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XylemSapAscentbyBulkFlow:AReview
• Themovementofxylemsapagainstgravityismaintainedbythetranspira7on‐cohesion‐tensionmechanism
• Transpira7onlowerswaterpoten7alinleaves,andthisgeneratesnega7vepressure(tension)thatpullswaterupthroughthexylem
• Thereisnoenergycosttobulkflowofxylemsap
Stomataregulatetherateoftranspira<on
• Leavesgenerallyhavebroadsurfaceareasandhighsurface‐to‐volumera7os
• Thesecharacteris7csincreasephotosynthesisandincreasewaterlossthroughstomata
Stomata:MajorPathwaysforWaterLoss
• About95%ofthewateraplantlosesescapesthroughstomata
• Eachstoma
– Flankedbyapairofguardcells
– Controlthediameterofthestomabychangingshape
2
MechanismsofStomatalOpeningandClosing
• Changesinturgorpressureinguardcellsopenandclosestomata
– Primarilyfromthereversibleuptakeandlossofpotassiumions
• K+co‐transportedbyprotonpumps
Fig. 36-17
Radially orientedcellulose microfibrils
Cellwall
Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed
VacuoleGuard cell
(a) Changes in guard cell shape and stomatal opening and closing (surface view)
Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed
(b) Role of potassium in stomatal opening and closing
H2O H2O
H2O
H2O H2O
H2O H2O
H2O
H2OH2O
K+
S<muliforStomatalOpeningandClosing
• Generally,stomataopenduringthedayandcloseatnighttominimizewaterloss
• Stomatalopeningatdawnistriggeredby:
– Light
• Bluelightreceptorss7mulateK+uptake
– CO2deple7on
– Internal“clock”inguardcells
• Func7onseveninthedark
• circadianrhythm
– 24‐hourcyclesofbehavior
3
EffectsofTranspira<ononWil<ngandLeafTemperature
• Plantslosealargeamountofwaterbytranspira7on
– Ifnotreplacedplantwillwilt
• Transpira7onalsoresultsinevapora<vecooling
– Canlowerthetemperatureofaleafandpreventdenatura7onofvariousenzymesinvolvedinphotosynthesisandothermetabolicprocesses
Adapta<onsThatReduceEvapora<veWaterLoss
• Xerophytes
– plantsadaptedtoaridclimates
– Haveleafmodifica7onsthatreducetherateoftranspira7on
• Crassulaceanacidmetabolism(CAM)
– Specializedformofphotosynthesiswherestomatalgasexchangeoccursatnight
Fig. 36-18 Ocotillo (leafless)
Oleander leaf cross section and flowers
Cuticle Upper epidermal tissue
Ocotillo leaves
Trichomes(“hairs”)
Crypt Stomata Lower epidermaltissue
100 µm
Ocotillo after heavy rain Old man cactus
4
SugarTransport
• Transloca<on
– Processoftranspor7ngtheproductsofphotosynthesisthroughphloem
MovementfromSugarSourcestoSugarSinks
• Phloemsap
– Aqueoussolu7onthatishighinsucrose
– Travelsfromasugarsourcetoasugarsink
• Sugarsource
– Anorganthatisanetproducerofsugar,suchasmatureleaves
• Sugarsink
– Anorganthatisanetconsumerorstorerofsugar,suchasatuberorbulb
• Astorageorgancanbebothasugarsinkinsummerandsugarsourceinwinter
• Sugar
– Mustbeloadedintosieve‐tubeelementsbeforebeingexposedtosinks
• Dependingonthespecies
– Maymovebysymplas7corbothsymplas7candapoplas7cpathways
• Transfercells
– Modifiedcompanioncellsthatenhancesolutemovementbetweentheapoplastandsymplast
• Viaprotonpumpco‐transport
MovementfromSugarSourcestoSugarSinks
5
Fig. 36-19
Mesophyll cellCell walls (apoplast)
Plasma membranePlasmodesmata
Companion(transfer) cell
Sieve-tubeelement
High H+ concentration CotransporterProtonpump
