Unit V: Plant Physiology
Chapter11Unit V: Plant Physiology(Functional Organisation)Transport in PlantsLearning ObjectivesThe learner will be able to,Over 450 million years ago (the Ordovicianperiod in Paleozoic era) plants migratedfrom their own sophisticated water worldto newly formed land. The land had harshenvironment; water availability was deeperand so plants struggled for getting water fortheir very existence. Some of them failedto survive and rest adopted themselves tothe new world. The biggest adaptationsfollowed for their survival was buildingtheir own water absorbing systems todraw water from deep inside the land. Thecreation and updating of water absorbingsystem (vascular tissues) led to the diversityof the plant kingdom. The gregariousgrowth of prehistoric pteridophytes,gymnosperms and present-day floweringplants led to the biggest challenge in thetransport of water from root to severalmeters high trees against gravity. In thischapter, we will study the events takingplace between the gain of water in rootsand loss in leaves and the mechanismsbehind the basic physical and biologicalprocesses in the movement of water, gasesand minerals in plants. Further, we studyhow food material synthesized in the leafcan be transported to various utilizingand storage areas against struggles andchallenges. Recall knowledge of basic physicaland biological processes studied inprevious classes. Classify, differentiate and comparethe process of active and passivetransport. Understand the mechanism ofabsorption of water. Analyse the various theories inascent of sap. Understandtheprocessoftranspiration and Compare thevarious types of transpiration. Discuss the mechanism of phloemtranslocation. Understand the process behindmineral absorption.Chapter Outline11.1 Types of transport11.2 Cell to Cell transport11.3 Plant water relations11.4 Absorption of water11.5 Ascent of Sap11.6 Transpiration11.7 Translocation of organic solutes11.8 Mineral absorption57
ii. Active transport: It is a biologicalprocess and it runs based on theenergy obtained from respiration. Itis an uphill process.Diffusion: The net movement ofmolecules from a region of theirhigher concentration to a regionof their lower concentration alonga concentration gradient until anequilibrium is attained.11.2 Cell to Cell TransportCell to cell or short distance transportcovers the limited area and consists of fewcells. They are the facilitators or tributariesto the long-distance transport. Thedriving force for the cell to cell transportcan be passive or active (Figure 11.1). Thefollowing chart illustrate the various typesof cell to cell transport:Characteristics of diffusioni. It is a passive process, hence no energyexpenditure involved.ii. It is independent of the living system.iii. Diffusion is obvious in gases andliquids.Cell to cell transportPassive TransportDiffusionActive TransportFacilitated DiffusionChannel ProteinCarrier ProteinChannel ProteinCarrier ProteinPumpsFigure 11.1: Cell to cell transportA11.2.1 Passive Transport1. DiffusionWhen we expose a lightened incense stickor mosquito coil or open a perfume bottlein a closed room, we can smell the odoureverywhere in the room. This is due to theeven distribution of perfume moleculesthroughout the room. This process iscalled diffusion.HIGH CONCENTRATIONLOW CONCENTRATIONLorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibheuismod tincidunt ut laoreet dolore magna aliquam erat volutpat. Ut wisi enim adminim veniam, quis nostrud exerci tation ullamcorper suscipit lobortis nisl utaliquip ex ea commodo consequat. Duis autem vel eum iriure dolor in hendrerit invulputate velit esse molestie consequat, vel illum dolore eu feugiat nulla facilisis atvero eros et accumsan et iusto odio dignissim qui blandit praesent luptatum zzrildelenit augue duis dolore te feugait nulla facilisi.Lorem ipsum dolor sit amet, cons ectetuer adipiscing elit, sed diam nonummy nibheuismod tincidunt ut laoreet dolore magna aliquam erat volutpat. Ut wisi enim adminim veniam, quis nostrud exerci tation ullamcorper suscipit lobortis nisl utaliquip ex ea commodo consequat.Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibheuismod tincidunt ut laoreet dolore magna aliquam erat volutpat. Ut wisi enim adBIn diffusion, the movement ofmolecules is continuous and random inorder in all directions (Figure 11.2).Figure 11.2: Distribution of molecules indiffusion (A) Initial stage (B) Final stage59
