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:e-mailyuka88@affrc.go.jpReceived27February2006;accepted26April2006.

8 JARQ41(1)2007

Y.Nakano

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

P P P

0

4

8

12

16

0

4

8

12

16

0

4

8

12

16

Freq

uenc

y (%

)

DFT

Freq

uenc

y (%

)

DFT+Air

10 100 1,000

Length of lateral roots (cm)

Freq

uenc

y (%

) WSC

0

10

20

30

40

50

60

0 5 10 15 20 25 30

Distance from root tip of the first order laterals (cm)

Num

ber o

f roo

t hai

rs in

slic

es

of la

tera

l axe

sDFTDFT+AirWSC

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.

10 JARQ41(1)2007

Y.Nakano

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

0

1

2

3

Nitr

ate

abso

rptio

n ra

te

per p

lant

(mg·

plan

t–1·h

–1)

NSNS

NS

*

0

2

4

6

8

10 20 30 40 50

Temperature (°C)

Nitr

ate

abso

rptio

n ra

te p

er

DW

(μg·

mg

root

DW

–1·h

–1)

**

NS

NS

0

10

20

30

wat

er a

bsor

ptio

n ra

te

per p

lant

(g·ro

ot–1

·h–1

)

NSNS

NS

NS

0

20

40

60

80

wat

er a

bsor

ptio

n ra

te

per D

W (m

g·g

root

DW

–1·h

–1)

***

******

0

50

100

150

200

17 27 33 450

0.2

0.4

0.6

0.8

1

1.2

bb

a

b

bb

aba

Temperature (°C)

Roo

t res

pira

tion

rate

(μ

mol

O2·m

g D

W–1

·h–1

)

Wat

er a

bsor

ptio

n pe

r roo

t re

spira

tion

(g·μ

mol

O2–1

)

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.

11

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.

12 JARQ41(1)2007

Y.Nakano

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.

0

2

4

6

8S

hoot

dry

wei

ght

(g·p

lant

–1)

0

0.5

1

1.5

8 10 12 14

Days after transplanting

Roo

t dry

wei

ght

(g·p

lant

–1)

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.

13

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

Roots in thenutrientsolution

retain0.4500.085

export5.348.82

import0.648

import0.035

exportretain

0

200

400

600

800

1,000

1,200

1,400

Percentage of humid air

Frui

t yie

ld (g

·pla

nt–1

)

0

50

100

150

200

Frui

t wei

ght (

g·fru

it–1)

LSDz

LSDy

0 25 50 75 100

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.

14 JARQ41(1)2007

Y.Nakano

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

2

3

4

5

6

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

Fig. 11. Effects of percentage of humid air spaces in the root-zone on respiratory rate of tomato root segments:Rootsinthenutrientsolution, :Rootsinthehumidair.

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ResponseofHydroponicTomatoRootstoStress

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