Plant Physiology-Water relations

This Power Point Presentation intends to present the basic idea of water-plant relationship.

1. Welcome to
Plant water Relations
By
N. Sannigrahi, Associate Professor,
Department of Botany,
Nistarini College, Purulia (W.B) India

2. Wateristheuniversalsolventanditistheelixiroflife.The
plantsabsorbwaterandsolublemineralsaltsfromthesoilby
theirrootsystem.Thisfunctionofabsorptionisfacilitatedby
theunicellularroothairspresentontheroots.These
unicellularroothairsenterintheinterspacesofthesoil
particles,irregularly.Theseroothairsabsorbthewaterfound
intheformofthinfilmsaroundthesesoilparticles.
CellasaphysiologicalunitCellsarethebuildingblocksfrom
whichlivingthingsaremade.Asinglecellmaymakeupan
entireorganism,orgroupsofcellsmaybelooselyorganized
andlivetogether.Wateravailabilityincellensurestheosmotic
pressureofcellandthesustainableflowofthedifferent
nutrients,sucroseandotherlifesupportingelementsarealso
maintained.Themovementofstomataandsysmonastic
movementistheoutcomeofwaterabsorption.,

4. Thewateristhemostimportantfactorforthevitalfunctions
oftheplants.Theplantscannotsurviveinitsabsence.Wateris
averyimportantsolvent,andusuallyconsistsmostofthepart
oftheprotoplasm.Manybiochemicalreactionsgoingoninthe
plantsarecatalyzedbymanyenzymes,formedinthe
protoplasm.Usually75%quantityofwaterisfoundinthe
cytoplasm.Usuallyintheleaves,thequantityofwateris75%,
whereasinthestemsitis60%.Inseveralhydrophytes,e.g.,
algae,etc.,thisquantityexceedsupto98%.Inthesameway,
thequantityofwaterinthexerophytesisusually60%orlesser
thanthis.
Inthedormantseeds,thisquantityofwaterisonlyabout10%.
Thisis,ofcourseverymuchlessforanyvitalfunction,and
therefore,thegerminationoftheseseedsisonlythenpossible,
whentheygetsufficientmoisture.

5. Aboutthreeformsofwaterarefoundinthesoil.Justafterthe
rains,becauseofgravitationsomeamountofwateralongwith
somemineralsaltsgoesinthelowerstrataoftheearth.Thisis
calledgravitationalorfreewaterandcannotbeusedbythe
plants.
Besidesthis,everyparticleofsoilholdssomeimbibedwater
init.Thiswaterisholdupinthesoilparticlewithsuchagreat
imbibitionsforce,thatitcannotbeseparatedfromitforthe
useoftheplant.Thisiscalledhygroscopicwater,whichalso
cannotbeabsorbedfromthesoilbytheplants.Besidesthese
eachsoilparticleissurroundedbyaloosefilmofwater;this
filmisattractedbythecapillaryforcetothesoilparticle,and
suchwaterfilmiscalledcapillarywater.Nowitistobe
noticed,thatonlythiscapillarywatermaybeabsorbedbythe
roothairsoftheplants.

6. Water is tasteless, transparent, highly cohesive and adhesive;
only strong UV light is absorbed.
Water has high tension, has high specific heat and high heat of
Vaporization, high boiling point and high thermal conductivity
Water has high dipole moment due to high electro negativity
of Oxygen than Hydrogen.
Water is a universal solvent and can ionize,
All macromolecular components like DNA, protein remain
dissolved in water,
Maximum density of water at 4 ℃, an anomalous property of
water,
Elements more electropositive than Hydrogen like Ca, Na, Li,
K & Cs bind to hydroxides.