i. Size of molecule: Smaller moleculesdiffuse faster.iv. Diffusion is rapid over a shorterdistance but extremely slow over alonger distance.ii. Solubility of the molecule: Lipidsoluble substances easily and rapidlypass through the membrane. But watersoluble substances are difficult to passthrough the membrane. They must befacilitated to pass the membrane.v. The rate of diffusion is determined bytemperature, concentration gradientand relative density.Significance of diffusion in Plantsi. Gaseous exchange of O2 and CO2between the atmosphere and stomataof leaves takes place by the processof diffusion. O2 is absorbed duringrespiration and CO2 is absorbed duringphotosynthesis.ii. In transpiration, water vapour fromintercellular spaces diffuses intoatmosphere through stomata by theprocess of diffusion.iii. The transport of ions in mineral saltsduring passive absorption also takesplace by this process.Types of Membrane PermeabilityA solution is made up of soluteparticles dissolved in a solvent and thepermeability of the above componentsdepends on the nature of cellmembranes, which is given below:Impermeable: Inhibit the movementof both solvent and solute molecules.Example: Suberised, cutinesed orliginifid cell walls.Permeable: They allow diffusionof both solvent and solute moleculesthrough them. Example: Cellulosic cellwall.Diffusion forsterilization insurgical theatresSemi permeable: Semi permeableallow diffusion of solvent moleculesbut do not allow the passage of solutemolecule. Example: Parchment paper.Surgical theatres mustbe free from germs to prevent infectionduring surgeries. A mixture ofFormalin and Potassium permanganateproduces enormous fumes which willkill all pathogens in an enclosed area.This method is known as fumigationand operates by diffusion.Selectively permeable: All biomembranes allow some solutes to passin addition to the solvent molecules.Example: Plasmalemma, tonoplast, andmembranes of cell organelles.In facilitated diffusion, molecules crossthe cell membrane with the help of specialmembrane proteins called transportproteins, without the expenditure of ATP.2. Facilitated DiffusionCell membranes allow water and nonpolarmolecules to permeate by simple diffusion.For transporting polar molecules such asions, sugars, amino acids, nucleotides andmany cell metabolites is not merely basedon concentration gradient. It depends on,There are two types of transportproteins present in the cell membrane.They are channel protein and a carrierprotein.60
of ions or molecules. The pump action isan example of active transport. Example:Na -K -ATPase pump (Table 11.1).MEMBRANEOUTSIDETable 11.1 Comparison of differenttransport mechanismsINSIDEUNIPORTSYMPORTANTIPORTPassive transportFigure 11.5: Direction of transportPropertytransports two types of moleculesacross the membrane in the samedirection.Nature ofprocessRequirementiii. Antiport or Counter Transport: Anantiport is an integral membranetransport protein that simultaneouslytransports two different molecules,in opposite directions, across themembrane.for presenceof membraneproteinSelectivity ofmoleculeSaturation of11.2.2 Active TransporttransportUphillThe main disadvantage of passive transportprocesses like diffusion is the lack of controlover the transport of selective molecules.There is a possibility of harmful substancesentering the cell by a concentration gradientin the diffusion process. But selectivepermeability of cell membrane has a greatcontrol over entry and exit of molecules.Active transport is the entry of moleculesagainst a concentration gradient and anuphill process and it needs energy whichcomes from ATP. Passive transport useskinetic energy of molecules moving downa gradient whereas, active transport usescellular energy to move them against agradient. The transport proteins discussed infacilitated diffusion can also transport ions ormolecules against a concentration gradientwith the expenditure of cellular energy asan active process. Pumps use a source offree energy such as ATP or light to drivethe thermodynamically uphill y toinhibitorsActiveSimple Facilitated transportdiffusion diffusionPhysicalBiological YesYesCheck your grasp!What are the similarities anddifferences between co- transport andcounter transport?Solution:Similarity: In both system two moleculesare involved for the unidirectionaltransport.Difference: In co-transport, twomolecules are transported togetherwhereas, in counter transport twomolecules are transported in oppositedirection to each other.62
