L3 - measuring transpiration rates

• Measuring transpiration:

  -can be difficult due to the practical difficulties of collecting evaporated water

  -rate of water uptake can be measured using a potometer

  -90 to 95% of the water taken up by the plant is lost in transpiration, so it gives us a good estimation of the rate of transpiration

  -the other 5 to 10% is used for photosynthesis
• Root pressure: a force generated by the roots that pushes water and mineral ions up the xylem. The mechanism involves active transport of mineral ions into the xylem, this lowers the water potential in the xylem so water moves in by osmosis
• Transpiration pull: transpiration (the evaporation of water from aerial parts of the plant) produces a tension or ‘pull‘ on the water in the xylem vessels by the leaves. Water molecules are cohesive so water is pulled up through the plant
• Capillary action: the ability of water molecules to move upwards in a fine tube. The polar molecules form hydrogen bonds with neighbouring water molecules and with polar molecules in the xylem walls, both cohesion and adhesion
• Factors that affect the rate of transpiration:

  -light

  -humidity

  -temperature

  -air movement

  -water availability
• If light intensity increases, the rate of photosynthesis increases too as more stomata open to allow more carbon dioxide to diffuse in. This means more water vapour is able to diffuse out
• If temperature increases, transpiration rate does too. This is because water molecules have more kinetic energy so water will evaporate into the air spaces faster, increasing the water vapour potential gradient, causing faster diffusion. Hotter air can also hold a higher amount of water vapour
• If humidity increases, transpiration decreases as there is more water vapour in the area surrounding the leaf. The water vapour gradient then decreases causing slower diffusion
• If air movement increases, transpiration does too as surrounding water vapour is blown away from the leaf, increasing the water vapour potential gradient, causing faster diffusion
• If soil-water availability increases, then the transpiration rate does too because when the soil is dry, the plant is under stress which reduces the rate of transpiration