As the water moves upward from roots to the leaves, a lot of it evaporates through the stomata present on the lower surface of a leaf. This process of evaporation of water from leaves and other aerial parts of the plant is called transpiration.
The xylem sap (water containing minerals) rises against the gravity without the help of any mechanical pump. The xylem sap is largely pulled upward by transpiration cohesion-tension mechanism, also called transpiration pull.
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i. Root Pressure (Pushing Xylem Sap):
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Root pressure is a pressure created due to continuous influx of water in the xylem vessels from the root hairs and root cortex. Root pressure causes guttation, the exudation of water droplets that can be seen on leaf surface.
In most plants, root pressure is not the major mechanism during the ascent of sap. At the most root pressure pushes the sap in the xylem vessels up to a certain height. Later on the sap moves with cohesion, adhesion and transpiration pull.
ii. Capillarity Nature of Xylem Vessels:
Xylem vessels are very narrow. This causes the water from a lower level to rise by a capillary action in order to fill up the vacuum created at the leaves due to loss of water by transpiration.
iii. Pulling xylem sap: Transpiration Cohesion- tension mechanism or transpiration pull:
Stomata are the sites of exchange of C02 and 02 between photosynthetic tissues and atmosphere. They are also sites of transpiration. The air in these stomata is saturated with water vapour since it is in contact with the most walls of the mesophyll cells.
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On most of the days, the air is drier outside the leaf, i.e. it has a lower water concentration outside than inside the leaf. Thus, the gaseous water diffuses outside the leaf through the stomata and there is a loss of water during transpiration.
This leads to a generation of tension (negative pressure) in the leaf due to the unique physical property of water. The thin film of water vapour present in the mesophyll cells replaces the water vapour which is lost from the leaf stomata by transpiration. During this process water is pulled on by the adhesive and cohesive forces between the molecules of two different substances.
This tension is the pulling force or suction force which draws water from the leaf xylem through the mesophyll cells toward stomata. The water lost via transpiration is replaced by the water that is pulled out of the leaf xylem.
Iv. Cohesion and Adhesion of Water:
The transpiration pull on water extends from the leaves up to the root tip and even into the soil solution. This cohesion of water is due to hydrogen bonding between water molecules. Each water molecule is attached (adhered) to adjacent water molecule and this pull is relayed from molecule to molecule down the entire column of water in the xylem.