xylem sap ascent by bulk flow: a revie · xylem sap ascent by bulk flow: ... a review •the...

1

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