Theories of Water Translocation
Transport of water and mineral ions from the root to the leaves is by three ways:
- Root pressure
- Cohesion tension
The gradient of water concentration that exists across the cortex creates a pushing force called "root pressure", i.e. a pressure that "pushes" the water across. Root pressure can be demonstrated by cutting a stem at soil level. After sometime, droplets of water can be seen exuding from the cut surface. In some plants, the process occurs naturally as "guttation", when droplets of water are forced through special pores (hydathodes) on the leaf edges. It is likely that the process of root pressure requires energy, as metabolic poisons halt the process.
While root pressure might account for some of the upward movement of water in small herbaceous plants, it is insufficient to overcome the force of gravity and push water through tall plants. To determine what else is involved in ensuring water gets to the sites of photosynthesis, let us look at the plant's water transporting tissue, the xylem.
Root pressure is caused by mineral ions, which are actively transported into xylem vessels in the root by endodermal cells. This makes the water potential of the xylem more negative and causes water to enter the xylem by osmosis.
Water evaporating from the leaves drives the movement of water from the roots. The evaporation creates a WP (Wall Pressure) gradient and the properties of water and the xylem vessels allow this pressure to be transmitted all the way to the roots even in the tallest trees. This is known as the cohesion tension theory of water transport. Conditions always allow evaporation of water from the leaves. When water evaporates from a leaf mesophyll cell, this cell's WP will become more negative. Water from an adjacent cell will then move into this cell by osmosis, as a result of the WP difference between them. This 'chain' of WP differences continues back to the xylem sap. Water moves smoothly and continuously along this WP gradient. The continual movement of water from the roots to the leaves is often called the transpiration stream. This can be thought of as the movement of water from a less negative to a more negative water potential. Water moves from less negative in soil to the more negative in air surrounding the leaves. This provides a very steep gradient from soil to air and is one of the driving forces of the transpiration stream.
- Xylem vessels are full of water.
- Tension is set up in water column as water leaves xylem in the leaves.
- Tension is transmitted back to the root due to cohesion of water molecules. Water has high cohesion because it is a polar molecule and H-bonding occurs between water molecules.
- The column of water under tension does not break because of adhesion. The water molecules tend to 'stick' to xylem walls. This supports the column.
Capillarity refers to a rise in water in tubes of small diameter kept in a vessel containing water. This rise in water is due to the forces of adhesion and cohesion. Adhesive forces attract molecules of different kinds, whereas cohesion forces attract molecules of the same kind to each other. Force of gravity also affects water uptake by capillarity.
According to this theory, water is first taken in due to the force of adhesion between water and the walls of thin xylem vessels. As the water flows upwards along the wall, strong cohesive forces between water molecules come into play to pull the water upward. This upward pull of water continues until the forces of adhesion and cohesion are balanced by the downward force of gravity.
The Reasons Why Plants Need Water
The movement of water into the roots of a plant from the soil takes place chiefly through myriads of tiny, fine root hairs extending between the particles of soil.
Water enters through the roots and is conducted upwards through the plant, to the cells. The water-conducting tissue, called the xylem, is found in the central parts of the root, vascular bundles near the outside of the stem and in the veins of the leaves, especially those near the upper surface.