Ascent of sap

This PPT provides detail explanation about Ascent of sap, Transpiration pull, Cohesion and adhesion theories and experiments

1. ASCENT OF SAP Dr. K. GANESH KUMARI ASSISTANT PROFESSOR DEPARTMENT OF PLANT SCIENCE SRIMAD ANDAVAN ARTS AND SCIENCE COLLEGE (AUTONOMOUS) TRICHY - 5

2. Introduction The upward movement of sap from the root system to the aerial parts of the plant through the xylem is known as ascent of sap It distributes water and minerals to stem and leaves. This is also known as water conduction or translocation of water Sap is an aqueous solution of water, dissolved mineral salts and some organic solutes present in the xylem Hence, it is often known as xylem sap.

3. Rate of movement of Sap The rate of movement of sap varies with diameter of the conducting vessels. Wide vessels – radii of 100 to 200 µM - water rises up to the rate of 16 to 45 mh -1 Small vessels – radii of 25 to 75 µM - Water rises up to the rate of 1 to 6 mh -1

4. Path of Ascent of sap In plants, ascent of sap takes place from roots to leaves only through xylem. Water enters the xylem vessels or tracheid of the root through the cortical cells Xylem terminates as small branches in the mesophyll tissue It forms a continuous column for the ascent of sap to the leaves

5. Means xylon in greek – wood Mostly dead cells Made up of no. of elements Xylem Vessels Elongated or drum shaped structures Conducts water and minerals vertically Have perforation plate Tracheids Elongated cells with tapering ends No perforation at the end walls but have pores on lateral walls Xylem Parenchyma It does not help in conducting Living cell Xylem fibres Support Store starch Xylem transport Fibre Tracheid     Vessel    Xylem parenchyma

6. Xylem is the principal water conducting tissue in plant. It can be shown by an experiment

7. Experiment 1 Balsam Plant experiment A leafy twig of balsam plant is cut under water to avoid air bubbles and the cut end is placed in a beaker with eosin water Coloured lines will be seen moving upwards in the stem as the stem is the transparent in nature If the sections are cut at this stem, only the xylem will be filled with coloured water.

8. Experiment 2 Ringing experiment A leafy twig of woody plant is cut under water to avoid the entry of air bubbles and its bark is cut out of a ring of 1inch length. Like wise another twig is cut from the same plant and its wood element is carefully removed from it for at least 1cm height with out damaging the bark at that site. The cut ends of these two twigs are kept dipped in water for 3 – 4 hours

9. Cont … While observing these twigs, the leaves of debarked twig remain fresh above the ringed portion but those of wood eliminated twig show the sign of wilting Therefore it is clear that in debarked twig water is continuously supplied to the leaves through the xylem, so its leaves remain fresh.

10. Transpiration pull and Cohesion theory According to this theory, the ascent of sap is due to transpiration pull from the top of plant and the cohesion and adhesion properties of water. It is a convincing theory for explaining the ascent of sap in tall trees. This theory is now popularly known by various names such as Dixons and Jolly’s theory, cohesion theory, cohesion-tension theory and transpiration pull theory. This theory is based on the cohesive and adhesive properties of water and transpiration pull.

11. Cohesion Attraction between water molecules are called cohesion. Water molecules remains joined to each other due to the presence of H-bonds between them. Although H-bond is very week, when they are present in enormous numbers, a very strong mutual force of attraction or cohesive force develops between water molecules. Hence, the water molecules form an continuous water column in the xylem

12. Cont … Cohesive force provides a tensile strength to the water column. The magnitude of cohesive force is very high (260 bars) and hence the water column cannot be broken by the force of gravity. Further, this force is enough for lifting water to a height of 120 m and for overcoming the resistance offered by barriers in the way of water movement.

13. Adhesion The adhesion property (attraction between the water molecules and the walls of xylem) further ensures the continuity of water column in the xylem. Xylem vessels form continuous tubes extending from roots to the top of the plants. One end of xylem tube is connected with the root hairs (via pericycle, endodermis and cortex) and the other end is connected with the substomatal cavity in the leaves via mesophyll cells. This tube is filled with sap in the form a continuous water column due to cohesion and adhesion

14. Transpiration pull The pulling force developed in the water of xylem due to transpiration is called transpiration pull. When transpiration takes place in leaves at the upper parts of the plant, water evaporates from the intercellular spaces of the leaves and goes to the outer atmosphere through the stomata. As consequence, more water is released into the intercellular spaces from the mesophyll cells

15. Cont … The mesophyll in turn draw water from the xylem of the leaf. Thus, a water tension is created in the xylem of leaves. This tension is transmitted downward to water in the xylem of root through xylem of stem. Because of this tension, water is pulled upward to the leaves. Since sap exists as a continuous column, it moves upward in the xylem of stem and leaves to reach the transpiring surfaces at the top of the plant.

16. Cont … Thus transpiration at the leaf surfaces pulls water from the roots to get it into the leaves. The water column is just like a steel rope which is extended from substomatal cavities in the leaves to the roots. If this rope is pulled from the top, the entire rope will move upward. In plants, the pull is generated by the process of transpiration and hence it is known as transpiration pull.

17. Demonstration of water lifting power of transpiration Pull Cut a fresh leafy twig under water and fix it in a long narrow glass tube which is filled with water. Place the lower end of the tube in a beaker containing mercury Make the joint between stem and glass tube air tight. After sometime, due to transpiration pull, the mercury will gradually rise in the glass tube.

18. Objection to Cohesion theory Due to the variation of temperature , the xylem vessels enlarges during the day time and shrinks in the night. While the vessel size is increasing, there are fair chances for the entry of gas bubbles in the water column. The air bubbles in the conducting channels will break the continuity of the water column. This phenomenon is known as Cavitation. It was demonstrated by Milburn and Johnson (1966)

19. Cavitation overcomes The impact of cavitation is overcome by the presence of many columns of vessels side by side; it eliminates the injurious effect of temporary cavitation. When the tension is relieved by rain or simply at night, the gases are dissolved in the solution forming a continuous column.

20. Thank you