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Transpiration |
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{{about|plant transpiration|transpiration in human and animal physiology|sweating|and|hyperhydrosis}} |
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[[File:transpiration Overview.svg|thumb|Overview of transpiration.<br> |
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1-Water is passively transported into the roots and then into the [[xylem]]. <br> |
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2-The forces of cohesion and adhesion cause the water molecules to form a column in the xylem. <br> |
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3- Water moves from the xylem into the mesophyll cells, evaporates from their surfaces and leaves the plant by diffusion through the stomata.]] |
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[[Image:Tomato leaf stomate 1-color.jpg|thumb|[[Stoma]] in a [[tomato]] leaf shown via colorized [[scanning electron microscope]]]] |
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[[Image:Afternoon Clouds over the Amazon Rainforest.jpg|thumb|The [[clouds]] in this image of the [[Amazon Rainforest]] are a result of transpiration.]] |
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'''Transpiration''' is the process of [[water]] movement through a [[plant]] and its [[evaporation]] from aerial parts, such as from [[leaf|leaves]] but also from [[Plant stem|stems]] and [[flower]]s. Leaf surfaces are dotted with pores which are called [[stomata]], and in most plants they are more numerous on the undersides of the foliage. The stomata are bordered by [[guard cell]]s and their stomatal accessory cells (together known as stomatal complex) that open and close the pore.<ref>{{citation| url = | publisher = Freeman, Scott| title = Biological Science| edition = 3| author = Benjamin Cummins| year = 2007| page = 215 }}</ref> Transpiration occurs through the stomatal apertures, and can be thought of as a necessary "cost" associated with the opening of the stomata to allow the diffusion of [[carbon dioxide]] gas from the air for [[photosynthesis]]. Transpiration also cools plants, changes [[osmotic pressure]] of cells, and enables [[mass flow]] of [[Plant nutrition|mineral nutrients]] and water from roots to [[shoot]]s. |
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Mass flow of liquid water from the roots to the leaves is driven in part by [[capillary action]], but primarily driven by water potential differences. In taller plants and trees, the force of gravity can only be overcome 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]]. |
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== Regulation == |
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Plants regulate the rate of transpiration by the degree of stomatal opening. The rate of transpiration is also influenced by the evaporative demand of the atmosphere surrounding the leaf such as [[humidity]], [[temperature]], wind and incident sunlight. [[Soil]] water supply and soil temperature can influence stomatal opening, and thus transpiration rate. The amount of water lost by a plant also depends on its size and the amount of water absorbed at the roots. Transpiration accounts for most of the water loss by a plant, but some direct evaporation also takes place through the cuticle{{citation needed|date=May 2013}} of the leaves and young stems. Transpiration serves to evaporatively cool plants as the escaping water vapor carries away heat energy. |
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This table summarizes the factors that affect the rates of transpiration. |
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{| class="wikitable" |
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|- |
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! Feature !! How this affects transpiration |
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|- |
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| Number of leaves || More leaves (or spines, or other photosynthesizing organs) means a bigger surface area and more stomata for gaseous exchange. This will result in greater water loss. |
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|- |
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| Number of stomata || More stomata will provide more pores for transpiration. |
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|- |
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| Size of the leaf || A leaf with a bigger surface area will transpire faster than a leaf with a smaller surface area. |
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|- |
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| Presence of [[plant cuticle]] || A waxy cuticle is relatively impermeable to water and water vapour and reduces evaporation from the plant surface except via the stomata. A reflective cuticle will reduce solar heating and temperature rise of the leaf,{{citation needed|date=May 2013}} helping to reduce the rate of evaporation. Tiny hair-like structures called [[trichome]]s on the surface of leaves also can inhibit water loss by creating a high humidity environment at the surface of leaves.{{citation needed|date=May 2013}} These are some examples of the adaptations of plants for conservation of water that may be found on many [[xerophytes]]. |
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|- |
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| Light supply || The rate of transpiration is controlled by stomatal aperture, and these small pores open especially for photosynthesis. While there are exceptions to this (such as night or "[[CAM photosynthesis]]"), in general a light supply will encourage open stomata. |
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|- |
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| Temperature || Temperature affects the rate in two ways: |