Significance of imbibition11.3 Plant Water Relationsi. During germination of seeds, imbibitionincreases the volume of seed enormouslyand leads to bursting of the seed coat.Water plays an essential role in the lifeof the plant. The availability of waterinfluences the external and internalstructures of plants as protoplasm is madeof 60-80% water. Water is a universalsolvent since most of the substancesget dissolved in it and the high tensilestrength of water molecule is helpful inthe ascent of sap. Water maintains theinternal temperature of the plant as wellas the turgidity of the cell.ii. It helps in the absorption of water byroots at the initial level.ActivityImbibition experimentCollect 5 gm of gum from Drumsticktree or Babool tree or Almond tree.Immerse in 100ml of water. After 24hours observe the changes and discussthe results with your teacher.11.3.1 ImbibitionColloidal systems such as gum, starch,proteins, cellulose, agar, gelatin whenplaced in water, will absorb a large volumeof water and swell up. These substances arecalled imbibants and the phenomenon isimbibition.Examples: 1. The swelling of dryseeds 2.The swelling of wooden windows,tables, doors due to high humidity duringthe rainy season.The Power ofImbibition11.3.2 Water Potential (Ψ)The concept of water potential wasintroduced in 1960 by Slatyer and Taylor.Water potential is potential energy of waterin a system compared to pure water whenboth temperature and pressure are kept thesame. It is also a measureof how freely watermolecules can move in aparticular environmentorsystem.Waterpotential is denoted byIn olden days, smallwooden pegs wereinserted into crevices of rocksfollowed by continuous hydration.Due to imbibition the volume ofwooden peg increases and cuts offrocks precisely.The gluten from wheat can take asmuch as 300% of its own weight63
withdrawal of water from the cell decreasesthe water potential and the cell becomesflaccid.the Greek symbol Ψ (psi) and measuredin Pascal (Pa). At standard temperature,the water potential of pure water is zero.Addition of solute to pure water decreasesthe kinetic energy thereby decreasing thewater potential. Comparatively a solutionalways has low water potential than purewater. In a group of cells with differentwater potential, a water potential gradientis generated. Water will move from higherwater potential to lower water potential.3. Matric Potential (ΨM)Matric potential represents the attractionbetween water and the hydrating colloidor gel-like organic molecules in the cellwall which is collectively termed as matricpotential. Matric potential is also knownas imbibition pressure. The matricpotential is maximum (most negativevalue) in a dry material. Example: Theswelling of soaked seeds in water.Water potential (Ψ) can be determined by,1. Solute concentration or Solutepotential (ΨS)2. Pressure potential (ΨP)11.3.3 Osmotic Pressure and OsmoticPotentialBy correlating two factors, water potentialis written as,When a solution and its solvent (purewater) are separated by a semipermeablemembrane, a pressure is developed in thesolution, due to the presence of dissolvedsolutes. This is called osmotic pressure(OP). Osmotic pressure is increasedwith the increase of dissolved solutes inthe solution. More concentrated solution(low Ψ or Hypertonic) has high osmoticpressure. Similarly, less concentratedsolution (high Ψ or Hypotonic) has lowosmotic pressure. The osmotic pressureof pure water is always zero and itincreases with the increase of soluteconcentration. Thus osmotic pressurealways has a positive value and it isrepresented as π.ΨW ΨS ΨPWater Potential Solute potential Pressure potential1. Solute Potential (ΨS)Solute potential, otherwise known asosmotic potential denotes the effect ofdissolved solute on water potential. In purewater, the addition of solute reduces itsfree energy and lowers the water potentialvalue from zero to negative. Thus the valueof solute potential is always negative. In asolution at standard atmospheric pressure,water potential is always equal to solutepotential (ΨW ΨS).Osmotic potential is defined asthe ratio between the number of soluteparticles and the number of solventparticles in a solution. Osmotic potentialand osmotic pressure are numericallyequal. Osmotic potential has a negativevalue whereas on the other hand osmoticpressure has a positive value.2. Pressure Potential (ΨP)Pressure potential is a mechanical forceworking against the effect of solutepotential. Increased pressure potentialwill increase water potential and waterenters cell and cells become turgid. Thispositive hydrostatic pressure within thecell is called Turgor pressure. Likewise,64
