Transpiration

Transpiration is the evaporation of water from the surface of a leaf.  A plant loses around 90% of its water from transpiration.

Water is on the spongy mesophyll cells in a leaf. However, when it evaporates, it diffuses out of the leaf into the atmosphere. This is because the area on the outside of the leaf has a lower water potential than that inside of the leaf where the water vapour is currently situated. The xylem is a continuous tube from the roots to the leave therefore, as the water is being lost, more water is being drawn up through the xylem to replace it thus creating a flow of water.

Note: the water continues up the xylem due to intermolecular forces.

There are various factors that influence the rate of transpiration such as:

1. Light intensity
2. Temperature
3. Humidity
4. Wind intensity

1. If the light intensity increases, the stomata open to absorb carbon dioxide for photosynthesis. They do this as the guard cells [of stomata] gain water through osmosis. This results in them becoming turgid and opening the stomata. When the stomata are opened, water vapour is lost hence the transpiration rate increases.

2. If you increase the temperature, more water would be evaporated from inside the leaf on the spongy mesophyll. Transpiration is the evaporation of water from the surface of a plant hence greater heat = more evaporation = greater rate of transpiration. Remember, water can only escape from leaves in a gas form.

3. If you increase the humidity of the atmosphere this means that the volume of water vapour in an atmosphere increases. Therefore there is a smaller difference in water potential between the inside of the leaf and in the atmosphere. As a result, less water evaporates and diffuses into the atmosphere. Hence the rate of transpiration decreases.

4. If you increase the wind intensity of the atmosphere, more water molecules are going to be removed from the outside of the leaf therefore decreasing the water potential. To balance the concentration gradient,  more water diffuses into the atmosphere to replace the water vapour hence the rate of transpiration increases.

Prevention

Sometimes, the rate of transpiration is too great for a plant and they are losing a lot of water. To prevent this from happening, leaves are covered in wax. There are also much fewer stomata on the top of the leaf than at the bottom to restrict the water vapour from diffusing. The top of the leaf is more exposed to the light intensity,temperature, wind intensity more than the bottom side of the leaf. When flaccid, the guard cells will close. Every cells'  'turgor pressure' is lost meaning the plant is no longer supported and will die.

Turgor pressure and plant support

Remember, turgid cells are those that contain a large volume of water. They have a large turgor pressure which supports the plant and keeps it growing upwards and not wilting. Turgor pressure is the pressure of the water contained within the cell acting upon its cell wall. If all cells are turgid they are strongly pushing towards each other thus propelling the plant to grow upwards. A cell with a lack of water would have lost its turgor pressure and would wilt.

Experiments to investigate the role of environmental factors in the rate of transpiration

A potometer is simply used to measure the rate of water taken up through a plant.

Set up a basic potometer comprising of a beaker of water with a known volume, a capillary tube and a plant and rubber tubing.

Using a hairdryer, desk lamp and candle, situate each object one at a time near to the plant for a time of an hour.

Then measure how much water has been taken up by the plant by subtracting the new volume of water in the beaker by the new volume.

To ensure no water has been lost by evaporation [from the beaker] place a layer of oil on top.