Transpiration
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{{dablink|Transpiration may also refer to [[sweating]] and [[hyperhydrosis]].}}
'''Transpiration''' is the [[evaporation]] of water from the aerial parts of [[plant]]s, especially [[leaf|leaves]] but also [[Plant stem|stem]]s, [[flower]]s and [[root]]s. 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 [[Plant nutrition|mineral nutrients]] and water from [[root]]s to [[shoot]]s. 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 [[Earth's atmosphere|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 [[gallon]]s (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 [[Crop (agriculture)|crops]] tends to fall between 200 and 1000 (i.e., crop plants transpire 200 to 1000 kg of water for every kg of dry matter produced) {{Harv|Martin|Leonard|Stamp|1976|p=81}}.
Transpiration rate of plants can be measured by a number of techniques, including [[potometer]]s, [[lysimeter]]s, porometers, and heat balance sap flow gauges.
[[Desert]] plants and [[conifer]]s have specially adapted structures, such as thick cuticles, reduced leaf areas, sunken stomata and hairs to reduce transpiration and conserve water. Many [[cactus|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.
== See also ==
*[[Evaporation]]
*[[Active Transport]]
*[[Osmosis]]
*[[Hyperhydrosis]]
*[[Transpirational pull]]
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== References ==
* [http://extension.oregonstate.edu/mg/botany/transpir.html A description of transpiration, including a short animation illustrating the process]
* {{Harvard reference |
Surname1=Martin | Given1=J. |
Surname2=Leonard | Given2=W. |
Surname3=Stamp | Given3=D. |
Title=Principles of Field Crop Production (Third Edition) |
Publisher=New York: Macmillan Publishing Co., Inc. | Year=1976 |
ID=ISBN 0-02-376720-0}}
== External links ==
*[http://www.ramin.com.au/creekcare/trees.shtml Transpiration by Trees]
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{{Botany}}
[[Category:Hydrology]]
[[Category:Plant physiology]]
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