitalelevationmapoftheAutonomousRegionofMadrid(Spain).The field area devoted to this crop was 20 ha. The soil was an Alfisol (USDA soiltaxonomy), more specifically, Haploxeralf Calcic. At this site, the climate is XericMediterranean, sub-type Mild Meso-Mediterranean, commonly described as continental-Mediterranean.Basedonhistoricalrecords(1957to2000),theclimatedataoftheaverageyearare:13.4°Cmeantemperature,60%relativehumidity,1.8ms-1windspeed,309.8calcm-2 day-1 global radiation, 429.7 annual rainfall and 744.9 mm y-1 ETP (Thornthwaite)

(Mauri,2000).Plantmaterial for thiswork came from a previous crop ofC.cardunculus var.altilisconductedbytheAgro-EnergyGroupofUPM,whichwasoriginatedfromaseedpopulationselectedforbiomassproduction(seealsoCurtetal.,2002).CropconditionsandcropmanagementThe field was subsoiled, ploughed and harrowed in September to be physicallyprepared for sowing. Specific soil characteristics were determined at the design of thetreatments (see next sub-section). In order to describe the climate conditions during theexperiment, the records of temperature andprecipitationwere compiled from the closestmeteorologicalstation,locatedinSanFernandodeHenares(Madrid,Spain).Thecropwasestablishedfromseeds.SowingwascarriedoutinOctoberbymeansofaseed driller (4-5 kg seeds ha-1) working with 0.80 cm separation between rows. Plantsdevelopedaccordingthenaturalcycleofthisplantspecies,driedupinsummertimeandre-grewsoonafterthefirstautumnrainfalls.Asdescribedintheliterature,theyearofsowingis for the establishment of the crop; in dry Mediterranean conditions the growth of theabovegroundplantpartsisusuallylowinthefirstyearandnoharvestisperformeduntiltheendofthesecondgrowthcycle(firstproductiveyear).Fertilizationwasconductedsoonaftertheautumnre-growth;450kgha-1of15:15:15complexfertilizerwereapplied.Preventivepestcontrol(a.i.dimethoate)wascarriedoutinFebruary againstCassida (see Iglesias et al., 1999).Harvestingwas accomplished the firstweekofSeptember,oncethe2ndgrowthcyclewasover.TreatmentsInorder tostudytheeffectofplantdensityoncropperformanceandbiomassyield,twoattachedplotssizing1haeachweresignpostedinautumnforplantdensitytreatments.Oneplot represented theCynara standardplantationdesign in rows 0.80 cmapart; plantdensitywasmaintained as itwas at that time (Treatment SR). The other plot aimed at adouble-row plantation design; it was achieved by lifting two out of four crop rows(Treatment PR). This way the new-designed plot would contain about half the plantpopulationoftherestofthefield(Figure2).Soilcharacteristicsweredeterminedaccordingstandardmethods(MAPA,1994)andonaveragewerepH8.3,EC0.18dSm-1,organicmatter0.5%,P18ppm,K(AcNH4)463ppm.In summertime (firstweekof September)both treatmentswere evaluated forplanttraits and biomass production. Samplingswere performed by hand, taking at random sixreplicationsof32m2each,pertreatment.Theparametersstudiedwerethefollowing:plantdensity, stemdensity, number ofwell-developed heads, plant height, harvestable biomass(stem+caulineleaves+heads),wholebiomass(harvestablebiomassplusbasalleaves)anddrymattercontent.Drymattercontentwasdeterminedafterdryingat105°Cuntilconstantdryweight.Yieldswereexpressedonadrybiomassbasis.StatisticalanalysisComparisonbetweentreatmentswasperformedbymeansofanANOVAtestoverthefollowing parameters: plant density, stem density, number ofwell-developed heads, plantheightandbiomassproduction.StatgraphicsCenturionXVIv.16.08.08softwarewasusedforthementionedanalyses.Significancewasacceptedatp≤0.05level.

Figure2.Viewoftheplots.Left:single-rowdesign.Right:double-rowdesign.RESULTSANDDISCUSSIONIndry-farmingunderaMediterraneanenvironment,themostimportantfactoristheprecipitation,intermsofbothquantityanddistribution.Attheexperimentsitetheaverageprecipitationisnearly430mmandthedryperiodtypicallylasts3months,i.e.,fromJunetoSeptember.Inourexperiment,theestablishmentyear(1stgrowthcycle)wasdrierthantheaverageyear,asshownbytherecordsofprecipitation(394.4mm)andevapotranspiration.Onthecontrary,the2ndcyclewasaboveaveragewith534.4mmannualprecipitation(Table1).RainfalldistributionwastypicallyMediterranean(Figure3).Table1. Climatedata.Tmax,absolutemaximumtemperature;tm,meantemperature;Tmin,absolute minimum temperature; pp, precipitation, ETo, evapotranspiracion; Rg,globalradiation.