7. Allplantsandotherlivingorganismsrequirefreeenergyto
growandreproduce.
Inthermodynamics,freeenergyrepresentspotentialtodo
work.Accordingtothermodynamiclawseverycomponentof
asystempossessesfreeenergycapableofdoingworkunder
constanttemperatureconditions.
Thepotentialenergyofwateriscalledwaterpotential.Water
potentialisregardedasthetendencyofwatertoleavea
system.Itisoftenusedwhileexplainingthedirectioninwhich
waterwillflowfromonecelltoanother,orfromonepartof
theplanttoanothersuchasfromsoiltoroot,fromrootto
leaves,fromleavestoair,orfromsoiltoair.Thedifferent
otherstepsarefollowedincourseoftheconductionofwater
alongtheconductionprocess.

8. Thepotentialenergyofwateriscalledwaterpotential.Water
potentialisregardedasthetendencyofwatertoleaveasystem.Itis
oftenusedwhileexplainingthedirectioninwhichwaterwillflow
fromonecelltoanother,orfromonepartoftheplanttoanother
suchasfromsoiltoroot,fromroottoleaves,fromleavestoair,or
fromsoiltoair.
Wateralwaysmovesfromaregionofhigherwaterpotentialtothat
oflowerwaterpotential.Itcanalsobesaidthatthedifferencein
waterpotentialbetweentwopointsisameasureoftheamountof
work,i.e.,energy,neededtomovewaterfromonepointtotheother.
Osmoticmovementofwaterinvolvescertainworkdoneandinfact
themaindrivingforcebehindthismovementisthedifference
betweenfreeenergiesofwaterontwosidesoftheselectively
permeablemembrane.Forwaterfreeenergymoleculeisknownas
waterpotential(Ψ
w).

9. Waterpotentialismeasuredintermsofpressure.Common
measurementunitofwaterpotentialisPascal,Pa;1
Megapascalrepresents10bars,i.e.,1Megapascal=10bars.
Onebarisclosetooneatmosphereofpressure,i.e.,1bar=
0.987atmosphereofpressure.Atatmosphericpressurewater
potentialofpurewateriszero,andtherefore,allsolutionsat
atmosphericpressurehavelowerwaterpotentialsthanwater,
i.e.,theyhaveanegativevalue.
Waterpotentialofanysolutionisinfluencedbythree
factors:
(i)Concentration,
(ii)Pressureand
(iii)Gravity.

10. Thiscanberepresentedbyfollowingequation:
Ψ
w=Ψ
s+Ψ
p+Ψ
g
i.e.,Ψ
seffectofsolutes(i.e.,solutepotentialorosmotic
potential)
Ψ
peffectofpressure(i.e.,pressurepotentialorhydrostatic
pressure)
Ψ
geffectofgravity(i.e.,gravitypotential)
Thismeans,purewaterhasahigherpotentialthanthewater
insideacell.Inotherwordsofpurewateriszero,and
therefore,waterpotentialinsideplantcellsisnegative.
Solute Potential Ψ
s:
Solutes present in a cell reduce the free energy of water, or the
water potential.

11. PressurePotential(Ψ
p)orHydrostaticPressure:
Thepositivehydrostaticpressureiscalledturgorpressure.The
pressurepotentialforpurewaterinanopenbeakeriszero.
Gravitypotential(Ψ
g):
Thistermrepresentstheeffectofgravityonwaterpotential.It
dependsontheheightofwater.Ifverticalheightislessthan
fivemeters,theΨ
gisnegligible.InaplantcellonlyΨ
sandΨ
p
areimportant,andconsidered,i.e.,Ψ
w=Ψ
s+Ψ
p.
OSMOSIS:Themovementofsolutionfromhigherfreeenergy
tolowerfreeenergy(Moresolute)acrossthesemi-permeable
membraneiscalledosmosis.Theminimumpressureexerted
thecelltostoptheinflowofthesolutionhavinghigherfree
energyiscalledosmoticpressure.Itisveryimportantforthe
celldynamism.

