Transpiration Notes

Transpiration

Transpiration is the loss of water in the form of vapor from the aerial parts of the plant, primarily through the stomata.

Magnitude of Transpiration

Trees can lose substantial amounts of water through transpiration. For example, a beech tree may lose water equal to five times the fresh weight of its leaves in a summer day, and an acre of beech wood may lose approximately 30003000 gallons of water per day.

Types of Transpiration

  • Foliar Transpiration: Transpiration from leaves.

    • Stomatal Transpiration: Occurs through stomatal openings. Most foliar transpiration is stomatal.

    • Cuticular Transpiration: Occurs through the cuticle, accounting for only about 5%5\% of total foliar transpiration.

  • Lenticular Transpiration: Loss of water vapor through lenticels in fruits and woody stems.

System of Stomatal Transpiration

Mesophyll cells are filled with water, and their wet cell walls are in contact with intercellular spaces, which connect to the atmosphere via stomata. If the water-vapor pressure inside the leaf is higher than in the air, water diffuses outward through the stomata.

Leaves with stomata only on their lower surfaces lose approximately 97%97\% of their total transpiration from that surface.

Stomata

Stomata are minute elliptical pores in the epidermis of plants. A stoma consists of:

  • Stoma: An individual stoma of corn is approximately 55 μm wide by 2626 μm long.

  • Guard Cells: Two kidney-shaped epidermal cells that bound the pore. They contain green plastids and have a thickened wall towards the stoma and a thin outer wall.

  • Subsidiary Cells (Accessory Cells): Epidermal cells surrounding the guard cells in some species.

In dicots, guard cells are generally surrounded by epidermal cells while subsidiary cells are present only in few species. Stomata are surrounded by reniform guard cells in dicots and by dumbbell-shaped guard cells with bulbous ends in grasses of monocots. Cellulose microfibrils are radially arranged in the kidney-shaped guard cells of dicots and in the bulbous portions of the dumbbell-shaped guard cells of monocots.

Distribution of Stomata

The position and distribution of stomata vary in plants. Stomata are classified as follows:

  1. Anisocytic stomata: The subsidiary cells of the guard cell pair, in which the tree surrounding subsidiary cells are of unequal size.

  2. Paracytic stomata: A stomatal complex in which one or more of the subsidiary cells that flank the stoma are two or three parallel with the long axis of the guard cells.

  3. Haplocheilic: In some gymnosperms, applies to a type of stoma in which the 2 guard cells are derived from a single mother cell and the subsidiary cells are derived from a different initial.

  4. Syndetocheilic: In some gymnosperms, applied to a type of stoma in which the 2 guard cells and the subsidiary cells are all derived from a single mother cell.

  5. Anomocytic stoma: Subsidiary cells surrounding the guard cell pair are not morphologically distinct from other epidermal cells.

Mechanism of Stomatal Opening and Closing

Stomatal movements are due to changes in the volume and shape of guard cells, caused by turgidity and flaccidity.

In dicots, turgid guard cells bulge outward, causing the inner thick walls to pull apart and open the stoma. When guard cells lose turgidity, the thick walls revert to their original position, closing the stoma. In monocots, bulbous ends of guard cells swell due to high turgor pressure, pulling the ends in opposite directions and opening the stoma.

Theories of Stomatal Movements

Photosynthetic Production in the Guard Cells

Von Mohl (1856) proposed that chloroplasts in guard cells manufacture osmotically active substances via photosynthesis, increasing osmotic pressure and causing stomatal opening. This hypothesis is not accepted because:

  • Increased CO2CO_2 concentration should lead to increased opening, but it results in partial closure.

  • Chloroplasts of guard cells are often incapable of significant photosynthesis.

Starch-Sugar Hypothesis

Lloyd (1908) found that starch in guard cells increases by night and decreases by day. The interconversion of starch and sugar controls the turgidity of guard cells.

Sayre (1926) found widest opening at pH 4.2−4.44.2-4.4 when starch content was low, and closure at higher or lower pH, associated with increased starch content. Removal of CO2CO_2 by photosynthesis increases pH, converting starch into sugar.

The enzyme phosphorylase (Hanes, 1940) catalyses the reaction:

Starch+n(inorganic phosphate)→phosphorylasen(glucose−1−phosphate)Starch + n(inorganic \, phosphate) \xrightarrow{phosphorylase} n(glucose-1-phosphate)

Yin and Tung (1948) found phosphorylase in guard cells. Yemm and Willis (1954) showed stomatal closure is caused by conversion of sugar into starch.

In light, CO<em>2CO<em>2 is used in photosynthesis, lowering acidity (increasing pH) and favoring starch to sugar conversion. In darkness, CO</em>2CO</em>2 accumulates, increasing acidity (lowering pH) and favoring sugar to starch conversion.

Steward's Scheme

Steward (1964) criticised the scheme because the osmotic pressure of the guard cells will increase unless glucose-1-phosphate is further converted to glucose and inorganic phosphate. Steward's scheme is shown below:
At higher pH stomata open in the following manner:

Glucose−1−PO<em>4→phosphoglucomutaseGlucose−6−PO</em>4→phosphataseGlucose+PO4Glucose-1-PO<em>4 \xrightarrow{phosphoglucomutase} Glucose-6-PO</em>4 \xrightarrow{phosphatase} Glucose + PO_4

Closing of the stomata occurs when glucose converts first into glucose-1-PO4 with the help of ATP in the presence of enzyme hexokinase and oxygen.

Glucose+ATP→hexokinaseGlucose−1−PO4Glucose + ATP \xrightarrow{hexokinase} Glucose-1-PO_4

Glucose−1−PO<em>4→phosphorylaseStarch+PO</em>4Glucose-1-PO<em>4 \xrightarrow{phosphorylase} Starch + PO</em>4

Scarth's theory suffers from three drawbacks:

(i) It cannot explain as to how a change in CO2CO_2 inside the leaf can raise the pH from 4.54.5 to 7.07.0.

(ii) Secondly sugar has never been observed to appear in the guard cells when starch disappears at the time of the opening of the stomata. On the other hand starch gets converted into organic acids.

(ii) In monocots the guard cells do not form starch at all.

Glycolate Theory

Zelitch (1963) proposed that low CO2CO_2 concentration results in glycolate production in guard cells during the day. Glycolate is oxidized to glyoxylate, which is converted to carbohydrate. The soluble sugars raise the osmotic pressure of guard cells. The glyoxylate is reduced by NADPH to glycolate. According to Zelitch the ATP produced in the glycolate-glyoxylate shuttle causes active pumping of water into the guard cells.

The theory, however, has been rejected due to the following reasons:

(i) It fails to explain the opening of the stomata in darkness in succulents.

(ii) If fails to explain the opening of stomata in light in complete absence of CO2CO_2 i.e., glycolate.

(iii) Stomata have been found to open even in the absence of photophosphorylation.

(iv) It fails to explain the fact as to why blue light is more effective than red light in causing stomatal opening.

(v) Stomata in certain plants have been seen to close at midday without any change in the starch content.

Active K+K^+ Transport Mechanism

Fujino (1959, 1967) proposed that stomatal opening and closing result from active transport of potassium into and out of guard cells. Any of the following processes can initiate stomatal opening:

  • Disappearance of starch from guard cells

  • Production of organic acids, particularly malic acid

  • Excretion of H+H^+ from guard cells

  • Uptake of K+K^+ ion into vacuoles of guard cells

  • Uptake of Cl−Cl^- into the vacuoles

The conversion of starch into organic acids is supported by the identification of phosphoenolpyruvic acid carboxylase (PEPC) within guard cells. Increase in organic acid content of epidermal cells with stomatal opening has been reported.

