WaterTransport inMaizeRoots' MEASUREMENTOF HYDRAULICCONDUCTIVITY, SOLUTEPERMEABILITY, AND OF REFLECTIONCOEFFICIENTS OF EXCISEDROOTSUSINGTHE ROOT PRESSUREPROBE

Aroot pressure probe hasbeenusedtomeasure theroot pressure (P,) exerted byexcised mainroots ofyoungmaize plants (ZeaMaysL.). Defined gradients ofhydrostatic andosmotic pressure could besetup between rootxylem andmedium toinduce radial water flows across the root cylinder inbothdirections. Thehydraulic conductivity oftheroot (Lpr) wasevaluated fromrootpressure relaxations. Whenpermeating solutes wereadded tothemedium, biphasic rootpressure relaxations wereobserved withwater andsolute phases androot pressure minim (maxima) which allowed theestimation ofpermeability (Ps,) andreflection coefficients (5ff) ofroots. Reflection coefficients were: ethanol, 0.27; mannitol, 0.74; sucrose, 0.54; PEG1000, 0.82; NaCl, 0.64; KNO3,0.67, andpermeability coefficients (in10-meters persecond): ethanol, 4.7; sucrose, 1.6; andNaCI, 5.7. Lp,wasverydifferent forosmotic and hydrostatic gradients. Forhydrostatic gradients Lp,was1.10-7 meters persecond permegapasal, whereas inosmotic experiments thehydraulic conductivity wasfound tobeanorder ofmagnitude lower. Forhydrostatic gnrdients, theexosmotic Lp,wasabout 15%larger thantheendosmotic, whereas inosmotic experiments thepolarity inthewater movement was reversed. Theseresults either suggest effects ofunstirred layers atthe osmotic barrier intheroot, anasymmetrical barrier, and/or mechanical effects. Measurements ofthehydraulic conductivity ofindividual root cortex cells revealed anLpsimilar toLp,(hydrostatic). Itisconcluded that, inthepresence ofexternal hydrostatic gradients, watermoves primarily intheapoplast, whereas inthepresence ofosmotic gnrdients this component ismuchsmaler inrelation tothecell-to-cell component (symplasmic plus transcellular transport).