Transpiration is the loss of water vapour from plants (predominantly from the leaves as a consequence of gas exchange)

• Light energy converts the water within the leaves into vapour, which then evaporates and diffusesfrom the leaf via the stomata
  

• New water is absorbed from the soil by the roots, creating a difference in pressure between the leaves (low) and roots (high)
  

• Water will flow – via the xylem – along the pressure gradient to replace the water lost from the leaves (transpiration stream)
  

The rate of transpiration by a terrestrial plant can be affected by a number of abiotic factors, including:

• Sunlight – Light stimulates the opening of stomata, as gas exchange is required for photosynthesis to occur

• Temperature – High temperatures increase the vaporisation of water, leading to greater diffusion from the leaf

• Wind – The flow of air functions to remove the water vapour surrounding the leaf, increasing diffusion rates  

• Humidity – A higher concentration of water vapour in the air will decrease the rate of diffusion from the leaf

Factors Affecting Transpiration

Transpiration

Transpiration begins with the conversion of water into vapour within the spongy mesophyll of the leaf

• Some of the light energy absorbed by leaves is converted into heat, which evaporates the water within the leaf tissue
  

• This vapour diffuses out of the leaf via stomata, creating a negative pressure gradient within the leaf
  

• This negative pressure causes water to be drawn out of xylem vessels and through cell walls by capillary action, generating tension
  

• This tension draws water up the xylem vessels in opposition to gravity – moving in a continuous stream via mass flow
  

The flow of water through the xylem vessels relies on two key properties of water – cohesion and adhesion

• Water molecules are polar and can form hydrogen bonds that function to drag water molecules up the xylem via cohesion
  

• The xylem wall is also polar and forms intermolecular associations with water molecules to generate further tension via adhesion
  

The water vapour lost from the leaf is replaced by water taken up from the soil by the roots (maintaining a pressure gradient)

• Plants may actively pump protons into the soil to displace mineral ions that then diffuse into the roots (indirect active transport)
  

• Water will then be absorbed into the roots via osmosis – moving towards the region with the higher solute concentration

• The rate of water uptake will be regulated by specialised water channels (aquaporins) on the root cell membrane
  

• The epidermis of roots may have cellular extensions called root hairs, which further increases the surface area for absorption
  

Transpiration Stream