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Ascent of sap [autosaved]
- 1. Transport of Water Kamaljit Sidhu Associate Professor Department of Botany Khalsa College For Women Civil lines Ludhiana
- 3. Ascent of Sap • The upward movement of water from roots towards the top of the plant is known as ascent of sap • Root is the absorbing end of a plant • Aerial parts bearing the leaves consume the absorbed water • Movement against the force of gravity • Rate of ascent of sap is 75 cm min-1for transpiring plants • Water is transported through xylem • Passage of water through sieve tubes or xylem vessels and tracheid's
- 4. Evidences that xylem is the water conducting tissue • Take a freshly cut leafy shoot under water • Dip the cut end of the shoot in 2% Eosine solution • After sometimes the veins would become red • Even the cut end would become reddish • Cut the section and observe it under the microscope • Tracheids and vessels are found to be coloured • This shows that the coloured water passes through xylem • Appearance of red colour in the leaf veins indicate that the coloured water travels through the vascular bundles
- 7. Theories of Ascent of Sap • Resistance is present in the path of water flow from root to the top • This resisting force is in the form of gravitational force, perforation plates of xylem , end walls of vessels and tracheid , irregular walls of water channels • Two types of –Vital force theories and Physical force theories • Vital force theories-Upward movement of water by activity of living cells of the plant bordering xylem • Westermaier (1884)-The upward movement of water takes place through the xylem parenchyma cells , tracheids and vessels act as water reservoirs in which water is held by capillarity
- 8. Vital force theories • Relay pump theory-Water travels up from the roots to the leaves in the stair case movement • Two forces influencing-atmospheric pressure and osmotic pressure • Parenchyma cells absorb water from the xylem vessels by osmosis • Water rises upward in them by air pressure • OP falls so that the xylem absorbs the same water at a higher level
- 10. Objections to Vital force theories
- 11. Physical Force Theories • Root pressure theory-Put forward by Priestly • Force developed in the xylem of the root which push the sap towards the top • Root pressure of 1-2 atm in all the plants,Grape-vine- 5.6 atm • Not found in gymnosperms • Water rises upwards even after removal of roots • Rapidly transpiring plants,no root pressure • Root pressure in favourable season like spring orrainy season • Amount of sap exuded does not form more than 4% of total transpiration • Absorption of detopped plant is quite low • Root pressure does not exceed1 -2 atm • Commonly found at night when transpiration is low
- 12. Physical Force Theories • Capillarity-Bohm(1863) water rises up through xylem vessels due to capillarity • Capillarity is the phenomena when water rises up in the narrow tubes due to surface tension • Objections-value of capillarity is very small water can rise up to only 1m • Xylem vessels do not have uniformity of lumen • Cannot operate in plants having tracheids in the xylem • As the water rises up due to capillarity the diametre of lumen would decrease a)Tall plants would have narrow vessels b)Autumn wood efficient than spring wood
- 13. Physical Force Theories • Chain theory: Put forth by Jamin , plants have a column of alternate air and water, water moves upwards due to the expansion of air,does not exist in plants • Imbibition theory:Adsorption of water by hydophillic colloids,put forth by Unger,water travels up the vessels and tracheids as they have hydrophillic colloids,but very slow as they hold the water firmly • Atmospheric pressure- put forth by Boehm,Due to transpiration,the pressure falls below the atmospheric pressure,to fill the gap pulls the water from below Objections-No free surface at the lower end,water rises only 10m
- 14. Physical Force Theories • Cohesion tension theory-Put forth by Jolly and Dixon also known as Dixons theory of Ascent of sap ,main features are • Continuous water column- Continuos water column of water from roots through stem into the leaves,present in tracheary elements,not pulled down by the force of gravity as it is pulled up by transpiration,cohesion,adhesion surface tension keeps it in place • Withdrawl of water-Intercellular spaces of leaves are saturated with water vapours ,connected outside through stomata, outside has a higher DPD,so water diffuses out, decreases the water potential of the leaf cells in contact with intercellular spaces ,they withdraw water from deep mesophyll cells, creates a DPD gradient • Transpiration pull-The tension in the water column due to transpiration is known as transpiration pull • Cohesion or tensile strength of water-Transpiration pull is unable to break the water column due to strong cohesion force among water molecules due to strong h-bonds so it is stretched-tensile strength • Ascent of sap-Due to the tension created by transpiration water is pulled up passively from below to the top like a rope
- 15. Evidences to support Dixons Theory • The rate of water absorption and ascent of sap depends the rate of transpiration • The branch cut from the rapidly transpiring plant water snaps away showing that it is under tension • Development of tension in the xylem channels by probe experiments • Evaporation of water from a porous pot could not produce the same tension as transpiration • Cut shoots of clematis could draw a column of mercury up 1.4-2m • Mac dougal developed a self recording instrument for measuring the • changes in the diameter-Dendrograph • Cohesive strength of xylem sap is adequate to meet the stress of • transpiration pull • Energy requirednto lift the water to a maximum height is very small • The op of the leaf cells show dirunal variation,maximum in the afternoon • Value of op of the cell sap in different plant types is trees>shrubs>herbs
- 16. Objections of Dixons Theory • Theoretical value of cohesion force of water is 15000-20000atm while practically it is only45atm • Gases are dissolved in xylem water , when xylem is under tension, gases converted into bubbles, form larger bubbles-cavitation,favoured by increase in temperature , height and tension • Ascent of sap continues even after water column is broken • Water column breaks down frequently in herbaceous plants • According to Dixon-tracheids are more efficient than vessels but not true • In some plants slight shaking breaks the water column but not same for trees, even severe shaking does not break