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1) An increased rate of evaporation due to a temperature rise will hasten the loss of water. |
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<br> 2) Decreased relative humidity outside the leaf will increase the water [[potential gradient]]. |
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| Relative humidity || Drier surroundings gives a steeper water potential gradient, and so increases the rate of transpiration. |
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| Wind || In still air, water lost due to transpiration can accumulate in the form of vapor close to the leaf surface. This will reduce the rate of water loss, as the water potential gradient from inside to outside of the leaf is then slightly less. Wind blows away much of this water vapor near the leaf surface, making the potential gradient steeper and speeding up the diffusion of water molecules into the surrounding air. Even in wind, though, there is some accumulation of water vapor in a thin [[boundary layer]] of slower moving air next to the leaf surface. The stronger the wind, the thinner this layer, and the steeper the water potential gradient. Also, the bigger the leaf, the greater the average thickness of the boundary layer, which means a bigger leaf will have a slightly slower transpiration rate per unit area (although a higher transpiration rate overall). |
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|- |
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| Water supply || [[Irrigation in viticulture|Water stress]] caused by restricted water supply from the soil may result in stomatal closure and reduce the rates of transpiration. |
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|} |
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<Gallery> |
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File:Transpiration Temperature Graph.svg|The effect of temperature on the transpiration rate of plants. |
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File:Transpiration WindVelocity Graph.svg|The effect of wind velocity on the transpiration rate of plants. |
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File:Transpiration Humidity Graph.svg|The effect of humidity on the transpiration rate of plants. |
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</Gallery> |
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[[Image:Xerophyte.png|thumb|300 px|Some [[xerophyte]]s will reduce the surface of their leaves during water deficiencies (left). If temperatures are cool enough and water levels are adequate the leaves expand again (right).]] |
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A fully grown [[tree]] may lose several hundred of gallons of water through its leaves on a hot, dry day.{{citation needed|date=June 2013}} 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 (''i.e.'', crop plants transpire 200 to 1000 kg of water for every kg of dry [[matter]] produced).<ref>{{Citation |
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| last=Martin | first=J. | last2=Leonard | first2=W. |
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| last3=Stamp | first3=D. |
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| title=Principles of Field Crop Production (Third Edition) |
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| publisher=New York: Macmillan Publishing Co., Inc. | year=1976 |
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| isbn=0-02-376720-0}}</ref> |
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Transpiration rates of plants can be measured by a number of techniques, including [[potometer]]s, [[lysimeter]]s, porometers, [[photosynthesis system]]s and heat balance sap flow gauges. Isotope measurements indicate transpiration is the larger component of [[evapotranspiration]].<ref>{{cite journal|last=Jasechko|first=Scott|coauthors=Zachary D. Sharp, John J. Gibson, S. Jean Birks, Yi Yi & Peter J. Fawcett|title=Terrestrial water fluxes dominated by transpiration|journal=Nature|date=3 April 2013|doi=10.1038/nature11983|url=http://www.nature.com/nature/journal/vaop/ncurrent/full/nature11983.html|accessdate=4 April 2013|pmid=23552893|volume=496|issue=7445|pages=347–50}}</ref> |
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[[Desert]] plants 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. |
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{{clear}} |
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==See also== |
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*[[Antitranspirant]] – a substance to prevent transpiration |
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* [[Eddy covariance]] flux (aka eddy correlation, eddy flux) |
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* [[Hydrology (agriculture)]] |
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* [[Latent heat flux]] |
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* [[Soil plant atmosphere continuum]] |
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* [[Weap|Water Evaluation And Planning system (WEAP)]] |
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== References == |
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{{Reflist|2}} |
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== External links == |
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* [http://www.ramin.com.au/creekcare/trees.shtml Transpiration by Trees] |
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* [http://ga.water.usgs.gov/edu/watercycleevapotranspiration.html USGS The Water Cycle: Evapotranspiration] |
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{{Botany}} |
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[[Category:Hydrology]] |
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[[Category:Plant physiology]] |
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Revision as of 18:37, 24 February 2014
Transpiration