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Ascent of sap


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Ascent of sap

  1. 1. [Upward movement of water and othercomponents from roots to aerial parts of the plant ] 1
  2. 2. Ascent ofwater andminerals….•Upwardmovement ofwater andminerals iscalled as ascentof sap ortranslocationof sap. 2
  3. 3. 3
  4. 4. Structures involved in theprocess of ascent of sap …• Water mainlyabsorbed by roothairs• As xylemelements arepresent all over thelength that’s whyxylem is mainlyresponsible forascent of sap 4
  5. 5. •Xylem cells•Xylem vessels•Xylem tracheids•Pits 5
  6. 6. •Tracheids have large septalperforations that providecontinuous system and do notafford much resistance for thesmooth flow of water.•The xylem parenchymaprovides access for the lateralmovement of water towardscortical cells.•It contains many inorganic andorganic components absorbedby the root system.•So the trachea and tracheidswith their large lumen and end toend association act as excellentpipelines for the movement ofwater, minerals . 6
  7. 7. RATE OF MOVEMENTUsing dyes and radioactiveisotopes, it has beendetermined that theaverage rate of movementof water in xylem is 60-75cms/minute. This is undernormal transpiringconditions. However, thisrate is not constant and itvaries depending upon theenvironmental conditions,particularly RelativeHumidity (RH) of theatmosphere. 7
  8. 8. Mesophyll cells (transpiration occurs Increase in DPD Absorption of H2O from near by cells Leads to increase in DPD from cell to cellLeads to pulling of water through xylem vessels of leaf Xylem midrib xylem of root Continuous intake of water from root hairs 8
  9. 9. •vital theories•Root pressure theory•Physical theories Vital theories are stated here .. •Godlewski’s relay pump theory •Pulsation theory •Root pressure theory 9
  10. 10. • Due to rythmaticchange in OP ofxylem elements•Living cells absorbwater by osmosis•And finally water ispumped into xylemvessel due tolowering of pressurein living cells•Thus staircase typeof movement occurs 11
  11. 11. •Living cells of innermost layerof cortex had rhythmicpulsations.•These pulsations areresponsible for the upwardmovement of water.•Fine needle attached togalvanometer was inserted inplant stem•Galvanometer shows slowoscillations but when ittouched cortex cells it turnedviolent indicating that thecells were pulsating. 12
  12. 12. •When watered plant is cut nearbase xylem sap flow out at theend with a pressure.•Mainly observed in herbaceousplants•Water oozes out due to bleeding•This is due to hydrostaticpressure inside root c/a rootpressure•Root pressure appear due toosmotic pressure.•If root is supplied with isotonicor hypertonic solution rootpressure disappears. 13
  13. 13. 1)Capillarity :water rises in narrow tubes due to the force of surface tension• It stated that it is the capillary action of narrow xylem tubes and it is further supported by atmospheric pressure2) Imbibitional theory : ascent of sap takesplace due to imbibitional forces through walls ofxylem cells.•Magnitude of imbibitional force may be up to 1000atm,•But discarded due to fact that water moves inlumen of vessels rather than through the walls. 14
  14. 14. 15
  15. 15. COHESION TENSION THEORY…….•Theory wasproposed by Dixonand Jolley, but thetheory has beenexpanded .•This theory is alsocalled as Suctionforce theory. 16
  16. 16. COHESIVE FORCESWhen fine water columns aresubjected to a transpirationpull as great as 1000 bars ormore, the water column islikely to snap, but it does notbecause inter molecularforces that hold the watermolecules together aregreater than the opposingforces,. 17
  17. 17. Such forces are called cohesive forces; they are nothing buthydrogen bonds between the water molecules. The overallstrength of water column in such narrow xylem elementshas been estimated to be many folds higher than thetranspiration pull and the gravitational pull put together 18
  18. 18. •Transpirational pull develops a negative pressureor tension in xylem sap which is transferred to theroots.•DPD of water in mesophyll cells increases.•So water is absorbed by cell wall from protoplasm•DPD of xylem cells increases.•When transpiration is rapid but absorption isslow, water in xylem cells is passed into the state oftension.• 19
  19. 19. •Leaves loose water by stomata by the processof transpiration•Water transpired through the mesophyll cellspaces also•DPD increases and water potential decreasesin mesophyll cells.•Due to this water moves along concentrationgradient.•Now the water is continuously pulled bycohesive and adhesive forces•This upward force is called as transpirationalpull. 20
  20. 20. Flow chart showing cohesion tension theory of Dixon Strong force of attraction in water molecules {cohesion} Tensile strength of water + adhesive force of water Water molecules remain adhere to xylem vessel walls 21
  21. 21. EVIDENCE IN SUPPORT OF COHESION...1. Transpiration pull developed in the aerial regionsin the air is more than 1000 bars.2. Cohesive force that holds the water molecules in acolumn as narrow as xylem vessels is very strong andthey withstand the opposing transpiration andgravitational pulls.3. Because of the opposing forces, tension developsin the water column. As a result, xylem elements becomeslightly narrow. This results in the contraction andexpansion of the stem which has been demonstrated byusing dendrometer. The diurnal behavior of rhythmiccontraction and expansion is a good evidence for thewater column to be in tension when the transpiration is 22rapid or not.
  22. 22. 4. The forces that operate cohesiontranspiration pull are just passive forces and nometabolic energy is involved in thisphenomenon, because present the movement ofwater upwards. 23
  23. 23. Guttation : •While transpiration loss of water in the form of liquid is c/a guttation •Such plants have pore in the epidermis near xylem elements c/a hydathode. •Thus rate of expiration increases. 24