12. Osmosisofdifferenttypesasstatedbelow:
Endosmosis-Movementofsolventfromexternalsourcesto
acrossthesemi-permeablemembrane.
Endosmosis-Themovementofwaterfromcelltotheexternal
surfaceacrossthemembraneonthebasisofthefreeenergyof
thetwosolutiontakingpartinthisactivities.
CelltoCellosmosis-Thecloselayingcelltakepartinosmosis
eventhedifferenceofosmoticpressureoftheadjoiningcells.
Veryoftenreverseosmosismaytakenintoaccountwiththe
helpofexertingpressureasevidencedinthepurificationof
salinewaterintopotablewater.
Themembranemaybedifferenttypes-impermeable
membrane,permeablemembrane,semi-permeablemembrane
andselectivelypermeablemembrane.

13. Osmoticpressurecanbecalculatedbythefollowing
relationship:
OP=CRT
C=Molarconcentrationofsolution.
R=Gasconstantwhichis0.082
T=Absolutetemperature273°C
1.Hypertonicsolution(Hyper=Morethan):
Asolutionwhoseosmoticconcentration(solutepotential)is
morethanthatofanothersolutionsorcellsap(tonoplasm)is
calledhypertonicsolution.Ifacellisplacedinhypertonic
solutionwatercomesoutofthecellbytheprocessof
exosmosisandcellbecomesflaccid.

14. 2.Hypotonicsolution(Hypo=Lessthan):Asolutionwhose
osmoticconcentration(solutepotential)islessthanthatof
anothersolutionorcellsap(tonoplasm)iscalledhypotonic
solution.Ifacellisplacedinhypotonicsolution,waterenters
intothecellbytheprocessofendosmosisandcellbecomesof
turgid.Thehypotonicnatureofthesoilisaveryimportant
factorsofwaterabsorption.
3.Isotonicsolution(Iso=Same):Asolutionwhoseosmotic
concentration(solutepotential)isequaltothatofanother
solutionorcellsap(tonoplasm)iscalledisotonicsolution.Ifa
cellisplacedinaisotonicsolution,therewillbenochangein
thecellandthesolution.Theisotonicconditionmaintainsa
stedysourceoftheosmoticsystemofthecell.

15. OsmoticPressure:Osmoticpressureisalsodefinedas‘The
hydrostaticpressuredevelopedinasolutionduetotheosmotic
flowofsolvent(suchaswater)whenthesolutionisseparated
frompuresolventbyasemi-permeablemembrane.Osmotic
pressuretermwascoinedbyPfeiffer.Osmoticpressureis
measuredinatmosphere,barsorPascal’s.Instrumentusedfor
measuringO.P.iscalledOsmometer.Nowadays,theterm
osmoticpotential(i.e.solutepotential=Ψs)ispreferredover
osmoticpressure.Numerically,osmoticpotentialisequalto
osmoticpressurebutoppositeinsign.Osmoticpressurehasa
positivevaluewhileosmoticpotential(Ψs)hasequalbut
negativevalue.

16. Osmoticpressureisdirectlyproportionaltotheconcentration
ofdissolvedsolutesinthesolution.Moreconc.solutionhas
higherosmoticpressure.
Osmoticpressureofsolutionisalwayshigherthanitspure
solvent.
Osmoticpressuredoesnotincreasebytheadditionof
insolublesoluteinthesolution.
Thus,duringosmosisthemovementofsolventmolecules
takesplacefromthesolutionwhoseosmoticpressureislower
(i.e.,lessconcentratedorhypotonic)intothesolutionwhose
osmoticpressureishigher(i.e.,moreconcentratedor
hypertonic).

