Transpiration and Stomata

• Transpiration rate is affected by four main things
• Light intensity - the brighter the light, the greater the transpiration rate. Stomata begin to close as it gets darker. Photosynthesis can't happen in the dark, so they don't need to be open to let CO2 in. When the stomata are closed, very little water can escape
• Temperature - the warmer it is, the faster transpiration happens. When it's warm the water particles have more energy to evaporate and diffuse out of the stomata
• Air flow - the better the air flow around a leaf (e.g. stronger wind), the greater the transpiration rate. This means there's a high concentration of water particles outside the leaf as well as inside it, so maintaining a low concentration of water in the air outside the leaf. Diffusion then happens quickly, down a concentration gradient
• Humidity - the drier the air around a leaf, the faster transpiration happens. This is like what happens with air flow. If the air is humid there's a low of water in it already, so there's not much of a difference between the inside and the outside of the leaf. Diffusion happens fastest if there's a really high concentration in one place, and a really low concentration in the other
• You can estimate the rate of transpiration by measuring the uptake of water by a plant. This is because you can assume that water uptake by the plant is directly related to water loss by the leaves (transpiration)

  
• Set up the apparatus as in the diagram, and then record the starting position of the air bubble. Start a stopwatch and record the distance moved by the bubble per unit time, e.g. per hour. Keep the conditions constant throughout the experiment, e.g. temperature and air humidity

  
• This piece of apparatus is called a potometer

  
• Guard cells are adapted to open and close stomata
• Guard cells have a kidney shape which opens and closes the stomata ina leaf

  
• When the plant has lots of water the guard cells fill it and go plump and turgid. This makes the stomata open so gases can be exchanged for photosynthesis

  
• When the plant is short of water, the guard cells lose water and become flaccid, making the stomata close. This helps stop too much water vapour excaping

  
• Thin outer walls and thickened inner walls make the opening and closing work

  
• Guard cells are alsp sensitive to light and close at night to save water without losing out on photosynthesis

  
• You usually find more stomata on the undersides of leaves than on the top. The lower the surface is shaded and cooler - so less water is lost trough the stomata than if they were on the upper surface

  
• Guard cells are adapted for gas exchange and controlling water loss within a leaf