748J.Zhuetal.

700600Seed number/plant

500400

3002001000Individual seed dry weight (g)

0.2

thatRCAreducesthemetaboliccostsofsoilexploration,leadingtogreaterwateracquisitionindryingsoil.WesuggestthatRCAdeservesconsiderationasatraittoimprovethedroughttoleranceofmaizeinbreedingprograms.

ACKNOWLEDGMENTS

ThisresearchwassupportedbyUSDA-CSREESgrant207-35100-18365toJPL,SMKaepplerandKMB.WeacknowledgeDrShawnKaepplerattheUniversityofWis-consinforprovidingseedsandRobertSnyderfortechnicalassistance.WealsothankAmyBurtonforassistanceinquanti cationofaerenchyma.

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0.1

33

248

331

34

77

284

364

Low RCAHigh RCA

Recombinant inbred lines

Figure13.Seednumberperplantandindividualseeddryweightofsevengenotypesat oweringstageinwell-watered(WW)andwater-stressed(WS)conditionsinthe eld.Data

shownaremeans SEofthemean(n=4).Meanswiththesamelettersarenotsigni cantlydifferent(P 0.05).

rootexplorationofdeepersoil,greaterwateracquisition,andgreatershootbiomassandyieldunderstress(Figs2,3,4,6,8,11,12&13).Collectively,theseindicate(1)anincreaseofRCAindifferentroottypesatvariousdevelop-mentalstagesinwaterstresstobeapositiveadaptationforplants;and(2)themagnitudeofRCAresponsestowaterstressmayberelatedtoroottype,plantdevelopmentalstage,anddroughtseverity.

Roothydraulicconductivityisanimportantparameterforplantwateracquisition.Roothydraulicconductivitywasreducedundernitrogen,phosphorusandsulfurde ciencies(Karmokeretal.1991;Clarksonetal.2000).Fanetal.(2007)reportedthataerenchymaformedunderphosphorusde ciencycontributestothereducedroothydrauliccon-ductivityinmaizerootsinasolutionculturesystem.Poten-tialtradeoffsforRCAunderwaterstress,includingreducedhydraulicconductance,meritfurtherresearch.

WeobservedsubstantialgeneticvariationforRCAamongsevenmaizegenotypesunderwater-stressedcondi-tions.Ourresultsareentirelyconsistentwiththehypothesis

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