Crop establishment (1st growth cycle)

First productive year (2nd growth cycle)

Tmax (ºC) 37.5 39.3 tm (ºC) 14.0 14.1 Tmin (ºC) -6.6 -7.0 pp (mm year-1) 394.4 534.4 ETo (mm year-1) 1233.3 1162.2 Rg (MJ m-2) 207.7 195.3

Figure3. Ombrothermic diagram; tm, mean monthly temperature (°C); pp, monthlyprecipitation(mm).ActualplantdensityByactualplantdensityitwasmeanttheactualplantpopulationdeterminedatharvesttime. In the standard plantation design (Treatment SR), the value found for actual plantdensity was 27,786 plants ha-1 which represented a high plant population since it wasequivalent toonly0.45mplant spacingwithina row.For thedouble-rowdesign (PR) theactualplantpopulationwas12,655plantsha-1.Thisvaluewasequivalentto0.49mdistancebetweenplantsinaplantrow,avaluesimilartoSR,butinthePRtreatmentplantsbenefitedfrommorespaceatonerowsideduetothedouble-rowdesign.IntheopinionofFernandez(2009) the recommended plant density of cynara energy crop in Spain is about 15,000plantsha-1.Therefore,theactualplantdensity inourdouble-rowdesignwouldbeclosetotherecommendedvaluewhileSRwouldrepresentahighlypopulatedplantation.PlanttraitsandbiomassproductionTheCynaracropperformedwellinbothtreatments.Plantsgrewmoreinheight(215vs.205cm),producedmoreheads(5.56vs.4.38headsplant-1)andyieldedmorebiomass(0.83 vs. 0.69 kg dm-1whole biomass plant-1) in the double-row treatment than in singlerows.Inaddition,biomasspartitioningwasbetterinPRthaninSRsincetheproportionofbasalleaveswaslowerinPRthaninSR(26.0vs.28.1%onwholebiomassbasis).Therefore,itwasrevealedthatareducedplantdensity(12,655vs.27,786plantsha-1)enhancedCynaraplantgrowth.The results of plant traits, biomass production and biomass partitioning of bothtreatment,intermsofcroparea,areshowninTables2and3.Itisworthmentioningthatthevaluesfoundforthecoefficientsofvariationweregenerallywithintherangeoftheexpectedvalues for field trials (<25%), in spiteof the fact that theplantationwasestablished fromseedsandthatthisplantspeciesisallogamous.Atthedateofharvesttheabovegroundplantparts were fully dry; the dry matter content was 85.5 and 89.1% on average, for thetreatmentsPRandSR.In termsof croparea, theyields inharvestablebiomassamounted to8.2and13.3 tdm-1ha-1forthePRandSRtreatments,respectively,asaresultofthevaluesfoundforactualplantdensity(Table2).Thesevaluesareconsistentwiththeyieldsreportedforalong-termexperimentofCynaraconductedinanothersiteofMadrid,whichrangedfrom3.4to25.2tdm-1ha-1year-1dependingontherainfallvaryingfrom280to588mmyear-1(Fernandezet

al.,2005).Table2. Meanvaluesofplantandstemdensity,canopyheight(Hmax),heightoftheshorteststem(Hmin)andyield inharvestablebiomass for the treatmentsofdouble rows(PR) and single rows (SR). Between brackets, coefficients of variation (%).Significance:***p<0.001,ns=notsignificant.Parameter PR SR Significance Plants m-2 1.3 (11.5) 2.8 (13.0) *** Stems m-2 2.0 (22.0) 4.6 (18.9) *** Heads m-2 10.7 (22.6) 19.7 (15.5) *** H max (cm) 214.8 (4.6) 204.6 (5.8) ns H min (cm) 90.3 (18.1) 92.5 (14.0) ns Yield (kg dm-1 m-2) 0.82 (11.9) 1.33 (13.4) ***

Table3. Resultsofbiomasspartitioning (harvestablebiomass,%ondryweightbasis) forthe treatments of double rows (PR) and single rows (SR). Between brackets,coefficientsofvariation(%).Significance:ns=notsignificant.Plant fraction PR SR Significance Cauline leaves 25.6 (18.7) 19.7 (24.6) ns Stems 29.1 (13.0) 31.6 (20.5) ns Heads 45.3 (9.7) 48.7 (11.1) ns Concerning crop water efficiency at the field level, it was calculated as the ratiobetween harvestable biomass and annual rainfall. In our experiment it was estimated at1.53-2.49gL-1dependingonthedensitytreatment,forthedryconditionsofcentralSpain.Theanalysisofvariance showedhigh significantdifferencesbetween treatments forthevariables:actualplantdensity(plantsm-2),stemdensity(stemsm-2),numberofheads(nm-2) and biomass production (dry weight m-2) but not for height (Hmax and Hmin) orbiomasspartitioning(%)(Table3).Itbecameevidentthattheyieldwasverysensitivetothefactor studied although possible changes in plant architecture – plant height, biomasspartitioning–werenotsignificant.TheenhancedgrowthofCynaraunderthePRtreatmentdid not compensate the effect of the lower plant density and resulted in lower yields inbiomassandheads.

CONCLUSIONThe performance of C. cardunculus, grown according a standard plantation design(singlerows,0.80mapart),wascomparedtoadouble-rowplantationdesignthathalvedtheplantdensity.Inbothtreatmentsthecropperformedwellandtheresultsofplanttraitsandbiomass production were consistent with other experiments in dry farming. The trendobservedwasthatplantsgrewmoreinheightandproducedmoreheadsandbiomassinthedouble-row treatment than in single rows. However, in terms of crop area, that apparentenhancedgrowthdidnotcompensatetheeffectofthereducedplantdensityoncropyield.Thereforeitcanbeconcludedthatthereducedplantdensity–plantationdesignindoublerows–doesnotresultinayieldimprovementofthisenergycrop.ACKNOWLEDGEMENTSFinancial support from the EU Commission (Project BIOCARD, FP6-19829) and theMinistryofScienceandInnovationofSpain(ProjectPROBIOCOM,RTA2012-00082-C02) isgratefullyacknowledged.LiteraturecitedCurt,M.D.,Sanchez,G.,andFernandez,J.(2002).ThepotentialofCynaracardunculusL.forseedoilproductionin

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