review response of tomato root systems to environmental ... · with image-processing software...
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7
Introduction
InJapan,theareaofvegetablesundersoillessculturehasgraduallyincreasedto1,192ha(2003)24,whereasthetotalareaofvegetablecultivationhasdecreasedabout2%yearly22.Morethan40typesofsoillesscultureareprac-ticedinJapan8,butfewstudieshavecomparedthestructureandfunctionofrootsystemsamongthem,betweenwhichclose interrelationships should exist. Soilless cultureallowsreproduciblecontroloftheroot-zoneenvironment.Nevertheless, evenunder soilless culture, roots couldexperiencemultipleenvironmentalstresses.Especiallyinsystemswithoutsolidmedia,changesinnutrientcon-centration,pH,temperature,andoxygenconcentrationwouldhavelargeimpactsonplantgrowth.Rootsystemsaltertheirinternalstructureandexternalarchitecturetominimizesuchimpactsandtoacquireoxygen,waterandnutrientsinthemostefficientway12.Weevaluatedthestructuralandfunctionalresponsesoftomatoesgrowninsoillessculturetotheroot-zoneenvironment.Wefocusedonrootsinhumidairandrootsinnutrientsolution.
Rootsinhumidairadaptmuchbettertochangesinnutrientconcentration,pHandtemperaturethandothoseinsolution25.Rootsystemsoftomatoesgrownbycapil-laryhydroponics21andinnutrientfilm2developinbothhumidairandsolution;thesetworootpartscouldplaycomplementaryroles.Therefore,quantitativeevaluationisneededtoclarifytheresponsesofbothtypesofrootsystemtotheroot-zoneenvironment.
External and internal root structures of tomato plants grown hydroponically in humid air or in nutrient solution
Weexaminedtheeffectsofroot-zoneenvironment,humidairandnutrientsolutionontheexternalandinter-nalstructuresoftomatoroots17.Youngtomatoplantsweregrowneither inwet-sheetculture(WSC), inwhichallrootsdevelopinhumidair,orhydroponicallybythedeepflowtechnique(DFT)withorwithoutaeration(Fig.1).Dissolvedoxygenconcentrationwas45%to62%ofsatu-rationinDFT,and91%to96%inDFT+Air.
GrowthoftomatoplantsinDFT+AirandWSCwas
REVIEWResponse of Tomato Root Systems to Environmental Stress under Soilless Culture
Yuka NAKANO*DepartmentofFruitVegetables,NationalInstituteofVegetableandTeaScience(Taketoyo,Aichi470–2351,Japan)
AbstractWeinvestigatedtheeffectsofroot-zoneenvironment,humidityaroundtherootsandnutrientsolutionontheactivityandmorphologyoftomatorootsgrowninwet-sheetculture(exposedtoair)ordeepflowtechnique(submergedinsolution).Differencesinrootexternalandinternalstructurebetweentreatmentgroupscouldbeinterpretedasadaptiveresponsestotherootenvironment.Theexposedrootscouldadaptmorereadilytoextremesoftemperaturethanthoseinthesolution.Thoseadaptationsoccurredthroughshort-termphysiologicalresponsesandlong-termadditivemorphologicalresponses.Wealsoevaluatedthefacilitatingeffectsoftheflowofnutrientsolutiononrootrespirationandnutri-entuptakerate.Wheretherootsystemwassplitbetweenhumidairandnutrientsolution,rootsinthesolutionabsorbedandsuppliednitrogenmoreefficientlyperdryweightthandidrootsinair.Splitrootsystemsbetweenhumidairandnutrientsolutionshowedstablegrowthoftomatoplants.Ourobserva-tionsofrootplasticitywillhelptheestablishmentofgrowingsystemsthatsupporthighyieldandstableproduction.
Discipline: HorticultureAdditional key words:nutrientabsorption,rootrespiration,rootstructure,root-zonetemperature
JARQ41(1),7–15(2007)http://www.jircas.affrc.go.jp
*Correspondingauthor:[email protected];accepted26April2006.
