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2.1.2 Transpiration

Module by: Zamekile Sondzaba. E-mail the author

Transpiration

Transpiration is the evaporation of water from the leaves of plants. Water is lost from the leaf through special pores called stomata. Stomata are found on both surfaces of the leaf but there are usually more on the ventral (lower surface ) of the leaf. This is to reduce the amount of transpiration that will occur because the top of the leaf is exposed to more sunlight than the bottom.

http://education.uoit.ca/lordec/ID_LORDEC/transpiration_pull/

This interactive website explains transpiration pull. Plants use the process of transpiration pull to move water from the soil up into the leaves.

Heat from the environment causes the water in the sub-stomatal air spaces to evaporate out of the stomata. This process is called transpiration.

Transpiration is therefore defined as the loss of water vapour from the leaves of a plant.

Transpiration only occurs during the day when the stomata are open. At night the stomata are closed.

Excess water diffuses into the sub-stomatal air spaces to replace that which has been lost from the leaves

Water diffuses from the xylem of the leaf into surrounding mesophyll cells.

Water circulates to supply plants with their water requirements.

Water is pulled from the xylem of the stem to the xylem of the leaves. The xylem is found in the veins of the leaf.

Rate of transpiration

This increases in conditions of …

  • High light intensity (bright sunlight)
  • Increased temperatures (hot weather)
  • Wind
  • Low humidity (dry conditions)

Light Plants transpire more rapidly in the light than in the dark. This is largely because light stimulates the opening of the stomata. Light also speeds up transpiration by warming the leaf.

Temperature Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises due to the increased kinetic energy of the water molecules. At 30°C, a leaf may transpire three times as fast as it does at 20°C.

Wind When there is no breeze, the air surrounding a leaf becomes increasingly humid thus reducing the rate of transpiration. When a breeze is present, the humid air is carried away and replaced by drier air. So a steep diffusion gradient is maintained.

Humidity The rate of diffusion of any substance increases as the difference in concentration of the substances in the two regions increases. When the surrounding air is dry, diffusion of water out of the leaf goes on more rapidly.

Soil water A plant cannot continue to transpire rapidly if its water loss is not made up by replacement from the soil. When absorption of water by the roots fails to keep up with the rate of transpiration, loss of turgor occurs, and the stomata close. This immediately reduces the rate of transpiration (as well as of photosynthesis). If the loss of turgor extends to the rest of the leaf and stem, the plant wilts.

The volume of water lost in transpiration can be very high. It has been estimated that over the growing season, one acre of corn (maize) plants may transpire 1.5 million litres of water. As liquid water, this would cover the field with a lake 38 cm deep. An acre of forest probably does even better.

This diagram shows a potometer which is used to measure the rate of transpiration. As the leafy twig transpires, the air bubble moves to the left. The quicker the air bubble moves the faster the leafy twig is transpiring.

Figure 1
Figure 1 (Picture 11.png)

Suggestion: Can we have a

graph relating to

transpiration rate over time?

Also add an arrow to show

movement of the bubble

TOWARDS the plant for

greater clarity. [clark]

The diagram below shows a summary of the movement of water from the roots to the leaf.

Figure 2
Figure 2 (Picture 12.png)

Why do plants need water?

Plants need water to maintain turgor pressure. This helps provide support for the plant as when a cell absorbs water the cell membrane pushes against the cell wall. The cell is now turgid. If there isn’t enough water in the plant the membrane moves away from the cell wall and the cell is now flaccid. This is when a plant begins to wilt and it will eventually die. Wilting is the loss of rigidity of the non-woody parts of plants and occurs when the turgor pressure falls towards zero. Lower water availability can results from drought conditions, high salinity, saturated soil conditions, low temperatures or bacterial or fungal infections that affect the vascular system of the plant,

When the environment is extremely humid (moist) the rate of transpiration is very low. In some plants, the leaves secrete water onto the surface of the leaves through specialised pores called hydathodes. Hydathodes are the open ends of xylem vessels at the edges of the leaves in certain species of plants such as strawberries and some grasses.

Investigation – the effect of environmental conditions on transpiration rate (using a simple potometer)

A potometer measures the rate of transpiration by measuring the movement of water into a plant. The following experiment uses a simple hand made photometer.

Aim: To assess the effect of different environmental conditions (e.g. temperature) on transpiration rate.

Apparatus

  • a drinking straw
  • a soft green leafy shoot
  • Vaseline
  • Marking pen
  • Play dough / putti
  • Plastic bag
  • Elastic band
  • Ruler

Method

Perform the following steps under water

  1. Cut the stem of the leafy shoot under water .
  2. Test to make sure the stem of the leafy twig will fit snug tightly into the top of the straw.
  3. Remove the leafy shoot from the straw and set aside.
  4. Fill the straw with water. Place your finger over one end of the straw to stop the water from running out.
  5. Put the leafy shoot into the open end and seal it with play dough while removing it from water (KEEP FINGER ON THE STRAW!)

Perform the following steps above water

  1. Seal with Vaseline. Make sure it is air tight and water tight – if not, all the water will run out when you take your finger off the straw.
  2. Mark the water level on the straw.
  3. Place your photometer under one of the following conditions for one hour:
  • as is, in a warm, sunny place (no wind)
  • as is, in a warm, windy place
  • with a plastic bag tied around the leaf, in a warm, sunny place.
  • A shady place
  1. After an hour: use the marking pen to mark the change in water level on the straw.
  2. Measure the distance the water moves.

Results

  • Draw a table and record the class’ results.
  • Plot a bar graph to compare the distances the water moved in the different straws.

Discussion

  • Why is it important to cut the stem under water?
  • What does the water movement in the straw indicate?
  • Which four external environmental factors are you investigating?
  • Under which condition is water loss from the leaf the greatest?

Conclusion

  • What can you conclude from this investigation?
  • Give two ways in which you can improve your experimental results.

More information about potometer experiments can be found on the following websites:

http://www.practicalbiology.org/areas/advanced/exchange-of-materials /transpiration-in-plants/measuring-rate-of-water-uptake-by-a-plant-shoot- using-a-potometer,62,EXP.html

http://www.practicalbiology.org/areas/advanced/exchange-of-materials /transpiration-in-plants/

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