Raschke believes the secretion of H+H^+ ion from the guard cells to be of primary importance in stomatal opening. The H+H^+ ions are made available by dissociating organic acids for exchange with K+K^+ ions. Quantitatively most of the counter charges for K+K^+ in the guard cells are provided by organic anions (malate). Cl−Cl^- ions balances only a small percentage of K+K^+ ions.

There are several evidences to support the occurrence of malate in the vacuole of the guard cells at the time of opening of stomata:

(i) The fungal toxin fusicoccin induces phosphorylation of PEPcase, an enzyme which is responsible for synthesis of malate.

(ii) Decrease in the amount of starch in the guard cell parallels formation of malate in the guard cells.

(iii) Malate has been found to be up to six times in amount in open stomata as compared to that of closed stomata.

(iv) The enzyme PEPcase is present in large amount within the guard cells.

The transport of malate into the vacuole, as well as uptake of K+K^+ and Cl−Cl^- are active processes. The energy requirement for stomatal movement seems to come from respiration.

Accumulation of ions in the guard cells is induced by ATP-powered proton pump which is located in the plasma membrane. There are two evidences to support the involvement of proton pump in the stomatal movements.

  1. The fungal toxin fusicoccin promotes active proton pump as well as stomatal opening.

  2. Secondly, it has been found that vanadate inhibits the proton pump as well as the opening of the stomata.

Turgor (of up to about 5050 atm) is produced in the guard cells by the osmotic pressure exerted by imported potassium ions in association with organic anions (malate) and imported chloride ions.

The stomatal closure is believed to be brought about by a passive or highly catalyzed movement of K+K^+ (and Cl−Cl^−) from the guard cells to the epidermal tissue in general and the subsidiary cells in particular, if they are present.

It is believed that Ca++Ca^{++} ions are indirectly responsible for initiating a series of steps resulting in opening of anion channels to permit the release of malate as well as Cl−Cl^- ions. This results in the opening of K+K^+ channels for their efflux.

Proton Transport Theory

The proton transport concept proposed by Levitt (1974) combines the classical pH theory and active K+K^+ ion transport theory.

  1. Light-induced proton transport from the cytoplasm into the chloroplasts of the guard cells creates a negative potential which will not favour a continued proton transport. This negative potential may lead to an influx of positively charged K+K^+ ions from the surrounding cells so that the negative potential decreased and proton transport continues to take place.

This results in the development of a maximum Ap. The pH of the cytoplasm is raised to 8-9 and that of the chloroplast is lowered to 5. At a higher pH CO<em>2CO<em>2 converted to HCO</em>3HCO</em>3.

K+ ions have threefold role in the opening of stomata:

(i) K+K^+ ions permits maximum H+H^+ transport into the chloroplast.

(ii) K+K^+ ions maintain high cytoplasm pH, to permit the continued conversion of PEP+HCO3 to R(COOH).

(iii) K+K^+ ions and R(COO), together raise the osmotic potential of the guard cells to cause the stomatal opening.

The complete scheme of Levitt is as follows:

A. All chloroplast-containing cells (mesophyll and guard cells)

  1. Cytoplasm: light induced →H+H^+ transport → chloroplasts

  2. pH of cytoplasm (8-9) pH of chloroplask(5)