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morevigorousthanthatinDFT(Table1). Theshoot-to-rootratiodecreasedintheorderDFT+Air>WSC>DFT,butcomputerizedimageanalysisoftherootsystemswithimage-processingsoftware(NIH-Image)revealednodifferencesinthetotalrootlengthsorrootsurfaceareasamongthethreetreatments.DFThadalargerproportionofshorterlateralrootsthanDFT+AirandWSC(Fig.2),suggestingthatoxygendeficitinDFTinhibitedrootelon-gation.Aerenchyma,consideredtobeanadaptationtoanaerobicconditions9,waspresent in thesteleonly in
Tomato plantIrrigation tube Air pump Multi-hole tube Root-proof sheet
PumpAir stone Nutrient solution Non-woven fabric
Styrene foamDFT DFT+Air WSC
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Fig. 1. Schematic diagrams of deep flow technique without aeration (DFT), DFT with aeration (DFT+Air) and wet-sheet culture (WSC) for the cultivation of tomato
Nutrientsolutionineachcontaineriscirculatedbyapump.
Fig. 2. Frequency distribution of total length of individual lateral roots of tomato seedlings grown in different hydroponic systems
Lateralrootsarefirst-orderlateralsincludinghigher-orderlaterals.DetailsoftreatmentsaredescribedinFig.1.
Fig. 4. Numbers of root hairs on 250-µm lengths of lateral roots of tomato seedlings grown in different hydroponic systems
VerticallinesindicateSD(n=3).
9
ResponseofHydroponicTomatoRootstoStress
DFT.RootsinWSChadlargercorticalcells,metaxylemandstele,moredepositsofligninlamellaeintheexoder-mis,andmoreroothairsthanrootsinDFT(Figs.3&4).Enlargedcorticalcellsandincreaseddepositsofligninarealsofoundunderdroughtstress4,26.Waterdeficitpromotesthedevelopmentofroothairs13,whichplayanimportantroleinuptakeofmineralsandwater.Accordingly,thegreaterallocationofphotosynthates,namelydrymatter,to theroots inWSCthaninDFT(Table1)couldbearesponsetothedrierconditionsinWSC.Thesechangesinexternalandinternalstructuresoftherootscouldbeinterpretedasadaptiveresponsestotherootenvironment,thatis,anoxiainDFTandwaterdeficitinWSC.
Fig. 3. Transection of tomato roots with phloroglucinol staining of lignin lamellae (arrowheads) within exodermal wallsSampleswerepreparedfromfirst-orderlateralrootsdevelopedineachhydroponicsystem.a:Deepflowtechniquewithoutaeration(×200),b:Deepflowtechniquewithaeration(×200),c:Wet-sheetculture(×200),d:Wet-sheetculture(×400).Rectanglemarked(d)incistheimageind.
Table 1. Effects of root-zone environment on the growth of tomato seedlingsz
Hydroponicsystemy
Dryweight(g·plant–1)w S/Rx,w
Shoot Root
DFT 1.29 b 0.25 b 5.0 cDFT+Air 1.79 a 0.28 ab 6.3 aWSC 1.73 a 0.30 a 5.6 bz Dataareshownasthemeanof20–21samplesat8daysfromtransplanting.
yDFT:Deepflowtechniquewithoutaeration,DFT+Air:Deepflowtechniquewithaeration,WSC:Wet-sheetcul-ture.
xShootdryweight/rootdryweight.wValuesfollowedbythesameletterwithinacolumnarenotsignificantlydifferentatΡ=0.05.
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Influence of growing temperature on activity and structure of roots in humid air and in nutrient solution
Theroot-zonetemperaturedirectlyaffectsnutrientabsorption1andrespiration11andindirectlyaffectsrootdevelopment5 andallocationofphotosynthates3,7. Westudiedtheshort-termandlong-termeffectsofroot-zonetemperatureonrootphysiology15,18.
1. Short-term influence of growing temperatureWecomparedshort-termdifferences inwaterand
nitrate absorption between tomato roots in humid air(WSC)androotsinnutrientsolution(DFT)atfourroottemperatures,17,27,33,and45ºC18.Tomatoseedlingswere transplanted into acryl growing containers withDFTorWSCingrowthchamberssettoa35ºC(day)/22.5ºC(night)temperaturecycle.At9to11daysaftertransplanting,eachgrowingcontainerwasimmersedinawaterbathwithtemperaturecontrolledat17,27,33,or45ºC.Ratesofwaterandoxygenabsorptionbytomatorootsweremonitoredbyusinganelectronicbalanceandoxygensensorduringdaytime.
