1. There are two mechanisms of water absorption in plants - active and passive absorption. Active absorption utilizes ATP and accounts for less than 10% of absorption, while passive absorption is responsible for over 90% of absorption and is driven by transpiration. 2. There are several theories that attempt to explain the mechanism of water ascent in plants, including root pressure theory, atmospheric pressure theory, and imbibition theory. 3. The most widely accepted theory is the transpiration pull and cohesion tension theory proposed by Dixon and Jolly. It states that transpiration is necessary to pull water up through the plant and that the cohesive and adhesive properties of water allow it to ascend to the leaves.

1. Mechanisms of water
absorption and ascent of sap

2. Mechanism of water absorption
Water is absorbed by two methods:
1. Active absorption
• Root plays active role and shoot plays passive role
• ATP is utilized
• Transpiration not responsible for active absorption
• Accounts for < 10% absorption
• Involve symplast (Protoplasm+plasmodesmata) movement

3. 2.Passive absorption
• Root plays passive role and shoot plays active role.
• ATP is not utilized.
• Involves both apoplast(cell wall + intercellular space) and symplast
movement.
• Maximum absorption (>90%) of water occurs through passive
absorption.
• Rapid absorption takes place.
• Transpiration is responsible for passive absorption.

4. Fig: different pathways of water absorption

5. Mechanisms of ascent of sap
There are several theories on ascent of sap,which are as follows:
1. Vital Theories:
i) Relay pump theory:
•Given by Godlewski
•Through xylem parenchyma
ii) Pulsation theory:
•Given by J.C.Bose.
•Through innermost cortical cells.

6. 2.Root pressure theory:
• Given by Priestly
• A sort of hydrostatic pressure develops in the root due to the
accumulation of absorbed water which is responsible for ascent of
sap
Objection
• Not observed in Gymnosperms
• Pressure not enough to reach to top Of plant
• Root pressure fluctuate in soil type, temperature, Moisture content

7. 3.Physical force theory
i) Atmospheric pressure theory/ capillary force theory
• Given by Boehm.
• Atmospheric pressure raise up the water to fill the vacuum created
by continuous evaporation of water from the plant surfaces.
• Water ascends in the xylem vessel through capillary action.
OBJECTION
Capillary action isnot applicable to very tall plants.

8. ii) Imbibition theory
• Proposed by Unger and supported by Sachs.
• Upward movement of water takes place by Imbibition through the
thick walls of the xylem cells and sclerenchyma Cells of the phloem.
Objection
Ascent of sap stops if the lumens of xylem vessels and tracheids are
blocked with wax

9. iii) Transpiration pull and cohesion tension
theory
• Most widely accepted theory
• Proposed by Dixon and jolly.
• It operates only in passive water absorption.
• Based on cohesive and adhesive properties of water.
• Transpiration is necessary for ascent of sap.

11. Conclusion