Mechanisms of water absorption and ascent of sap.

water absorption and ascent of sap

1. Mechanisms of water absorption and ascent of sap

2. Mechanism of water absorption Water is absorbed by two methods: 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: 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 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.

10. Conclusion