Waterabsorptionratesperplantwerealmostequalinbothsystemsatalltemperatures(Fig.5).Ontheotherhand,nitrateabsorptionrateperplantwasgreaterinWSCthaninDFTat45ºC,althoughnosignificantdifferenceswerefoundatother temperatures. Rootrespiration inDFTincreasedastheroottemperatureincreasedfrom17to33ºC,butdecreasedat45ºC,whilewaterabsorptionperrootrespirationdecreasedwithincreasingtemperature(Fig.6).Theseresultssuggestthatrootsinthehumidair
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Fig. 5. Effects of root zone temperature on water and nitrate absorption rates of tomato seedlings in two hydroponic systems
:Deepflowtechnique,:Wet-sheetculture.Dataaremeansof3or5samples.Waterandnitrateabsorptionratesweremeasuredduringtheperiod10:00to14:30.*, **, and *** indicate significant difference atP=0.05,0.01and0.001,respectivelywithFisher’sPLSDtest.DW:Dryweight.
Fig. 6. Effects of root zone temperature on root respiration rate and water absorption per unit of respiration in deep flow technique (DFT)
:Rootrespiration,:Waterabsorptionperroot.Valuesaremeansof3or4measurements±SD.DifferentlettersrepresentsignificantdifferencesatP=0.05.
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ResponseofHydroponicTomatoRootstoStress
wouldbemoretolerantofsupraoptimaltemperaturethanthoseinthesolution.
2. Long-term influence of growing temperatureWealsoanalyzedthelong-termeffectsofroot-zone
temperatureonrootdevelopment15. TomatoseedlingsgrowinginWSCorDFTwereraisedingrowthchamberskeptataconstant15,25or35ºCfor6to12daysuntilthefive-leafstage.Weevaluatedtheadaptabilityoftherootsystemstohighorlowtemperaturebycomparingtherootactivityandstructurebetweenthesystems(Fig.7).Atalltemperatures,WSCtomatoplantsgrewlargerthanDFTplants.ThebleedingrateofxylemsapperwholerootsystemwashigherinWSCthaninDFTat15and35ºC,andtherootrespirationrateperdryweightwashigherinDFTthaninWSCatalltemperatures(Table2).TherootrespirationratemeasuredinDFTshoweddifferenttrendsfromthegrowthandbleedingratesofxylemsap,soweuseditinfurtherexaminationasanindexofphysiologicalactivity.
TherootsystemsinWSChadmorefirst-orderlater-alsandgreaterprojectedareasthanthoseinDFTat15and35ºC,butweresimilartothoseinDFTat25ºC(Table3).Thefractaldimensionoftherootsystems,character-izingtheircomplexity23,washigherinWSCthaninDFTat15ºC,butlowerat35ºC.ThehigherrootdensityandtheshorterorequivalentmeanlengthinDFTthaninWSCatalltemperatures(Table3)meansthatrootelongationwasinhibitedbyhightemperature.Therewasahighcor-relationbetweentotalrootlength,projectedareaandthefractaldimensionsat25and35ºC.
Theseresultsindicatethatrootsinhumidairwouldadaptmorereadilytohighorlowtemperaturethanthosein solution. Thisagreeswellwith theobservationbyYamazaki(1986)25.
Table 2. Effects of growing temperature on the bleeding rate of xylem sap and root respiration of young tomato seedlings in the two types of hydroponic systemz
Hydroponicsystemy
Bleedingratex,w Rootrespirationratew
mg·plant–1·h–1 mg·rootlength(m)–1·h–1 µmolO2·root–1·h–1 µmolO2·mgDW–1·h–1
15ºC 25ºC 35ºC 15ºC 25ºC 35ºC 15ºC 25ºC 35ºC 15ºC 25ºC 35ºC
WSC 460 414 140 15.3 12.9 7.3 22.9 13.5 14.4 0.093 0.102 0.164DFT 191 339 69 12.8 9.6 5.2 19.8 26.2 12.4 0.156 0.266 0.221
*** NS *** NS * NS NS ** NS *** *** *zDataareshownasthemeanof5or6samples.yWSC:Wet-sheetculture,DFT:Deepflowtechnique.xAverageofthebleedingratemeasuredduringtheperiodfrom7:30to12:00.wNS,*,**,and***indicatenosignificantdifferenceorsignificantdifferenceatP=0.05,0.01and0.001,respectivelywithFisher’sPLSDtest.
Fig. 7. Digitized images of tomato root systems grown at 15, 25 and 35ºC in the two hydroponic systems
WSC:Wet-sheet culture, DFT:Deep flow tech-nique.Eachbarindicates5cm.