  3. CO2CO_2, ATP etc.

  4. RUBP → (CH2O)(CH_2O) in the chloroplast

  5. [CO2CO_2] intercell decreases

B. Guard cells only

  1. Cytoplasm: CO<em>2CO<em>2 <0.01% → HCO</em>3HCO</em>3

  2. Cytoplasm: HCO<em>3HCO<em>3 + PEP PEPCPEPC R(COOH)</em>2(COOH)</em>2 + Pi

  3. Cytoplasm: R(COOH)<em>2(COOH)<em>2 → R(COO)</em>2(COO)</em>2+2H+</p></li><li><p>Cytoplasm:H^+</p></li><li><p>Cytoplasm:K^+influx→effluxinflux → effluxH^+((H^+, K^+ATPase)</p></li><li><p>ChloroplastATPase)</p></li><li><p>Chloroplast(CH_2O)+Pi→PEP</p></li><li><p>R(000)+2+ Pi → PEP</p></li><li><p>R(000) + 2K^+decrease(osmoticpressureincreases)</p></li><li><p>decrease (osmotic pressure increases)</p></li><li><p>H_2OendosmosisPt(turgorpressureincrease)</p></li><li><p>Stomaopens.</p></li></ol><p>RoleofMalateandOtherIons</p><p>Talbottetal.(1996)suggestthatinmanyplants,theonlyanionstobalancethechargesofendosmosis Pt (turgor pressure increase)</p></li><li><p>Stoma opens.</p></li></ol><p>Role of Malate and Other Ions</p><p>Talbott et al. (1996) suggest that in many plants, the only anions to balance the charges ofK^+ionsinguardcellsareions in guard cells areCl^-andmalate.Thereareseveralstrongevidencesinfavourofmalateasthepredominantanionofguardcells:</p><p>(i)Increaseinmalateconcentrationislinkedtodecreaseintheconcentrationofstarchprovingthatstarchisconvertedintoorganicacidwhichdissociatesintoand malate. There are several strong evidences in favour of malate as the predominant anion of guard cells:</p><p>(i) Increase in malate concentration is linked to decrease in the concentration of starch proving that starch is converted into organic acid which dissociates intoH^+ionsandmalateions.</p><p>(ii)ThefungaltoxinfusicoccincausesincreaseintheactivityofPEPcaseaswellasinincreaseintheconcentrationofmalateandatthesametimeitalsopromotestheopeningofthestomata.</p><p>(iii)ThepresenceofenzymePEPcaseinlargeamountsintheguardcellsindicatesitsparticipationinthestomatalopening.</p><p>(iv)Theconcentrationofmalateisverylowinclosedstomata.Itsconcentrationgoesupto5to6timesintheguardatthetimeofopeningofstomata.</p><p>(v)TheroleoffusicoccinisreversedbygrowthinhibitorABAwhichcausesclosureofstomata.</p><p>AccordingtoTalbottandZeiger(1998)theopeningofstomataiscorrelatedwithincreaseinsucrosecontentoftheguardcellsandthestomatalclosurewithdecreaseinthesucrosecontentparticularlyintheevening.Accordingtoanotherviewsucrosemaybetakenupapoplasticallyfromthemesophyllbytheguardcellstoincreasetheconcentrationofsolutes.Theybelievethatopeningofthestomatainthemorningmaybeduetoions and malate ions.</p><p>(ii) The fungal toxin fusicoccin causes increase in the activity of PEPcase as well as in increase in the concentration of malate and at the same time it also promotes the opening of the stomata.</p><p>(iii) The presence of enzyme PEPcase in large amounts in the guard cells indicates its participation in the stomatal opening.</p><p>(iv) The concentration of malate is very low in closed stomata. Its concentration goes up to 5 to 6 times in the guard at the time of opening of stomata.</p><p>(v) The role of fusicoccin is reversed by growth inhibitor ABA which causes closure of stomata.</p><p>According to Talbott and Zeiger (1998) the opening of stomata is correlated with increase in sucrose content of the guard cells and the stomatal closure with decrease in the sucrose content particularly in the evening. According to another view sucrose may be taken up apoplastically from the mesophyll by the guard cells to increase the concentration of solutes. They believe that opening of the stomata in the morning may be due toK^+ionanditscounterionswhereasthesucrosetakesovertheirjobintheafternoon.</p><p>Redandbluelightareeffectiveincausingstomatalopening.WhiletheredlighthasapositiveeffectontherateofphotosynthesisaswellasontheproductionofATPandthusindirectlypromotesstomatalopeningthebluelighthasbeenobservedtopromotetheopeningofthestomatadirectlybycausingbiosynthesisofmalateandalsobyprotonextrusionbytheguardcells.</p><p>Theopeningofstomataisduetoosmoticallyactivesolutes,whichmaybecausedbyanyoneormoreofthefollowing:</p><p>Accumulationofion and its counter ions whereas the sucrose takes over their job in the afternoon.</p><p>Red and blue light are effective in causing stomatal opening. While the red light has a positive effect on the rate of photosynthesis as well as on the production of ATP and thus indirectly promotes stomatal opening the blue light has been observed to promote the opening of the stomata directly by causing biosynthesis of malate and also by proton extrusion by the guard cells.</p><p>The opening of stomata is due to osmotically active solutes, which may be caused by any one or more of the following:</p><p>Accumulation ofK^+,,Cl^-ionsandmalateinsidethevacuole.</p><p>Conversionofstarchintosucrose.</p><p>Synthesisofsucrosebychloroplastsoftheguardcellsduringphotosynthesis.</p><p>Sucroseobtainedbyapoplastyfromthemesophyll.</p><p><strong>ScotoactiveClosureinNon−Succulents</strong></p><p>Indarkness,ions and malate inside the vacuole.</p><p>Conversion of starch into sucrose.</p><p>Synthesis of sucrose by chloroplasts of the guard cells during photosynthesis.</p><p>Sucrose obtained by apoplasty from the mesophyll.</p><p><strong>Scotoactive Closure in Non-Succulents</strong></p><p>In darkness,H^+ionsdiffuseintothecytoplasmofguardcells,correlatedwitheffluxofions diffuse into the cytoplasm of guard cells, correlated with efflux ofK^+ions.TheflowcontinuestillRions. The flow continues till R(COO)2ionsareavailableinthecytoplasmtocombinewithions are available in the cytoplasm to combine withH^+ionstoformRions to form R(COOH)2molecules.Levittproposedthefollowingsteps:</p><ol><li><p>Chloroplast:gradientinduced→molecules. Levitt proposed the following steps:</p><ol><li><p>Chloroplast: gradient induced →H^+diffusioncytoplasm</p></li><li><p>Cytoplasm:Rdiffusion cytoplasm</p></li><li><p>Cytoplasm: R(COO)2+2+ 2H^+→R→ R(COOH)2</p></li><li><p>Cytoplasm:Unbalanced</p></li><li><p>Cytoplasm: UnbalancedK^+effluent→subsidiarycellschloroplastdiffusion</p></li><li><p>Cytoplasm:Reffluent → subsidiary cells chloroplast diffusion</p></li><li><p>Cytoplasm: R(COOH)2→decarboxylation→RH2+2→ decarboxylation → RH2+2CO2</p></li><li><p>Chloroplast:High[</p></li><li><p>Chloroplast: High [CO_2]+RUBPetc.→(CH,O)calvincycleenzymes</p></li><li><p>LossofR] + RUBP etc. → (CH,O) calvin cycle enzymes</p></li><li><p>Loss of R(COO)_2++K^+increase(osmoticpressuredecreases)</p></li><li><p>increase (osmotic pressure decreases)</p></li><li><p>H_2Oexosmosis(TurgorPressure)</p></li><li><p>Stomacloses.</p></li></ol><p>ScotoactiveOpeningandPhotoactiveClosureinSucculents(CAMPlants)</p><p>Succulentscarryonorganicacidmetabolismduringthenightandcarbohydratemetabolismduringtheday.Duringnighttheenzymephosphoenol−pyruvatecarboxylase(PEPcase)bringsaboutareactionbetweenphosphoenol−pyruvate(PEP)obtainedfromstarchandexosmosis (Turgor Pressure)</p></li><li><p>Stoma closes.</p></li></ol><p>Scotoactive Opening and Photoactive Closure in Succulents (CAM Plants)</p><p>Succulents carry on organic acid metabolism during the night and carbohydrate metabolism during the day. During night the enzyme phosphoenol-pyruvate carboxylase (PEPcase) brings about a reaction between phosphoenol-pyruvate (PEP) obtained from starch andCO_2toformatemporaryproduct,afourcarbonoxaloacetateandaninorganicphosphate.TheoxaloacetateisimmediatelyreducedtomalatebyNADH.Themalicacidisstoredinthevacuolarsap.Thereisaconsequentfallinwaterpotentialresultinginendosmosis,turgor,bulgingoftheguardcellsandopeningofthestoma.</p><p>DuringthedaytimethemalatedissociatesintoPEPandCO2duetoPEPcaseactivity.Theto form a temporary product, a four carbon oxaloacetate and an inorganic phosphate. The oxaloacetate is immediately reduced to malate by NADH. The malic acid is stored in the vacuolar sap. There is a consequent fall in water potential resulting in endosmosis, turgor, bulging of the guard cells and opening of the stoma.</p><p>During the day time the malate dissociates into PEP and CO2 due to PEPcase activity. TheCO_2releasedentersthePCRcycletomanufacturestarch.Thepyruvatealsogivesrisetostarchviatrioses.</p><p>Insucculentsquitefrequentlystomataopenduringthenight.Initiallythestomataclosewhendarknesssetsin.Thisresultsindeficiencyofreleased enters the PCR cycle to manufacture starch. The pyruvate also gives rise to starch via trioses.</p><p>In succulents quite frequently stomata open during the night. Initially the stomata close when darkness sets in. This results in deficiency ofO2intheleavessinceitisusedupinrespiration.Thein the leaves since it is used up in respiration. TheO2deficiencyismorepronouncedinthethickerleaves.Thisresultsindecreaseinthemitochondrialactivityasaresultofwhichanaerobicrespirationtakesover.Themitochondrialinducedprotontransporttocytoplasmstopsandtheresultantacidificationofthecytoplasmisremoved.</p><p>ThepHofthecytoplasmrisesandPEPwhichisnowavailableinlargeramountduetotheabsenceofthemitochondrialKrebscyclewillbeconvertedtoR(COOH)2byprocess.Oncethestomataopens,deficiency is more pronounced in the thicker leaves. This results in decrease in the mitochondrial activity as a result of which anaerobic respiration takes over. The mitochondrial induced proton transport to cytoplasm stops and the resultant acidification of the cytoplasm is removed.