It is equal to the difference of osmoticpressure and turgor pressure of a cell.The following three situations are seen inplants:11.3.4 Turgor Pressure and WallPressureWhen a plant cell is placed in pure water(hypotonic solution) the diffusion of waterinto the cell takes place by endosmosis. Itcreates a positive hydrostatic pressure onthe rigid cell wall by the cell membrane.Henceforth the pressure exerted by the cellmembrane towards the cell wall is TurgorPressure (TP). DPD in normal cell: DPD OP – TP. DPD in fully turgid cell: Osmoticpressure is always equal to turgorpressure in a fully turgid cell. OP TP or OP-TP 0. Hence DPD offully turgid cell is zero.The cell wall reacts to this turgorpressure with equal and opposite force,and the counter-pressure exerted by thecell wall towards cell membrane is wallpressure (WP). DPD in flaccid cell: If the cell is inflaccid condition there is no turgorpressure or TP 0. Hence DPD OP.11.3.6 OsmosisTurgor pressure and wall pressuremake the cell fully turgid.Osmosis (Latin: Osmos-impulse, urge) is aspecial type of diffusion. It represents themovement of water or solvent moleculesthroughaselectivelypermeablemembrane from the place of its higherconcentration (high water potential) tothe place of its lower concentration (lowwater potential).TP WP Turgid.ActivityFind the role of turgor pressure insudden closing of leaves when wetouch the ‘touch me not’ plant.Types of Solutions based on concentrationi. Hypertonic (Hyper High; tonic solute): This is a strong solution (lowsolvent/ high solute / low Ψ) whichattracts solvent from other solutions.11.3.5 Diffusion Pressure Deficit (DPD)or Suction Pressure (SP)Pure solvent (hypotonic) has higherdiffusion pressure. Addition of solute inpure solvent lowers its diffusion pressure.The difference between the diffusionpressure of the solution and its solvent ata particular temperature and atmosphericpressure is called as Diffusion PressureDeficit (DPD) termed by Meyer (1938).DPD is increased by the addition of soluteinto a solvent system. Increased DPDfavours endosmosis or it sucks the waterfrom hypotonic solution; hence Renner(1935) called it as Suction pressure.ii. Hypotonic (Hypo low; tonic solute):This is a weak solution (high solvent/low or zero solute / high Ψ) and itdiffuses water out to other solutions(Figure 11.7).iii. Isotonic (Iso identical; tonic soute):It refers to two solutions having sameconcentration. In this condition the netmovement of water molecule will be zero.The term hyper, hypo and isotonic arerelative terms which can be used only65
in comparison with another solution.Thistle funnel experiment1. Types of osmosisBased on the direction of movement ofwater or solvent in an osmotic system,two types of osmosis can occur, they areEndosmosis and Exosmosis.i. Endosmosis: Endosmosis is defined asthe osmotic entry of solvent into a cellor a system when it is placed in a purewater or hypotonic solution.For example, dry raisins (high soluteand low solvent) placed in the water,it swells up due to turgidity.Figure 11.6: Thistle Funnel ExperimentMouth of a thistle funnel is tied withgoat bladder. It acts as a semipermeablemembrane. Pour concentrated sugarsolution in the thistle funnel andmark the level of solution. Placethis in a beaker of water. After sometime, water level in the funnel risesup steadily. This is due to the inwarddiffusion of water molecules throughthesemipermeablemembrane(Figure 11.6).ii. Exosmosis: Exosmosis is defined asthe osmotic withdrawal of water froma cell or system when it is placed in ahypertonic solution. Exosmosis in aplant cell leads to plasmolysis.2. Plasmolysis (Plasma cytoplasm;lysis breakdown)When a plant cell is kept in a hypertonicsolution, water leaves the cell due toexosmosis. As a result of water loss,protoplasm shrinks and the cell membraneis pulled away from the cell wall and finally,the cell becomes flaccid. This process isnamed as plasmolysis.Conversely, if water in the beaker isreplaced by a sugar solution and sugarsolution in the thistle funnel replacedby water, what will be happen?Wilting of plants noticed under thecondition of water scarcity is an indicationof plasmolysis. Three types of plasmolysisoccur in plants: i) Incipient plasmolysisii) Evident plasmolysis and iii) Finalplasmolysis. Differences among them aregiven in table 11.2.SignificancePlasmolysis is exhibited only by livingcells and so it is used to test whether thecell is living or dead.Figure 11.7: Types of solution based onconcentration66