17. PlantCellsasOsmoticSystems:
Livingcellsinplantsformosmoticsystemsduetothe
presenceofsemi-permeableplasmamembraneandthecellsap
havingacertainosmoticpressure.Plasma-membraneactually
isnottrulysemi-permeableasitallowscertainsolutestopass
throughitandhence,itisknownasselectivelypermeableor
differentiallypermeablemembrane.Thetonoplastorthe
vacuolarmembranealsopossessesthesamenature.The
solventincaseofplantsisalwayswater.Thecellwallis
permeable.
Ifalivingplantcellortissueisplacedinwaterorhypotonic
solution(whoseO.P.islowerthanthatofcellsap)water
entersintothecellsapbyosmosis.Thisprocessiscalledas
endosmosis.Asaresultofentryofthewaterintothecellsap,

18. apressureisdevelopedwhichpressestheprotoplasmagainst
thecellwallandthecellbecomesturgid.Thispressureis
calledasturgorpressure.Consequenceoftheturgorpressure
isthewallpressurewhichisexertedbytheelasticcellwall
againsttheexpandingprotoplasm.Atagiventimeturgor
pressure(T.P.)equalsthewallpressure(W.P.).Ifontheother
hand,theplantcellorthetissueisplacedinhypertonic
solution(whoseO.P.ishigherthanthatofcellsap)thewater
comesoutofthecellsapintotheoutersolutionandthecell
becomesflaccid.Thisprocessisknownasex-osmosis.Cellor
tissuewillremainassuchinisotonicsolution.

19. SignificanceofOsmosisinPlants:
(1)Largequantitiesofwaterareabsorbedbyrootsfromthe
soilbyosmosis.
(2)Celltocellmovementofwaterandothersubstances
dissolvedinitinvolvesthisprocess.
(3)Openingandclosingofstomatadependupontheturgor
pressureoftheguardcells.
(4)Duetoosmosistheturgidityofthecellsandhencethe
shapeorformoftheirorgansismaintained.
(5)Theresistanceofplantstodroughtandfrostincreaseswith
increaseinosmoticpressureoftheircells.
(6)Turgidityofthecellsoftheyoungseedlingsallowsthemto
comeoutofthesoil.

20. Plasmolysis:Innormalconditiontheprotoplasmistightly
pressedagainstthecellwall.Ifthisplantcellortissueis
placedinahypertonicsolution,watercomesoutfromthecell
sapintotheoutersolutionduetoex-osmosisandthe
protoplasmbeginstocontractfromthecellwall.Thisiscalled
asincipientplasmolysis.Iftheouterhypertonicsolutionis
verymuchconcentratedincomparisontothecellsap,the
processofex-osmosisandcontractionorshrinkageof
protoplasmcontinuesandultimatelytheprotoplasmseparates
fromthecellwallandassumesasphericalform.This
phenomenoniscalledasplasmolysisandthecellorthetissue
issaidtobeplasmolysed.Becauseofthepermeablecellwall
thespaceinbetweenthecellwallandplasma-membranein
plasmolysedcellsisfilledwithouterhypertonicsolution

21. Ifaplasmolysedcellortissueisplacedinwater,processof
end-osmosistakesplace.Waterentersintothecellsap,thecell
becomesturgid,andtheprotoplasmagainassumesitsnormal
shapeandposition.Thisphenomenoniscalleddeplasmolysis.
AdvantagesofPlasmolysis:
1.Itindicatesthesemi-permeablenatureoftheplasma-
membrane.
2.Thisphenomenonisutilizedinsaltingofmeatandfishes
andadditionofconcentratedsugarsolutiontojamsandjellies
tocheckthegrowthoffungiandbacteriawhichbecome
plasmolysedinconc.solution.
3.ItisalsousedindeterminingtheO.P.ofthecellsap

22. Theaquaticandsubmergedaquaticplantsabsorbwaterfrom
theirbodysurfaceandtheepiphytesabsorbviaVelamen.But
mostoftheterrestrialplantsabsorbwaterfromunicellularroot
hairs.Somemoss,fungiabsorbwaterfromRhizoidsandin
lichenittakesplacefromRhizinelikefibrousstructure.
Irrespectiveofthemediumoftheabsorptionofwaterbythe
plants,thesoilwaterbasicallybelongstothefollowing
categories-Gravitationalwater,Hygroscopicwater,
Chemicallyboundwater,Capillarywater.Butmostofthe
wateravailablebytheplantsarecapillarywaterandthiswater
sourceisavailablebytheplantsamidstthesmallporespresent
inbetweenthesoilparticles.Theepiblemabasicallyoriginates
someunicellularroothairsandthecapillarywaterisusedby
theseroothairsasmediumoftheabsorptionofwater

