Ascent of sap and transpiration

Ascent of sap- Transpiration pull theory, Transpiration-Types of transpiration, mechanism of transpiration-Starch sugar interconversion theory and Potassium pump theory

1. Ascent of s ap and transpiration Dhole N. A. Department of Botany, Digambarrao Bindu ACS College, Bhokar

2. Ascent of SAP : The water is absorbed by the roots and whish is distributed to all parts of the plant body through the conducting strands. This movement of water, from base of the stem to apex of the plant. This upward movement of water from base to apex of the plant is called as Ascent of Sap . Some of the plant grow up to 100 to 200 feet or more. Path of ascent of sap: The path of movement of water in plant is xylem. Mechanism of ascent of sap: Various theories have been proposed by many persons to explain the mechanism of ascent of sap Vital theory Root pressure theory Physical force theory Transpiration pull theory

3. Transpiration pull theory: Transpiration Pull Theory was first proposed by Dixon and Joly (1894)  . This theory is based on following points : 1. Cohesive and Adhesive properties of water mol­ecules to form a continuous water column in the xylem. 2. Transpiration pull exerted on this water col­umn.

4. The walls of mesophyll cells in leaf lose water vapour through stomata and cuticular transpiration. Sufficient quantity of water is transpired through intercellular spaces of the mesophyll cells. This is because the dry air outside the leaf has lower water potential than the moist air of leaf. As a result, water diffuses out of stomata and diffusion pressure deficit (DPD) increases . As a result, water is sucked from adjoining mesophyll cells and ultimately from vascular tissue. This tension is transmitted to the roots . The molecules of water show cohesion and molecules of water & vessel wall show adhesion . Because of cohesive and adhesive forces, water column does not break and water is pulled upward by the force called as transpiration pull.

5. Transpiration: The loss of water in the form of water vapors from the aerial parts of the plant is called as transpirations. The large amount of water absorbed by roots, only 2-3% water utilized for metabolism and remaining 97-98% water is evaporated through transpiration. Types of Transpiration: Stomatal transpirations Cuticular transpirations Lenticular transpirations

6. Stomatal transpiration Cuticular transpiration Lenticular transpiration Types of transpiration • On the basis of the passages through which plants give out water in the form of vapor transpiration is of three types:

7. 1. Stomatal transpiration: • Transpiration that occurs through stomata called stomatal transpiration. This type of transpiration only occurs in its presence of sunlight (in daytime). Because stomata open in the sunlight and close in the darkness. In this method plants give out 80-90% water in the form of vapor.

8. 2. Cuticular transpiration: Transpiration that occurs through the cuticle or cracks of thin cuticle layer of leaves and stems is said to be cuticular transpiration. This is a day-night process. In this process, 5-10% water is given out in the form of vapor.

9. 3. Lenticular transpiration: Sometimes transpiration occurs through lenticels , the small opening in the corky tissue of covering stems , and this type of transpiration is said to be the lenticular transpiration. In this process, only 0.1% water is evaporated or water lost.

10. Aerial parts of whole plant Cutin ( cuticular transpiration) 3%~10% Stomata ( stomatal transpiration) ~ 90% Lenticels (lenticular transpiration) 0.1%

11. Structure of stomata :

12. Mechanism of opening & closing of stomata:

13. Different theories for opening and closing of stomata: Starch-Sugar inter conversion theory Potassium pump theory 1. Starch-Sugar inter conversion theory: Scarth (1932) As per this theory, inter conversion of starch and sugar is because of change in pH of guard cells.

14. Starch-Sugar inter conversion theory:

15. 2. Potassium pump theory: Levitt (1974) The entry of potassium ion has role in the opening and closing of stomata.

16. Significance of transpiration: It creates suction force and help in the ascent of sap. It affect the diffusion pressure deficit , thereby indirectly helping diffusion process through the cells. It affects the absorption of water & minerals by roots. It helps in evaporating excess amount of water. It helps translocation of food from one portion of the plant to the other. It maintains a suitable temperature for the leaves. It brings about the opening and closing of stomata which indirectly influence the process of photosynthesis and respiration.

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