Low H+ concentration
Key
Apoplast
Symplast Mesophyll cellBundle-sheath cell
Phloemparenchyma cell
SucroseATP
H+
H+ H+
S
S
Fig. 36-19a
Key
Apoplast
Symplast
Mesophyll cellCell walls (apoplast)
Plasma membranePlasmodesmata
Companion(transfer) cell
Sieve-tubeelement
Mesophyll cellBundle-sheath cell
Phloemparenchyma cell
• Inmanyplants
– Phloemloadingrequiresac7vetransport
• ProtonpumpingandcotransportofsucroseandH+
– Enablethecellstoaccumulatesucrose
• Atthesink
– Sugarmoleculesdiffusefromthephloemtosink7ssues
• followedbywater
MovementfromSugarSourcestoSugarSinks
6
BulkFlowbyPosi<vePressure
• Sapmovesthroughasievetubebybulkflowdrivenbyposi7vepressure
– Calledpressureflow
• Increasingpressureatsourceend
• Reducedpressureatsinkend
Animation: Translocation of Phloem Sap in SummerAnimation: Translocation of Phloem Sap in Summer
Animation: Translocation of Phloem Sap in SpringAnimation: Translocation of Phloem Sap in Spring
4
Fig. 36-20
3
2
1
1
2
34
Vessel(xylem)
Sieve tube(phloem)
Source cell(leaf) Loading of sugar
Uptake of water
Unloading of sugar
Water recycled
Sink cell(storageroot)
Sucrose
H2O
H2O
Bul
k flo
w b
y ne
gativ
e pr
essu
re
H2OSucrose
Bul
k flo
w b
y po
sitiv
e pr
essu
re
• Thepressureflowhypothesis
– Explainswhyphloemsapalwaysflowsfromsourcetosink
• Experimentshavebuiltastrongcaseforpressureflowasthemechanismoftransloca7oninangiosperms
MovementfromSugarSourcestoSugarSinks
7
Fig. 36-21
Sapdroplet
25 µm
Sieve-tubeelement
Stylet Sap droplet
Aphid feeding Stylet in sieve-tubeelement
Separated styletexuding sap
EXPERIMENT
Thesymplastishighlydynamic
• Thesymplastisaliving7ssueandisresponsiblefordynamicchangesinplanttransportprocesses
Plasmodesmata:Con<nuouslyChangingStructures
• Plasmodesmata
– Canchangeinpermeability(openorclose)inresponseto
• Turgorpressure
• Cytoplasmiccalciumlevels
• CytoplasmicpH
• Plantvirusescancauseplasmodesmatatodilate
– Normally~2.5nm
• Dilateto>10nm
– Allowingvirusestofitthrough
8
ElectricalSignalinginthePhloem
• Thephloemallowsforrapidelectricalcommunica7onbetweenwidelyseparatedorgans
– Responsibleforrapidmovementsintouchsensi7veplants
• Venusflytrap
– Canini7atechangesin
• geneexpression
• Photosynthesis
• Respira7on
Phloem:AnInforma<onSuperhighway
• Phloem
– isapassagewayforsystemictransportofmacromoleculesandviruses
• Systemiccommunica7on
– Helpsintegratefunc7onsofthewholeplant
• Iesignalsforconversionofvegeta7vemeristemstofloralmeristems
Youshouldnowbeableto:
1. Describehowprotonpumpsfunc7onintransportofmaterialsacrossmembranes
2. Definethefollowingterms:osmosis,waterpoten7al,flaccid,turgorpressure,turgid
3. Explainhowaquaporinsaffecttherateofwatertransportacrossmembranes
4. Describethreeroutesavailableforshort‐distancetransportinplants
9
5. Relatestructuretofunc7oninsieve‐tubecells,vesselcells,andtracheidcells
6. Explainhowtheendodermisfunc7onsasaselec7vebarrierbetweentherootcortexandvascularcylinder
7. Defineandexplainguha7on
8. Explainthisstatement:“Theascentofxylemsapisul7matelysolarpowered”
9. Describetheroleofstomataanddiscussfactorsthatmightaffecttheirdensityandbehavior
10. Tracethepathofphloemsapfromsugarsourcetosugarsink;describesugarloadingandunloading