4.2 - Transport of Water and Mineral Salts

Water Potential

• Water moves from a region of high water potential to a region of low water potential by osmosis.
• Mineral ions are actively pumped into the vacuoles of root hair cells, making their cell sap water potential lower than the water potential in the soil.
• Water from the soil enters root hair cells and epidermal cells by osmosis.
• Water continues to move by osmosis across the cortex, endodermis and pericycle into the root xylem vessels.

Pathways of Water and Mineral Salts from Soil to Xylem

• Root hairs absorb water into the root system.
• Water moves from root cells to the xylem through the apoplast and symplast pathways.

Apoplast Pathway

• Water moves through spaces between cellulose fibres in the cell wall.
• Casparian strips in the endodermal cell walls are impermeable to water, so water cannot continue through the apoplast pathway at the endodermis.
• Water crosses the endodermis through the symplast pathway.

Symplast Pathway

• Water moves through the cytoplasm and plasmodesmata.

Mechanisms of Water Transport from Roots to Leaves

• Water and mineral salt movement from the soil to the leaves is aided by root pressure, capillary action and transpirational pull.

Root Pressure

• Moves water from the soil into the root xylem vessels by osmosis.
• Continuous osmosis across the cortex, endodermis and pericycle creates root pressure.
• Root pressure pushes water into the root xylem vessels and then into the stem xylem vessels.
• Root pressure alone cannot transport water to the leaves and shoots of tall plants.

Capillary Action

• Moves water upwards in the stem against gravity.
• Cohesion force acts between water molecules.
• Adhesion force acts between water molecules and the xylem wall.
• Cohesion and adhesion produce a continuous pulling force that moves water through the xylem vessel.

Transpirational Pull

• During transpiration, water diffuses as water vapour from the intercellular spaces to the surroundings through open stomata.
• Spongy mesophyll cells lose water and have lower water potential than adjacent cells.
• Water moves from adjacent cells to spongy mesophyll cells by osmosis.
• This produces transpirational pull, which pulls water molecules from the leaf xylem vessel towards the outside of the leaf.

Guttation

• Guttation is the secretion of water droplets through a special structure at the end of leaf veins without involving the stomata, caused by high root pressure.
• It occurs when root pressure is high and the transpiration rate is low, usually at night and early morning when air humidity is high and the surrounding temperature is low.
• Root pressure pushes water to the leaves and stems.

Differences Between Guttation and Transpiration

Guttation	Transpiration
Occurs at night and early morning	Occurs on hot and windy days
Occurs only in herbaceous plants	Occurs in all plants
Releases water droplets	Releases water vapour
Releases water through a special structure at the end of leaf veins	Releases water through stomata
Occurs when root pressure is high	Controlled by stomatal opening and closing
Releases water rich in minerals	Releases pure water

• Both occur through the leaf and cause permanent water loss from the plant.

Plants That Do Not Undergo Guttation or Transpiration

• Without guttation:
  • effective root pressure cannot be maintained, disrupting water absorption by root hair cells when relative humidity is high;
  • plant waste substances cannot be eliminated;
  • pressure in leaf veins rises and may burst the veins, exposing leaves to pathogen infection and causing them to fall.
• Without transpiration:
  • the optimum plant temperature cannot be maintained; increased temperature may denature enzymes and disrupt photosynthesis and respiration;
  • mineral ions such as potassium ions cannot be transported from the roots to the leaves for photosynthesis;
  • water transport is disrupted, causing the plant to wilt;
  • the plant may eventually die.