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Effects of flow rate of hydroponic nutrient solution on growth and ion uptake by tomato seedlings
Inmanyhydroponicsystems,nutrientsolutioniscir-culatedforaeration.Evenapartfromtheeffectsofaera-tion,circulationitselfhasaneffectonplantgrowth,butnoquantitativeanalysishadbeendone.Wedeterminedtheeffectsofstirringofnutrientsolutiononrespirationandionuptakebytomatoroots,andtheeffectsofincreasedflowrateofnutrientsolutionongrowthoftomatoseed-lingscultivatedbyDFT14.
Weusedstirrersinbufferwithamanometerat500,700,900,and1,200rpmtomeasuretherespirationrateofdetachedrootsculturedinDFT.Therespirationrateincreasedwiththestirringrate.Eventhoughthelevelofdissolvedoxygen(DO)droppedrapidly,therootrespira-tionratestayedconstant.
Tomatoseedlingsweretransferredintocontainerswithnutrientsolutioncontaining15N-labelednitrateinagrowthchamber.Absorptionofwaterand15Nalsotendedtoincreasewithincreasedstirringrate,andthisincreasewasalsoindependentofDO.
Wedetermined theeffectsof the flowrateof thenutrientsolution(0.35,1.5and3.6L·min–1)inDFTonseedlinggrowth.Higherflowratesstimulatedshootandrootgrowth(Fig.8).DOvaluesinthenutrientsolutionforflowratesof0.35,1.5and3.6L·min–1were1.1–3.4ppm,1.8–4.4ppmand2.6–5.8ppm, respectively. ChangesintheflowrateofthesolutionmayaffectplantgrowththroughchangesinthelevelofDOinthesolutionandintherateoftransferofoxygenandionsclosetotherootsurfaces,wherethediffusionprocessisdominant.Theseresultssupportthehypothesisthataboundarylayerexistsontherootsurfaceandgetsthinnerastheflowofnutrient
solutionincreases19.
Effects of a combination of roots in humid air and roots in nutrient solution
Asdiscussedabove,tomatorootsinthehumidairandinthenutrientsolutiondifferedintheirmorphologyandphysiology.Toallowplantstocopewithroot-zonestresses,weproposeasoillessculturesystemthatdevel-opsbothtypesofroot.
Table 3. Effects of growing temperature on the number of first order lateral roots, total projected root area, average root diameter, and fractal dimension in tomato root in the two types of hydroponic systemz
Hydroponicsystemy
Firstorderlateralrootsw Averagerootdiameter(mm)w
Totalprojectedrootarea(cm2)w
Fractaldimensionx,w
No.(No.·plant–1)
Meanlength(cm)
Density(No.·cmtaprootaxis–1)
15ºC 25ºC 35ºC 15ºC 25ºC 35ºC 15ºC 25ºC 35ºC 15ºC 25ºC 35ºC 15ºC 25ºC 35ºC 15ºC 25ºC 35ºC
WSC 108 112 89 6.0 5.5 6.5 4.0 4.7 5.7 0.58 0.43 0.43 179.7 123.1 76.7 1.74 1.66 1.59DFT 80 107 111 4.6 6.1 4.6 4.5 5.2 21.3 0.58 0.43 0.44 79.4 142.3 53.1 1.68 1.63 1.64
* NS * * NS *** NS NS *** NS NS NS *** NS ** * NS *zDataareshownasthemeanof5or6samples.yWSC:wet-sheetculture;DFT:deepflowtechnique.xThefractaldimensionwasdeterminedusingtheaverageofthelocalfractalbymass-radiusmethod(Ketipearachchi&Tatsumi,2000)10.
wNS,*,**,and***indicatenosignificantdifferenceorsignificantdifferenceatP=0.05,0.01and0.001,respectivelywithFisher’sPLSDtest.
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Fig. 8. Effect of flow rate of nutrient solution on growth of tomato seedlings
:0.35L·min–1,:1.5L·min–1,:3.6L·min–1.
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ResponseofHydroponicTomatoRootstoStress
1. Absorption and distribution of 15N by tomato roots divided between humid air and nutrient solution
Wedividedtherootsystemsofplantsbetweenhumidairandnutrientsolutionandanalyzedtheabsorptionanddistributionof15Nbythedifferentparts16.Equalconcen-trationsofK15NO3solutionwereadministeredsimultane-ouslytothenutrientsolutionbyanutrienttube,andthehumidairthroughnon-wovenfabricandanutrienttube.Attheendof72-hexposureto15N,weanalyzedtherootsandshoots.
