744J.Zhuetal.

0–20

20–4040–6060–8080–100100–120120–140

0–20

Soil layer (cm)

20–4040–6060–8080–100100–120120–140

0–2020–4040–6060–8080–100100–120120–140

15

30

45

Figure4.Rootlengthdistributionin

soillayersofsixgenotypes(lowRCAlinesontheleft,andhighRCAlinesontheright)at35dafterplanting(DAP)inbothwell-watered(WW)and

water-stressed(WS)conditionsinthemesocosms.Datashownaremeans SEofthemean(n=4).**showssigni cantlydifferentpairedmeansinWSandWW(P 0.05).

60

75

15

30

45

60

75

90

Root length (m)

Fieldstudy

Volumetricsoilmoisturewasmaintainedaround30%atboth25cmand50cmdepthsinwell-wateredconditionsthroughoutthegrowingseason(Fig.6).Forwater-stressedplants,volumetricsoilmoistureprogressivelydecreasedfrom30%to10%at25cmdepth,andto15%at50cmdepthinrainoutshelters(Fig.6).

WaterstressincreasedRCAinthehighRCAlinesat56DAPinthe eld(Fig.7).Lines34,77,284and364(highRCAlines)averaged12.6%RCA,whilelines33,248and331(lowRCAlines)averaged1.98%RCA.Inwell-wateredconditions,therewasnosigni cantdifferenceforRCAamongthelines(P<0.05).Water-stressedplantshaddoubledrootlengthdensityandrootnumberdensityat40–50cmdepthinthesoilcomparedwithwell-wateredplantsat56DAPinallsevenlines(Fig.8).Rootlengthdensityat40–50cmdepthwassigni cantlygreaterinthefourhighRCAlinescomparedwiththethreelowRCAlinesat56DAP(Fig.8).ThefourhighRCAlinesaveraged3.4timestherootnumberdensityat40–50cmdepthcom-paredwiththethreelowRCAlinesunderwaterstress(Fig.8).Resultsfromminirhizotronmeasurementsfurthercon rmedthathighRCAlineshaddeeperrootingintermsofrootlengthperframeandrootnumberperframecom-paredwiththelowRCAlinesat42DAP(Fig.9).Therewassigni cantcorrelationbetweenrootlengthdensitymea-suredbysoilcoringandbytheminirhizotrontechnique(R2=0.63,P<0.01)(Fig.10).Rootnumberdensitywasalsosigni cantlycorrelated(R2=0.31,P<0.01)betweensoilcoringandminirhizotrontechniques(Fig.10).

Mid-dayleafrelativewatercontent(RWC)at42DAPinthehighRCAlineswassigni cantlygreaterthanforthelowRCAlinesinwater-stressedconditions,whiletherewasnosigni cantdifferenceamonggenotypesforRWCinwell-wateredconditions(Fig.11).

Shootbiomassofthesevenlineswasreducedbyanaverageof46%underwaterstressat oweringcomparedwithwell-wateredplantsinthe eld(Fig.12).Underwaterstress,thefourlineswithhighRCAhad43%moreshootbiomassat oweringcomparedwiththethreelineswithlowRCA.ThehighRCAlinesaveragedeighttimestheyieldofthelowRCAlinesunderwaterstress(P<0.001)(Fig.12).Theincreasedyieldunderwaterstresswascausedbyincreasesofbothkernelnumbersperplantandindividualkernelweight(Fig.13).

DISCUSSION

OurresultssupportthehypothesisthatRCAincreasesdroughttolerancebyreducingrootmetaboliccosts,permit-tinggreaterrootgrowthandwateracquisition.Inmeso-cosmsundercontrolledwaterstressconditions,threehighRCAlineshadlowerspeci crootrespiration,greatermaximumrootdepthandgreatershootbiomassthanthreelowRCAlines.Inthe eldunderwaterstressconditionsinrainoutshelters,fourhighRCAlineshadgreaterrootdepth,greaterleafrelativewatercontentandsubstantiallygreatergrainyieldthanthreelowRCAlines.

FortheanalysisofthephysiologicalfunctionofRCA,weusedRILs,whichpermitthecomparisonofcloselyrelatedgenotypeswithidenticalgeneticbackgrounds,yetwithout

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