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Ascent of sap

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Suresh Babu
Suresh Babu

The document discusses the mechanism of ascent of sap in plants. There are three main theories discussed: vital theories which propose that pumping occurs via contractions of cells, root pressure theory where pressure from root tissues forces water up the xylem, and transpiration pull theory which is the most accepted. Transpiration pull theory states that water loss from transpiration in leaves creates a tension that is transmitted downwards through the continuous water column in the xylem, pulling water up from the roots to replace what is lost.

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ASCENT OF SAP By Dr.M.V.Suresh Babu M.Sc., M.Phil.,Ph.D., Assistant Professor of Botany E_mail: mvs.ced2010@gmail.com
ASCENT OF SAP Definition: The upward movement of water and mineral salts in xylem vessels through the roots to aerial parts of the plants is known as ascent of sap. Mechanism of Ascent of sap : A number of theories have been given to explain about the mechanism of ascent of sap. A) Vital theories B) Root pressure theory C) Physical force theory
A) Vital theories 1.Relay pump theory Godslewski According to this theory the pumping of water takes place upwards due to the vital activities of xylem parenchyma and xylem rays. 2. Pulsation theory -- J. C. Bose According to this theory water is pumped up due to the contraction and expansion of innermost cortical cells which creates a pulsation causing upward movement of water.
B) Root pressure theory The pressure exerted by the root tissues that would force liquid up the xylem. The basis for root pressure is a higher solute concentration, and accordingly a more negative water potential, in the xylem sap than in the soil solution. This water potential draws water into the stele, once there, the water has nowhere to go but up, so it rises in the vessels and tracheids. The best evidence of root pressure is guttation and bleeding.
Manifestation of root pressure: A) Guttation Good evidence that shows the phenomenon or root pressure guttation, A process inn which liquid water is forced out through openings at the margins of leaves (Hydathodes) due to root pressure. Guttation occurs only under conditions of 1. high atmospheric humidity and 2. plentiful water in the soil, which occur most commonly at night.
B) Bleeding Exudation of sap from injured parts of the plants is called bleeding. Examples: Exudation of milky latex from euphorbia, cut stumps of some plants, such as Coleus.
Experimental studies of root pressure: If a well watered plant is cut near its base, the xylem sap is seen to flow out through cut end with a pressure. This pressure is actually the hydrostatic pressure developed in the root system called root pressure
Limitations: Root pressure, however, cannot account for the ascent of sap in trees. Root pressure seldom exceeds 0.10.2 MPa (12 atmospheres). If root pressure were driving sap up the xylem, we would observe a positive pressure potential in the xylem. Materials can be transported upward in the xylem even when the roots have been removed
3. Transpiration pull theory The most accepted theory of water movement through plants. Proposed by DIXON and JOLLY in 1894.
Ascent of sap
According to this theory:- Due to transpiration water lost in the form of vapour from the leaves. Due to water loss mesophyll cells becomes concentrated. DPD of mesophyll increases. These mesophyll cells draw water from adjoining mesophyll cells which in turn from xylem elements of leaf. Xylem filled with continuous water column from leaf to root due to cohesive force. A tension is generated at the top called transpiration pull. This tension transmitted downwards up to roots through stem. Due to which water moves upwards.

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Ascent of sap

  • 1. ASCENT OF SAP By Dr.M.V.Suresh Babu M.Sc., M.Phil.,Ph.D., Assistant Professor of Botany E_mail: mvs.ced2010@gmail.com
  • 2. ASCENT OF SAP Definition: The upward movement of water and mineral salts in xylem vessels through the roots to aerial parts of the plants is known as ascent of sap. Mechanism of Ascent of sap : A number of theories have been given to explain about the mechanism of ascent of sap. A) Vital theories B) Root pressure theory C) Physical force theory
  • 3. A) Vital theories 1.Relay pump theory Godslewski According to this theory the pumping of water takes place upwards due to the vital activities of xylem parenchyma and xylem rays. 2. Pulsation theory -- J. C. Bose According to this theory water is pumped up due to the contraction and expansion of innermost cortical cells which creates a pulsation causing upward movement of water.
  • 4. B) Root pressure theory The pressure exerted by the root tissues that would force liquid up the xylem. The basis for root pressure is a higher solute concentration, and accordingly a more negative water potential, in the xylem sap than in the soil solution. This water potential draws water into the stele, once there, the water has nowhere to go but up, so it rises in the vessels and tracheids. The best evidence of root pressure is guttation and bleeding.
  • 5. Manifestation of root pressure: A) Guttation Good evidence that shows the phenomenon or root pressure guttation, A process inn which liquid water is forced out through openings at the margins of leaves (Hydathodes) due to root pressure. Guttation occurs only under conditions of 1. high atmospheric humidity and 2. plentiful water in the soil, which occur most commonly at night.
  • 6. B) Bleeding Exudation of sap from injured parts of the plants is called bleeding. Examples: Exudation of milky latex from euphorbia, cut stumps of some plants, such as Coleus.
  • 7. Experimental studies of root pressure: If a well watered plant is cut near its base, the xylem sap is seen to flow out through cut end with a pressure. This pressure is actually the hydrostatic pressure developed in the root system called root pressure
  • 8. Limitations: Root pressure, however, cannot account for the ascent of sap in trees. Root pressure seldom exceeds 0.10.2 MPa (12 atmospheres). If root pressure were driving sap up the xylem, we would observe a positive pressure potential in the xylem. Materials can be transported upward in the xylem even when the roots have been removed
  • 9. 3. Transpiration pull theory The most accepted theory of water movement through plants. Proposed by DIXON and JOLLY in 1894.
  • 11. According to this theory:- Due to transpiration water lost in the form of vapour from the leaves. Due to water loss mesophyll cells becomes concentrated. DPD of mesophyll increases. These mesophyll cells draw water from adjoining mesophyll cells which in turn from xylem elements of leaf. Xylem filled with continuous water column from leaf to root due to cohesive force. A tension is generated at the top called transpiration pull. This tension transmitted downwards up to roots through stem. Due to which water moves upwards.