Discuss the factors responsible for ascent of xylem sap in plants
Xylem sap ascends mainly due to forces generating in the foliage of plants as a result of active transpiration. Thus, the factors which enhance the rate of transpiration are also the factors responsible for ascent of xylem sap in plants.
Various factors responsible for ascent of xylem sap in plants are as follows:
(i) Capillarity: There is limited rise of water in narrow tubes or capillaries due to forces of cohesion amongst molecules of water and their property of adhesion to other substance.
(ii)Root pressure: It is positive pressure that pushes sap from below due to active absorption by root
(iii)Transpiration pull: Transpiration in aerial parts brings the xylem sap under negative pressure or tension due to continuous withdrawal of water by them. Water column does not break due to its high tensile strength related to high force of cohesion and adhesion.
Transpirational pull is responsible for the ascent of water in the xylem. This ascent of water is dependent on the following physical factors:
Cohesion – Mutual attraction between water molecules
Surface tension – Responsible for the greater attraction between water molecules in liquid phase than in gaseous phase
Adhesion – Attraction of water molecules to polar surfaces
Capillarity –Ability of water to rise in thin tubes
These physical properties of water allow it to move against gravity in the xylem.