Transpiraton
Monacia Williams, Contributor

Hello again, students! I hope you all enjoyed your holidays and that the new year will be productive. I also hope your minds are still focused on the lessons.

This week, we are going to look at the factors that influence the movement of water vapour from the plant. Last week, we learnt that this movement was known as transpiration. In order to understand fully what happens in transpiration, you need to bear in mind some concepts you learnt some time ago. You learnt that substances move by one of two means, either by diffusion or osmosis. You also learnt that water moves by osmosis and other substances move by diffusion.

This could create some confusion if you forget that what is actually moving from the plant is water vapour, a gas, not water, a liquid, and that this movement is not from the cell across a cell membrane, but from the intercellular air spaces in the leaves. This means then that the movement of the water vapour is by diffusion, not osmosis. Since the movement is by diffusion, the main factor which affects diffusion will also affect the movement of water vapour.

Do you recall what needs to be present for diffusion to take place? Did you answer a diffusion gradient? If you did, you are correct! You will find that all the factors that affect transpiration are factors that affect the existence of the diffusion gradient.

As we prepare to look at these, first a brief recap.

• Water evaporates from the spongy mesophyll cells into the intercellular air spaces.
• The air spaces become filled with water vapour.
• This creates a diffusion gradient between the air spaces and the outside.
• Water vapour diffuses through the stomata into the air.
• Movement of water from the spongy cells create an osmotic gradient between the cells and the xylem, so water moves into the cells to replace the water that is lost.
• We need also to remember that, although there are times when transpiration can have a deleterious effect on the plant, it does provide the following benefits:
• It provides the leaves with water for photosynthesis.
• It transports the dissolved minerals from the roots to the other parts of the plant.
• It helps to provide turgidity for herbaceous plants.
• The evaporation of water from the leaves cools the plant.

The movement of water is governed by the opening and closing of the stomata, hence the transpiration rate is greater during the day when the stomata are open to facilitate photosynthesis. There are times during the day when the stomata will close to prevent the loss of water even though the light intensity is ideal for photosynthesis to occur. This means that the need for the plant to conserve water is greater than the need to carry out photosynthesis. In environments where water is at a premium, many plants have developed adaptations to aid them in the conservation of water.

Factors affecting transpiration

The rate at which the plant takes up water depends on the rate at which the plant loses this water in the form of vapour. This is known as the transpiration rate. Transpiration rate can be measured experimentally. These measurements can be used to test the effect of the environmental factors on transpiration. The measurements are made using an instrument called a potometer. This is a specially designed instrument used to measure the rate of water uptake in a leafy twig. Important: You need to pay particular attention to the design and operation of this instrument because many questions on transpiration are centred on transpiration experiments.

• Light intensity

During bright sunlight, the stomata will be fully open to facilitate the movement of carbon dioxide but, as already stated, there are times during the day, from noon until about 2 p.m., when the stomata will close to prevent excessive water loss.

• Temperature

On a hot day, any water vapour around the leaves is going to evaporate quickly, hence a diffusion gradient will always be present. Therefore, water vapour will continue to move from the intercellular spaces through the stomata in order to achieve equilibrium. Hence, transpiration rates will be high - transpiration rates increase with rises in temperature.

• Humidity

When the relative humidity of the air is high, the transpiration rate will be low. Relative humidity is a measure of the water vapour content of the air, so if the humidity is high, this means that the air contains a lot of water vapour. If the water vapour content of the air is high, and that of the intercellular spaces is also high, this would remove the diffusion gradient, and hence, diffusion would not take place. When the relative humidity is low, the reverse happens, and the transpiration rate increases. Therefore, transpiration rate decreases with an increase in relative humidity.

• Wind

Windy conditions increase the rate of transpiration because the wind blows away the water vapour from around the leaves. This maintains the diffusion gradient; thus, the transpiration rate remains high. If the air is still, the water vapour remains around the leaves and the gradient is not maintained, hence the transpiration rate remains low. Transpiration rate increases with an increase in wind speed.

Next, we will look at some questions on transpiration. See you then, have a great week!

Monacia Williams teaches at Glenmuir High School. Send questions and comments to kerry-ann.hepburn@gleanerjm.com