What factors affect the rate of transpiration.

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Date of investigation: 22nd May 2001

Title: Transpiration

Question: What factors affect the rate of transpiration.

Factors:

Various environmental factors affect the rate of transpiration in a plant.

Here are a few: -

Temperature

A temperature increase means that the water will evaporate more rapidly from the leaves of a plant causing more water to be absorbed through the roots. The thermal energy also helps to make the chemical reaction between carbon dioxide and water happen more often to make glucose more efficiently. As photosynthesis only use’s 1%-2% of the water taken out by the roots this causes little change on the rate of transpiration.

Availability of water

The plant will have trouble transpiring steadily if the amount of water loss is more then the uptake of water by the plant. If the rate of transpiration exceeds the absorption of water, loss of turgor occurs. (The explanation of turgid can be found in my background information.) As a result of this the stomata closes, which leads to an immediate reduction in the rate of transpiration and photosynthesis. The loss of turgor in the plant causes it to wilt.

Light intensity

The sunlight warms up the plant causing more evaporation to occur on the surface of the leaves. The presence of light also stimulates the opening of stomata. The size of stomata this has a great impact on the rate of transpiration. The larger the stomata the more water vapour diffused from the stomata. This is the reason plants transpire more rapidly during the day.

Wind factor

When the air starts to fill with water vapour the humidity starts to affect the plant. The plant can only diffuse water vapour through the stomata if the leaf cells contain more vapour pressure then the air outside. If the air is humid the rate of transpiration decreases rapidly. When the wind is acting on the air around the plant it transports the molecules away, decreasing the vapour pressure in the air.

Humidity

Water loss from plants occurs when the vapour pressure in the air is less then that in the leaf cells. If humidity starts to build then the vapour pressure outside will be greater and so the rate of transpiration will decrease.

Root surface area

A larger surface means the plant is able to absorb more water increasing the rate of transpiration.  

Background information:

A plant takes up water trough its roots. Water is then transported upwards trough the xylem cells. (If you place a plant cutting in dyed water you will be able to highlight the xylem cells.) The water travels upwards trough a process called osmosis.

This is when a cell with a high concentration of water transports the excess water to a cell with a lower concentration of water. The water enters trough the semi permeable cell wall. When the sap vacuole is full of water it is turgid. When plant cells are turgid they can handle the pressure because they have a cell wall. In animal cells when the cells are turgid they swell up and bust. This is called haemolysis.

If a plant cell is placed in a strong sugar or salt solution water passes out by osmosis.

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These cells now contain little water in their sap vacuole. They are now flaccid.

The cells become plasmolysed when more water is diffused and the cytoplasm starts to pull away from the cell wall as the vacuole shrinks. The cells begin to wilt.

As the water reaches the leaves 1%-2% is used up during photosynthesis. The remaining water is diffused out if the stomata.

Stomata are openings normally found on the underside of a leaf. They allow water vapour and waste gases such as oxygen to be diffused out of the leaf.

The stomata are surrounded by two crescents ...

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