</p><p>The pH of the cytoplasm rises and PEP which is now available in larger amount due to the absence of the mitochondrial Krebs cycle will be converted to R(COOH)2 by process. Once the stomata opens,O_2becomesavailableagainandthemitochondriaareactivatedagain.Themitochondrianowstartreleasingbecomes available again and the mitochondria are activated again. The mitochondria now start releasingH^+intothecytoplasm.ThepHofthecytoplasmdecreasesresultingintheclosureofthestomata.Thisexplainstherepeatedopeningandclosingofstomatafrequentlyseeninthedark.</p><p>PresentStatusoftheTheories</p><p>Thoughquitesatisfactoryexplanationshavebeenputforwardbyseveralworkersintherecentpastregardingthephotoactiveopeningofthestomatacertainaspectsoftheproblemremainunexplainedasyet.theyareasfollows:</p><p>(i)TheroleofABAonthestomatalmovementshasnotbeenexplainedinasatisfactorymanner.</p><p>(ii)AccordingtoDomes(1971)stomataoccurringonthetwosurfacesofaleafshowdifferentdegreesofopeningunderthesameconditionsoflightintensityandcarbondioxideconcentration.</p><p>(iii)Thestomatashowdiurnalrhythmseveniftheleavesarekeptineithercontinuousperiodoflightordarkness.</p><p>(iv)HallandKaufman(1975)observedthatslightdecreaseinrelativehumiditycausessharpincreaseinstomatalclosure.Therelationshipbetwenrelativehumidityoftheatmosphereandthehumidityoftheleafonthestomatalapertureisnotunderstood.</p><h4id="722efdd1−6de4−4d1f−afba−dbf04ceb1b50"data−toc−id="722efdd1−6de4−4d1f−afba−dbf04ceb1b50"collapsed="false"seolevelmigrated="true">AspectsofPhysiologyofStomatalMovements</h4><h5id="611682e8−fb2c−4e3d−9bb8−b3bf4ad07bc5"data−toc−id="611682e8−fb2c−4e3d−9bb8−b3bf4ad07bc5"collapsed="false"seolevelmigrated="true">StomatalOpeningasResponsestoCarbonDioxide</h5><p>Removalofcarbondioxidefromtheatmospheresurroundingtheleavescausesstomatoopenmorewidely.Conversely,thestomatacloseinlightiftheinto the cytoplasm. The pH of the cytoplasm decreases resulting in the closure of the stomata. This explains the repeated opening and closing of stomata frequently seen in the dark.</p><p>Present Status of the Theories</p><p>Though quite satisfactory explanations have been put forward by several workers in the recent past regarding the photoactive opening of the stomata certain aspects of the problem remain unexplained as yet. they are as follows:</p><p>(i) The role of ABA on the stomatal movements has not been explained in a satisfactory manner.</p><p>(ii) According to Domes (1971) stomata occurring on the two surfaces of a leaf show different degrees of opening under the same conditions of light intensity and carbon dioxide concentration.</p><p>(iii) The stomata show diurnal rhythms even if the leaves are kept in either continuous period of light or darkness.</p><p>(iv) Hall and Kaufman (1975) observed that slight decrease in relative humidity causes sharp increase in stomatal closure. The relationship betwen relative humidity of the atmosphere and the humidity of the leaf on the stomatal aperture is not understood.</p><h4 id="722efdd1-6de4-4d1f-afba-dbf04ceb1b50" data-toc-id="722efdd1-6de4-4d1f-afba-dbf04ceb1b50" collapsed="false" seolevelmigrated="true">Aspects of Physiology of Stomatal Movements</h4><h5 id="611682e8-fb2c-4e3d-9bb8-b3bf4ad07bc5" data-toc-id="611682e8-fb2c-4e3d-9bb8-b3bf4ad07bc5" collapsed="false" seolevelmigrated="true">Stomatal Opening as Responses to Carbon Dioxide</h5><p>Removal of carbon dioxide from the atmosphere surrounding the leaves causes stoma to open more widely. Conversely, the stomata close in light if theCO_2concentrationisincreased.Photosynthesisintheleafmesophyll,aswellasthechloroplastoftheguardcellslowersthecarbondioxideconcentrationoftheguardcellsleadingtothestomatalopening.Converselyinlowlightintensityorindarknessrespirationresultsinaccumulationofcarbondioxidewhichleadstostomatalclosure.</p><h5id="ac4b816d−08dd−4eba−a7bf−e3c8b62fd25e"data−toc−id="ac4b816d−08dd−4eba−a7bf−e3c8b62fd25e"collapsed="false"seolevelmigrated="true">EffectofWaterStressoftheMesophyll</h5><p>Itcauseslossofturgidityinthemesophyllcellswhichbringsaboutpartialorcompleteclosureofthestomata.Waterisbelievedtocontrolstomatalmovementsintwoway:hydropassivecontrolorhydroactivecontrol.Intheformertheoverallwaterpotentialoftheplantaffectsthestomatalmovement.Inthecaseofhydroactivecontrolabscisicacid(ABA)playsanimportantroleintheclosureofstomata.Stomatagetclosedwhenthereisdeficiencyofwatereventhoughlightandtemperatureconditionsarefavourable.</p><p>HeathandMansfield(1962)foundincreaseinthesensitivityofthestomatatocarbondioxidewhentheleaveswereunderwaterstrain.Thisincreasedsensitivitytocarbondioxideisbelievedtobeplant′sprotectivemechanism(byclosingthestomata)underconditionsofwatershortage.</p><h5id="2fc59543−6246−45e5−9b56−f2db6884093e"data−toc−id="2fc59543−6246−45e5−9b56−f2db6884093e"collapsed="false"seolevelmigrated="true">EffectofTemperature(Mid−dayclosure)</h5><p>Mid−dayclosureofstomataathightemperaturehasbeenobservedinmanyplants.AccordingtoHeathandOrchard(1957)thetemperatureincreasearoundthemid−dayresultinginstomatalclosureiscausedbyanincreaseincarbondioxideconcentration.Theincreaseinleaftemperaturecausesanincreaseintherateofrespiration,whichresultsinaccumulationofcarbondioxide.Thishypothesiswassupportedbyobservationthatthestomatalclosureathightemperaturecanbepreventedwhenaleafisinitiallykeptinanatmospherefreeofcarbondioxide.</p><h5id="3ac366d1−7454−47b2−a51c−46a4ee579f36"data−toc−id="3ac366d1−7454−47b2−a51c−46a4ee579f36"collapsed="false"seolevelmigrated="true">RoleofABAinStomatalClosure</h5><p>Thephytohormoneabscisicacid(ABA)reducestheturgoroftheguardcellssothatthestomagetsclosedandfurtherlossofwaterisprevented.</p><p>AccordingtoMacRobbie(1998),thereductioninturgoroftheguardcellsisduetoeffluxofconcentration is increased. Photosynthesis in the leaf mesophyll, as well as the chloroplast of the guard cells lowers the carbon dioxide concentration of the guard cells leading to the stomatal opening. Conversely in low light intensity or in darkness respiration results in accumulation of carbon dioxide which leads to stomatal closure.</p><h5 id="ac4b816d-08dd-4eba-a7bf-e3c8b62fd25e" data-toc-id="ac4b816d-08dd-4eba-a7bf-e3c8b62fd25e" collapsed="false" seolevelmigrated="true">Effect of Water Stress of the Mesophyll</h5><p>It causes loss of turgidity in the mesophyll cells which brings about partial or complete closure of the stomata. Water is believed to control stomatal movements in two way: hydropassive control or hydroactive control. In the former the overall water potential of the plant affects the stomatal movement. In the case of hydroactive control abscisic acid (ABA) plays an important role in the closure of stomata. Stomata get closed when there is deficiency of water even though light and temperature conditions are favourable.</p><p>Heath and Mansfield (1962) found increase in the sensitivity of the stomata to carbon dioxide when the leaves were under water strain. This increased sensitivity to carbon dioxide is believed to be plant's protective mechanism (by closing the stomata) under conditions of water shortage.</p><h5 id="2fc59543-6246-45e5-9b56-f2db6884093e" data-toc-id="2fc59543-6246-45e5-9b56-f2db6884093e" collapsed="false" seolevelmigrated="true">Effect of Temperature (Mid-day closure)</h5><p>Mid-day closure of stomata at high temperature has been observed in many plants. According to Heath and Orchard (1957) the temperature increase around the mid-day resulting in stomatal closure is caused by an increase in carbon dioxide concentration. The increase in leaf temperature causes an increase in the rate of respiration, which results in accumulation of carbon dioxide. This hypothesis was supported by observation that the stomatal closure at high temperature can be prevented when a leaf is initially kept in an atmosphere free of carbon dioxide.</p><h5 id="3ac366d1-7454-47b2-a51c-46a4ee579f36" data-toc-id="3ac366d1-7454-47b2-a51c-46a4ee579f36" collapsed="false" seolevelmigrated="true">Role of ABA in Stomatal Closure</h5><p>The phytohormone abscisic acid (ABA) reduces the turgor of the guard cells so that the stoma gets closed and further loss of water is prevented.</p><p>According to Mac Robbie (1998), the reduction in turgor of the guard cells is due to efflux ofK^+ionaswellasoftheanions.Accordingtohimthelossinturgoroftheguardcellsisalsoduetoexitofsucroseandconversionoforganicacidmalatetoosmoticallyinactivestarch.</p><p>McAinshetal.(1990)foundthatABAincreasesthecytosoliccalciumwhichactivatestwotypesofanionchannelsviz.slow−activatingsustained(S−type)andrapidtransienttype(R−type)anionchannels.Thereleaseofanionsthroughthesechannelscausesachangeinthemembranepotential,whichinhibitsinwarddiffusingion as well as of the anions. According to him the loss in turgor of the guard cells is also due to exit of sucrose and conversion of organic acid malate to osmotically inactive starch.</p><p>Mc Ainsh et al. (1990) found that ABA increases the cytosolic calcium which activates two types of anion channels viz. slow-activating sustained (S-type) and rapid transient type (R-type) anion channels. The release of anions through these channels causes a change in the membrane potential, which inhibits inward diffusingK^+ionsandatthesametimepromoteseffluxofions and at the same time promotes efflux ofK^+ ions from the guard cells.

