Everything about Transpiration totally explained
Transpiration is the
evaporation of water from the aerial parts of
plants, especially
leaves but also
stems,
flowers and
roots. Leaf transpiration occurs through
stomata, and can be thought of as a necessary "cost" associated with the opening of stomata to allow the diffusion of
carbon dioxide gas from the air for
photosynthesis. Transpiration also cools plants and enables
mass flow of
mineral nutrients and water from
roots to
shoots. Mass flow is caused by the decrease in hydrostatic (water) pressure in the upper parts of the plants due to the
diffusion of water out of
stomata into the
atmosphere. Water is absorbed at the roots by
osmosis, and any dissolved mineral nutrients travel with it through the
xylem.
The rate of transpiration is directly related to the degree of stomatal opening, and to the evaporative demand of the atmosphere surrounding the leaf. The amount of water lost by a plant depends on its size, along with the surrounding
light intensity,
temperature,
humidity, and
wind speed (all of which influence evaporative demand).
Soil water supply and soil temperature can influence stomatal opening, and thus transpiration rate.
A fully grown tree may lose several hundred
gallons (a few cubic meters) of water through its leaves on a hot, dry day. About 90% of the water that enters a plant's roots is used for this process. The
transpiration ratio is the ratio of the mass of water transpired to the mass of dry matter produced; the transpiration ratio of
crops tends to fall between 200 and 1000 (for example, crop plants transpire 200 to 1000 kg of water for every kg of dry matter produced) .
Transpiration rate of plants can be measured by a number of techniques, including
potometers,
lysimeters, porometers, and heat balance sap flow gauges.
Desert plants and
conifers have specially adapted structures, such as thick cuticles, reduced leaf areas, sunken stomata and hairs to reduce transpiration and conserve water. Many
cacti conduct photosynthesis in
succulent stems, rather than leaves, so the surface area of the shoot is very low. Many desert plants have a special type of
photosynthesis, termed
Crassulacean acid metabolism or CAM photosynthesis in which the stomata are closed during the day and open at night when transpiration will be lower.
Further Information
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