transpiration

• what 5 environmental factors impact the rate of transpiration?

  Light intensity

  temperature

  relative humidity

  air movement (wind)

  water availability
• what is transpiration?

  Transpiration is the loss of water vapour from the upper parts of the plant- particularly by the leaves. Some water may evaporate through the upper leaf surface, but this is limited by the waxy cuticle. Most water vapour escapes through the stomata, the stomata open to allow gaseous exchange to occur for photosynthesis. the majority of water vapour will be lost during the day time because there is sufficient light.
• describe the typical pathway taken by water in most plants

  1. water enters the leaf through the xylem, and moves via osmosis into the cells of the spongy mesophyll. it may also pass along the cell walls via the apoplast pathway.
  2. water evaporates from the cells of the spongy mesophyll.
  3. water vapour moves via diffusion out of the leaves through open stomata. this relies on a difference in the concentration of the water vapour molecules in the leaf compared with the outside- the water potential inside the plant must be higher than the outside, this is known as the water vapor potential gradient.
• transpiration may be an inevitible consequence of gaseous exchange, but it is also essential for the plant to survive. as water vapour is lost from the leaf, it must be replaced by the roots. this draws water up the stem as a transportation stream- consequently this movement:

  • transports useful mineral ions up the plant
  • maintains cell turgidity
  • supplies water for growth, cell elongation, and photosynthesis
  • supplies water that, if evaporated, can keep the plant cool.