    Stomatal Opening Independent of Carbon Dioxide Concentration

    Many investigators have found light-induced opening of the stomata without any reduction in the concentration of carbon dioxide. Photosynthetic phosphorylation could provide ATP required to drive ion-transport pumps.

    Role of Blue Light in Stomatal Opening

    Schwartz and Zeiger (1984), Tallman (1992) and Assmann (1993) have found a specific blue light response in the stomatal movements. Karlsson (1986) has found the effect maximum on stomatal opening at 450 nm. He also found that if blue light is given in certain quantity then red light increases the amount of stomatal opening.

    The blue light photoreceptors have not been identified as yet. Roth-Bejerano and Itai (1987) considered phytochrome to be a possible photoreceptor. In Arabidopsis, two gene products Phot 1 & Phot 2 protein have been identified to be the blue-light photoreceptors. Srivastava and Zeiger (1995) and others believe the carotenoid Zeaxanthin to be the blue-light receptor.

    Blue light initiates sensory transduction cascades, after it is received by a chromophore. There are three kinds of photoreceptors which mediate three types of blue light responses viz. cryptochromes, phototropins, and zeaxanthin.

    There are several evidencs to show that blue light stimulates stomatal opening through zeaxanthin:

    (i) The sensitivity of the guard cells to blue light is proportionate to the concentration of zeaxanthin.

    (ii) The degree of opening of the stoma depends upon the concentration of zeaxanthin.

    (iii) The action spectrum of blue light stimulated opening of stomata is parallel to the action spectrum of zeaxanthin.

    (iv) The inhibitor 3 mM dithiothreitol (DTT) inhibits stomatal opening as well as synthesis of zeaxanthin.

    (v) The phot-1/phot-2 mutant lacks blue light stimulated stomatal opening.

    (vi) In some species, the conversion of C3 metabolism to C4 metabolism immediately puts to a stop the synthesis of zeaxanthin.

    Temperature-induced opening

    Francis Darwin (1898) was the first to discover that increased temperature results in the opening of the stomata even in darkness which has been confirmed by Mansfield (1965). It is suggested that at high temperature starch may be hydrolysed to maltose in darkness. In light temperature might operate in conjunction with other factors which influence starch hydrolysis, such as carbon dioxide removal and exposure to blue light.

