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BIOC40H3 (14)
Lecture

Lecture 3

5 Pages
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Department
Biological Sciences
Course Code
BIOC40H3
Professor
Connie Soros

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Plant Physiology – Lecture 3  Aswediscussedinthelastlectureonwater andplants, thereis aninherent conflict betweenaplant’sneedtotakein CO2 for photosynthesisandto conservewater. Thisis because bothCO2and watermust usethesamepathway throughstomates, with CO2 goingin, andwater beinglost out ofthepores  We discussedthe special properties ofwater last lecture, (mostofthem, dependent ontheextensive hydrogenbondingin water molecules)  now wewilllook atmanyofthewaysthat plantshaveevolvedadaptationstomovewaterwithintheplant, control waterloss fromleavesandtoreplacethewater thatislost. WATER IN THE SOIL  Water potential ( ) of soilscanbedividedintothreecomponents 1. osmoticpressure( ) 2. hydrostatic pressure( ) 3. gravitational potential )  Sincesoluteconcentrationinsoiltendstobe verylow, theosmoticpressure ) isgenerallynegligible.  Forwet soilsthe hydrostaticpressure( )is veryclose zero, but assoildriesout, thehydrostaticpressure decreases and becomes negative  Remember that water hasa highsurfacetensionthattendstodecrease air-waterinterfaces, but because ofhighadhesiveforces inwater, italsoclingstothesurfaces ofsoil particles. Therefore, asthewater contentofsoildecreases, thewaterrecedesinto theinterstitial spaces(smallest channels)between soilparticlesformingair-water surfaceswith curvaturesthat correspondto thebalancebetweenthetendencytominimizethesurfaceareaofthe air-waterinterfaceandtheattractionofwater forsoil particles  Water under acurvedsurfacewilldevelopanegative hydrostaticpressure( ).Assoil dries out, water isfirstremovedfrom thelargest spaces betweensoilparticlesandsubsequentlyfrom successivelysmaller spacesbetweenthe soilparticles.  Asthesoilbecomes drier,thecurvature oftheair-water surfacesinporesincreases causingthe hydrostaticpressureto become increasinglynegative(greater tension). Thelastwater potential component is thegravitational pote)tial (  Unlikeinwater, wherethis parameter is usuallynegligible gravityplaysanimportantroleinsoil drainage and gravitational potentialisproportionaltoelevation, higherat higher elevationandlower at lowerelevation WATER ABSORPTION BY ROOTS  Effectivewater absorptionbytherootsinvolvesintimate contact betweenthe surfaceoftheroot andsoil  The amount of surfacecontact betweenthe soil androotsismaximized by root hairs. Root hairsarefilamentousoutgrowths ofroot epidermalcellsthatgreatlyincreasethesurface areaoftheroot providinggreater capacityforabsorptionofions and water from the soil  Asmentionedinthefirst lecture, water enterstherootsmostreadilyat thetips(also usuallythelocation ofmost abundantroot hairs), oldermorematureareasoftheroot(closertothebaseoftheplant)haveoftendevelopedanouterlayer of protective tissuecalledan exodermis(or hypodermis)whichcontainshydrophobicmaterial(suberinina casparianstrip)initswalls makingthetheseregionsoftherootlesspermeabletowater  By decreasingthepermeabilityintheolderregionsoftheroot, tensions can extend further downintothe root system allowing water uptakefrom thedistal (farthestaway) regions oftheroot. PATHWAYS FOR WATER INTHE ROOT  Therearethreedifferentpathwaysthatwater canflow throughintheroot, the apoplast, thesymplast, and thetransmembrane pathway 1. Apoplast ­ acontinuous system ofcellwalls, intercellular airspaces oflivingcellsandthelumens(cavities)ofnon- livingcells(eg. Xylemelementsand fibers) ­ Intheapoplasticpathwayofroots, watermovesthrough cellwallsand extracellular spaceswithout crossinganymembranes ­ Water doesnot passthroughthelivingpartsofcells. Water movementthroughtheapoplastisobstructed at theendodermisbythe casparianstrip. Thecasparian stripis aband withintheradial walls ofthe endodermisthatisimpregnatedwith suberin(awax-like, hydrophobic substance)atthis point, water and solutesareforcedto passthroughtheplasmamembrane andintothelivingpartofthe plant(symplast) ­ Waterwillpassthroughtheaquaporins(water selectivechannelsof integral proteinswithinthe bilayer of theplasmamembrane) ­ Evidenceforthisisfoundinstudies (eg. Siefritzet al. 2002, Martreet al. 2002)wherethe genesfor aquaporinshavebeen down-regulated, resultinginreducedhydraulic conductivityoftherootsandeither plantsthatwilteasily orcompensate by producinglargerootsystems ­ Aquaporins canberegulated(ie.Gated) inresponsetointercellular pH (increasedintracellular pH can occur inresponsetolow temperature oranaerobic conditions). Anincreasecytoplasmic pH altersthe conductanceofaquaporinsinrootcells greatlyreducingtheirpermeabilitytowater. 2. Symplast ­ Consists oftheentirenetwork of cellcytoplasminterconnected byplasmodesmata ­ Inthispathwaywatertravelsacrosstherootintheliving parts ofcellsvia plasmodesmata. 3. Transmembranepathway ­ Istheroutewatertakes, byentering acellon onesideandexitingontheother sidebeforeenteringthe next cellin the series ­ Inthispathway, watercrossestheplasmamembrane ofeachcellinitspathtwice(first entering, then exitingthecell). Theremay alsobe transportacrossthetonoplast(vacuolemembrane) GUTTATION  When soilwater pressureishighand transpirationratesarelow, some plantswilldeveloprootpressureanda positive pressure inthexylem (iftranspirationishigh, water istakenintotheleavesand lost totheatmosphereveryrapidlysothat apositive pressurenever developsinthe xylem)  When a positive pressuredoesoccur, plantswilloftenproduceanexudation ofliquiddropletsontheedges oftheirleavesdue toroot pressure. Thisphenomenon iscalled guttation. The positivexylem pressurewill causetherelease ofxylem sap through specializedporescalled hydathodesthat areassociatedwith vein endings attheleafmargin  Guttationisthedewdropsthat youseeontipsofgrass leavesinthemorning. WATER TRANSPORT IN THE XYLEM  Xylem constitutesthelongestpartofthe pathwayofwatertransport, oftenmorethan99.5%ofthewater transportpathway.  The structure of xylemcontributestothemovement ofwater fromthe rootstotheleavesandnegativehydrostaticpressure generatedbytranspirationintheleavespullswaterthroughthexylem  Pressure-drivenbulkflow isresponsibleforlong distancetransportofwater inthe xylem. Thewateratthetopofatree develops alargetension(anegative hydrostaticpressure) whichpullswaterthroughthexylem. Thismechanism is calledthe cohesion-tensiontheoryofsapascent becauseitreliesonthecohesive (mutual attractionbetweenwater molecules)properties ofwater inorder tosustainlargetensionsinthexylemwater columns  The negativepressurethat causeswater tomoveup throughthexylem develops atthe surfaceofthe cellwalls ofleaves  The cellwallsarecomposedofhydrophilic cellulosemicrofibrils and other hydrophiliccomponents, that causewater to adhere(adhesiveproperty)tothewall  Asw
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