23. MechanismofAbsorptionofWater:
Inhigherplantswaterisabsorbedthroughroothairswhichare
incontactwithsoilwaterandformaroothairzonealittle
behindtheroottips.Roothairsaretubularhairlike
prolongationsofthecellsoftheepidermallayer(when
epidermisbearsroothairsitisalsoknownaspiliferouslayer)
oftheroots.Thewallsofroothairsarepermeableandconsist
ofpecticsubstancesandcellulosewhicharestrongly
hydrophilic(waterloving)innature.Roothairscontain
vacuolesfilledwithcellsap.Whenrootselongate,theolder
hairsdieandnewroothairsaredevelopedsothattheyarein
contactwithfreshsuppliesofwaterinthesoil.(1)Active
AbsorptionofWater:Inthisprocesstherootcellsplayactive
roleintheabsorptionofwaterandmetabolicenergyreleased
throughrespirationisconsumed.

24. Activeabsorptionmaybeoftwokinds:
(a)Osmoticabsorptioni.e.,whenwaterisabsorbedfromthe
soilintothexylemoftherootsaccordingtotheosmotic
gradient.
(b)Non-osmoticabsorptioni.e.,whenwaterisabsorbed
againsttheosmoticgradient.
(2)PassiveAbsorptionofWater:
It is mainly due to transpiration, the root cells do not play
active role and remain passive. (1a) Active Osmotic
Absorption of Water:
Firststepintheosmoticabsorptionofwateristheimbibition
ofsoilwaterbythehydrophiliccellwallsofroothairs.
OsmoticPressure(O.P.)ofthecell-sapofroothairsisusually
higherthantheO.P.ofthesoilwater.Therefore,theDiffusion

25. PressureDeficit(D.P.D.)andthesuctionpressureintheroot
hairsbecomehigherandwaterfromthecellwallsentersinto
themthroughplasma-membrane(semi-permeable)byosmotic
diffusion.Asaresult,theO.P.,suctionpressureandD.P.D.of
roothairsnowbecomelower,whiletheirturgorpressureis
increased.Now,thecorticalcellsadjacenttoroothairshave
higherO.P.,suctionpressureandD.P.D.incomparisontothe
roothairs.Therefore,waterisdrawnintotheadjacentcortical
cellsfromtheroot-hairsbyosmoticdiffusion.
Inthesameway,thewaterbycelltocellosmoticdiffusion
graduallyreachestheinnermostcorticalcellsandthe
endodermis.Osmoticdiffusionofwaterintoendodermistakes
placethroughspecialthinwalledpassagecellsbecausethe
otherendodermalcellshaveCasparianstripsontheirwalls
whichareimpervioustowater

26. Waterfromendodermalcellsisdrawnintothecellsof
pericyclebyosmoticdiffusionwhichnowbecomesturgidand
theirsuctionpressureisdecreased.Inthelaststep,wateris
drawnintoxylemfromturgidpericyclecells.(Inrootsthe
vascularbundlesareradialandprotoxylemelementsarein
contactwithpericycle).Itisbecauseinabsenceofturgor
pressureofthexylemvessels(whicharenon-elastic),the
suctionpressureofxylemvesselsbecomeshigherthanthe
suctionpressureofthecellsofthepericycle.Whenwater
entersintoxylemfrompericycle,apressureisdevelopedin
thexylemofrootswhichcanraisethewatertoacertainheight
inthexylem.Thispressureiscalledasrootpressure.