    Temperature reduces the length of time in the opening of the stomata by one-half for every rise of 10°C.</p><h4id="75fd58ab−2eda−48bf−91d2−6bb89f7f8b96"data−toc−id="75fd58ab−2eda−48bf−91d2−6bb89f7f8b96"collapsed="false"seolevelmigrated="true">FactorsAffectingTranspiration</h4><h5id="015f7b72−ed3f−4ab4−828a−229de56915f6"data−toc−id="015f7b72−ed3f−4ab4−828a−229de56915f6"collapsed="false"seolevelmigrated="true">Light</h5><p>Lightincreasestherateoftranspirationdirectlybycausingtheopeningofthestomata.Lightalsoaffectstherateoftranspirationbyincreasingthepermeabilityoftheprotoplasmicmembrane,whichcausesaneasypassageofwatertotheatmosphere.</p><h5id="11e26a4c−a849−4352−93c7−b3e43bb70fea"data−toc−id="11e26a4c−a849−4352−93c7−b3e43bb70fea"collapsed="false"seolevelmigrated="true">HumidityoftheAir</h5><p>Atlowrelativehumiditytheatmosphereisdryandunsaturatedandhasgreatcapacitytoabsorbwater.Onthecontraryiftheatmosphereisveryhumidandrelativelysaturateditscapacitytoabsorbwaterislowandtherateoftranspirationissloweddown.</p><h5id="234be04e−a60b−4983−b4ee−3307aaaccc22"data−toc−id="234be04e−a60b−4983−b4ee−3307aaaccc22"collapsed="false"seolevelmigrated="true">Temperature</h5><p>Theeffectofariseintemperature,therefore,isprincipallyanincreaseinthesteepnessofthewater−vapourpressure,whichenhancestherateoftranspiration.Therateofdiffusionofwatervapourthroughthestomaisalsoincreasedwiththeincreaseoftemperature.Hightemperaturealsohelpsinincreasingtherateoftranspirationbyloweringthehumidityoftheair.Italsoincreasestranspirationbycausingstomatatoopenquicklyandwidely.</p><h5id="d4591e5c−bcdc−4ede−9c01−7ad2d425ea32"data−toc−id="d4591e5c−bcdc−4ede−9c01−7ad2d425ea32"collapsed="false"seolevelmigrated="true">Wind</h5><p>Theblowingwindremovestheaccumulatedhumidityandbringsfreshaircapableofabsorbingwaterandthustherateoftranspirationissomewhatenhanced.Agentlebreezeismoreeffectiveinincreasingtherateoftranspirationthanwindofgreatervelocity.MartinandClements(1935)andSatoo(1955)foundthathighvelocityofwindincreasestherateoftranspirationinitiallybuttheincreaseisfollowedbyagradualdeclineintherateoftranspiration</p><h5id="276a2f31−6f66−425b−ac1e−5e80ed998a29"data−toc−id="276a2f31−6f66−425b−ac1e−5e80ed998a29"collapsed="false"seolevelmigrated="true">AvailableSoilWater</h5><p>Ifavailablesoilwaterissuchthattherateofabsorptionofwaterissloweddown,therateoftranspirationiscorrespondinglydecreased.</p><h5id="e3bcbc0d−c92b−47d0−963f−d9949213ee31"data−toc−id="e3bcbc0d−c92b−47d0−963f−d9949213ee31"collapsed="false"seolevelmigrated="true">AtmosphericPressure</h5><p>Atloweratmosphericpressureathighaltitudes,therateoftranspirationisincreasedbuttheincreaseisoffsetbytheprevailinglowtemperatureattheseheights.</p><h5id="818bb3d2−4c2c−4a9a−b112−5322045210b0"data−toc−id="818bb3d2−4c2c−4a9a−b112−5322045210b0"collapsed="false"seolevelmigrated="true">StructuralFeaturesofPlants</h5><p>Ratesofstomataltranspirationdependsuponthesize,positionanddistributionofstomataontheleaves.Thepresenceofthickcuticle,wax,etc.reducestherateofcuticulartranspiration.Variousadaptationsinthexerophytesaremeantforreducingtherateoftemperature.</p><h4id="65f2f450−d81e−44d4−8c4a−da00849fa901"data−toc−id="65f2f450−d81e−44d4−8c4a−da00849fa901"collapsed="false"seolevelmigrated="true">DailyPeriodicityofTranspiration</h4><p>Innighttherateoftranspirationisinsignificant.Inthemorningthereisasteadyriseinthetranspirationrateuntilthemaximumrateisachievedbynoon.Thengraduallyfalluntiltheprocesscomestoastandstillbeforedarknesssetsin.Whenallthestomataofaleafareclosedtheprocessoftranspirationisstopped.</p><h4id="79375cf9−74ac−4959−a2a0−92650edd1525"data−toc−id="79375cf9−74ac−4959−a2a0−92650edd1525"collapsed="false"seolevelmigrated="true">SignificanceofTranspiration</h4><p>Thesignificanceoftranspirationinplantsisoneofthemostcontroversialsubjectsinplantphysiology.Someworkerswouldconsidertranspirationtobeofgreatbenefittotheplant.Otherswouldregardtheprocesstobeentirelyharmfulanduseless.</p><h5id="f54865bd−03ec−4768−8f04−f56d9be55950"data−toc−id="f54865bd−03ec−4768−8f04−f56d9be55950"collapsed="false"seolevelmigrated="true">SupposedRoleintheMovementofWater(AscentofSap)</h5><p>Workerswhobelievetranspirationtobeofgreatimportancetoplantsclaimthatitbringsaboutrapidtranslocationofwaterintheplant.Theyforgetthattherapidascentofsapwouldnotoccuriftherewasnolossofwaterintranspiration.</p><h5id="f2b52bd9−4ca6−4d69−95d1−f6380e49f6a7"data−toc−id="f2b52bd9−4ca6−4d69−95d1−f6380e49f6a7"collapsed="false"seolevelmigrated="true">SupposedRoleintheAbsorptionandTranslocationofMineralSalts</h5><p>Transpirationisbelievedtoberesponsiblefortheabsorptionofdissolvedsaltsalongwithwaterandtheirtranslocationintheplantalongwiththetranspirationstream.</p><p>Thisviewhas,however,beenfoundtobeuntenablesincethemechanismofabsorptionofsaltsisindependentoftheabsorptionofwater.Moreover,thelawsofdiffusionwouldnotfavourtheentryofsaltsalongwiththediffusingwatermoleculesduringendosmosis.</p><h5id="8ad34a8d−9c00−4e2b−b6d2−ac7ed1cb4334"data−toc−id="8ad34a8d−9c00−4e2b−b6d2−ac7ed1cb4334"collapsed="false"seolevelmigrated="true">SupposedRoleintheRegulationofTemperature</h5><p>Transpirationissupposedtopreventtheheatingoftheleafandhelpsinregulatingthetemperatureoftheleaves.</p><p>Thisso−calledadvantageoftranspirationisalsoanerroneousconceptforitisknownthattranspirationbyitselfisinsufficienttocausetotaldissipationoftheabsorbedradiantenergy.Theregulationoftemperatureinthemisinfactbroughtaboutbythephysicalprocessofradiationofextraheatintotheouteratmosphere.</p><p>Conclusion</p><p>Theplantsare,therefore,variouslymodifiedtokeeptherateoftranspirationtotheminimum.Curtis(1926)has,therefore,rightlycalledtranspirationasthe′necessaryevil′.</p><h4id="01025471−6a30−46d4−8e28−64470d79a905"data−toc−id="01025471−6a30−46d4−8e28−64470d79a905"collapsed="false"seolevelmigrated="true">Guttation</h4><p>Whenconditionsaresuchthattheabsorptionofwatergreatlyexceedstranspiration,theexcessofwaterescapesthroughstructurescalledhydathodespresentatthetipsofveinsofleaves.Guttationnormallyoccursatnightbutmayalsooccurinthedaytime,iftheplantsaregrowinginmoist,soilunderhumidconditions(Kramer,1949,1959).</p><p>Thefluid,whichoozesoutinguttation,normallycontainsavarietyofdissolvedinorganicandorganicsubstances.Whentheguttatedliquidevaporatesthesaltsetc.areconcentratedontheleafmarginsandmaycauseinjurytotheleaf.</p><h4id="96901b2b−891e−4b35−83ac−2c01467c4ed7"data−toc−id="96901b2b−891e−4b35−83ac−2c01467c4ed7"collapsed="false"seolevelmigrated="true">Wilting</h4><p>Plantsnormallymaintainturgidityintheircellstocarryonvariousphysiologicalprocesses.However,mesophyllcellsoftheleavesofthelandplantshavebeenfoundtolosesometurgidityonbrightwarmdays.Thisisduetoatemporaryincreaseintherateoftranspirationoverabsorption.Thispartiallossofturgiditydoesnotcausevisiblewiltingandisknownasincipientwilting.</p><p>Ifthesoilwaterissolowthatitisunavailabletotheplant,therewillbeagenerallossofturgorthroughouttheplantandthewholeplantwillbewilted.Thisisknownaspermanentwiltingsincetheplantscannotrecoverfromthistypeofwilting.</p><p>TheseseedlingscanbepreventedbysprayingtheseedlingswithABAwhichwilldecreasetherateoftranspirationandpreventwiltingoncetheseedlingsgetacclimatizedtofieldconditions.</p><h5id="f1fb9a42−c594−4aba−a7b7−858941e55156"data−toc−id="f1fb9a42−c594−4aba−a7b7−858941e55156"collapsed="false"seolevelmigrated="true">Transpiration</h5><p>Transpirationisthelossofwatervaporfromplantaerialparts,mainlythroughstomata.</p><h6id="9178d7e6−6b15−46fa−a643−b35159369964"data−toc−id="9178d7e6−6b15−46fa−a643−b35159369964"collapsed="false"seolevelmigrated="true">MagnitudeofTranspiration</h6><p>Treescanlosesignificantwater;beechtreesmaylosewaterequaltofivetimesthefreshweightoftheirleavesonasummerday.</p><h6id="e4b4584b−f370−4c2b−b3f7−937923642059"data−toc−id="e4b4584b−f370−4c2b−b3f7−937923642059"collapsed="false"seolevelmigrated="true">TypesofTranspiration</h6><ul><li><p><strong>FoliarTranspiration:</strong>Fromleaves.</p><ul><li><p><strong>StomatalTranspiration:</strong>Throughstomata;mostfoliartranspiration.</p></li><li><p><strong>CuticularTranspiration:</strong>Throughthecuticle(about.