• The?effect of environmental factors on the rate of transpiration?in plants can be measured using a piece of equipment called a?potometer
  • Note that while potometers are used to measure transpiration rates, they?technically measure the rate of water uptake?rather than the rate of transpiration, as a small amount of the water taken up by a plant?will be used in photosynthesis
    • Because the amount of water used in photosynthesis is so small in relation to the total amount of water that passes through a plant, the?rate of water uptake can reasonably be used to represent the rate of transpiration
• Different types of potometer exist
  • Bubble potometers?measure the?movement of an air bubble along a water-filled tube?connected to a plant shoot as water is drawn up by the shoot
    • The position of the air bubble is?recorded at the start of an experiment, and then a researcher can either measure?how far the bubble moves in a set amount of time, or?time how long it takes for the bubble to move a certain distance
  • Mass potometers?measure the?change in mass of a water-filled test tube?connected to a plant shoot as it loses water over a set amount of time
• The effect of?various environmental factors?on transpiration can be measured by placing the potometer in different conditions e.g.
  • Wind speed
  • Humidity
  • Light intensity
  • Temperature

Investigating the effect of light intensity on the rate of transpiration

Apparatus

• Plant shoot
• Cutting board
• Scalpel/scissors
• Paper towels
• Potometer
• Volume scale
• Beaker
• Capillary tube
• Stopwatch
• Vaseline

Method

• Cut a shoot?underwater
  • This is done to?prevent air from entering the xylem; this could block the movement of water through the plant
• Set up the apparatus?as shown in the diagram, ensuring that it is?airtight, and using vaseline to seal any possible gaps
• Dry the leaves?of the shoot
  • Any water present on the leaves might affect the rate of transpiration as it?could block the stomata
• Remove the capillary tube from the beaker of water to?allow a single air bubble to form?and then place the tube back into the water
• Set up a light source?from which the?light intensity can be varied
  • This could be achieved by varying the light bulbs used or by varying the distance between the light source and the plant shoot
• Allow the plant to adapt to the new environment for 5 minutes
• Record the starting location?of the air bubble, leave for a?set period of time, and then?record the end location?of the air bubble
• Change the light intensity?by a measurable amount e.g. moving the lamp 10cm further away from the plant shoot
• Reset the bubble?by opening the tap below the reservoir
• Repeat?the experiment at the new light intensity, and again at a?range of different intensities

• A potometer can be used to?test hypotheses about the effect of various environmental factors, including temperature or humidity, on?transpiration rates
• Environmental factors can be investigated in the following ways
  • Air movement
    • A?fan on different settings?could be used to vary the flow of air around a plant shoot
  • Humidity
    • Enclosing the plant shoot in a?plastic bag?can increase the humidity
    • A?humidifier or dehumidifier?could be used to give a measurable variation in humidiy
  • Light intensity
    • A?lamp at different distances?or with?different types of light bulb?can be used to vary light intensity
  • Temperature
    • A?thermometer or temperature probe?can be used to find surroundings with different air temperatures
    • A?heater or air conditioner?can be used to give a measurable variation in temperature
• A researcher would need to be aware of the importance of?controlling any variables other than the variable of interest?to ensure that any results were valid e.g. placing a plant shoot in different rooms could be a way of varying temperature, but might bring the risk of also varying light levels and humidity; these variables would need to be controlled

Factors affecting the rate of transpiration

• Air movement
  • More air movement leads to increased rates of transpiration
    • The air outside a leaf usually contains a?lower concentration of water vapour?than the air spaces inside a leaf, causing water vapour to?diffuse out of the leaf
    • When the air is relatively still,?water molecules can accumulate?just outside the stomata, creating a?local area of high humidity
    • Less water vapour will diffuse out?into the air due to the?reduced concentration gradient
    • Air currents, or wind, can?carry water molecules away from the leaf surface, increasing the?concentration gradient?and causing more water vapour to diffuse out
• Temperature
  • Higher temperatures lead to higher rates of transpiration, up to a point at which?transpiration rates will slow?
    • An increase in temperature results in an?increase in the kinetic energy?of molecules
    • This increases the rate of transpiration as?water molecules evaporate out of the leaf?at a faster rate
    • If the temperature gets too high the?stomata close to prevent excess water loss
    • This dramatically?reduces the rate of transpiration
• Light intensity
  • Higher light intensities will increase the rate of transpiration?up to a point at which?transpiration rates will level off?
    • Stomata close in the dark?and their closure greatly reduces the rate of transpiration
    • Stomata open when it is light?to enable gas exchange for photosynthesis; this increases the rate of transpiration
    • Once the stomata are all open any increase in light intensity has no effect on the rate of transpiration
• Humidity
  • Higher humidity levels reduce the rate of transpiration?
    • If the humidity is high that means the?air surrounding the leaf surface is saturated with water vapour
    • This causes the rate of transpiration to decrease as there is?no concentration gradient?between the inside of the leaf and the outside
      • At a certain level of humidity, an?equilibrium?is reached; water vapour levels inside and outside the leaf are the same, so there is no net loss of water vapour from the leaves