Transport of water

For B.Sc. III (Medical) students

1. TRANSPORT OF WATER for B.Sc. III (Medical) Dr Jasmine Brar Email : [email protected]

2. The upward movement of water from root towards the top is known as ascent of sap upto a distance of 100-130 m in Eucalyptus, Sequoia etc. The rate of ascent of sap is upto 75 cm min-1 for rapidly transpiring dicots & 10 cm-1 in gymnosperms. This happens by upward movement of water through xylem vessels: Ringing Experiment Stain Test Dixon’s test on Blockage of Lamina Mechanical Pressure Labelled Water

3. Demonstration of path of Ascent of Sap Shoot with ring of bark & pith removed remains turgid B. Shoots with xylem removed shows wilting of leaves

4. Eosine Test to Demonstrate the of path of Ascent of Sap

5. Leafy shoot freshly cut under water dipped in 2% eosine solution. Tracheids & vessels will be found coloured indicating that coloured water travels through vascular bundles. Dixon Test on Blockage of lamina ( Tilia europea , Lime) Cut end of branches were placed in water, melted paraffin & melted gelatin at 50 ºC for 40 min. Branches were than transferred to fresh water at 13 ºC

6. After 15.30 hrs leaves of control remain fresh, leaves in paraffin were completed wilted & leaves in gelatin were partially wilted. They were shifted to safranin for 1.15 hrs & sections were cut after staining. The walls in paraffin & gelatin were stained upto height of 20 cms & xylem cavities were found to be completely blocked in paraffin & partially in gelatin .

7. Mechanical Pressure: In soft stem eg Vines ascent of sap can be prevented by applying mechanical force as it will close the lumen of xylem channels. Labelled water: Isotopically labelled water supplied to roots of potted plants will be detectable after sometime in xylem of stem.

8. Theories of Ascent of Sap General route : 1. Gravitational Force 2. Perforation of plates of xylem 3. End walls of vessels & tracheids 4. Irregular walls of water channels 5. Passage of water from xylem parenchyma into xylem channels Lot of resistance in the path of water from root to top of Shoot.

9. 1. Vital Force Theories for Ascent of Sap 2. Physical Force Theories for Ascent of Sap Vital Force Theories: Upward movement of water by Activity of living cells of plant bordering xylem. Godlewaki (1884): Water move up like a stair-case path (relay pump/clambering theory) under influence of Air pressure Osmotic pressure of parenchyma cells & medullary rays Osmosis play important role.

10. Theories of Ascent of Sap Godlewski’s (1884) Theory of Ascent of Sap

11. Pulsation Theory of Bose (1923) Inserted fine electric probe into stem of Desmodium . The galvanometer needle exhibited small oscillations in the outer part of stem & in innermost cortical cells the oscillations become rapid & permanent. Conclusion : Pulsatory activity of innermost cortical cells & such pulsations were present even when root pressure was nil & transpiration = 0. Temperature Anaesthetics Effect Ascent of Sap Poisons

12. Force Acting: The sensitive cells absorb water from outside & pump the same into vessels. Bennedict , 1927: Pulsation activity cannot account for observed rate of Ascent of sap (8000-27000 times more rapid). Movement of water is supported by Creation of bioelectric potential in xylem walls. Osmotic relations b/w living cells &newly differentiated xylem elements ( Priestley) .

13. Pulsation Theory of Bose (1923)

14. Objections to Vital Force Theories Boucherie (1840) demonstrated that a poisonous fluid supplied to cut end ascended to the top. Experiment repeated by Strasburger (1891) 75 year oak tree (22 m) was cut from base & placed in picric acid solution (poison) & it reached the top of plant. Similar results were obtained when stems were killed by exposure to high temp of 90ºC

15. 1. Root Pressure: Priestley Basis: Root pressure developed in the xylem push the sap upwards. 1-2 atm ( Merwin , 1909); Grape vine: 5.6 atm & upto 10-15 atm. Objections: Root pressure has not been found in all plants. Gymnosperms have 0 values & they are tallest trees. Physical Force Theories

16. Removal of roots cause transient drop in transpiration. A tension/negative pressure is found in rapidly transpiring plants. Root pressure is seen in favourable periods (Spring/rainy) When xylem sap is hypertonic & transpiration rate is low & in summers, when water requirements are high root pressure is almost nil. Kramer, (1969): Amount of sap exuded by root pressure is only 4% of total transpiration of intact plant.

17. Demonstration of root pressure

18. 2. Capillarity: Water rises in narrow tubes (xylem vessels) due to force of tension: capillarity. Water can only rise upto 1 m in dia of 0.03 nm. Hence, Applicable to small sized plants. Vessels does not have uniformity of lumen on account of thickenings. Cannot operate in tracheids due to presence of end walls. Soil water is not connected with xylem vessels directly.

19. Free surface must be present in xylem vessels for the force to operate. Rise of water ˃ narrow vessels; whereas tall trees have large diameter of xylem vessels. Autumn wood is more efficient than spring wood for water conduction which is not true as spring wood has better conduction.

