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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 onboth 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 willoccur 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 uptranspiration 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 ofthe 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 humidair 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 thesurrounding 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 keepup 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 tothe 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 maytranspire 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.

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Source:  OpenStax, Siyavula: life sciences grade 10. OpenStax CNX. Apr 11, 2012 Download for free at http://cnx.org/content/col11410/1.3
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