27. ActiveNon-OsmoticAbsorptionofWater:
Sometimes,ithasbeenobservedthatabsorptionofwatertakes
placeevenwhentheO.P.ofthesoilwaterishigherthanthe
O.P.ofcell-sap.Thistypeofabsorptionwhichisnon-osmotic
andagainsttheosmoticgradientrequirestheexpenditureof
metabolicenergyprobablythroughrespiration.
(2)PassiveAbsorptionofWater:
Passiveabsorptionofwatertakesplacewhenrateof
transpirationisusuallyhigh.Rapidevaporationofwaterfrom
theleavesduringtranspirationcreatesatensioninwaterinthe
xylemoftheleaves.Thistensionistransmittedtowaterin
xylemofrootsthroughthexylemofstemandthewaterrises
upwardtoreachthetranspiringsurfaces.

28. Duringabsorptionofwaterbyroots,theflowofwaterfrom
epidermistoendodermismaytakeplacethroughthree
differentpathways:
(i)Apoplasticpathway(cellwallsandintercellularspaces),
(ii)Trans-membranepathway(bycrossingtheplasma
membranes)andiii)Symplastpathway(through
Plasmodesmata).
Themechanismofwaterabsorptiondescribedearlier,in-fact
belongstothesecondcategory.Therelativeimportanceof
thesethreepathwaysinwaterabsorptionbyrootsisnotclearly
established.However,acombinationofthesethreepathways
isresponsiblefortransportofwateracrosstheroot.

29. Differentfactorsplayregulatoryroleoftheabsorptionof
waterasfollows:
EXTERNALFACTORS:
Availabilityofsoilwater-Physicallydrysoilaswellas
physiologicallydrysoilgenerallyreducethewaterabsorption
duetoleastwaterandhighdegreeofsalinityrespectively.The
lowoxygencontentinthesoilreducetherateofabsorption.
Soiltemperature-lowtemperatureincreasetheviscosityof
watercausingminimumabsorptionalongwithleast
permeabilityandreductionofrootgrowth.Hightemperature
hasdetrimentaleffectonplantsabsorption.
Soilaeration-IncreaseofCO2insoilreducetherateof
absorption&leastaerationsoilhavesameissue.

30. Concentrationofsoilsolution-Thephysiologicallydrysoil
containshighdegreeofsalinity.Thisconditionalsodecrease
ofsoilwaterabsorption.
INTERNALFACTORS:
Rootpressure-Rootpressurepromoteswaterabsorptionandit
hasdirectroleinwaterabsorption.
Transpiration-Thetranspirationincreasesthesuctionpressure
bypromotingthetranspirationpullandithaspositiveeffect
onwaterabsorption.
Metabolism-Therespirationofratehaspositiveaffectonthe
rateoftheabsorptionofsoilwater.\
Structureofroot-Rootshootratiobecomesmore,thewater
absorptionalsoincreases.

31. 1.Ifanycellprotoplasmdensity0.2Msucrosesolution
equivalent,whatwillbeitsosmoticpressure
2.Ifosmoticpressureofacellis3bar,DPD=2.1bar,what
willbetheTurgorpressureofthecell?
Ifacellhaveosmoticpotential0,244MegsPascal,Pressure
potential,0.402Mpa,Whatwillbetheosmoticpotentialof
cell?
IfacellhaveOsmoticpressureis2.5bar,DPD=1.8bar,what
willbethevalueofOsmoticpressure?
Physiologicallydrysoilisunfitforwaterabsorption-why?
IftherhizospherehasminimumOxygenavailability,whatwill
betheaffectonwaterabsorption?

32. ACKNOWLEDGEMENT
1. Google for images
2. Different websites for contents
3. Plant Physiology-Mukherjee & Ghosh
4. A textbook of Botany-Hait, Bhattacharya & Ghosh
5.Essentials of Biophysics-Narayanan
6.Plant Physiology-Devlin
7. Plant physiology-Nogle & Fritz
Disclaimer: This presentation has been made as open source
of learning without any financial interest.