7.2.2 Transpiration in Plants

• The movement of water through a plant's xylem occurs due to the evaporation of water vapour from the leaves and the cohesive and adhesive properties exhibited by water molecules
• The water potential gradient is the driving force behind the movement of water from the soil (high water potential) to the atmosphere (low water potential), via the plant's cells
  • Plants are constantly taking water in at their roots and losing water via the stomata (in the leaves), maintaining the water potential gradient between the roots and the leaves
• Around 99 % of the water absorbed is lost through evaporation from the plant's stem and leaves in a process called transpiration
• Transpiration refers to the loss of water vapour via the stomata by diffusion
  
  • Note that this is different to the transpiration stream which is the movement of water from the roots to the leaves
• Transpiration is important to the plant in the following ways
  • It provides a means of cooling the plant via evaporative cooling
  • The transpiration stream is helpful in the uptake of mineral ions
  • The turgor pressure of the cells (due to the presence of water as it moves up the plant) provides support to leaves (enabling an increased surface area of the leaf blade) and the stem of non-woody plants

The loss of water vapour from the leaves of plants (transpiration) results in a lower water potential in the leaves. This creates a concentration gradient between the roots and leaves and causes water to move upwards

Movement of water through leaves

• Certain environmental conditions (e.g. low humidity, high temperatures) can cause a water potential gradient between the air inside the leaves (higher water potential) and the air outside (lower water potential) which results in water vapour diffusing out of the leaves through the stomata (transpiration)
• The water vapour lost by transpiration lowers the water potential in the air spaces surrounding the mesophyll cells
• The water within the mesophyll cell walls evaporates into these air spaces resulting in a transpiration pull
• This transpiration pull results in water moving through the mesophyll cell walls or out of the mesophyll cytoplasm
  • Movement of water through the cell walls of a plant is said to occur via the apoplast, or apoplastic, pathway
  • Movement of water from the cytoplasm of a cell occurs via the symplast, or symplastic, pathway
• The pull from the water moving through the mesophyll cells results in water leaving the xylem vessels through pits (non-lignified areas), which then causes water to move up the xylem vessels to replace this lost water (due to the cohesive and adhesive properties of the water). This movement is called the transpiration stream
  • When rates of transpiration are high the walls of the xylem are pulled inwards by the faster flow of water

The role of the stomata

• Transpiration is mainly controlled by the pairs of guard cells that surround stomata (singular stoma)
• Guard cells open the stomata when they are turgid and close the stomata when they lose water
• When the stomata are open there is a greater rate of transpiration and of gaseous exchange
• When the stomata close, transpiration and gaseous exchange decrease
• As stomata allow gaseous exchange (CO₂ in and O₂ out) they are generally open during the day

Transpiration Diagram

Water enters the leaf as a liquid from the xylem and diffuses out as water vapour through the stomata. This loss of water by evaporation and transpiration results in a water potential gradient between the leaves (low) and roots (high) causing water to move up the plant in the transpiration stream