Transpiratrion: Types, mechanism, factors & significance

This PPT intends to explore the basic mechanism of transpiration along with the factors and significance.

1. PLANT PHYSIOLOGY
TRANSPIRATION, SIGNIFICANCE AND
FACTORS CONTROL TRANSPIRATION
by
N. Sannigrahi, Associate Professor,
Dept. of Botany, Nistarini College,
Purulia (W.B) India

2. TRANSPIRATION

3. TRANSPIRATION
Transpirationisaprocessinwhichlargeamountsof
waterartecontinuouslylostintheformofvaportothe
atmospherefromtheevaporatingsurfacesoftheaerial
partsoftheplants.Onlyasmallportionoftheabsorbed
waterisretainedforthemaintenanceofcellular
metabolismandturgidity.Waterevaporatescontinuously
fromthecellssurfaceexposedtoairandthisreplacedby
theabsorptionfromthesoil.Thetranspirationis
markedlydifferentfromevaporationandguttation.
Evaporationiscompletelyaphysicalphenomenonin
whichliquidwaterischangedtoitsvaporstatethrough
anyopensurface.Theguttationisobservedinthe
herbaceousplantswherewateriddroppedfromthe
marginoftheleafbyaspecialorgancalledhydathodes
throughaspecialtissuecalledepithem.

4. TYPES OF TRANSPIRATION
Thelossofwaterinthevaporformfromtheexposedpartsofa
plantiscalledtranspiration.Thelossofwaterdueto
transpirationisquitehigh2litersperdayinSunflower,36—45
litersinAppleandupto1toneperdayinElmtree.Rather98-
99%ofthewaterabsorbedbyaplantislostintranspiration.
Hardly0.2%isusedinphotosynthesiswhiletheremainingis
retainedintheplantduringgrowth.
Youngstems,flowers,fruits,etc.alsotranspirealot.
Maturestemstranspireverylittle.Transpirationfrom
stemsiscalledcaulinetranspiration.Dependinguponthe
plantsurfacetranspirationisofthefollowingfourtypes—
stomatal,cuticular,lenticularandbark.

5. SYSTEMS OF TRANSPIRATION

6. TURGOR PRESSURE & STOMATA
Stomatafunctionasturgor-operatedvalvesbecausetheir
openingandclosingmovementisgovernedbyturgorchanges
oftheguardcells.Whenever,theguardcellsswellupdueto
increasedturgor,aporeiscreatedbetweenthem.Withtheloss
ofturgorthestomataporesareclosed.
Stomatagenerallyopenduringthedayandcloseduringthe
nightwithafewexceptions.Theimportantfactorswhich
governthestomataopeningarelight,highpHorreducedCO
2
andavailabilityofwater.Theoppositefactorsgovernstomata
closure,viz.,darkness,lowpHorhighCO
2anddehydration.
Thus,thechangeofturgorpressurewithintheguardcellsare
oneoftheimportantfactorsintheopeningandclosingof
stomata.

7. STOMATA-MICROSCOPIC VIEW

8. OPENING AND CLOSING OF STOMATA
Theopeningandclosingofstomatawasaverydebating
oneandfourimportanttheorieshavebeenadvocatedto
extinguishthisdebateTheyareasfollows:
1.Theoryofphotosynthesisinguardcells,
2.Starch-Sugarinterconversiontheory,
3.TheoryofGlycolatemetabolism,
4.PotassiumpumptheoryandroleofABAoractive
Potassiumpumptheory.
Thepotassiumpumptheoryisthemostimportantone
whichmainlyconsiderstheinfluxofpotassiumionsand
effluxofhydrogenionsintheguardcellstomaintainthe
openingandclosingofstomataatthecostofthe
involvementoftheABA.

9. MECHANISM OF TRANSPIRATION
Therearenumberoftheoriestoexplaintheopening&closingofthe
mechanismsofstomatabutthemostacknowledgedtheoryistheinfluxofK
ionandeffluxofHioninguardcellsasgivenbyLevittin1974.
MalateorK
+
ionPumpHypothesis(ModernTheory):
ThemainfeaturesofthetheorywereputforwardbyLevitt(1974).
Accordingtothistheory,pHoftheguardcellcanriseduetoactiveH
+
uptakebyguardcellchloroplastsormitochondria,CO
2assimilationby
mesophyllandguardcells.AriseinpHcauseshydrolysisofstarchtoform
organicacids,especiallyphosphoenolpyruvate.Starch→Hexose
Phosphate→PhosphoenolPyruvate.
Phosphoenolpyruvatecanalsobeformedbypyruvicacidofrespiratory
pathway.WiththehelpofPEPcarboxylase(PEPcase),itcombineswith
availableCO
2toproduceoxalicacidwhichgetschangedintomalicacid.
MalicaciddissociatesintoH
+
andmalate.H
+
ionspassoutoftheguard
cellsactively.Inexchange,K
+
ionspassinwardly.SameCI
–
ionsmayalso
enterguardcellsalongwithK
+
ions.Guardcellsmaintaintheirelectro
neutralitybybalancingK
+
withmalateandCl
–