</p><h4 id="75fd58ab-2eda-48bf-91d2-6bb89f7f8b96" data-toc-id="75fd58ab-2eda-48bf-91d2-6bb89f7f8b96" collapsed="false" seolevelmigrated="true">Factors Affecting Transpiration</h4><h5 id="015f7b72-ed3f-4ab4-828a-229de56915f6" data-toc-id="015f7b72-ed3f-4ab4-828a-229de56915f6" collapsed="false" seolevelmigrated="true">Light</h5><p>Light increases the rate of transpiration directly by causing the opening of the stomata. Light also affects the rate of transpiration by increasing the permeability of the protoplasmic membrane, which causes an easy passage of water to the atmosphere.</p><h5 id="11e26a4c-a849-4352-93c7-b3e43bb70fea" data-toc-id="11e26a4c-a849-4352-93c7-b3e43bb70fea" collapsed="false" seolevelmigrated="true">Humidity of the Air</h5><p>At low relative humidity the atmosphere is dry and unsaturated and has great capacity to absorb water. On the contrary if the atmosphere is very humid and relatively saturated its capacity to absorb water is low and the rate of transpiration is slowed down.</p><h5 id="234be04e-a60b-4983-b4ee-3307aaaccc22" data-toc-id="234be04e-a60b-4983-b4ee-3307aaaccc22" collapsed="false" seolevelmigrated="true">Temperature</h5><p>The effect of a rise in temperature, therefore, is principally an increase in the steepness of the water-vapour pressure, which enhances the rate of transpiration. The rate of diffusion of water vapour through the stoma is also increased with the increase of temperature. High temperature also helps in increasing the rate of transpiration by lowering the humidity of the air. It also increases transpiration by causing stomata to open quickly and widely.</p><h5 id="d4591e5c-bcdc-4ede-9c01-7ad2d425ea32" data-toc-id="d4591e5c-bcdc-4ede-9c01-7ad2d425ea32" collapsed="false" seolevelmigrated="true">Wind</h5><p>The blowing wind removes the accumulated humidity and brings fresh air capable of absorbing water and thus the rate of transpiration is somewhat enhanced. A gentle breeze is more effective in increasing the rate of transpiration than wind of greater velocity. Martin and Clements (1935) and Satoo (1955) found that high velocity of wind increases the rate of transpiration initially but the increase is followed by a gradual decline in the rate of transpiration</p><h5 id="276a2f31-6f66-425b-ac1e-5e80ed998a29" data-toc-id="276a2f31-6f66-425b-ac1e-5e80ed998a29" collapsed="false" seolevelmigrated="true">Available Soil Water</h5><p>If available soil water is such that the rate of absorption of water is slowed down, the rate of transpiration is correspondingly decreased.</p><h5 id="e3bcbc0d-c92b-47d0-963f-d9949213ee31" data-toc-id="e3bcbc0d-c92b-47d0-963f-d9949213ee31" collapsed="false" seolevelmigrated="true">Atmospheric Pressure</h5><p>At lower atmospheric pressure at high altitudes, the rate of transpiration is increased but the increase is offset by the prevailing low temperature at these heights.</p><h5 id="818bb3d2-4c2c-4a9a-b112-5322045210b0" data-toc-id="818bb3d2-4c2c-4a9a-b112-5322045210b0" collapsed="false" seolevelmigrated="true">Structural Features of Plants</h5><p>Rates of stomatal transpiration depends upon the size, position and distribution of stomata on the leaves. The presence of thick cuticle, wax, etc. reduces the rate of cuticular transpiration. Various adaptations in the xerophytes are meant for reducing the rate of temperature.</p><h4 id="65f2f450-d81e-44d4-8c4a-da00849fa901" data-toc-id="65f2f450-d81e-44d4-8c4a-da00849fa901" collapsed="false" seolevelmigrated="true">Daily Periodicity of Transpiration</h4><p>In night the rate of transpiration is insignificant. In the morning there is a steady rise in the transpiration rate until the maximum rate is achieved by noon. Then gradually fall until the process comes to a standstill before darkness sets in. When all the stomata of a leaf are closed the process of transpiration is stopped.</p><h4 id="79375cf9-74ac-4959-a2a0-92650edd1525" data-toc-id="79375cf9-74ac-4959-a2a0-92650edd1525" collapsed="false" seolevelmigrated="true">Significance of Transpiration</h4><p>The significance of transpiration in plants is one of the most controversial subjects in plant physiology. Some workers would consider transpiration to be of great benefit to the plant. Others would regard the process to be entirely harmful and useless.</p><h5 id="f54865bd-03ec-4768-8f04-f56d9be55950" data-toc-id="f54865bd-03ec-4768-8f04-f56d9be55950" collapsed="false" seolevelmigrated="true">Supposed Role in the Movement of Water (Ascent of Sap)</h5><p>Workers who believe transpiration to be of great importance to plants claim that it brings about rapid translocation of water in the plant. They forget that the rapid ascent of sap would not occur if there was no loss of water in transpiration.</p><h5 id="f2b52bd9-4ca6-4d69-95d1-f6380e49f6a7" data-toc-id="f2b52bd9-4ca6-4d69-95d1-f6380e49f6a7" collapsed="false" seolevelmigrated="true">Supposed Role in the Absorption and Translocation of Mineral Salts</h5><p>Transpiration is believed to be responsible for the absorption of dissolved salts along with water and their translocation in the plant along with the transpiration stream.</p><p>This view has, however, been found to be untenable since the mechanism of absorption of salts is independent of the absorption of water. Moreover, the laws of diffusion would not favour the entry of salts along with the diffusing water molecules during endosmosis.</p><h5 id="8ad34a8d-9c00-4e2b-b6d2-ac7ed1cb4334" data-toc-id="8ad34a8d-9c00-4e2b-b6d2-ac7ed1cb4334" collapsed="false" seolevelmigrated="true">Supposed Role in the Regulation of Temperature</h5><p>Transpiration is supposed to prevent the heating of the leaf and helps in regulating the temperature of the leaves.</p><p>This so-called advantage of transpiration is also an erroneous concept for it is known that transpiration by itself is insufficient to cause total dissipation of the absorbed radiant energy. The regulation of temperature in them is in fact brought about by the physical process of radiation of extra heat into the outer atmosphere.</p><p>Conclusion</p><p>The plants are, therefore, variously modified to keep the rate of transpiration to the minimum. Curtis (1926) has, therefore, rightly called transpiration as the 'necessary evil'.</p><h4 id="01025471-6a30-46d4-8e28-64470d79a905" data-toc-id="01025471-6a30-46d4-8e28-64470d79a905" collapsed="false" seolevelmigrated="true">Guttation</h4><p>When conditions are such that the absorption of water greatly exceeds transpiration, the excess of water escapes through structures called hydathodes present at the tips of veins of leaves. Guttation normally occurs at night but may also occur in the daytime, if the plants are growing in moist, soil under humid conditions (Kramer, 1949, 1959).</p><p>The fluid, which oozes out in guttation, normally contains a variety of dissolved inorganic and organic substances. When the guttated liquid evaporates the salts etc. are concentrated on the leaf margins and may cause injury to the leaf.</p><h4 id="96901b2b-891e-4b35-83ac-2c01467c4ed7" data-toc-id="96901b2b-891e-4b35-83ac-2c01467c4ed7" collapsed="false" seolevelmigrated="true">Wilting</h4><p>Plants normally maintain turgidity in their cells to carry on various physiological processes. However, mesophyll cells of the leaves of the land plants have been found to lose some turgidity on bright warm days. This is due to a temporary increase in the rate of transpiration over absorption. This partial loss of turgidity does not cause visible wilting and is known as incipient wilting.</p><p>If the soil water is so low that it is unavailable to the plant, there will be a general loss of turgor throughout the plant and the whole plant will be wilted. This is known as permanent wilting since the plants cannot recover from this type of wilting.</p><p>These seedlings can be prevented by spraying the seedlings with ABA which will decrease the rate of transpiration and prevent wilting once the seedlings get acclimatized to field conditions.