3. Deplasmolysis4. Reverse OsmosisThe effect of plasmolysis can be reversed,by transferring them back into water orhypotonic solution. Due to endosmosis,the cell becomes turgid again. It regains itsoriginal shape and size. This phenomenonof the revival of the plasmolysed cellis called deplasmolysis. Example:Immersion of dry raisin in water.Reverse Osmosis follows the sameprinciples of osmosis, but in the reversedirection. In this process movement ofwater is reversed by applying pressure toforce the water against a concentrationgradient of the solution. In regularosmosis, the water molecules move fromthe higher concentration (pure water hypotonic) to lower concentration(salt water hypertonic). But in reverseosmosis, the water molecules move fromthe lower concentration (salt water hypertonic) to higher concentration (purewater hypotonic) through a selectivelypermeable membrane (Figure 11.9).Potato OsmoscopeUses: Reverse osmosis is used forpurification of drinking water anddesalination of seawater.Figure 11.8: Demonstration ofEndosmosis by Potato OsmoscopePressurePure Wateri. Take a peeled potato tuber andmake a cavity inside with the helpof a knife.Salt Waterii. Fill the cavity with concentratedsugar solution and mark the initiallevel.iii. Place this setup in a beaker of purewater.iv. After 10 minutes observe the sugarsolution level and record yourfindings (Figure 11.8).v. With the help of your teacherdiscuss the results.Instead of potato use beetroot or bottleguard and repeat the above experiment.Compare and discuss the results.MembraneMovement of WaterFigure 11.9: Reverse OsmosisCheck your grasp!If a cell in the cortex with DPD of 5atmis surrounded by hypodermal cells withDPD of 2atm, what will be direction ofmovement of water?Solution: Water will move from lowDPD to high DPD (hypodermis 2 atmto cortex 5 atm).67
Table 11.2: Difference between plasmolysis types.Incipient plasmolysisEvident plasmolysisNo morphologicalWilting of leaves appear.symptoms appear in plants.Final plasmolysisSevere wilting anddrooping of leaves appear.The plasma membranePlasma membrane completely Plasma membranecompletely detaches fromseparates only at the corner detaches from the cell wall.from the cell wall of cells.cell wall with maximumshrinkage of volume.It is reversible.It is reversible.It is irreversible.11.4 Absorption of WaterTerrestrial plants have to absorb water fromthe soil to maintain turgidity, metabolicactivities and growth. Absorption of waterfrom soil takes place in two steps:1. From soil to root hairs – eitheractively or passively.Figure 11.10: Structure of Root Hair2. From root hairs further transport inthe lateral direction to reach xylem, thesuperhighway of water transport.11.4.2 Path of Water Across Root CellsWater is first absorbed by root hairand other epidermal cells throughimbibition from soil and moves radiallyand centripetally across the cortex,endodermis, pericycle and finally reachesxylem elements osmotically.11.4.1 Water Absorbing OrgansUsually, absorption of water occurs inplants through young roots. The zoneof rapid water absorption is root hairs.They are delicate structures which getcontinuously replaced by new ones.Root hairs are unicellular extensions ofepidermal cells without cuticle. Root hairsare extremely thin and numerous and theyprovide a large surface area for absorption(Figure 11.10).There are three possible routes ofwater (Figure 11.11). They are i) Apoplastii) Symplast iii) Transmembrane route.1. ApoplastThe apoplast (Greek: apo away;plast cell) consists of everything external68
