Root cortical aerenchyma improves the drought tolerance of m(7)

746J.Zhuetal.

consistentwiththeresponsetolowphosphorusavailability(Fanetal.2003;Zhu&Lynch2004).HigherRCAlineshavemorecorticalairspaceinroottissues(Figs1&7),whichmayreducerootcarboncostforrootmaintenanceinhigherRCAlines.Thus,higherRCAlinesmayhavemorecarbonandenergyavailableforrootconstruction,permittingexplorationofdeepersoilsunderwaterstress(Figs3,4,6,9&10).Covariateanalysisshowedthattherewasnosigni -cantrelationshipbetweentotalrootdryweightandper-centageofRCA(F=0.65,P=0.57).Inotherwords,RCAwasnotdrivenbythesizeofrootsystemorthesizeoftheplant(Fig.5).WefoundhighRCAlineswithdeeperrootingunderwaterstresshadhigherleafrelativewatercontent(Fig.11).Takentogether,these ndingsdemonstrateRCAplaysanimportantroleforplantadaptationtodrought.Wefoundthatseminalrootrespirationperunitlength(y1)wassigni cantlyassociatedwithpercentageofRCA

50Root length 40–50 cm deep

4030

2010080Root number 40–50 cm deep

ef

WSWW

a

ab

abc

cd

dde

de

WSWW

c–1

)Total root length (cm frame

60

bc

15

40

20

33

248

331

34

77

10

5

284364

Low RCAHigh RCA

Root counts (No. frame-1)

Recombinant inbred lines

Figure8.Rootlengthdensity(upperpanel)androotnumberdensity(lowerpanel)at56dafterplanting(DAP)from40to50cmdeepsoilcoresegmentsforsevengenotypesinbothwell-watered(WW)andwater-stressed(WS)conditionsinthe eld.Datashownaremeans SEofthemean(n=4).Meanswiththesamelettersarenotsigni cantlydifferent(P 0.05).

25201510

acquisition,sincedeepersoildomainsgenerallyhavegreaterwatercontentcomparedwithshallowsoildomains,asevidencedinthisstudybytimedomainre ectometry(TDR)measurementsinthe eld(Fig.6)andsoilwaterpotentialmeasurementsat35DAPinmesocosms.

Thereisincreasingevidencethatrootmetaboliccostsareanimportantcomponentofplantgrowthandadaptationunderlowphosphorusavailability(Zhu&Lynch2004;Lynch&Ho2005;Zhu,Kaeppler&Lynch2005c).Rootcostcanbeestimatedastotalbelow-groundexpenditureofcarbon,whichisusedforrootconstruction,rootmainte-nance,andionuptake(Lambers,Atkin&Scheurwater1996;Lambersetal.2002;Lynch&Ho2005).Inthisstudy,speci crootrespirationperunitoflengthandweightweresigni cantlyreducedbywaterstress(Fig.3),whichis

5

33

248

331

34

77

284

364

Low RCAHigh RCA

Recombinant inbred lines

Figure9.Rootlengthperframe(upperpanel)androotcounts

perframe(lowerpanel)byminirhizotrontechniqueat42dafterplanting(DAP)at50cmdeepsoilforsevengenotypesinbothwell-watered(WW)andwater-stressed(WS)conditionsinthe eld.Datashownaremeans SEofthemean(n=4).Meanswiththesamelettersarenotsigni cantlydifferent(P 0.05).

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