Dryweightandpercentagedrymatterweresignifi-cantlyhigherinrootsinthehumidairthaninthenutri-entsolution.Percentagenitrogencontentsofrootsinthehumidairandinthenutrientsolutionwere1.52%and0.38%,respectively.Respirationratewashigherinrootsinthenutrientsolutionthanthoseinthehumidair,by2timesonawhole-rootdryweightbasisandby3timesonadry-weightbasis.Rootsinthehumidairexportedless15Ntootherpartsoftheplantthandidrootsinthesolu-tion;theformerretained5timesasmuch15Nthattheyabsorbedthanthelatterandimportedmore15Nthanthelatterdid(Fig.9).
Rootsinthesolutionabsorbedandsuppliednitro-genmoreefficientlyperdryweightthandidrootsinthehumidair.Thelargeaccumulationofdrymatterbyrootsinthehumidairmayincreasenitrogenabsorption,eventhoughtheefficiencyislow.
2. Influence of percentage of air and solution spaces in the root-zone on vegetative growth and fruit yield of tomato grown in wet-sheet culture
Somesoillessculturesystemshavehumidairandsolutionspacesintheroot-zone.Althoughthosesystemsaresuitableforgrowingvegetables,theappropriateratioofspacesandtheirphysiologicalroleswereunknown.Soweinvestigatedtheeffectsofpercentagesofhumidairandsolutionspacesintheroot-zoneonvegetativegrowthandfruityieldoftomato20.Percentagesofairspaceintheroot-zonewere0(allrootsweresubmergedinnutrientsolution),25,50,75,and100%(theentirerootsystemwasdevelopedonthewetsheetandexposedtothehumidair).
Enlargingtheairspaceintheroot-zoneupto50%increasedthedryweightofshootsandroots.Leafexpan-sionwassuppressedatboth0%and100%airspace.Fruityieldandweightwereheavierwhentheairspaceexceeded50%(Fig.10),butthepercentagesofdrymatterandsol-ublesolidswerelower(5.1–5.3%)thanat25%(6.0%)andat0%(7.1%).Thephotosyntheticrateofleaveswassignificantlylowerat0%airspace.Respirationratesofrootsnearthebasewerealmostequalinalltreatments,whereasthoseofrootsfrommiddletotipwerehigherat
50%and75%airspacethanatotherproportions(Fig.11).Totalrootrespirationincreasedastheairspaceincreasedto75%.
Partial exposure of roots to humid air promotedthephysiologicalactivityofthewholerootsystemandincreasedyield.Hence,forstablegrowthoftomatoplantsinsoillessculture,therootsneedexposuretobothenvi-ronmentsidenticallyinsoilculture.
Shoot
Roots in thehumid air
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Fig. 9. Schematic diagram of nitrogen flow from roots in the nutrient solution and in the humid air (µg·g DW–1·h–1)
Fig. 10. Effects of percentage of humid air space in the root-zone on fruit yield and weight of single-truss tomato
:Fruityield,:Fruitweight.VerticalbarsrepresentLSDatP=0.05.zLSDonfruitweight.yLSDonfruityield.
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Responses of root system to environment and stress adaptability
Weundertook a series of experiments to test theplasticityofrootsinresponsetovariationsintheroot-zoneenvironment.Inhumidair,rootsadapttomildwaterstressbychangingtheirmorphologyandfunction,result-inginalargeandwellbranchedrootsystem.Ontheotherhand,rootsinnutrientsolutionhavefewbranchesandroothairs.Rootsinhumidairhavelowerrespirationandnutrientabsorptionthanrootsinnutrientsolutiononadryweightbasis,butalmostthesameonarootsystembasis.Theadaptationsinrootsinthehumidairallowedplantstowithstandtemperaturestress.Wherethereislittlelikeli-hoodofstress,asinahighlyregulatednutrientsolutionwithplentyofresources,the‘optimum’rootsystemwillbesmall.Thebalancebetweenrootdevelopment(costs)andwaterandnutrientavailability(benefits)willreflectthestressadaptabilityofroots6.Whenseveralstressesoverlapcoincidentally,variousresponseswouldinteractwitheachotherand,hence,totalabilityofrootswouldbedefined.Minimizingtheriskofstressesshouldbeabasicstrategy,butourobservationsofrootplasticitywillhelptheestablishmentofgrowingsystemsthatsupporthighyieldandstableproduction.
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0
1
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4
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Percentage of humid air
Res
pira
tion
rate
(μ m
ol O
2 g D
W–1
min
–1)
0 25 50 75 100
A B AAAA B B BC BC C C CD D D DE E
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