Factors affecting the rate of transpiration

• what is the transpiration rate?

  the rate at which water lost from plants
• what are the general factors that affect the rate of transpiration?

  • genetic factors such as those controlling the number, distribution and size of the stomata
  • environmental factors such as temperature, humidity and air movement - these 3 factors affect the water potential gradient between the water vapour in the leaf and the atmosphere, so they affect the rate of transpiration - light intensity also affecst transpiration
• what is the harmful aspect of transpiration?

  water loss
• what are the useful aspects of transpiration?

  • water uptake
  • water distribution
  • ion distribution
  • evaporative cooling
• how does water affect the rate of transpiration?

  • a temperature increase lowers the water potential of the atmosphere
  • it increases the kinetic energy of water molecules, accelerating their rate of evaporation from the walls of the mesophyll cells and, if stomata are open, speeds up their rate of diffusion out into the atmosphere
  • the higher temperature causes the water molecules to diffuse away from the leaf more quickly, reducing the water potential around the leaf
• how does water move down a water potential gradient?

  • by osmosis when it crosses a membrane
  • by evaporation when it transpires out of a leaf
• how does humidity affect the rate of transpiration?

  • theres a water potential gradient between leaf and atmosphere and when stomata are open, water vapour diffuses out the leaf, down water potential gradient
  • transpiration in still air causes accumulation of a layer of saturated air at surfaces of leaves
  • water vapour slowly diffuses away, leaving concentric rings of decreasing humidity the further away from the leaf you go-sometimes called ‘diffusion shells’
  • water vapour diffuses down this gradient of relative humidity(also a gradient of water potential)away from the leaf
• what does increased humidity mean for the rate of transpiration in terms of water potential?

  higher the humidity, the higher the water potential
• what is the relative humidity of the air inside the leaf and why?

  its 100% as the air inside a leaf is saturated with water vapour
• how does air speed affect the rate of transpiration?

  • movement of the surrounding air blows away the layer of humid air at the leaf surface
  • the water potential gradient between the inside and the outside of the leaf consequently increases and water vapour diffuses out through the stomata more quickly
  • the faster the air is moving, the faster the concentric shells of water vapour get blown away, the faster transpiration occurs
• how does light intensity affect the rate of transpiration?

  • in most plants, the stomata open wider as the light intensity increases, increasing the rate of transpiration
  • so stomata tend to open widest in the middle of the day, less widely in the morning and evening and to be closed at night
• how do all the environmental factors come together to affect the rate of transpiration?

  more water is lost on a dry, windy day than on a humid, still day - this is because the walls of the spongey mesophyll cells are saturated with water which evaporates and moves down a gradient of water potential from leaf to the atmosphere, which has a low humidity, the wind having reduced the thickness of the layer of saturated air at the leaf surface
• what is this a graph of?

  the effect of temperature on transpiration rate

  
• what is this a graph of?

  the effect of humidity on transpiration rate

  
• what is this diagram showing?

  water vapour diffuses down a relative humidity and water potential gradient out of the leaf

  
• what does this diagram show?

  the shells of water vapour have been blown away, making the gradients of relative humidity and water potential steeper so the water vapour diffuses out faster

  
• what does this graph show?

  the effect of air speed on transpiration rate