Figure 11.11: Path of water across root cellsto the plasma membrane of the livingcell. The apoplast includes cell walls,extra cellular spaces and the interior ofdead cells such as vessel elements andtracheids. In the apoplast pathway, watermoves exclusively through the cell wall orthe non-living part of the plant withoutcrossing any membrane. The apoplast is acontinuous system.11.4.3 Mechanism of Water AbsorptionKramer (1949) recognized two distinctmechanisms which independently operatein the absorption of water in plants.They are, i) active absorption ii) passiveabsorption.1. Active AbsorptionThe mechanism of water absorption due toforces generated in the root itself is calledactive absorption. Active absorption maybe osmotic or non-osmotic.2. SymplastThe symplast (Greek: sym within; plast cell) consists of the entire mass of cytosolof all the living cells in a plant, as well as theplasmodesmata, the cytoplasmic channelthat interconnects them.i. Osmotic active absorptionThe theory of osmotic activeabsorption was postulated by Atkins(1916) and Preistley (1923). According tothis theory, the first step in the absorptionis soil water imbibed by cell wall of the roothair followed by osmosis. The soil wateris hypotonic and cell sap is hypertonic.Therefore, soil water diffuses into roothair along the concentration gradient(endosmosis). When the root hair becomesfully turgid, it becomes hypotonic andwater moves osmotically to the outer mostcortical cell. In the same way, water entersinto inner cortex, endodermis, pericycleand finally reaches protoxylem. As theIn the symplastic route, water has to crossplasma membrane to enter the cytoplasmof outer root cell; then it will move withinadjoining cytoplasm through plasmodesmataaround the vacuoles without the necessity tocross more membrane, till it reaches xylem.3. Transmembrane routeIn transmembrane pathway watersequentially enters a cell on one side andexits from the cell on the other side. Inthis pathway, water crosses at least twomembranes for each cell. Transport acrossthe tonoplast is also involved.69
sap reaches the protoxylem a pressure isdeveloped known as root pressure. Thistheory involves the symplastic movementof water.in the rate of respiration and also the rateof absorption of water.Objections to osmotic theory: 1.Thecell sap concentration in xylem is notalways high. 2. Root pressure is notuniversal in all plants especially in trees.In passive absorption, roots do not playany role in the absorption of water andis regulated by transpiration only. Due totranspiration, water is lost from leaf cellsalong with a drop in turgor pressure. Itincreases DPD in leaf cells and leads towithdrawal of water from adjacent xylemcells. In xylem, a tension is developedand is transmitted downward up to rootresulting in the absorption of water fromthe soil.In passive absorption (Table 11.3),the path of water may be symplastic orapoplastic. It accounts for about 98% ofthe total water uptake by plants.2. Passive Absorptionii. Non-Osmotic active absorptionBennet-Clark (1936), Thimann (1951)and Kramer (1959) observed absorptionof water even if the concentration of cellsap in the root hair is lower than that ofthe soil water. Such a movement requiresan expenditure of energy released byrespiration (ATP). Thus, there is alink between water absorption andrespiration. It is evident from the fact thatwhen respiratory inhibitors like KCN,Chloroform are applied there is a decreaseConcept Map - Movement of water in an osmotic systembased on various parametersLow / Zero soluteconcentrationHigh solventConcentrationHypotonicHigh osmoticpotential(Zero)Low WaterPotential(Negative value)High WaterPotential(Zero)LowDPDHighDPDHigh OsmoticPressure(Positive value)Low OsmoticPressure(Zero)PUREWATERSALTWATER70High soluteconcentrationLow solventConcentrationHypertonicLow osmoticpotential(Negative Value)
Table: 11.3 Differences between ActiveAbsorption and Passive AbsorptionActivePassiveabsorptionabsorptionActive absorption The pressure fortakes place by the absorption is notactivity of root and developed in rootsroot hairsand hence rootsplay passive roleTranspiration has Absorptionno effect on active regulated byabsorptiontranspirationThe root hairsThe absorptionhave high DPDoccurs due toas compared totension createdsoil solution andin xylem sap bytherefore water is transpiration pull,taken by tensionthus water is suckedin by the tensionRespiratory energy Respiratory energyneedednot requiredIt involvesBoth symplastsymplasticand apoplastmovement of water movement of waterinvolvedFigure 