4.2 - Transport of Water and Mineral Salts

Plants need water to transport mineral salt from the roots to other parts of the plants. Water also gives turgidity to the plants so that they remain fresh. Can you imagine how the water moves inside a very tall tree from the roots to the ends of the leaves? In order to move such a distance, the movement of water and minerals is actually supported by transpirational pull, capillary action and root pressure.

Transpirational pull happens when water is evaporated through the stoma. This attracts water out of the leaves.

Capillary action is formed from the adhesion force and cohesion force of water molecules. This allows water movement along the stems against gravitational pull.

Root pressure also helps to move water from the soil into xylem vessels via osmosis.

Figure 4 - The movement of water from the soil to the xylem vessels.

1. In root hair cells , the water potential is lower compared to the surrounding soil. This is because the concentration of mineral ions is higher as they are pumped into the vacuole through active transport.
2. Water from the soil diffuses into the root hair cells and epidermal cells via osmosis.
3. The water potential becomes higher in the root hair cells and this promotes the movement of water into the cortex via osmosis.
4. Consequently, water osmosis happens continuously across the cortex, endodermis and pericycle layers.
5. This creates root pressure that pushes the water into the xylem vessels root and then the xylem vessels of the stem.

Figure 5 - Water movement into the xylem vessels of stem and leaves.

1. When water reaches the xylem vessels of the stem, pressure from the root is not enough to move the water up to the leaves. Therefore, capillary action of the xylem formed from the adhesion and cohesion forces as well as transpirational pull are needed to facilitate water movement.
2. Adhesion and cohesion forces formed between the water molecules promotes the movement of water continuously through the xylem vessels of the stem.
3. During transpiration, water is removed as water vapor to the outer environment through the opened stoma.
4. Spongy mesophyll cells lose water and causes the water potential to be lower than the surrounding cells. As a result, water from the neighboring cells diffuses into the spongy mesophyll cells via osmosis. This creates transpirational pull that pulls the water out of the leaves.

Figure 6 - Apoplast pathway and simplest pathway are the 2 pathways used by water during movement from the root cells to the xylem. Casparian strips found at the cell wall of endodermis are not permeable to water and only allow water movement through the simplest pathway.

Guttation in Plants

Guttation is the secretion of water droplets which are formed at the end of the leaf veins as a result of high root pressure. This process does not involve the stomata. This phenomenon is often seen when the root pressure of plants become high and the rate of transpiration is low, normally when the humidity of the environment is high.

Figure 7 - Guttation on the surface of a leaf.

Comparison between Guttation and Transpiration

• Similarities

  • Both processes happen in the leaves of the plants
  • Both involve the loss of water permanently

• Differences

  • Guttation

    • Plants involved: Herbaceous plants only
    • Time: Happens during the night and early in the morning
    • Form of water: Water droplets
    • Structures involved: Special structure
    • Factor: High root pressure
    • Water composition: Rich in minerals

  • Transpiration

    • Plants involved: All plants
    • Time: Happens during the day when it is hot and windy
    • Form of water: Water vapor
    • Structures involved: Stomata
    • Factor: Opening and closing od stomata
    • Water composition: Pure water only

The Condition of Plants that Do Not Undergo Transpiration and Guttation

Effects to plants that do not undergo guttation

• Effective root pressure cannot be maintained. The process of water absorption will be affected when the humidity of the environment is high.
• Waste products of the plants cannot be eliminated.
• Pressure at the leaf veins becomes high and causes them to burst. The leaves are more prone to pathogenic infection and eventually fall off.

Effects to plants that do not undergo transpiration

• Optimum plant temperature cannot be maintained. The increased in the temperature of the plants will destroy the enzymes and affect the biochemical processes which include photosynthesis and respiration.
• Mineral ions such as potassium ions cannot be transported to the leaves for photosynthesis.
• The whole process of water transportation is affected and the plants will wilt.
• The plants can die if this persists for a long time without transpiration.