Transpiration

• what is transpiration?

  loss of water vapour from plant leaves as a result of evaporation of water from cell surfaces inside the leaf, the water vapour them diffuses into the atmosphere through the stomata
• what 3 processes does transpiration involve?

  • osmosis of water from xylem to mesophyll cells
  • evaporation of water from the surface of mesophyll cells into intercellular spaces
  • diffusion of water vapour from the intercellular spaces out through the stomata
• what is the transpiration stream?

  • the uninterrupted stream of water, which is taken up by the roots and is transported via xylem vessels to the leaves, where it will eventually evaporate into the air
  • this means that water leaving the xylem into the leaves must be replaced from below
• why is the transpiration stream advantageous for the plant?

  • water is required in leaves for photosynthesis
  • water is required for cells to grow and elongate
  • water keeps cells turgid
  • the flow of water can carry useful mineral ions up the plant
  • evaporation of water can keep the plant cool
• what is meant by 'cohesion-tension theory'?

  • the best current model explaining the movement of water through a plant during transpiration
• what is the change in diameter of trees during day and night?

  • when transpiration is at its height during the day, the tension in the xylem vessels is at its height too
  • tree diameter shrinks
  • at night, transpiration is at its lowest, tension in xylem vessels is at its lowest and diameter of tree increases
• how does cutting xylem support the cohesion-tension theory?

  • air is drawn into the xylem rather than water leaving out of cut end
  • plant can no longer mover water up the stem as the continuous stream of water molecules held together by cohesive forces had been broken
• how do guard cells control the loss of water vapour when closing?

  • without light (e.g. night) there is no photosynthesis, so no sugars made for respiration, so no ATP from respiration for active transport of K+ ions - so then diffuse from high concentration to a low concentration, water then follows due to water potential difference, closing the stomata
• how does the stomata open?

  • chloroplasts absorb light, which is used to make glucose through photosynthesis, sugars then used to make ATP through respiration to move K+ ions into the guard cells via active transport
  • this means water will follow due to the difference in water potential - resulting in guard cells swelling and opening
• what does more light mean?

  • more open stomata - they can have different degrees of openness due to different amounts/ intensities of light
• what does a potometer measure?

  • rate of water uptake
• how do you set up a potometer?

  • lay capillary tube and rubber connector under water and fill
  • select a leaf shoot and keep end underwater whilst you select and trim if necessary with a knife
  • quickly insert shoot into rubber connector making sure its tight - underwater!
  • place bottom end of capillary tube into beaker of water
  • smear Vaseline around the join between shoot and rubber connector to ensure airtight seal
  • leave for 5 mins to allow water to be drawn up end of capillary tube and form a small air bubble
  • time the movement of air bubble along the capillary tube for a set distance
• what is the importance of cutting the stem underwater?

  to ensure no air gets into xylem vessels which would block water from moving up (cavitation)
• why is it important not to get water on the leaves?

  • water may block stomata which will affect the rate of water uptake (transpiration)
  • affects water potential gradient (water outside so water won't move up and out)
• how do you set up a mass potometer?

  • cut end of leafy shoot under water to prevent cavitation
  • fill conical flask with water to 250cm3 mark and mark level clearly with a Chinagraph pencil
  • place cut end in flask of water and seal with a cotton wool plug
  • insert rubber bung into the flask and seal the join with Vaseline
  • put conical flask with the shoot onto a chemical balance and record mass
  • leave apparatus for 1 hour, recording mass every 10 mins
  • at end, check water level in flask and calculate difference in water level from starting level
• why is a mass potometer more accurate than a capillary tube potometer?

  has a higher resolution
• how does number of leaves affect rate of transpiration?

  • a plant with more leaves has a larger surface area over which water vapour can be lost - increases rate
• how does number, size and position of stomata affect rate of transpiration?

  • if leaves have many larger stomata, then water vapour is last more quickly - increases rate
  • if stomata are on the lower surface, water vapour loss is slower as they are not as exposed to harsh weather
• how does the presence of waxy cuticle affect rate of transpiration?

  • reduces evaporation from leaf surface - decreases rate
• how does light intensity affect rate of transpiration?

  • in light, the stomata are open to allow gaseous exchange for photosynthesis
  • increase light intensity gives increasing numbers of open stomata, increasing rate of water vapour diffusing out therefore increasing evaporation from surface of leaves - increase rate
• how does temperature affect rate of transpiration?

  • increases water loss:
  • increase rate of evaporation from cell surface so that the water vapour potential in leaf rises
  • increases rate of diffusion through stomata because water molecules have more kinetic energy
  • decrease relative water vapour potential in air, allowing more rapid diffusion of molecules out of leaf
• how does the relative humidity affect the rate of transpiration?

  • higher relative humidity in air will decrease rate of water loss
  • this is because there will be a smaller water vapour potential gradient between the air spaces in the leaf and the air outside
• how does air movement or wind affect rate of transpiration?

  • air moving out of the leaf will carry away water vapour that has just diffused out of the leaf
  • this will maintain a high water vapour potential gradient
• how does water availability affect rate of transpiration?

  • if there is little water in soil, then the plant can't replace the lost water
  • water loss in plants is reduced when stomata are closed or when the plants shed leaves in winter
• why is water loss from transpiration unavoidable?

  stomata will be open to allow for gas exchange for photosynthesis so in this process water vapour will move up and out of stomata
• what features do all plants employ to reduce water loss due to transpiration?

  • guard cells - open and close stomata to control
  • waxy cuticle prevents evaporation of water
  • closed stomata at night
  • presence of stomata on lower leaf surface reduces loss of water vapour
  • stress response