</p><h5 id="f1fb9a42-c594-4aba-a7b7-858941e55156" data-toc-id="f1fb9a42-c594-4aba-a7b7-858941e55156" collapsed="false" seolevelmigrated="true">Transpiration</h5><p>Transpiration is the loss of water vapor from plant aerial parts, mainly through stomata.</p><h6 id="9178d7e6-6b15-46fa-a643-b35159369964" data-toc-id="9178d7e6-6b15-46fa-a643-b35159369964" collapsed="false" seolevelmigrated="true">Magnitude of Transpiration</h6><p>Trees can lose significant water; beech trees may lose water equal to five times the fresh weight of their leaves on a summer day.</p><h6 id="e4b4584b-f370-4c2b-b3f7-937923642059" data-toc-id="e4b4584b-f370-4c2b-b3f7-937923642059" collapsed="false" seolevelmigrated="true">Types of Transpiration</h6><ul><li><p><strong>Foliar Transpiration:</strong> From leaves.</p><ul><li><p><strong>Stomatal Transpiration:</strong> Through stomata; most foliar transpiration.</p></li><li><p><strong>Cuticular Transpiration:</strong> Through the cuticle (about5\%%oftotal).</p></li></ul></li><li><p><strong>LenticularTranspiration:</strong>Throughlenticelsinfruitsandwoodystems.</p></li></ul><h6id="a7c74460−3a5a−49b9−9755−d0f5d52e53d0"data−toc−id="a7c74460−3a5a−49b9−9755−d0f5d52e53d0"collapsed="false"seolevelmigrated="true">SystemofStomatalTranspiration</h6><p>Watervapordiffusesthroughstomataiftheleaf′swater−vaporpressureishigherthanintheair.</p><p>Leaveswithstomataonlyontheirlowersurfacesloseaboutof total).</p></li></ul></li><li><p><strong>Lenticular Transpiration:</strong> Through lenticels in fruits and woody stems.</p></li></ul><h6 id="a7c74460-3a5a-49b9-9755-d0f5d52e53d0" data-toc-id="a7c74460-3a5a-49b9-9755-d0f5d52e53d0" collapsed="false" seolevelmigrated="true">System of Stomatal Transpiration</h6><p>Water vapor diffuses through stomata if the leaf's water-vapor pressure is higher than in the air.</p><p>Leaves with stomata only on their lower surfaces lose about97\%%oftheirtotaltranspirationfromthatsurface.</p><h6id="55661c85−a54d−40bd−9e70−b33b501b0291"data−toc−id="55661c85−a54d−40bd−9e70−b33b501b0291"collapsed="false"seolevelmigrated="true">Stomata</h6><p>Minuteellipticalporesintheepidermis,includingstoma,guardcells,andsometimessubsidiarycells.</p><ul><li><p><strong>Stoma:</strong>Individualpore(e.g.,corn:of their total transpiration from that surface.</p><h6 id="55661c85-a54d-40bd-9e70-b33b501b0291" data-toc-id="55661c85-a54d-40bd-9e70-b33b501b0291" collapsed="false" seolevelmigrated="true">Stomata</h6><p>Minute elliptical pores in the epidermis, including stoma, guard cells, and sometimes subsidiary cells.</p><ul><li><p><strong>Stoma:</strong> Individual pore (e.g., corn:5μmwidebyμm wide by26μmlong).</p></li><li><p><strong>GuardCells:</strong>Kidney−shapedcellsboundingtheporewiththickenedwallstowardsthestoma.</p></li><li><p><strong>SubsidiaryCells:</strong>Epidermalcellssurroundingguardcellsinsomespecies.</p></li></ul><h6id="ac14fcf3−c710−4176−be3c−5f64fd40c918"data−toc−id="ac14fcf3−c710−4176−be3c−5f64fd40c918"collapsed="false"seolevelmigrated="true">DistributionofStomata</h6><p>Varioustypesexist,includinganisocytic,paracytic,haplocheilic,syndetocheilic,andanomocytic.</p><h6id="339b4133−6847−48d0−a667−ca8d200d63d2"data−toc−id="339b4133−6847−48d0−a667−ca8d200d63d2"collapsed="false"seolevelmigrated="true">MechanismofStomatalOpeningandClosing</h6><p>Movementsareduetochangesinguardcellvolumeandshape,influencedbyturgidity.</p><p>TheoriesofStomatalMovements</p><p>Activeμm long).</p></li><li><p><strong>Guard Cells:</strong> Kidney-shaped cells bounding the pore with thickened walls towards the stoma.</p></li><li><p><strong>Subsidiary Cells:</strong> Epidermal cells surrounding guard cells in some species.</p></li></ul><h6 id="ac14fcf3-c710-4176-be3c-5f64fd40c918" data-toc-id="ac14fcf3-c710-4176-be3c-5f64fd40c918" collapsed="false" seolevelmigrated="true">Distribution of Stomata</h6><p>Various types exist, including anisocytic, paracytic, haplocheilic, syndetocheilic, and anomocytic.</p><h6 id="339b4133-6847-48d0-a667-ca8d200d63d2" data-toc-id="339b4133-6847-48d0-a667-ca8d200d63d2" collapsed="false" seolevelmigrated="true">Mechanism of Stomatal Opening and Closing</h6><p>Movements are due to changes in guard cell volume and shape, influenced by turgidity.</p><p>Theories of Stomatal Movements</p><p>ActiveK^+TransportMechanism</p><p>Stomatalmovementsresultfromactivepotassiumtransportintoandoutofguardcells.</p><p>RoleofMalateandOtherIons</p><p>Malateisapredominantanion;sucroselevelsalsocorrelatewithstomatalmovements.</p><p>ScotoactiveClosureinNon−Succulents</p><p>Indarkness,processesleadtostomatalclosure.</p><p>ScotoactiveOpeningandPhotoactiveClosureinSucculents(CAMPlants)</p><p>Succulentsuseorganicacidmetabolismatnightandcarbohydratemetabolismduringtheday.</p><h6id="0a51dd28−5b52−450a−b595−1da2109ba804"data−toc−id="0a51dd28−5b52−450a−b595−1da2109ba804"collapsed="false"seolevelmigrated="true">AspectsofPhysiologyofStomatalMovements</h6><p>StomatalOpeningasResponsestoCarbonDioxide</p><p>LowerTransport Mechanism</p><p>Stomatal movements result from active potassium transport into and out of guard cells.</p><p>Role of Malate and Other Ions</p><p>Malate is a predominant anion; sucrose levels also correlate with stomatal movements.</p><p>Scotoactive Closure in Non-Succulents</p><p>In darkness, processes lead to stomatal closure.</p><p>Scotoactive Opening and Photoactive Closure in Succulents (CAM Plants)</p><p>Succulents use organic acid metabolism at night and carbohydrate metabolism during the day.</p><h6 id="0a51dd28-5b52-450a-b595-1da2109ba804" data-toc-id="0a51dd28-5b52-450a-b595-1da2109ba804" collapsed="false" seolevelmigrated="true">Aspects of Physiology of Stomatal Movements</h6><p>Stomatal Opening as Responses to Carbon Dioxide</p><p>LowerCO2causesstomatatoopen;highercauses stomata to open; higherCO2causesthemtoclose.</p><p>EffectofWaterStressoftheMesophyll</p><p>Waterstressleadstostomatalclosure,possiblyviaabscisicacid(ABA).</p><p>EffectofTemperature(Mid−dayclosure)</p><p>Hightemperaturescancausestomatalclosureduetoincreasedrespirationandcauses them to close.</p><p>Effect of Water Stress of the Mesophyll</p><p>Water stress leads to stomatal closure, possibly via abscisic acid (ABA).</p><p>Effect of Temperature (Mid-day closure)</p><p>High temperatures can cause stomatal closure due to increased respiration andCO_2accumulation.</p><p>RoleofABAinStomatalClosure</p><p>ABAreducesguardcellturgor,preventingwaterloss.</p><p>StomatalOpeningIndependentofCarbonDioxideConcentration</p><p>Lightcaninducestomatalopeningregardlessofaccumulation.</p><p>Role of ABA in Stomatal Closure</p><p>ABA reduces guard cell turgor, preventing water loss.</p><p>Stomatal Opening Independent of Carbon Dioxide Concentration</p><p>Light can induce stomatal opening regardless ofCO_2$$ levels.

    Role of Blue Light in Stomatal Opening

    Blue light promotes stomatal opening, possibly via zeaxanthin.

    Temperature-induced opening

    Increased temperature can lead to stomatal opening even in darkness.

    Factors Affecting Transpiration

    Light

    Increases transpiration by opening stomata.

    Humidity of the Air

    Low humidity increases transpiration; high humidity slows it.

    Temperature

    Increases transpiration by increasing the water-vapor pressure gradient.

    Wind

    Gentle breezes enhance transpiration; high-velocity winds may initially increase transpiration but lead to a decline.

    Available Soil Water

    Reduced water availability decreases transpiration.

    Structural Features of Plants

    Features like thick cuticles reduce transpiration.

    Daily Periodicity of Transpiration

    Transpiration peaks at noon and is minimal at night.

    Significance of Transpiration

    Supposed Role in the Movement of Water (Ascent of Sap)

    Transpiration helps water translocation in plants.

    Supposed Role in the Absorption and Translocation of Mineral Salts

    Transpiration aids salt absorption and translocation.

    Supposed Role in the Regulation of Temperature

    Transpiration helps regulate leaf temperature.

    Conclusion

    Transpiration