11.12: Balsam plant and eosindye experiment11.5.1 The Path of Ascent of SapThere is no doubt; water travels up along thevascular tissue. But vascular tissue has twocomponents namely Xylem and Phloem.Of these two, which is responsible for theascent of sap? The following experimentwill prove that xylem is the only elementthrough which water moves up.11.5 Ascent of SapIn the last chapter, we studied about waterabsorption from roots to xylem in a lateraldirection and here we will learn aboutthe mechanism of distribution of waterinside the plant. Like tributaries jointogether to form a river, millions of roothairs conduct a small amount of water andconfluence in xylem, the superhighway ofwater conduction. Xylem handles a largeamount of water to conduct to many partsin an upward direction.The water within the xylem along withdissolved minerals from roots is called sapand its upward transport is called ascentof sap.Cut a branch of balsam plant andplace it in a beaker containing eosin(red colour dye) water. After sometime, a red streak appears on the stemindicating the ascent of water. Removethe plant from water and cut a transversesection of the stem and observe it underthe microscope. Only xylem element iscoloured red, which indicates the path71
into the inner cortex of the stem, thegalvanometer showed high electricalactivity. Bose believed a rhythmicpulsating movement of inner cortex like apump (similar to the beating of the heart)is responsible for the ascent of sap. Heconcluded that cells associated with xylemexhibit pumping action and pumps thesap laterally into xylem cells.of water is xylem. Phloem is not coloredindicating that it has no role in the ascentof sap (Figure 11.12).Mechanism of Ascent of SapIn ascent of sap, the biggest challenge is theforce required to lift the water to the topof the tallest trees. A number of theorieshave been put forward to explain themechanism of the ascent of sap. They are,A. Vital force theories, B. Root pressuretheory, and C. Physical force theory.Objections to vital force theoriesi. Strasburger (1889) and Overton(1911) experimentally proved that livingcells are not mandatory for the ascent ofsap. For this, he selected an old oak treetrunk which when immersed in picricacid and subjected to excessive heat killedall the living cells of the trunk. The trunkwhen dipped in water, the ascent of saptook place.11.5.2 Vital Force TheoriesAccording to vital force theories, living cellsare mandatory for the ascent of sap. Basedon this the following two theories derived:1. Relay pump theory of Godlewski(1884)Periodic changes in osmotic pressureof living cells of the xylem parenchymaand medullary ray act as a pump for themovement of water.ii. Pumping action of living cells shouldbe in between two xylem elements(vertically) and not on lateral sides.2. Pulsation theory of J.C.Bose (1923)11.5.3 Root Pressure TheoryBose invented an instrument calledCrescograph, which consists of an electricprobe connected to a galvanometer(Figure 11.13). When a probe is insertedIf a plant which is watered well is cut a fewinches above the ground level, sap exudesout with some force. This is called sapexudation or bleeding. Stephen Hales,father of plant physiology observed thisphenomenon and coined the term ‘RootPressure’. Stoking (1956) defined rootpressure as “a pressure developing in thetracheary elements of the xylem as a resultof metabolic activities of the root”. But thefollowing objections have been raisedagainst root pres
11.1 Types of transport 11.2 Cell to Cell transport 11.3 Plant water relations 11.4 Absorption of water 11.5 Ascent of Sap 11.6 Transpiration 11.7 Translocation of organic solutes 11.8 Mineral absorption Over 450 million years ago
Physiology with the teaching and research support agencies, suggest changes in the syllabus of Plant Physiology subjects of undergraduate and postgraduate courses and offer subsidies for the teaching of Plant Physiology. The XVII Brazilian Congress of Plant Physiology, was held in Cuiabá / MT, from June 9 to 12, 2019.
physiology · Exercise physiology · Gastrointestinal and kidney physiology · Heart and circulatory physiology · Molecular and cellular physiology · Muscle physiology · Physiome/systems biology Respiration physiology · Senses Editorial Board of The Journal of Physiological Science
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