Transport of Water and Mineral Salts

4.2

Transport of Water and Mineral Salts

Importance of Water

Water is very important to the growth of plants because water helps to move mineral salts from the soil to the stems and leaves.
Other than that, water also helps in giving turgidity to plant cells so that the plant remains fresh.

Transpirational Pull, Capillary Action and Root Pressure

Transpirational Pull

In leaf,water always evaporates from spongy mesophyll cells to leaves and finally to atmosphere via stomata.
This process is known as transpiration.
This creates a force which transport water and mineral salts from root xylem up to leaf continuously due to cohesive and adhesive forces of water.
Transpirational pull is the main force in transportation of water and mineral salts.

Capillary Action

Created by adhesion force and cohesion force of water in fine xylem vessels.

Root Pressure

Force created in root due to the presence of endodermis.
Endodermal cells transport mineral salts actively from cortex to vascular tissues of root.
This produces a concentration gradient between cortex and vascular tissue.
Water diffuses from cortex to vascular tissue across the endodermis.
Root pressure is created by pushing water up to stem.

Water Movement from the Soil to the Xylem Vessel

The water potential in the root hair cells is lower compared to water in the soil. This is because the mineral ions are actively pumped by the root hair cells into the vacuole, causing the cell sap of the root hair to have low water potential compared to the soil.
Water from the soil diffuses into the root hair cells and epidermal cells via osmosis.
The high water potential in the root hair cells causes the water to diffuse from the root hair cells into the cortex via osmosis.
This condition causes osmosis to continuously occur throughout the cortex, endodermis and pericycle layers.
This causes root pressure to push water into the xylem vessels of the root and then into the xylem vessels of the stem.

Water Movement in Xylem Vessel of Stem and Leaf

Water Movement in the Xylem Vessels of Stem

For tall plants, root pressure cannot transport water to the leaves, especially the shoots.
Therefore, the movement of water molecule in the xylem vessel is also helped by the capillary action of xylem produced by adhesion and cohesion forces and also transpirational pull.
Adhesion and cohesion forces produce a pulling force which continuously moves water in the xylem vessel.

Water Movement in the Xylem Vessel of the Leaves

When transpiration process happens, water diffuse out as water vapour from the spaces between the cells to the surroundings through opened stoma.
Spongy mesophyll cells lose water and they have low water potential towards adjacent cells.
Water molecules diffuse from neighbouring cells via spongy mesophyll cells by osmosis.
This movement produces a force called transpirational pull that pulls water molecules in the xylem vessel of the leaves to the outside of the leaves.

Guttation

Guttation is the process of removing water slowly in the form of droplets through hydatod at the margin of the leaf.
Guttation occurs at night where the rate of transpiration is low and the relative air humidity is high.
Root hair is the main force in guttation.
This phenomenon can be observed at the leaf margin of herbaecious plant.
If the plants do not undergo transpiration and guttation:
- Water and mineral salts cannot be trasnported from root to shoot.
- The rate of photosynthesis is low due to deficiency of water.
- Growth of plant is retarded because lack of mineral salts.
- Cooling effect of plant is low.

The Comparison between Guttation and Transpiration
Guttation	Transpiration
Similarities

Differences
Happens at night and early morning.	Happens on hot and windy days.
Only happens in herbaceous plants.	Happens in all plants.
Water is released in the form of water droplets.	Released as water vapour.
Water is released through a special structure at the end of the leaf veins.	Released through stomata.
Happens when root pressure is high.	Controlled by the stomatal opening and closing
Releases water that is rich in minerals.	Releases pure water.

Effects towards Plants that do not Undergo Guttation

Without guttation, effective root pressure cannot be maintained. Therefore, water absorption by root hair cells is disrupted in a surrounding with high relative humidity.
Without guttation, plant waste substances cannot be eliminated.
If guttation does not occur, the leaf vein pressure becomes high and causes the leaf vein to burst. This leads to the leaves being exposed to pathogen and eventually fall.

Effects towards Plants that do not Undergo Transpiration

Without transpiration, optimum temperature of plants cannot be maintained. Increase in temperature can denature enzymes and disrupt biochemical processes such as photosynthesis and respiration.
Without transpiration, mineral ions such as potassium ions cannot be transported from the roots to the leaves for photosynthesis.
Without transpiration, water transport throughout the plants will be disrupted and causing the plants to wilt.
Plants can die in the long run.