20. 3. Chain Theory: Jamin said that plants have a column consisting of alternate air-water. Water move upwardly due to expansion of air. Such chain of air-water does not exist in plants. 4. Imbibition Theory: Force of imbibition = 100-1000 atm Unger, (1868) Sachs, (1874, 78, 79) Xylem contain hydrophillic colloids. The value must be Infinitesimal small & adherence of water to xylem vessels not at all corelated with imbibition .

21. Atmosphere Pressure Theory: Boehm, (1809): pressure of transpiring surface falls below atm pressure. Water rise up to fill this gap. Objections: No free surface at lower end of plant which is required for operation of atmosphere pressure. Max height water can rise is 10 m even if complete vaccum is created.

22. Cohesion Tension/Cohesion Transpiration Pull Theory Dixon & Jolly (1894) Continuous Water Column. Withdrawl of water due to DPD deficit b/w atmosphere & inner surface of leaves. Development of tension/transpiration pull Cohesion or tensile strength of water. Ascent of Sap

23. Continuous water column /Hydrostatic System Theory (Mac Dougal , 1936) From root through stem into leaves. It is present in tracheary elements. It forms continuous system through unthickened areas, therefore, blockage of few tracheary elements does not break the continuity of the column ( Scholander , 1957). Water column is maintained by transpiration pull & hence not pulled by gravity. Cohesion, Adhesion, Surface tension maintains the water column.

24. ii. Withdrawl of water: Outside air has lower water potential/higher DPD as compared to saturated air inside leaf so water vapours diffuse out of leaves. It decreases the Ψ w of leaf cells as low as -30 bars & these cells withdraw water from deeper mesophyll cells due to gradient in DPD. Deeper mesophyll cells withdraw water from parenchyma cells bordering the vascular tissues of veins. Movement of water occurs through apoplast (Weatherly, 1963)

25. iii. Development of tension/transpiration: Due to continuous withdrawl of water from the xylem by transpiring leaf the whole water column of plant comes under tension/strain. It is called transpiration pull & negative hydrostatic pressure & has value upto 20 atm.

26. iv. Cohesion or tensile strength of water Transpiration pull is unable to break the continuity of water column as water molecules have strong cohesive force (H-bonding). The value of cohesion force has been calculated as 45-207 atm by Dixon & Jolly. Cohesive force is also called tensile strength. There acts force of adhesion b/w water molecules & walls of tracheary elements & walls are strong enough to resist collapsing when sap is under tension.

27. v. Ascent of sap: Due to transpiration water column is pulled up passively from below to top of plant like a rope. Tension of 1 atm is sufficient to pull water upto 10 m height; 10-20 atm is sufficient to raise water to the tallest trees by overcoming all the resistances.

28. Path of water through the plant

29. Cohesive force due to H-bonding b/w H2O

30. Evidences The rate of ascent of sap closely follow rate of transpiration . Evaporation from atmometer produce tension in water column which can raise mercury. Shoot attached to a tube having water & dipping in mercury cause mercury to raise due to transpiration pull . Branch is cut from rapidly transpiring plant, water snaps away from cut end proving that water column is under tension.

31. With the help of dendrograph it has been found that tree trunks shrink during the day & expand during night. Contraction is caused by narrowing of tracheary elements when water is under tension. The maximum tension observed is 10-20 atm which is sufficient to pull water to the tallest tress upto 130 metres in height. Gymnosperms does not have vessels instead they have tracheids so there is less ascent of sap in them.

32. Difference b/w rate of transpiration & Absorption

33. Dendrogarph Diurnal variation in diameter of tree trunk

34. Daily variation in osmotic pressure in leaves

35. Objections A tension upto 100 atm has been reported in xylem sap while cohesive force of sap can be as low as 45 atm. Overlapping cuts do not stop ascent of sap though they break continuity of water column.

36. Defunctioning of water system of whole plants Gases like CO2, O2, N2 dissolved in air shall form submicroscopic air bubbles under tension which fuse to form large air bubbles by process of cavitation & break the continuity of column & stop the ascent of sap. Cavitation is favoured by: Increase in temperature Height Tension It decreases water conduction by 30% (growing season) & 60-100% (winters)

37. Blocking of narrow water channels due to expansion of air bubbles as a result of increase in temperature & tension. Air bubbles at low temperature /tension Air bubbles at high temperature /tension

38. Movement of 32P in ascent of sap Without cuts With 2 overlapping saw cuts Ascent of Sap continues even when continuity of water is broken by overlapping cuts (Mackay & Weatherly (1973)

39. Transpiration Pull theory is most widely accepted: Endodermis filters out gas bubbles from entering xylem. Xylem vessels are not continuous & are connected by pits (air bubbles cannot pass through pit membrane). Plants possessing vessels also have tracheids for alternate pathway free from air bubbles. 4. Plants with secondary growth produce new xylem channels every year.

40. 5. Air bubbles get slowly re-absorbed in liquid water when tension is low at night. 6. Positive root pressure also help in shrinking air bubbles. 7. Ascent of sap occurs even after overlapping cuts due to repair of cavitation ( Leapold & Kriedemann , 1975)

41. Lateral movement of water Lateral movement of water also occur which feed the living cells of cortex & pith by cell to cell movement of water. It also occurs in tracheids & vessels of xylem. It can be seen by cutting the roots of tree on one side. No difference in water content of leaves of the affected side is observed.

42. Thanks