10. MECHANISM
Inthecombinedstatetheypassintothesmallvacuolesandincrease
theosmoticconcentrationoftheguardcells.Asaresultguardcells
absorbwaterfromthenearbyepidermalcellsthroughendosmosis,
swellupandcreateaporeinbetweenthem.
Duringstomataclosure,theH
+
ionsdiffuseoutoftheguardcell
chloroplasts.ItdecreasespHoftheguardcellcytoplasm.Anymalate
presentinthecytoplasmcombineswithH
+
toformmalicacid.
Excessofmalicacidinhibitsitsownbiosynthesis.HighCO
2
concentrationalsohasasimilareffect.Un-dissociatedmalicacid
promotesleakageofions.AsaresultK
+
ionsdissociatefrommalate
andpassoutoftheguardcells.
Formationofabcisicacid(asduringdroughtormidday)also
promotesreversalofH
+
=↔K
+
pumpandincreasesavailabilityof
H
+
insidetheguardcellcytoplasm.LossofK
+
ionsdecreases
osmoticconcentrationofguardcellsascomparedtoadjacent
epidermalcells.

11. FACTORS OF TRANSPIRATION
Thiscausesexosmosisandhenceturgidityoftheguardcells
decreases.Itclosestheporebetweentheguardcells.Simultaneously
theorganicacidsaremetabolizedtoproducestarch
Stomatamovementsareinfluencedbyanumberof
environmentalfactorslikelight,temperature,humidity,
wateravailabilityandCO
2concentration.Internalor
endogenousfactorsincludegrowthhormones,organicacids,
K
+
,Сl
–
andH
+
ions.
LIGHT: In the majority of plants the stomata open in light
and close in darkness. The light intensity required for
stomata opening is quite low (250 ft. candles in Tobacco).
Even moon light is sufficient in some cases.
Both red and blue parts of light are effective though the
latter is slightly more effective. However, in succulents or
CAM plants (crassulaceanacid metabolism), the stomata
remain closed during daytime. They open only during dark,
e.g., Agave, Opuntia, and Pineapple.

12. FACTORS
TEMPERATURE :Q
10forstomatalopeningistwo.At38°-
40°C,stomatacanopenincompletedarkness,whileat0°C
theyremainclosedevenincontinuouslight.Normallyhigh
temperatureabove30°Creducesstomatalopeninginmany
species.
HUMIDITY:Inhumidenvironmentthestomata
remainopenedforlongerperiodswhileindry
environmenttheyremainclosedforlongerperiods.
WATER STRESS:Plantsundergowaterstressif
availabilityofwaterislessthantherateof
transpiration.Waterstress(=waterdeficit=moisture
deficit)bringsaboutstomatalclosureduetoABAand
riseinDPDofepidermalcells.
Itcausesclosureofstomata.

13. FACTORS
CARBONDIOXIDE:LowCO
2concentrationusuallyinduces
openingofstomatawhilehighCO
2concentrationclosesthesame.In
someplantsmerebreathingovertheleavescausesstomataclosure.
However,guardcellsaresensitivetoCO
2concentrationonlyfrom
theirinnerside(i.e.,concentrationintheleafinterior).Stomataofa
planttransferredtodarkCO
2freeenvironmentwillremainclosed
buttheywillopeninlightwheninternalCO
2isutilized.
pH:RiseinpHisknowntoberequiredforopeningof
stomatawhileafallinpHinducesclosureofstomata.
HORMONES :Cytokininsareessentialforopeningof
stomatawhileabscisicacidtakespartinstomataclosure.
MINERALS:Stomataopeningdependsuponavailabilityof
K
+
ionsfromadjacentepidermalcells.Anumberofother
mineralsarealsoessentialforstomatamovements,e.g.,P,
N,Mg,Ca,etc.

14. SIGNIFICANCE OF TRANSPOIRATION
Transpirationplaysaveryimportantroletogovernthe
metabolicandphysiologicalactivitiesofplants.Theyare
asfollows:
1.Transpirationallowstranspirationpulltoliftthewater
againstthegravitationalpullingforceasexplainedin
adhesionandcohesionofwaterandtranspirationpull
theoryduringascentofsap.
2.Alargeamountofwaterinlostbuttheprocessis
essential.Hence,itiscallednecessaryevil.
3.Thecoolingroleofthetranspirationallowstheleaves
tofunctionnormallyevenatthebrightestsunlight
withoutinjury.
4.Ithasmultipleeffectonmineraltransport,water
movementandtomaintainoptimumturgidityofthe
cells.